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Single Channel Directly Detected Optical-OFDM towards Higher Spectral Efficiency and Simplicity in 100 Gb/s Ethernet and Beyond Lenin Mehedy, Masuduzzaman Bakaul, and Ampalavanapillai Nirmalathas

Abstract—Recently proposed IEEE P802.3ba 100 Gb/s Ethernet (100 GbE) standard has adopted 100 Gb/s transmission over 10 km and 40 km of single mode fiber (SMF) using four channel (4 x 25 Gb/s) wavelength division multiplexed (WDM) systems, which is neither cost effective nor spectrally efficient compared to a single channel system exploiting the combination of higher order modulations and optical orthogonal frequency division multiplexing (OOFDM). This paper demonstrates that a spectrally efficient (4 bit/s/Hz) single channel 100 Gb/s system can be designed based on 64-quadrature amplitude modulation (64-QAM) and directly detected O-OFDM (DDO-OFDM) with an effective OFDM signal bandwidth of 24 GHz. Such a system can not only offer error-free (at bit error ratio of 10-3 without forward error correction) transmissions over the targeted maximum distance of 40 km SMF but also achieves a power margin of 16 dB without any in line amplifier or dispersion compensation. This confirms that the proposed system has the potential to offer 100 Gb/s Ethernet for both point-to-point short communication links and 1:32 split passive optical networks (PON). Bit rate of the system is then increased to 1 Tb/s employing WDM and found it to be equally potential for point-to-point short communication links and 1:16 split PON. Finally the reach-limits of both of the proposed systems were quantified. Index Terms—Optical OFDM; Ethernet; incoherent; direct detection; DDO-OFDM; spectral efficiency

Manuscript received November 25, 2011. Lenin Mehedy and Masuduzzaman Bakaul are with the National ICT Australia (NICTA), Department of Electrical and Electronic Engineering, The University of Melbourne, Victoria 3010, Australia (e-mail: [email protected], [email protected]). Ampalavanapillai Nirmalathas is with the Department of Electrical and Electronic Engineering, The University of Melbourne, Victoria 3010, Australia (e-mail: [email protected]).

I. INTRODUCTION he explosive growth in bandwidth hungry applications

demanded a bandwidth requirement of 100 Gb/s in Thas next generation communication networks [1,2]. The IEEE P802.3ba Ethernet Task Force was formed in 2007 to standardize the existing 100 Gb/s technologies and devise a common platform to persuade the successful deployments commercially [3]. Recently, the taskforce has finalized the recommendations and proposed to use 4 (four) parallel WDM channels each carrying on-off-keyed (OOK) 25 Gb/s data to achieve an aggregated bit rate of 100 Gb/s, over a maximum distance of 40 km single mode fiber (SMF). The schematic of such a link is shown in Fig. 1 (a). Shown in Fig. 1 (a), ten 10 Gb/s data streams, with an aggregate bit rate of 100 Gb/s, are at first converted to four parallel 25 Gb/s data streams using a 10:4 serializer. Each of the streams is then used to modulate an optical carrier at 1310 nm band, multiplexed and transmitted over the targeted lengths of SMF to the destination, where the WDM channels are demultiplexed before photo-detection and recovery of the data. All the demultiplexed channels are however amplified with semiconductor optical amplifiers (SOAs) prior to photo-detection to compensate for various link losses. This proposition allows the IEEE P802.3ba Task Force to limit the bandwidths of opto-electronics and radio frequency (RF) devices to 25 GHz at the expense of additional cost and complexities from three sets of additional transmitting, receiving and MUX/DEMUX devices [3-5]. Since the standard recommends 1310 nm optical band for signal transport and amplification, upgradeability of the system to longer reach will be restricted due to higher fiber loss at 1310 nm band and limited amplification capabilities of SOAs compare to the fiber loss at 1550 nm band and amplification capabilities of erbium-doped-fiber-amplifiers (EDFAs). Moreover, future Ethernet, which is expected to support Tb/s [6], will be even more difficult to achieve by extending this 1310 nm band and OOK based 100 Gb/s WDM system due to a lot more stringent system requirements [7]. This paper, therefore, considers these potential challenges of the proposed 100 Gb/s Ethernet (100 GbE) standard, and explores a viable alternative based on a combination of directly detected

