Single carrier cyclic prefixed (SCCP) communications are a close relative of multicarrier communications. Both types of systems are robust to multipath, provided that the channel delay spread is shorter than the guard interval between transmitted blocks. If this condition is not met, a channel shortening equalizer can be used to shorten the channel to the desired length. Previous work on channel shortening has largely been in the context of digital subscriber lines, a wireline system that allows bit allocation, thus it has focused on maximizing the bit rate for a given bit error rate (BER). We propose and evaluate a channel shortener that attempts to directly minimize the BER for an SCCP system. The problem is shown to be analytically distinct from the analogous problem in multicarrier systems.

In modern DSL systems, crosstalk is a major source of performance degradation. Crosstalk cancellation schemes have been proposed to mitigate the effect of crosstalk. However, the complexity of crosstalk cancellation grows with the square of the number of lines in the binder. Fortunately, most of the crosstalk originates from a limited number of lines on a limited number of tones. As a result, a fraction of the complexity of full crosstalk cancellation suffices to cancel most of the crosstalk. The challenge is then to determine which crosstalk to cancel on which tones, given a certain complexity constraint. This paper presents an algorithm based on a dual decomposition to optimally solve this problem. The proposed algorithm naturally incorporates rate constraints and the complexity of the algorithm compares favourably to a known resource allocation algorithm, where a multi-user extension is made to incorporate the rate constraints.

In a previous paper, we proposed a bitrate maximizing (BM) design criterion for the time-domain equalizer (TEQ) in a discrete multitone receiver. This BM-TEQ and the closely related BM per-group equalizers (PGEQ) get close to the performance of the so-called per-tone equalization (PTEQ). In this paper, we show that the BM-TEQ criterion, despite its nonlinear nature, is well suited for a recursive Levenberg-Marquardt (RLM) based design. This adaptive BM-TEQ also allows us to track slow variations of the transmission channel and the noise. This RLM-based design uses the same second-order statistics (SOS) as the earlier presented recursive least-squares (RLS) based adaptive PTEQ and opens up a complete range of adaptive BM equalizers: from the computationally efficient RLS-based PTEQ with largest memory cost, over the RLM-based BM-PGEQ with intermediate memory cost, towards an RLM-based BM-TEQ with considerably smaller memory cost, but larger equalizer updating complexity.

Sampling clock synchronization in discrete multitone systems, such as digital subscriber line modems can be done with a phase locked loop. This requires expensive analog hardware, such as a voltage controlled oscillator. However, it is possible to use a cheaper free-running oscillator and tackle the problem in the digital domain. Using resampling or interpolation, the timing correction becomes transparent for subsequent equalizers. This article describes a novel resampling technique using Chebyshev polynomials.

Asymmetric digital subscriber line (ADSL) employs the discrete multi-tone modulation (UMT) as the transmission format. To separate up- and downstream signals, the ADSL standard allows the use of echo cancellation resulting in improved bit rates, reach and/or noise margins. DMT-based modems typically use the mixed time/frequency (MTF) domain echo canceling (EC) scheme of M. Ho et al. (Jul. 1996), where the echo-filter is estimated in the frequency domain by means of the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal is excited, the MTF adaptation process leads to an inferior estimate of the echo channel, unless extra power on unused echo tones is transmitted. Since transmitting extra power on such tones is often undesired, we present in this paper an alternative EC scheme, the circulant decomposition canceler (CDC), which works without extra power requirements and which exhibits comparable complexity as M. Ho et al. (Jul. 1996). In addition, we have shown how the CDC scheme can be incorporated into a receiver structure using per tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far end signal.

A time-domain equalizer (TEQ) is inserted in discrete multitone (DMT) receivers to impose channel shortening. Many algorithms have been developed to initialize this TEQ, but none of them really optimizes the bitrate. We present a truly bitrate-maximizing TEQ (BM-TEQ) cost function that is based on an exact formulation of the subchannel signal-to-noise ratio as a function of the TEQ taps. The performance of this BM-TEQ comes close to the performance of the per-tone equalizer.

