Equalization for discrete multitone transceivers to maximize bit rate

Abstract

In a discrete multitone receiver, a time-domain equalizer (TEQ) reduces intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Current TEQ design methods such as minimum mean-squared error (MMSE), maximum shortening SNR (MSSNR), and maximum geometric SNR (MGSNR) do not directly maximize bit rate. In this paper, we develop two TEQ design methods to maximize bit rate. First, we partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel SNR definition. Then, we derive a nonlinear function of TEQ taps that measures bit rate, which the proposed maximum bit rate (MBR) method optimizes. We also propose a minimum-ISI method that generalizes the MSSNR method by weighting the ISI in the frequency domain to obtain higher performance. The minimum-ISI method is amenable to real-time implementation on a fixed-point digital signal processor. Based on simulations using eight different carrier-serving-area loop channels, 1) the proposed methods yield higher bit rates than MMSE, MGSNR, and MSSNR methods; 2) the proposed methods give three-tap TEQs with higher bit rates than 17-tap MMSE, MGSNR, and MSSNR TEQs; 3) the proposed MBR method achieves the channel capacity (as computed by the matched filter bound using the proposed subchannel SNR model) with a five-tap TEQ; and 4) the proposed minimum-ISI method achieves the bit rate of the optimal MBR method.

A blind adaptive TEQ for multicarrier systems

“…There are currently many methods which, when given a channel, can compute the optimal equalizer (for some metric) [1]- [3]. There are also many suboptimal and/or adaptive approaches, such as [4]- [7].…”

Infinite length results for channel shortening equalizers

“…For pointto-point systems that use bit allocation, no bits can be allocated to these subchannels, so the bit rate suffers. This has been observed in [8]. For broadcast systems, the bit error rate on these subchannels will become very large, though this may be mitigated somewhat by coding across frequencies.…”

“…More recently, Melsa, Younce, and Rohrs [5] proposed the maximum shortening SNR (MSSNR) method, which attempts to minimize the energy outside the window of interest while holding the energy inside fixed. This approach was generalized to the min-ISI method in [8], which allows the residual ISI to be shaped in the frequency domain. A blind, adaptive algorithm that searches for the TEQ maximizing the SSNR cost function was proposed in [10].…”

“…In broadcast systems, the true performance measure is the bit error rate for a fixed bit allocation. Optimizing the MSE or SSNR does not necessarily optimize the bit rate [3], [7], [8], [11], [12] or the bit error rate. This paper analyzes two families of TEQ design methods amenable to cost-effective real-time implementation: the MMSE and MSSNR designs.…”

Low complexity MIMO blind adaptive channel shortening

“…4 shows a plot of the shortening SNR [3] achieved by FRODO using the optimal and heuristic delays. The performance was averaged over ADSL carrier serving area loops 1 through 8 [4]. The heuristic delay provides reasonable performance for TEQs with at least 8 taps, and nearly optimal performance for TEQs with at least 32 taps.…”