Spread-spectrum Multiple-access Systems Research Articles

On the MAX/TC criterion for code acquisition and its application to DS-SSMA systems

We analyze in depth the MAX/TC criterion for code acquisition in spread-spectrum transmissions. The uncertainty region is divided into sectors: inside each sector, the cell with associated maximum test variable is selected (MAX criterion), and sectors are serially tested according to the threshold crossing (TC) criterion. The most commonly used analysis tools for code acquisition, namely the direct and the flow-graph approaches, are extended to the MAX/TC criterion. An accurate but simple approximation to the acquisition time probability density function is found in terms of a gamma distribution, which is shown to yield much better accuracy than the Gaussian distribution. The general formulation is applied to the particular case of direct-sequence spread-spectrum multiple-access (DS-SSMA) systems, showing that sector size is a fundamental parameter to be optimized in the minimization of acquisition time, and that in some cases MAX/TC with parallel architecture performs marginally better than MAX/TC with serial architecture.

Performance of chip coding in FFH SSMA systems

This paper proposes chip coding using convolutional codes for fast frequency-hopping spread-spectrum multiple-access (FFH-SSMA) systems. Its performance is evaluated and compared with that of repetition coding which is most commonly used in FFH systems. In our proposal, an information symbol is encoded by a convolutional code of rate 1/n and n chips are transmitted through a frequency hopper. We consider three models when perfect side information (PSI) is available, no side information (NSI) is available, and the ratio threshold test (RTT) is used. The probability density functions (PDFs) of the received signal are derived for asynchronous hopping systems. The performances of convolutional coding with threshold decoding and Viterbi decoding are presented under the constraint of fixed total bandwidth and information bit rate. In comparison with repetition codes, it is found that convolutional codes drastically enhance the system performance. It is identified that Viterbi decoding with RTT offers the most significant performance improvement.

Estimated bit error probability of DS-SSMA/MDPSK with differential phase detector on satellite mobile channel

The performances of direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with M-ary differential phase shift keying (MDPSK) and differential phase detector (DPD) are estimated on the satellite mobile channel (SMC) which includes Gaussian and Rayleigh channels as the special cases. The numerical results are presented for the system with one of three types of chip waveforms (rectangular, cosine and root Nyquist filtering (RNF)), and binary or complex random signature sequences. They show that the system with the complex random signature sequences outperforms the system with the binary random signature sequences on Gaussian and Rician (K = 10) channels, while on a Rayleigh channel both systems perform similarly; the system with 2 × I users and BDPSK signalling format outperforms that with I users and QDPSK for Gaussian and Rician (K = 10) channels; and from the point view of both bandwidth efficiency and antimultipath effects, the cosine pulse shaping is the best choice of the three pulse shape functions, although the system with the RNF has the highest bandwidth efficiency.

A reservation random-access protocol for voice/data integrated spread-spectrum multiple-access systems

Most code-division multiple-access (CDMA) systems described in the literature provide only one single service (voice or data) and employ the strategy of "one-code-for-one-terminal" for code-assignment. This assignment, though simple, fails to efficiently exploit the limited code resource encountered in practical situations. We present a new protocol called reservation-code multiple-access (RCMA), which allows all terminals to share a group of spreading codes on a contention basis and facilitates introducing voice/data integrated services into spread-spectrum systems. The RCMA protocol can be applied to short-range radio networks, and microcell mobile communications, and can be easily extended to wide area networks if the code-reuse technique is employed. In RCMA, a voice terminal can reserve a spreading code to transmit a multipacket talkspurt while a data terminal has to contend for a code for each packet transmission. The voice terminal will drop a long delayed packet while the data terminal just keeps it in the buffer. Therefore, two performance measures used to assess the proposed protocol are the voice packet dropping probability and the data packet average delay. Theoretical performance is derived by means of equilibrium point analysis (EPA) and is examined by extensive computer simulation.

On the decimations of Frank sequences

Sets of decimated Frank (1962) sequences with optimal correlation properties were first considered by Alltop (1984). By regarding decimated Frank sequences as special cases of the generalized Frank sequences proposed by Suehiro and Hatori (1988), the set size of decimated Frank sequences can be doubled without sacrificing the optimality of the correlation properties. Besides, the proposed sequences form representatives of generalized Frank sequences of different classes. They thus provide an efficient way to generate signature sequences for spread spectrum multiple access systems with dynamic signature assignments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Code diversity transmission in a slow-frequency-hopped spread spectrum multiple-access communication system

At every hop in a conventional frequency-hopped spread spectrum multiple-access (FH-SSMA) system, each transmitter sends its data using a single frequency bin. In this paper, a scheme in which several frequency bins are simultaneously used by a transmitter is studied. It is found that such a code diversity scheme can yield a significant reduction in symbol error rate over that of a conventional FH-SSMA system. A priority system can also be implemented by assigning different diversity degrees to the transmitters.

