The frequency diversity of multicarrier direct-sequence signaling can potentially offer robust performance in frequency-selective channels. The paper focuses on the acquisition of multicarrier signals in channels containing fading and partial-band interference. The maximum-likelihood decision rule for parallel acquisition in frequency-selective fading and partial-band interference is derived. Several simpler, near-optimal decision rules are also discussed. The performance of these decision rules is compared to that of equal-gain combining for multicarrier acquisition. Results show that the decision rules designed specifically for partial-band interference give significantly better performance. Methods of acquisition with a limited number of correlators are also discussed. Finally, the potential benefits of estimating the signal strength on each subcarrier prior to acquisition are examined.

This paper investigates the design and performance of hybrid sequential and nonsequential acquisition schemes for direct-sequence spread-spectrum (DS/SS) systems in the presence of multiple-access interference. Carrier phase and chip boundary information are not available at the time of acquisition. Simulation results show that the sequential scheme significantly outperforms the nonsequential scheme. For a DS/SS system using the sequential acquisition scheme, the acquisition-based capacity is evaluated and compared with the post acquisition-based capacity. Results show that for certain values of processing gain the system capacity is limited by the acquisition-based capacity.

This paper investigates the design of hybrid chip-asynchronous acquisition schemes for direct sequence spread-spectrum communication systems. Both sequential and nonsequential tests are considered. Two novel sequential tests are considered that are based on the sequential probability ratio test (SPRT). Pipelining is used in the sequential schemes to ensure efficient utilization of the correlators. Simulation results show that the sequential tests perform significantly better than a nonsequential test.

This paper investigates the problem of acquisition in unslotted distributed direct-sequence packet radio networks. A noncoherent serial acquisition scheme is considered that uses passive matched filters to detect packet headers. A new Gaussian approximation is derived to model the effects of multiple-access interference during acquisition. This approximation takes into account oscillator inaccuracies and Doppler shifts, and its validity depends only on the presence of a sufficient number of chips in the acquisition header. In particular, the accuracy of the approximation is not contingent on the presence of a large number of equal-power interferers. False alarm processing is modeled and a simple expression for the probability of acquisition is derived. The problem of threshold selection is considered and it is shown that a single threshold can be chosen to provide acceptable performance in a wide range of channel conditions.

The performance of frequency-hop spread-spectrum (FHSS) communication systems using hybrid automatic-repeat-request (ARQ) can be improved by combining current and prior transmissions at the receiver. Two methods for combining packets in systems that employ interleaving and Reed-Solomon (RS) coding are presented and analyzed for the partial-band interference channel. These methods use majority logic combining at the codeword level to make retransmission decisions. Bounded distance errors-and-erasures decoding and erasure generation by means of Viterbi's ratio threshold test (RTT) are incorporated in the analysis. Results of the analysis show that, with comparable packet error probabilities, the packet-combining schemes provide significant gains in throughput when compared with systems that do not employ combining.

In this letter, formulas for the total number of decodable and undecodable vectors are derived for a general (n,k) q-ary linear block code. These formulas are used to find the probabilities of decoder error and decoder failure for Reed-Solomon codes under errors-and-erasures decoding. The resulting analytical expressions are computationally efficient and allow accurate calculation of very small values of decoder failure probabilities. The formulas are used to analyze the performance of type-I hybrid automatic-repeat-request (HARQ) protocols with two decoding diameters.

We present an algorithm that permits a receiver to calculate the probability of packet error in parallel with the Viterbi decoding process. This packet reliability value may be used to request retransmissions in a type-I hybrid ARQ scheme. It is shown that this scheme can be used to guarantee any required bound on the packet error probability. In addition, this scheme can be used in conjunction with packet combining. The performance of this scheme is compared with that of the Yamamoto-Itoh algorithm and is shown to provide a significant improvement in throughput.

Erasure generation techniques are investigated in conjunction with symbol interleaving as methods to improve the performance of meteor-burst communication systems. Erasure generation via Viterbi's (1982) ratio-threshold test (RTT) and a Bayesian scheme are both considered. It is found that a system using fixed-rate coding, interleaving, and erasure generation can significantly outperform more complex variable-rate coding schemes which do not use erasures.

