Cooperative relaying in energy and spectrum efficient broadband wireless networks

Abstract

In recent years, wireless data traffic has grown rapidly. Unfortunately, there is a severe shortage of radio spectrum and interference management becomes a prominent and significant challenge. To achieve higher data rates, future wireless technologies will have to utilize and manage the resources more effectively and efficiently. Cooperative relaying has been recognized as one promising technology for future communications by both academia and industry. In this thesis, we shall investigate the benefits of cooperative relaying in achieving higher energy and spectrum efficiency in the single-user channel and multi-user interference channel (IC), respectively. For the single-user channel, we will focus on the multiple-input multiple-output (MIMO) relaying scheme. Relay transmission can deal with path-loss by replacing a long hop with multiple short hops, and MIMO transmission can combat small-scale fading with diversity gains, both of which are beneficial to energy efficiency. We will investigate two practical issues regarding MIMO relaying, namely, the optimal relay placement and the effect of relay antenna correlation. In relation to the first issue, we consider a decode-and-forward (DF) MIMO relaying channel, and minimize the outage probability by optimizing the relay position. It is shown that relay position optimization improves the diversity gain of a system whose adjacent hops have different diversity orders. The diversity improvement ensures a more reliable transmission, and as a result, the receiver can have a better quality of service (QoS) while the transmit power is greatly decreased. In relation to the second issue, we study the effect of the correlation among relaying antennas by comparing the MIMO relaying with distributed relaying. We prove that there exists a unique correlation threshold for linear antenna arrays, with which we can determine the operating regions for MIMO and distributed relaying schemes. Furthermore, we show that this correlation threshold is insensitive to the number of antennas and multiplexing gain, but quite responsive to the relay position. The understanding of these practical aspects shall provide useful insights for future relaying system design. As the wireless environment becomes more crowded, interference will become a major limitation in achieving a higher throughput. Coexistence of multiple users may enhance the spectrum efficiency of the whole network, but an effective interference management scheme is critical. In the second half of the thesis, we will investigate how cooperative relaying can be utilized to improve the performance of the multi-user IC, under rich and poorly scattered propagation environment, respectively. For the first case, we consider an amplify-and-forward (AF) dual-hop MIMO IC under the assumption that all channel matrices are with full rank. We derive the upper bound for the maximum achievable degrees of freedom (DoF) tuple via relay-aided MIMO interference alignment (IA). To check the tightness of the bound, we further propose an iterative IA algorithm for a given feasible DoF tuple. It is shown that the proposed algorithm can achieve the upper bound for the sum DoF in the low and high DoF regions, and can outperform the conventional single-hop MIMO IA. For the second case, i.e., under a poorly scattered propagation environment, the system will suffer from rank-deficient channel matrices. As opposed to the former case, the directions of the interfering signals observed by the receivers are limited by the column space of the channel matrices, which makes it more difficult to manage the interference. We characterize the exact DoF of a 2-user dual-hop MIMO IC with rank-deficient channel matrices. A relay-aided interference management scheme is proposed and is found to be DoF optimal. The result shows that the multiple relaying paths can reduce the negative effect of the channel rank-deficiency by enhancing the total rank of the equivalent end-to-end channel matrix between each pair of users. Under both rich and poorly scattered propagation environment, cooperative relaying shows clear advantage in managing interference and achieving higher throughput as compared to single-hop IC.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology