Analog and digital modulation formats of optical fiber communication within and beyond

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  • 1. INTERNATIONAL JOURNAL OF ELECTRONICS AND International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEMECOMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)ISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 4, Issue 2, March – April, 2013, pp. 198-216 IJECET© IAEME: www.iaeme.com/ijecet.aspJournal Impact Factor (2013): 5.8896 (Calculated by GISI) ©IAEMEwww.jifactor.com ANALOG AND DIGITAL MODULATION FORMATS OF OPTICAL FIBER COMMUNICATION WITHIN AND BEYOND 100 GB/S: A COMPARATIVE OVERVIEW S.K Mohapatra1 Department of ECE, Trident Academy of Technology. BPUT, Bhubaneswar,Odisha,India. R. Bhojray2 Department of ECE, Trident Academy of Technology. BPUT, Bhubaneswar,Odisha,India. S.K Mandal3 Department of ECE, Trident Academy of Technology. BPUT, Bhubaneswar,Odisha,India. ABSTRACT For transferring data to increase performance and implementation simplicity different analogue and digital techniques are used in fiber optic communication channel. Different digital modulation formats maximizes spectral efficiency and also improves tolerance to transmission impairments. This paper reviews a comparative analysis for the different digital modulation formats within 100Gb/s and beyond the 100Gb/s. A brief overview over different transmission systems transmitting huge amount of data at channel bit rates up to 1Tb/s or beyond this. In this specific article we survey in a comparative tabular manner to analyse the advantages of digital modulation formats over old analogue modulation formats. Keywords: Optical fiber communication ,Analog modulation format, Digital modulation format , ≤ 100Gb/s , ≥ 100Gb/s. 198
  • 2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME1. INTRODUCTION In communication technology, a large bandwidth was the universal demand forthe industrial and consumer application. For this, advanced modulation techniquesused in optical communication in terms of high bandwidth data communication. Inadvanced modulation formats, the ones that make use of not only amplitude but alsoother signal domains, such as phase and the state of polarization are moresophisticated techniques to encode the electrical data pattern onto an optical carrier.This produces an enhancement in the functionality and an increase in the spectralefficiency compared to the analogue modulation formats of fiber opticcommunication. The major advantage of using fiber optic digital modulation formats is that theuse of digital signals reduces hardware complexity, noise and interference difficultiesare compared to the analogue signal where large number of wave forms will berequired resulting in a large bandwidth for the symbol to be transmitted [4]. Over the past years various digital modulation formats designed which aremainly consists of 2.5,10,25,40 and 100 Gb/s wave length channels. But for todayoptical communication systems, data rate per channel increases to beyond 100 Gb/s.The 100 Gb/s Ethernet (GbE) interfaces have been published by the IEEE standard802.3ba [6] in 2010 for 10 Km and 40 Km reach, using 4 channels with 25 Gb/s. Theline side bit rate of about 112 Gb/s (OTU4 bit rate) and the OTN multiplex with theclient data and standard Reed Soloman FEC has been defined by ITU-T standardG.709 [7] published in 2009. Since 2010, 100 Gb/s bandwidth systems slowly progress in the differentoptical communication networks for industrial applications. The moderncommunication optical systems requires data transmission at a higher rate i.e beyond100Gb/s (Exa:- 200Gb/s, 400 Gb/s, 1000 Gb/s and even 1T bit/s Ethernet). For shortreach clients side applications 100 Gb/s transmission desired with 10 No. Of 10 Gb/sor 4 No. Of 25 Gb/s [6]. For high transmission capacity, serial transmission of a hugenumber of digital wave division multiplexing channels at narrow channel spacing isbasically designed. This segment comparatively reviews the digital modulationtechnological options for serial transmission of within and beyond 100 Gb/s. In the introductory part we elaborate the bit rate in Mb/s with respect to thewavelength of optical source, the repeater spacing and optical carrier. Then in the firstpart we express on 40Gb/s optical systems and represent an overview on themodulation formats starting from binary amplitude shift keying to M-QAM.Thesecond section is expressing on 100Gb/s systems and give an overview on thedifferent features,system tolerances and main characteristics. In the third section wefocus on the digital modulation formats for systems beyond 100 Gb/s. 199
  • 3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME2. CLASSIFICATION OF OPTICAL FIBER MODULATION TECHNIQUES Sl. Modulation formats Type Notation No. Analog modulation Amplitude modulation/Intensity AM/IMSC 01. formats modulation sub-carrier Frequency modulation/Intensity FM/IMSC modulation sub-carrier Phase modulation PM ON-OFF keying/binary amplitude shift OOK/ BASK keying. Binary frequency shift keying BFSK Binary phase shift keying BPSK Differential phase shift keying DPSK Return to zero DPSK RZ-DPSK Quadrature phase shift keying QPSK Differential QPSK DQPSK Return to zero DQPSK RZ-DQPSK 02. Digital modulation Return to zero DPSK-3ASK RZ-DPSK-3ASK formats Polarization division multiplexing PM-QPSK/DP-QPSK QPSK PM-orthogonal frequency division PM-OFDM-QPSK/DP- multiplexing QPSK OFDM-QPSK Optical polarization FDM-RZ-DQPSK OP-FDM-RZ-DQPSK Polarization division multiplexing- PM-DQPSK or DP- DQPSK DQPSK M-ary quadrature amplitude