Non-orthogonal multiple access (NOMA) on shared resources has been identified as a promising technology in 5G to improve resource efficiency and support massive access in all kinds of transmission modes. Power domain and code domain NOMA have been extensively studied and evaluated in both literatures and 3GPP standardization, especially for the uplink where large number of users would like to send their messages to the base station. Though different in the transmitter side design, power domain NOMA and code domain NOMA share the same need of the advanced multi-user detection (MUD) design at the receiver side. Various multi-user detection algorithms have been proposed, balancing performance and complexity in different ways, which is important for the implementation of NOMA in practical networks. In this paper, we introduce a unified variational inference (VI) perspective on various universal NOMA MUD algorithms such as belief propagation (BP), expectation propagation (EP), vector EP (VEP), approximate message passing (AMP) and vector AMP (VAMP), demonstrating how they could be derived from and adapted to each other within the VI framework. Moreover, we unveil and prove that conventional elementary signal estimator (ESE) and linear minimum mean square error (LMMSE) receivers are special cases of EP and VEP, respectively, thus bridging the gap between classic linear receivers and message passing based nonlinear receivers. Such a unified perspective would not only help the design and adaptation of NOMA receivers, but also open a door for the systematic design of joint active user detection and multi-user decoding for sporadic grant-free transmission.

In this paper, we study the equalization design for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with insufficient cyclic prefix (CP). In particular, the signal detection performance is severely impaired by inter-carrier interference (ICI) and inter-symbol interference (ISI) when the multipath delay spread exceeding the length of CP. To tackle this problem, a deep learning-based equalizer is proposed for approximating the maximum likelihood detection. Inspired by the dependency between the adjacent subcarriers, a computationally efficient joint detection scheme is developed. Employing the proposed equalizer, an iterative receiver is also constructed and the detection performance is evaluated through simulations over measured multipath channels. Our results reveal that the proposed receiver can achieve significant performance improvement compared to two traditional baseline schemes.

Non-orthogonal multiple access (NoMA) as an efficient way of radio resource sharing has been identified as a promising technology in 5G to help improving system capacity, user connectivity, and service latency in 5G communications. This paper provides a brief overview of the progress of NoMA transceiver study in 3GPP, with special focus on the design of turbo-like iterative multi-user (MU) receivers. There are various types of MU receivers depending on the combinations of MU detectors and interference cancellation (IC) schemes. Link-level simulations show that expectation propagation algorithm (EPA) with hybrid parallel interference cancellation (PIC) is a promising MU receiver, which can achieve fast convergence and similar performance as message passing algorithm (MPA) with much lower complexity.

This paper aims to evaluate the system level performance of the newly proposed non-orthogonal multiple access scheme sparse code multiple access (SCMA) with uplink grant-free transmissions for 5G massive machine-type communication (mMTC) scenario. The challenge lies in the modeling and abstraction of physical layer actions as the link-to-system interface, including the features of overloading, grant-free transmission, as well as non-linear receiver. Specifically, we propose to use single bound to simplify the maximum-likelihood (ML) receiver modelling with only linear computational complexity, while still guaranteeing sufficient accuracy requirement. Extensive simulations verify the correctness of the method under a series of practical assumptions. Evaluation results show that non-orthogonal multiple access schemes such as SCMA with grant-free can provide significant system capacity gain over orthogonal multiple access such as grant-free OFDMA (about 3.5 times as shown in the evaluated scenario) in terms of packet arrival rate at given system outage (defined as 1% packet drop rate). It is noted that the proposed method has been just agreed in 3gpp RAN1 86 meeting and included in the technical report.

Full-duplex (FD) networks promise nearly doubled spectrum efficiency for single links with beyond 100 dB self-interference cancellation capability. However, the performance of FD networking is not that straightforward, as there will be several new types of interference introduced. Hence, it is critical to exploit some radio resource management techniques to combat the annoying interference and inevitably needs to reserve part of radio resources in HD mode to obtain the essential auxiliary information. It is naturally to ask which physical channels and signals could be left in the FD mode and how much system gain FD cellular networks could bring? In this paper, we try to answer these questions. We first propose the feasibility to leave one kind of physical channels or signals in the FD mode and then highlight some potential resource management schemes. We will also demonstrate the system gain of FD networks by leveraging our proposed design and the management techniques. Our simulation results show FD communications leads to promising performance improvement.

