Yifei Yuan

Non-orthogonal multiple access for 5G: solutions, challenges, opportunities, and future research trends

The increasing demand of mobile Internet and the Internet of Things poses challenging requirements for 5G wireless communications, such as high spectral efficiency and massive connectivity. In this article, a promising technology, non-orthogonal multiple access (NOMA), is discussed, which can address some of these challenges for 5G. Different from conventional orthogonal multiple access technologies, NOMA can accommodate much more users via nonorthogonal resource allocation. We divide existing dominant NOMA schemes into two categories: power-domain multiplexing and code-domain multiplexing, and the corresponding schemes include power-domain NOMA, multiple access with low-density spreading, sparse code multiple access, multi-user shared access, pattern division multiple access, and so on. We discuss their principles, key features, and pros/cons, and then provide a comprehensive comparison of these solutions from the perspective of spectral efficiency, system performance, receiver complexity, and so on. In addition, challenges, opportunities, and future research trends for NOMA design are highlighted to provide some insight on the potential future work for researchers in this field. Finally, to leverage different multiple access schemes including both conventional OMA and new NOMA, we propose the concept of software defined multiple access (SoDeMA), which enables adaptive configuration of available multiple access schemes to support diverse services and applications in future 5G networks.

IMT-advanced relay standards [WiMAX/LTE Update]

There are two candidates for IMT-Advanced (4 G) standards, LTE-Advanced by 3 GPP and 802.16 m by IEEE. This article focuses on relay architectures in 16 m and LTE-A, and discusses design principles and trade-offs leading to decisions in each standards group. Basically, 16 m relay and LTE-A Release 10 relay are very similar technologies where the relay is essentially an orthogonal frequency-division multiple access base station with a wireless backhaul link. However, some open issues, such as mobility, power saving, multihop architecture, transparent relaying, multi-arrier transmission, and cooperative transmission, are still left as challenges for engineers and researchers. This article provides insights to both relay standards that could be helpful for readers to fully comprehend practical ways of incorporating relays into 4 G wireless broadband networks.

Mobile relay in LTE-advanced systems

Voice and data communications on high speed vehicles encounter bad channel condition, high call drop rate, serious signaling congestion and excessive power consumption of UE. Mobile relay technology which features on-board relay node is expected to improve the quality of service for passengers. However, the design for fixed relay in LTE-Advanced system cannot meet the requirements of mobile relay. In this article, the architecture for mobile relay is presented. The key techniques of supporting mobile relay are investigated, such as the group mobility, the local service support, the multi-RAT and RAN sharing, with the corresponding solutions. The potential system optimization, for example, the self-optimization network, power saving, measurement and system information acquisition are also presented where the efficiency of mobile relay can further be improved. Simulation and numerical results demonstrate the feasibility of the mobile relay.

Non-orthogonal transmission technology in LTE evolution

Non-orthogonal transmission, although not entirely new to the wireless industry, is gaining more attention due to its promised throughput gain and unique capability to support a large number of simultaneous transmissions within limited resources. In this article, several key techniques for non-orthogonal transmission are discussed. The downlink technique is featured by MUST, which is being specified in 3GPP for mobile broadband services. In the uplink, grantfree schemes such as multi-user shared access and sparse code multiple access, are promising in supporting massive machine-type communication services. The multi-antenna aspect is also addressed in the context of MUST, showing that MIMO technology and non-orthogonal transmission can be used jointly to provide combined gain.

LTE-Advanced coverage enhancements

Various technologies in LTE/LTE-Advanced have significantly improved the data throughput of 4G systems. However, the coverage of LTE networks, a very important performance metric to operators, receives relatively less attention. In this article we describe the motivations of LTE coverage enhancements from several aspects. Through link budget analysis, the limiting link and channels are identified. Then potential solutions to LTE coverage enhancements are discussed in a comprehensive manner, with the focus on schemes of specification impact. This article provides insights on how to design practical schemes to improve the coverage of LTE/ LTE-Advanced systems.

Experimental investigation on a vertical sectorization system with active antenna

Cell splitting is an effective way of implementing network densification in order to boost the capacity of a mobile system. Traditionally, the cell splitting procedure has been carried out in the azimuth dimension. However, another approach known as vertical sectorization has been gaining momentum recently, where the sectors of a conventional cellular system are split vertically instead, creating inner and outer cells by using AAS technology. Then vertical sectorization can use the same communication resources in both inner and outer parts of the cells. Performance of vertical sectorization has been extensively studied in the literature using advanced system simulations. However, field measurements to characterize the actual performance of vertical sectorization with AAS have been scarce so far. In this article, we first identify key issues that may hinder the smooth implementation of commercial cellular systems using vertical sectorization. Then we present the results of a measurement campaign that was carried out to study these limitations and to identify the actual vertical radiation patterns when AAS technology is used. Finally, we reflect on some problems that remain open to make future cellular systems using vertical sectorization with AAS commercially viable.