modulation M-QAM Minimum shift keying MSK Gaussian MSK GMSK Single carrier modulation formats SC Multi carrier modulation formats MC3. ANALOG OPTICAL FIBER MODULATION TECHNIQUES The optic baseband transmission in which the signal is carried on a light beam modulatedat the baseband frequencies of the information. In this analog modulation the optic power variesin proportion to the input current known as Intensity modulation.3.1 AM/IM Subcarrier modulation Conventional AM places message on a carrier whose frequency is much greater than themessages . AM of a single sinusoid can be written as i = Is (1+m cosωm) cosωsct (1)where ωsc is the subcarrier frequency.We can add a dc current I0 to the above current and drivean optic source with the result producing IM of a light beam by an AM signal. This is AM/IMmodulation that generates optic power. P=P0 + Ps(1+m cosωm) cosωsct (2) 200
  • 4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME3.2 FM/IM Subcarrier modulation Adding a dc current to the FM equation , i = Is cos (ωsct+β sinωmt) (3)where β is the modulation index and intensity modulating an optic source with it producesFM/IM modulation. For the sine wave optic power varies asP=P0 + Ps cos(ωsct+β sinωmt) (4)The detected current has the same waveform as the optic power. As the FM bandwidth is largerthan the AM bandwidth , fewer FM messages can be fitted within the fiber’s limited range offrequencies.3.3 PM Subcarrier modulation For analog optical communication , phase modulation is a advanced modulation formatwhich is a electro-optic effect principle that generates a phase shift [3]. That phase shift is linearlyproportional to the applied field. The phase demodulation process uses heterodyne detectionwhich forces a sinusoidal non linearity on the demodulated signal as comparison to the amplitudedemodulator [3].4. OPTICAL FIBER DIGITAL MODULATION TECHNIQUES Modulation is a method by which digital information is imprinted onto an optical carrierand in its most general sense also including CODING to present transmission errors. In opticalfibers the electromagnetic waves with frequencies of nearly 200THz are used to transferinformation from one point to another. In optical fiber communication systems the modulation ofboth amplitude and phase of the carrier allows for an improved utilisation of the complex plane,where information symbols are mapped, yielding an increased spectral efficiency.4.1 OOK/BASK The original signals are modulated onto high frequency optical carriers in optical fibercommunication systems. In ASK format the baseband signal is multiplied by a carrier frequency ,thus the binary 0 is transmitted with 0Watt and binary 1 with A Watt. The demodulation processat the receiving end is performed efficiently by applying photodetectors, which converts theoptical signal to the electrical signal. 4-ary ASK digital modulation formats are developed ,having M=2b where b is the number of bits per symbol used to double the transmission capacity.Table-11 summarizes the gain factor with respect to DBPSK. Table-12 shows the poorinformation capacity and bandwidth efficiency.4.2 BFSK BFSK is a data signal converted into a specific frequency in order to transmit it overoptical fiber media to a destination point. The choice of the frequency deviation depends on theavailable bandwidth. The total bandwidth of a FSK signal is given approximately by 2∆f+2Bwhere B is the bit rate. When ∆݂ ‫ , ܤ ب‬the bandwidth approaches 2∆f and is nearly independentof the bit rate. Table-12 compares the information capacity with OOK which is slightly better,but in Table-12 the bandwidth with OOK is not so efficient. 201
  • 5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME4.3 BPSK In BPSK modulation technique, the binary data are modulated onto the optical carrierreferring to the phase difference between binary 0 and 1. The binary 1is signed as sinωt and 0 issigned as -sinωt. Its demodulation process is so complex than other digital modulationformats.Table-9 summarizes the advantages and disadvantages of BPSK modulation format. Ithas small error rate as shown in Table-9.4.4 DPSK In optical transmission systems, the DPSK is preferred because of its high robustness tonon linear propagation[8] and to a smaller extent, to polarization mode dispersion(PMD) [9]. It isa fast and stable modulation format and well suited for many optical applications. It has someadvantages to the binary PSK, as a lower phase error rate and a no need to know the absolutephase. The information capacity is twice the BFSK as pointed in Table-12.4.5 NRZ/RZ-DPSK The multichannel parallel format conversions from the non return to zero DPSK(NRZ-DPSK) to the return to zero DPSK(RZ-DPSK) using a single semiconductor opticalamplifier(SOA). The simultaneous conversions are based on the cross phase modulation (xpm)effect , which is induced by a synchronous optical clock signal with high input power. The XPMadds an identical phase shift onto every input bit, resulting in the phase difference unchanged[5].The input spectra are broadened and subsequent filter is utilized to extract the specific part toform a RZ pulse 6 channel NRZ-DPSK signals at 40Gb/s can be converted to the correspondingRZ-DPSK signals with nearly 0.8 to -1dB power penalty for all the channels. The OSNRsensitivity at BER=2 ×10-3 (dB) is very small, i.e. 12.5 according to Table-7.4.6 QPSK In QPSK, two bits in the bit stream are taken and four phases of the carrier frequency areused to represent the four combinations of the two bits. There are different phases of the carrierare used to represent the four possible