Unlocking Potentials of MIMO Full-duplex Radios for Heterogeneous Networks (UPFRONT)

Lead Research Organisation: University College London
Department Name: Electronic and Electrical Engineering


This project will develop multi-antenna full-duplex technology to achieve highly efficient spectrum usage in HetNets (heterogeneous networks). Full-duplex radios are much more than just doubling the capacity as perceived in current literature. More explicitly speaking, we believe that it is making communication technologies impossible become possible. The implications of full-duplex communications are transformative. Full duplex, which permits simultaneous transmission and reception, motivates a fundamental rethinking of the ways wireless networks are designed and optimised.

However, the fact that the power of the transmitting signal is so much larger (over 100 dB) than that of the receiving signal has fuelled the belief that it is impossible to separate them on a single channel. Recently, this picture has begun to change, following the pioneering work by Choi et al. from Stanford University who demonstrated a working full-duplex radio on a single channel.

We envisage that full-duplexing can transform the operations of wireless networks and is expected to have massive benefits in HetNets. HetNets are widely regarded as one key wireless technology for the provision of future wireless communications (including 5G) by complex interoperation between macrocells and small cells. The enormous interest for HetNets is due to their capability of providing high regional capacities and flexible coverage, and more importantly their low infrastructure costs. In HetNets, a mosaic of wireless coverage is obtained by a variety of wireless coverage zones from macrocell to small cell such as, pico- and femtocell. Of increasing interest to mobile operators are the customers' installed femtocells that can greatly improve indoor coverage but share the same frequency band as the macrocells. There is a huge scope of research in resource allocation and physical-layer design and optimisation in HetNets.

This project will exploit the full potential of MIMO full-duplexing in HetNets by designing a holistic solution that interconnects antenna design, physical-layer signal processing, and network resource allocation to address the inherent challenges of full-duplexing and realise its massive end-to-end benefits. To achieve this goal, UPFRONT first proposes new antenna design specially for wideband MIMO full-duplexing, which is substantially more challenging than the existing narrowband single-antenna case. Next we leverage the powerful MIMO signal processing to handle the overlooked in-tercell interference and new interference introduced by the full-duplex operation, which are critical to deliver the end-to-end benefits of full-duplex HetNets but were not well studied before. Furthermore, UPFRONT will explore the unexplored full-duplexing opportunities to address the networking-wide resource allocation challenges associated with the adoption of full-duplexing small cells under the greater macrocell structure sharing the same mobile spectrum. The outcomes of UPFRONT will elucidate the importance of a holistic approach to full-duplexing design and have impact in fundamental and practical research of future wireless networks.

Planned Impact

UPFRONT proposes a transformative step towards highly efficient HetNet by exploiting the hidden potential of full duplex radios and MIMO antennas. The Real Wireless report to Ofcom in 2012 predicted that there is a spectrum crunch heading for the UK by 2020, unless around 300MHz more cellular and 350MHz more Wi-Fi spectrums are made available. Creating additional bandwidth out of the existing one using full-duplexing technology is therefore of great importance. In addition, indoor wireless coverage in the UK has been disappointing, and the size of the problem is significant. A recent consumer survey for the Small Cell Forum showed that 22% experienced poor voice quality at least 1-3 times a month. A further survey amongst business users supports the problem being worse in business premises with as much as 39% of UK businesses reported to suffer from poor in-building mobile services today. It was reported that 7.5 million dedicated in-building wireless solutions might be needed across the UK to provide just a baseline 2Mbps service to all indoor users.

UPFRONT tackles this problem by devising technical solutions to improve the spectral efficiency and resolve the critical interference problem in HetNets. The vision is that full-duplex HetNets will become the dominant in-building solution for future-generation wireless communications. On the other hand, full-duplexing is being considered seriously as a key enabler for the future Wi-Fi standard 802.11ax to achieve four times throughput of the current 802.11ac standard. In addition, there is a huge potential for full-duplexing in the network architecture of C-RAN (cloud-random access network) and also mm-wave communications.

