Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability analysis method is derived for a multicode DS-CDMA system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of multiple code sequences on the bit-error probability, whereas Gaussian approximations do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using Gaussian approximations. A two-step analytical procedure enables deriving an explicit, compact form for the characteristic function of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.

This paper considers an improved single-user detection technique for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems using long sequence spreading (random-CDMA) Most of the known detection schemes for DS-CDMA suffer from either poor performance under power-imbalance (near-far like) conditions, excessive complexity, or incompatibility with systems employing long sequence spreading. To address these problems, this paper considers a signal-to-noise ratio maximizing linear time-invariant filter for one-shot bit symbol detection exploiting some information about the interferers. This filter, referred to as the chip-delay locked matched filter (CLMF), exploits the cyclostationarity in multiple-access interference, and it can offer good near-far resistance while remaining suitable for systems with long sequence spreading. The CLMF requires knowledge of interferers chip delays and signal powers; however, knowledge of their pseudonoise sequences is unnecessary. This paper also demonstrates the improvement in performances offered by the CLMF over other single-user receivers such as the conventional matched filter and noise-whitening matched filter performance is evaluated in terms of probability of outage for single-rate and dual-rate DS-CDMA systems using bandwidth-efficient chip pulses, over a single-path additive white Gaussian noise channel. Errors in the interferer chip delay estimates degrade the CLMF performance. However, if the root-mean-square value of these errors is less than 5% of the chip interval, then this degradation is small.

In this paper, we examine the performance of the iterative minimum mean-squared error (MMSE) adaptive receiver structure over asynchronous direct-sequence code-division multiple-access (DS-CDMA) channels. The receiver base-station uses estimates of the detected users' symbols to improve the convergence rate of the adaptive algorithm. The iterative algorithm used is based on the parallel interference cancellation (PIC) algorithm, and we examine the transient behaviour of both the least-mean-square (LMS) and the recursive-least squares (RLS) algorithms in asynchronous (i.e., reverse-link) DS-CDMA systems. Our results demonstrate a significant convergence improvement for the asynchronous case. It is shown that for the more severe asynchronous case, the convergence speed of the iterative algorithm is even better than the one achieved for the synchronous channel model. Also, for systems with no power control, the iterative algorithm is shown to be near-far resistance.

Coherent reception of quadrature phase-shift keying (QPSK) modulated signals in an uplink channel of a direct-sequence code-division multiple access (DS-CDMA) system is considered. The receivers, commonly referred as the conventional matched-filter receiver and the RAKE receiver, are analyzed when the transmission of data is performed over the additive white Gaussian noise (AWGN) channel and the multipath slow Rayleigh fading channel, respectively. Both analyses take into account inaccuracies in the carrier phase estimations resulting from a carrier phase estimator.

In this paper, we derive exact form expressions for the Bit Error Probability (BEP) and throughput of cooperative Direct Sequence-Code Division Multiple Access (DS-CDMA) systems using incremental selective relaying which combines selective relaying with incremental relaying protocols in the presence of multipath propagation. The derived results are valid for any multipath intensity profile of the channel and any path delays. They also consider the correlation of the multipath gains. Simulation results along with analytical studies of BEP and throughput prove that the combination of incremental relaying with selective relaying in cooperative DS-CDMA systems improve significantly the throughput performance and can achieve the maximum possible spatial diversity when it is required by destination.

This paper considers sequence acquisition in direct-
\nsequence spread-spectrum (DS/SS) systems that use Gold
\nsequences as the multiple access pseudonoise (PN) sequence.
\nWhen long PN sequences are used, the design of an acquisition
\nscheme requires the characteristics of partial correlation of PN
\nsequences. For ease of analytic design, the partial correlation
\nof Gold sequences is approximated using a piecewise linearized
\nupper bound model. Since the acquisition of PN sequence in direct-
\nsequence code-division multiple access (DS-CDMA) systems
\ncan be formulated as testing two simple hypotheses, we apply fixed
\nsample size (FSS) and sequential tests to detection of phase alignment.
\nWhen a sliding correlator-type structure is employed for
\nthe acquisition scheme, the acquisition performance of Gold-sequence-
\nbased DS-CDMA systems is analyzed and compared to
\nthat of -sequence-based systems. Analytical results show that
\nthe use of Gold sequences requires a mean acquisition time quite
\ncomparable to the use of -sequences. Finally, analytical results
\nare verified by computer simulation.

Highly reliable data transmission using Reed-Solomon(RS)/convolutional concatenated coding in conjunction with hybrid type-I automatic retransmission request (ARQ) is investigated. The study is based on an analysis of the tradeoff between error correction and error detection capabilities of RS codes for the direct-sequence code-division multiple-access (DS-CDMA) systems. Two RS decoding schemes are considered: error-only decoding (decoding without side information) and error-and-erasure (EE) decoding. In the EE scheme, the inner code is decoded with a modified Viterbi algorithm, which produces reliable information along with the decoded output. Numerical results illustrate the importance of properly selecting the design parameters for the purpose of maximizing spectral efficiency. The results also show that an E/sub b//N/sub 0/ gain of 0.1 to 0.5 dB can be obtained by using EE decoding, depending on the selected parameters.

