Among traditional methods of doing antenna processing, one can mention transmit diversity, receive diversity or beamforming, which can be uplink and/or downlink (Figure 3.12).
None of these methods is handling multiple bit streams in the way that modern MIMO systems do.
With these traditional methods, the channel capacity is still bounded by the Shannon limit: C = log2(1 + SNR.T.R) bits/s/Hs, in which T and R are the number of transmit and receive antennas, respectively.
Beamforming downlink or uplink still suffers some capacity limitation as compared to MIMO with multiple bit streams: channel capacity is still low and fading depth still large.
The paragraph below recalls the basics of beamforming principles. Such principles can still be applied on OFDM signals, although these are broadband signals. Indeed, narrowband beamforming can be implemented on individual subchannels. Notice that, since the broad-band channels are frequency-selective, the spatial channel characteristics vary from subcarrier to subcarrier. By default, different beamforming vectors will be needed on different subchannels.
Adaptive Rx beamformer example
The goal of a network of antennas with a formation of beams is to recreate at the exit y(t) of the network the conditions of optimal combination, that is to say ...