MMSE Equalization of Interference on Fading Diversity Channels

Abstract

Adaptive equalization is used in digital transmission systems with parallel fading channels. The equalization combines the diversity channels and reduces intersymbol interference due to multipath returns. When interference is present and correlated from channet to channel, the equalizer can also reduce its effect on the quality of information transfer. Important applications for interference cancellation occur in diversity troposcatter systems in the presence of jamming, diversity high frequency (HF) systems which must cope with interfering skywaves, and space diversity line-of-sight (LOS) radio systems where adjacent channel interference is a problem.In this paper we develop the general formulation for minimum mean square error (MMSE) equalization of interference in digital transmission diversity systems. The problem formulation includes the use of available receiver decisions to assist in MMSE processing. The effects of intersymbol interference are included in the analysis through a critical approximation which assumes sufficient processor capability to reduce IS1 effects to levels small enough for satisfactory communication. The analysis also develops the concept of additional implicit or intrinsic diversity which results from channel multipath dispersion. It shows how the M M S E processor sacrifices diversity to suppress interference even when the interference arrives in the main beams of the receiver antenna patterns. The condition of near synchronous same-path interference is also addressed. Because the spatial angle of arrival of the interference may result in delay differences between interference signals in different antenna channels, interference delay compensation may be required. We show that this effect is compensated for with a small number of appropriately spaced equalizer taps.

An adaptive adjacent channel interference cancellation technique

“…In reference [5], a digital filter with adjustable number of taps is proposed. Here an estimate of interference is used to add or remove taps at the tails of square-root raised-cosine (SRRC) filters.…”

Techniques for applying interference cancelers to high-frequency band wireless link systems employing multilevel modulation

“…An interference canceler (IC), which adaptively removes interference signals at receivers, is one of the effective interference suppression techniques [11][12][13][14][15][16]. Generally, there are two types of ICs.…”

Performance evaluation of linear equalizers in the presence of adjacent channel interference

“…The mathematical formulation of adaptive equalizers is largely limited to the investigation of dispersive channels with stationary gaussian noise (Monsen 1971(Monsen ,1974(Monsen , 1984. The problem of how to handle degradations introduced by ACI in the analysis has not been addressed.…”

“…The problem of how to handle degradations introduced by ACI in the analysis has not been addressed. Although considerable work has been done on the design of receiver filters in an ACI environment (Cruz and Simpson 1981, Korn and Seth 1982, Kalet 1977, Tahernezhadi and Cruz 1986, they have not approached the formulation from the adaptive equalization standpoint, with the exception of the work by Monsen (1984), where it was assumed that the symbol estimates of interfering channels are known to the receiver. Here, however, no knowledge of estimated symbols of interfering channels is assumed.…”