Theoretical and Measured Performance of a DFE Modem on a Fading Multipath Channel

Abstract

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.. ;:..

Distributed Space-Time Cooperative Schemes for Underwater Acoustic Communications

“…Due to the orthogonality of , we have (20) where is the identity matrix and we define as the result of the spectral factorization of the coefficient of the identity matrix in (20). Hence (21) The output vector is given by (22) whose components can be expressed as (23) (24) and the output noise has PSD given by (25) Thus, and are independent and the problem of jointly detecting and from decouples: is detected from and from .…”

“…We employ the technique described in [20], and, for mathematical tractability, consider the particular scenario in which the channels have only one multipath component. This case corresponds, for example, to the scenario where propagation occurs via a single surface reflection.…”

“…The feedback coefficients, which operate on the previously decoded symbols, are denoted by . As usual in the literature, we make the key assumption that the previous symbols are decoded perfectly [20]. While this is not the case for low signal-to-noise ratio (SNR), it is a good approximation in the high SNR regime and, furthermore, it is the typical scenario where diversity analysis is performed.…”

Upper-bound performance of a wide-band adaptive modem

“…This was also applied by Monsen [10] who asserted that the interference characteristics do not significantly alter the mean BER performance. To verify this, several experiments were conducted using fixed modulation schemes in a fading multipath environment, where the mean BER performance for each of these schemes, was numerically calculated as follows: (4) where MODN can be for binary phase-shift keying (BPSK), 4-QAM, 16-QAM, or 64-QAM.…”

Trellis-coded CPFSK and soft-decision feedback equalization for micro-cellular wireless applications

“…In this case, the equalizer needs to combat the linear ISI caused only by the fading channel. It is well known that a DFE is more effective than a linear equalizer in combating ISI in the presence of moderate to severe amplitude distortion (e.g., in the case of indoor radio channels) [12,13]. Decision feedback equalization uses previously detected symbols to synthesize the postcursors of the discrete impulse response that are subsequently subtracted from the received samples through a feedback loop.…”