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Fig. 1. Schematic diagram of 100 Gb/s Ethernet optical interfaces. (a) IEEE P802.3ba four channel 100 Gb/s Ethernet standard, (b) proposed optical OFDM based single channel 100 Gb/s Ethernet

(incoherent) optical OFDM (DDO-OFDM) and higher order modulation formats such as quadrature amplitude modulation (QAM). The schematic of such a scheme is shown in Fig. 1(b). In this approach, instead of four WDM channels, a single channel 100 Gb/s transmission with a spectral efficiency of 4 bit/s/Hz, excluding the spectral gap and considering data-modulated subcarriers only, will be realized at 1550 nm band using DDO-OFDM system and higher order QAM (e.g. 64-QAM). Such a system has the potential to open up opportunities for future upgrade to longer reach as well as Tb/s transmission by exploiting the mature dense-WDM (DWDM) and optical amplification technologies currently being used in existing long-haul OOK systems. Recently 100 Gb/s and beyond optical transmission systems have been demonstrated based on coherent and directly detected O-OFDM using a combination of polarization division multiplexing (POL-MUX), higher order modulations and/or self-coherence [8-14]. Most of these demonstrations have been focused on long–haul systems and adopted system architectures enabling the transmissions at longer distances, where systems’ cost and complexity were never of paramount importance. However, unlike the long-haul systems, the picture is quite different in access domain and other short-haul point-to-point applications, where cost-effectiveness and simplicity are the key enablers of the prospective technologies [15,16]. Since higher order modulations such as 128-QAM and 64-QAM are readily possible with the current state-of-arts [17-19], this paper explores a combination of 64-QAM with low-cost and simpler directly detected O-OFDM system to achieve a

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spectrally efficient single channel 100 Gb/s transmission system, where a single optical carrier is used to transport 100 Gb/s data over a 40 km un-amplified SMF link, limiting the OFDM-signal bandwidth within 24 GHz without any polarization domain multiplexing. Also, the proposed system is designed by using the basic IEEE 802.11a OFDM PHY [20] specifications such as the numbers of subcarriers, pilot subcarriers, training sequences, the choice of modulation formats, and the amount of cyclic prefix etc.; so that the existing wireless OFDM standard may potentially be considered as a benchmark for future selection of 100 Gb/s O-OFDM specifications in short-haul communication systems [21]. Part of this investigation can be found in [22] whereas this paper extends the investigations to study the feasibility of 1 Tb/s transmission using wavelength division multiplexing (WDM) of ten similar 100 Gb/s DDO-OFDM channels. Effectiveness of this 1 Tb/s DDO-OFDM system has also been demonstrated over 40 km SMF to meet the longest reach target of the IEEE P802.3ba’s 100 Gb/s standard. We further investigate the maximum reach of the proposed systems over SMF links without any inline amplifier or dispersion compensation. Simulation results confirm that the proposed 100 Gb/s and 1 Tb/s DDO-OFDM systems can extend the reach to 67 km and 49 km respectively, beyond which special attention will be needed in dispersion management. This paper is organized as follows: section II describes the single channel 100 Gb/s DDO-OFDM system based on the IEEE 802.11a OFDM PHY specifications, section III presents performances of the 100 Gb/s system, section IV describes the 1 Tb/s DDO-OFDM system and its performances, section V discusses the transmission reachlimits of these systems and finally section VI is the conclusion.

II. SINGLE CHANNEL 100 GB/S DDO-OFDM SYSTEM Shown in Fig. 2, functionality of the proposed single channel 100 Gb/s DDO-OFDM system is verified with a simulation model developed in the commercial simulation tool VPItransmissionMakerTM V7.6. The key simulation steps and parameters are described in the subsections in more detail for clarity.

