Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 44

The MISO system utilizes M_T transmitting antennas with pre-processing or precoding to perform diversity at the transmitting end. And the obvious difference from reception diversity is that the transmitter may not have knowledge of the MISO system channel. Since the channel characteristics can be estimated at the receiving end, while the channel information needs to be fed back from the receiver to the transmitter at the transmitting end, there are basically two ways to obtain direct transmit diversity.

•When the transmitter has complete channel knowledge, beamforming can be achieved by various optimization metrics such as SNR and SINR to obtain diversity and array gain.

•When the transmitter has no channel information, the so-called space–time coding pre-processing can be used to obtain the diversity gain, but the array gain cannot be obtained.

In the following, different beamformers will be evaluated and several indirect transmit diversity techniques will be discussed, which can convert space diversity into time or frequency diversity.