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This paper addresses the problem of adjacent channel interference between minimum-shift keying (MSK)-type signals transmitted through Rayleigh fading channels. A receiver, based on the multiple-input, multiple-output antenna diversity concept and the maximum-likelihood criterion, allows the transmission of MSK-type signals with very small channel spacings, greatly improving the bandwidth efficiency of these signals. The concept of antenna diversity is used to greatly reduce the effects of adjacent channel interference for channels with Rayleigh fading. The results show that it is possible to transmit bit-synchronized MSK-type signals with a very small normalized channel spacing (e.g., /spl Delta/fT/sub b/=/spl plusmn/0.4) between channels, under almost any power imbalance between signals, with very small degradation in performance.

This paper addresses the problem of adjacent channel interference between constant-envelope signals, e.g., MSK-type signals. A technique, based on the BLAST concept, is used to greatly reduce the effects of adjacent channel interference (ACI) and co-channel interference (CCI), for channels with Rayleigh fading. The results show that it is possible to transmit these signals with a very small spacing between channels, under almost any power imbalance between signals, with very small degradation in performance! For example, it is shown that for MSK-type signals it is possible to operate at a channel spacing of 0.4/T/sub b/ with very little degradation in detectability performance, due to ACI. All the benefits of antenna diversity are achieved with very little loss due to ACI.

The demand for high data rate communication systems and the lack of available bandwidth has led to much research effort into MIMO systems using multiple transmit and receive antennas. It has been shown that MIMO systems can improve the capacity of rich Rayleigh fading channels by a factor which is linearly proportional to the minimum number of transmit and receive antennas. The V-BLAST architecture has been proposed for realizing high spectral efficiencies over flat Rayleigh fading indoor wireless channels. Multitone transmission, or OFDM, has been used in a number of systems. OFDM eliminates the need for a complex equalizer for overcoming ISI caused by delay spread or frequency-selective fading. However, it has been shown that for a single-input-single-output (SISO) system operating over a frequency-selective channel, if the transmitter "knows" the channel characteristics, the "water-pouring" technique can be used to optimize the transmitted bit rate and approach the channel capacity. In this paper, we use the "water-pouring" approach for combined OFDM-MIMO systems, and show that it is possible to achieve very good bandwidth efficiencies for un-coded systems.

It is theoretically shown that the capacity of a voiceband channel operating through a digital network is at least 56 kbps (for a 3.5 kHz baseband bandwidth). The capacity may actually be higher but how close one can come to 64 kbps, the limit imposed by the digital network itself, is unknown. The transmission method is based on generating nonuniformly pulsed PAM (pulse amplitude modulation) signals which at the network sampling instants go through one of the slicing levels of the A/D converter of the network. To accomplish this, both the slicing levels and the sampling clock times must be known. This is a hard practical problem and one can only conjecture if a reasonable solution will be possible. The signal may be recovered using a Viterbi detector to reduce intersymbol interference effects, but at present, the number of states required for this detector is very large. >

The idea of using multitone signaling for wireless mobile communications has become increasingly popular in the past few years. This article describes the use of multitone signals and antenna combining in a system which is capable of "learning" the characteristics of the wireless transmission channel. The channel is assumed to be a frequency-selective channel which is slowly changing. By utilizing both the "water-pouring" technique of multitone signaling and antenna diversity combining, it is possible to maximize the bit rate through the channel. The new part of this system is the use of antenna diversity combining in this transmission scheme.

Frequency division multiplexed, orthogonal M -ary FSK modulated signals transmitted over a frequency hopping channel (with one channel use per hop) interfere with each other unless perfectly synchronized in time and frequency with the receiver. In order to maintain the interfering "crosstalk" energy at harmless levels, even in the case of grossly power unbalanced users, the frequency separation between assigned signaling frequencies must be expanded beyond the orthogonal spacing. It is found that the required bandwidth expansion can be significantly reduced by amplitude windowing at the receiver, at a cost of a slight decrease in detectability. This effect is evident through examination of both worst case, single source crosstalk and average, multiple user crosstalk.

The maximum bit rate of multitone QAM (quadrature amplitude modulation) over a general linear channel is found. First, the overall bit rate for an AWGN channel with a two-level transfer function is maximized, using a multitone QAM system. The power distribution between the tones and the number of bits/symbol per tone is optimized for a given symbol error rate. Extending these results to the general channel, it is shown that the optimum power division for multitone signals is similar to the water-pouring solution of information theory. Furthermore, multitone QAM performance is about 9 dB worse than the channel capacity, independent of the channel characteristics. The multitone results throughout are compared to those of an equivalent single-tone linearly equalized system. The comparison shows that the multitone system is useful for some channels, e.g. those with deep nulls in the transfer function. The maximum bit error rate over a twisted-pair channel which is performance dominated by near-end crosstalk (NEXT) is also found. >

A method is presented for comparing the performance of different quadrature-carrier modulation techniques. It is based on the calculation of the mean-square crosstalk between adjacent digital communications channels. This method was used to determine the crosstalk of a number of constant envelope modulation schemes, including Staggered Quaternary Phase Shift Keying (SQPSK) and Minimum Shift Keying (MSK). The results, as have previous results, show the improved performance of MSK over SQPSK. By utilizing the general equation for mean-square crosstalk as a measure of performance, a search was made for constant envelope modulation methods with reduced crosstalk. A number of methods were found which have better mean-square crosstalk performance than MSK. Finally this paper presents a method for reducing degradation due to crosstalk by using mismatched windows in the standard correlation detector. An example of this technique for detection of MSK signals is analyzed.

A QAM signal transmitted over a channel with linear distortion and additive white Gaussian noise can be linearly equalized at the receiver to eliminate intersymbol interference. If the QAM signal is power constrained and a given symbol error rate is required, we show that it is possible to maximize the bit rate of this system by optimizing the symbol rate and the number of bits/symbol. Ideal linear equalization is assumed at the receiver to overcome the distortion introduced by the channel. As an example, a Gaussian channel is chosen, and the bit rate is maximized for this channel. The QAM maximization is especially useful for channels with slowly decaying channel attenuation characteristics, e.g., the twisted-pair channel.