## Abstract

This paper proposes a novel non-orthogonal optical multicarrier access system based on filter bank and sparse code multiple access (SCMA). It offers released frequency offset and better spectral efficiency for multicarrier access. An experiment of 73.68 Gb/s filter bank-based multicarrier (FBMC) SCMA system with 60 km single mode fiber link is performed to demonstrate the feasibility. The comparison between fast Fourier transform (FFT) based multicarrier and the proposed scheme is also investigated in the experiment.

© 2015 Optical Society of America

## 1. Introduction

The exponentially growth of traffic demand is pushing network bandwidth requirement even further. In order to cope with this demand, access network is moving from the conventional on-off keying (OOK) time division multiplexing (TDM) to advanced modulation formats such as quaternary pulse amplitude modulation (4PAM) and duobinary modulation to offer enhanced operation and speed [1–3 ]. As the access speed evolves to 40Gb/s beyond, the TDM-based single carrier access scheme is challenging to support larger number of subscribers and finer granularity. However, the timing and synchronization of high-speed signal are cost-inefficient for access network. Recently, multicarrier technique has been seen as a powerful solution to provide larger capacity and higher granularity level, such as wavelength division multiplexing and orthogonal frequency division multiplexing (OFDM) [4–6 ]. Overall, the OFDM based optical access network has received wide attention due to its high spectral efficiency and flexible spectrum allocation. Considering the computation complexity, the fast Fourier transform (FFT) is adopted to generate the orthogonal subcarriers in OFDM. However, this delicate orthogonality can be upset even by relatively small frequency synchronization error, resulting in inter-carrier interference (ICI) [7]. The frequency synchronization and orthogonality would be more rigorous for upstream signal due to the upstream optical carrier de-correlation. The induced interference is proportional to the number of access users. The cyclic prefix (CP) is needed to resolve the dispersion-induced ICI; nevertheless it reduces the bandwidth efficiency of the access signal. Although the ICI can be compensated without CP in OFDM system, it needs additional channel algorithm and overhead, which would reduce the spectral efficiency and increase the complexity of the signal processing at the receiver.

As the rapid development of the digital signal processing (DSP) technology, the filter bank based new technique is proved to be more promising than FFT-based digital subcarrier method [8, 9 ]. There have been many filter bank-based access schemes proposed in 5th generation (5G) wireless communication. It does not need CP insertion and shows higher robustness to residual frequency offset by taking advantage of low spectral leakage of prototype filters. Besides, it can improve the system flexibility by proper designing the prototype filters and grouping the frequency clusters. There have been some researches about off-set QAM (OQAM) OFDM, which can be seen as a special form of filter bank based scheme [10, 11 ]. However, the multicarrier generation in OQAM-OFDM is still based on FFT operation. The generalized filter bank-based multicarrier (FBMC) is needed to be investigated to improve the performance of multicarrier system [12]. On the other hand, advanced coding such as sparse code multiple access (SCMA) can be applied to improve the access signal flexibility and performance [13]. It offers directly mapping from data bits to multi-dimensional complex vectors and reduces the interference among different users, which allows the network to enable massive connectivity. The SCMA is essentially a kind of coding technique, which is evolved from electrical code division multiplexing (ECDM). The filter bank belongs to multicarrier technique, which is alternative to the FFT-based OFDM technology. We have proposed ECDM-OFDM technology before, which utilizes the advantages of CDM and OFDM to improve the signal flexibility and performance [14]. The combination of SCMA and filter bank can further reduce the subcarrier interference and improve the spectral efficiency.

In this paper, to our best knowledge, we firstly propose and demonstrate a novel non-orthogonal optical multicarrier access system based on filter bank and SCMA. The codebooks of SCMA can be flexible assigned to different ONUs according to the network traffic. Compared with traditional OFDM based access network, no CP is required and the frequency offset can be released. An experiment of 73.68 Gb/s FBMC/SCMA signal is successfully demonstrated over 60 km fiber link.

