However, the realization of MIMO technique is limited over the uplinks transmission because of high cost and the user equipment size. Hence, to overcome this MIMO limitation, the virtual-MIMO (V-MIMO) is presented. The V-MIMO system has been used in various systems such as ultra-wide band (UWB) by Srivastava & Mohan (2016) and wireless networks by Li et al. (2016). V-MIMO is constructed by a number of individual user equipment (UE), where each UE has a single transmitter antenna. Then, the data stream is being transmitted independently either on the same subcarrier or sub-channel. In the uplink transmission, the V-MIMO can be constructed by ensuring the number of receiver antennas should be at least the same as the number of transmitting antennas. The user pairing is one of the important processes in V-MIMO system, in which different users with single-antennas are selected to constitute a V-MIMO. By implementing user pairing, the multi-user diversity can be achieved. However, when more antennas are presented either at transmitter or receiver, less multi-user diversity will occur. Hence, it is essential to have a scheduling criteria when performing user pairing in V-MIMO such as random pairing scheduler (RPS), proportional fair (PF) or round robin (RR) by Chen et al. (2008). The general outline of the V-MIMO is presented in Figure 2.2.

Figure 2. 2: General outline of V-MIMO system

2.1 User Grouping or User Pairing

User pairing approach has been done by Chao et al. (2007). The Adjustable Determinant Pairing Scheduling (ADPS) is proposed as scheduler criterion when performing the user paring. The proposed scheme is developed to overcome the weakness of traditional algorithm for user paring. ADPS considers fast fading and slow fading, but the traditional algorithm only considers slow fading. Hence, the maximum throughput is difficult to achieve. The authors consider both slow fading and fast fading in their research work. The analysis has been made to compare the proposed algorithm with the random pairing scheduling (RPS) and determinant pairing scheduling (DPS). The random pair scheduling (RPS) has been used in V-MIMO system for pairing user into a group. Basically, the first user is selected by using the pairing algorithm such as round-robin (RR). For the second user selection, this method will select the second users randomly from the remaining set of users. This algorithm can perform the user pairing with lower computational complex