Journal of Electrical Engineering and Electronic TechnologyISSN: 2325-9833

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Review Article, J Electr Eng Electron Technol Vol: 3 Issue: 1

Study in F.O.C. Multiplexing Techniques-A Review

Devendra KumarTripathi*, Pallavi Singh, Shukla NK and Dixit HK
J.K. Institute of Applied Physics and Technology, University of Allahabad, U.P, India
Corresponding author : Devendra Kumar Tripathi
J.K. Institute of Applied Physics and Technology, Department of E&C University of Allahabad, U.P, India
E-mail: [email protected]
Received: July 27, 2013 Accepted: January 20, 2014 Published: January 24, 2014
Citation: Tripathi DK, Singh P, Shukla NK, Dixit HK (2014) Study in F.O.C. Multiplexing Techniques-A Review. J Electr Eng Electron Technol 3:1. doi:10.4172/2325-9833.1000110

Abstract

Study in F.O.C. Multiplexing Techniques-A Review

Communication network bandwidth requirement is growing at stunning rate, consequently numerous multiplexing techniques have been proposed by contributors for the augment in spectral efficiency from time to time. In this view the paper presents profound review on numerous multiplexing techniques for fiber optic communication, such as wavelength division multiplexing, optical time division multiplexing, optical code division multiplexing, optical orthogonal division multiplexing, mode division multiplexing, space division multiplexing, polarization division multiplexing and duty cycle division multiplexing technique.

Keywords:

Keywords

DWDM; ONU; SCM; SHCD; PDM-DQPSK; OOFDM; UHD; DOCDMA

Introduction

Primary requirement of public has been to communicate since primordial days. It twisted growing attention in formulating communication schemes for transporting messages from one to another place. With introduction of high performance computer processors fetched a lot of benefits for digital communication over that of analog communication, comprising much more aspects, storage and quicker processing. It created massive amount of information, that is growing exponentially each year and to be transported over data transport networks. Different kinds of communication system emerged over the past decades and fundamental inspirations for every category were to get better transmission fidelity, enhance the data rate and enhancing transmission distance amid different destinations, which can be obtained by using optical fiber optic communications. It is a method of transmitting information from one destination to other by launching pulses of light throughout an optical fiber. The light forms electromagnetic carrier wave which were modulated to transmit information signal. It was developed in the 1970s and revolutionized the communications industry and has played a significant role in the discovery of the information age due to of its benefits over electrical transmission. Optical fiber communication systems have basically replaced copper wire communications in core networks of the developed world because of its benefits over the traditional media. In the older times 1800’s first wires were put down for telegraph and trend has been to augment the quantity of information which may be transported in a specified interval. In the very beginning year’s telegraphs, telephones and several other straightforward communication techniques have been exercised by the time need for speed and bandwidth have further grown up so burdening the pre-existing network. Thus the straightforward and apparent answer was to insert additional lines of communication. However wire-laying and maintenance cost was too expensive, consequently new solution to the difficulty has to be devised and the key solution was the multiplexing techniques [1]. The process of making the most efficient utilization of the existing channel capability is referred as Multiplexing and different techniques that can accomplish this include are wavelength division multiplexing (WDM) [2-51], sub carrier multiplexing (SCM) [52-55], time division multiplexing (TDM) for optical domain OTDM [56-64], Mode division multiplexing (MDM) [65-78], Space division multiplexing (SDM) [79-95], Duty cycle division multiplexing (DCDM) [96], Polarization division multiplexing(PDM) [97-121], frequency division multiplexing (FDM) for optical domain OOFDM [122-207], code division multiplexing (CDM) for optical domain OCDMA [208-316]. Recent literature review for each has been described as follows.

Wavelength Division Multiplexing (WDM)

