Introduction - Interference Cancellation Using Space-Time Processing and Precoding Design - page 2

complexity of the maximum-likelihood detection increases exponentially as a func-
tion of the number of antennas, the number of users, and the bandwidth efficiency
(measured in bits per channel use). Therefore, usually it is not practical.
Applying a linear transformation to the codeword before transmission is called
linear precoding. When there are some feedback available at the transmitter, pre-
coding techniques can be used to improve the system performance. For example, in
a point-to-point multiple-input multiple-output (MIMO) channel, the performance
of the system can be improved significantly using precoders [ 10 - 16 ]. In a multi-
antenna multiple access channel, one can utilize the channel information to improve
the performance of the system while achieving interference cancellation using array
processing methods mentioned above. For example, in [ 8 ], post-processing SNR is
maximized for a given linear receiver by selecting the QOSTBC with the minimum
quaternionic angle as well as realizing interference cancellation. In [ 17 ], limited feed-
back is used to adapt the phase of a transmitted signal to improve the performance
of the system.
The common goal and the main characteristics of the above multi-user systems are
the small number of required receive antennas and the low complexity of the array-
processing decoding. A receiver does not need more than J receive antennas and the
decoding is symbol-by-symbol or pairwise using low-complexity array-processing
methods. One drawback, however, is that if we demand lowcomplexity, themaximum
diversity of NM is not achievable. Our motivation is to utilize the channel information
at the user transmitters to increase the diversity of the system while keeping the low
complexity of the decoding. In other words, unlike the above-mentioned methods,
we do not use receive antennas to cancel the interference. Instead, we use the chan-
nel information at the transmitter to design precoders that align different groups of
signals along orthogonal directions. As a result, interference suppression is achieved
without utilizing the receive antenna resources and therefore full diversity is achieved
naturally.
1.2 Interference Cancellation and Detection for Multiple Access
Channel with Quantized Feedback
Although the performance of the scheme in [ 18 ] is better than that of the former
proposed schemes, perfect channel information is needed at transmitters. It is not
practical in reality. We investigate the use of limited feedback to achieve interference
cancellation as well as full diversity. Limited feedback has been used extensively
in the case of the single-user MIMO systems. It has been shown that the capac-
ity and performance of the point-to-point MIMO systems can be increased signif-
icantly using limited feedback [ 10 - 16 ]. There are few examples of multi-antenna
multi-user systems with limited feedback in the literature. In [ 8 ], post-processing
SNR is maximized for a given linear receiver by selecting the QOSTBC with the
minimum quaternionic angle as well as realizing interference cancellation. In [ 17 ],