combinations of two bits : 00, 01, 10 and 11. It doubles theline rate compared to OOK by coding two bits in one symbol, applying 50Gbaud to get 100Gb/s.The output signal of the transmitter has mainly constant optical power and the information iscarried in the four phase states of the optical phase of the emitted light. Table-7 summarizes thatthe PMD tolerance without compensation higher than different DPSK formats. It has better errorperformance over BPSK and BFSK according to Table-9.4.7 DQPSK It is the four level version of DPSK. DQPSK transmits two bits for every symbol (bitcombination being 00, 01, 11 and 10) and has an advantage over DPSK is that it has narroweroptical spectrum which tolerate more dispersion (both chromatic and polarization mode), allowsfor stronger optical filtering and enables closer channel spacing. As a result, DQPSK allowsprocessing of 40Gb/s data rate in a 50GHz channel spacing system. The bandwidth saving ofDQPSK over both DD-OOK and DPSK suggest that DQPSK can improve the reach andefficiency of WDM systems according to Table-11. 202
  • 6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME4.8 RZ-DQPSK To get RZ-DQPSK signal, two phase modulators are cascaded for the modulation of theoptical phase by 0 to ߨ/2 and by 0 to ߨ/4 applying binary modulation signals or a single phasemodulator driven by an electrical 4-level modulation signals. The 100Gb/s transmission usingDQPSK modulation format has widely been demonstrated over either lab or field fibers at100Gb/s[10-12], with FEC overhead at 107Gb/s[13-17] and at 111Gb/s[18] and at 112Gb/s OTU-4channel bit rate[19, 20]. Table-8 compares the OSNR value of RZ-DQPSK with NRZ-DQPSK. AlsoTable-8 summarizes the modulated bandwidth which is doubles with respect to the NRZ-DQPSKformat.4.9 RZ-DPSK-3ASK This is a very fundamental mixer of ASK modulation and phase modulation. In RZ-DPSK-3ASK modulation format 2.5 bits are coded in one symbol which leads to a symbol rate of 43Gbauds[21-24] for support of the OTU4 line rate [7] of 112Gb/s. The OSNR tolerance of this modulationformat is limited, as a result, the transmission reach is also limited [25,26]. The main application areais in the metro where its estimated reach is less than 500 according to Table-13.4.10 PM-QPSK or DP-QPSK The 100Gb/s PM-QPSK transmission process [27] running at a symbol rate of 25-28Gbaud iswidely applied with off-line signal processing of the electrical signals which are measured by 4-channel high speed real time oscilloscopes acting as fast A/D convertors[28,29]. As per Table-13, itscompatibility with 10Gb/s and 40Gb/s is positive. The long haul OIF is the perfect application area forthis modulation format.4.11 PM-OFDM-QPSK or DP-OFDM-QPSK Another commercially available 100Gb/s transponder applies two narrow spaced (20GHz)optical carriers each modulated with PM-QPSK format based on 14Gbaud modulation [30,27]. Thismodulation format has been denoted as DP or PM-OFDM-QPSK and requires the hardware of two50Gb/s PM-QPSK transmitters and receivers. The compatibility with 10Gb/s and 40Gb/s is negativeshall be set according to Table-13.4.12 OP-FDM-RZ-DQPSK To carry two optical carriers, there are two polarizations can be used to eliminate the fastautomatic optical polarization demultiplexures [27]. The two carriers can be multiplexed anddemultiplexed with optical fibers. This modulation format based on 28Gbaud and has been entitled asorthogonal polarization frequency division multiplex RZ-DQPSK. But also to the separation of twooptical carriers in two polarizations only 100GHz channel spacing is supported. The compatibilitywith 10Gb/s and 40Gb/s is positive (which is negative in case of PM-OFDM-QPSK) shall be setaccording to Table-13.4.13 PM-DQPSK or DP-DQPSK By applying polarization division multiplexing (PM), we can reduce the symbol rate. As aresult the line rate is doubles or the symbol rate becomes half [27]. This leads to 100Gb/s polarizationmultiplexed DQPSK signals or dual polarization (DP) with a symbol rate of 28Gbaud to support theOTU 4 line rate. The 28Gbaud modulation formats supports the 100G DWDM transmission with 50GHz channel spacing. The two DQPSK signals are combined orthogonally polarized using apolarization beam combiner According to Table-13 the symbol rate is summarized as 28 which isalmost equal to OP-FDM-RZ-DQPSK optical digital modulation format. 203
  • 7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME4.14 M-QAM Recently QAM scheme with polarization multiplexing is utilized to achieve a channel rateof 200Gb/s with 16QAM. In an M-QAM, m bits are transmitted in a single time slot [27].Optimizing the SE of signals with M-QAM constellations by Nyquist filtering towards Nyquist-WDM [32] is currently of high research interest and has already been demonstrated at submarinetransmission configurations[33] using RZ at PM-QPSK. Polarization multiplexed 16 QAMsignals have been realized by multilevel generation using passive combination of binary signalsto achieve 224 Gb/s channel rate(200G+FEC overhead)[45-47] and for 448 Gb/s channel rate[48]. Using polarization multiplexing and QAM modulation format transmission lengths between670 up to 1500km have been demonstrated [45-47]. RF-assisted optical Dual-carrier 112Gb/spolarization-multiplexed 16QAM is applied to achieve 112Gb/s channel rate[49]. DP-64QAMformat has been applied to achieve a 240Gb/s channel with 12 bits/symbol [50]. QAMmodulation is reported for lower bit rate channels of 100Gb/s using 32QAM[51], 100Gb/s using35QAM[52], 112Gb/s and 120Gb/s using 64QAM[53,54], 56Gb/s with a spectral efficiency