Amongst various multiplexing methods wavelength division multiplexing (WDM) systems are an innovation, which is essentially frequency division multiplexing in the optical frequency domain, where on single optical fibers there are multiple communication channels at different wavelengths [2]. One of the significant approach in reducing the cost per information bit in wavelength division multiplexed fiber optic Communication systems are to augment spectral efficiency that is to fill highest possible information in the limited bandwidth of inline optical amplifiers. The conception of WDM has been pursued since first commercial light wave systems became available in 1980, further 1.33 μm window upgraded to 1.55 μm resulting in channel spacing of 250nm. Considerable attention were paid during 1980’s toward reducing channel spacing and multichannel systems with a channel spacing of less than 0.1nm had been demonstrated by 1990 [2-5]. One of the simplest WDM scheme has been illustrated in Figure 1 it may contain different types of optical filter such as post-amplifier or booster, in-line amplifier and preamplifier. On the transmitter side N-independent users and information data were modulated onto N-high frequency carriers each with unique wavelength (λ). These wavelengths may be spaced based on ITU-T standards. A wavelength multiplexer combines these optical signals and couples them into a single fiber. On the receiving end demultiplexer is essential to split the optical signals into suitable channels. This is done with N-optical filters whereby their cut-off frequencies were set based on the transmitted light source frequency. Total capacity of a WDM link depends on how much close the channels able to be spaced in the existing transmission window. The major disadvantage of WDM is the low channel utilization and spectral efficiency, because one wavelength is required per user. Therefore, for multiplexing n users, n wavelengths or light sources with n filters are required, which increases the cost of the system. The goals of all other multiplexing techniques were to increase channel utilization and channel capacity of the WDM systems. However in late 1980s with the advent of tunable lasers that have extremely narrow line width, one then can have very closely spaced signal bands. This is the basis of dense WDM (DWDM) and during decade 1990 WDM systems were developed most stridency [6,7].
Figure 1: A simple WDM system.
WDM systems allocated into different wavelength patterns such as coarse WDM (CWDM) and denseWDM (DWDM). Dense wavelength division multiplexing uses the identical transmission window but with denser channel spacing. DWDM merges multiple optical signals so that they may be amplified as a group and transported over a single fiber to increase capacity. With coarse wavelength division multiplexing in contrast to conventional WDM and DWDM employs enhanced channel spacing to permit less sophisticated and hence cheaper transceiver designs. For CWDM wideband optical amplification is unavailable so restricting the optical spans to some tens of kilometers WDM, DWDM and CWDM were based on the identical notion of employing different wavelengths of light on a single fiber but vary in the spacing of the wavelengths, and number of channels and the capability to amplify the multiplexed signals in the optical space.
In the WDM/DWDM scheme, the existing fiber cables were used to transmit increased number of wavelengths instead of single wavelength carried by the fiber cables in the early 1990’s. For WDM in the very beginning started with only four signal wavelength channels per fiber in the mid of 1990’s, but current optical transmission systems have been transporting up to 128 wavelength channels in a single fiber has been vogue in market. The erbium-doped fiber amplifier (EDFA) had prepared WDM highly striking because it could simultaneously amplify many WDM channels by the mid of 1990s. It has allowed the capacity of fiber-optic communication systems to scale in the wavelength domain by two orders of magnitude as compared to single-channel systems. Although much commonly deployed systems support thirty two to sixty four channels with total system capacity created Tb/s (Tera-bits/s) a milestone in system revelations by major vendors in 1998. Until ~2000, obtaining a closer WDM channel spacing was a matter of benefitting towards the stability of lasers and of building extremely frequency selective optical filters. But pre-2000, the augment in optical spectral efficiency, has been consequently because of enhancements in different device technologies. Along with efficient commercial WDM systems have come into view near around year 1995 and their entire communication capacity had surpassed 1.6Tb/s by the year 2000.So undoubtedly with the introduction of WDM systems has led a virtual upheaval in the designing of light wave systems and employing it 1.6 Tb/s communication capacities has been accounted [8-12]. Another development with ultradense wavelength division multiplexing method large number of information channels may be transmitted using channel to channel spacing nearer as 12.5 GHz [13,14]. Optical networks employing wavelength division multiplexing (WDM) are believed to be the next generation networks that can meet the ever-increasing demand for bandwidth of the end users [15]. Along with using narrowly spaced channel a spacing, up to 2.5 Tb/s transmission is reported [16] by multiplexing 256×12.5Gb/s channels, and transmitted over 2000 km standard single mode fiber (SSMF).
Performance investigation with wide band amplification in addition to spectrally-efficient modulation/multiplexing techniques, wide-band optical amplification also essential to realize 10-Tb/s-class total capacity. Conventional C- and L-band EDFAs have the gain bandwidth of about 4THz. Larger transmission capacity has been also achieved by utilizing the S- and L-bands [17]. Another good approach towards bandwidth-extension occurred, was with parallel arrangement of two (C+L) in 1997 [18] or three (S+C+L) in 1999 [19] fiber amplifiers and 13.7-THz signal bandwidth have been obtained over the S-, C-, and L-bands by using three types of fiber amplifiers. Also 10.9 Tb/s transmission experiment has been demonstrated in year 2001 [20]. Investigation for triple bands (S+C+L) 10.92 Tb/s transmission were also experimentally reported by using 273 WDM channels and 50 GHz spacing with40-Gb/s/ch WDM signal [21]. To realize cost-effective WDM systems and hence extending the gain bandwidth optical amplifiers have been an attractive approach. Extended L-band amplifiers such as Phosphorous co-doped silicate EDFA (P-EDFA), Bismuth-oxide-based EDFA and telluride based EDFA offer wide continuous gain bands. There has been realization of 7-THz bandwidth and demonstrated 14Tb/s transmission by means of a P-EDFA in year 2006. The hybrid configuration of Raman and EDFA also expand the single gain bandwidth with only minor modification to the conventional EDFA configuration. With 10.2-THz bandwidth 20.4-Tb/s optical transmissions have been established by using a hybrid of P-EDFA and Raman amplifiers in year 2007 [22]. All-Raman amplification has been also attractive to obtain a wide continuous gain bandwidth, longer reach 3000kmand 12.2-THz bandwidth has been obtained by using silica fiber Raman amplifiers in 2005 [23]. Along with hybrid Silica/Telluride fiber Raman amplifiers have attained 201.8-nm bandwidth, revealed for 1001 channels with 25-GHz channel to channel spacing for transmission reach of 120kmyear 2002 [24].
Further total capacity extension achieved by employing Raman amplification technologies and high SE modulation techniques. From time to time numerous approaches were devised, among them 50 GHz DWDM transmission at 10.7Gb/s over up to 1200 km on NZDSF without optical in-line or post-compensation is demonstrated with duo binary signals and receiver-based MLSE-EDC, with system flexibility suitable for add/drop optical networking in year 2006 [25]. Developments to enhance the capacity study hybrid- WDM (HWDM) systems with equal transmission technique, were one of the natural alternatives to allocate both IM and PSK signals simultaneously. Mixed WDM system which carries signals by its different bit rates or dissimilar code formats individual wavelength and offer its suppleness in making use of diverse modulation formats, so transmission performance of mixed 10Gbit/s optical transport systems the dispersion levels were tumbling in the system get better network transmission performances. Study on RZ-OOK and RZDPSK with dense OTDM-WDM schemes revealed, RZ-DPSK performed increasingly better than RZ-OOK in a higher spectral density with Q gain rising from 3 to 5 dB, whereas the RZ-DPSK do better than RZ-OOK by 4dB [26]. The study for 80×42.7 Gb/s and WDM of 100 GHz channel to channel spacing along with dispersionmanaged fiber spans, RZDPSK modulation, balanced detection and all-Raman amplification got the 5200km transmission reach [27].
Later on illustrated fiber optic systems of high bit rate OTDM together with WDM and hence providing enormous optical transmission capability. Hybrid WDM-TDM multiplexing 40GHz pulse source were achieved by extremely nonlinear propagation of a sine-modulated signal into a Raman-pumped consistent dispersion shifted fiber, and pulse repetition rate were synchronous to an external electrical clock , OTDM multiplexing to 80 GHz has been demonstrated in year 2004 [28]. Another investigation with WDM–OTDM hybrid simulation experimentations conceded out and investigated for the choice of most appropriate modulation format, dispersion compensation technique and erbium doped fiber amplifier (EDFA) noise figure for WDM–OTDM hybrid link, where communication link lengths employed beyond 500 km at high Q-values was attained [29]. The optical transmission with subcarrier multiplexing (SCM) is affected by the beat interference and generated because of beating of the subcarrier frequency components throughout the photo detection process. The maximum number of subcarrier channels which may be multiplexed in optical channel for communication over a single mode fiber and estimated impact of optical beat interference (OBI) on the communication performance of an SCM-WDM system [30,31]. Later on performance study with SCM-WDM optical communication system in the presence of optical beat interference (OBI) and observed that there has been a considerable augment in the signal to interference ratio (SIR) by application of Miller code compared to NRZ and Manchester code for the similar data rate [32]. In 2011 proposed study were for the cost-effective burst mode bit discrimination circuit for dual rate reach extender based on 10 gigabit Ethernet passive optical network and this illustrated method in a coexisted gigabit Ethernet passive optical network with 10 gigabit Ethernet passive optical network. It showed that the dual rate burst mode receiver sensitivity of – 32dBm for 1.25 Gbit/s signal and -27dBm for 10.3 Gbit/s signal, correspondingly [33].
Performance investigated (2010) with OFDM modulated ROF communication system with photonic crystal fiber (PCF) and SC source that can select mm-wave carriers at any space from the frequency comb [34]. Afterward (2011) experimental investigation performed for proposed symmetric WDM-PON scheme with colorless ONU and illustrated that the system tolerance to the intrachannel crosstalk has been very much enhanced when crosstalk signal locates at comparatively higher frequency band [35]. One of the capable solution to carry hundred plus Gb/s data rates were WDMbased architecture large coverage and effortless upgradeability, has been by the induction of orthogonal frequency division multiplexing (OFDM) technique into WDM-PON. This may does supplementary augment in the suppleness of the communication network in addition to upgrading to long reach metro access networks, had been concentrated investigation by a lot of research groups modern era [36-39]. Proposed study (2012) for new WDM Terabit access network based on pre-DFT-OFDM and single photonic crystal fiber (PCF) with super continuum (SC) light source, pre-DFT technology assisted to alleviate fiber nonlinearity of OFDM signal. Performance investigation with 2.5THz wide frequency comb supported symmetric 2.56Tb/s WDM pre-DFT OFDM access over sixty kilometer with sixteen hundred supported ONUs has been established and the architecture can be illustrated as one capable technique for the prospect terabit optical metro/access [40].
Further efforts to break “10G barrier” outcome in 40Gb/s transponders that have been employed a little further complex modulation for instance Differential Phase Shift Keying (DPSK) and Optical Duobinary (ODB).All of these realizations have been commercially fruitless and polarization mode dispersion (PMD) turns out to be a major trouble at such data transmission rates above 10Gb/s. Since their optical reach has been insufficient with long haul application. The technological advancement that permitted the “10Gbps Speed Limit” to be wrecked has been about the beginning of coherent optical technologies. In the beginning with 40Gb/s and thereafter for 100Gb/s long haul optical transmission as illustrated in Figure 2. In this figure the years illustrated has been based on commercial accessibility and not on “hero researches” [41]. Later on (2010-11) optical communication technologies have attained maturity point in the market whereby they could authentically permit 100 Gb/s coherent signals to be sent over the similar and a little bits greater distances as 10G IM-DD, and at the same time grouping of augmented video traffic, mobile devices with elevated resolution display technology and with the use of cloud-based storage of content were aiding to fuel the sustained increase in internet need at a rate of between 35% and 50% per year. Consequently there were rising need for the extra capacity, spectral efficiency, and get better in expenditure-per-bit that coherent 100 Gb/s technologies may offer. It has produced launch pad for the bunch-market acceptance of this technology beginning with 2012. As a disparity while coherent 40Gb/s realizations begin on the market in middle of 2000 it remains further costly than either 10Gb/s IM-DD and coherent 100Gb/s and so were “compressed” into market places such as submarine communication networks [41,42].
Figure 2: Technology enablers for DWDM capacity (ref-white paper- www.infinera.com-USA).
In the latter stage, communication era ongoing with further improvement in optical coherent technologies, were bean implausible technical accomplishment and permitting at least a factor of fivefold augment in the total fiber capability as contrast to the leading 10G IM-DD accomplishment (8Tb/s compared to 1.6Tb/s in the C-Band). The coherent accomplishment consisting of at least high order amplitude/phase modulation, polarization multiplexing, Coherent detection employs local oscillator laser in the receiver end. But with the emergence of the commercial coherent technologies only a few of DWDM vendors have made noteworthy development towards practical and dependable coherent transmission realizations, and frequently vendors have perpendicularly incorporated supply chain replicas where they possess the academic property to their coherent algorithms as described during the 2010 period and hence permitted upgrading towards 100G communication per wavelength, with far greater spectral efficiency and corresponding optical performance as compared to 10G IM-DD communication systems [41].
Later on new trend of investigations emerged with concentrated studies on the multi pumped fiber Raman amplifiers have been further probed in order to optimize performance with pump wavelengths, powers under counter propagating pumping configurations have illustrated that optical gain bandwidth of Raman amplifier has been enhanced by augmenting the number of counter propagating pumps which consequence decrease in gain ripple the optical gain spectrum of these amplifiers, with the characteristics for wide band fiber Raman amplifiers may be employed in applications of present WDM optical transmission networks in year 2012 [43]. Afterward (2012) study with least allowed channel to channel spacing were developed along with mixed WDM transmission systems to attain the utmost spectral efficiency for system’s. Therefore fiber optic communication systems were regarded as under the perception of next generation optical communication networks, thus an ordered representation for the future blueprint of backbone optical communication networks [44].
Further investigations illustrated for the multiplexed system of 640 GB/s data rate (16×40 GB/s) with RZ modulation with different channel spacing and results demonstrate first a better link superiority for wavelengths nearer to the central wavelength and established that further raise in the channel spacing results in the quality factor augment [45]. Later on investigating (2012) performance of the WDM system about effect of different dispersion values on the WDM systems and showed that CSRZ signal has been far less sensitive to fiber nonlinear effects and hence showed better robustness against communication impairments, NRZ system have enhanced dispersion tolerance [46]. Investigated transmission performance with a 10×10Gb/s low-cost photonic integrated transceiver comprising of a reflective SOA-based laser array, also a reflective EAM array over 825 km under mixed 10G/40G traffic conditions at 50 GHz grid [47]. Later study (2013) was illustrated to optimize seven-multicore EDFA for DWDM systems. Every core of MC-EDF was pumped by LD autonomously along with bundled fan-in consequential in MCEDFA having with gain of 15 dB and NF of 6 dB within C-band [48].
Performance investigated with number of simulations for an analytical model long-haul DWDM communication system employing in-line phase insensitive fiber optic parametric amplifiers (FOPA). Comparative performance study FOPA with EDFA, illustrated that a broad band long-haul DWDM transmission system based on FOPAs as in-line amplifiers may be realistic [49]. Other proposed experimental illustration about an optical wireless DWDM system at 60 GHz with optical incoherent heterodyne upconversion utilizing an optical frequency comb for numerous users with wire line and wireless services were simultaneously supported [50]. But because of the software defined networking (SDN) and flexible grid optical transport technology were two main key technologies which permitted the communication network operators to tailor their infrastructure based on application requirements. Consequently minimize the additional capital and operational costs needed for hosting the new applications. Accordingly established about design and accomplishment of a novel open flow based SDN integrated control plane permitting flawless operation across heterogeneous state-of-the-art optical and packet transport domains and experimentally estimate for the open flow protocol extensions for the flexible DWDM grid transport technology along with its incorporation with fixed DWDM grid and layer-two packet switching [51].