5G wireless networks are expected to support new applications demanding ultra high reliability and ultra low latency, such as self-driving cars, industrial control, and real-time gaming. Such services are known as ultra-reliable and low-latency communications (URLLC) targeting at 99.999% transmission correctness within 1 ms delay bound. Current Long Term Evolution (LTE) system is far from supporting such services, especially for uplink. Even with around 10 times shortened transmission interval, due to the waste of time in requesting and receiving the scheduling grant, only one shot uplink transmission can be supported within 1 ms, which is very hard to meet the reliability requirement. Grant-Free transmission is thus proposed to effectively save the time of requesting/waiting for grant and thus relax the transmission interval design in frame structure. This paper presents the design of grant-free transmission for URLLC services and also the evaluation results. We shall show that the saved time can be used for more Hybrid Automatic Repeat reQuest (HARQ) retransmissions and thus further enhance the reliability. Moreover, contention based grant-free transmission with proper HARQ design can well meet the reliability requirement. Finally, the performance can be further enhanced with non-orthogonal multiple access schemes such as Sparse Code Multiple Access (SCMA) and advanced receivers such as Message Passing Algorithm (MPA) or Expectation Propagation Algorithm (EPA) receiver.

Non-orthogonal multiple access has been extensively discussed in New Radio (NR), the study item working on 5G air interface in 3GPP. Sparse code multiple access (SCMA) is one of the proposed MA schemes. In this paper, the basic concept of SCMA is introduced, including codebook mapping, multiple access procedure, and advanced receivers. Then, link level simulations are applied to verify the design of SCMA, which show that SCMA has many excellent properties: shaping and diversity gain by sparse codebooks, resilient to inter- user interference, and robust to codebook collision, thus it is a promising candidate MA scheme for 5G.

A user-centric no-cell (UCNC) framework has been proposed which enables different level coordination over a large number of TRPs. This paper presents some key technologies in UCNC framework with the focus on PoC of UL grant- free transmissions in UCNC architecture. Massive connectivity support is one of the three important scenarios in 5G networks. The challenges to support massive connectivity lie in the cost of signaling overhead and transmission latency. An uplink grant- free transmission based on SCMA (sparse code multiple access) design is proposed. The proposed scheme can provide different levels of overloading to efficiently meet the massive connectiv- ity requirements. Extensive lab tests have been conducted to verify the performance matrices. Taken LTE as a baseline for comparison purpose, it is shown that the signaling overhead by using uplink grant-free transmission can be reduced by around 80% and the user plane transmission latency in ECO state can be reduced by around 93%. Moreover, massive connectivity is efficiently supported with SCMA scheme that yields around 230% gain over OFDMA in terms of supported active users.

Sparse code multiple access (SCMA) scheme is considered to be one promising non-orthogonal multiple access technology for the future fifth generation (5G) communications. Due to the sparse nature, message passing algorithm (MPA) has been used at the receiver to achieve close to maximum likelihood (ML) detection performance with much lower complexity. However, the complexity order of MPA is still exponential with the size of codebook and the degree of signal superposition on a given resource element. In this paper, we propose a novel low complexity iterative receiver based on expectation propagation algorithm (EPA), which reduces the complexity order from exponential to linear. Simulation results demonstrate that the proposed EPA receiver achieves nearly the same block error rate (BLER) performance as the conventional message passing algorithm (MPA) receiver with orders less complexity.

A 5G mobile network has evolved from a cell centric radio access network to a user centric one. A user-centric no-cell (UCNC) framework has been proposed, which enables physical layer coordination over a large number of transmission and reception points for each mobile user to always experience cell-center-like communications. This paper presents some key technologies in the UCNC framework. The focus is on proof of concept of UL grant-free transmissions in the UCNC architecture. Massive connectivity supporting a huge number of devices is one of the three important scenarios in 5G networks. The challenges to support massive connectivity lie in the cost of signaling overhead and transmission latency. An uplink grant-free transmission based on sparse code multiple access (SCMA) design is proposed. The proposed scheme can provide different levels of overloading to efficiently meet the massive connectivity requirements. Grant-free transmission conducted in RRC connected state can reduce signaling overhead, while in energy conserved operation (ECO) state, it can significantly reduce the user plane transmission latency. Extensive laboratory testing has been conducted in a collaborative project of Huawei and Telefonica to verify the performance matrices. Taking LTE as a baseline for comparison purposes, it is shown that the signaling overhead by using uplink grant-free transmission can be reduced by around 80% and the user plane transmission latency in ECO state can be reduced by around 93%. Moreover, massive connectivity is efficiently supported with the SCMA scheme that yields around 230% gain over OFDMA in terms of supported active users.

