Direct-sequence Spread-spectrum Multiple-access Research Articles

Quaternary DS-SSMA Communications Employing Generalized Detector over Rayleigh Fading Channels

This paper deals with an employment of the complex modulators and generalized receivers constructed based on the generalized approach to signal processing (GASP) in noise in quaternary direct-sequence spread-spectrum multiple-access (DS-SSMA) communication systems with complex signature sequences in flat Rayleigh fading. Owing to availability of large sets of complex spreading sequences possessing perfect correlation characteristics, the interest to use the complex spreading sequences in DS-SSMA communication systems has increased. The complex spreading sequences investigated in the present paper are based on the recently introduced orthogonal unified complex Hadamard transform (UCHN) sequences. In the present paper, the complex processing in modulators and generalized receivers is also employed with the purpose to take advantage of the correlation properties of complex signature sequences. We derive the average bit error rate (BER) for quarternary synchronous communications system based on the generalized approach to signal processing in noise first, and then the BER for quaternary asynchronous ones is evaluated using the characteristic function approach. We presented some results based on the Gaussian approximation method for asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise Computer modeling results demonstrate that the communication systems using UCHT spreading sequences perform generally better than the GOLD sequences and the 4-phase family A sequences. Comparative analysis between the asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise demonstrates superiority over the asynchronous communication systems employing the correlation receiver.

Noncoherent unitary space-time modulated DSSS systems in multipath channels

In this paper, in order to effectively apply unitary space-time modulation to the direct-sequence spread-spectrum multiple-access (DSSS-MA) networks, we propose a low-rate, noncoherent, unitary, and space-time modulated DSSS system supporting any number of transmit antennas based on Walsh matrices. The proposed scheme simultaneously performs bandwidth spreading and space-time coding and outperforms those using high-rate, conventional unitary space-time constellations. Furthermore, the proposed scheme allows for a simple detector structure based on fast Walsh transforms.

Evaluating viterbi decoded reed-solomon block codes on a complex spreaded DS/SSMA CDMA system

In recent years, the Viterbi Algorithm (VA) has been shown to be an efficient trellis decoder for binary and non-binary linear blocks. In this simulation study the performance of VA decoded non-binary Reed-Solomon (RS) block codes, employed in a wideband Code Division Multiple Access (CDMA) system, operating in multipath fading channel conditions, is considered. The RAKE receiver-based Direct Sequence Spread Spectrum Multiple Access (DS/SSMA) Quadrature Phase Shift Keying (QPSK) system employed in this study uses Complex Spreading Sequence (CSS), such as Zadoff-Chu (ZC), Double Sideband (DSB) Constant Envelope Linearly Interpolated Root-of-Unity (CE-LI-RU) filtered General Chirp-like (GCL), Analytical Bandlimited Complex (ABC) and Quadriphase (QPH) sequences. This paper firstly describes the notion of VA decoding of linear block codes. Background information is then given on the filtered and unfiltered CSS families employed in the simulation study. Next, the complex Direct Sequence Spread Spectrum Multiple Access (DS/SSMA) Quadrature Phase Shift Keying (QPSK) transmitter and RAKE receiver simulator structures, as well as a novel complex multipath fading channel simulator structure are presented. This is then followed by a description of this study's versatile multi-user multipath fading performance evaluation platform, which is constructed using the complex RAKE receiver-based DS/SSMA QPSK communication system and complex multipath fading channel simulator structures. Lastly, simulated Bit Error Rate (BER) performances are presented for uncoded wideband systems and wideband systems employing VA decoded non-binary RS block codes in realistic multi-user multipath fading channel conditions. The effect of Multi-User Interference (MUI), as well as that of Channel State Information (CSI) on the decoding performance of the VA are also considered.

An analysis on the state-dependent nature of DS/SSMA unslotted ALOHA

In this paper, we present a novel approach to analyze the throughput of direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system. In the unslotted system, the departure rate of interfering transmissions is proportional to the number of current interferers that can be regarded as the system state. In order to model this state-dependency, we introduce a two-dimensional state transition model that describes the state transition of the system. This model provides a more rigorous analysis tool for the DS/SSMA unslotted ALOHA systems with both fixed and variable packet lengths. Numerical results reveal that this analysis yields an accurate system performance that coincides with the simulation results. Throughout the analysis we have discovered that the state-dependency of the departure rate causes interference averaging effect in the unslotted system and that this effect yields a higher throughput for the unslotted system than for the slotted system when supported by a strong channel coding.