Subcarrier Multiplexing (SCM)

Subcarrier Multiplexing (SCM) is technique for multiplexing numerous diverse communications signals so that transmitted along a single optical fiber. SCM is employed in the passive optical network (PON) access infrastructures as an option of WDM.SCM employs different approach in contrast to WDM. WDM optical carrier is modulated with a baseband signal of typically hundreds of megabits per second. For the SCMA architecture baseband data is initially modulated on a GHz wide subcarrier which then afterward modulated on the optical carrier in this manner every signal reside in a different segment of the optical spectrum surrounding the centre frequency of the optical carrier. On the receiving section receiver were tuned to the right subcarrier frequency filtering out extra subcarriers and operation of multiplexing and demultiplexing single subcarriers were done electronically, conversion into the optical carrier were performed on the multiplexer side. SCM must be employed in combination with WDM to obtain benefit, most of the obtainable fiber bandwidth, may be used effectively for lower-speed, lower-cost multiuser systems. An important benefit of SCM is that microwave devices were much more grown up than optical devices; the stability of a microwave oscillator and the frequency selectivity of a microwave filter were more superior to their optical equivalent. As well as the low phase noise of RF oscillators constructs coherent detection in the RF domain easier as compared to optical coherent detection and new modulation techniques may be employed. One of accepted relevance of SCM technology in fiber optic systems was analog cable television (CATV) delivery [52]. A general block diagram has been shown in Figure 3 below.
Figure 3: General structure of Sub carrier multiplexing.
Transmission performance of structures with RF subcarrier multiplexing (SCM) has been widely influenced by the beat interference generated due to beating of the subcarrier frequency components through the photo detection process, boundaries the acceptable subcarrier power, the optical modulation index and maximum number of subcarrier channels which maybe multiplexed in an optical channel for transmission over a single mode fiber (SMF), investigated on the impact of optical beat interference(OBI), on the transmission performance of an SCM WDM system [53,54]. In the investigated performance in a SCM-WDM optical transmission system in presence of optical beat interference (OBI). There was an important augment in the signal to interference ratio (SIR) by using Miller code in contrast to NRZ and Manchester for similar data rate [55].

Optical Time Division Multiplexing (OTDM)

Numerous low bit rate data signals are multiplexed, or merged to form a high bit rate data signal on the time shared basis. As transmission medium were time shared by a number of incoming signals so this method is normally called time division multiplexing (TDM) and for electrical domain TDM is called ETDM. For TDM systems with n number of users having the same pulse width of T (seconds) were multiplexed then the pulse width of the multiplexed data signals were T/n, a simple block diagram has been shown Figure 4. Multiplexer and demultiplexer were required to operate at frequency identical to the entire aggregate bit rate that is n times quicker as compared to the bit rate of a single user. Multiplexer interleaves low speed streams to get the high speed streams, interleaving may be done on a bit-by-bit or packet-by-packet basis. Framing is required and receiving terminal must be able to recognize the timing of every bit properly and it needs receiving system to individually synchronize in time with the start of each frame, with each slot in a frame, and each bit within a slot. It is done by adding up framing and synchronization bits to the data bits, called overhead bits. Optical Time Division Multiplexing (OTDM) system method is the extend time division multiplexing by optically combining number of lower speed electronic baseband digital channels, a similar concept to electrical TDM, only that it is implemented in optical domain. Very initial 100-Gb/s OTDM transmission trial over a 36-km fiber link has been already accounted in 1993, along with very low timejitter phase lock loop timing extraction [56].
Figure 4: Point to point OTDM simple structure.
Lots of development with OTDM technologies has approached towards huge data rates and longer reach transmission distance around 2000 [57,58]. Clock-recovery mechanism operating at the base bit rate is essential on the receiver end to drive and synchronize the demultiplexer [10]. OTDM needs narrow RZ pulses to be capable to interleave data of diverse customers within a bit interval and narrow pulses need high spectral width, this system turn into vulnerable to CD and polarization mode dispersion (PMD) in addition to creating requirement for higheroptical signal-to-noise ratio in the wavelength channels due to very small pulses and higher OSNR attained by using a higher signal power, creates system sensitive to fiber nonlinearity. In the experimental result for 160 gbps OTDM link demonstrated that use of PMD compensates within 24msalong with 2.5 ps of first order and 15ps of the second order, significant improvement attained around 2006 [59]. Transmission Study of 160-Gb/s over 4320 km, 1.28-Tb/s over 240 km, and 2.56-Tb/s over 160-km fiber link, with all-Raman amplification, alternating polarization case,170-Gb/s transmission over a record length of 4320 km in 2006 [60] and accompanied by OTDM along with DQPSK modulation format and polarization multiplexing of 1.28-Tbit/s transmission Over 240 km, 2.56-Tb/s transmission over 160-km had been illustrated in 2006 [61]. Investigating for OTDM with 16x10- Gb/s 100-km transmission system with home-produced multiplexer were verified experimentally, demultiplexer were based on two cascaded electro absorption modulator (EAM) and a clock recovery unit form a feedback loop, Error-free demultiplexing from 160-Gb/s to 10-Gb/s after over 100-km transmission, power penalty about 3.5dB with the bit error rate of 10-9 has been achieved (2009) [62]. Later on investigated for a four channel OTDM systems (all-channel multiplexer and de- multiplexer) along with a Mach–Zehnder modulator and SMZ de-multiplexer to observe the impact of forward error correction (FEC) on OTDM system, significantly improvement were found that with FEC in OTDM transmission in 2010 [63]. Afterward study (2012) demonstrated stable transmission system of uncompressed ultra-high definition (UHD) video signal by means of 344-Gb/s dual-polarization optical time division multiplexing (OTDM) with channel identifiable clock recovery [64].

Mode-Division Multiplexed transmissions (MDM)

With development of digital signal processing mode-divisionmultiplexing is getting more concentration from the working people of fiber optic community. It can be a replacement, sustain and another way of growing capacity need over single fiber. Modedivision multiplexed transmissions have got special concentration because this permits to enhance the amount of signals per fiber core. Using it many individual signals were transmitted through propagation modes, any particular core, on a particular wavelength. Realization of any mode multiplexer/demultiplexer can be done with optical devices without DSP in electrical domain; a lot of MUX/ DEMUX modes were proposed [65-67], Mode-division multiplexed transmission systems transmission performance reported remained ambiguous. Primary investigations based on MDM technique employed merely two or three modes as LP01, two degenerate LP11 modes for communication lengths of 4.5 km [68], 10 km [69], 26 km [70], 33 km [71], and 40 km [72]. More recent investigations were illustrated about larger communication lengths by with advantage of the MIMO processing in the digital signal processing’s (DSP). In work illustrated about MDM-WDM transmission with LP01 and two degenerate LP11 modes, with 6×6 time-domain MIMO equalizer, FMF, few modes (FM)-EDFA were used transmitted up to 50 km [73]. Study of MDM-WDM (LP01 and LP11) with few-mode fiber, signal were recuperated employing 6×6 MIMO equalizer at receiver following amplification by a few-mode EDFA, transmitted over 50 km [74]. Further highest capacity reported work for the investigation of simultaneous transmission of six spatial and polarization modes each carrying 40 Gb/s QPSK channels over 96 km of a small differential group delay FMF. Channels were successfully detected by offline digital signal processing’s (DSP) based on coherent detection and MIMO processing [75]. Later on successfully communicated over 137-km (few-mode fiber) with 8 dB of mode-equalized distributed Raman gain employing backward pumping scheme [76]. Although spatial division multiplexing (SDM) were not only restricted to MDM so the other investigations illustrated about transmission of very huge data capacity over multi-core fibers (MCF) and longer reach communication [77,78]. Although transmission systems reach is not the solitary performance indicator. Present significance of MDM has been strongly connected to the quantity of modes that may be carried by the communication system. Therefore it is logical to imagine about more number of modes application in optical transport systems for applying primary modifications, although it is unclear about where is the threshold.

Space Division Multiplexing (SDM)

