Experimental Demonstration of 39Gbps for FDM PON Aurélien Lebreton
(1-2)
(1)
, Benoît Charbonnier , Jérôme Le Masson (1) Chanclou
(2-3)
(2)
, Rongping Dong , Philippe
(1)
Orange Labs, 2 av. Pierre Marzin, 22307 LANNION, France,
[email protected] Lab-STICC, Universié de Bretagne-Sud, Lorient, France,
[email protected] (3) Ecoles de Saint-Cyr Coëtquidan, Guer, France,
[email protected] (2)
Abstract We demonstrate experimentally a downstream capacity of 39Gbps based on FDM PON architecture using a new resources allocation algorithm with 11.5GHz electrical bandwidth. Introduction The demand of bandwidth per user increase since few years due to the occurrence of services requiring high data rate (such as HD TV – 3D TV …). In coming years, this demand will continue to expand and operators have to evolve their access network to respond to this request. FSAN and ITU are standardizing a system (Next Generation Passive Optical Networks NGPON2 [G.989.1]) with a 40Gbps capacity for the downstream, 10Gbps for the uplink, 64 users per feeder with a passive reach of 20km [1] [2]. This next generation of access network has to coexist with older systems presently deployed and has to provide 1Gbps per user using Wavelength Division Multiplexing combined with Time division multiplexing, WDM/TDM, as shared access method. Even if WDM is used to reduce the line rate (10Gbps x 4 wavelengths), the use of TDM/TDMA [3] impose a high Digital Signal processing (DSP) at ONU side to demodulate only 1Gbps among the 10Gbps received data stream, which is source of cost and power consumption. Other architectures have been explored, such as Orthogonal Frequency Division Multiplexing (OFDM) [4], to increase the global data rate but are also source of cost due to the stringent requirements for high speed DSP (40GS/s). An alternative to the WDM/TDM could be the use of Frequency Division Multiplexing (FDM) combined with WDM. Indeed, in a previous paper we have demonstrated [5] that only 500MS/s is sufficient to provide 1Gbps per user, simplifying the need of high DSP at ONU side, reducing cost and power consumption. We have also experimentally evaluated [6] for the downstream link, a capacity of about 20Gbps on 1 wavelength using 5.5GHz electrical spectrum. In this paper we experimentally demonstrate a 39Gbps capacity using 11.5GHz electrical bandwidth with 20km of Single Mode Fiber (SMF). We first report our experimental test bed. Then
we detail our improved resource allocation algorithm and finally we generate simultaneously all users and verify experimentally the allocation by measuring the Bit Error Rate (BER) for each user. Experimental setup To evaluate the performance and the global capacity of our proposed system, we first probe the transmission channel for different frequencies and optical budgets with a QPSK reference signal of 250MHz electrical bandwidth, covering the range of parameters that we will use.
Fig 1: Experimental test bed
Our experimental test bed for downlink performance evaluation is represented in Fig 1. A Distributed Feed-Back (DFB) laser at 1.5μm emitting +9dBm of continuous optical power is externally modulated by a Mach Zehnder Modulator (MZM) with 10GHz electrical bandwidth biased at quadrature. The modulating signal is generated by the Downstream Transmitter and is a broadband signal whose spectrum extends from 500MHz to 12GHz. We evaluate the transmission quality by measuring the Error Vector Magnitude (EVM) that we convert into Signal to Noise Ratio (SNR) for different center frequencies, Optical Budgets and RF power modulating the MZM. Resources allocation algorithm The proposed algorithm performs a resource allocation with the following degrees of freedom: power level, carrier frequency, signal bandwidth and spectral efficiency. The entire system is subject to several constraints: power budget, channel frequency response, aggregated bandwidth and information reliability. For a predefined set of possible constellations (like M-QAM) and a targeted bit error rate, a function b = γ(snr) establishes the relationship
k>1.. N @
k>1.. N @
Two-users resource allocation For only two adjacent users, the max-min resource allocation can be reduced into a two dimensional optimization problem. The objective here is to find {w1, w2, p1, p2} in order to maximize the minimum of D1/D1target and D2/D2target, subject to the constraints (w1+w2)≤W' and (p1+p2)≤P'. At optimality, a solution of this problem should satisfy D1/D1target = D2/D2target. Fig 2 gives an example of the representation of D1/D1target and D2/D2target as functions of p1 and w1 with w2=W' -w1 and p2= P'- p1. For any fixed value of p1 such as 0≤ p1≤P' and p2= P'- p1, it is possible to find w1 and w2 such as w1+w2=W' that satisfy D1/D1target = D2/D2target. As illustrated in Fig 3 (top), it corresponds to the intersection point of the curves of weighted data rates for the two users as a function of their respective bandwidth. This point can be efficiently determined using the bisection method. The set of the intersection points for all values of p1 in [0, P'] is represented by the bold curve in Fig 2 (three-dimensional view) or in Fig 3 (bottom, two-dimensional view). The final step is to find the maximum of this set using a onedimensional search method.