## 2. Principle

Figure 1
illustrates the schematic of the proposed non-orthogonal optical multicarrier access system. At the optical line terminal (OLT), different data streams are firstly coded with codebooks. The SCMA encoder can be seen as a map from bit data to multi-dimensional complex codebooks. The principle of SCMA mapping is shown in Fig. 2
, where we adopt six codebooks and four subcarriers as an example. For each sub-stream, the incoming bits are mapped to a codeword selected from the codebook. The codewords are combined and carried over shared subcarriers of multicarrier system. Same codebook can be assigned to different ONUs among different frames. Assuming the length of codeword is *M* and the sparse vectors have *N* non-zero positions, the number of codebooks can be given by

*k*is the index of subcarrier,

*x*is the transmitted symbols,

^{k}*τ*is the symbol spacing,

_{0}*υ*is the subcarrier spacing and

_{0}*f(t)*is the prototype filter of FBMC represented by

*J*is the number of bins and

*a*is the

_{j}*j*

^{th}filter coefficient. At the optical network units (ONUs), the matched filters for different subcarriers are adopted to filter out the symbols. The received symbols are detected with maximum a posteriori (MAP) detection [15], which is given byThere

*x’*is the received symbols. The original bit stream can be further recovered according to the dedicated codebook at the ONU. Due to the sparsity property, the sparse vectors have

*N*non-zero positions with

*M*codeword length (

*N*<<

*M*). The complexity of the decoding will turn out to be

*O*(|X|

^{N}), which is significantly reduced compared to

*O*(|X|

^{M}) for non-sparse case.

## 3. Experimental validation

Figure 3 illustrates the experimental schematic for the proposed non-orthogonal optical multicarrier access system. At the OLT, a CW laser with linewidth of 300 kHz and optical power of 9 dBm serves as the light source, which is centered at 1551.72 nm. It is modulated by the FBMC/SCMA signal with the aid of an optical I/Q modulator. The signal is generated offline with digital signal processing. Six bit sequences are generated with SCMA encoder before filtering. In the experiment, the number of codebooks is set to be 6 and the size of each codebook is 4, in which the codeword is labeled by 2-bit data. The length of codeword is equal to four tones. It means that the subcarrier number of filter bank for each SCMA frame is four. The sparse codewords are multiplexed and only three of them collide over each subcarrier, which has been shown in Fig. 2. The filter bank has better spectrum confinement compared with conventional filter such as rectangular filter. The prototype filter uses finite impulse response (FIR) filter, which is consisting of group of cosine filters with different filter coefficients. In the experiment, the filter tap is set to be 32. The frequency characteristic of the prototype filter is shown in Fig. 3(a). It can be seen that the prototype filter has got fast decaying side-lobes. In the experiment, 256 subcarriers are used for data and each subcarrier occupies a bandwidth of 48 MHz, which results in bandwidth of 12.28 GHz for both I and Q parts. Considering the overload factor is 1.5 and each codeword is labeled by 2 bits, the signal rate is 73.68 Gb/s. A digital-to-analog convertor (DAC) at 50 Gs/s is adopted to produce the electrical signal. The electrical spectrum of the I part is shown in Fig. 3(b). Then output optical signal from the optical I/Q modulator is further amplified by a commercial Er-doped fiber amplifier (EDFA) with noise figure of 4.2 dB before transmission. It is propagated through a 60-km standard single mode fiber (SMF) with total loss of 13.2 dB.

At the ONU, another CW light with same linewidth is used as the local oscillator (LO). The coherent optical receiver is consisting of a 90° optical hybrid and two balanced photodetectors. Then the detected signals are sampled by an analog to digital convertor (ADC) with 100 Gs/s for further digital signal processing. The sampled signals are fed into the matched filters to retrieve the symbols for SCMA decoder. A variable optical attenuator is placed after the fiber for bit error rate (BER) measurement.

The downstream transmission performance is investigated and the measured result is shown in Fig. 4
, where we have tested three data streams. It can be seen that the three data streams exhibit similar performance and the power penalty is less than 0.3 dB after 60 km transmission. In the following analysis, we have measured the averaged performance of the six data streams. Figure 5
compares the performance of FBMC/SCMA signal with FBMC/LDS signal under same bandwidth. The LDS is also a kind of sparse code technology which expands symbols to a sequence of complex symbols by using a CDM signature. It carries data over spread conventional QAM points. The FBMC/SCMA signal outperforms the FBMC/LDS signal by 1.29 dB at a BER of 1 × 10^{−3}. It is mainly because that SCMA enjoys the inherent shaping gain due to additional degrees of freedom in the codebook design. There are *L* codebooks each with *M* codewords for SCMA, but LDS only has got *L* signature. SCMA carries data over multi-dimensional constellations, which enables SCMA to benefit from the shaping gain.