Space Division Multiplexing is approximately as matured as fiber optic communication (FOC) itself, has been reported in past 1979 for further augmentation of fiber capacity and most apparent approach grown up with fabrications of multicore fibers [79]. Now day’s investigations on SDM are adopting digital compensation and coherent detection having competent of prevail over complex impairments, has been acknowledged as customary part of high performance optical systems. Crosstalk among nearby channels was apparent probable deficiency and needs to be tackled, because SDM bunches spatial channels closely into every fiber. Crosstalk’s are unattractive and minimized electronically on the receiver side with coherent detection, made viable tactics for precise communication requisite. In the past decades hike in near around ten times in each fourth year have been demonstrated in Figure 5 and transmission technology capable to sustain with persistent augmentation of increasing capacity needs, since huge data were transmitted over same fiber by upgrading apparatus at the fiber ends, so transmission cost of increased data transmission were affordable. With augment in number of fibers in optical networks shall get its limit of capability in upcoming years [80]. Predictable assertion of SDM is to make available the next leap in capacity per fiber, as shown in Figure 5, although at the same time it allow huge decrease in price tag per bit and hence the enhanced energy efficiency [81], a difficult test. SDM is dissimilar from WDM that lets to allocation of key parts .As in WDM dispersion compensation module and EDFA may be simply shared by a lot of WDM channels with minimum extra complexity. Benefits of SDM has been much speculative and presumed that a lot of system parts may be ultimately included and engineered to sustain this good novel platform. So due to growing need main investigations have been concentrated to ascertain capability of SDM [82].
Figure 5: Evolution of transmission capacity in optical fibers as evidenced by state of the art laboratory Transmission demonstrations over the years (Morioka et al. [82]).
SDM has attained much significance since it has showed great potential for triumph over the capacity crisis, transmission has been illustrated for a number of fiber types, multimode fibers [74,85], strongly coupled multi core fibers [83] and weakly coupled multi core fibers [84]. Number of modes per core augmented by number of cores in the fiber and hike in capacity achieved with application of parallel single mode fiber (SMF). SDM systems can be made viable with parallel single mode fiber combine devices such as inline amplifiers also with shorten DSP need hence controllable power consumption, Few mode fiber(FMF)which were based on EDFA investigated [87] and illustrated in [74,86], with Raman amplification in Few mode fiber(FMF) were investigated [88,89]. The maximum capacity showed so far over few mode fibers (FMF), applied both wave length division multiplexing and mode-division multiplexing, were 88×3×112- Gb/s employing just C-band [74]. Other studied benefit of SDM systems were that diverse channels have the identical environmental disturbances during transmission and hence minimized path length deviations, spectral efficiency were get better by spectrally interleaving pilot-tone along with signal [89], reported work on SDM have shown option of utilizing SHCD along with transmitted pilot tone through space-division-multiplexed channel, rest is employed for signal channels. Investigations about SDM systems using FMF explicated overall system architecture, sub system modules for MDM transmission and significant components [90-94].
As shown in Figure 6 mode multiplexing is achieved with spatialmode multiplexer (S-MUX), signals were carried by diverse spatial modes were instigated into the few mode fibers(FMF), all modes on the similar wavelengths were processed as an unit, as SDM super channel, after transmission over FMF, the received signals were mode demultiplexed, then detected with coherent receivers, converted from optical(O) to electrical(E) domain, MIMO algorithm to compensate crosstalk ,mode coupling and lastly employed to DSP module, channel capacity enhanced by N times to that of single mode system [89,94]. Investigation reported photonic mesh networks SDM which were based on reconfigurable add-drop multiplexers with associated SDM-WDM transmitters and receivers, super channels, the strategy for switching [94]. In current report about performance of a selfhomodyne coherent detection (SHCD) system with nineteen core multi-core fiber (MCF) and sixteen wavelength-division-multiplexed channels, illustrated that with pilot-tone transmitted on a single MCF core and information carrying signals on the remaining cores, were well-suited with SDM transmission, potentially relaxing laser line width, DSP needs due to phase noise cancellation, although inter core crosstalk may have impact on core selection and performance [95].
Figure 6: Architecture of N × N SDM transmission system utilizing coherent MIMO digital signal processing. MUX/DEMUX: multiplexer/demultiplexer, coherent receiver.

Duty Cycle Division Multiplexing (DCDM)

Duty cycle division multiplexing (DCDM) is a new kind of the multiplexing technique introduced in 2007. In this technique, different users sign with different RZ duty cycles and then combine together synchronously to form a multilevel step shape signal Figure 7. The multiplexing process can be performed either in electrical domain (E-DCDM) or optical domain (O-DCDM). Key advantage of the DCDM is inherent self-symbol synchronized system. However, disadvantage of DCDM system is that it required high OSNR in comparison to the binary signaling such as RZ or NRZ. The performance of DCDM is improved using optical multiplexer by adding the cost in multiplexer, which required one modulator for each multiplexing user. In terms of transmitter and receiver complexity, DCDM has simple transmitter and receiver. At the same time, DCDM allows high speed aggregate bit rate to be recovered at symbol or baud rate, which made DCDM receiver very economic. Furthermore, this technique permits more users to be allocated in WDM channel, which contributes towards improvement in SE [96].
Figure 7: (a) Schematic of E-DCDM system (b) eye diagram, and (c) demultiplexer [96].

Polarization Division Multiplexing (PDM)

Polarization Division Multiplexing (PDM) is one of the competent, emerging technique to enhance data rates without enhancing symbol rates, a physical layer method for multiplexing signals transmitted on the e.m waves employing polarization of the e.m waves to discriminate amid the dissimilar orthogonal signals. In this method, two separately modulated data channels with similar wavelength, but orthogonal polarization states were concurrently transmitted in a single fiber. PDM were generally employed all together with optical QAM or modulations letting transmission rates of 100 Gbit/s or even high over a single wavelength. Group of PDM wavelength signals then be passed over WDM infrastructure and hence augmenting its capability. Difficulty with practical use of PDM over fiber optic transmission systems were variations in the polarization state that arise constantly over time because of physical changes in the fiber surroundings. For long haul system these variations are boundless consequently resulting in unpredictable rotation of the polarized light’s Jones vector over the complete sphere. Nonlinearities in it were Polarization mode dispersion, polarization-dependent loss(PDL) and XPolM (cross-polarization modulation) were the phenomena’s could be reasons in PDM systems. Therefore PDM were usually employed in coincidence with advanced channel coding techniques. PM-QPSK and PM-DQPSK modulations were used, PolDM having benefits that it can be implemented without major changes to the existing systems. PolDM can be implemented by adding a transceiver and associated polarization multiplexer/demultiplexer at each end of the fiber link, while leaving rest of the system unaffected [97-100]. Investigations in POLMUX showed enhanced sensitivity to polarization effects, such as PMD and PDL, in contrast to single polarization case penalties occur from PMD and PDL induced crosstalk among the demultiplexed channels and from OSNR degradation by PDL [101- 103], coherent detection schemes a verified method to compensate PMD and PDL [104,105], in contrast, low complex 100-Gb/s POLMUX systems relying on direct detection may potentially make possible cost-effective terabit/s-capacity transport network [106,107]. Study in [108], experimental estimation were enlarged to 100-Gb/s POLMUX RZ-DQPSK, outcomes validate decreased PMD tolerance because of increased bit-rate. In other reported study in polarization demultiplexing, with coherent detection, digital signal processing, considered cumbersome in the optical domain, simply done in the electrical domain, even though there were still attention to do polarization demultiplexing using optical methods [109,110]. So PDM has been considered as approximately regular alternative for now day’s optical coherent systems. Dispersion-managed, timeinterleaved return-to-zero ILRZ PDM formats [111,112], periodicgroup- delay (PGD) for inline dispersion compensators [114], and including some PMD along the transmission links were used for upgrading, Later on simulated for coherent systems polarizationdivision- multiplexed quadrature-phase-shift-keying (PDM-QPSK) ,showed that inter-channel cross-polarization modulation (XPolM) induced nonlinear polarization scattering degrade the transmission performance, to mitigate used time-interleaved RZ-PDM formats, periodic-group-delay PGD dispersion compensators, also with careful add of polarization-mode-dispersion (PMD) [113-115].
Investigations with 8×200-Gbit/s PDM-DQPSK transmission system over 1200 km standard single mode fiber (SSMF) in lab environment with CS-RZ-DQPSK showed, bit error rate (BER) of 1e-4obtained for OSNR value of 23.29dB, new automatic optical polarization demultiplexing method for PDM signals, usefulness of this demultiplexing method has been experimentally indicated in a 2×10 Gb/s on-off-keying (OOK) PDM transmission system [116,117].
Later on revealed, the study for impact of PDL-induced crosstalk, experimental and numerical simulations with direct detection systems for 100-Gb/s POLMUX RZ-DQPSK with applied optical narrow filtering on PDL revealed that RZ time-interleaving highly robust to PDL [118], another experimental illustrations showed 2688-km multi-span transmission employing WDM of ten signals of 50-GHz spaced 128-Gb/s PDM-QPSK, space-division multiplexed in a low-crosstalk 76.8-km seven-core fiber, attaining a record net aggregate per-fiber spectral-efficiency-distance product of 40,320 kmb/s/Hz, 40×433.6-Gb/s N-WDM (Nyquist wavelength-divisionmultiplexing) OTDM with EDFA, PDM-CSRZ-QPSK, post filter, 1-bit MLSE, BER of all channels were less than the pre forwarderror- correction (pre-FEC) limit after 2800-km SMF-28 transmission [119,120]. Present illustration for PM-QPSK signals of 100G, with dual-polarization Volterra series nonlinear equalizer applied in frequency-domain to diminish nonlinearities. Thus, the performance of nonlinear equalization considerably improved if both optical and electrical filtering were optimized and hence enabling the VSNE technique to do better than its SSF counterpart at huge input powers [121].

Optical Orthogonal Frequency Division Multiplexing (OOFDM)