Fig. 2. 100 Gb/s DDO-OFDM simulation setup

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A. Baseband OFDM signal generation To generate the baseband OFDM signal, the basic OFDM parameters such as inverse fast Fourier transform (IFFT) size, number of data subcarriers, number of pilot subcarriers, cyclic prefix (CP) length and allocation of subcarriers for data and pilot etc. are chosen according to the IEEE 802.11a OFDM PHY specification [20]. Therefore, an IFFT block size of 64 is chosen to map the pseudorandom-bit-sequence (PRBS) data, among which the center subcarrier is filled with zero, 48 subcarriers are associated with 64-QAM modulated data and 4 subcarriers are dedicated for binary-phase-shift-keyed (BPSK) pilot symbols. Two OFDM symbols in the preamble are used for training and a CP of sixteen samples is used to provide a guard interval between OFDM symbols. Assuming the OFDM subcarriers to be numbered from -32 to +32, the subcarrier allocation is shown in Fig. 3 [20]. Figure 3 shows that 48 data subcarriers (d0 to d47) are mapped on the subcarriers numbered -26 to -22, -20 to -8, -6 to -1, 1 to 6, 8 to 20, 22 to 26 and four pilot subcarriers are mapped on the subcarriers numbered -21, -7, 7, and 21. The remaining higher frequency subcarriers are filled with zeros for oversampling that facilitate low pass filtering of the signal after digital-to-analog-converter (DAC). Since BPSK modulation is used for pilots and preambles (training symbols), and higher order QAMs are used in data subcarriers, modulation formats vary across OFDM subcarriers causing considerable power variation across subcarriers. Therefore normalization is performed to ensure a constant average power across all subcarriers for all mappings [20]. Normalized data symbols ( Cnorm ) are generated by Cnorm = Ci × K mod , where modulation dependent normalization

K mod =

factor

M

calculated

by

, M is the highest number of constellation

M

∑C

K mod

is

2 n

n =1

points, |Cn| is the amplitude of the nth constellation point in the complex plain (i.e. Cn = In + jQn) and Ci is the ith data symbol in complex plain. The values of Kmod for different modulation formats are given in Table 1 for reference. A total of 502 OFDM symbols including two OFDM training symbols are generated for simulation. The Nyquist sampling rate (R) of the baseband OFDM signal is calculated using Eq. (1), where NFFT is the FFT size, Tsym is the OFDM symbol time including the guard interval, and Ncp is the number of samples used as CP.

R = 2 × NFFT × Since

1

1 Tsym

× (1 +

N CP NFFT

)

(1)

denote the symbol-rate/subcarrier, the useful

Tsym bandwidth (B) of the OFDM signal including the zero-filled center subcarrier can be calculated according to the Eq. (2), where NSD and NPILOT denote the number of data and pilot subcarriers respectively. Then the effective bit rate (b) can be calculated using Eq. (3).

Fig. 3. Subcarrier allocation for data and pilots

B = ( NSD + NPILOT + 1) × b = NSD ×

1 N × (1 + CP ) Tsym NFFT

1 × log 2 M Tsym

(2) (3)

Now, as the number of subcarriers (NSD) and Ncp are kept fixed according to the IEEE 802.11a OFDM PHY, a symbol rate of 350 MBd/subcarrier is chosen to achieve an overall bit rate of 100 Gb/s ( 48 × 350e 6 × 6 ) where data bits are encoded using 64-QAM (6 bit/symbol) [21]. Then the sampling rate of the baseband I and Q channel is 28 GS/s ( 64 × 350e 6 × 1.25 ) resulting in a total signal bandwidth (B) of 24 GHz ( 53 × 350e 6 × 1.25 ) allowing an effective spectral efficiency of 4 bit/s/Hz. It is worth noting that the used sampling rate of 28 GS/s is practicable since the commercially available analog-to-digital converters (ADCs) already have a maximum sampling rate of 56 GS/s for the future 100 Gb/s systems [23].

B. DDO-OFDM transmission and detection In DDO-OFDM setup, radio frequency (RF) up-conversion is needed to ensure the required spectral gap between the optical carrier and the lowest OFDM subcarrier [8,9]. This spectral gap accommodates the intermodulation products (subcarrier x subcarrier beat products) that are generated during direct detection because of using a square-law photodetector. Hence, for the desired 100 Gb/s DDO-OFDM signal, the base band O-OFDM signal, is mixed with a 36 GHz (24 GHz × 1.5) local oscillator (LO) to generate the required 24 GHz spectral gap between the carrier and the lowest OFDM subcarrier. The composite RF-OFDM signal is then used to drive a Mach-Zehnder modulator (MZM) biased (Vpi = 4.5 V) just above its intensity null point (Vpi = 5 V) to maintain a low carrier to OFDM sideband power ratio for improved receiver’s sensitivity [8,9]. A distributed feedback (DFB) laser module (LD) with a linewidth of 1 MHz is used as the optical source. An optical band pass TABLE I MODULATION DEPENDENT NORMALIZATION FACTOR Modulation