Full-duplex (FD) communications benefit from recent breakthroughs in analog and digital signal processing and demonstrate nearly doubled spectrum efficiency for point-to-point links with beyond 100 dB self-interference cancellation capability. However, FD cellular networks with base stations (BSs) in FD mode and user terminals (UEs) in half-duplex mode also introduce intra-cell inter-UE interference and other kinds of interference, which might even eat up the potential gain. In this paper, we propose to take advantage of power control and inter-UE interference cancellation (IC) techniques to cope with the bothersome intra-cell inter-UE interference. We verify that the power control solution for single-cell FD network with a given user pair exhibits an interesting binary feature, that is, either both the BS and the uplink UE transmit at the maximum power or one of them completely mutes to achieve the optimal sum rate. Moreover, the binary feature holds even when inter-UE IC is applied, thus showing appealing computational efficiency. We also demonstrate significant performance improvement by applying binary power control with inter-UE IC to both singlecell and multi-cell FD 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.

Sparse code multiple access (SCMA) is a novel non-orthogonal multiple access scheme, in which multiple users access the same channel with user-specific sparse codewords. In this paper, we consider an uplink SCMA system employing channel coding, and develop an iterative multiuser receiver which fully utilizes the diversity gain and coding gain in the system. The simulation results demonstrate the superiority of the proposed iterative receiver over the non-iterative one, and the performance gain increases with the system load. It is also shown that SCMA can work well in highly overloaded scenario, and the link-level performance does not degrade even if the load is as high as 300%.

The rapid traffic growth and ubiquitous access requirements make it essential to explore the next generation (5G) wireless communication networks. In the current 5G research area, non-orthogonal multiple access has been proposed as a paradigm shift of physical layer technologies. Among all the existing non-orthogonal technologies, the recently proposed sparse code multiple access (SCMA) scheme is shown to achieve a better link level performance. In this paper, we extend the study by proposing an unified framework to analyze the energy efficiency of SCMA scheme and a low complexity decoding algorithm which is critical for prototyping. We show through simulation and prototype measurement results that SCMA scheme provides extra multiple access capability with reasonable complexity and energy consumption, and hence, can be regarded as an energy efficient approach for 5G wireless communication systems.

In this paper, we consider energy efficient coverage planning in cellular networks. To save energy, each base station (BS) can work in sleep mode when there is no user in its coverage or the users can be served by neighbor base stations. With increasing coverage overlap, there is a tradeoff between the number of BSs per unit area and the proportion of active base stations. Analytical and numerical methods are presented to evaluate coverage planning schemes with different inter-BS distance. Evaluation results show that compared with the minimum coverage overlap scheme, the optimal planning scheme can reduce the network energy consumption by more than 20%, and the performance improvement depends on the user density, network topology, and the power consumption of sleep mode.

Traffic-aware resource allocation and base station (BS) sleep control are key methods for energy saving in cellular networks. In this paper, first, we consider the control problem of how to adapt transmit power according to flow-level traffic variations, which leverages the tradeoff between energy consumption and delay performance. Based on different time scales of traffic variations, two power adaptation strategies are investigated: load-aware and queue-aware. The two strategies adapt transmit power according to flow arrival rate and instantaneous number of flows, respectively. Optimal solutions are given for both strategies. Since the optimal solution of the queue-aware strategy has no explicit form, tight bounds are derived as an approximation. Simulation results show that the two strategies perform closely in terms of energy consumption and average delay, while the queue-aware strategy is better in the tail distribution of delay and is more robust to system parameter variations. Secondly, for the load-aware strategy, with more practical concerns like the total BS energy consumption and BS sleep control taken into account, the relationship between energy consumption and delay is explored and energy-optimal rate can be obtained under certain conditions. Two threshold-based BS sleep strategies are investigated where the optimal threshold and rate are derived respectively.

