A decision-feedback equalizer (DFE) is the basis of a recent development of a quadruple diversify troposcatter modem which can operate up to a data rate of 12.6 Mbit/s in a 99% bandwidth of 15 MHz. In this paper a theoretical approacii is developed for the calculation of average bit error rate (ABER), including the effects of intersymbol interference due to multipath and the finiteness of the transversal filters used to realize the DFE. By omitting the intersymbol interference effect, the calculation provides a lower bound which can be used to assess the intersymbol interference penalty for a particular DFE structure: The paper includes calculations of a DFE configuration which has a three tap forward filter with tap spacing equal to one-half a symbol interyal. Measured performance results from fading channel simulator tests of a three tap forward filter DFE are presented for data rates from 1.5 to 12.6 Mbit/s and for a wide range of multipath statistical conditions. The results for this DFE configuration show (1) excellent agreement between calculated and measured ABER, (2) a small intersymbol 'interference penalty when the 2u multipath spread is less than approxi$ately one-half the data symbol interval, and (3).successful operation at values of multipath spread up to twice the data symbol interval. In ,a sequel to this paper, the results of a field test of the DFE modem are presented. These live links test'results are consistent with both the calculated and simulator measured'data presented here.. ;:..
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.
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