Performance evaluation of multi‐cell direct‐sequence microcellular systems

AbstractThe performance of an asynchronous multi‐cell Direct‐Sequence Spread Spectrum Multiple‐Access (DS/SSMA) system is evaluated in lineal microcells with flat Rician fading and shadowing. Both the Bit Error Rate (BER) of the uplink (i.e., the user to the base station) and the downlink (i.e., the base station to the user) are evaluated. We develop a new analytical model to evaluate the uplink performance which accounts for the user's spatial distribution, the chip waveform, the power control, and macro‐selection diversity. On the downlink, since no power control is used, the desired signal follows a composite log‐normal noncentral chi‐square distribution. We propose an approximation method to reduce the computational complexity while still providing satisfactory precision. The analysis results show that the macro‐selection diversity can reduce the other‐cell interference effectively even for a channel with heavy shadowing. The Rician factor, the standard deviation of the shadowing, and the chip waveform have significant influence on the performance.

Estimated bit-error probability of DS-SSMA/MPSK with coherent detector on satellite mobile channel

In this paper we estimate the average bit-error probability of a direct-sequence spread-spectrum multiple-access (DS-SSMA) system using MPSK and a linear coherent detector on the satellite mobile channel (SMC). The formulas derived in this paper are suitable for various types of signature sequences and chip waveforms. The numerical results are presented for DS-SSMA using BPSK and QPSK with random signature sequences and three different chip waveforms: the rectangular, the cosine, and the root Nyquist filter (RNF) with rolloff rate, /spl beta/=0.4. The results show that the performance of a DS-SSMA system with complex random signature sequences is superior to a system using binary random signature sequences on Gaussian and Rician (K=10) channels, while on flat fading Rayleigh channels the performance for both systems is similar. In DS-SSMA, the RNF has the highest bandwidth efficiency of the three chip waveforms. The performance of DS-SSMA is also analyzed on the SMC using the Gaussian approximation (GA) method. Compared to the estimated results, GA gives a pessimistic approximation for binary random signature sequences on Gaussian and Rician channels.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hybrid slow frequency hopping/ direct sequence spread spectrum indoor communication systems with QPSK modulation

This paper analyses the performance of a spread-spectrum multiple access system based on hybrid direct sequence/slow frequency hopping techniques with QPSK modulation. The performance is analyzed considering multipath and multiple access interferences and an indoor channel having Rician distributed path gains. The system performance is investigated for 2 diversity techniques and compared, for given delay spread, bandwidth and bit rate, with the performance provided by bpsk modulation. It is shown that the larger symbol duration of QPSK is favorable to combat the effect of multipath propagation.

Matched median detectors for multiple-access channels

The performance of matched median detectors is studied in a coherent code-division multiple-access environment addressing asynchronous and synchronous communication. Two types of noise sources interfering with the data transmission are assumed: multiple-access interference and additive impulsive channel noise. A detector structure based on a multistage multiple-access rejection scheme and matched median filters is studied. Probability of error analysis is presented for a two-stage detector, and performance is characterized in terms of average bit-error probability. Calculations and simulations show that the multistage matched median structure performs well in situations where the energies of the code signals are different, therefore being able to combat the near-far problem characteristic of direct-sequence spread spectrum multiple-access systems.

Performance of binary DS‐SSMA communications through Nakagami fading channel

Abstract A binary asynchronous direct sequence spread spectrum multiple access system through Nakagami's m‐distributed channel is considered for non‐diversity receptions. Effects of fading and multiple access interferences on the average error probability are investigated. Approximation method to the average error probability is evaluated in two steps based on the moments of multiple access interferences and Nakagami distributions. Gauss quadrature rule is applied to evaluate the conditional probability conditioned on a fixed m‐distributed fading amplitude of the desired signal. Average error probability is then evaluated by the trapezoidal integration which integrates the conditional probability and the m‐distributed probability density function of the desired signal. This method provides a good approximation to the average error probability when the number of simultaneous users is large. Numerical results for the pseudorandom codes of code length 31 and Gold codes of code length 31 and 127 are presented.

An alternative derivation for the signal-to-noise ratio of a SSMA system

We present an alternative derivation for the signalto- noise ratio (SNR) of a phase-coded spread-spectrum multiple access (SSMA) system by use of ensemble-average approach. We show that our formula can be explicitly expressed in terms of system parameters and the expression for the SNR in [l] is just a special case when the user number is very large.

Adaptive retransmission control for slotted frequency-hopped spread-spectrum multiple access systems

The authors propose a simple adaptive retransmission control scheme for slotted frequency-hopped spread-spectrum multiple access systems. The scheme changes the retransmission probability adaptively according to the system state such that channel throughput can be maximised. It is shown that a performance quite close to the maximum possible throughput can be achieved.

