Band-limited Direct-sequence Code-division Multiple-access Systems Research Articles

Multiuser detectors for band-limited direct-sequence code-division multiple-access systems with multitone jamming and generalised-K fading

In this study, the authors analyse the bit-error rate (BER) performance of band-limited direct-sequence code-division multiple-access (DS-CDMA) systems employing two types of multiuser detectors, namely, linear decorrelator with whitening (LDW) and linear minimum mean-squared error (MMSE) detector over frequency-selective multipath channels with multitone jamming (MTJ). The authors consider the generalised-K distribution in order to include both shadowing and small-scale fading effects. Generalised-K distribution is generic and versatile as it can incorporate various composite fading and shadowing effects observed in wireless communication channels with mathematical tractability. The authors derive analytical BER expressions that are functions of band-limited pulse shapes, MTJ parameters and shadowing and fading parameters of the wireless channel. The system incorporates either a Nyquist pulse or a cosine band-limited pulse shape. Numerical results show that the systems with cosine pulse demonstrate better BER performance than the ones with Nyquist pulse in the presence of MTJ. Furthermore, in the presence of MTJ, MMSE-based systems demonstrate improved BER performance as compared to LDW-based systems. The authors also observe that the systems employing two-dimensional scheme, that is, the combination of space and multipath diversity, outperform the ones with multipath diversity scheme.

Analysis of MIMO Band-Limited DS-CDMA Systems in the Presence of Multitone Jamming Over Generalized-$K$ Fading Channels

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we study the effects of different pulse shapes on the bit-error-rate (BER) performance of a space-time transmit diversity scheme for band-limited direct-sequence code-division multiple-access (DS-CDMA) systems in the presence of multitone jamming (MTJ) over generalized-<formula formulatype="inline"><tex Notation="TeX">$K$</tex> </formula> fading channels. We consider a space-time system with two transmitter and multiple receiver antennas. Two types of band-limited pulse shapes, namely, spectrum raised cosine (SRC) and Beaulieu–Tan–Damen (BTD) pulses, are considered in the BER performance of the conventional matched-filter receiver, the decorrelator receiver, and the minimum-mean-square-error (MMSE) receiver. Numerical results show that the system with BTD pulse outperforms the system with SRC pulse, and the MMSE receiver gives the best BER performance among the three receivers considered. </para>

Analysis of BPSK modulated asynchronous band-limited DS-CDMA systems with diversity receivers over Generalised-K fading channels

The authors studied bit-error rate (BER) performance of asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems with various diversity-combining receivers over Generalised-K fading channels. The effects of band-limited pulse shapes, multitone jamming, multiple-access interference as well as both flat and frequency-selective fading are considered. The Generalised-K model is adopted in order to include the effects of shadowing and fading of a wireless channel. The authors consider binary phase-shift keying as the modulation technique. The analytical expressions are valid for any arbitrary value of Generalised-K distribution parameters. Two types of band-limited pulses, namely spectrum raised cosine and Beaulieu–Tan–Damen (BTD) pulses, are incorporated in the analysis. Numerical results show that the system with BTD pulse outperforms the one with SRC pulse for various diversity-combining receivers under various channel conditions. Furthermore, by incorporating a minimum mean-square error stage in the multipath diversity receiver, the BER performance can be further improved.

Second order polynomial class of chip waveforms for band-limited DS-CDMA systems

The conventional frequency domain square-root raised cosine (Nyquist) chip waveform has much poorer anti-multiple-access-interference (anti-MAI) capability than the optimal band- limited waveform in direct sequence code division multiple access (DS-CDMA) systems. However, the digital implementation of the optimal chip pulse is very costly due to the slow decaying rate of the time waveform. In addition, its eye diagram and envelope uniformity are worse than the Nyquist pulse for a wide range of roll-off factor, which will incur performance degradation due to timing jitters and post non-linear processing. In this paper, based on an elementary density function of a second-order polynomial, a class of second-order continuity pulses is proposed. From this class of pulses, we can find some members having faster decaying rate, bigger eye opening, more uniform envelope and stronger anti-MAI capability than the Nyquist waveform. The normalized-band-width-pulse-shape-factor product, the decaying rate of the tail of the time waveform, the opening of the eye diagram, and the envelope uniformity of the second-order continuity pulses are addressed in the paper that provide the basic information for the selection of the chip pulse for CDMA systems.

BER Performance of Bandlimited DS-CDMA Systems in Nakagami Fading

Accurate average bit-error rate analysis of asynchronous bandlimited binary direct-sequence code-division multiple-access (DS-CDMA) systems in Nakagami-m fading is studied. The fading is assumed to be flat and slow. The spectrum raised-cosine (SRC) and Beaulieu-Tan-Damen (BTD) pulse-shapes are employed. A new method which combines the characteristic function (CF) method with the improved Gaussian approximation (GA) is proposed for evaluating the BER of bandlimited DS-CDMA. A substantial computational complexity reduction is achieved. The well-known standard GA, Holtzman's simplified improved GA, and improved Holtzman's GA methods are also considered. The accuracies of the approximation methods are assessed using Monte Carlo simulation. For a system employing a deterministic sequence for the desired user and random sequences for the active interfering users, a CF method is employed to derive precise BER results. The new BTD pulse outperforms the SRC pulse in all situations

