We describe the ability of a linear equalizer/combiner or decision-feedback equalizer to suppress all received adjacent-channel, intersymbol, and cochannel interference. The emphasis is on values among transmitter bandwidth, receiver bandwidth, carrier spacing, and antenna diversity which provide the best opportunities for interference suppression. Through analyses of the number of degrees of freedom and constraints in generalized zero-forcing equalizers, and partial comparisons to calculations of equalizer minimum-mean-square performance, four results are obtained. First, with one antenna and a linear equalizer, arbitrarily large receiver bandwidths allow for marginal improvements in spectral efficiency through decreased carrier spacing, because the carrier spacing cannot be reduced to a value below the symbol rate without incurring unsuppressible interference. Second, large receiver bandwidths assist multiple antennas in improving the spectral efficiency in that carrier spacing values may go below the symbol rate, even in the presence of cochannel interference. Third, the use of equalizers and linear combiners, together with large receiver bandwidths, allows large transmitter bandwidths to be used. Fourth, for cochannel interference and intersymbol interference, the number of interferers that may be suppressible by a generalized zero-forcing linear equalizer/combiner increases linearly with the product of the number of antennas and the truncated integer ratio of the total bandwidth to the symbol rate.>

Equalization in cyclostationary interference, which occurs when similar neighboring digital communication systems cause interference (crosstalk), is considered. An expression is derived on the minimum mean square performance of a continuous-time infinite-length decision feedback equalizer in the presence of multiple cyclostationary interferers and additive white noise. This expression is calculated for a subscriber line system to show the performance improvements over the situation where the interference is stationary with the same power spectrum. Linear equalizer performance curves were also added to the comparisons. These results show two techniques which can provide opportunities for improved equalizer performance by enhancing the cyclostationarity of the interference. The first is by decreasing the misalignment of the phases of the transceiver clocks in the central office transmitters. The second is by using transmitter pulse bandwidths which are wide relative to the symbol rate.>