Fig 2: Weighted data rates for two users as a function of power and bandwidth distributions 4
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weighted user data rates
Problem Statement The dynamic resource allocation consists in finding the best assignment of power, carrier frequency and bandwidth regarding the user requirements. When user requirements are expressed as targeted data rates Dktarget, the problem statement can be expressed as the maximization of the weighted data rate Dk/Dktarget of the worst user under the constraints describe below. For a power budget P and an available bandwidth W, the resource allocation problem can be mathematically formulated as a max-min problem: § § Dk · · ¸¸ ¸ finding max ¨¨ min ¨¨ target ¸ pk ,bw k , f k k>1.. N @ D © k ¹¹ © (2) subject to ¦ pk d P and ¦ w k d W
user throughput
where gk(fk) is its gain-to-noise ratio obtained through channel probing using the experimental setup described in the previous section. The allocated power is denoted pk and wk is the allocated bandwidth.
Multiple users allocation method An exhaustive search of a solution for the maxmin-problem (2) is intractable for more than two users. The suboptimal method implemented here consists in working with couples of adjacent users using the method presented in section above. The purpose of this method is to select the two adjacent users presenting the higher difference in their allocated weighted data rates, performs the resource allocation for two users and iterates these actions until convergence. An illustration of the convergence of the algorithm is depicted in Fig 4, where each curve is the evolution of the weighted data rate for one user during the iterations.
0.2 0.4 0.6 bandwidth user 1 [GHz]
2 user throughput
between a given signal-to-noise-ratio and the number of bits b that can be transmitted. Using this function and a given power and bandwidth assignment, the achievable data rate for user k at carrier frequency fk is then given by: pk .g k f k D k w k .J snrk f k , whith snrk f k (1) wk
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Fig 3: Two-dimensional view for a fixed power distribution (top) and projection of the intersection curve (bottom)
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Fig 4: Illustration of the convergence of the proposed allocation resource algorithm
Signal generation and results The main challenge of using a large electrical bandwidth RF signal is to generate a constant power spectral density in low as well as in high frequencies bands. Because of the roll-off of the arbitrary waveform generator (AWG), there is power attenuation for user situated at the top of the spectrum. To overcome this issue, as depicted in Fig 5, we use 3 AWGs. AWG1 generate a signal for users situated between 500MHz to 6GHz. AWG3 generate the signal of the user we want
to evaluate performance when it is situated after 6GHz. Finally AWG2 generate the complementary signal of AWG3 between 6 and 12GHz.
BER (Fig 11). To assign the correct optical budget, we tune the VOA to the correct value.
Fig 5: Spectrum generation
Fig 11: BER performance for each user
Fig 6 to Fig 9 show an example of spectrum generated by each AWG. In this case, the user we want to evaluate performance is situated at 7,1GHz.
To calculate the total capacity, we multiply the bandwidth by the modulation rate for each channel and sum all data rate per user. Finally we found a reachable capacity of 39Gbps, using 11,5GHz electrical bandwidth by allocating 48 users.
Fig 6: Spectrum AWG1out
Fig 8: Spectrum AWG3out
Fig 7: Spectrum AWG2out
Fig 9: Whole Spectrum
Because the user signal is realized in base band and transposed, thanks to a RF local oscillator at the correct RF frequency, there is no power attenuation of the signal. By this way, it is now possible to generate and demodulate a correct signal over the full band. With the results of the resources allocation ® algorithm, we create with Matlab the corresponding 48 channels.
Fig 10: Results of the resources allocation for 48 users
Fig 10 shows the modulation rate, RF power, bandwidth and optical budget for each user. With the method described above, we demodulate each sub-band and evaluate the transmission performance by measuring the
Conclusion We have presented experimental results of our proposed FDMA PON for the downlink direction. We have tested a new algorithm and have verified it in term of performance by measuring the BER over the 11,5GHz electrical bandwidth and have obtained a total capacity of 39Gbps. Acknowledgments This work has been carried out within the framework of the FAON project which is partly funded by the Agence Nationale de la Recherche under reference 11-INFR-005-01. Funding from the European Commission through the projects FABULOUS is also acknowledged. References [1] P. Chanclou et al., « Network operator requirements for the next generation of optical access networks », IEEE Network, vol. 26, no 2, p. 8 䙲 14, avr. 2012. [2] H. Nakamura, “NG-PON2 Technologies”, OFC 2013, paper NTh4F.5 [3] Y. Luo et al., « Time- and WavelengthDivision Multiplexed Passive Optical Network (TWDM-PON) for NextGeneration PON Stage 2 (NG-PON2) », Journal of Lightwave Technology, vol. 31, no 4, p. 587 䙲 593, févr. 2013. [4] N. Cvijetic et al., « Terabit Optical Access Networks Based on WDM-OFDMAPON », Journal of Lightwave Technology, vol. 30, no 4, p. 493 䙲 503, févr. 2012. [5] A. Lebreton et al., « Low Complexity FDM/FDMA Approach for Future PON », OFC2013, paper [6] A.Lebreton et al., “Towards 40Gbps downstream FDM PON”, accepted paper for ONDM 2013 conference.