of11.8bit/s/Hz using DP-256QAM[55], 54Gb/s using DP-512QAM[ 56]. According to Table-15,we conclude a comparative analysis between different M-QAM modulation techniques havingdifferent bit rate (Gb/s).4.15 MSK The new optical minimum shift keying modulation scheme have the high spectralefficiency as compared to other digital modulation formats. The transmitter for optical MSKbased on two Marh-Zehnder modulators (MZM) similar to the transmitter for DQPSK. The MSKreceiver with one delays and add filter (DAF) and photodiodes for direct detect detection issimilar to the DPSK receiver. On the basis of error performance, the signal coherence andderivation ratio are largely unaffected which is reflected according to the Table-9. By the Table-12, the information capacity is shown as doubles the capacity of BFSK signal.4.16 GMSK Gaussian minimum shift keying is a simple optical binary modulation scheme which isviewed as a derivative of optical MSK modulation technique. In this format , the side lobe levelsof the spectrum are further reduced by passing the modulating NRZ data waveform through a pre-modulation Gaussian pulse shaping filter. The bandwidth of a optical GMSK system is defined bythe relationship between the pre-modulation filter bandwidth B and the bit period TB. Thedecision of BTB according to 0.2GMSK, 0.25GMSK, 0.5GMSK at 99.99% are 1.22,1.37 and 2.08respectively.4.17 Single carrier (SC) modulation formats For bit rates beyond 100Gb/s on a single carrier, higher level modulation schemes likeQAM with PM is used to get a channel rate of 200Gb/s with 16QAM. In this format 2×m bits aretransmitted per symbol. Various constellations [27.31] can be applied for PM-QAM modulationformat. Minimizing the SE of signals with M-QAM constellation by Nyquist filtering towardsNyquist-WDM[32] is currently of high research interest and has already been demonstrated atsubmarine transmission configurations[33] using RZ at PM-QPSK. Table-14 gives an overviewon single channel M-QAM options from 200Gb/s to 1Tb/s [27] 204
  • 8. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME4.18 Multicarrier (MC) modulation formats: Optical OFDM transmission is a MC modulation format which approach to support highbandwidth channels [34]. To form the IFFT, DSP is applied in the transmitter. Due to therectangular (almost) safe of O-OFDM signals high capacity transmission can be performed byclose allocation of multiple OFDM signals in the frequency domain without guard bands A number of transmission experiments using polarization multiplexed O-OFDM and PM-O-OFDM have been reported [27,35-41], transporting Tb/s super channels over submarinedistances[41]. Recently field transmission trials over installed standard SMF applying PM-OFDMformat in co-propagation with 112G DQPSK channels are reported using 253 Gb/s OFDM super-channels with subcarriers carrying QPSK signals and 400Gb/s super-channels 8QAM signals[42] over 768 Km and Tb/s super-channels over 454 Km [43] and 3560 Km [44]. This opticalOFDM transmission with PM-QPSK modulation overview is depicted in Table-13Generation Wavelength of Optical Bit rate Mb/s Repeater Spacing (Km) Loss source(µm) I 0.8 4.5 10 1 II 1.3 1.7×102 50 <1 III 1.55 1.0×104 70 <0.2 Iv 1.55 1.0×105 100 <0.002 V 1.55 >1.0×109 >100 <0.002Table-1: Generations of optical fiber communication which shows analysis between wavelength and bit rate within Mb/s. Level Line rate DS3 channel OC-1 51.84Mbps 1 OC-3 155.52Mbps 3 OC-9 466.56Mbps 9 OC-12 622.08Mbps 12 OC-18 933.12Mbps 18 OC-24 1.244 Gbps 24 OC-36 1.866 Gbps 36 OC48 2.488 Gbps 48 Table-2: Optical interfaces related between Line rate with different DS3 channels. Fiber Optical loss (dB/km) Size(µ) Type 780 nm 850 nm 1300 nm 1550 nm 9/125 SM 3.0 2.5 0.5-0.8 0.2-0.4 50/125 3.5-7.0 2.5-6.0 0.7-4.0 0.6-3.5 62.5/125 4.0-8.0 3.0-7.0 1.0-4.0 1.0-5.0 100/140 MM 4.5-8.0 3.5-7.0 1.5-5.0 1.5-5.0 110/125 15 200/230 12 Table-3: Performance analysis of optical fiber loss of analog modulation schemes. 205
  • 9. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEMEPARAMETERS CAUSES CRITICAL EFFECTS COMPENSATION POWER/CHANNE LAttenuation Material .Reduced signal power Shorter spans, purer fiber absorption/system levels material. .Increased bit errorsChromatic Wavelength Increased bit errors Use of compensation fiber ordispersion dependent group material. velocityPolarization Polarization state .Increased bit errors New fiber with low PMDmode dispersion dependent values; careful fiber differential group layng;PMD compensators delayFour wave Signal interference .Power transfer from Use of the fiber with CDmixing original Increased bit compensators; 10mW errors signal to other Unequal channel spacing frequencies .Channel crosstalkSelf phase Intensity dependent .Spectral broadening Use of the fiber with CDmodulation and refraction index .Initial pulse compensators;cross phase 10Mw compressionmodulation .Increased bit errorsStimulated Interaction of signal .Decreased peak power Careful power designRaman with fiber modulator .Decreased OSNRscattering structure 1mW .Optical crosstalkStimulated Interaction of signal .Signal instability Spectral broadening ofBrillouin with acoustic waves .Decreased peak power the light source.scattering 5mW .Decreased OSNR .Increased bit errors Table-4: Analysis of analog modulation of fiber optic transmission phenomena.Fiber type DESCRIPTION ZERO DISPERSION DISPERSION Dispersion slope at 1550 WAVELENGTH AT nm 1550 nmG 652 Non-dispersion 1300-1324 nm -17 ps/nm/km 0.057 ps/nm2/km shifted fiberG 653 Dispersion shifted 1500-1600 nm 0 ps/nm/km 0.07 ps/nm2/km fiberG 655A-C Non-zero dispersion Not specified but 1450- 4 ps/nm/km 0.045 to 0.1 to ps/nm2/km shifted fiber 1480 nmG 656 Negative Non-zero Not specified -5 ps/nm/km 0.05 to 0.12 ps/nm2/km dispersion shifted fiber Table-5: Performance analysis of different types of fibers with dispersion at 1550nm. 206
