Robert F. H. Fischer

Multilevel codes: theoretical concepts and practical design rules

This paper deals with 2/sup l/-ary transmission using multilevel coding (MLC) and multistage decoding (MSD). The known result that MLC and MSD suffice to approach capacity if the rates at each level are appropriately chosen is reviewed. Using multiuser information theory, it is shown that there is a large space of rate combinations such that MLC and full maximum-likelihood decoding (MLD) can approach capacity. It is noted that multilevel codes designed according to the traditional balanced distance rule tend to fall in the latter category and, therefore, require the huge complexity of MLD. The capacity rule, the balanced distances rules, and two other rules based on the random coding exponent and cutoff rate are compared and contrasted for practical design. Simulation results using multilevel binary turbo codes show that capacity can in fact be closely approached at high bandwidth efficiencies. Moreover, topics relevant in practical applications such as signal set labeling, dimensionality of the constituent constellation, and hard-decision decoding are emphasized. Bit interleaved coded modulation, proposed by Caire et al. (see ibid., vol.44, p.927-46, 1998), is reviewed in the context of MLC. Finally, the combination of signal shaping and coding is discussed. Significant shaping gains are achievable in practice only if these design rules are taken into account.

A new loading algorithm for discrete multitone transmission

A new loading algorithm for discrete multitone transmission is proposed. Thereby rate is not distributed according to channel capacity, but rate and transmit power are assigned to maximize the signal-to-noise ratio in each carrier. Because closed form expressions can be derived the algorithm is of very low complexity, even lower than the loading algorithm recently proposed by Chow et al. (see IEEE Transactions on Communications, no.2/3/4, p.773-5, 1995). Nevertheless achievable performance is higher or at least the same. Results for a typical high rate transmission over twisted pair lines are presented.

Precoding in multiantenna and multiuser communications

In this paper, Tomlinson-Harashima Precoding for multiple-input/multiple-output systems including multiple-antenna and multiuser systems is studied. It is shown that nonlinear preequalization offers significant advantages over linear preequalization which increases average transmit power. Moreover, it outperforms decision-feedback equalization at the receiver side which is applicable if joint processing at the receiver side is possible, and which suffers from error propagation. A number of aspects of practical importance are studied. Loading, i.e., the optimum distribution of transmit power and rate is discussed in detail. It is shown that the capacity of the underlying MIMO channel can be utilized asymptotically by means of nonlinear precoding.

Low-complexity near-maximum-likelihood detection and precoding for MIMO systems using lattice reduction

We consider the lattice-reduction-aided detection scheme for 2/spl times/2 channels recently proposed by H. Yao and G.W. Wornell (see Proc. IEEE Globecom, 2002). By using an equivalent real-valued substitute MIMO channel model, their lattice reduction algorithm can be replaced by the well-known LLL (Lenstra-Lenstra-Lovasz) algorithm (Lenstra, A.K. et al., 1982), which enables the application of the algorithm to MIMO systems with arbitrary numbers of dimensions. We show how lattice reduction can also be favourably applied in systems that use precoding and give simulation results that underline the usefulness of this approach.

Lattice-reduction-aided broadcast precoding

A precoding scheme for multiuser broadcast communications is described, which fills the gap between the low-complexity Tomlinson-Harashima precoding and the sphere decoder-based system of Peel et al. Simulation results show that, replacing the closest-point search with the Babai approximation, the full diversity order supported by the channel is available to each user, as in the system of Peel et al., and unlike Tomlinson-Harashima precoding, which suffers some diversity penalty. The complexity of the scheme is similar to that of Tomlinson-Harashima precoding.

MIMO precoding for decentralized receivers

This paper presents a modified version of Tomlinson-Harashima type precoding applicable to a setting with decentralized receivers, e.g. in a DS-CDMA downlink scenario. Precoding, i.e., nonlinear pre-equalization, is an interesting strategy for significantly simplifying signal processing at the receiver side (mobile terminals) at only moderate additional complexity at the base station.

The PAPR Problem in OFDM Transmission: New Directions for a Long-Lasting Problem

Energy efficiency matters in future mobile communications networks. The key driving factor is the growing energy cost of network operations that can make up as much as 50% of the total operational cost today [1]. In the context of green information and communication technology (ICT), this has led to many global initiatives such as the Green Touch consortium.

Dynamics limited precoding, shaping, and blind equalization for fast digital transmission over twisted pair lines

A new combined precoding/shaping technique for fast digital transmission over twisted pair lines is proposed. Major advantages of this “dynamics shaping” are: dynamics of the signal at the input of the decision device are reduced by a great amount. Thereby, A/D-conversion, adaptive equalization, and symbol timing are rather facilitated. A trade-off between signal dynamics at the transmitter output, decision device input and SNR-gain by noise whitening is offered. For dynamics limitation relevant in practice, gains up to 6 dB are achieved. Additionally, the transmitter can be fixed to a typical application because, in contrast to Tomlinson-Harashima or other precoding techniques, blind adaptive equalization is practicable to remove residual intersymbol interference in the case of a mismatch of precoding and actual cable characteristics. The residual SNR-loss is negligible in most applications. SNR-gains due to noise prediction, channel coding and signal shaping simply can be combined using dynamics shaping. Nevertheless, system complexity is of the order of other precoding/shaping techniques. Although numerical results are only presented for a HDSL-application in the German Telekom subscriber network, the proposed transmission scheme may simplify all kinds of high-speed data communications via copper lines, such as LANs, ADSL, CDDI, etc

Tomlinson-Harashima precoding in space-time transmission for low-rate backward channel

In this paper, Tomlinson-Harashima precoding, a nonlinear pre-equalization technique, is proposed for transmission over multiple-input/multiple-output channels. Instead of equalizing intersymbol interference (temporal equalization) here spatial equalization, i.e., equalization of multi-user interference is performed. If only a low-rate backward channel is available for communicating channel state information back from the receiver to the transmitter, a compromise precoder setting, calculated from (medium-term) average channel knowledge in combination with linear residual equalization at the receiver side is proposed. Compared to an optimal adjustment of the precoder, i.e., perfect channel state information at the transmitter, only small losses have to be accepted.

Peak-to-Average Power Ratio Reduction in MIMO OFDM

Peak-to-average power ratio (PAR) reduction in OFDM using antenna arrays (MIMO OFDM) is considered. In particular, generalizations of selected mapping (SLM) recently proposed in literature, are studied, and a new version, we call it directed SLM (dSLM), is introduced. It is shown that, in contrast to the other schemes, dSLM utilizes the potential offered by MIMO transmission-the complementary distribution function of the PAR exhibits a steeper (increased by a factor equal to the number of transmit antennas) decay. This effect is similar to the diversity gain in error performance when using MIMO transmission. We show that dSLM is very flexible and not restricted to any modulation format or OFDM frame size. Moreover, a variant of dSLM which does not require any side information to recover data at the receiver is presented.