Introduction to spatially multiplexed MIMO systems
We have observed in chapter 2 that the capacity of wireless communication links is increased by using multiple antennas at the transmitter and the receiver. To achieve these capacities, a transmission scheme, called Diagonal-Bell Laboratories Layered Space-time (D-BLAST) has been proposed by G. J. Foschini (1996). In an i.i.d. Rayleigh scattering environment, this processing structure leads to theoretical rates which grow linearly with the number of antennas (for NR
) with these rates approaching 90% of Shannon's capacity. But this has large computational complexity required for implementation of this scheme.
A simplified version, called Vertical BLAST (V-BLAST) has been proposed by G. J. Foschini et al., (1999). They have demonstrated spectral efficiencies of 20–40 bps/Hz at average signal-to-noise ratio (SNR) ranging from 24 to 34 dB could be achieved in indoor environments. The essential difference between D-BLAST and V-BLAST lies in the vector encoding process. The D-BLAST code blocks are organized along diagonals in space-time. In V-BLAST, however, the vector encoding process is simply a demultiplexing (DMUX) operation followed by independent bit-to-symbol mapping of each sub-stream. No inter sub-stream coding, or coding of any kind, is required.
Note that BLAST detection scheme can be done in one of the following ways:
1. interference nulling to reduce the effect of the other (interfering) signals on the desired one is employed in successive interference cancellation (SIC) which will be discussed in section 10.4;
2. ordering to select the sub-stream with the largest signal-to-noise ratio (SNR) or other criteria along with SIC, which will be discussed later in ordered successive interference cancellation (OSIC) (section 10.5).
Vertical/horizontal layered space-time transmission
Vertical Bell Laboratories Layered Space-time Transmission (V-BLAST) suggests simultaneous transmission of independent uncoded data sub-streams. The input data is separated into several substreams with demultiplexing (DMUX) operation which are associated with each transmit antenna.
This process is followed by independent bit-to-symbol mapping (M-ary) of each sub-stream and may be interleaved (optional) as depicted in Fig. 10.1 (a). We consider a V-BLAST system with NT
transmitting antennas. At the transmitter the data is passed through a serial-to-parallel converter (S/P converter) and transformed into NT
sub-streams, where each sub-stream is sent through a different transmit antenna.