Orthogonal Frequency Division Multiplexing is multicarrier modulation as well as multiplexing technique, multicarrier method in which the data information were carried on multiplexed setup in parallel over many lower rate subcarriers. Different carriers used to modulate the individual information signals were referred as subcarriers. OFDM has similarities with FDM but spectrally capable due to closer channel spacing as exhibited in Figure 8, spectral diagram of non-overlapping subcarrier frequency signals were prearranged in parallel illustrating conventional FDM where each being separated by fixed guard band. Benefits of OFDM in the optical domain are its huge spectral efficiency, tolerant to ICI and smaller multi path distortion [122,123]. OFDM for optical domain referred as Optical OFDM (OOFDM), for all optical OFDM system Mach-Zehnder Interferometric modulator converts electrical to optical signal. With serial to parallel and parallel to serial blocks convert data with high rate into low data rate and vice versa and data to symbol mapper and De-mapper block does modulation and input data bits grouped decrease data rate. Add and remove cyclic prefix (CP) diminish ISI effect, by adding partial symbol information of each cycle to the beginning of the symbol. Higher is the delay spread, higher is the length of cyclic prefix (CP), is chosen as one fourth of the symbol period. Afterwards symbols from parallel paths combined to make a serial data, as illustrated in the Figure 9.
Figure 8: A comparison bandwidth needs FDM vs. OFDM.
Figure 9: Basic structure of optical OFDM system.
Chang revealed the fundamental principle of OFDM overlap numerous channel spectra inside limited bandwidth without interference [124-126], with enormous capability LTE of 3GPP, mobile universal interoperability for microwave access, wireless, wired, LAN and long-haul communication, have played significant role [127-131]. Although OFDM have been studied in radio frequency(RF) domain for more than decades and analysis on OFDM for optical transmission started merely in the late part of 1990s [132], with key advantages of OFDM for optical communication have been first illustrated in [132,133].
For long-haul communication, two major alternatives of OOFDM were mostly investigated, contains incoherent OOFDM such as Intensity-Modulation, Direction-Detection (IMDD) OOFDM and coherent OOFDM. IMDD OOFDM has showed enormous potential for practical implementation in expenditureaware purpose situations such as LANs and MANs. Coherent optical OFDM (COOFDM) has been widely taken as talented contender for future long-haul high capacity optical communication systems [134-137]. Since net transmission data rates were augmented at a factor of 10 per year at the experimental level, examined transmissions based on optical FFT have been able up to 1Terabits/s transmission in single communication channel [138,139] and 10.8 Terabits/s [140] in the previous years. However illustrations of real-time optical OFDM with digital signal processing has enhanced 10Gigabits/s [141].
The straightforward implementation based on minimumexpenditure optical components and coherent optical OFDM (COOFDM) were proposed, scheme mitigated optical dispersion and Optical SNR sustained below2dB transmitted over 3000 km with SMF [142] need to achieve massive spectral efficiency and receiver sensitivity, ever since concentration in optical OFDM enhanced spectacularly. First demonstrated investigation on coherent OOFDM with line rate of 8 Gb/s in year 2007 [143]. Investigation with real time optical OFDM transmission besides offline digital signal processing and display of very first real time OOFDM taken place. Later OOFDM with optical phase modulator investigated for 40 GHz ROF symmetric system, after transmission over 50 km SMF illustrated low 0.5 dB for down link and for uplink penalty neglected (2009) [144]. Further net rate surpassed to 10Gb/s within one year as the speed of real-time OFDM development was too quick [145].
Further investigation for the transmission performance along with multimode fibers, SMF, adaptive cyclic prefix (ACP)and adaptively modulated optical OFDM(AMOOFDM) for fixed cyclic prefix duration of 25%, demonstrated that ACP has enhanced transmission capability by a factor of >2 (>1.3) for >1000 m MMFs (<80 km SMFs) with 1dB link loss margin augmentation [146]. While investigation employing CO-OFDM technique for the long-haul transmission of 100-Gb/s-class channels, long-haul high-capacity transmission experiments employing No-GI CO-OFDM; 13.4 Tb/s transmission effectively revealed over 3600 km of ITU-T G.652 single-mode fiber without employing optical dispersion compensation [147]. Later on, the study of optical transmission for more than six hundreds of kilometers SSMF has been successfully shown in with application of orthogonal-band-multiplexing that has been advantageous for orthogonal OFDM with 110-Gb/s [158], 56 Gb/s [159] and 41.25 Gb/s per single band established in [150]. Afterward investigation of the coherent optical orthogonal frequency-division multiplexing (CO-OFDM), coherent optical single-carrier frequency domain equalization block transmission (CO-SCFDE),based on adaptive modeling of phase noise, unified observation equations for dissimilar coherent optical block transmission systems were constructed, which leads to unified phase noise estimation and suppression [151].
The study to detect 1.2-Tb/s twenty four carrier NGICO-OFDM signal having 12.5-Gbaud PDM-QPSK carriers with 50-Gb/ s ADC employing three different techniques, with multi-band detection potential save number of essential optical components on receiver side [152,153]. Further investigated of high instantaneous signal peak w.r.t the average signal level that is PAPR,PAPR is proportional to number of carriers, and is given by PAPR (dB) = 10 log (N),where N is the number of carriers, problem of large PAPR illustrates poorer compassion to fiber nonlinearity and incompetent power consumption [154]. Numbers of techniques have been shown in to trounce this problem, one of them is coherent optical single carrier frequency division multiplexing (CO-SCFDM) techniques has been projected to surmount this problem (2010-11) [154-156]. SCFDM termed as discrete-Fourier-transform-spread (DFT-S) OFDM, which is an option to coherent optical OFDM that diminishes PAPR leading to further augmentation in nonlinear tolerance with frequency domain [157,158], applied for uplink communication in wireless LTE proposal with outstanding characteristic of significantly reducing PAPR than conventional OFDM [157,158]. Afterward, study for minimizing high PAPR, based on μ-law companding transform in coherent optical OFDM [159] and investigated for DD-OOFDM with flipped-exponential (FE) Nyquist pulse method get major improvement in PAPR [160]. Afterward for amplitude and phase shift keying (APSK) modulated CO-OFDM with and without differential encoding for 40 Gbit/s single-channel and 5×40 Gbit/s WDM, illustrated that in contrast with conventional 16QAM modulated OOFDM signal, 16(D) APSK modulated OOFDM signal has lower tolerance towards ASE noise, accumulated CD reduces PAPR, up to 1000 km single-channel transmission, recline needs for training symbols and pilot subcarriers so enhancing spectral efficiency [161]. Later on with timing offset estimation method for DDOOFDM systems, experimental results exhibits smaller MSE in methods and attains large timing estimation precision in DDOOFDM communication system [162]. Further with proper selection of FE Nyquist pulses for shaping different subcarriers of OFDM, optical DD-OFDM transmission system attains major improvement in PAPR [163]. Later on study in DD-OOFDM system with electronic pre-distortion method of companding transform (CT) to diminish PAPR reach about 3 dB when complementary CDF was 1×10−4, OFDM signals and improves receiver sensitivity [164]. Investigating OOFDM attains good performance in contrast to conventional OOFDM in terms of BER and PAPR with discrete Hartley transform (DHT) and constant envelope (CE) modulation [165].
Later on, performance investigated with more-compact phase modulators, lowers necessary bandwidth improved performance in OOFDM with replacement of complex (IQ) modulators [166]. Later on, with 5 ×200Gb/s(AOS) all optical sampling-OFDM method using PolMux-DQPSK system in single source, real-time detection using optical Fourier transform filter and established that method may coexist with conventional WDM channels, capable method improving transmission date rate [167]. For secured communication physical secure strategy with OFDM-PON scheme were examined through scrambling OFDM subcarriers provided secured solution year 2012 [168,169]. Security linked experimental study (2012) reveal that OFDM-PON has been one of the effective solution for physical secure communication, with high key sensitivity, sufficient key space, scalable granularity and dynamic key ensure secure communication, diminishes computational complexity of cipher text generation by 66% in 2012 [170]. Afterward with PONs as paratactic OFDMPON without special protecting fibers, Rayleigh backscattering and crosstalk mitigated as usual operational mode [171].
Afterward studied with single-polarization or polarizationmultiplexing in conjunction with OOFDM modulation format demonstrated to attain perfect estimation of OSNR for back-to-back and 800-km of transmission, performed good in presence of huge quantity of CD [172], with forty optical subcarriers closely spaced at 9.375 GHz transmitted SCFDM signals or 1.08 Tb/s OFDM, transmitting more than 2536 km or 3170 km SSMF, correspondingly, as compared to CO-OFDM, CO-SCFDM were nearly 1.0 dB further nonlinear tolerance. Achieved highest transmission reach in the experiment [173], also huge number (87) optical subcarriers gaped at 5.15625 GHz transmitted 1.45 Tb/s DFT-S-OFDM signals and transmitting over 480 km SSMF [174].
Afterward, spectrally efficient transmission services from Gb/s to Tb/s through closely spaced super-channels offered with Gridless UDWDM system with optical subcarrier inside super-channel comprised or dropped to realize sub wavelength distribution; superchannel be routed or transmitted as single unit [175]. Further gridless OFDM super-channels, adaptive WDM systems investigated with boundaries of spectral efficiency, transmission distance through five channels, for changeable modulation formats, channel spacing, symbol rate although OFDM signal with fix number of 120 data subcarriers [176]. Recent study (2013) with PDM-QPSK capability in 12.6 Tb/s with highest reach of 5120, 4400, 1120, 1024 km for SCFDM, OFDM, PDM-16-QAM (25.2 Tb/s), OFDM (at fine granularity) correspondingly, for UDWDM orthogonal transmission show better SE-distance product [177]. Later on study in OOFDMA dynamic resource allocation, integrates MANs with reconfigurable OFDMA, remote nodes (RNs) connected by single fiber OFDMA trees with the corresponding CN, illustrated metro-access integrated network possible and power distribution was sprightly [178].
Present development along with conventional least square (LS) channel estimation, achieved better channel estimation performance and regarded as suitable option for CO-OFDM scheme with tradeoff between complexity and performance [179], for 40 Gb/s COOFDM systems with mid link optical phase conjugation (M-OPC) based on FWM in SOA employed, results exhibit M-OPC diminish fiber nonlinearity impairment [180] and with adaptive estimation algorithm showed tremendous performance for CO-OFDM with novel two-stage LMS phase noise adaptive estimation algorithm [181]. Further, CO-OFDM with adaptive fiber nonlinearity pre compensation (AFNP) scheme mitigates nonlinearity show enhanced efficiency, cost efficient method in low-dynamic scheme [182], with self-correlation technique to extract nonlinear effects, after 1500km SSMF, 12.3 dB phase noise is induced andDC ratio is altered from 0% to 105%, as walk-off changed from 1.6 ps/km to 12.8 ps/km, about 10.6 dB reduced phase noise [183]. Afterward study in BER estimation by Eb/No and Monte-Carlo (MC) method with OSNR, chromatic dispersion (CD), laser line width and fiber nonlinearity impairments, 40 Gb/s CO-OFDM schemes BER based on Eb/No estimate precisely 10−16 levels with 105 bits, while MC needed minimum 1018 bits, Eb/No is good and offers an alternative to the time-consuming MC method [184]. Experiments exhibited with flexible method of generating wide bandwidth OFDM super channels by means of software-programmable wavelength selective switch to combine modulated pulses and for transmitted 279×40 Gbit/s over 400 km [185]. Afterward, for VCSELs in OFDM QPSK/16QAM with direct detection, over SMF (100 m and 5 km) and MMF (100 m and 1 km) small-range links, transmission set-up exhibits supremacy of 1550-nm single-mode VCSEL in contrast to its multi-mode 850-nm analogue [186]. Study of optical signal in terms of error vector magnitude (EVM), results show quality of OFDM signals be improved up to 4 dB in EVM, for 10 Gb/s, 16-QAM-encoded OFDM signals after 20 km SMF transmission with IMDD [187]. Later for in-band RF pilot-tone improved tolerance structures to laser line width and launch power into fiber for 400 Gb/s multiband 16QAM DFT-S OFDM system [188]. Together with pilot tones optical fast orthogonal frequency division multiplexing (F-OFDM-20Gb/s &4 ASK) illustrated single F-OFDM symbol with six pilot tones attained near-optimal estimation performance for 840-km dispersion [189].
Afterward, study with independent component analysis (ICA) applied for IQ imbalance system to encounter mirror interference, IQ imbalance compensated by ICA algorithm efficiently [190]. Investigation with double-carrier OOFDM signals transmitted exhibit that, overall DBP of system is doubled to 2×50 ps×10 Gb/s because of double-carrier transmission [191]. Afterward injectionlocked (ILO) ILO-OOFDM scheme with directly modulating RF OFDM signal on injection-locked laser better features demonstrated [192]. Investigation revealed, DDO-OFDM with adaptive code rate technique (ACT) and bit interleaver, sensitivity of 5.9-Gb/s 64QAM OFDM signal using ACT(9.43 Gb/s after encoding), is better more than 2 dB in contrast to with 5.9-Gb/s OFDM signal with 0.66 turbo coding rate (8.85 Gb/s after encoding) at BER of 1e-4 [193], for DDOOFDM with block wise signal-phase-switching (SPS) experimental illustrated that for 61Gbits/s SPS transmission over 80 km SSMF successfully attained along with single polarization and single photo detector [194].
Study for, low cost OOFDM having huge prospective for next generation optical access networks and PONs, adaptive scheme enable significant BER improvement, with dynamic bandwidth allocation (DBA) at MAC layer in uplink transmission of OFDMPON, algorithms capability in making system highly energy-efficient, experimental study in OFDMA-PON system show hierarchical modulation method improves power margin by 2.7 dB for, used to take extra users or increased transmission reach [195,196]. Later study showed, OOFDM with LMS-TEQ and decision feedback time domain equalizer (DF-TEQ) shrinks CP length, calculated based on the trade-off between OOFDM system efficiency and LMS-TEQ/DF Study for, low cost OOFDM having huge prospective for next generation optical access networks and PONs, adaptive scheme enable significant BER improvement, with dynamic bandwidth allocation (DBA) at MAC layer in uplink transmission of OFDMPON, algorithms capability in making system highly energy-efficient, experimental study in OFDMA-PON system show hierarchical modulation method improves power margin by 2.7 dB for, used to take extra users or increased transmission reach [195,196]. Later study showed, OOFDM with LMS-TEQ and decision feedback time domain equalizer (DF-TEQ) shrinks CP length, calculated based on the trade-off between OOFDM system efficiency and LMS-TEQ/DFTEQ complexity [197]. Afterward investigated with normalized least mean squares-time domain equalizer (NMLS-TEQ) cancels residual ISI employed to diminish CP length in DDOOFDM communication over 2400 km of SSMF, NLMS-TEQ cuts size of CP so performance is improved [198].
Performance investigation for hybrid OFDM systems such as, OFDM-FSO (Free Space Optics) transmission system incorporating OTSB and OSSB methods pledge considerable improved FSO link in contrast to conventional FSO systems [199]. Afterward, hybrid OFDM-ROF system (OFDM-OSSB-ROF) transmission system tested for different order of QAM sequence generator on high bit rate, with and without FBG compensation. It showed in case of hybrid ROF system with FBG, improvement in SNR ratio and total power received is achieved in contrast to hybrid ROF system without FBG [200]. Study with wired and wireless for hybrid OFDM–OSSB– ROF transmission system using 4 QAM sequence generator at high transmission rate, different laser spectral-widths. Improvement after a range of 10–100 km for both hybrid ROF systems on minimizing laser spectral-width from 10 MHz to 100 kHz at transmission bit rates of 10/20-Gbps [201]. Later on, design of fading resistant ODSB-FSO system using a simulated test-bed employing OFDM scheme to calculate FSO range with satisfactory SNR and BER with highest stream rate of 5 Gbps under impact of hazing, fog and clear weather conditions, showed that hybrid OFDM-ODSB-FSO transmission system promise improved free space link in contrast to traditional FSO systems under diverse weather conditions [202]. Later study, on WDM-ROF-PON architecture based on 16QAMOFDM modulation (10Gb/s) for bidirectional access networks in downstream and upstream signals transmitted over 40-km SSMF without optical dispersion compensation, show WDM-ROF-PON design capable contender in next generation optical networks [203]. Experimental study showed, with 20 Gb/s WDM-OFDM-PON for mm-wave signal generation by optical carrier suppression (OCS) process employing single-drive Mach–Zehnder modulator as a light sources with colorless ONU scheme to getBER under 2×10−3 [204]. Further investigation using OLT in WDM-OFDM-PON’s numerical computations showed about decrease of energy consumption, and energy efficiency improvement of 28.3% in optical line terminal (OLT) could be attained by means of OLT in WDM-OFDM-PON’s as compared to the conventional WDM-OFDM-PON [205,206]. Eventually study in integrated DWDM and OOFDM systems with OADM, for effects on transmitted channels, link length, operating optical signal wavelength, optical transmitted signal power, optical signal bandwidth, transmission bit rate, optical received power and BER at receiving side were examined, for fiber nonlinearity effect showed that systems Q is nearly 18 dB for this scheme operating at 9.953 Gb/s in SSMF without dispersion compensation up to 4500 km distance [207].

