nd, at 5 GHz, as the 802.11a interface, and in the ISM (Industrial, Scientific and Medical) band, at 2.4 GHz, as the 802.11g interface. The 802.11n interface uses the 20 MHz radio channel, as for the 802.11a/g interfaces, and offers the possibility of aggregating two adjacent radio channels in the U-NII band at 5 GHz (Figure 4.9). Sub-band A Sub-band B Sub-band C Channel number 20 MHz 40 MHz USA and Japan only Figure 4.9. Frequency plan 4.3.3. Frequency multiplexing For the 20 MHz radio channel band, for HT modes, the complex numbers are numbered 0 to 51 and are subsequently mapped onto OFDM (Orthogonal Frequency-Division Multiplexing) sub-carriers, numbered -28 to -22, -20 to -8, -6 to -1, 1 to 6, 8 to 20 and 22 to 28 (Table 4.6). 802.11n Interface Sub-carriers -21, -7, 7 and 21 are ignored and subsequently used for the insertion of pilot sub-carriers. Parameters NON_HT HT 20 MHz HT 40 MHz NSD Number of sub-carriers assigned to the DATA field NSP Number of sub-carriers assigned to pilots NST Total number of sub-carriers AF Spacing between sub-carriers 0.3125 MHz (20 MHz/64) Table 4.6. OFDM multiplexing parameters For the 40 MHz radio channel band, the complex numbers are numbered 0 to 107 and are subsequently mapped onto OFDM sub-carriers numbered -57 to -54, -52 to -26, -24 to -12, -10 to -1, 1 to 10, 12 to 24, 26 to 52 and 54 to 57 (Table 4.6). Sub-carriers -53, -25, -11, 11, 25 and 53 are ignored and subsequently used for the insertion of pilot sub-carriers. Sub-carrier 0, associated with the central frequency, is omitted and filled with the value of ZERO. 4.3.4. Space multiplexing 4.3.4.1. MIMO mechanism The MIMO mechanism consists of simultaneously transmitting m signals and receiving them on n antennas, with m <n, using the same radio channel. Each receiving antenna receives the m transmitted signals, each signal being modified by the transfer function between the transmitting and receiving antennas. There is thus a transmission matrix H of size m X n (Figure. 4.10). The MIMO mechanism, by spatially multip