lexing m signals, makes it possible to increase the rate of the radio channel with the same factor. The MIMO mechanism uses the transmission matrix H to perform spatial demultiplexing. Wi-Fi Integration to the 4G Mobile Network For the SU (Single User) MIMO mechanism, the m transmitted signals are destined for the same user. Figure 4.10. MIMO mechanism 4.3.4.2. STBC mechanism When the number of transmitters (m) is greater than the number of receivers (n), the additional transmitters are used to effect transmit diversity, thereby improving the quality of the received signal by protecting the transmission from fading. The STBC mechanism performs space and time diversity. The signal S corresponding to a spatial flux is divided into two parts, S1 and S2. The complex numbers of the S (= S1 + S2) constellation of Nss spatial streams (Nss = 1) are distributed over NSTS space-time flows (NSTS = 2) (Figure 4.11). Frequency Space * conjugate complex S = S1+S2 Process diversity diversity Frequency Figure 4.11. STBC mechanism 802.11n Interface 4.3.4.3. Beamforming Beamforming allows a transmitter, called a beamformer, to focus the energy of several sources of the same signal in the direction of the receiver, called a beamformee. Phase reception increases the signal-to-noise ratio of the received signal (Figure 4.12). The two received S signals are out of phase Signal S Process Result The two received S signals are in phase Figure 4.12. Beamforming mechanism For the explicit beamforming, a device measures the radio channel and uses this measurement to directly calculate the direction matrix. The active channel measurement is performed by transmitting a sounding to the beamformee, which responds with a frame that indicates how the sounding was received. For implicit beamforming, frames such as ACK control frames or data transmitted on pilot channels can be used to estimate the direction matrix. 4.3.5. Modulation and coding scheme The value of the modulation and coding scheme (MCS) determines the rate value from the following p