Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 50

When the transmitter has no channel information, multiple antennas can be used at the transmitter and receiver ends to achieve diversity and increase the system capacity. This can be realized by spreading the symbols over the antenna (space) and time using the so-called space–time coding. In the following, the space–time block code will be briefly introduced.

Similar to MISO system, two symbols c₁ and c₂ are simultaneously transmitted from antenna 1 and antenna 2 in the first symbol period, and the symbols – and are transmitted from antenna 1 and antenna 2 in the next symbol period.

Assuming that the flat fading channel remains unchanged in the two consecutive symbol periods, the 2 × 2 channel matrix can be expressed as

It is worth noting that the subscripts here represent the receive and transmit antenna labels instead of the symbol periods. The signal vector received by the receiving array in the first symbol period is

The signal vector received in the second symbol period is

where n₁ and n₂ are additive noise components per symbol period of the receive antenna array (the subscripts represent symbol periods instead of antenna labels). Therefore, the receiver produces a mixed signal vector

Similar to the MISO system, two symbols c₁ and c₂ are transmitted during two symbol periods of two transmit antennas. Therefore, matrix H_eff is orthogonal to all channel information, namely .

If , then

where n′ satisfies E{n′} = 0_2×1 and . The above equation shows that the transmission of the symbols c₁ and c₂ is completely decoupled, which means

The average output SNR is

The Alamouti algorithm of the 2 × 2 structure obtains the receive array gain (g_a = M_R = 2) but does not get the transmit array gain (because there is no channel information). The above method can get the full diversity

The diversity gain of both the Alamouti algorithm and the dominant eigenmode transmission is 4, but the array gain of the dominant eigenmode transmission is 3 dB larger than the Alamouti algorithm. Alamouti algorithm can also be used for any number of receive antennas , but the number of transmit antennas of the system should be less than or equal to 2.