Optical Code Division Multiple Access (OCDMA)

It is also a type of multiplexing, in which users allocated unique codes function as address and employed codes are different unique, a receiver with only matched code may receive message accurately, there were lots of method of encoding, yet since its beginning year 1983.As illustrated in Figure 10, shows a simple view of the optical CDMA design [208]. To exploit full advantage of fiber bandwidth CDMA basics used in wireless domain transformed in to optical domain termed as Optical CDMA (OCDMA), firstly employed for optical domain in the middle of 1980s [209-211]. It can be classified as director differential detection employed for spectral encoding, incoherent amplitude encoding, coherent phase encoding, spatial encoding, matrix encoding. Amongst many trends to realize OCDMA system, frequent division was in between incoherent [211,212] and coherent systems. Since only power of the signal has to be detected and any phase information of signal is not observed so incoherent amplitude encoding were beneficial.
Figure 10: General block diagram of OCDMA.
A kind of optical codes were projected by numerous authors and developed optical orthogonal codes presented their analysis and applications in year 1989. Use of optical orthogonal codes enables huge number of asynchronous users to communicate efficiently and consistently, theoretical lower and upper bounds on the highest probable size of OOCs resulted. In 1989 different parameters like data rate, code length, code weight, number of users and receiver threshold, evaluated performance of codes projected. Study showed improved system performance with diminishing multiple accessinterference (MAI) with ideal optical hard-limiters, used optical delay lines encode optical signals [213,214]. Optical codes with crosscorrelation CDMA investigated with balanced detection, application in quasi-synchronous CDMA with number of codes as bipolar, gold, optimal OOC’s showed more tolerance to jitter and comparative study for synchronous OCDMA with CDMA revealed that SCDMA accommodate larger number of users than CDMA [215-217]. Coherent phase encoding, phase modulated narrow optical pulses formed through electro-optic modulators and optical pulses encoded through tapped optical delay lines by using different delays, phase shifts in different branches studied. In 1992 investigated about optical phase coding, in conjunction with differential detection to minimize MAI. Also experimentally showed that the use of a master encoding network to provide reference for the system, to encode and decode transmitted data low-coherence source, phase codes ensuring minimum interference between users [218], and employed ‘master’ encoder to give reference for optical system. Matrix encoding wavelength-time encoder used super structured fiber-brag grating (SSFBG) and broadband light pulse utilized with SSFBG, Bragg’s wavelength of gratings were reflected back and output as encoded signal. Spectral components were time shifted due to positions of gratings in fiber and conjugate of SSFBG employed on decoder end to recover signal with employed strain to tune grating to dissimilar wavelengths, later on new schemes along with cyclic encoder/decoder through arrayed waveguide grating were examined [219,220]. FBG along with delay lines, arrayed waveguide gratings (AWG) with wavelength periodicity used for constructing programmable wavelength-time OCDMA coders [221-225]. Spatial encoding systems studied in 1994 with fibers as space channels for encoding OCDMA systems, realized such system with two-dimensional optical codes and spatial optical CDMA in year 1985 [226,227]. Study (1992) for temporal/spatial codes showed that performance of the temporal/ spatial codes enhanced compared to temporal codes as it permit for low bit times [228], studied for non-coherent spatial/spectral code, referred as 2-D perfect difference codes [229]. Spectral encoding with modulation employing small-cost broadband optical source, two types of spectral encoding were spectral amplitude and phase encoding investigated (1993-1995) [230,231], showed (1997) to compensate uneven spectral shape of laser source non uniform spectral encoding enhanced orthogonality in between coded waveforms of numerous users [232]. Studied (2001) new code family spectral amplitude encoding employing FBG array and transmitter/receiver structures [233]. Differential detection spectral coding to decrease MAI investigated for multimedia applications spectral amplitude coded OCDMA (2004) [234,235]. Heterodyne detection receiver with their effort on exposure of spectral amplitude-encoding OCDMA system studied (2005) [236]. To compensate non-ideal uneven spectra of optical sources non-uniform spectral phase encoding asynchronous spectral OCDMA with perfect difference codes (PDC) employing interference cancellation method investigated with synchronous systems only and mode-locked lasers, design improves orthogonality of encoded waveforms between the multiple users in 2006 [237]. Spectral phase encoding with pseudo-random (phase masked) spatial phase pattern included in between two lenses studied with OCDMA system, new coding schemes, coherent ultra-short light pulses, highresolution spectral phase coding of picoseconds and femtosecond pulses, revealed that performance improves greatly with increasing code length(1990) [238]. Alternatively, the Optical CDMA can be summarized as exhibited in Figure 11.
Figure 11: Classification of Optical CDMA.
OCDMA may be bifurcated in terms of signal modulation and detection methods as incoherent and coherent methods. Signal modulation based on optical phase coding is resultant frequently from extremely coherent source such as mode-locked laser and detection on receiver comprises knowledge in phase information of carriers for coherent OCDMA method. Techniques allow use of ultra-minute optical pulse for either spectrally coded time spread (SPECTS), WDM with OCDMA investigated for sixteen lasers within an 80-GHz tunable window by high resolution phase in the encoder/decoder for only four users with bit-error rate <10-9 year 2005 [239] or spatial Light Phase Modulator (SLPM),study with four users spectral phasecoded OCDMA system via nonlinear processing with ultralow power (~30 fJ/bit), full interference suppression were achieved [240,241], or directly time-spread coded by means of superstructure fiber Bragg grating (SSFBG) [242-244] or A waveguide grating configuration (AWG) multi-port in 2006 [245], analysis with bipolar orthogonal coding such as gold sequences for direct-sequence spread-spectrum (DS/SS), the acquisition performance of Gold-sequence-based DSCDMA systems were examined with m-sequence-based systems, show that use of Gold sequences requires mean acquisition time quite comparable to the use of m-sequences, allowed in application year 2000 [246].
Coherent methods were dominant, complex and costly due to need of laser source and phase control with accurate adjustment, for an incoherent scheme requires more standard methods of intensity modulation and easy detection along with incoherent source, for instance broadband amplified spontaneous emission source, while coherent techniques were based on the optical phase for signal modulation and detection. Normally used incoherent OCDMA methods illustrated as spectral-amplitude-coding (SAC) [247-250] and two-dimensional code, spatial coding [251-255] and Wavelength-Hopping Time-Spreading (WHTS), temporal (time) spreading [256]. Previous investigate OCDMA schemes were with low spectral efficiency [257], hybrid OCDMA systems emerged as an effort to enhance security for OCDMA systems for augmenting network capability higher than the single system. Coherent OCDMA systems compared to its performance with OTDMA system, exhibited hybrid OCDMA/WDMA system improved performance 2000 [258].
Transmission performance further enhanced with hybrid SACOCDMA over WDM system and carries added number of active users since WDM offers huge bandwidth of optical fibers, supports several parallel channels at realistic data rates [259], lot of works for hybrid schemes using dissimilar codes were investigated [260], but systems performance for all time degraded because of complex code [261]. Accordingly hybrid system, more flexible code designed in SAC-OCDMA over WDM hybrid system with modified double weight (MDW) code, diminishes MAI effect using complementary subtraction technique and flexible auto cross correlation achieved longer reach [262]. Hybrid SAC-OCDMA-WDM offered numerous advantages, as it can hold asynchronous operation, flexible bandwidth management, simplified network control, high speed connectivity with improved security and service differentiation [263].
Improvement achieved with hybrid coding systems combined with novel coding methods, wavelength-hopping time-spread (WHTS) encoding, 2-D coding scheme based on combining of spectrum encoding along with temporal encoding and grouping of space encoding along with 2-D coding, Space-Spread Wavelength- Hopping Time-Spreading encoding (SS-WH-TS) generated and deemed with 3-D coding proposal [264,265]. As for as security is considered for optical communication network provides plentiful tackle to both network suppliers and impostors [266]. Later showed that OCDMA has immense network security features for users, encoded signal is noise-like waveform that possibly not be obtainable to any eavesdropper devoid of identifying and knowing allocated code of approved subscriber, showed perception of transmission security with SAC-OCDMA, and with numerous SAC-OCDMA codes accurate probability of detected Spectral-Encoding Chip Bandwidth (SECB) incomplete code sequences revealed [267]. Low cost implementation in contrast to bipolar ones based on enhanced PN code with modified double weight (MDW) code system superior security and unipolar OCDMA code methods based on modified quadratic congruence (MQC) investigated [268]. Serious efforts towards designing newer and efficient hybrid OCDMA and WDM overlay schemes appeared to produce enhanced security for the Optical CDMA systems above alone system. Thus few of the hybrid WDM/OCDMA methods were illustrated in Table 1 with their concise form.
Table 1: Summary of few hybrid Optical CDMA Methods.
In the first method exhibited, the transmission was for only limited number of channels for both OCDMA and SONET with similar WDM window [269], experimental results of hybrid scheme indicated numerous OCDMA and usual OC-192 OOK channel transmitted in tandem and Optical CDMA signals reside in unused bandwidth of WDM channel. Illustrated in the second method, hybrid OCDMA-WDM overlay system investigated established in 2001 [270]. Hybrid operation was addressed along with narrowly spaced WDM subscribers. Idea of this system is that ultra-shortpulses used for spectral encoded/decoded OCDMA. Sub-picoseconds laser pulses used as short pulses for Optical CDMA coding make system highly complex. The hybrid operation is addressed with high closely spaced WDM users, with ultra-tiny-pulses used for spectral encoded/decoded Optical CDMA. Error-free detection on OCDMA receivers obtained with application of nonlinear fiber thresholder in decoder, WDM signal filtered out and concealed properly. Similar concepts were realized in optical domain, OCDMA encoded short pulses extend over time for spectral phase coding and decoder is with Fourier Transform for windowed data signals. WDMA method permitted electrical user signals to access optical networks, access moves were jointed in method to obtain large throughputs, quicker and more flexible to LANs [271,272]. Later shown in third method (2006), the coherent spectral phase encoded Optical CDMA utilized over an accessible WDM network for secure communication. Signals used safely encoded and temporally extended to be hidden beneath host channel, aim is to offer ad hoc security augmentation for an encoded signal. Only two channels were taken for investigation of the projected system; one for host of WDM system using OOK and other for safe M-ary signal. Noise from amplifier due to spontaneous emission included since composite signal was amplified. Hence, protected signal fully masked and enclosed by means of amplifier noise in addition to host channel and this united signal applied into optical fiber communication link [273].
