intaining a desired quality-of-service (QoS). This is because, with large N, the relay can transmit focusing its energy in the direction of the receiver, thereby limiting any signal in the other directions where receivers are not located. In this manner, the SLI can be reduced to a significant level. Therefore, by using large N together with low transmit power, it results in lower SLI, since the SLI is proportional to the transmit power [23]. 4.8.1.4 Active Cancellation There also exist active cancellation (time-domain and spatial suppression) tech- niques [39]; however, it requires the knowledge of SLI. With massive MIMO, the SLI channel is of large dimension, and therefore it becomes quite complex to estimate the channel. 4.9 Multi-Pair Full-Duplex Relay: Mathematical Model Consider a system model, as shown in Fig. 4.7, where multiple pair of full-duplex users wants to exchange information with each other via a full-duplex AF relay [2, 24, 40, 41]. The relay consists of N number of transmit antennas and N number of receive antennas. Each user consists of one transmit and one receive antenna. The user on the one side of the relay communicates with user on the other side of the relay. There is no direct path between the users on the two sides of the relay. Relay and the users transmit and receive in the same time-frequency resource resulting in SI. IUI, due to the full-duplex users on same side of the relay. E. Sharma and P. Singh S23.3 S24,4 S21,1 S2K-1 S22K-1,2K- S22K,2K Transmit Receive Fig. 4.7 Full-duplex multi-pair relay system [2, 24, 32] 4.9.1 Channel Model The channel between the transmit antenna of the kth user and the relay receive antenna array is denoted by the vector gk E CNx1 Similarly the channels between the relay transmit antenna array and the receive antenna of the kth user are denoted by the vector We define G = HuD1/2 = [g1 g2K and F = H D / 2 f2K]. Here the matrices Hu and H account for small-scale fading, with i.i.d. CN(0, 1) elements. The diagonal matrices Du and Dd account for large-scale