Antenna separation and capacity of BLAST in correlated channels

BLAST (Bell-labs LAyered Space-Time) is a communication technique for achieving very high spectral efficiencies in highly scattering environments using multiple transmit and receive antennas. The large capacities promised by the use of BLAST may be realized when transfer functions between all pairs of elements are largely uncorrelated. However, as the base station is usually placed above local clutter, the angular spectrum at the base station becomes narrow in the horizontal plane, on the order of 2 degrees at 1 km, which determines the minimum required spacing between antennas to limit signal correlation. The BLAST system is thus expected to be subject to a trade-off between capacity and array size. In this work the correlation between base station antennas is derived for different antenna spacings. The result is used to derive link capacity, when there is correlation among receive antennas and among transmit antennas. It is found that for a 16×16 BLAST system at 10 dB average SNR, an antenna separation of 4 wavelengths between nearest neighbors in a linear array allows one to achieve 36 bits/sec/Hz as compared to 42 bits/sec/Hz for the uncorrelated antenna case. Here it is assumed that there is correlation only at the base station and remote antennas are uncorrelated because the remote is in a cluttered environment. The use of dually polarized antennas, which can transmit and receive separately in two polarizations, is also explored and found to allow a reduction of the base station array size by half