Synchronous DS-SSMA system with improved decorrelating decision-feedback multiuser detection

An improved decorrelating decision-feedback multiuser detector (IDDFD) is proposed, whose complexity is linear in the number of users, but performs closely to the maximum likelihood detector (MLD), whose complexity grows exponentially with the number of users. The IDDFD also relaxes the requirement of correct power ordering which is essential to successfully cancel multiuser interference (MUI) for the standard decorrelating decision-feedback detector (DDFD). Simulation results show that in a direct-sequence spread-spectrum multiple-access (DS-SSMA) system using Gold codes of length 31 an IDDFD with only twice the complexity of the DDFD can simultaneously service 31 users with an error rate of each user close to the interference-free case. The IDDFD is then extended and studied to user numbers larger than the processing gain, complex signature waveforms, and MPSK signaling.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Average error probability for quadriphase modulated DS-SSMA communications through Nakagami fading channel

An asynchronous direct sequence spread spectrum multiple access system using quadrature phase shift keying modulation through Nakagami'sm-distributed fading channel is considered for nondiversity reception. Approximation to the average error probability is evaluated in two steps. Using the Gauss quadrature rule, the moments of the multiple interferences are used to evaluate the conditional probability conditioned on a fixed fading amplitude of the desired signal. The probability density function of the desired signal is Nakagami distributed. The average error probability which is the expected value of the conditional probability is evaluated by the trapezoidal integration method. This method provides a good approximation to the average error probability for a small to a fairly large number of users. Numerical results are presented for a set of Gold code of code length 127.

Multiple hops/symbol FFH-SSMA with MFSK modulation and Reed-Solomon coding for indoor radio

A multiple-hopes-per-symbol fast-frequency-hopped spread-spectrum multiple-access (FFH-SSMA) system for indoor radio communication is treated. The data are Reed-Solomon (RS) encoded and then modulate a carrier by means of M-ary frequency-shift-keying (MFSK) The indoor radio channel is described as a multipath (Rayleigh or Rician) slowly fading channel. The receiver uses predetection diversity by linearly combining the squared enveloped of the different hops of the same MFSK symbol. The bit error rate (BER) performane is evaluated, and how the performance is influenced by various parameters, such as the number of hops per symbol, the size of the symbol alphabet, the ratio of specular power to fading power, and the ratio of nonreference user power to reference user power, is investigated. >

Hybrid DS/SFH spread-spectrum multiple access with predetection diversity and coding for indoor radio

The authors derive close upper and lower bounds on the average bit error probability for hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access (DS/SFH-SSMA) systems with noncoherent DPSK demodulation, using predetection diversity (selection combining and equal gain combining) in conjunction with interleaved channel coding (Hamming

A simple, accurate method to calculate spread-spectrum multiple-access error probabilities

The exact calculation of error probabilities for direct-sequence spread-spectrum multiple-access (DS/SSMA) systems has been addressed in the literature. The exact calculation is computationally difficult, so emphasis has been on approximations and bounds. One particularly attractive approximation is to just use a signal-to-noise ratio in a Gaussian approximation, the 'standard approximation'. Unfortunately, that approximation is not generally accurate enough. An improved Gaussian approximation with good accuracy has recently been presented. The author derives an accurate Gaussian approximation which is also computationally very simple. >

Hybrid DS/SFH-SSMA with predetection diversity and coding over indoor radio multipath Rician-fading channels

The authors investigate the bit-error-rate (BER) performance of hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access (DS/SFH-SSMA) systems operating over a multipath Rician-fading channel (which models indoor radio propagation in factories). They consider both phase-shift-keying (PSK) modulation with coherent demodulation and differential phase-shift-keying (DPSK) modulation with noncoherent demodulation. Predetection multipath diversity (maximal ratio combining for coherent reception and equal gain combining for noncoherent reception) and simple interleaved channel coding are employed for improving the BER performance. The BER of both coherent and noncoherent hybrid systems is obtained using a Gaussian interference approximation. >

Multiple hops/symbol fast frequency-hopping spread spectrum multiple access with coding for indoor radio

The paper is concerned with the bit error rate (BER) performance of multiple hops/ symbol fast-frequency-hopped spread-spectrum multiple-access (FFH-SSMA) systems operating through an indoor radio multipath Rayleigh fading channel. M-ary frequency-shift-keying (MFSK) modulation with noncoherent square-law envelope demodulation and Reed-Solomon (RS) coding is considered. Results obtained in this study show that an FFH-SSMA system is not sensitive to the changes in the number of paths. Employing RS coding with an increased number M of MFSK signals yields a significant improvement in BER performance. Assuming a given system bandwidth, using a single hop per symbol is the best solution when the multiaccess interference can be large. Using several hops per symbol is more advantageous when the multiaccess interference is small.