Direct-sequence Spread-spectrum Transmission Research Articles

Per-survivor processing for underwater acoustic communications with direct-sequence spread spectrum

This paper proposes a receiver for direct-sequence spread spectrum transmissions in underwater acoustic channels, which combines a per-survivor processing (PSP) structure with sparse channel estimation. Specifically, the PSP structure establishes the trellis on the symbol level to render a small to moderate number of states, thus reducing the computational complexity. Meanwhile, the sparse channel estimation is performed on the chip level, where the orthogonal matching pursuit algorithm is used and a two-dimensional grid of path delay and Doppler scaling factor is incorporated in the dictionary construction. The effective combination of the PSP detection and sparse channel estimation achieves a good tradeoff between performance and complexity. Simulation and experiment results show that the proposed receiver outperforms the conventional RAKE receiver considerably, and most importantly, the proposed PSP receiver with an exact wideband dictionary maintains an excellent performance even for challenging underwater acoustic channels with large Doppler disparities on different paths.

Oversampled M-sequences for joint data and bit epoch estimation in DSSS transmissions

The maximum likelihood (ML) estimator for the bit synchronization epoch and data bit values in direct-sequence spread-spectrum (DSSS) transmission consists of determining the binary sequence that maximizes the correlation with the recovered data samples. This requires the exhaustive test of all sequences generated by the different bit combinations and alignments, resulting in a computationally intensive process. In this letter, the properties of maximum length sequences (m-sequences) are exploited for testing all the bit combinations and alignments in parallel, leading to a computationally efficient implementation of the ML estimator. The case where the data bits are modulated by an overlay or secondary code is also considered and the proposed algorithm is generalized to include the effect of this additional modulation.

Multi-channel blind deconvolution algorithm for multiple-input multiple-output DS/CDMA system

Direct sequence spread spectrum transmission can be realized at low SNR, and has low probability of detection. It is aly problem how to obtain the original users' signal in a non-cooperative context. In practicality, the DS/CDMA sources received are linear convolute mixing. A more complex multichannel blind deconvolution MBD algorithm is required to achieve better source separation. An improved MBD algorithm for separating linear convolved mixtures of signals in CDMA system is proposed. This algorithm is based on minimizing the average squared cross-output-channel-correlation. The mixture coefficients are totally unknown, while some knowledge about temporal model exists. Results show that the proposed algorithm can bring about the exactness and low computational complexity.

Opportunistic multipath for bandwidth-efficient cooperative multiple access

Within a new paradigm, where wireless user cooperation is viewed as a form of (opportunistic) multipath, we exploit the unique capabilities of direct-sequence spread spectrum transmissions in handling multipath to design a novel spectrally efficient protocol for wireless cooperative networks. We show how and why our proposed system achieves diversity without increasing bandwidth. After analyzing its performance, we deduce that user capacity can be significantly improved with respect to existing third generation cellular systems in the uplink

Performance analysis of a spreading sequence estimator for spread spectrum transmissions

Direct sequence spread spectrum transmissions (DS-SS) are now widely used for secure communications, as well as for multiple access. They have many interesting properties, including low probability of interception. Indeed, DS-SS transmitters use a periodical pseudo-random sequence to modulate the baseband signal before transmission. A receiver which does not know the sequence cannot demodulate the signal. In this paper, we propose a new method which can estimate the spreading sequence in a noncooperative context. The method is based on eigenanalysis techniques. The received signal is divided into windows, from which a covariance matrix is computed. We show that the sequence can be reconstructed from the two first eigenvectors of this matrix, and that useful information, such as desynchronisation time, can be extracted from the eigenvalues. The main achievement of the present paper is a performance analysis of the proposed spreading sequence estimation procedure. An analytical approach is first considered owing to matrix perturbation theory and Wishart matrix properties. Then, complementary Monte Carlo simulations are performed to show the effectiveness of the proposed method.

A Novel Four Dimensional CDMA Modem for Wireless Multimedia Communications

A description and analysis of a four dimensional (4D) Direct Sequence Spread Spectrum (DSSS) modem architecture, comprising a transmitter and a receiver, are presented. The DSSS modulator employs Complex Spreading Sequences (CSS). Two integrated synchronization loops are incorporated in the DSSS receiver, namely a four dimensional Decicion-Directed Costas Carrier Recovery Loop (DD-CCRL) combined with a 4D Decision-Directed Complex Delay-Locked-Loop (CDLL). These loops are analysed and the system performance in Additive White Gaussian Noise (AWGN) verified by means of computer simulation. It is shown that the new DSSS modulation method does not suffer any losses compared to a conventional BPSK DSSS modem, whereas the multidimensional spreading technique achieves four times the data throughput rate of a BPSK DSSS system and two times the throughput of a QPSK modulation method in the same bandwidth and with identical spreading sequence lengths. The multidimensional spreading technique therefore offers extra flexibility in terms of the choice of data rate and DSSS processing gain achievable for a given DSSS transmission bandwidth without loss of performance compared to BPSK and QPSK in AWGN. Similar results are achieved in fading channels. A prototype of the new multidimensional DSSS modulation method is presently implemented with forefront Field Programmable Gate Array (FPGA) technology with a view of realising a multimedia wireless communication system for rural areas.