Other (fourth) method similar plan used except the method of coding, safe signal was spectrally encoded by implementing hopping technique optical communication employing tunable delay lines for different frequencies called (WHTS) [274,275]. Dispersive element applied for time spreading, hiding Optical CDMA channel in WDM channel and hence makes an eavesdropper to detect data from safe signal. Later on fifth method (2011) MQC family code (1D) used for SAC-OCDMA in addition to MDW code and WDM signals superimposed on code pulses of OCDMA. Results exemplified about practicability of transmitting both OCDMA and WDM users on the similar spectrum band, attained with satisfactory performance in addition to superior data secrecy [276,277].
Hybrid WDM/OCDMA method codes of OCDMA applied on each WDM wavelengths, and broadly studied for large number of users, large data rates and new codes for OCDMA projected for hybrid construction. Performance investigated with initial WDM and SAC_ OCDMA scheme utilized in LANs 2005 [278] and to sustain large number of active clients by minimizing consequence of PIIN noise, WDM/SAC balanced inadequate block design code applied, double weight (DW) code family in hybrid WDM/SAC scheme explored in 2007 [279], limited capability systems, in 2006 demonstrated hybrid WDM/OCDMA scheme over fiber to the home systems and showed that maximal length sequence codes utilized by advantage of cyclic property of arrayed waveguide grating routers [280].
Further study of the novel OCDMA encoders/decoders for various WDM channels projected and investigated (2007), phase mask have been employed upon super-structure FBG based encoder/ decoder to build [281]. Later on explored design along with WDM and reconfigurable optical OCDMA/dense supported on quaternary phase coding gratings on number of channels, used code-reconfigurable apparatus for decoding supported upon thermo-optic effect, as well as continuous fixed phase-shift SSFBGs of reconfigurable grating studied, illustrated direct-sequence of OCDMA supported SSFBG encoding attuned with WDM technology but this experimental scheme showed low spectral efficiency [282,283].
Study (2009) for error probability in passive optical network (PON) have been projected taking into account optical orthogonal codes (OOC) for OCDMA over DWDM modeling evaluated for this scheme using DQPSK modulation with balanced detection. Results illustrated for twenty four asynchronous users modulated at 10Gbps data rates that bit error rate have been 10-9 or small [284]. Later on study (2009) with novel family code termed as WS- Shifted Prime (SP), SP codes for SAC OCDMA-supported passive optical network, with grouping of WDM and bidirectional WDM-PON employed to join OLT and ONUs, two SAC code word’s transmission allocated and leading to performance improvement since noise has been minimized and augments capacity against eavesdropping [285,286]. Afterward, novel coding technique (2010) employed for studying capacity of hybrid WDM/OCDMA scheme; with direct-sequence OCDMA method included to WDM and various users used only one codeword for communication with diverse users [287].
OCDMA can be also being bifurcated on the basis of dimension of codes, as one and two-dimensional accordingly spreading mode, WDM+OCDMA and MW OCDMA have been amongst twodimensional OCDMA schemes. In MW OCDMA scheme every address code applies set of symmetric prime-hop pulses along with diverse frequencies transmitted with diverse velocities leading to chromatic dispersion and with WDM+OCDMA alike set of optical orthogonal codes used again on every wavelength channel, WDM+OCDMA system show good performance for heavy traffic load. Later Multi-Wavelength OCDMA (MW OCDMA) and hybrid WDM+OCDMA were comparatively investigated. Later (2010) new code family based on OCDMA network for SAC/WDM codes Quasi- Cyclic Low-Density Parity-Check (QCLDPC) code and Extended Welch-Costas (EWC), showed good performance in contrast to the conventional SAC OCDMA [288,289].
Later on, study with highly cost-sensitive optical network units (ONUs) as low-cost option placed in homes ensuing in the growth of spectrum-sliced WDM (SS-WDM).OCDMA created many benefits of being based on intolerance in the time domain to reduce effects of pulse overlaps and hence expansion, performance improvement [290,291]. A generic modeling method presented for MAI and BER estimation, applicable to realistic networks with fully asynchronous operation and is valid for any type of code for effect of optical fiber group velocity dispersion on Wavelength-Hopping Time-Spreading (WHTS) OCDMA Networks and tested Prime-Hop and Bin’s one coincidence codes. Results showed incompatibility of standard SMF for use in WHTS-OCDMA networks in contrast to dispersion shifted fibers (DSFs) [292].
Later on performance investigated for the Modified frequency hopping (MFH), modified quadratic-congruence (MQC), MUI and PIIN minimizing since phase induced intensity noise (PIIN) problem due to spontaneous emission of broad band source and MUI limits system performance, MUI raises with concurrent users a main degradation factor for SAC-OCDMA system. Study show as received power raises PIIN noise for all of codes raises linearly, affect of PIIN minimized by increasing code weight so conserves ample S/N ratio over bit error probability [293]. Study with OCDMA showed that it is very commanding tool to get better performance of superior communication networks in particular when coupled with technologies which will allow device integration and packaging [294]. In the experimental demonstration of new receiver supporting different codes as well as one-dimensional optical codes and wavelength-hopping/ time-spreading two dimensional codes with small bit error rate, numerical results indicated that normally performance of proposed receiver succeeds over conventional OCDMA receivers [295]. Demonstrated studies (2012) with hybrid models OCDMA over WDM system, which supports different types of data in one system, in order to design a system, modified double weight (MDW) code is employed as signature address this code is with flexible auto cross correlation properties and suppress MAI in manifold amount, exhibited proposed system enhanced performance in contrast to OCDMA system [296].
Comparative study (2012) with spectrum-sliced wavelength division multiplexing (SS-WDM) and spectrum amplitude coding OCDMA systems with different light sources illustrated, that MAI effects has important impact on SS-WDM over SAC Optical CDMA systems and spectral efficiency at constant BER of 10-12. SSWDM show superior spectral efficiency than optical CDMA since no bandwidth expansion is desired [297]. Performance studied for hybrid WDM/OCDMA schemes, where codes of OCDMA employed on each of WDM wavelengths and necessary background of OCDMA, employing MQC family code, WDM interference signals is suppressed properly for detection of optical broadband CDMA using notch filters show good performance [298]. Studied (2012) for DOCDMA (dynamic optical code division multiple access) which is predictable to high-bandwidth communication systems and on the receiver end, a synchronized tunable optical filter (TOF) with the identical function used as decoder, dynamically modulated central wavelength of TOF as per functional code on the transmitter during bit period earlier the transmission of data. Results showed DOCDMA diminishes PIIN effect and enhances performance (number of users, BER) [299].
Recently (2013) studied novel modulation technique multicode pulse-position modulation (MCPPM), is grouping of multicode modulation (MCM) and PPM, takes benefit from both MCM (in mitigating the dispersion) and PPM. Results show with 2-D OCDMA using 4-4-MCPPM, 4-2-MCPPM performed well with 60 users (5Gbps/user) at the transmitted power of -7dBm attained power gain of 11 dB and 36 users (10G bps/user) with transmitted power of -2 dBm correspondingly [300]. Comparative study of DWDM and OCDMA, showed OCDMA is one of substitute multiplexing method to more conventional time division multiple access (TDMA) and wavelength division multiple access (WDMA) [301].
Study for hybrid FSK-OCDMA with modified prime code as signature sequence for coding techniques, using MAI cancelation technique in OCDMA, FSK-OCDMA showed better BER performance in contrast to PPM-OCDMA technique [302]. Later on with splitstep Fourier method based on the coupled nonlinear Schrödinger equation, 3-D Poincare sphere theory and ones matrix method for DS-OCDMA, show that for incident pulse’s width less than chip duration, good encoding/decoding performance may be obtained [303]. In comparative performance investigation of asynchronous incoherent OCDMA system with uncoded systems using wrapped overlapping pulse position modulation (WOPPM) and unwrapped OPPM under MAI environment, illustrated for constant throughput, performance of WOPPM is considerably superior to OOK and PPM [304]. Afterward investigation with extended 2D multi weight multilength optical orthogonal code (MWML-OOC) using both Poisson and binomial distributions of MAI, analyzed performance of 1D and 2D MWML-OOC which can support multirate transmission and QOS differentiation in the OCDMA networks. It illustrated overall performance BER considering the binomial distribution of MAI is better than that obtained from Poisson-modeled MAI for both 1D and 2D MWML-OOC, also extended 2D MWML-OOC supports large number of users with reduced chip times and improve the BER compared with its 1D and 2D counterparts [305]. Performance investigation with extended modified Reed-Solomon codes to construct a new family of 2-D codes for asynchronous OCDMA, 2-D optical codes compared with that of the multilevel prime codes, show unique partition property of new optical codes supports exchange between code cardinality and performance for meeting different system needs and support applications needing rapid switching of many code word’s [306]. Further experimental study showed that coherent spectral phase encoded OCDMA completely suppresses MAI, speckle noise for first time without requirement for fast nonlinear time gating [307], study for OCDMA with prime code families and with new MUI cancellation method complete removal of MUI effect illustrated [308].
For binary chip-asynchronous(BCA-OCDMA) with time spreading results reveal suitability of nonuniform signaling attained throughput of 2.5 bits per OCDMA chip, chip asynchronism leads to around threefold robust capacity gain over chip-synchronous OCDMA capacity with additive channel noise [309]. Investigated performance 6ch×10Gb/s transmission with novel code dynamic cyclic shift (DCS) code for SAC-CDMA networks, using FBG, show performance of DCS code is superior to random diagonal (RD) and modified frequency hopping (MFH), the bit-error rate of the DCS code considerably better than other codes [310]. Experimentally exhibited, asynchronous 4 x 40Gbps full-duplex DPSK-OCDMA PON employing 8-level phase-shifted, 320Gchip/s apodized SSFBG en/decoders at every ONU and a single multiport encoder/decoder at OLT were investigated [311], study with novel SFBGs with 2/4/8ch encoder/decoder show peak over highest cross-correlation level ratios being higher than 17 and 10.2 dB, correspondingly [312]. Investigation with a simple divided spectrum balanced detection (DSBD) for SACOCDMA systems in which phase induced intensity noise (PIIN) is reduced. Although SDBD is more complex and append more constrains on system components, theoretical results show that DSBD improvement over conventional balanced detection method [313].
Recently study (2013) with SAC-OCDMA-FSO communication scheme compared with FSO system using intensity modulation/ direct detection (IM/DD) method and Khazani–Syed (KS) code with spectral direct decoding (SDD) technique; results show that system performance is superior to system using IM/DD technique [314]. Afterward performance using LDPC code for 2-D T/W OCDMA system investigated using carrier-hopping prime code as signature sequence, exhibit use of LDPC code strictly minimize error floor in OCDMA system, effectively improve system performance [315]. Further new trend of study grown up with modified interleavedivision multiple-access (IDMA) for IM/DD optical networks, showed noteworthy improvement by means of optical IDMA system in BER performance in contrast to OCDMA systems under same conditions [316].