For the two-branch space-time (ST) block-coded direct-sequence code-division multiple-access (DS-CDMA) systems, the impacts of a time-varying multipath channel on the downlink transmission are analyzed. By considering the systems using the random binary spreading code (RBSC) and deterministic binary spreading code (DBSC), the effects of the multipath interference and multiuser interference are included in the analyses of the bit-error rate and bit-error outage. Also, for the performance analysis of the system employing the decision-feedback (DF) detector, the effect of error propagation is taken into account. It is known that enlarging the spreading factor can enhance the interference-rejection ability of a DS-CDMA system and, hence, can improve the performance. However, it also lengthens the symbol duration and, thus, stiffens the diversity penalty resulting from the channel variation within an ST-code-word duration. Thus, a moderate spreading factor should be chosen. In this paper, for the RBSC system using the simple-maximum-likelihood (SML) detector, we derive an optimum spreading factor that is optimum in the minimum-error-probability sense. Numerical results have revealed that the derived optimum spreading factor is a good estimate of the ones for the DBSC systems using the SML, zero-forcing, and DF detectors. Therefore, it is very useful for system designers in determining the system parameters

A serial concatenated convolutional and differential coding scheme is employed in a multi-user direct-sequence code-division multiple-access (DS-CDMA) system. The system consists of single-user detectors (SUDs), which are used to suppress multiple-access interference (MAI) with no requirement of other users' spreading codes, timing, or phase information. The differential code, treated as a convolutional code of code rate 1 and memory 1, does not sacrifice the coding efficiency and has the least number of states. The iterative process exchanges information between the differential decoder and the convolutional decoder. Both component decoders adopt the a posteriori probability (APP) algorithm. Numerical results in additive white Gaussian noise (AWGN) channels show that this concatenated coding scheme provides better performance and more flexibility than conventional convolutional codes in DS-CDMA systems, even in the sense of similar complexity. Further study shows that the performance of this coding scheme applied to DS-CDMA systems with SUDs improves by increasing the processing gain or the number of taps of the interference suppression filter, and degrades for higher near–far interfering power or additional near–far interfering users.

In most direct sequence code division multiple access (DS-CDMA) systems, the modulation scheme used is a simple one. In this paper, employing a combined modulation scheme in DS-CDMA is proposed. By combining two or more modulation schemes, the resultant symbol energy can be increased, and thus a better symbol error rate performance can be achieved. For the combined modulation scheme, the symbol error rate is determined by the less powerful one of the basic modulation schemes. So a proper selection of modulation combination is essential for power efficiency. In a DS-CDMA system, unlike a narrow-band system, the bandwidth efficiency is closely related to its power efficiency. Thus a combined modulation concept may result in a bandwidth efficient DS-CDMA system. In this paper, we present two DS-CDMA schemes by combining MFSK and coherent MPSK and differential BPSK. These two schemes are respectively denoted as (1) MFSK-MPSK/DS-CDMA, and (2) MFSK-DPSK/DS-CDMA. The proposed schemes have better performance than the conventional CDMA schemes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In this paper, we present a blind code-timing estimator for asynchronous DS (direct-sequence) CDMA (code-division multiple-access) systems. that utilize bandlimited chip waveforms. The received signal is first Fourier-transformed to the frequency domain, followed by a frequency deconvolution to remove the effect of the chip waveform. The code-timing is next derived from the output of a narrowband filter with a sweeping center frequency, which is designed to suppress interference of various sources, including, e.g., MUI (multiuser interference), ISI (inter-symbol interference), CCI (co-channel interference), and narrowband interference. The proposed estimator requires only the spreading code of the desired user. It can be used in frequency-flat or frequency-selective, time-invariant or time-varying fading channels. Moreover, the proposed estimator can be readily implemented using standard adaptive schemes, making it appealing not only for acquisition but also for tracking. Numerical results are presented to illustrate the performance of the proposed scheme.

It is difficult to assign independent radio frequency bands to wideband DS-CDMA (direct sequence code division multiple access) systems because of lack of radio frequency resources, so that CDMA overlay technique, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered to improve spectral efficiency. In the overlay situation, however, signals from existing narrowband systems are not necessarily transmitted with low power level enough not to give damage to DS-CDMA signals, so that an efficient rejection technique for high power narrowband interferers is required for successful CDMA overlay. We propose a high power narrowband interference rejection method which connects several narrowband interference cancellers with simple construction in series and show its rejection performance obtained from computer simulation.

The multiuser detection (MUD) performance analysis in a synchronous direct sequence code division multiple access (DS-CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.

In this paper, a new convolutionally coded direct sequence (DS) CDMA system is proposed. The outputs of a convolutional encoder modulate multiple band-limited DS-CDMA waveforms. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a convolutionally coded single carrier DS CDMA system with a RAKE receiver. At roughly equivalent receiver complexity, results demonstrates superior performance of the coded multicarrier system.

The ultimate limits of chaos-based asynchronous direct-sequence code-division multiple access systems are investigated using the concept of capacity taken from information theory. To this aim, we model the spreading at the transmitter and the sampling of the incoming signal at the receiver with a unique linear multi-input multi-output transfer function. We then assume the existence of a coding/decoding pair that is able to transmit information through this channel with a vanishing error probability. The capacity of the system is then identified with the maximum rate at which such an errorless link may operate. The capacity is a random quantity depending on the spreading sequences and, due to the asynchronism, on the users relative delays and phases. We then compare different spreading strategies [classical m and Gold codes, independent identically distributed (i.i.d.) and chaos-based codes] in terms of expected performance as well as of the probability that one method outperforms another. To ease further analytical investigations that should cope with expectations of logarithms, we measure capacity not only in bits per use but also in codewords per use. Some formal results along with extensive numerical evidence show that this does not alter the performance ranking. Such a ranking shows that chaos-based spreading always outperforms i.i.d. spreading and those trying to mimic it. This aligns with and complements what was already known about the ability of chaos-based techniques of minimizing multiple access interference.