Discrete multi tone (DMT) uses an inverse discrete Fourier transform (IDFT) to modulate data on the carriers. The rather high side lobes of the IDFT filter bank can lead to spurious emissions in unauthorised frequency bands. Applying a window function within the DMT symbol alleviates this, but introduces distortions that are generally not easy to compensate. In this paper a special class of window functions is derived for which this compensation only requires a modest amount of processing at the receiver.

Discrete multitone (DMT) offers an elegant way to achieve high capacity, dividing the spectrum into small bands and processing these individually. The per tone equalizer (PTEQ) optimizes the capacity for each band individually, thus optimizing the whole. However, it provides little protection against narrow band radio frequency interference (RFI), being spread over all tones because of the high side lobes of the DFT filter band used in the receiver. The use of windowing functions limits this noise spreading, but is difficult to combine with the PTEQ. This paper describes a method to combine the PTEQ with a raised cosine window, while keeping the complexity reasonable. Extensions to other windowing functions are also given.

A time-domain equalizer (TEQ) is inserted in discrete multitone (DMT) receivers to impose channel shortening and hence overcome the need for a too long cyclic prefix. Many algorithms have been developed to initialize this TEQ, but none of them really optimizes the bitrate. In this paper, we present a new bitrate maximizing TEQ (BM-TEQ), cost function that results in a TEQ design/initialization that outperforms any other TEQ design. In the derivation, we exploit the fact that the frequency-domain equalizers (FEQ) do not alter the SNR on the individual tones. The performance of this BM-TEQ comes close to the performance of the per-tone equalizer (PTEQ), an alternative DMT equalization structure that ensures consistently better performance than a TEQ. Finally, the presented BM-TEQ design is also used in a "per group" equalization scheme (PGEQ), which is intermediate (in terms of memory requirement and performance) between TEQ and PTEQ. The PGEQ design then encompasses BM-TEQ and PTEQ design procedures as extreme cases.

In the past, several papers have reported on echo canceller (EC) structures developed for discrete multitone transmission (DMT) based asymmetric digital subscriber lines (ADSL) transmission. The most commonly known scheme, developed by Ho, Cioffi and Bingham, is based on canceling the received echo noise efficiently in times as well as in the frequency domain. The time domain processing part, which is commonly called cyclic echo synthesis (CES), can be minimized by choosing the optimal temporal alignment between the echo transmitter and the EC in the modem. This paper improves previously published work by minimizing the CES complexity without choosing a specific temporal alignment. Moreover, the obtained structure is highly suitable for making the update process of the echo canceller coefficients independent of the far end signal, leading to improved convergence behavior.

A novel equalization technique for Discrete multitone (DMT) based modems is proposed which incorporates receiver windowing operations. The method is especially useful in an environment dominated by crosstalk and narrowband interference, such as digital subscriber line (DSL). The high sidelobe level of the DFT filters present in classical DMT receivers leads to considerable performance degradation due to intercarrier interference and broadband susceptibility to narrowband disturbers. Window functions can be used to alleviate this, but they are difficult to combine with equalization techniques, e.g. per tone equalization [1]. The presented method offers a way to do so without unduly increasing the complexity.

In classical discrete multi tone receivers, the equalizer structure consists of a (real) time domain equalizer (TEQ) combined with (complex) 1-taps frequency domain equalizers. Recently, an alternative receiver was introduced based on a separate per tone equalization, where the SNR on each tone and hence the bitrate is maximized. In this paper, a new adaptive initialization scheme for this per tone equalizer (PTEQ) is introduced, based on a combination of Least Mean Squares (LMS) and Recursive Least Squares (RLS) with inverse updating. It will be shown that the proposed method exhibits only slightly slower convergence compared to full RLS with inverse updating, while complexity as well as memory cost are reduced considerably.