  • 10. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEMEBit rate per Type of PMD delay Max. Insertion return physical plant Attenuation channel transmission limit(ps) CD at loss loss. verification profile 1550 nm(ps)2.5 Gbps OC- 40 18817 1550/1625 1550 1550/1625nm 1550-DWDM 48/STM-16 nm nm 1625nm10 Gbps OC- 10 1176 1550/1625 1550 1550/1625nm 1550-DWDM 192/STM-64 nm nm 1625nm40 Gbps OC- 2.5 73.5 1310/1550 1550 1310/1550nm 1550-DWDM 768/STM- nm nm 1625nm 25610 Gbps Ethernet 5 738 1310/1550 1550 1310/1550nm 1550- nm nm 1625nm Table-6: Transmission rate performance for NRZ fiber modulation coding format within 40Gb/s.Characteristics ODB/PSPT NRZ/DPSK NRZ- RZ-ADPSK RZ-DQPSK PM-QPSK ADSPKOSNR Sensitivity at 17.5 12.5 13 12.5 13.5 12.5BER=2×103 dBNominal range using 700 1600 1600 2200 1400 1700EDFAFilter tolerant Yes Affects Yes Yes Yes Yes(for 50 GHz channel rangespacing)PWM tolerance 2.5 3 3.5 3.5 6 10withoutcompensation(PS)Sensitivity to non- No No No No Yes Yeslinear distortionComplexity/cost Low Low Low Medium High High Table-7: Comparative analysis of digital optical modulation formats for 40Gb/s.Modulation Modulators Modulators used in OSNR Chromatic Modulated Clocktechniques used in receiver (dBm) dispersion bandwidth recovery of PSK at transmitter frequency 40Gb/sRZ DPSK 2 Nos. 1 delay intero - 15.6 50 160 40 ferometer (DI) & 2 photo detectors(PD)NRZ DPSK 1 Nos. 1DI +2PDs 18.5 74 80 40RZ DQPSK 2 Nos. 2DIs + 4PDs 17.7 161 80 20NRZ 2 Nos. 2DIs+4PDs 20.5 168 40 20DQPSK Table-8: Performance analysis of different PSK digital modulation schemes for 40Gb/s. 207
  • 11. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEMESl. Modulation Demodulation Error Advantages DisadvantagesNo. techniques performance performance1 BASK Easy demodulation Restricted in Hardware Poor BW linear region Implementations simple and low cost2 BFSK Matched filter Performs Same as bask Hardware detectors used well design of receiver is complex3 BPSK Receiver circuit is Small error Used only for Inefficient complex due to rate satellite phase shift communication. detection4 DPSK Receiver requires Required 3 Introduces the Efficient less memory dB less than complexities of than coherent BFSK receiver design PSK5 QPSK Phase shift Better over Bandwidth efficient Hardware detection is used BPSK and than BPSK design of BFSK receiver is complex6 MSK Direct inject to The signal Constant Envelope The spectrum NRZ data to coherence is not compact frequency and enough to modulator. derivation realize data ratio are rates largely unaffected by variations in input data7 QAM Coherent detection Small error Better transmission BW is same as rate than MSK ASK and PSK8 16 QAM Coherent detection Same as Producing a very BW is same as QAM spectrally efficient ASK and PSK transmission.9 64 QAM Coherent detection Same as Very efficient BW is same as QAM spectral efficiency ASK and PSK10 GMSK Bandwidth time The carrier is Constant envelope, It promotes product lag or lead by spectrally efficient ISI at higher 0 performance is 90 over bit bit rate measured by SNR period w.r.t transmission Vs BER BT resulting BER Table-9: Modulation parameters of Digital modulation techniques in 40Gb/s modulation formats. 208
  • 12. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME Modulation ∆v/Rb ∆v for Rb = 10 Gb/s 2 DPSK 3.0×10-3 30 MHz 4 DPSK 5.0×10-4 5 MHz 2 PSK 8.0×10-4 8 MHz 4 PSK 2.5×10-5 250 KHz 8 PSK 1.5×10-6 15 KHz 16PSK 2.4×10-7 24 KHz 8 QAM 9.0×10-6 90 KHz 16 QAM 6.9×10-3 69 KHz Tale 10: Comparative analysis of the Laser linewidths required to implement various modulation techniques by assuming a 0.5 dB penalty. Modulation Code rate Symbol rate Energy Gain (dB) Vs (GHz) DD-00K DBPSK DD-00K 15/16 42.7 --- -2.46 D-BPSK 15/16 42.7 2.46 --- 15/32 42.7 4.43 1.97 1/2 40.0 4.37 1.91 2/3 30.0 3.83 1.37 D-QPSK 3/4 26.7 3.40 0.94 7/8 22.9 2.41 -0.05 15/16 21.3 1.52 -0.94Table-11: Performance analysis of bandwidth of DQPSK over DD-OOK and DPSK suggest that DQPSK can improve the reach and efficiency. Sl.No. Modulation Points Symbols Information Derived BW efficiency formats capacity form 01 BASK 01 01 Poor ASK Poor 02 BFSK 01 01 Better than FSK Not efficient BASK 03 BPSK 02 02 2 BFSK PSK Only for high speed data transfer 04 DPSK 01 02 2BFSK PSK Only for medium speed communication 05 QPSK 04 04 2BFSK PSK High 06 MSK 04 04 2BFSK OQPSK Lower than QPSK 07 QAM 02 04 Better than ASK & Less than other BASK PSK techniques 08 16 QAM 04 04 Better than ASK & Less than other QAM PSK techniques 09 64 QAM 06 04 Better than ASK & Less than other QAM PSK techniques 10 GMSK 04 04 Same as QAM FSK Excellent Table 12: Parametric comparison of optical fiber digital modulation formats for 40Gb/s 209