Kmod

BPSK 4-QAM 8-QAM 16-QAM 32-QAM 64-QAM 128-QAM

1 1/√2 1/√6 1/√10 1/√20 1/√42 1/√82

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filter (OBPF) module is used to suppress the lower sideband of the optical signal. The optical single side band and carrier (OSSB+C) formatted O-OFDM signal is then amplified using an optical gain block (5 dB noise figure) to maintain an optical power of 4 dBm at the input of the fiber, which is the maximum launch power recommended by the IEEE P802.3ba task force [3]. The generated SSB DDOOFDM signal is then transported over a block of SMF and directly detected using a PIN photodiode (PD) at the receiver. The SMF has an attenuation of 0.2 dB/km, −20

dispersion of 16 ps/nm/km, nonlinear index of 2.6 × 10 m2/W, and PMD of 0.1 ps/√km. Another OBPF is used before the PD to avoid any out of band noise. The PD has a responsivity of 0.9 A/W with shot noise and thermal noise (1e-12 A/√Hz) enabled. The detected RF OFDM signal is then amplified (noise spectral density of 5e-12 A/√Hz) and down mixed with another 36 GHz LO to recover the I and Q channels. Assuming perfect I/Q-imbalance compensation at the receiver, the received I and Q channels are combined to form a string of complex numbers (I + jQ). This string of complex numbers is then divided into groups of 80 (16 + 64) numbers to form a series of OFDM symbol blocks. The CP, comprising of 16 complex numbers in the beginning, is removed from each OFDM block and remaining 64 complex numbers are then fed into the inputs of a 64-point FFT module. FFT operation then helps to recover the transmitted QAM symbols associated with the OFDM data subcarriers. The recovered QAM data symbols are then equalized using zero-forcing equalizer utilizing the BPSK based training sequences [22]. Finally, the equalized QAM data symbols are passed to the QAM demodulator for symbol decision and the data bits are restored.

C. Bit-error-ratio (BER) calculation using Error vector magnitude (EVM) Since small number of OFDM symbols (only 500) are transmitted to reduce simulation time, the total number of transmitted bits ( 500 × 48 × 6 ) is not enough to allow low BER measurements. Therefore BERs are estimated from the root-mean-square EVM (EVMRMS) measurements of the system using Eq. (4) [22,24]:

 1 2 1 −    3log L   2 L BER =  ×Q  2 2  2   L − 1   EVM RMS × log 2 M log 2 L 

  (4)   

Here, L is the maximum number of levels in M-QAM (e.g. L is 4 for 8-QAM), the function Q(x) is evaluated

 x  ,  2

by 0.5 × erfc 

where erfc ( . ) is the complementary

error function. EVMRMS of the system is calculated by Eq. (5) Lp

EVM RMS where

2  NST
∑  ∑ xik − xik   = i =1  k =1 NST × Lp × Pavg

(5)

xik is the normalized ideal constellation symbol in

the complex plain corresponding to the estimated symbol

Figure 4. BER vs. EVM in an optical back-to-back setup with 0 Hz laser linewidth and 0 km fiber along with theoretical BERs as continuous lines. EVM thresholds for different BERs can be found from this figure

x
ik , x
ik − xik

denotes the magnitude of error vector, NST

is the total number of data and pilot subcarriers (i.e. NST = NSD + NPILOT), Lp is the total number of received OFDM symbols,

Pavg is the average power of the constellation that

is 1 (one) since normalization is applied (or otherwise

Pavg =

1 2 K mod

).

