Linear Processing Techniques For Multi-Antenna Relaying Systems In The Presence Of Interference

In recent years, with the rapid development of different kinds of multimedia services, there is a soaring demand for higher transmission rate and larger system capacity. It is well known that mobile-network densification is the primary solution for the explosive growth of data traffic. However, it will inevitably lead to an interference-limited communication environment, which has become the bottleneck for the system performance. Since the existing LTE systems are designed to operate with a one-cell frequency reuse, co-channel interference caused by the cell-edge users at the neighboring cells becomes a serious problem deteriorating the quality of service (QoS) of cellular networks. Toward this end, one effective solution that has received wide acceptance is the deployment of wireless relays, which has been incorporated in wireless standards such as LTE-Advanced. Relays can significantly reduce the transmission path-loss between the cell-edge users and the base stations, thereby effectively reduce the required transmission power for the cell-edge users and considerably relieve the inter-cell interference problem. However, co-channel interfer-ence problem cannot be solved by simply adding several relays in the cellular networks. It requires more sophisticated system design. Currently, the main measure employed by LTE systems to deal with the inter-cell interference is inter-cell interference coordination (ICIC), the basic aim of which is to, if possible, avoid scheduling transmissions to/from terminals at the cell border simultaneous-ly in neighboring cells, thereby avoiding the worst-case interference situations. One of the main drawbacks of this strategy is that its performance is not so reliable and highly depends on to what extent the base stations can be coordinated. Furthermore, ICIC may not achieve the best system efficiency since it fail to take fully advantage of the resources. It should be noted that multi-antenna technique provides extra spatial degrees of freedom which can be efficiently utilized for interfer-ence cancellation. Such spatial domain operation can effectively release the precious time and frequency resources, and lead to a higher system efficiency. Motivated by this, this paper presents an explorative research on linear processing techniques for multi-antenna relaying systems in the presence of interference. Firstly, we consider a typical relay-assisted transmission scenario in LTE systems and model it as a general three-node network. We assume that the considered system subject to arbitrary num-ber of co-channel interference (CCI) and additive white Gaussian noise (AWGN). To combat the CCI we propose three heuristic two-stage relay precoding schemes, i.e., the relay first applies the1) maximal-ratio combining (MRC)2) zero-forcing (ZF)3) minimum mean-squared error (MMSE) principle to combine the signal from the source, and then steers the transformed signal towards the destination with the maximum ratio transmission (MRT) technique. We provide a comprehen-sive comparison among all three schemes in terms of outage and capacity performance. We also derive the achievable diversity order for all three schemes. Furthermore, we extend our analysis to large scale antennas scenario, and show that both the ZF/MRT and MMSE/MRT schemes are capable of completely eliminating the CCI, while perfect interference cancelation is not possible with the MRC/MRT scheme. Our results provide significant insights on the selection of proper linear processing schemes according to different situations in practice.Then, we consider a more flexible relaying system, where the relay is not required to be teth-ered to the grid for power supply, instead, it can collect energy from the ambient radio-frequency (RF) signal via a "power splitting" receiver. The effect of multiple antennas and CCI on the per-formance of the considered system is studied. In such system, CCI is a double-edge sword, while it corrupts the desired signal, it provides additional energy. The implementation of the multiple antenna enables us to utilize the signal processing techniques to exploit CCI as a favorable factor. To achieve this, firstly we study the outage probability and ergodic capacity of an interference free system, which serves as a benchmark for the performance in the presence of CCI, and then we ex-tend the analysis to an interfering environment, where we propose three different linear processing schemes, namely,1) MRC/MRT,2) ZF/MRT and3) MMSE/MRT. A comprehensive evaluation of all three schemes is presented. In addition, the optimal power splitting ratio minimizing the outage probability is characterized. Our results show that the implementation of multiple antennas increases the energy harvesting capability, hence, significantly improves the system’s performance. Moreover, it is demonstrated that the CCI could be potentially exploited to substantially boost the performance, while the choice of a linear processing scheme plays a critical role in determining how much gain could be extracted from the CCI.