Conclusion

This paper has presented an in-depth review of the multiplexing techniques exercised with fiber optic communication such as OTDM, WDM, SCM, OOFDM, PDM, MDM, SDM and DCDM to augment spectral efficiency of an optical network. For instance, SCM is a technique for multiplexing various diverse communications signals so that they may be transmitted along a single fiber. MDM transmissions have got special concentration because it enhances the amount of signals per fiber core and SDM bunches spatial channels closely into every fiber, very much significant since potential for triumph over the capacity crisis. PDM is physical layer capable multiplexing scheme enhances data rates without enhancing symbol rates but it experiences channel crosstalk and due to need of high speed polarization controllers augments system complexity and cost. With DCDM method diverse users signed with dissimilar RZ duty cycles to share the identical WDM channel, may diminish clock recovery frequency extensively. OOFDM is multicarrier technique in which data information were carried in multiplexed setup in parallel over many lower rate subcarriers and different carriers used to modulate individual information signals referred as sub-carriers but with complex transmitter and receiver necessity for accurate synchronizations, although for higher bit rate, longer reach OOFDM is preferred. OCDMA is also a kind of multiplexing with security features but number users is limited, extremely competent and strong contender from multiple access systems and Optical IDMA (OIDMA) system with modified interleave-division multiple-access (IDMA) for IM/DD optical networks showed noteworthy performance improvement in contrast to OCDMA systems under same condition.
Capacity augmentation is achieved through hybrid multiplexing techniques and amongst many hybrid techniques PDM and QPSK/ DQPSK or grouping of them has got most attention in the optical fiber communication systems. Further, WDM-OTDM, SCM-WDM with Miller codes, POLMUX RZ-DQPSK, BCA-OCDMA, DOCDMA, OCDMA using MCPPM, MDW-OCDMA, balanced detection (DSBD) for SACOCDMA, integrated DWDM-OOFDM systems with OADM, OLT in WDM-OFDM-PON, adaptively modulated optical OFDM (AMOOFDM), and OFDM–OSSB–ROF with QAM sequence generator has demonstrated noteworthy performance improvement. A supplementary spectral augmentation can be further achieved with application of advanced fiber, modulation methods, solit on pulse transmission and use of advanced digital signal processing techniques such as electronic dispersion compensation, FEC (Read Solomon, concatenated codes). Consequently future prospect is the necessity to design an all in one system, a single novel universal automatic multiplexing design (DWDM/OCDMA/OIDMA/MDM/OTDM/ PDM/SDM/SCM/hybrid) with choice of mode (multiplexing) selection depending upon ones need and some sort of programming involved on input end which can be helpful to select a particular multiplexing mode depending upon particular application.

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