Fast-acquisition algorithm for joint maximum-likelihood data and bit synchronization epoch estimation in burst direct-sequence spread-spectrum transmission

We develop a fast-acquisition simplified joint maximum-likelihood (ML) estimation algorithm of the bit synchronization epoch and data bit values for a short-burst direct-sequence spread-spectrum (DSSS) transmission, in a white Gaussian noise environment. With respect to the optimal algorithm, the simplified procedure reduces the computational cost by more than one order of magnitude at no accuracy expense for carrier-to-noise ratios (CNRs) above 35 dB/spl middot/Hz.

Chip-level differential encoding/detection of spread-spectrum signals for CDMA radio transmission over fading channels

This paper is concerned with the design and performance evaluation of a direct-sequence spread-spectrum transmission and signal-detection technique for application to code-division multiple-access mobile radio networks. Unlike the conventional differential encoding/detection of data symbols, such a technique envisages differential encoding/detection of the spreading code chips to counteract the fast time-selective fading encountered in mobile radio transmissions. Binary PSK modulation on a Rician frequency-flat-fading channel is assumed, with Gaussian and Rayleigh channels as particular cases. The bit-error rate performance of the receiver is analytically evaluated as a function of the relevant system parameters, and the impact on the receiver performance of a carrier frequency offset is also investigated.

Control of the spectra of DS-SS split signals by interleaving

Direct-sequence spread-spectrum transmissions are increasingly employed in terrestrial radio-location systems to minimise interference with narrowband communications services in shared bands. Further flexibility in band planning is afforded by a scheme to generate from the high-rate sequence an integral number of lower-rate, narrow-bandwidth, 'split-signal' sequences which are transmitted in available gaps in the spectrum. The paper shows that the bandwidth and power density of the transmission may be further controlled by overlapping or interleaving the spectra of the split signals without unacceptable degradation of positioning performance due to their mutual interference.

On the detection of bandlimited direct-sequence spread-spectrum signals transmitted via fading multipath channels

Recent investigations suggest bandlimited direct-sequence spread-spectrum transmission for use in CDMA mobile radio systems. In this paper, transmission systems with signal bandwidths not exceeding the chip rate are considered. First, the optimum noncoherent single-user receiver and various suboptimum approximations thereof are derived. These receivers utilize the fact that the received signals can be sampled at chip rate. This results in receiver concepts, which are well suited for fully digital implementations. Then, it is shown that the main result on the structure of the optimum single-user receiver is directly applicable to optimum noncoherent multiuser joint detection and interference cancellation. In this paper, noncoherent detection is considered. By this, we mean that only the channel statistics and not the channel itself is known to the receiver. It is shown that due to this less restrictive assumption, more powerful channel coding and interleaving schemes can be employed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Selective fading on LOS microwave links: classical and spread-spectrum measurement techniques

A survey is presented of techniques for measuring selective fading on line-of-sight (LOS) microwave links, particularly over a wide band. Classical pulse methods, including pulse compression, as well as the much used frequency-sweep approach are covered. Methods utilizing both frequency-hopping and direct-sequence spread-spectrum transmissions are also discussed. An analysis of this latter method (used by the US National Bureau of Standards and by the authors) is included and a relationship is established between the true channel transmittance and that estimated by this approach. The advantages and disadvantages of the various schemes are outlined with the aim of assisting an experimenter in the choice of technique in a given experimental context. >

Performance of Nondiversity Receivers for Spread Spectrum in Indoor Wireless Communications

In this work we have considered direct-sequence spread-spectrum transmission for indoor wireless communications. We have modeled the indoor communications medium, which is a multipath fading channel, by a discrete set of Rayleigh faded paths. We have proposed new analytical techniques to evaluate the probability of error for the receiver terminals studied in this work. Numerical results reveal that, for the nondiversity receivers considered here, Rayleigh fading is very hostile to this form of modulation/multipleaccess technique. The results also indicate that either some form of operation to prevent Rayleigh fading or diversity operation to counteract Rayleigh fading is required.