  • 13. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME OOK OOK-VSB DQPSK RZ-DPSK-3ASK PM- OP-FDM- PM- PM_OFModulation DQPSK RZ- QPSK DM_QP formats DQPSK SK Bits/symbol 1 1 2 2.5 2×2 2×2 2×2 2×2×2 Spectral 0.5 1 1 2 2 1 2 2 efficiency Estimated < 500 < 500 1000 < 500 600 1500 1500 2000 reach(Km)Compatibilit Positive positive positive positive positive positive positive negativey with 10G & 40G Application Short reach Short reach metro metro metro Long haul Long haul Long area OIF haulConstellation ×2 Symbol rate 112 112 56 44 28 28 28 14Coherent/non Non-coherent Non- Non- Non-coherent Non- Non- Coherent coherent -coherent coherent coherent coherent coherent Product No No No No No Yes Yes Yes available Green field ------ ------ ------ ------- ------ ------- Yes Yes OSNR 17.5 18.5 15.5 >20 15.5 15.5 <15 <15tolerance(dB)@BER4×10-3 CD ±5 ±5 ±20 ±30 ±90 ±90 >> >>tolerance(ps/ nm)@ 2dB penalty Max. DGD 4 4 9 10 18 18 >> >>tolerance(ps) @ 2 dB penalty Power Positive Positive positive Negative positive Negative positive Negativeconsumption Practical E&E/O E&E/O CD & CD & adaptive Opt. 2×50G None 2×50G issues Components Component adaptive PMD-comp at old polarizatio superior interface CD and adapt. s CD & PMD-comp fibers n solution s PMD adapt PMD at old fibers demux,C copmensation compensati D& on adaptive PMD comp at old fibersEffectiveness ----- ------ For metro ------- -------- -------- For long -------- of cost haul Table13: Vital features of 100 Gb/s digital modulation formats. 210
  • 14. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEMEModulat PM PM PM PM- PM- PM- PM- PM- PM- PM- PM-ion - - - 8QAM 16QA 16QA 32QA 32QA 64QA 64QA 256Qformat BP QP QP M M M M M M AM SK SK SKBit 100 100 400 400 200 400 400 1000 400 1000 400rate(Gb/s)Bits/Sy 2×1 2×2 2×2 2×3 2×4 2×4 2×5 2×5 2×6 2×6 2×8mbolConstellation ×2 ×2 ×4 ×4 ×8Symbol 28- 28- 112 75-85 28-32 56-64 45-51 112- 37-43 93- 28-32rate(Gb 32 32 - 128 107d) 128OSNR( 10. 12. 18. 20.2 19.2 22.2 24.2 28.2 26.7 30.7 >30dB)@ 8 2 2minimum BaudrateOSNR( 8.2 9.8 15. 17.8 16.8 19.8 21.8 25.8 24.3 28.3 >30dB)@ 8maximum BaudrateChannel 50 50 200 133 50 100 80 200 67 166 50SpacingNo. of 44 88 22 33 44 44 55 22 66 26 88C-bandchannelsPenalty 00 00 06 08 07 10 12 16 14.5 18.5 >20Vs.100G(dB)Total 8.8 8.8 8.8 13.3 17.6 17.6 22 22 26.4 26 35capacity(Tb/s) Table14: Comparative overview of different modulation formats for 100Gb/s, 200Gb/s, 400Gb/s and 1000Gb/s by taking reference to theoretical 40Gb/s values. 211
  • 15. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME5. COMPARISON The different optical analog and digital modulation formats Table1-14 compares withdifferent parameters. In the introductory comparison Table-1 views on different wavelengthof optical source in um range from 1st generation to latest generation by which the bit rate ismuch more greater than previous generations. In the analog modulation format the types ofoptical fibers having different sizes Table-3 summarizes their optical losses per km range.We compare the different optical fiber parameters like attenuation , CD, self phase and crossmodulation, stimulated Raman Scattering in Table-4. The dispersion at 1550nm of differentfiber types like G.652,G.653,G655A-C and G656 overviews in Table-5. The bit rates perchannel (2.5Gb/s,10Gb/s,40Gb/s,etc) is illustrated in Table-6 which compares PMD delaylimit in ps with respect to return loss. It shows that the return loss especially same for ≤40Gb/s. Under SONET, 256 (optical carrier) optical interfaces are defined and their line ratescompares which is pointed in Table-2. Table-7 gives a comparative overview on filter tolerant for 50 GHz channel spacingwith PMD tolerance without compensation. The RZ-ADPSK and NRZ-ADPSK comparestheir nominal range EDFA are 2200 and1600 respectively. The No. of delay interoferometerand photodetectors used for different optical modulation formats like RZ-DPSK,NRZ-DPSK,RZ-DQPSK,NRZ-DQPSK (i.e phase shift keying modulation formats) is summarizedin Table-8. The modulated bandwidth of different BPSK schemes compares in Table-8. TheOSNR of different PSK formats and their error performances Table-8,9 for 40Gb/s isillustrated. The QAM ,16QAM and 64QAM modulation formats having their demodulationperformances compares in Table-9. The Laser line widths required to implement variousoptical digital modulation techniques by using a 0.5 dB penalty having 10Gb/s is exhibited inTable-10. In Table-11, the bandwidth saving of differential quadrature PSK over both DD-OOK and DPSK suggest that DQPSK can improve the efficiency of the different opticaldigital modulation formats. The GMSK error performance promotes ISI at higher bit ratetransmission compares with other optical digital modulation formats having excellentbandwidth efficiency Table-9,12. Various constellations can be applied for PM-QAMmodulation formats , e.g circular QAM symbol constellations or quadratic constellation withdifferent sizes as depicted in Table-13,14[27]. The different symbol rates of differentmodulation formats are compared with their area of application , estimated reach and spectralefficiency in Table-13. Table -14 reflects an comparative overview on single channel M-QAM options like PM-16QAM of 200Gb/s , PM-8QAM of 400Gb/s ,PM-32QAM of1000Gb/s ,PM-64QAM of 1000Gb/s ,PM-256QAM of 1000Gb/s by taking 40Gb/s value asreference , which considering polarization multiplexing for all options.6. CONCLUSION By the comparative analysis of the analog and digital optical modulation formatswhich is carried out in this article gives a conclusion that, for the excellent application , thedigital modulation format is highly applicable. At 40Gb/s the system designer has a soleconsideration for the techniques like BASK ,BFSK ,BPSK ,DPSK and DQPSK does notunder the region of consideration and the system designer has to think in terms of bettermodulation techniques like the MSK , GMSK and PM-QPSK , where PM-QPSK has provedits performance over the other two in the area of fiber optic communication. The 100Gb/smodulation format has been defined by an OIF framework and multisource agreement. 212
