On April 23, 2018, Guangxia Zhou successfully defended his Ph.D. thesis Rate-Splitting Methods for Interference Management in Cellular Wireless Systems. The Ph.D. examination committee was formed by Prof. Christian Becker (Institute of Electrical Power and Energy Technology) and the two reviewers Prof. Gerhard Bauch and Prof. Robert Schober (FAU Institute for Digital Communications).

Abstract

The explosively growing cellular data demand requires using radio frequency spectrum more efficiently. As illustrated in Figure 1, this potentially leads to harsh interference. Thus, it calls for methods that can mitigate the performance degradation introduced by interference.

Figure 1   A heterogeneous network using the same frequency

The state-of-the-art interference mitigation techniques are generally characterized into three categories, namely, treating interference as noise, orthogonal resource allocation and interference exploitation. In this thesis, a rate-splitting scheme is used as a framework to characterize an interference channel, and to identify in which regime a specific interference mitigation technique should be used.  Simulation results show that interference exploitation has a great potential for cell range expansion scenarios. Thus, this thesis focuses on designing a rate-splitting based interference exploitation.

In principle, the superiority of the original rate-splitting scheme is based on two assumptions: a fully joint decoding procedure and an optimal power allocation strategy. This leads to two key design aspects that are investigated in this thesis.

Firstly, fully joint decoding multi-user codes is technically infeasible at reasonable complexity, since the complexity is exponential in the product of the number of users and the code word length, in general. This thesis shows that conventional detection-decoding techniques used for a multi-access channel (MAC) cannot provide desirable results within reasonable complexity and thereby substantially reduce interference mitigation gains. Thus, a multi-layer rate-splitting scheme shown in Fig 2 is proposed to tackle this obstacle. Simulation results show that this scheme can achieve the same maximum sum rate as that achieved by joint decoding in relatively low complexity. In short, the multi-layer rate splitting scheme is recognized as a key component in developing a practical rate-splitting based interference exploitation technique.

Figure 2   Transmission structure for the multi-layer rate-splitting scheme

Secondly, the optimal power allocation strategy is found to be a major challenge for all rate-splitting based interference exploitation techniques. The original solution is to search all possible power allocations to get the largest achievable rate region. This thesis investigates the power allocation strategies for two cases: strong and weak interference channels, respectively. The results show that the multi-layer rate splitting with power splitting algorithms can achieve or closely approach the boundary of a Han-Kobayashi achievable rate region. Based on simulation results, the multi-layer rate-splitting scheme combined with the sum mutual information maximization with specified input constellations can be a good candidate to achieve the required performance in a general weak interference channel.