III. PERFORMANCE OF 100 GB/S DDO-OFDM SYSTEM To validate the simulation model, the system is at first simulated in a back-to-back setup with 0 Hz laser linewidth, 0 km of fiber and varying the additive white Gaussian noise (AWGN) of the system. The BER performance of the system in such back-to-back setup is plotted against the EVM of the system in Fig. 4 where corresponding theoretical BERs estimate from Eq. (4) are also plotted as continuous lines for reference. As shown in Fig. 4, the performance of the simulation model closely matches with the theoretical prediction as expected. Figure 4 also confirms the EVM thresholds for different modulation orders to achieve a certain BER. For example, for usual forward error correction (FEC) limit of 10-3 BER, the EVM thresholds are roughly -23 dB and -10 dB for 64 QAM and 4-QAM respectively. Now, to measure the optical transmission performance, the laser linewidth is set to 1 MHz and the signal is transmitted over 40 km of SMF without any inline amplifier or dispersion compensation. The relevant spectra of the signal at different points of the link are shown in Fig. 5. As shown in Fig. 5 (a), I channel of the signal has a bandwidth of 12 GHz after suppressing the aliasing signals using a low pass filter after the DAC. Then after up-converting the OFDM signal to 36 GHz, the RF spectrum has an OFDM signal bandwidth of 24 GHz and a total signal bandwidth of 48 GHz including the spectral gap of 24 GHz as shown in Fig. 5 (b). The optical spectra after the modulator and before photodetector are shown in Fig. 5 (c) and (d) respectively, which confirm that lower side band

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Fig. 7. Effect of laser linewidth on the 100 Gb/s DDO-OFDM signal over 40 km SMF

Fig. 5. Spectra of 100 Gb/s DDO-OFDM signal, (a) RF spectra before RF up-converter (I branch), (b) RF up-converted OFDM signal, (c) Optical single sideband and carrier (OSSB+C) formatted DDO-OFDM signal, (d) Optical spectrum before direct detection, (e) RF spectrum after direct detection, (f) ) received RF spectrum after RF down-converter (I branch)

is successfully suppressed using optical band-pass filter. The RF spectrum after photodetector in Fig. 5 (e) clearly shows that the intermodulation products are effectively accommodated within the spectral gap leaving the actual OFDM signal un-contaminated. The last RF spectra in Fig. 5 (f) is the down-converted and low pass filtered I channel, which is subsequently sampled and processed using receiver’s OFDM digital signal processing module. The performances of this 100 Gb/s DDO-OFDM system at backto-back (0 km SMF) and over 40 km length of SMF are shown in Fig. 6. The insets of Fig. 6 are the unequalized and equalized constellations after 40 of SMF at a received optical power of -16 dBm. As shown in Fig. 6, the sensitivity of the receiver after 40 km SMF is -18 dBm at the FEC limit. Thus there is a power margin of 14 dB as the maximum received optical power after 40 km is -4 dBm. Figure 6 also shows that at the FEC limit receiver’s sensitivity decreases by 2 dB after 40 km SMF. This

Fig. 6. Performance of the 100 Gb/s DDO-OFDM system over 40 km SMF with 1 MHz laser linewidth

performance penalty after 40 km can be attributed to chromatic dispersion since the phase coherency between the optical carrier and the OFDM subcarriers vanishes due to fiber dispersion and it introduces significant phase noise in direct detection of the signal [25,26]. Therefore, to further improve the performance of the system after 40 km of SMF by reducing this phase incoherency between optical carrier and OFDM subcarriers, we simulate the system with different narrower laser line-widths (LLWs) and the performance is shown in Fig. 7. As expected, the receiver’s sensitivity improves by 1 dB and 2 dB respectively with LLW of 500 kHz and 100 kHz. Thus the power margin increases to 16 dB with laser line-widths of 100 kHz, which will enable deployment of the proposed 100 Gb/s DDOOFDM system in traditional FTTH infrastructure with a split ratio of 1:32.

IV. 1TB/S DDO-OFDM SYSTEM AND ITS PERFORMANCES 1 Tb/s DDO-OFDM system is designed by wavelength division multiplexing of similar ten 100 Gb/s DDO-OFDM channels in a 100 GHz grid. Laser linewidths of the systems is varied from 100 kHz to 1 MHz with 5 mW average power, insertion loss of the WDM multiplexer is set to 2 dB and a fifth order Gaussian filter (75 GHz) is used at the receiver for channel demultiplexing. To avoid data redundancy, performance of the center channel (i.e. fifth channel at 193.1 THz) is measured, which is expected to suffer more from adjacent channel interferences, compare to channels at the edges. Power per channel is controlled in such a way that the total average output power reaches 10 dBm, which is the maximum total average power as proposed in the IEEE P802.3ba’s 100 GbE standard [3]. We simulate a total of 22 OFDM symbols comprised of two training symbols followed by 20 data symbols to reduce simulation time. Relevant spectra of the 1 Tb/s system after 40 km of SMF are shown in Fig. 8 (a)-(d). Figure 8 (a) shows the received optical spectrum of the 1 Tb/s DDO-OFDM signals. Figure 8 (b) is the optical spectrum of the demultiplexed channel 5 (at 193.1 THz) before detection. RF spectrum after direct detection is presented in Fig. 8 (c), which shows that the 100 Gb/s OFDM signal bandwidth is only 24 GHz that is up-