Accurate computation of bit error probabilities of band-limited asynchronous DS-CDMA systems using higher order moments

An existing method called the simplified improved Gaussian approximation (SIGA) was previously applied to compute the bit error probabilities (BEPs) of band-limited asynchronous direct sequence code division multiple access (DS-CDMA) systems employing general pulse shaping (IEEE Trans. Commun., vol. 50, p. 656, 2002). The SIGA method uses moments up to the second order and is more accurate than the standard Gaussian approximation (SGA) (IEEE Trans. Commun., vol. 50, p. 656, 2002). In this paper, a new method that uses moments up to the fourth order is proposed for computing the BEP. The method is derived from a five-point Chebyshev interpolation formula and is inherently more accurate than the SIGA. Like the SIGA, the new method requires the evaluation of only closed-form expressions and the error function. The new method achieves higher accuracy with a modest increase in computational complexity.

An Improved Design of Chip Waveforms for Band-Limited DS-CDMA Systems

This paper introduces an efficient and improved design of chip waveforms to minimize the multiple-access interference in band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. For ease of implementation, the DS-CDMA system employs a time-limited chip waveform, whereas its band limitation is ensured by the low-pass filters at both the transmitter and receiver ends. The design uses sinusoids to synthesize the time-limited chip waveform so that the portion of its spectrum across the specified bandwidth is as flat as possible. It is shown that by using a simple series expansion (with only a few terms) the synthesized chip waveforms significantly outperform the spreading/despreading waveforms previously proposed, particularly for large values of the chip duration-bandwidth product.

Performance Analysis of a Linear MMSE Receiver for Bandlimited Random-CDMA Using Quadriphase Spreading Over Multipath Channels

This paper analyzes the bit-error-rate (P/sub e/) performance of a linear minimum mean-square error (LMMSE) receiver for bandlimited direct-sequence code-division multiple-access systems which use quadriphase spreading with aperiodic pseudonoise (PN) sequences. The analysis is based on the improved Gaussian approximation (IGA) with focus on chip pulse shaping. It shows that the IGA reduces to the standard Gaussian approximation (SGA) if 1) random quadriphase spreading is employed, 2) the spreading factor takes moderate to large values, and 3) the chip pulse excess bandwidth (BW) is zero. Hence, the SGA, known for its inaccuracy in low regions of P/sub e/, remains an accurate approximation even when the number of active users in the system is small as long as the aforementioned conditions are met. The analysis holds for either matched or different transmit and receive filters. Consequently, closed form conditional P/sub e/ expressions are derived for the coherent selective RAKE and the LMMSE receivers and verified with Monte Carlo simulations. Numerical results are presented to illustrate the performance improvement achieved by the LMMSE receiver which, in contrast to the coherent selective RAKE receiver, not only suppresses interference when the excess BW of chip pulse is nonzero, but also harnesses the energy of all paths of the desired user. Under the examined scenarios tailored toward current narrowband system settings, the LMMSE receiver achieves 60% gain in capacity over the selective RAKE receiver. A third of the gain is due to interference suppression capability of the receiver while the rest is credited to its ability to collect the energy of the desired user diversified to many paths. Future wideband systems will yield an ever larger gain.

An improved Gaussian approximation for probability of bit-error analysis of asynchronous bandlimited DS-CDMA systems with BPSK spreading

This paper analyzes probability of bit-error (P/sub e/) performance of asynchronous bandlimited direct-sequence code-division multiple-access systems with binary phase-shift keying spreading. The two present methods of P/sub e/ analysis under bandwidth-efficient pulse shaping: the often-cited standard Gaussian approximation and the characteristic function (CF) method suffer from either a low accuracy in regions of low P/sub e/ (< 10/sup -3/) or a prohibitively large computational complexity. The paper presents an alternate method of P/sub e/ analysis with moderate computational complexity and high accuracy based on a key observation. A sequence of chip decision statistics (whose sum yields a bit statistic) forms a stationary, m-dependent sequence when conditioned on the chip delay and phase offset of each interfering signal. This observation permits the generalization of the improved Gaussian approximation previously derived for the rectangular pulse and the derivation of a numerically efficient approximation based on the CF method. Numerical examples of systems using the square-root raised-cosine and IS-95 pulses illustrate THE P/sub e/ performance, user capacity and the accuracy of the proposed method.

A simple and accurate method of probability of bit error analysis for asynchronous band-limited DS-CDMA systems

This paper considers probability of bit error (P/sub e/) analysis in asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. It presents a simple and accurate method of P/sub e/ analysis. The proposed method can serve as an attractive alternative to the only two techniques currently available for band-limited systems: the standard Gaussian approximation (SGA) and the characteristic function method. The former is prone to inaccuracy while the latter, large computational complexity. The method generalizes the simplified improved Gaussian approximation (SIGA) derived previously for rectangular pulses. This paper also outlines a generalization of another method referred to as the improved Gaussian approximation (IGA). Numerical examples demonstrate the far greater accuracy of the generalized SIGA with respect to the SGA. The examples consider the IS-95 and square-root raised cosine (Sqrt-RC) pulses as well as uniform and nonuniform received power conditions.