UPFRONT's industry partners are highly interested in the project and are able to exploit the project results. For InterDigital, a world-leading wireless R&D company, it has strong interests in exploring full-duplexing solutions for more efficient broadband networks years ahead of market deployment. It will support UPFRONT by bringing the expertise in cellular networks, W-LAN (IEEE 802.11ac/ax) and mm-wave (together with NYU Wireless, at New York University). China Mobile as the world's largest mobile network operator, is committed to R&D on key technologies for 5G, and it sees full-duplex technology the next game changer that can help alleviate the spectrum crunch. They are keen on technologies for enhancing self-interference cancellation, full-duplex MIMO, full-duplex networking technologies and prototype platform. In addition, China Mobile will generously offer access to their newly built 5G in-field evaluation system in Tianjin, China. This will allow our 3 RAs to pay a 2-week visit to carry out field tests of the proposed techniques in this project, and it also provides them invaluable training opportunities to gain hands-on experience of the real-world wireless systems.

All partners will take an active role in the definition of use case scenarios and exploitation paths. Our industry partners will particularly help maximise impact on the telecommunications sector in the UK, a sector which includes large numbers of smaller businesses and emerging industries. UPFRONT will also provide a firm foundation for the consortium to leverage further funds via EU HORIZON 2020. This will enable full system development, bringing in EU companies and broadening the research to include other application areas and management specialists.

In summary, UPFRONT is timely because the explored full-duplex technology could provide a solution to spectrum crunch that concerns the whole wireless industry, improve the in-door user experience, and open up abundant opportunities to redesign future-generation wireless systems.
Description Existing cellular transmission technologies and standards primarily rely on the radio spectrum below 5 GHz. Moreover, the downlink (transmitting base station to receiving user) and uplink (transmitting user to receiving base station) functionalities are typically separated in time or frequency - i.e., half-duplex mode - in order to avoid the self-interference and cross downlink-uplink interference.

Due to the rapid increase in traffic demands under limited amount of bandwidth, the simultaneous downlink and uplink operation over the same radio frequency resources - i.e., full-duplex mode - has become a potential solution for further improving the spectral efficiency. In this project, we have investigated the fundamental performance trends of full-duplex cellular networks.

Our main findings can be summarized as follows:

a. We have devised a unified mathematical framework using tools from information theory for the design, modeling, and analysis of full-duplex cellular networks. The proposed model can be readily used to calculate the spectral efficiency for various cellular network configurations. In particular, to fully account for the self-interference phenomena, we have derived the distribution of the bi-directional channel power gain over multi-user multiple-input multiple-output (MU-MIMO) Rician fading channels with arbitrary statistics. The generalized closed-form expression of the self-interference distribution allows for performance analysis under arbitrary self-interference cancellation capability. The validity of the theoretical findings is confirmed through extensive Monte-Carlo simulations.

b. We illustrate that the performance of full-duplex versus half-duplex cellular networks is severely limited particularly in the uplink direction due to the overwhelming interference when using conventional transmissions technologies such as baseline single-input single-output (BL-SISO) and MU-MIMO. Hence, we propose the use of massive MIMO technology for improved resilience against self-interference and cross downlink-uplink interference. We show that with massive MIMO-enabled full-duplex technology, spectral efficiency improvements in the order of hundred can be achieved compared to full-duplex BL-SISO. On the other hand, we show that the performance gain of full-duplex versus half-duplex massive MIMO cellular networks largely depends on the self-interference cancellation capability at the user side. Further, the anticipated two-fold increase in throughout is only achievable as the number of antennas tends to be asymptotically large.
Exploitation Route The proposed analytical framework can serve as a benchmark tool for the performance analysis and optimization of generalized full-duplex wireless systems. This makes it possible for interested parties to further investigate solutions and scenarios where the use of full-duplex technology is beneficial. Some promising aspects that can be immediately studied using the proposed framework include energy efficiency and secrecy performance of full-duplex cellular networks.
Sectors Digital/Communication/Information Technologies (including Software)