MIMO. One of the main technologies to account for the rapid growth of data traffic is introduction of multiple-input multiple-output (MIMO) systems. The concept of MIMO has emerged in the late 90s and was introduced to WiFi systems about 2006 [20] [21] and to the cellular networks shortly after [22]. In MIMO technology, BSs are equipped with multiple antennas and therefore, are able to communicate with multiple UEs simultaneously [23]. Unlike the traditional model where BSs served users by orthogonalising the channel, MIMO enables them to achieve higher rates and therefore increasing SE by serving more users in the same time and fre- quency resource [24]. Therefore, the more antennas a BS has, the more degrees of freedom it can provide and consequently a higher SE will be achieved [25]. Multiple works have investigated PHYLS in MIMO wiretap including [26–28] where the transmitter, receiver and EDs are equipped with more than one anten- nas. Moreover, beamforming with artificial noise (AN) in MIMO was addressed in [29] to compensate for imperfect channel state information (CSI). In [30] the af- fect of power allocation between the transmitted information and AN on PHYLS for multiple-input single-output (MISO) communication has been analysed. In [31] a regularized channel inversion precoding for a network with multiple-antenna BS and single-antenna UE system is proposed to maximise secrecy rate. In addi- tion, [32] has looked into the importance of fading channels on secrecy performance from an information theoretic point of view where average secrecy rate and secrecy outage probability is introduced as performance metrics. Following this, [33] has investigated the secrecy outage probability in a single-input multiple-output (SIMO) with maximal ratio combining in a Rayleigh fading environment.
MIMO. MIMO technology improves the channel capacity and the performance of communication system using multiple antenna without additional frequency assignment or transmission power. MIMO technology is divided by STC(Space Time Coding) and SM(Spatial Multiplexing) that these two methods are related on trade off. It is determined method Figure 4. 2x2 SM block diagram As shown in Figure 4, SM can increase data rate, as independent date symbols are transmitted at each transmit antenna. Signals received each antenna consist of the sum of transmitted data. which is chosen, according to communication environment ⎛ y1 ⎞ ⎛ h11 h12 ⎞⎛ s1 ⎞ ⎛ n1 ⎞ and user's demands. ⎜ ⎟ = ⎜ y h ⎟⎜ ⎟ + ⎜ h s n ⎟ = H ⋅ s + n (3) ⎝ 2 ⎠ ⎝ 21 22 ⎠⎝ 2 ⎠ ⎝ 2 ⎠ Various methods are investigated in order to detect each transmit signals in vector of receiving signals. Typical MIMO detection methods are ML(Maximum Likelihood), ZF(Zero Forcing) and MMSE(Minimum Mean Square Error) , etc.. SM can not get enough gain when multiple antenna is used by only one side, receiver or transmitter. Multiplexing gain is the same as minimum value of the number of transmitter / receiver antenna.