  • 16. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME In the next advanced technology of 400Gb/s , the symbol rate up to 32Gbaud is highlyreused present electronic technique. The symbol rate of 32Gbaud is compatible with opticalROADM technologies. So there is a argument in future 400Gb/s and 1Tb/s bit rates that ifsupports the ITU-T grid or not. But this challenge has a solution with high symbol ratescompared to presently used. If two carriers with 200Gb/s PM-16QAM modulated having32Gbaud is applied ,then the above argument may be solved with a spectral efficiency of 4.But single carrier PM-MQAM is a better solution for the above problem.REFERENCES[1] Fiber optic communications-wikipediaEn.wikipedia.org/wiki/fiber optic communication.[2] Fiber optic communication ,Joseph C.Palais ,fourth edition[3] Bryan M. Haas and Thomas E.Murphy “a simple linearized phase modulatedanalog optical transmission system” IEEE photonics technology letters vol.19,No.10,May15,2007.[4] D.K Sharma,A.Mishra and R.Saxena “analog and digital modulation techniques :An overview ” TECHNIA- international journal of computing scienceand communicationtechnologies. Vol 3,No.1,july 2010.[5] Yu Yu,Bingrong zou,Wenhan Wu and Xinliang zhang “all optical parallel NRZ-DPSK to RZ-DPSK format conversion at 40 Gb/s based on XPM effect in a single SOA”.the international online journal of optics, optic express,vol-19,issue 15[6] IEEE Std 802.3ba-2010, Amendment to IEEE Std 802.3-2008: Media Accesscontrol parameters, physical layers, and management parameter for 40 Gb/s and 100 Gb/soperation, June 2010.[7] ITU-T Recommendation G.709, Interfaces for the Optical Transport Network (OTN),December 2009.[8] M. Rohde,C.Caspar,N.Heimes,M.Konitzer,E-J.Bachus and N.Hanik, “Robustness ofDPSK direct detection transmission format in standard fiber WDM systems”Electron.Lett,vol 36,pp.1483-1484,2000.[9] C.Xie,L.Moller,H.Haunstein and S.Hunsche “comparison of system tolerance topolarization mode dispersion between different modulation formats” , IEEE photons.Technol.Lett.vol.15,pp.1168-1170,Aug.2003[10] I. Morita et al., High speed transmission technologies for 100-Gb/s-classEthernet, in:ECOC 2007, invited paper Mo1.3.1.[11] M. Daikoku et al., 100-Gb/s DQPSK transmissionexperiment without OTDMfor 100G Ethernet transport, J. Lightw. Technol. 25 (1) (2007).[12] M. Daikoku, I. Morita, H. Taga, H. Tanaka, T. Kawanishi, T. Sakamoto, T.Miyazaki,T. Fujita,100 Gb/s DQPSK transmission experiment without OTDMfor 100G Ethernettransport, in: OFC 2006, post-deadline paper PDP36.[13] P.J. Winzer et al., 10 _ 107-Gb/s NRZ-DQPSK transmission at 1.0 b/s/Hz over 12 _ 100km including 6 optical routing nodes, in: Proc. OFC 2007, post deadline paper PDP24.[14] P.J. Winzer et al., 2000 km-WDM transmission of 10 _ 107 Gb/s RZ-DQPSK, in:,ECOC 2006, Cannes, post-deadline paper Th4.1.3.[15] Xiang Zhou, Jianjun Yu, Mei Du, Guodong Zhang, 2 Tb/s (20 _ 107 Gb/s) RZDQPSKstraight-line transmission over 1005 km of standard single mode fiber (SSMF) withoutRaman amplification in: Proc. OFC 2008, paper OMQ3.[16] G. Raybon et al., 107-Gb/s transmission over 700 km and one intermediate ROADMusing Lambda Xtreme_ transport system, in: OFC 2008, paper OMQ4. 213
  • 17. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME[17] Mei Du et al., Unrepeatered transmission of 107 Gb/s RZ-DQPSK over 300 kmNZDSF with bi-directional Raman amplification, in: Proc. OFC 2008, paper JThA47.[18] A. Sano et al., 14-Tb/s (140 _ 111-Gb/s PDM/WDM) CSRZ-DQPSK transmissionover 160 km using 7-THz bandwidth extended L-band EDFAs, in: ECOC 2006, post-Deadline paper Th4.1.1.[19] W. Idler et al., WDM field trial over 764 km SSMF with 16 _ 112 Gb/s NRZDQPSKco-propagating with 10.7 Gb/s NRZ, in: ECOC 2010, paper We.8.C.5.[20] W. Idler et al., 16 _ 112 Gb/s NRZ-DQPSK WDM transmission over 604 km SSMFincluding high PMD fibers, in: OECC 2010, paper 9B1-2.[21] Brian Teipen, Impact of modulator characteristicson multi-level signal Transmissionperformance, in: ITG Workshopp Fachgruppe 5.3.1, 2008 (Kiel).[22] Michael H. Eiselt et al., Requirements for 100-Gb/s metro networks, in: OFC 2009,paper OTuN6.[23] Brian Teipen et al., 100 Gb/s DPSK-3ASK modulation format for metro networks:experimental results, in: ITG Photonische Netze, 2009.[24] Brian Teipen et al., 107 Gb/sDPSK-3ASK optical transmission over SSMF, in:OFC 2010, paper NMB1.[25] M. Eiselt, B. Teipen, DPSK-3ASK transmission optimization by adapting modulationLevels, in: APOC 2008, paper 3171-17. [26] M. Eiselt, B. Teipen, DPSK-3ASKtransmission optimization by adapting modulation Levels, in: APOC 2008, paper 3171-17.[27] Eugen Lach,Wilfried Idler “modulation formats for 100 G and beyond” ELSEIVIERoptical fiber technology 17(2011) 377-386.[28] D. van den Borne et al., Coherent equalization versus direct detection for 111- Gb/sEthernet transport, in: LEOS Summer Topical Meeting, 2008, paper MA2-4.[29] E. Torrengo et al., Influence of pulse shape in 112-Gb/s WDM PDM-QPSKtransmission, IEEE Photon. Technol. Lett. 22 (23) (2010).[30] K. Roberts et al., 100G and beyond with digital coherent signal processing, IEEECommun. Mag. (2010) 62–69.[31] Xiang Zhou, Jianjun Yu, Advanced coherent modulation formats and algorithms: higher-order multi-level coding for high-capacity system based on 100 Gbps channel, in: OFC2010, paper OMJ3.[32] G. Bosco et al., Performance limits of Nyquist-WDM and CO-OFDM in high speed PM-QPSK systems,IEEE Photon. Technol. Lett. 22 (2010) 1129–1131.[33] J.X. Cai et al., 20Tbit/s capacity transmission over 6860 km, in: Proceedings OFC, 2011, PDPB4.[34] WilliamShieh, OFDM for adaptive ultra high-speed optical networks, in: OFC2010, paper OWO1.[35] Fred Buchali et al., Nonlinear limitations in a joint transmission of 100 Gb/s OOFDMand 40 Gb/s DPSK over a 50 GHz channel grid, in: OFC 2010, paper OTuL4.[36] X. Liu et al., Single coherent detection of a 606-Gb/s CO-OFDM signal with 32-QAMsubcarrier modulation using 4 _ 80-Gsamples/s ADCs, in: ECOC 2010, post-deadlinepaper PD2.6.[37] S.L. Jansen et al., Coherent optical 25.8-Gb/s OFDM transmission over 4160- kmSSMF, J. Lighw. Technol. 26 (1) (2008).[38] A. Sano et al., 30 _ 100-Gb/s all-optical OFDM transmission over 1300 km SMF with10 ROADM nodes, in: ECOC 2007, post-deadline paper PD 1.7.[39] Xiang Liu et al., Transmission of a 448-Gb/s reduced-guard-interval COOFDM signalwith a 60-GHz optical bandwidth over 2000 km of ULAF and five 80-GHz-gridROADMs, in: OFC 2010, post deadline paper PDPC2. 214
