Jianglei Ma

Uplink contention based SCMA for 5G radio access

Fifth generation (5G) wireless networks are expected to support very diverse applications and terminals. Massive connectivity with a large number of devices is an important requirement for 5G networks. Current LTE system is not able to efficiently support massive connectivity, especially on the uplink (UL). Among the issues that arise due to massive connectivity is the cost of signaling overhead and latency. In this paper, an uplink contention-based sparse code multiple access (SCMA) design is proposed as a solution. First, the system design aspects of the proposed multiple-access scheme are described. The SCMA parameters can be adjusted to provide different levels of overloading, thus suitable to meet the diverse traffic connectivity requirements. In addition, the system-level evaluations of a small packet application scenario are provided for contention-based UL SCMA. SCMA is compared to OFDMA in terms of connectivity and drop rate under a tight latency requirement. The simulation results demonstrate that contention-based SCMA can provide around 2.8 times gain over contention-based OFDMA in terms of supported active users. The uplink contention-based SCMA scheme can be a promising technology for 5G wireless networks for data transmission with low signaling overhead, low delay, and support of massive connectivity.

SCMA for downlink multiple access of 5G wireless networks

Sparse code multiple access (SCMA) is a new frequency domain non-orthogonal multiple-access technique which can improve spectral efficiency of wireless radio access. With SCMA, different incoming data streams are directly mapped to codewords of different multi-dimensional cookbooks, where each codeword represents a spread transmission layer. Multiple SCMA layers share the same time-frequency resources of OFDMA. The sparsity of codewords makes the near-optimal detection feasible through iterative message passing algorithm (MPA). Such low complexity of multi-layer detection allows excessive codeword overloading in which the dimension of multiplexed layers exceeds the dimension of codewords. Optimization of overloading factor along with modulation-coding levels of layers provides a more flexible and efficient link-adaptation mechanism. On the other hand, the signal spreading feature of SCMA can improve link-adaptation as a result of less colored interference. In this paper a technique is developed to enable multi-user SCMA (MU-SCMA) for downlink wireless access. User pairing, power sharing, rate adjustment, and scheduling algorithms are designed to improve the downlink throughput of a heavily loaded network. The advantage of SCMA spreading for lightly loaded networks is also evaluated.

Filtered OFDM: A new waveform for future wireless systems

A spectrally-localized waveform is proposed based on filtered orthogonal frequency division multiplexing (f-OFDM). By allowing the filter length to exceed the cyclic prefix (CP) length of OFDM and designing the filter appropriately, the proposed f-OFDM waveform can achieve a desirable frequency localization for bandwidths as narrow as a few tens of subcarriers, while keeping the inter-symbol interference/inter-carrier interference (ISI/ICI) within an acceptable limit. Enabled by the proposed f-OFDM, an asynchronous filtered orthogonal frequency division multiple access (f-OFDMA)/filtered discrete-Fourier transform-spread OFDMA (f-DFT-S-OFDMA) scheme is introduced, which uses the spectrum shaping filter at each transmitter for side lobe leakage elimination and a bank of filters at the receiver for inter-user interference rejection. Per-user downsampling and short fast Fourier transform (FFT) are used at the receiver to ensure a reasonable complexity of implementation. The proposed scheme removes the inter-user time-synchronization overhead required in the synchronous OFDMA/DFT-S-OFDMA. The performance of the asynchronous f-OFDMA is evaluated and compared with that of the universal-filtered OFDM (UF-OFDM), proposed in [1], [2].

Filtered-OFDM - Enabler for Flexible Waveform in the 5th Generation Cellular Networks

The underlying waveform has always been a shaping factor for each generation of the cellular networks, such as orthogonal frequency division multiplexing (OFDM) for the 4th generation cellular networks (4G). To meet the diversified and pronounced expectations upon the upcoming 5G cellular networks, here we present an enabler for flexible waveform configuration, named as filtered-OFDM (f-OFDM). With the conventional OFDM, a unified numerology is applied across the bandwidth provided, balancing among the channel characteristics and the service requirements, and the spectrum efficiency is limited by the compromise we made. In contrast, with f-OFDM, the assigned bandwidth is split up into several subbands, and different types of services are accommodated in different subbands with the most suitable waveform and numerology, leading to an improved spectrum utilization. After outlining the general framework of f-OFDM, several important design aspects are also discussed, including filter design and guard tone arrangement. In addition, an extensive comparison among the existing 5G waveform candidates is also included to illustrate the advantages of f-OFDM. Our simulations indicate that, in a specific scenario with four distinct types of services, f-OFDM provides up to 46% of throughput gains over the conventional OFDM scheme.

