Beyond MIMO

Beyond MIMO

Written by John Haine, on 1 Jul 2018

This article is from the CW Journal archive.

Massive MIMO is a defining technology for 5G but the varying needs of applications and subscribers need a rethink of MIMO technologies. Prof John Haine reports on a CW radio SIG event which explored some of the options.

Almost since THE beginning of electrical communications engineers have invented new ways to increase the capacity of links. Great ingenuity went into finding ways to share telegraph lines between multiple simultaneous telegraphists in both directions. When radio and thermionic valves arrived, multiple frequencies (they called them wavelengths then!) started to be used to avoid neighbouring transmitters interfering with each other; and two-way radio systems also used "push to talk" as a primitive way to share channels in time. Multiple access to shared radio channels became really important when cellular systems arrived, to allow operators to maximise the number of users they could serve with their initially meagre allocations of spectrum. The key point about radio signals, that they propagate, also leads to neighbouring systems interfering, and multiple access schemes essentially try to use the shared radio channel to serve the maximum number of users by separating them in frequency, time, and space. Even after more than a century of development, new ways are being found to do this more efficiently and on 14th December last year a Radio SIG event at Girton College explored the most recent developments.

MIMO started out as a way to transmit higher data rates over radio systems using multiple antennas at each end of the link. Now widely used in LTE, the next innovation is "Massive MIMO" (M-MIMO), where a large number of antennas (maybe more than 100) are used at the base station to serve many terminals, each with only one or a few, with complex signal processing behind each base station antenna. The Communications Systems and Networks research group at Bristol University is a world leader in the technology and Prof. Mark Beach presented their work, which uses programmable test equipment from National Instruments at each end of the link. The system has achieved record-breaking spectrum efficiency, with nearly 3Gbit/s being transmitted to 22 terminals indoors using only a 20MHz channel at 3.5GHz. Excellent performance has also been demonstrated outdoors with pedestrian and vehicular mobility, in collaborations with Lund University (Sweden) and British Telecom Research at Martlesham. Another interesting feature of M-MIMO is the channel "hardening" against impairments such as multipath which can greatly improve reliability, as required in some critical 5G applications. Though there are many implementation issues to be resolved, such as how to fit all the base station antennas to masts, how to do radio planning, and how to integrate M-MIMO into future software-defined networks, the technique is a good candidate to be part of 5G.

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Needs drive technology choices

In the days when radio networks carried mainly voice traffic, managing radio resources was comparatively easy. Today and in future though, whatever multiple-access technique is used, the traffic is a complex mix of real-time voice and perhaps video, non-time critical data, and possibly data needing latency and delivery guarantees for example for autonomous vehicles or critical control applications. The "Radio Resource Manager" (RRM) now has to take account not just of the radio channel conditions for every potential user but also their varying communications needs; and perhaps the availability of multiple networks such as LTE and Wi-Fi or other system in unlicensed spectrum. Mike Fitch of BT drawing on results from the EU H2020 project Speed-5G presented a new approach to RRM and showed simulation results for the quality of experience for different user classes with different types of traffic. Such techniques will be essential in future 5G systems which will not just use dedicated spectrum but dynamically share other available networks.

Wallace Kuo on behalf of Alain Mourad of InterDigital presented another spin on how multiple radio technologies could be adapted, contrasting the new approach being taken in the EU-Taiwan 5G-CORAL project. This combines many disparate radio access networks with "edge" and "fog" computing – i.e. the availability of virtualisable distributed storage and processing resource located close to the network nodes. Again the objective is to make the best use of all the available radio resources that could serve a terminal according to its position and traffic requirements.

Much of the development of these new multiple access systems is aimed at optimising broadband communications for the kind of "smartphone" use cases that have become important since the launch of 3G and now dominate mobile traffic. The same systems are generally far from optimum for typical IoT use cases. Juan Nogueira of Flex much simpler systems optimised for IoT, with limited bandwidth and focused on devices in difficult locations that have to operate for long periods using primary batteries. The presentation ranged from the simplest such as Sigfox with very limited messaging capacity and virtually no security; through LoRa which has greater capacity and in-built encryption; and ended by giving an overview of the developments in 3GPP cellular technology derived from LTE, especially NB-IoT, which promise to optimally address the use cases. This emphasised that any long-term solutions to the radio access problem must consider the needs of the IoT which can be very different from mobile broadband – may require different solutions to be deployed.

Too much is a good thing

Most of the more recent developments in multiple access have sought to make the signals being transmitted "orthogonal" in some sense – the ultimate expression of this being the OFDMA used in LTE. An alternative approach is to "overload" the channel by intentionally introducing controlled interference. This has been used for a long time, for example in the GMSK modulation used in GSM, and in the way that co-channel interference is managed in cellular. Konstantinos Nikitopoulos of the 5G Innovation Centre at Surrey University presented techniques where this is taken to extremes and the interference is in effect cancelled by complex maximum-likelihood signal processing which is becoming possible even in terminals using massively parallel processors. Up to 45% increase in throughput and 100x lower latency compared to conventional "linear" methods has been shown to be achievable. This increases the capacity of the channel; and new techniques for signalling have been developed based on the methods to give much more efficient sharing of the bandwidth between multiple users. Such methods depend on very complex signal processing in both the base station and terminal, and perhaps it remains to be seen if the progress in VLSI will allow this to be applied particularly in the terminal where power consumption is critical.

What was clear from the presentations is that there is a way to go in using the radio channel to its best effect to accommodate the huge growth foreseen in data demand. In the future we will see perhaps significant developments at the physical layer, using techniques such as non-orthogonal modulation and massive MIMO; combined with system approaches that enable a terminal to access a wide range of networks, and optimally combining signals to match the user's demands from moment to moment. This poses huge challenges to the radio design in the terminal, which is already a major problem. And can a commercial and regulatory framework be found that will not inhibit approaches that best serve users' needs? These are aspects that perhaps should be explored in future events.

John Haine
Visiting Professor - University of Bristol (Communication Systems & Networks Research Group)

John Haine has spent his career in the electronics and communications industry, working for large corporations and with four Cambridge start-ups. His technical background includes R&D in radio circuitry and microwave circuit theory; and the design of novel radio systems for cordless telephony, mobile data, fixed wireless access and IoT communications. He has led standardisation activities in mobile data and FWA in ETSI, and contributed to WiMax in IEEE. At various times he has been involved in and led fund-raising and M&A activities. In 1999 he joined TTP Communications working on research, technology strategy and M&A; and after the company’s acquisition by Motorola became Director of Technology Strategy in Motorola Mobile Devices. After leaving Motorola he was CTO Enterprise Systems with ip.access, a manufacturer of GSM picocells and 3G femtocells. In early 2010 he joined Cognovo, which was acquired by u-blox AG in 2012. He led u-blox' involvement in 3GPP NB-IoT standardisation and the company's initial development of the first modules for trials and demonstrations. Now retired from u-blox he is an Honorary Professor in Electronic and Electrical Engineering at Bristol University, where he chairs the SWAN Prosperity Partnership Project external advisory board . He was founder chair and is Board Member Emeritus of the IoT Security Foundation. He served on the CW Board chaired the Editorial Board of the CW Journal.  John has a first degree from Birmingham (1971) and a PhD from Leeds (1977) universities, and is a Life Member of the IEEE.

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