A new asymptotic analysis is presented to establish whether or not commonly used techniques to approximate the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) systems still apply when the systems are strictly band limited. The results apply to any linear data modulation and spreading scheme. The interference components, including the interchip interference (ICI) of a linear receiver output, are shown to be asymptotically jointly Gaussian random variables conditioned on the phases and delays of the interfering signals. Expressions for the error probabilities of band-limited systems are also developed from this result.

In this paper, the optimal design of a wideband overlay system is considered for communications over a white Gaussian noise channel corrupted by the signal from an existing system. Under the assumptions that the symbol transmission rate of the new system is an integer multiple of that of the existing system and that the transmit filter is time invariant, the optimum transmitter and receiver waveforms are derived that jointly minimize the average mean-squared error (MSE) at the output of the linear receiver of the new system. It is shown that this joint optimization technique leads to significant performance improvement through complete interference avoidance over the overlay systems with receiver-only optimization, and over those with no transmitter and receiver optimization. It is also shown that, if the existing system does not induce any self interference, the derived solution reduces to the generalized Nyquist condition for no mutual as well as self interference. The case where the symbol transmission rate of the existing system is an integer multiple of that of the new system is also briefly discussed.

In this paper, a linear receiver is proposed and its performance is analyzed for direct-sequence spread-spectrum communications with wideband data-like interference. Two groups of communicators employing generalized random spreading are considered, and the signals are assumed to have harmonically related cyclostationarity periods. First, the output signal-to-interference ratio (SIR) of a general correlation receiver is derived in the frequency domain, and the key system parameters that affect the structure of the interference are identified. Then, a linear interference suppression scheme based on the maximum SIR (MSIR) criterion is proposed, and a receiver structure similar to the cyclic Wiener filter is derived. Numerical results show that the proposed linear MSIR receiver significantly outperforms the conventional correlation receiver by effectively suppressing the wideband data-like interference.

Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms.

Gaussian approximations to the overall interference at the output of linear receivers are commonly used to analyze the performance of asynchronous DS/SSMA communication systems. In this paper, a new asymptotic analysis is presented to establish the validity of Gaussian approximations for the systems employing more general chip waveforms of interest, which include strictly band-limited waveforms. The transmit and receive chip waveforms, can be either time-limited or band-limited with their energy normalized to unity.

A new class of direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with continuous phase modulation (CPM) is defined. The signals are unique in that the spreading is done by adding an extra phase term to the information phase while maintaining phase continuity, constant envelope, and efficient bandwidth usage. The spreading phase is formed from the spreading code and is independent of the information phase, which allows despreading to be done separately before data detection, which, in turn, allows a simple CPM detector to be employed for data detection. The information phase is considered in the minimum-shift keying (MSK) format, and a serial-MSK-type spread-spectrum receiver is considered for performance analysis. Expressions for the signal-to-noise ratio, the power spectral density, and the probability of bit error are developed, along with methods for computing their values to an arbitrarily close approximation. Numerical results show that the proposed system is an attractive alternative to the conventional DS-SSMA systems.

Direct-sequence spread-spectrum multiple-access (DS/SSMA) communications, strictly band-limited transmitter chip waveforms with excess bandwidth in the interval between zero and one, pseudo-random spreading sequences, an additive white Gaussian noise channel, and matched filter receivers are considered. First, a new expression for the average bit error rate (BER) is derived for systems with quaternary phase-shift keying (QPSK) spreading, the conventional matched filter receiver, a coherent detector for binary phase-shift keying (BPSK) data symbols, and chip waveforms that result in no interchip interference. The expression consists of a well known BER expression based on the standard Gaussian approximation to multiple-access interference and a few correction terms. It enables accurate BER evaluations without any numerical integration for various choices of system parameters of interest. The accuracy of the expression is guaranteed as long as the conditional Gaussian approximation to the cross-correlation coefficients between the desired user's spreading sequence and the interfering users' spreading sequences is valid. The expression well reflects the effect of filtering on the system performance. Extensions of the expression are discussed for systems with QPSK spreading and different detection schemes, systems with BPSK spreading, and systems with different transmit and receive filters. Monte Carlo simulation results are also provided to verify the accuracy.

Slot synchronization is a critical step for fast and reliable cell search in intercell asynchronous direct sequence-code division multiple access systems. To increase reliability, observations over a number of slots may be combined. In this paper, combining schemes of multiple observations are studied for slot synchronization. The optimal combining rule is determined based on detection theory. It is found that two known combining schemes correspond to special cases of the optimal combining. These schemes may not work well in typical environments, since the schemes are optimized for specific environments. To improve slot synchronization performance in typical environments, a new combining scheme is proposed. The performance of the proposed combining scheme as well as other combining schemes is analyzed for Rayleigh fading channels with frequency offset. Numerical analysis shows that the proposed combining scheme significantly outperforms other combining schemes in typical environments.

Slot combining is a method to achieve fast code acquisition in inter-cell asynchronous DS/SS systems by employing multiple observations. The optimal slot combining is difficult to implement, since it requires information on channel and frequency offset. Two practical combining schemes, a noncoherent combining and a coherent combining, are found to be special cases of the optimal combining. In this paper, a novel differentially coherent combining scheme is proposed. This combining scheme may be viewed as a combination of differential detection and coherent combining techniques. The performances of the differentially coherent combining as well as the optimal combining, noncoherent combining, and coherent combining are analyzed for Rayleigh fading environments with frequency offset. It is found that the differentially coherent combining scheme significantly outperforms the noncoherent combining and coherent combining schemes for a typical range of Doppler spread and frequency offset.

Performance analysis schemes for wide-band M-ary frequency-shift keying systems with a limiter-discriminator-integrator receiver are presented along with analysis and simulation results for 4-ary and 8-ary systems. The probability distribution of clicks is required for performance analysis and is determined for Rayleigh fading channels.