Weighted circularly convolved filtering in OFDM/OQAM

A novel technique for removing the edge time transitions, a.k.a. tails, as well as the half symbol offset overhead of OFDM/OQAM signal is proposed. The proposed technique, which is called weighted circular convolution, totally removes the signal overheads, while preserving the real-orthogonality of the prototype filter, and as such, does not incur any ISI/ICI on the demodulated OQAM symbols. It is based on extension of the finite-length OQAM sequence to infinite-length such that the resulting signal is periodic. This periodicity together with special structure of OFDM/OQAM enables one to transmit only the desired overhead-removed portion of signal. This is equivalent to a weighted circular convolution instead of linear convolution in the OFDM/OQAM modulator/demodulator. The spectral sidelobe augmentation due to the sharp edge transitions of the overhead-removed signal is resolved by a weighted time-domain windowing. The proposed overhead-removal technique is applicable to an OFDM/OQAM burst of arbitrary length (either even or odd) and, among other benefits, significantly improves the spectral efficiency especially for short-burst signals.

Low Complexity Techniques for SCMA Detection

Sparse code multiple access (SCMA) is a codebook- based non-orthogonal multiplexing technique. In SCMA, the procedure of bit to QAM symbol mapping and spreading of CDMA are combined together and incoming bits are directly mapped to multi-dimensional codewords of SCMA codebook sets. Due to the sparse nature of codewords, SCMA enjoys the low complexity reception, taking advantage of a near optimal message passing algorithm (MPA). This makes SCMA a candidate for supporting massive connectivity in future 5G networks, where the number of users can potentially be higher than the codeword length (spreading factor). To this end, more efficient reception techniques are needed on top of what MPA delivers. In this paper, some complexity reduction techniques are presented to further reduce the SCMA decoding complexity. These techniques are considered from two perspectives: i) transmitter-side technique, by designing SCMA codebooks with a specific structure providing low complexity of detections, and ii) low complexity decoding techniques taking advantage of the SCMA codebook structure. The proposed techniques are evaluated in terms of both complexity and performance. It is shown that significant amount of complexity reduction is possible using the proposed techniques with negligible performance penalty, which paves the way of supporting various applications in future 5G systems using SCMA.

Radio access virtualization: Cell follows user

Virtual radio access (VRA) technology wherein groups of cooperative transmit points (TPs) form virtual TPs (VTPs) to serve user equipments (UEs) continue to be a thriving subject of research in future generations of wireless networks. In this paper, we propose a technique that uses UE-centric metrics to provide multiple partitions of a wireless network into VTP sets. Our technique guarantees that all UEs enjoy a required gain in at least one VTP; effectively eliminating the edge UE experience in the network. To further enhance the performance of the proposed VRA technique in practical scenarios wherein there is a large load imbalance in the network, we also introduce a new concept of soft UE-TP association in which each UE is partially associated with multiple TPs. The use of our soft association concept when forming VTP sets facilitates load-balancing among various TPs. Finally, a technique is also offered to select the best VTP set at each scheduling resource unit. Numerical simulations are used to demonstrate the performance of our virtualization techniques.

SCMA: A Promising Non-Orthogonal Multiple Access Technology for 5G Networks

Sparse code multiple access (SCMA) is a code domain non-orthogonal multiple-access technique introduced for future 5G wireless networks. This paper describes the basic ideas of SCMA, the SCMA codebook design, the encoder and decoder, as well as the SCMA enabled transmission schemes for different application scenarios, including uplink grant-free contention-based transmission, downlink multi-user superposition, and downlink open-loop CoMP for ultra-dense networks even with moving users. We shall show from design principles and application examples that SCMA can resolve some major issues of current wireless systems and establish itself as a strong candidate for 5G networks.

Adjustable ultra narrow-band pulse for asynchronous 5G M2M communications

An adjustable pulse bandwidth single carrier waveform for asynchronous low-cost low-power 5G M2M applications is proposed. The solution uses ultra narrow-band pulse to improve coverage, which is a much more efficient coverage enhancement solution than existing repetition solutions. Frequency localized pulse shaping is used, which allows asynchronous transmission and packing more machine-type devices (MTDs) for the same spectrum resources. In addition, bandwidth of each MTD is configurable based on long term channel quality and system load, which supports higher throughput and lower power consumption. Performance comparison with the repetition solution of 3GPP LTE demonstrates significant gain in terms of number of supported connections.

Turbo-coded single-carrier faster-than-Nyquist transmission

Faster than Nyquist (FTN) transmission is investigated for a point-to-point AWGN link. The FTN is interpreted as a form of coding at the pulse shape level, and accordingly, the so-called Mazo phenomenon is interpreted as a coding gain. Then, it is shown by simulation that such a gain can disappear in a coded FTN transmission with a powerful FEC code, such as turbo code. This result undermines the FTN transmission as an effective technique in an actual communication system.