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Fig. 8. Spectra of 1 Tb/s DDO-OFDM signal after 40 km SMF, (a) optical spectrum of 10 channels, (b) demultiplexed center channel (at 193.1 THz) before direct detection, (c) RF spectrum after direct detection, (d) received RF spectrum after RF downconverter and low pass filter (I branch)

converted to a 36 GHz RF so that the intermodulation products could be effectively accommodated within the spectral gap. After electrical amplification and downconversion of this received OFDM signal, the RF spectrum of the signal (I branch) is shown in Fig. 8 (d). Transmission performance of this 1 Tb/s DDO-OFDM signal at back-toback (0 km SMF) and over 40 km length of SMF is shown in Fig. 9 with different LLWs. It shows that the sensitivity of the receiver with 1 MHz LLW is -15.5 dBm after 40 km SMF, which is 4.5 dB higher compared to that of the backto-back (0 km SMF) setup. However, receiver’s sensitivity after 40 km SMF can be improved significantly up to 4 dB by using narrower LLW of 100 kHz as expected [26] and shown in Fig. 9. To investigate the effect of fiber nonlinearity on the system, we simulate the performance of the 1 Tb/s system with different transmission power per channel, keeping LLW fixed at 100 kHz and the results are shown in Fig. 10 and Fig. 11. Figure 10 shows, as expected, receiver sensitivity improves when power per channel decreases. However, this sensitivity improvement is insignificant when channel powers are below 0 dBm. Shown in Fig. 11, receivers’ sensitivity degrades with the increase of channel powers and this performance degradation can be attributed

Fig. 9. Performance of 1Tb/s DDO-OFDM signal over 40 km SMF

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Fig. 10. Performance of 1Tb/s DDO-OFDM signal over 40 km SMF with different transmission power per channel

to fiber nonlinearity. Also shown in Fig. 11, the maximum power margin of 13 dB can be achieved when transmission power is 2 dBm/channel, which will ensure successful operation of the 1 Tb/s system over a 1:16 split PON.

V. MAXIMUM TRANSMISSION DISTANCE WITHOUT INLINE AMPLIFICATION OR DISPERSION COMPENSATION In order to investigate the maximum point-to-point transmission distances of the proposed single channel 100 Gb/s system without any inline amplification or dispersion compensation, the system is simulated over different lengths of SMF with 100 kHz LLW and results are shown in Fig. 12. Figure 12 confirms that the 100 Gb/s DDO-OFDM system can operate over at most 67 km of SMF without any inline amplifier or dispersion compensation. Figure 12 also shows that the system performance degrades rapidly after 60 km SMF. To find the responsible fiber impairments for such phenomenon, the system is simulated without chromatic dispersion (CD) in SMF and results are plotted in Fig. 12. As shown in Fig. 12, system performance without CD does not degrade sharply beyond 60 km SMF. The reason is that CD creates a phase mismatch between the optical carrier and the OFDM subcarriers, which results in electrical noise during direct detection at the receiver [26]. Therefore the results confirm that the maximum

Fig. 11. Receivers’ sensitivity (at the FEC limit) and achievable power margin of 1Tb/s DDO-OFDM signal over 40 km SMF with different transmission power per channel

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Fig. 12. Reach limitation of the proposed single channel 100 Gb/s DDO-OFDM system and effect of fiber impairments

transmission distance of such a 100 Gb/s point-to-point system is up to 67 km. Similarly, the 1 Tb/s system is simulated with transmission power of 0 dBm/channel over different lengths of SMF to find its maximum transmission distance. As shown in Fig. 13, the system performances over usual SMF reaches the FEC limit at around 49 km. Without nonlinearity (NL) in the SMF, as shown in Fig. 13, performance of the system improves compared to standard SMF, but there is still a sharp decrease in performance beyond 40 km. To find the reason, the system is simulated again by disabling CD parameter of the fiber and the results are also shown in Fig. 13. The plot without CD is quite linear and the abrupt degradation beyond 40 km disappears. Therefore, to extend the reach of 1 Tb/s system beyond 49 km, dispersion management will be required.

