Wireless links form a critical component of communication systems that aim to provide ubiquitous access to information. However, the time-varying characteristics (or “state”) of wireless channels caused by the mobility of transmitters, receivers, and objects in the environment make it difficult to achieve reliable communication. Adaptive signaling exploits any channel state information (CSI) available at the transmitter to provide the potential to significantly increase the throughput of wireless links and/or greatly reduce the receiver complexity. As such, adaptive signaling has attracted significant research interest in the last decade and has found application in numerous commercial wireless systems, ranging from cellular data systems to wireless local area networks (WLANs). However, one of the great challenges of wireless communications is that it is difficult to obtain perfect CSI due to the inherently noisy and outdated nature of CSI available at the transmitter. Over the last decade, we have championed the idea of choosing the appropriate transmitted signal based on statistical models for the current channel state conditioned on the channel measurements. In this semi-tutorial paper, we first review how this class of methods has been developed for single-antenna systems, and then present novel recent designs for multiple-antenna systems. Key to the development in each case is the development of the error characterization given the outdated estimates and the use of such to allocate data rate and power over time and possibly space. In general, the focus is on rate allocation, while power allocation is done through a pruning method. Numerical results will demonstrate in both the single-antenna and multiple-antenna cases that such an approach provides a robust method for improving system data rate versus the standard practice of employing link margin to compensate for such uncertainties.

Channel state information(CSI) at the transmitter, even imperfect, can improve the capacity of a multiple-input multiple-output(MIMO) communication system and substantially reduce the receiver complexity. Eigenbeamforming has been proven to be the preceding scheme that not only achieves the channel capacity, but also minimizes the mean square error (MSE) at the output of the receiver in this case. This paper presents a novel adaptive modulation scheme that loads power and integer bits over the eigenmodes in a MIMO communication system according to outdated CSI at the transmitter. An un- coded narrowband system is considered in a time-varying and spatially uncorrelated fading channel. Simulations indicate that the proposed scheme provides a much higher spectral efficiency than the standard strategy which simply adds energy margin to achieve the target error performance.

This article presents a blind predictive decision-feedback equalization (DFE) scheme motivated by the work of Labat et al. (see idid., vol.46, p.921-30, July 1998). The proposed scheme outperforms another previously proposed blind predictive DFE scheme, and also eliminates the filter interchange required in the scheme of Labat et al.

An orthogonal frequency division multiplexing (OFDM) system operating over a wireless communication channel effectively forms a number of parallel frequency-nonselective fading channels, thereby obviating the need for complex equalization and thus greatly simplifying equalization/decoding. However, the OFDM system also exhibits two weaknesses relative to its single-carrier counterparts: (1) the diversity achieved by the OFDM system can be less than a single-carrier system employing the same error control code in a signaling environment rich in diversity and (2) the baseband transmitted signal can exhibit significant amplitude fluctuation over time, thereby precluding efficient transmit amplifier operation. In this paper, nonstandard multidimensional signal sets matched to the OFDM framework are prescribed that address both of these issues. The proposed signal sets are chosen to maximize the diversity achieved by an uncoded system under a constraint to control the peak-to-mean envelope power ratio (PMEPR) of the baseband transmitted waveform. The cost of employing the proposed signal sets is an increase in decoding complexity, as essentially a small amount of controlled equalization has been added to the receiver; thus, the resulting system can be viewed as a hybrid between an OFDM system and a standard single-carrier system. Numerical results are presented which suggest that: (1) the system can provide an attractive alternative to a standard OFDM system in terms of required average transmitted SNR versus receiver complexity and (2) the system yields a modest reduction in PMEPR versus a standard OFDM system.

Blind equalization, which refers to the technique by which an unknown dispersive channel is equalized without the aid of a training sequence, has generated a lot of interest in previous years. However, blind equalizers are notorious for the slow convergence of their filter coefficients to the optimal values, which has prompted a flurry of activity into the development of numerous algorithms and structures to speed up their convergence. This paper is motivated by the approach presented by Labat, Macchi and Laot (see IEEE Transactions on Communications, p.921-30, vol.46, no.7, 1998) and presents a scheme to hasten convergence of the coefficients of the filters of a predictive decision feedback equalizer (PDFE). The proposed scheme is seen to ameliorate many of the drawbacks of the scheme proposed by Labat et al. Numerical results are presented that demonstrate the superiority of the proposed scheme versus the scheme suggested by Tong and Liu (see Proceedings of the IEEE International Conference on Acoustics, Speech, and Signal Processing, p.3901-3904, no.5, 1997).

When operating in a wireless transmission environment with available diversity greater than the minimum Hamming distance of its error control code, orthogonal frequency division multiplexing (OFDM) systems can suffer a diversity loss relative to their single-carrier counterparts when both systems are employing identical error control codes. A method of trading complexity for increased diversity in wireless OFDM systems is considered. The method employs a multidimensional signal set, where each signal point in the set is a linear transform of a signal point drawn from a multidimensional signal set that is the product of standard two-dimensional signal sets, each rotated about the origin. The peak-to-mean envelope power ratio (PMEPR) of the proposed system is considered, and it is shown that the PMEPR can be reduced further at the sample points, albeit moderately, by a modification of the proposed signal sets on certain subsets of the subcarriers. Finally, performance curves for the proposed system are given for environments with a varying amount of frequency diversity available in the channel.