Volterra functional expansions are power series with memory that explicitly characterize the input/output relationship of a finite-memory single-valued nonlinear black box whenever the latter is time-invariant and stable. The purpose of this paper is to introduce and report on the strengths and weaknesses of two distinct Volterra-series-type classes of adaptive nonlinear techniques for the quantitative measurement of second- and third-order harmonic distortion present in the communications channel of a, possibly signal space coded, QAM data communication system. Our investigation was originated as part of a request for new nondisruptive impairment measurement algorithms that would reliably characterize in real-time the performance of voiceband modems over both D1 conditioned private lines and the direct-distance-dialing (DDD) public switched network. Though, contrary to earlier studies, we do not contemplate nonlinear distortion compensator design, experimental evidence obtained by implementing the simplest of our nondisruptive algorithms on a digital signal processor chip verifies that over a certain range of signal-to-second- and third-order intermodulation distortion ratios our measurements are consistent with those obtained via the disruptive digital four tone test under a variety of voiceband telephone channel impairment conditions. Unfortunately, our experiments also indicate that neither the digital four-tone test nor any of our nondisruptive techniques have chances to prove useful for greater than 5% of the existing voiceband channels. However, this is entirely due to the special nature of the telephone channel environment and is not likely to dilute the importance of our proposal for other types of nonlinear environments such as radio channels or satellite links.>

The authors propose and evaluate a receiver architecture which combines the power of a decision feedback equalizer (DFE) with trellis coding, while allowing for minimal decoding delay in such a way that the total gain of the system is additive. The system is based on a structure that transposes the feedback filter of the DFE into the transmitter and, for high-order constellations, provides negligible increase in transmitter power. The first known hardware realization of a high bit rate digital subscriber line (HDSL) system that achieves the coding gain provided by a trellis code in addition to the equalization gain provided by the DFE is presented. A system whose complexity of implementation is comparable to that of a typical DFE and an independent Viterbi decoder is proposed.>

Implementing a frequency division multiplexing (FDM)-based modem for a multichannel application requires a bandwidth allocation strategy. That is, one must decide how to allocate total bandwidth to individual channels such that (a) the desired aggregate rate can be achieved and (b) the desired individual channel bit rates can be achieved. An optimal strategy for bandwidth allocation is proposed. The multitone channel result is used to specify a scheme to allocate four separate channels. The allocation strategy assigns to each subchannel a constellation that is equal in size to that of an equivalent single-band modem operating at the aggregate rate. In this manner, bandwidth is allocated proportional to the desired bit rate on a particular subchannel. Power allocation will be done to maintain a constant probability of error between all channels. This technique will allow the user to split total bandwidth to meet error performance objectives, provided that the desired aggregate rate does not exceed the maximum bit rate allowed by channel conditions.>

Adaptive or predictive carrier recovery systems, which are essential in high-performance quadrature-amplitude-modulated (QAM) data communications systems to correct for phase jitter and frequency offset, are considered. Analytical and experimental results are presented for two structures that implement a predictive carrier recovery system. These systems, which adapt their structure to match the spectral properties of the impairments, avoid the conflict between a wide bandwidth (to track fast jitter) and a narrow bandwidth (to minimize output noise) inherent in most carrier recovery loops. Such a system increases the likelihood that very bandwidth-efficient modems (e.g., 7 b/s/Hz for 19.2 kb/s voiceband modem applications) can provide reliable transmission in the presence of severe phase jitter and frequency offset. In particular, the predictive carrier recovery systems can track sinusoidal jitter present at more than one frequency as well as generalized time-varying phase jitter processes. Both finite-impulse-response (FIR) and infinite-impulse-response (IIR) adaptive phase tracking systems are considered. Prior limitations on adaptive IIR filters are overcome by designing a structure that is guaranteed to be stable and to possess only a global minimum as the filter coefficients converge to their desired values.>