Hybrid Beamforming and Relay Selection for End-to-End SNR Maximization in Single-User Multi-Relay MIMO Systems  
[PERSON] 1,2 [PERSON] 1,2 [PERSON] 2,3 [PERSON] 1,2,* and [PERSON] 1,2  
1 Department of Information and Communication Engineering, Sejong University, Seoul 05006, Republic of  
Multi-hop communication is an essential technology which reduces the dependency on the network infrastructure and this paradigm makes it possible for multiple satellites or multiple un-manned aerial vehicles (UAVs) to communicate [7; 8]. In addition, multi-hop communication provides assistance in controlling the problem of deep fading over long-distance transmission, when mm-Wave frequency band or Terahertz frequency spectrum is employed for signal transmission [9; 10]. The notion of relay-assisted communication is incorporated in MIMO systems to enhance network coverage and to make long-distance transmission reliable by improving the link quality. Regenerative and non-regenerative operations are two main signal processing strategies at relay nodes. In regenerative schemes, the received signal is decoded first at each intermediate hop before transmitting to the next hop, while non-regenerative schemes forward the received signal after amplification. Low complexity and high security are the characteristics of non-regenerative techniques [11]. The authors of [12] proposed the beamforming algorithm for amplify-and-forward (AF) relay networks by using statistical channel state information at the transmitter. While the authors of [13] presented a distributed beamforming method which attempts to maximize the received SNR. This technique requires full instantaneous channel state information (SCI) at communicating nodes to determine power allocation. It is of great practical importance to investigate the performance of hybrid beamforming design for mm-Wave MIMO relay systems. In general, this paradigm increases the probability to have LOS communication between source and destination indirectly [14; 15]. Fully digital massive MIMO systems have tremendous advantages in providing high spectral efficiencies. However, equipping each antenna with a dedicated RF chain significantly increases the cost and power consumption of such systems. Hence, hybrid beamformers have attracted a lot of attention as a means to provide a trade-off between spectral and energy efficiencies, [16; 17]. In such systems, a small number of RF chains are connected to a large number of antennas via a network of phase shifters and/or switches. Fully digital precoding for mm-Wave massive MIMO systems is not feasible in actual practice as it requires one dedicated RF chain for each antenna element in an array [18]. The RF chain contains analog-to-digital/digital-to-analog converter (ADC/DAC), amplifiers, mixers, and low-noise amplifiers, and therefore it is quite expensive and power hungry as well. In turn, it increases hardware complexity, cost, and power consumption to the system [19]. To overcome this problem, the notion of hybrid beamforming was introduced to achieve a good compromise between complexity and performance, where full complexity precoding is decomposed into the analog component for beamforming gain and a low-dimensional digital baseband processing unit to obtain the multiplexing gain [18; 19; 20]. A network of phase shifters/switches is required for practical implementation of the analog RF processor as it follows an element-wise constant amplitude constraint. However, there is no restriction on digital baseband precoder/combiner except power constraint. Usually, two types of hybrid precoding architectures are commonly used: (1) fully-connected architecture in which each RF chain is connected to all antenna elements in an array, and (2) partially-connected architecture in which each RF chain is connected to a sub-set of antennas [21]. It is worthwhile to mention here that fully-connected architecture is one of the most popular implementations for hybrid beamforming due to its great potential in achieving performance close to the fully-digital counterpart.