We consider subcarrier assignment and power allocation in OFDMA wireless mesh networks. Traditional full spatial reuse increases bandwidth efficiency but ignores multiuser diversity in the multi-hop network. Our method enables fractional spatial reuse with which multiuser diversity can also be exploited. The original optimization problem is decomposed into three tractable subproblems. First, subcarrier numbers on each link are decided according to end-to-end requirement and node power constraint. Then a novel link grouping method is proposed to utilize fractional spatial reuse in the network. Finally, a tabu-based subcarrier assignment algorithms is designed to assign subcarriers to groups to exploit multiuser diversity. Performance is evaluated under various network sizes to investigate the impact of spatial reuse and simulation results show our proposed scheme achieves multiuser diversity in spatial reuse scenarios.

Multihop cellular networks employ relay stations to enhance end-to-end link quality in terms of capacity, coverage and reliability. Techniques like cooperative relaying and spatial reuse can further improve the network performance. Efficient resource allocation algorithms are required to exploit the potential advantages. In this paper we first introduce three relaying schemes, and show that there is a trade-off between cooperative diversity and spatial reuse. Then we propose an efficient resource allocation algorithm to fairly allocate the resource among users in multihop cellular networks. The algorithm exploits both spatial reuse and cooperative diversity. Simulations show that the proposed algorithm outperforms the algorithms with only cooperative diversity or only spatial reuse.

This paper considers the problem of link scheduling with non-preemptive constraint in wireless mesh networks. In real-world implementation, there is often a constraint that a link can only transmit once and occupy consecutive time slots during a frame. We refer to it as the non-preemptive constraint. To date, only few scheduling algorithms in the literature has taken such constraint into consideration. In this paper, we show that optimal non-preemptive link scheduling (NPLS) problems are generally NP-hard and are provably harder to solve than link scheduling without such a constraint. To tackle the problem, a low-complexity list link scheduling (LLS) algorithm is proposed to approximate the optimal NPLS. Our analysis shows that with a randomly selected link-ordering list, throughput degradation of LLS compared to the optimal NPLS is bounded even in the worst case. By carefully constructing the link-ordering list, the performance of LLS can be further greatly improved. In this paper, we propose three schemes to construct link-ordering lists. The performance of the proposed schemes is evaluated through simulations.

In this paper, we concentrate on IEEE 802.11e based WLANs with mixed Voice-over-IP (VoIP) and data services. Although the enhanced distributed coordination access mechanism of IEEE 802.11e can differentiate VoIP and data services, the unsaturated feature of VoIP traffic and the unbalance between downlink and uplink degrade the capacity for VoIP seriously. To address these issue, we propose a queueing system based analytical model to investigate the contention between unsaturated voice traffic and saturated data traffic. Moreover, our queueing analysis indicates that the bottleneck of increasing the capacity for VoIP is the downlink. Based on the capacity analysis, we propose an admission control mechanism to guarantee the Quality-of-Service of admitted VoIP sessions. Extensive simulation results are given to illuminate the efficiency of the proposed scheme.

Introducing mobile data sinks into wireless sensor networks (WSNs) improves the energy efficiency and the network lifetime, and is demanded for many application scenarios, such as battlefield vehicle security, mobile data acquisition, and cellular phone based sensor networks. However, highly mobile sink nodes cause frequent topology changes, resulting in high packet loss rate and poor energy efficiency of traditional reactive WSN routing algorithms. A directional-antenna-assisted reactive routing protocol for WSNs, IDDA (Interest Dissemination with Directional Antenna) is introduced to resolve this problem. Different from traditional interest diffusion routing protocols, IDDA exploits the antenna directivity to prearrange interest dissemination along the direction of motion. IDDA enhances important performance metrics in a target detection application scenario, namely, energy efficiency, packet delivery ratio, and target detection ratio. An analytical model is established to calculate the optimal width of the antenna beam pattern and optimal transmitting power. Extensive simulation results show that IDDA outperforms the traditional directed diffusion protocol in all three aforementioned metrics, which guarantees that IDDA can be applied to WSNs with highly mobile data sink nodes.