We present a framework for determining the exact decoder error and failure probabilities for linear block codes in a frequency-hop communication channel with an arbitrary number of conditional symbol error and erasure probabilities. Applications are demonstrated for type-I hybrid ARQ systems by deriving equations for the packet error probability and throughput. Because these quantities are too small to be obtained by simulation, the framework provides exact results which are unobtainable by previous work.

This paper presents a novel multitone transmission scheme that employs a nonlinear binary code to specify multitone signal constellations. The scheme, which can be viewed as a generalization of M-ary frequency shift keying, is amenable to noncoherent demodulation, and it is therefore useful for communication over fading channels. We determine the performance of this modulation scheme when it is used in conjunction with a Reed-Solomon code for error control. Methods for determining and erasing unreliable multitone constellations are presented and their performance is evaluated when they are used in conjunction with errors-and-erasures decoding. The transmission scheme is evaluated with respect to complexity, bandwidth efficiency, and performance in the presence of Gaussian noise and frequency- and time-selective fading.

The use of block coding and errors-and-erasures decoding can enhance performance in frequency-hop (FH) communication systems, provided that a good scheme is employed to determine which symbols to erase. The problem of making erasure decisions from collections of receiver outputs is investigated in this paper. Methods to determine which received symbols to erase are derived from Bayesian decision theory. Decision rules are developed for a system with M-ary orthogonal signaling, noncoherent demodulation, and frequency-selective fading. One result is a class of Bayesian schemes in which erasure decisions are made independently from symbol to symbol. Within this class is a rule that uses signal amplitude estimates for improved performance. A second result is a Bayesian technique in which erasure decisions are mutually dependent and are made collectively for each codeword. These techniques are analyzed and compared with the performance of receivers that use erasure techniques that require multiple applications of bounded distance decoding. The performance of the Bayesian techniques for dependent erasures are also compared with the performance of receivers that do not permit erasures. It is found that each of the Bayesian techniques offers substantial performance gains over errors-only decoding, and the dependent erasure scheme provides the best performance among the techniques of lower complexity.

The use of side information can provide increased coding gains in frequency-hop multiple-access communication systems. The internal generation of this side information by the receiver is investigated in the paper. Methods to determine which received signals to erase prior to decoding are derived from Bayesian decision theory. The performance of synchronous and asynchronous frequency-hop systems with Reed-Solomon coding is analyzed for channels with Rayleigh fading, multiple-access interference, and wideband Gaussian noise. The performance of the Bayesian technique is compared with that of receivers that make erasure decisions using the Viterbi ratio-threshold test. The Bayesian method is found to provide better performance.>

A sequential multihypothesis test known as the M-ary sequential probability ratio test (MSPRT) is generalized to account for nonuniform decision costs. Bounds on error probabilities and asymptotic expressions for the stopping time and error probabilities are given. A key result of this correspondence is a proof that the generalized MSPRT is asymptotically efficient.

The sequential testing of more than two hypotheses has important applications in direct-sequence spread spectrum signal acquisition, multiple-resolution-element radar, and other areas. A useful sequential test which we term the MSPRT is studied in this paper. The test is shown to be a generalization of the sequential probability ratio test. Under Bayesian assumptions, it is argued that the MSPRT approximates the much more complicated optimal test when error probabilities are small and expected stopping times are large. Bounds on error probabilities are derived, and asymptotic expressions for the stopping time and error probabilities are given. A design procedure is presented for determining the parameters of the MSPRT. Two examples involving Gaussian densities are included, and comparisons are made between simulation results and asymptotic expressions. Comparisons with Bayesian fixed sample size tests are also made, and it is found that the MSPRT requires two to three times fewer samples on average.>

The erasure of unreliable symbols improves the performance of most types of error-control coding if a good method is used to decide which symbols should be erased. Bayesian decision theory is employed to obtain such a method for use in frequency-hop communications with Reed-Solomon coding and errors-and-erasures decoding. The performance of frequency-hop communications with Bayesian erasure insertion is analyzed for channels with both partial-band and wideband Gaussian noise. The Bayesian technique is compared with Viterbi's ratio-threshold test, and these are compared to receivers that do not erase and use errors-only decoding. Comparisons are also made with receivers that erase all the symbols that are affected by the partial-band interference. When interference is strong, large coding gains result from the Bayesian method, and error probabilities are reduced by several orders of magnitude.>