This paper proposes two low-complexity coding schemes for intersymbol interference (ISI) channels that perform close to the channel capacity. The first scheme is a serial concatenation of an outer code and a spectral shaping inner code. The second scheme is a parallel concatenation of two component trellis codes that are designed to be spectrally matched to the channel. Analysis using extrinsic information transfer (EXIT) functions and bit-error-rate (BER) simulations shows that both schemes surpass the independent identically distributed (i.i.d.) channel capacity and outperform other existing schemes.

It is well-known that iterative equalization/decoding offers good coding gains for intersymbol interference (ISI) channels. Recently, precoded ISI channels have been considered with turbo equalization which results in significant interleaving gains due to the recursive nature of precoded ISI channels. This paper presents new results pertaining to the design of codes for precoded ISI channels. An analytical proof is presented to show that precoding results in a loss in reliability during the first iteration for all 2-tap ISI channels. We compute the ratio of probability of error between nonprecoded and precoded ISI channels, which is closely related to the reliability, via a recursion. Based on this and the convergence properties of precoded and non-precoded channels, it is shown that by using a mixture of precoded and non-precoded parts, better performance can be achieved. Due to the recursiveness introduced from precoding, the codes required to achieve good performance are very simple codes and, hence, the resulting schemes provide good bit error rate performance at low receiver complexity.

We study packet combining techniques for retransmission schemes over intersymbol interference (ISI) channels. Two types of combining schemes are investigated, namely, maximum-likelihood combining (MLC) and iterative combining (IC). By first employing a precoding technique and then by interpreting the ISI channel as a trellis code, the transmissions of the same data packet at different times through the channel can be treated as the parallel concatenation of recursive trellis codes. If interleavers are used in between retransmissions, "turbo" coding gains can be achieved by iterative equalization. It is shown that IC provides excellent performance and outperforms other forms of combining in terms of frame error rate performance both analytically and through simulations.