VI. CONCLUSION Simulation results confirm that a single channel 100 Gb/s DDO-OFDM system can be effectively operate over 40 km SMF with an OFDM bandwidth of 24 GHz, which could be a viable alternative to 4 x 25 Gb/s WDM approach suggested by the IEEE P802.3ba standard for future 100 Gb/s Ethernet deployments. The proposed system can also be scaled up to 1 Tb/s by WDM of ten similar channels offering error free operation (at BER 10-3 without forward error correction) over 40 km SMF. These single channel 100 Gb/s and WDM 1 Tb/s systems also achieve power margins of 16 dB and 13 dB respectively after 40 km SMF, which confirm their potentials for both point-to-point short communication links and next-generation PONs with split ratios of 1:32 and 1:16 respectively. Moreover, the transmission distances of these systems can be extended beyond 40 km, for which dispersion management would be necessary.

ACKNOWLEDGMENT NICTA is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence program

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Fig. 13. Reach limitation of the proposed WDM 1 Tb/s DDOOFDM system and effect of fiber impairments

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> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < [14] A. Al Amin, H. Takahashi, I. Morita, and H. Tanaka, “100Gb/s direct-detection OFDM transmission on independent polarization tributaries,” IEEE Photon. Technol. Lett. , vol. 22, no. 7, pp. 468–470, 2010. [15] M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, and R. Waterhouse, “Spectrally efficient hybrid multiplexing and demultiplexing schemes toward the integration of microwave and millimeterwave radio-over-fiber systems in a WDM-PON infrastructure,” J. Opt. Netw., vol. 8, no. 5, pp. 462-470, May 2009. [16] M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, and R. Waterhouse, “Simplified multiplexing scheme for wavelengthinterleaved DWDM millimeter-wave fiber-radio systems,” in proc. the European Conference on Optical Communication (ECOC’2005), vol. 4, pp. 809-810, September, 2005 [17] F. Buchali, R. Dischler, “Optimized sensitivity direct detection O-OFDM with multi level subcarrier modulation,” in proc. conference on Optical Fiber Communication (OFC 2008), pp. 1-3, 2008, Paper OMU3. [18] J. Yu, X. Zhou, Yue-Kai Huang, S. Gupta, Ming-Fang Huang, T. Wang, P. Magill, “112.8-Gb/s PM-RZ-64QAM optical signal generation and transmission on a 12.5GHz WDM grid,” in proc. conference on Optical Fiber Communication (OFC 2010), pp. 1-3, 2010, Paper OThM1. [19] S. Okamoto, T. Omiya, K. Kasai, M. Yoshida, M. Nakazawa, “140 Gbit/s coherent optical transmission over 150 km with a 10 Gsymbol/s polarization-multiplexed 128 QAM signal,” in proc. conference on Optical Fiber Communication (OFC 2010), pp. 1-3, 2010, Paper OThD5. [20] IEEE Std 802.11™-2007 (Revision of IEEE Std 802.11-1999), Part 11: wireless LAN MAC and PHY specifications, clause 17, pp. 591-635, 2007. [21] L. Mehedy, M. Bakaul, A. Nirmalathas, “115.2 Gb/s optical OFDM transmission with 4 bit/s/Hz spectral efficiency using IEEE 802.11a OFDM PHY,” in proc. the 14th OptoElectronics and Communications Conference (OECC), pp. 1-2, July 2009 [22] L. Mehedy, M. Bakaul, A. Nirmalathas, “Spectrally-efficient 100 Gb/s transmission in next-generation optical access networks employing directly detected optical-OFDM,” in proc. Australasian Telecommunication Networks and Applications Conference (ATNAC), pp. 55-59, October 2010. [23] I. Dedic, “56Gs/s ADC : enabling 100GbE,” in proc. conference on Optical Fiber Communication (OFC 2010), pp. 1-3, 2010, Paper OThT6. [24] R. A. Shafik, M. S. Rahman, and A. H. M. R. Islam, “On the extended relationships among EVM, BER and SNR as performance metrics,” in proc. 4th International Conference on Electrical and Computer Engineering, pp. 408-411, December, 2006 [25] Z. Zan, M. Premaratne, and A. J. Lowery, “Laser RIN and linewidth requirements for direct detection optical OFDM,” CLEO’08, pp. 1-2, 2008, Paper CWN2 [26] W.-R. Peng, “Analysis of laser phase noise effect in directdetection optical OFDM transmission,” J. Lightw. Technol., vol. 28, no. 17, pp.2526-2536, Sept.1, 2010 Lenin Mehedy (S’08) received his B.Sc.Eng. degree in computer science and engineering from Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh, in November 2004 and M.S. degree in computer engineering from Kyung Hee University, Suwon, Korea, in February 2007. Currently he is working toward the Ph.D. degree in electrical and electronic engineering at the University of Melbourne, Australia. He is a Graduate Researcher in the Networked Systems group of Victoria Research Laboratory at the National ICT Australia (NICTA), Australia. He has written more than 20 technical articles and holds a patent in Korea. His research interests include graph

