Analysis and Design for Cell-Free Massive MIMO

Demand for wireless traffic (data traffic and the number of connected devices) has grown exponentially. With the dramatic growth of wireless devices (such as smart-phones, tablets, and laptops) and services/applications (such as real-time video calls, high-quality movies, real-time 3D games, and Internet of Things), the wireless traffic demand will continue to gain momentum. According to Cisco, global mobile data traffic will reach 43 Exabyte per month by 2021, which is about a 7-fold increase over 2016, and a 3-fold increase over 2018. By 2021, there will be 11.6 billion mobile-connected devices, which corresponds to 1.5 devices per capita. In addition, there is growing concern about green communications (low energy radiation/consumption), security and privacy. To summarize, future wireless networks have to satisfy the following requirements: (1) manage at the same time many (thousands or even millions) devices; (2) each needs a high throughput (spectral efficiency) to support many services/applications; (3) high energy efficiency; and (4) high-level security services.

Mobile network is one of the core networks in wireless communications. In conventional mobile networks, a coverage area is divided into cells. Each cell is served by one base station. We have seen many revolutions related to the cellular wireless network since its first trial was introduced in the 1970s. One of the inherent limitations of cellular networks is inter-cell interference which is the interference from other users in other cells. In particular, users at the cell boundaries perform badly due to the strong inter-cell interference which limits the performance of the whole networks. To reduce this bottleneck, network multiple-input multiple-output (MIMO) was introduced in the 2000s. In network MIMO, the base stations cooperate to serve the users. More precisely, the coverage area is divided into several disjoint clusters, each is jointly served by several base stations. Basically, network MIMO is equivalent to the cellular network with larger cell size and each cell is served by multiple base stations. There are still concepts of cells and cell boundaries, and hence, the inter-cell interference persists. In addition, network MIMO requires complicated signal co-processing with high backhaul overhead and deployment costs to reduce the effect of inter-cell interference.

In this project, we are targeting a new practical, useful, and scalable version of network MIMO called "cell-free massive MIMO". In cell-free Massive MIMO, a number of access points, which are distributed at random in a very wide area (e.g. over an entire city), serve simultaneously many users randomly distributed in the same area. There are no concepts of cells or cell boundaries. Cell-free massive MIMO reaps all benefits obtained from massive MIMO and network MIMO, and hence, it is expected to offer many advantages compared with the conventional wireless systems: 1) huge data throughput; 2) huge energy efficiency; 3) and high coverage probability. Thus, cell-free Massive MIMO is a disruptive technology for next generations of densified wireless systems. Since cell-free massive MIMO a very new research topic, a number of issues and questions still need to be tackled before rolling-out it into practice. The main objective of this project is to develop a complete, useful, and practical cell-free massive MIMO system which includes signal processing schemes, channel estimation, pilot assignment schemes, power controls, and AP selection schemes.

The application of the project results will contribute to the reduction of the ICT sector's contribution to global warming, through reduced power consumption and improved energy efficiency. It will also influence many dynamic economic sectors within the UK: telecom equipment manufacturing, telecom operators, smart cities, e-health, surveillance sector, military equipment and automotive companies.

This project develops novel, low-cost, and eco-friendly solutions for future wireless systems. The project will influence broad communities and facilitate the development of vibrant businesses around the proposed technology. The following list summarises potential impacts:

1) The new knowledge will be timely for standards bodies (IEEE, ITU-IMT 2020), government agencies (Ofcom), network operators (Vodafone-UK, O2-UK), and hardware manufacturers (BAE, Thales) which are currently investing an extensive amount of resources into the research of 5G and beyond wireless systems, in order to deliver high and ubiquitous data rates to millions/billions of users. It is expected that the outcomes of the project will help these authorities to reap the theoretical benefits of deploying cell-free massive MIMO to serve the UK citizens.

2) Cell-free massive MIMO is very promising research topic and is expected to meet the demands of future wireless networks. Undoubtedly, the project has huge potential to boost research in this field, attract industrial partners, suggest ideas which will lead to research patents, and also develop solutions that will be incorporated in future communication standards. Our research outcomes will speed up the realisation of 5G and beyond networks, which are expected to enable new applications in various domains and environments ranging from entertainment to health, transport and industry, stimulating economic development in many sectors of the economy. Thus, our research results will broaden the UK competitiveness and turn the opportunities in upcoming market segments into new products and employment growth.

3) Cell-free massive MIMO can serve simultaneously thousands/millions of users in urban environments with aggregate data rates 1000x faster compared to current 4G networks. This new technology can be applied in the future networked UK society including smart cities, e-healthcare, smart vehicles, and energy management. Consequently, the results of the project will create new jobs, start-up companies, thereby creating wealth and better services for the UK population, once fully commercially exploited. In addition, the UK's ICT market is the most dynamic in the EU, characterised by a high percentage of early adopters of innovative technologies, open and transparent market, and importantly, excellent engineering skills. This project, while helps maintain the UK's research excellence, will contribute to building up the next generations of highly skilled researchers as a step towards establishing world-class research systems.

4. We recall that the overall ICT footprint has represented about 2 percent of global CO2 emissions in 2007. This percentage is predicted to increase to more than 4% in 2020, which is more than one-third of the present annual emissions of whole of United Kingdom. Furthermore, there is an increasing concern about green communications, which connect to the effects of the radiation emitted from wireless devices on the human body. Different from the conventional cellular systems where the base station sends out the information to a wide area, in cell-free massive MIMO, all access points work together to send out concentrated beams of information to the desired users at desired locations. In addition, cell-free massive MIMO can be implemented using rather simple components with low cost and footprint. As a result, cell-free massive MIMO offers huge energy efficiency with low carbon footprint. The proposed research will contribute substantially to the reduction of the ICT sector's contribution to global warming. Note that ICT is accountable for more than 10% of the UK electricity consumption. Furthermore, the radiation emitted from wireless devices on the human body will be reduced significantly.