An asymptotic analysis of band-limited DS/SSMA communication systems

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.

Performance of Multicarrier DS/SSMA Over Fast Rayleigh Fading Channels With Doppler Diversity

Differential encoding is known to simplify receiver implemen- tation because it bypasses channel estimation.However, over rapidly fading wireless channels, extra transceiver modules are necessary to enable differ- ential transmission.Relying on a basis-expansion model for time- and fre- quency-selective (doubly selective) channels, we derive such a generalized block-differential (BD) codex and prove that it achieves maximum Doppler and multipath diversity gains, while affording low-complexity maximum- likelihood decoding.We further show that existing BD systems over fre- quency-selective or time-selective channels follow as special cases of our novel system.Simulations using the widely accepted Jakes model corrobo- rate our theoretical analysis. Abstract—The significant loss in performance of multicarrier di- rect-sequence spread-spectrum multiple-access (DS/SSMA) systems over fast-fading channels is investigated.First, the channel model for individual subchannels is obtained using a canonical decomposition to a wide-sense stationary, uncorrelated scattering channel.Next, a receiver structure, which features both Doppler diversity and frequency diversity, is presented and analyzed.It is found that large Doppler spreads cause the average magnitude of the desired signals to diminish dramatically.Expressions for the bit-error rate are derived for both uncoded and coded systems by applying the standard Gaussian approximation.Numerical results, which show that Doppler diversity is preferable over frequency diversity in fast-fading channels, are provided for different combinations of the diversity orders.Numerical results, which show that the intersymbol and intersubchannel interference can be effectively suppressed, are also pro- vided.It is show that the multiple-access interference tends to degenerate into a wide-sense stationary process with a statistical periodicity destroyed by the fast-fading channel. Abstract—We present a setting for decoding of low-density parity-check (LDPC) codes jointly with channel estimation, suitable for transmission over memoryless compound channels.We show that the performance of the combined scheme can be rigorously evaluated by means of density evo- lution, and focus on density evolution as a tool for designing a channel esti- mator that matches not only to the channel, but also to the LDPC ensemble, as well.The utility of the concept is exemplified for a compound binary sym- metric channel and an unknown-phase additive white Gaussian channel.

Performance evaluation for quaternary DS-SSMA communications with complex signature sequences over Rayleigh-fading channels

Performance of quaternary direct-sequence spread-spectrum multiple-access (DS-SSMA) systems with complex signature sequences and complex modulators and receivers in flat Rayleigh fading is investigated in this paper. Due to the availability of potentially large sets of complex sequences with good correlation characteristics, the interest of using complex spreading sequences in DS-SSMA has increased dramatically. The complex spreading sequences investigated in this paper include the recently introduced orthogonal unified complex Hadamard transform (UCHT) sequences. In this paper, complex processing in modulators and receivers is also employed in order to take advantage of the correlation properties of complex signature sequences. The average bit error rate (BER) for quaternary synchronous systems is obtained first, and then the BER for quaternary asynchronous systems is evaluated using characteristic function approach. Result based on Gaussian approximation method is also presented for asynchronous systems. The numerical examples illustrate that the systems based on UCHT spreading sequences perform generally better than the Gold sequences and the 4-phase family A-sequences.

Evaluation of SIR statistics in a DS/CDMA system with signal-level-based power control and multipath dispersion

The statistical evaluation of the estimated short-term signal-to-interference ratio (SIR) for power control is presented in many-to-one reverse link. As mentioned in other previous works, the statistical evaluation shows that the estimated short-term SIR can be approximated by a log-normal distribution. The analysis has applications to a cellular system employing direct-sequence spread-spectrum code-division multiple access (CDMA) with M-ary orthogonal modulation on the uplink. Copyright © 2004 AEI.

Dual-Phase Continuous Phase Modulation for Spread-Spectrum Multiple-Access Communication

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.