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theory, ubiquitous computing, optical communication and signal processing. Mr. Mehedy is a student member of IEEE and Optical Society of America (OSA). Masuduzzaman Bakaul (S’02–M’06) received the B.Sc.Eng. degree from Bangladesh University of Engineering and Technology (BUET) and Ph.D. degree from the University of Melbourne, Australia, both in electrical and electronic engineering in 1996 and 2006, respectively. In 2006, he joined the National ICT Australia (NICTA), a premier Australian research centre of excellence in ICT, where he is currently a Senior Researcher with the Networked System theme. He is also an Adjunct Fellow with the Department of Electrical and Electronic Engineering, University of Melbourne, Australia since 2006. From 1997-2001, he worked as an optical engineer for Fiber Optic Network Solutions (FONS) Bangladesh LTD, a subsidiary of FONS Corp., MA, USA, which has been acquired by ADC recently. His research interests include integration of optical and wireless technologies in last mile communications, microwave photonics, millimeter-wave RF in indoor and outdoor communications, optical performance monitoring, OFDM-over-fiber, and Photonics ADC on a chip. He has written more than 60 technical articles in these areas that include a book in millimeter-wave radio-over-fiber systems. His paper in IEEE LEOS’2005 conference was awarded the LEOS/Newport/Spectra-Physics Research Excellence Award. He has also contributed to NICTA’s commercialization activities through his research resulting in a start-up company. Dr. Bakaul was one of the TPC co-chairs of APCC/ATNAC’2010 and was the session chairs and members of TPC for various international conferences. Ampalavanapillai Nirmalathas (S’94– M’98–SM’03) obtained his B.E. and Ph.D. in Electrical and Electronic Engineering from the University of Melbourne in 1993 and 1998 respectively. He is currently a Professor and Head in the Department of Electrical and Electronic Engineering at the University of Melbourne, Australia. He was the Research Group Manager of the Networked Systems Group of Victoria Research Laboratory at the National ICT Australia (NICTA) and also the acting Lab Director of VRL in 2007. Between 2000 and 2004, he was the Director of Photonics Research Laboratory (Melbourne Node of Australian Photonics CRC) and also the Program Leader of Telecommunications Technologies Program. He has written more than 200 technical articles and currently holds 2 active international patents. His research interests include microwave photonics, semiconductor lasers, fiber-radio systems, optical access networks, optical performance monitoring and WDM packet switched networks. Prof. Nirmalathas is a member of Steering Committees of Asia Pacific Conference on Lasers and Electro-Optics (representative of IEEE LEOS) as well as the International Conference on Optical Internet (COIN) and a member of technical subcommittee on microwave photonics for the IEEE LEOS. He was also Guest Editor for Special Issue on Opto-Electronics and Communications of the IEICE Transactions in Communications. He also served as the General Co-Chair of 2008 IEEE Topical Meeting on Microwave Photonics. He is currently an Associate Editor of IEEE/OSA Journal of Lightwave Technology. He is a Senior Member of IEEE and a Fellow of the Engineers Australia.