Figure 6
depicts the measured BER as a function of residual frequency offset (RFO) for multicarrier signals with our proposed prototype filter and rectangular filter. Both of them adopt SCMA and the only difference is filtering. For filter bank, we adopt the proposed prototype filter stated in Eq. (3). The rectangular filter has got same tap as the prototype filter and the frequency characteristic is shown in Fig. 3(a) with red line. It can be seen that with the same subcarrier spacing, FBMC/SCMA is more robust against the RFO. When the RFO is zero, the BER of FBMC/SCMA signal is a little worse than the rectangular shaped signal, which is mainly due to the stronger filtering of FBMC compared with rectangular filter. When the RFO increases to ± 8MHz, the FBMC/SCMA signal can achieve a BER under 1 × 10^{−3} while the rectangular shaped signal gets a BER above 1 × 10^{−3}.

Figure 7
compares the performance of FBMC/SCMA signal with traditional IFFT-based OFDM signal. The bandwidths of FBMC and OFDM are both 12.28 GHz for I and Q parts. The electrical spectra of FBMC and traditional OFDM are shown in Fig. 7(i) and Fig. 7(ii) respectively. Compared with traditional OFDM, the FBMC has avoided long spectral tails and exhibited greatly suppressed side-lobes. It can be observed that FBMC/SCMA and OFDM signal have got receiver sensitivities of −18.3 dBm and −17.2 dBm at BER of 1 × 10^{−3} respectively. The total number of OFDM subcarriers is set to be 256 and 8 pilot subcarriers are used for phase estimation. For OFDM signal, the CP length is 1/16 of symbol length. It means that FBMC/SCMA signal gets better spectral efficiency and the 6.25% absence of the CP can be used for further channel coding such as forward error correction (FEC). Figure 7 also depicts the performance of filtered OFDM signal, where the receiver sensitivity is about −17.4 dBm at BER of 1 × 10^{−3}.

Figure 8
illustrates the performance of upstream signals with different numbers of ONUs, where we investigate both the FBMC/SCMA signal and traditional OFDM signal. Symmetric structure is adopted for downstream and upstream signals. The upstream performance is demonstrated under colorless link, which means all the ONUs adopt same wavelength. For OFDM signal, the number of subcarriers is 256 and CP length is 1/16 of symbol length. The ONUs share the subcarriers and time-slots. For FBMC/SCMA signal, power penalty of 0.3 dB can be observed when the number of ONUs increases from two to four. The optical beating noise is suppressed due to the code gain. However, the BER of traditional OFDM signal is beyond 1 × 10^{−3} due to the optical beating noise between the ONUs. As the increase of ONU number, there is obvious performance deterioration due to the enlarging noise.

In downstream, traditional OFDM offers lower complexity while the filter bank multicarrier provides higher bandwidth efficiency. The lower complexity of OFDM is due to the fundamental assumption that the subcarriers are a set of perfectly synchronized orthogonal tones. In upstream, because the synchronization among different ONUs is required, traditional OFDM is more challenging and has a higher complexity than filter bank multicarrier.

## 4. Conclusion

We have proposed and experimentally demonstrated a novel multicarrier access system based on filter bank and SCMA. A 73.68 Gb/s FBMC/SCMA signal with 6 codebooks is successfully achieved over 60 km SMF fiber. Compared with traditional IFFT-based multicarrier access, it provides higher spectral efficiency and better performance for both downstream and upstream signals. Our experimental results suggest the proposed scheme a promising way for future multicarrier access network.

## Acknowledgments

The financial supports from National NSFC (No. 61425022/61522501/61307086/61475024/61275074), National High Technology 863 Program of China (No. 2013AA013403, 2015AA015501, 2015AA015502, 2015AA015504) and Beijing Nova Program (No.Z141101001814048) are gratefully acknowledged. The project is also supported by the Universities Ph.D. Special Research Funds (No. 20120005110003/20120005120007).

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