Performance analysis of a DS/SSMA unslotted ALOHA system with two user classes

In this paper, we propose a direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system with two user classes and analyze the throughput of the proposed system. Mobile stations (MSs) are divided into two classes according to priority or traffic characteristics such as delay-intolerant and delay-tolerant. Different permission probabilities are assigned to each class so that the appropriate quality of service can be provided. We assume that the generation of class 1 and 2 messages are Poisson distributed and the message is divided into several packets before transmission. The system is modeled as a two-dimensional Markov chain under the assumption that the number of packets transmitted immediately by both user classes is geometrically distributed and the packet length is constant. We calculate the packet success probability and the throughput as a function of the signal-to-noise ratio (SNR) during packet transmission, considering the number of overlapped class 1 and 2 messages and the amount of their time overlap. Moreover, we show that the proposed system differentiates user messages according to class and maintains a high throughput even under heavy traffic conditions using access control based on the channel load.

Average bit-error-rate performance of band-limited DS/SSMA communications

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.

Chip waveform design for DS/SSMA systems with aperiodic random spreading sequences

The dependence of the error performance and spectral efficiency of direct-sequence spread-spectrum multiple-access (DS/SSMA) systems with matched filter receivers on the chip waveform is examined. The actual shape of the chip waveform, as well as its energy, is found to influence the statistical properties of the multiple-access interference (MAI). An approach to design waveforms that may result in interchip interference (ICI) is proposed and a criterion for design based on the conditional Gaussian approximation of the MAI for systems with aperiodic random spreading sequences is derived. For a simplified system, a closed-form solution for optimal band-limited waveforms is obtained for excess bandwidth less than or equal to one by using a performance metric that includes the effect of ICI. Numerical results, based on an analytical method, as well as Monte Carlo simulations, are provided to evaluate the performance of the proposed waveforms in general systems with conventional matched filter receivers.

Performance evaluation for GMSK DS∕SSMA systems based on a closed-form expression for output SNR

The performance of asynchronous Gaussian minimum shift keying (GMSK) direct-sequence spread-spectrum multiple-access (DS/SSMA) systems is evaluated using Pursley's approach. Taking into consideration the interchip interference caused by the Gaussian lowpass filter, a closed-form expression for signal-to-noise (SNR) at the output of a coherent receiver is derived and then used for system analysis. A system optimisation or evaluation based on output SNR is efficiently carried out, compared with Monte-Carlo simulation.

A multicarrier DS/SSMA system with reduced multiple access interference and higher data rate in Rician fading channels

We propose to use multiple signature sequences (instead of one) per user in a multicarrier direct-sequence spread spectrum multiple access system in Rician fading channels. Every user has a distinct set of spreading sequences, with a different spreading sequence for each carrier in each user's set. By analysis and computer simulation, we show that when these sets of sequences are chosen to be mutually orthogonal complementary sets of sequences, multiple access interference (MAI) is reduced in Rician fading channels. We also show that we can pack symbols more closely together, resulting in a higher data rate. The proposed system with a distinct set of deterministic spreading sequences per user has lower MAI and higher data rate than another system with one distinct random spreading sequences per user in Rician fading channels.

Performance analysis of DS/SSMA unslotted ALOHA system with variable length data traffic

We analyze the throughput of a direct-sequence spread spectrum multiple access (DS/SSMA) unslotted ALOHA system with variable length data traffic. The system is analyzed for two cases: (1) systems without a channel load sensing protocol (CLSP) and (2) systems with a CLSP. The bit-error probability and the throughput are obtained as a function of the signal-to-noise ratio (SNR) during message transmission, considering the number of overlapped messages and the amount of time overlap. We assume that the generation of data messages is Poisson distributed and that the messages are divided into packets before transmission. The system is modeled as a Markov chain under the assumption that the number of packets in a message is geometrically distributed with a constant packet length. The throughput variance of the DS/SSMA unslotted ALOHA system with variable length data traffic is obtained as the Reed-Solomon code rate varies. Results show that a significant throughput improvement can be obtained by using an error-correcting code.

Performance of a spatio-temporal matched filter receiver for DS/SSMA communications

A spatio-temporal matched filter receiver for direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with aperiodic random quadriphase spreading sequences is derived, and the system performance is analyzed. It is shown with the method of characteristic functions that the cross-correlation coefficients between the desired user's and the interfering users' spreading sequences tend, in distribution, to independently and identically distributed circularly symmetric complex Gaussian random variables as the processing gain goes to infinity. Based on this Gaussian approximation, the structure of the spatio-temporal matched filter receiver is derived and a bit error rate formula is obtained. Using Monte-Carlo simulations, as well as analytical methods, it is shown that the spatio-temporal matched filter receiver achieves a significant performance improvement over the conventional, temporal, and spatial matched filter receivers by effectively suppressing the multiple access interference.

