Analysis and Design for Cell-Free Massive MIMO

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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.