Adaptation Techniques and Enabling Parameter Estimation Algorithms for Wireless Communications Systems - Adaptive Signal Processing in Wireless Communications - page 19

1.4.1.3 Adaptive Interference Cancellation Receivers
The impairment sources in wireless mobile radio systems are numerous. Co-channel
interference, which is caused by the reuse of carrier frequencies in nearby cells, is one
of the major contributors. Another major interference source is adjacent channel inter-
ference, which is caused by the spectral overlap between adjacent channel users. Also,
thermal noise and other impairment sources that are commonly modeled as additive
white Gaussian noise (AWGN) degrade the performance of a receiver. The statistics of
these disturbance sources are different. Conventional receivers commonly assume that
the impairment at the receiver is white, which causes performance loss if the actual
impairment is colored. By exploiting the statistics of the impairments, better receivers
can be designed. For example, interference whitening is one such technique that partially
suppresses the interference and optimizes the demodulator performance. However, at
any given time, the kind of disturbance that is dominant at the receiver is not known
before. In order to achieve the best possible performance in all situations, the receiver
should estimate the possible disturbance source and adapt the receiver to the second-
order statistics of the impairment. Such an adaptive receiver described in [56] improves
the performance of the maximum likelihood-based receiver.
The interference can also be suppressed by employing interference cancellation tech-
niques in the receivers. For example, joint demodulation (JD) of co-channel signals is
a powerful technique for cancelling co-channel interference. In [57], it was shown that
the capacity of the IS-136 system can be increased significantly by using a JD receiver.
However, the JD receiver given in [57] works well only when there is a single dominant
interferer, the mobile speed is low, and the channel is nondispersive. Otherwise, the
conventional single-user demodulator (CD) works better than joint demodulation at the
targeted operating SINR level. A simple and efficient solution to the above problem is an
adaptive receiver that adapts the detector to the system conditions. Figure 1.9 illustrates

Conventional detector

Rx signal

Conventional
acquisition

Conventional
demodulation

Control
unit

Filter

Desired user data

Joint
acquisition

Joint
demodulation

Rx samples

Joint detector

FIgure 1.9 Example for adaptive interference cancellation receiver. A complex joint demodulation
and a less complex single-user demodulation used adaptively based on the measured parameters.