A binary direct-sequence spread-spectrum multipleaccess system with random sequences in flat Rayleigh fading is considered. A new explicit closed-form expression is obtained for the characteristic function of the multiple-access interference signals. It is shown that the overall error rate can be expressed by a single integral whose integrand is nonnegative and exponentially decaying. Bit-error rates (BERs) are obtained with this expression to any desired accuracy with minimal computational complexity. The dependence of the system BER on the number of transitions in the target user signature chip sequence is explicitly derived. The results are used to examine definitively the validity of three Gaussian approximations and to compare the performances of synchronous systems to asynchronous systems.

A dynamic medium access control (MAC) protocol is proposed for a finite-user slotted channel with multipacket reception (MPR). This protocol divides the time axis into transmission periods (TPs) where the ith TP is dedicated to the transmission of the packets generated in the (i - 1)th TP. At the beginning of each TP, the state (active or idle) of each user is estimated based on the length of the previous TP and the incoming traffic load. By exploiting the information on the state of users and the channel MPR capability, the number of users who can simultaneously access the channel in the current TP is chosen so that the expected length of this TP is minimized. As a result, the MPR capability is more efficiently utilized by the proposed protocol as compared to, for example, the slotted ALOHA with optimal retransmission probability. Furthermore, the proposed protocol requires little on-line computation. Its simplicity is comparable to that of slotted ALOHA. It can be applied to random access networks with spread spectrum and/or antenna array.

Abstract- An alternative code-division multiple-access user is determined by modulating the phase of the desired (CDMA) scheme to spread spectrum (SS), called spread time transmitted power spectral density. ’ That is, the available (ST) is proposed for badlimited multiple-access channels. bandwidth is partitioned into M subbands, or frequency bins, ST-CDMA can be considered the be-frequency dual of SS-CDMA. In ST-GDMA pslendsraodan (PN) sequences are where M is the processing gain. Each bin is assigned a phase assigned to each user, and the Fourier tcrursf@rm of the (Le., 0. T, f7(./2), which depends on the user’s PN-sequence. transmitted pulse for a given user is d&mined by modulating The pulse is then obtained by taking the inverse Fourier transthe phase of the desired transmitted spectnnn by the user’s form of the resulting frequency response. At the receiver, the PN-skquence. The t”Btasd data for a parthdnr mer can desired data can be recovered by sampling the output of a filter be recoverad by sampling t8c output of a Plter matchd tcE tbe w”s pulse. ImplemenEwtloqs rare dascrilred in which wrface matched to the specific user pulse. Alternatively, the receiver acoustic wave devices are used to perform the matched fislteL.ing can compute the Fourier transform of the time-windowed or Fourier transformation. Averaged signal-to-intdremoe received signal at each symbol interval, which, in the absence

A performance comparison is presented between two types of code-division multiple-access wireless local area networks: centrally controlled and ad hoc networks. Based on a finite-population model, the network throughput, the average packet delay, and the network first exit time are derived for both systems. Two aspects of the performance comparison are addressed: 1) the comparison between the centrally controlled and the ad hoc architecture; and 2) the impact of spreading gain and error control coding on both systems. The efficiency of bandwidth utilization is investigated by normalizing the network performance with respect to the consumed bandwidth. Evaluations of these performance comparisons are also provided.

In this paper, we first investigate the effect of residual code phase offset on the direct-sequence code-division multiple-access code acquisition scheme using a noncoherent matched filter receiver. We then propose a new code acquisition scheme that is robust to the variation of the residual code phase offset and outperforms the conventional scheme. When the code phase offset normalized to the chip duration between two sequences is within the advancing step size, the sum of two successive matched filter outputs has a constant value regardless of the residual code phase offset when the noise is absent. Based on this observation, we propose a new code acquisition scheme and investigate the performance of the scheme. The proposed code acquisition scheme is analyzed in an additive white Gaussian noise and multiple-access interference environment. Finally, numerical results are given to show that the proposed scheme is more robust to the variation of the residual code phase offset and has better performance than the conventional scheme on average.

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Many techniques for calculating bit-error probabilities (BEPs) of direct-sequence spread-spectrum multiple-access systems (DS-SSMA) have been reported. Among them are the following three techniques: 1) the standard Gaussian approximation; 2) the improved Gaussian approximation; 3) and the simplified improved Gaussian approximation. We extend these techniques to derive the BEPs of multicode DS-SSMA systems. We assess the three techniques by comparing their results with the result of computer simulation.

Abstract — In previous research, we considered SSMA (spread spectrum multiple access) communication systems using spreading sequences of Markov chains and pointed out that the theoretical evaluations of bit error probabilities in such systems based on the SGA (standard Gaussian approximation) is rather optimistic comparing with those based on the CLT (central limit theorem). The theoretical evaluations of bit error probabilities in such systems based on the CLT were in the forms of multiple integrals. In this report, we give approximations for these forms and show that they agree properly with the experimental results. I.

Non-coherent detectors for initial code synchronization (acquisition) of BPSK direct sequence spread spectrum signals on an AWGN channel are analyzed. In addition to the thermal noise, in many applications such detectors are faced with the "self-noise", due to the partial period correlations. Under the random code sequences assumption, in this paper an exact analysis of the non-coherent correlator's detection performance is carried out by using the theory of circularly symmetric random variables. The exact analysis shows that the familiar Gaussian approximation to the distribution function of the code self-noise is justified for all cases of practical interest. Furthermore, the overall detection performance was found to be determined asymptotically by the sum of the thermal and correlator's self-noise. In most cases of practical interest, this asymptotic result provides a very good approximation to the actual detection performance of a non-coherent correlator, improving the approximati...

The standard Gaussian approximation (SGA) for error analysis of direct-sequence code-division multiple-access (DS-CDMA) systems is very optimistic in many cases. Improved Gaussian approximation (IGA) is a technique that produces accurate error probabilities, but is still computationally intensive. Simplified IGA (SIGA) has complexity similar to that of SGA and, at the same time, provides sufficient accuracy. In this paper, we consider SIGA for DS-CDMA systems employing random sequences in a band-limited scenario. The validity of IGA for band-limited systems is established in a rigorous mathematical sense. Then a key parameter in SIGA is derived via a frequency -domain approach. Applications to a number of typical chip waveforms, including the popular sinc and raised-cosine pulses, are investigated. Performance comparison with IGA-based lower and upper bounds shows that SIGA yields very accurate probability of error.