This paper is concerned with the efficient transmission of channel state information (CSI) in the uplink, and data in the downlink, of a closed-loop transmit diversity system. Two ideas were explored: 1) the employment of analog CSI feedback whereby estimates of the downlink complex gains were fed back periodically to the base-station at a rate of only 1 analog symbol per estimate per frame, and 2) the use of position dependent modulation (PDM) to counter the signal-to-noise ratio variation across a downlink data frame. Simulation results indicated that as far as the bit-error-rate (BER) is concerned, analog feedback is superior to digital feedback because it does not experience unrecoverable quantization errors in the feedback channel estimates. The simulation also indicated that at a given target BER, PDM can attain a substantially higher throughput than fixed modulation

Multiple-symbol differential detection (MSDD) and pilot symbol-aided detection (PSAD) are two detection schemes that have been proposed for detecting data transmitted over fast fading channels. When the parameters of these detectors are chosen properly, they can provide an error performance that is close to ideal coherent detection. In this paper, we apply these two detection methods to a multiuser receiver for direct sequence CDMA. The multi-user receiver is implemented in the form of a multi-channel linear equalizer (LE), followed by either a bank of MSDD or a bank of PSAD. With this receiver structure, the front end equalizer is not required to perform any channel estimation. Fading gain estimation and compensation are done by the MSDD or the PSAD. It was found that both MSDD and PSAD perform substantially better than conventional differential detection when the signal-to-noise ratio (SNR) is large (greater than 20 dB) and/or when the fade rate is high (greater than 1 percent the symbol rate).

This paper presents a method for the exact calculation of the union bound on the error performance of trellis-coded modulation (TCM) in fading channels. Previous analyses have approximated the error expression to allow the use of the transfer function, or restricted their attention to the most common error events, or both. In contrast, we combine the transfer function, which counts all error events, with exact calculation of event probabilities, and produce an exact expression for the union bound. Although computationally intensive, the technique produces the tightest bound yet published. In addition, it is very general and is applicable to coherent, differential, and pilot-based detection, and to phase-shift keying (PSK)- and quadrature amplitude modulation (QAM)-types of constellations.

This paper is concerned with the subject of coding and detection for DS/CDMA systems operating under very fast Rayleigh fading. Specifically, we compare the performance of various chip level detectors, including the chip level pilot-assisted coherent detector, chip level differential detector, chip level non-coherent detector, and the chip level maximum likelihood detector. The true union bounds on the bit error rate of these detectors were computed, with the fade rate, coding rate, and the spreading factor as parameters. It was found that these detectors are very robust against fast fading. A reasonable level of data integrity can be maintained even if the fade rate is as large as 10% of the chip rate. Among the various detectors, the pilot-aided chip level coherent detector has the best performance while the chip level differential detector is the worst. Surprisingly enough, the optimal pilot-aided detector, under the assumption of ideal interleaving, uses a transmission format that consists of an alternating pattern of pilot and coded data symbols. It was also found that chip level differential detection performs better than its symbol level counterpart only when the fade rate exceeds 6% of the chip rate.

We present the true union bound of a DS/CDMA system that employs convolutional coding and a quadratic detector. In our study, the total bandwidth expansion factor caused by coding and spreading is fixed and the channel is assumed to exhibit Rayleigh flat fading. The objective is to assess the performance of various quadratic detectors as a function of the spreading factor and the fade rate. Specifically, four detectors were considered: chip level differential detection, chip level maximum likelihood sequence estimation, noncoherent detection, and perfect coherent detection. It was found that in the presence of fast Rayleigh fading, different detectors behave differently when the spreading factor varies. For example, the chip level differential detector and the ideal coherent detector favour a small spreading factor and a low rate code, while the MLSE prefers a high rate code and a high spreading factor. For the noncoherent detector, there actually exists an optimal system configuration at certain fade rates.

We present a coherent detection technique for continuous phase modulation (CPM) operating in the Rayleigh flat fading channel. The technique is based on the idea of inserting periodically data dependent pilot symbols that force the CPM signal to pass through known phase states. This transmission format enables the receiver to extract from the received signal the channel fading gains at regularly spaced instants. When coupled with proper channel estimation filters, very accurate channel state information (CSI) can be estimated at the receiver for fading compensation. Moreover, the accuracy of the CSI can be further refined by adopting a multiple-pass decoding approach. The paper discusses (a) the pilot symbol encoding technique required to force a M-level CPM scheme with a modulation index of p/M, p is an integer, to return periodically to a set of known phase states, (b) the optimal channel estimation filters, (c) a trellis-based precoding technique that can reduce the bit error rate in M-level CPM systems by close to 50%, and (d) a multiple-pass channel estimator/demodulator. Analytical and simulation results are presented for minimum shift keying (MSK), Gaussian MSK, and four-level continuous phase frequency shift keying with a modulation index of 1/4. It is observed that our pilot symbol-assisted CPM schemes exhibit no irreducible error floor even at a channel fade rate of three percent the symbol rate. The implicit phase coding in CPM and the accurate CSI provided by the pilot symbols lead to a diversity effect in the bit error rate curves of these modulation schemes.