| Recently, the cooperative communication has been a hot topic, which represents animportant direction in the future wireless communications. In this thesis, the main focusis on dealing with the interference problems that the cooperative communication isconfronted with. In contrast to the conventional way to passively deal with interferencewith frequency reuse or power control, the emerging view on the network system designconsiders how to treat the interference more proactively. The inter-relay interference(IRI) problem and interference alignment (IA) are mainly studied in the thesis, somenovel interference mitigation methods are proposed and the performance of theproposed algorithms is analyzed.Firstly, relay technique plays an important role in the cooperative communication,which is not only able to expand the communication coverage but also brings thediversity gain that can decrease the bit error rate (BER) and improve the reliability, butthe half-duplex relay halves the system spectrum efficiency. The two-path cooperativesystem does not change the spectrum efficiency, however, it suffers the IRI and how tosuppress IRI is one of the important topics. Firstly, the existing approaches such aspartial interference cancellation, self interference cancellation and full interferencecancellation are clearly analyzed in the thesis, but they all confront two non-negligibleissues. One is the time synchronization between the two relay nodes in the two-pathrelay system, i.e. when one relay is transmitting the signal to the destination, the signalshould be completely received by the other relay at the same time. The other issue isthat the perfect channel state information (CSI) for all the links in the system should beacquired at the relay or destination. A new IRI mitigation method is proposed by theusage of the autocorrelation matching theory. The proposed method is not onlyinsensitive to the synchronization but also reduces the burden of the destination, and itsperformance does not rely on the whole CSI of all links in the system that should beknown at the destination.IA is a new way of interference coordination to treat interference. From the angle of the information theory, IA can significantly improve the system’s degree of freedom(DoF), so it is an efficient way to improve the spectrum efficiency. The essence of IA isto make all the interference directions fall into a certain interference subspace, then, itsnull subspace can be utilized to transmit the source signals. Based on the conclusion ofthe current research development of IA, the constraints of IA achievement in MIMOcellular system are deeply discussed. Given the different conditions including the localCSI, system CSI and opportunistic communications, the different IA methods areanalyzed, moreover, an opportunistic interference alignment method is proposed foruplink in MIMO cellular networks and the theoretical results are derived in the threecells system. The theoretical results show two properties: When OIA is implemented inthe uplink condition, the value of the interference leakage is determined by the numberof cooperation cells, users in each cell and transmit antennas of each user, but it isunrelated to the number of antennas of the base stations which only needs to satisfy theZF condition. Since the significant effect of the interference power is considered in IAsystem, when the interference power is high enough, the diversity gain of the uplinkOIA scheme in MIMO cellular networks is approximately1(unity), i.e. user selectionand beamforming that are implemented by the proposed method can not bringadditional diversity gain.In addition, the majority of the existing papers about IA only give the IA schemeand the achievable DoF. However, the novel uplink opportunistic IA approach isproposed in the thesis, moreover, the outage probability and diversity-multiplexingtradeoff of the proposed method is derived as well. |
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