On rate-variable multidimensional DS/SSMA with sequence sharing

A general model for M-ary direct sequence-spread spectrum (DS/SS) communications is presented to provide multiple services in direct sequence-spread spectrum multiple access (DS/SSMA). The main idea is to allow each user to use parallel orthogonal spreading sequences, depending upon its needs. More specifically, each service level is associated with certain combinations of the sequences. We use a code, which we refer to as a "mapping code", to form a multidimensional signal constellation. Thus, the data rate can be changed by changing the mapping code. Further, techniques of sequence sharing (reuse) and sequence arrangement (allocation), suitable for packet switching, are proposed; this allows support of multimedia services, while resulting in a reduction of sequence consumption. We evaluate error rates in the presence of multiple access interference (MAI) and additive white Gaussian noise (AWGN). Numerical results are given to show the effects of the system configuration, the data rates, and other system performance measures.

Overlapping M-ary frequency shift keying spread-spectrum multiple-access systems using random signal sequences

A multiple-access spread-spectrum communication system using binary frequency shift keying (BFSK) or M-ary frequency shift keying (MFSK) and noncoherent demodulation is considered. In contrast to previous work typically assuming that the frequency shift keying (FSK) tones are nonoverlapping after direct-sequence (DS) spreading, here we consider a spread-spectrum multiple-access (SSMA) system under the assumption that the DS spread signals of different FSK tones are only orthogonal over the information symbol duration. Consequently, the frequency band of a spread FSK tone may be fully or partially overlapping with the other spread signals. An estimate of the variance of the multiple-access interference is obtained by assuming that the phase angles and time delays of the received signals are mutually independent random variables, provided that random signature sequences are employed for spreading. On the basis of the above assumptions, the bit error rate (BER) of our DS spread-spectrum multiple-access (DS-SSMA) and that of our hybrid DS slow frequency-hopping spread-spectrum multiple-access (DS-SFH SSMA) systems using FSK modulation is analyzed, when the channel impairments are constituted by a combination of additive white Gaussian noise (AWGN) and multiple-access interference. From our analysis and the numerical results, we concluded that, for a given system bandwidth and for a certain value of M, the system's BER performance can be optimized by controlling the amount of overlapping and that the systems with optimized overlapping outperformed the systems using no overlapping.

Performance of a type-II hybrid ARQ protocol in slotted DS-SSMA packet radio systems

The application of a type-II hybrid ARQ protocol in a slotted direct-sequence spread-spectrum multiple-access (DS-SSMA) packet radio system is investigated. Both the static performance and the dynamic performance of such a system are analyzed. In the physical layer, packet error and packet success probabilities are computed using the improved Gaussian approximation technique, which accounts for the bit-to-bit error dependence within a packet. In the data-link layer, two-dimensional Markov chains are employed to model the system dynamics. Based on this model, the performance of the type-II hybrid ARQ protocol is upper and lower bounded by considering, respectively, a superior scheme and an inferior scheme. Steady state throughput and delay performances of the two bounding schemes are obtained. Moreover, it is shown that for each fixed input load, there is an optimal retransmission probability under the finite user population assumption. Bounds on this optimal retransmission probability are also given.

TCM/SSMA communication systems with cascaded sequences and PAM/QAM signal sets

An expression in matrix form for the multiple-access interference (MAI) in an asynchronous direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with cascaded sequences (CVs), arbitrary chip waveforms, and trellis-coded modulation (TCM) with a pulse amplitude modulation (PAM) or quadrature amplitude modulation (QAM) signal set is obtained. TCM provides significant coding gain while the CVs decrease the correlation between the MAI of adjacent data intervals. The expression is used to calculate arbitrarily accurate probability density functions (PDFs) of the MAI in the TCM system and to derive an accurate approximation of the MAI variance. It also helps illustrate some properties of the MAI by separating contributing parameters into different matrices. We derive an approximation of the upper union bound on the bit-error probability and investigate its applicability. The results show that CV schemes can greatly reduce the pairwise error probabilities (PEPs) until the length of the CV becomes greater than that of the error weight sequence (EWS) under consideration.