  • 18. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME[40] Xiang Liu et al., Efficient digital coherent detection of a 1.2-Tb/s 24-carrier no-guard-interval CO-OFDM signal by simultaneously detecting multiple carriers per sampling,in: OFC 2010, paper OWO2.[41] S. Chandrasekhar et al., Transmission of a 1.2-Tb/s 24-carrier no-guard interval coherentOFDM super channel over 7200-km of ultra-large-area fiber, in: ECOC 2009, post-deadline paper PD 2.6.[42] R. Dischler et al., Transmission of 3 _ 253-Gb/s OFDM-super channels over764 kmfield deployed single mode fibers, in: OFC 2010, PDPD2.[43] R. Dischler et al., Terabit transmission of high capacity multiband OFDM superchannels on field deployed single mode fiber, in: ECOC 2010, paper Tu.3.C.6.[44] T. Xia et al., Field experiment with mixed line-rate transmission (112 Gb/s, 450 Gb/s,and 1.15 Tb/s) over 3560 km of installed fiber using filter less coherent receiver andEDFAs only, in: OFC 2011, PDPA3.[45] A.H. Gnauck et al., 10 _ 224-Gb/s WDM transmission of 28-Gbaud PDM 16-QAM on a50- GHz grid over 1200 km of fiber, in: OFC 2010, post deadline paper PDPB8.[46] P.J. Winzer et al., Spectrally efficient long-haul optical networking using 112- Gb/spolarization- multiplexed 16-QAM, J. Lighw. Technol. 28 (4) (2010).[47] M. Alfiad et al., Transmission of 11 _ 224 Gb/s POLMUX-RZ-16QAM over 1500 km ofLongLine and pure-silica SMF, in: ECOC 2010, paper We.8.C.2.[48] P.J. Winzer et al., Generation and 1200-km transmission of 448-Gb/s ETDM 56-GbaudPDM 16-QAM using a single I/Q modulator, in: ECOC 2010, post dead line paperPD2.2.[49] B.-E. Olsson et al., RF-assisted optical dual-carrier 112 Gb/s polarization multiplexed16-QAM transmitter, in: OFC 2010, paper OMK5.[50] A. Sano et al., 240-Gb/s polarization-multiplexed 64-QAM modulation and blinddetection using PLC- LN hybrid integrated modulator and digital coherent receiver, in:ECOC 2009, post-deadline paper PD 2.2.[51] Y. More et al., 200-km transmission of 100-Gb/s 32-QAM dual-polarization signalsusing a digital coherent receiver, in: ECOC 2010, paper 8.4.6.[52] X. Zhou et al., 64-Tb/s (640 _ 107-Gb/s) PDM-36QAM transmission over 320 km usingboth pre- and post-transmission digital equalization, in: OFC 2010, Post deadline paperPDPB9.[53] J. Yu et al., 112.8-Gb/s PM-RZ-64QAM optical signal generation and transmission on a12.5 GHz WDM grid, in: OFC 2010, paper OThM1.[54] A. Sano et al., 100 _ 120-Gb/s PDM 64-QAM transmission over 160 km using linewidth-tolerant pilotless digital coherent detection, in: ECOC 2010, post deadline paperPD2.4.[55] M. Nakazawa et al., 256 QAM (64 Gb/s) coherent optical transmission over 160 km withan optical bandwidth of 5.4 GHz, in: OFC 2010, paper OMJ5.[56] S. Okamoto et al., 512 QAM (54 Gb/s) coherent optical transmission over 150 km withan optical bandwidth of 4.1 GHz, in: ECOC 2010, post-deadline paper.[57] S.Revathi, G.Aarthi “Performance analysis of Wave Length Division and Sub CarrierMultiplexing using different modulation techniques” International Journal ofEngineering Research and Applications (IJERA),Vol. 1, Issue 2, pp.317-320[59] P. Hofmann, E. E. Basch, S. Gringeri, R. Egorov, D. A. Fishman, and W. A. Thompson,“DWDM Long Haul Network Deployment for the Verizon GNI Nationwide Network,”Proc. Optical Fiber Communication Conf. (OFC „05), Vol. 2, (2005). 215
  • 19. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME[60] B. Wedding and B. Franz, “Unregenerated optical transmission at 10 Gbit/s via 204 kmof standard single mode fiber using a directly modulated laser diode,” ElectronicsLetters, 29(4):402–403, February 1993.[61] B.Wedding, B. Franz, and B. Junginger, “10-Gb/s optical transmission up to 253 km viastandard single mode fiber using the method of dispersion-supported transmission,”Journal of Lightwave Technology, 2(10):1720–1727, October 1994.[62] M. Rhode, C. Caspar, N. Heimes, M. Konitzer, E. J. Bachus, and N. Hanik, “ Robustnessof DPSK direct detection transmission format in standard fiber WDM systems,”Electronics Letters, 26(17):1483–1484, August 2000.[63] C. Wree, J. Leibrich, and W. Rosenkranz, “RZ-DQPSK format with high spectralefficiency and high robustness towards fiber nonlinearities,” European Conference onOptical Communication (ECOC), 4(9.6.6), September 2002.[64] Abhijit Banerjee and B. N. Biswas, “Stability Analysis of a Modified PSK HomodyneOptical Receiver”, International Journal of Electronics and Communication Engineering &Technology (IJECET), Volume 3, Issue 2, 2012, pp. 32 - 40, ISSN Print: 0976- 6464, ISSNOnline: 0976 –6472.[65] Ami Munshi and Srija Unnikrishnan, “Performance Analysis of Radar Based onDS-BPSK Modulation Technique”, International Journal of Electronics and CommunicationEngineering & Technology (IJECET), Volume 4, Issue 2, 2013, pp. 137 - 143, ISSN Print:0976- 6464, ISSN Online: 0976 –6472. 216