Performance bounds on chip-matched-filter receivers for bandlimited DS/SSMA communications

Performance bounds on chip-matched-filter (CMF) receivers for bandlimited direct-sequence spread-spectrum multiple-access (BL-DS/SSMA) systems with aperiodic random spreading sequences are obtained. First, the optimum transmit-receive chip waveform pairs that maximize the conditional signal-to-interference ratio are derived. This leads to performance bounds on CMF receivers when the conditional Gaussian approximation for cyclostationary multiple-access interference (MAI) is exploited. The bounds are used to examine the dependence of the MAI suppression capability of the CMF receivers on the excess bandwidth of the system and the delay profile of multiple-access users. The system employing the flat-spectrum chip waveform pair is shown to have near-optimum average bit-error rate performance among the fixed CMF (FCMF) receiver systems. Numerical results are provided for an adaptive CMF receiver and for FCMF receivers employing several different fixed chip waveforms.     

A DS-CDMA system using despreading sequences weighted by adjustablechip waveforms

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique     

Performance Limits in DS-CDMA Timing Acquisition

This paper addresses timing acquisition aspects in direct-sequence code division multiple access (DS-CDMA) systems. Various chip waveform shaping schemes are considered, including both one-chip long full-response pulses, and partial-response ones occupying several chip periods. Different figures of merits are considered in a comparative analysis that seeks to establish performance limits in terms of correct timing detection capability, false alarm rate, bandwidth occupancy, multiple-access interference (MAI), and inter-chip interference (ICI). A waveform design algorithm is formulated to optimize system performance in terms of signal-to-interference-ratio (SIR) subject to other signalling constraints, and a solution based on the use of prolate spheroidal wave functions (PSWF) is derived. Numerous waveform design examples are then constructed to illustrate acquisition detection capability versus system load for both faded and unfaded cases. A comparative assessment of the performance of conventional signalling waveforms against the optimized ones is also presented. In particular, the numerical results show that the half-sine pulse used in minimum shift keying (MSK) is quasi-optimal within the full-response category, while root-raised cosine (RRC) Nyquist filtering with 22% rolloff (used in third generation CDMA standards) is also close to optimal when considering many-chip-long pulses.     

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.     

An optimal signal design for band-limited asynchronous DS-CDMAcommunications

An optimal signal design for band-limited, asynchronous, direct-sequence code-division multiple-access (DS-CDMA) communications with aperiodic random spreading sequences and a conventional matched filter receiver is considered in an additive white Gaussian noise (AWGN) channel. With bandwidth defined in the strict sense, two optimization problems are solved under finite bandwidth and zero interchip interference constraints. First, a chip waveform optimization is performed given the system bandwidth, the data symbol transmission rate, and the processing gain. A technique to characterize a band-limited chip waveform with a finite number of parameters is developed, and it is used to derive optimum chip waveforms which minimize the effect of multiple-access interference (MAI) for any energy and delay profile of users. Next, a joint optimization of the processing gain and the chip waveform is performed, given the system bandwidth and the data symbol transmission rate. A sufficient condition for a system to have lower average probability of bit error for any energy profile is found, and it is used to derive some design strategies. It is shown that the flat spectrum pulse with the processing gain leading to zero excess bandwidth results in the minimum average probability of bit error. Design examples and numerical results are also provided     

Asynchronous multiple-access interference suppression and chipwaveform selection with aperiodic random sequences

A linear decentralized receiver capable of suppressing multiple-access interference (MAI) for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems with aperiodic random signature sequences is proposed. Performance bounds on this receiver are also obtained. Using them as performance measures, the problem of chip waveform selection in DS-CDMA systems with the proposed receiver under the near-far scenario is investigated. In particular, the performance of several practical chip waveforms is compared. An LMS-type adaptive algorithm is developed to obtain the parameters needed in the receiver, which only requires the signature sequence and coarse timing information of the desired user     

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.     

Effect of chip waveform shaping on the performance of multicarrier CDMA systems

This paper studies the effect of chip waveform shaping on the performance of band-limited multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) systems. The performance criterion is the average multiple access interference at the output of a correlation receiver. A criterion based on the elementary density function is introduced for the performance comparison of various chip waveforms. It is demonstrated that the performance of MC-DS-CDMA systems is quite insensitive to the chip waveform shaping. Moreover, the optimum chip waveform for MC-DS-CDMA systems is practically the same as that of a single-carrier DS-CDMA system.     

Interference control and chip waveform design in multirate DS-CDMAcommunication systems

We study the interference effects in a multirate DS-code-division multiple-access (CDMA) system. Optimum chip waveform selection with arbitrary shapes is analyzed using a time domain approach. The problem is posed as an interference minimization problem under energy and time-bandwidth constraints and prolate spheroidal wave functions are used to arrive at a solution. Various factors affecting the interference are identified and the trade-off between competing factors is analyzed. The effect of the interchip interference on the optimum chip waveform design is also quantified under a practical bandwidth constraint. We study the benefits of employing two different chip waveforms for two classes of users. We compare the performance of systems employing two different chip waveforms with that of a single-chip waveform system such as IS-95. We show that when the power imbalance is large, it is advantageous to employ two different chip waveforms for different classes of users     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

Multicarrier DS/SFH-CDMA systems

Multicarrier direct-sequence/slow-frequency-hopping (MC DS/SFH) code-division multiple-access (CDMA) systems are proposed, in which multiple carriers are modulated by the same DS waveform and hopped in frequency according to a random hopping pattern. The receiver dehops the received signal with the same pattern, provides RAKE receivers for each carrier, and combines the outputs with a maximal ratio combiner (MRC). The performance of the proposed system is investigated over a frequency-selective Rayleigh-fading channel and compared to that of the MC DS-CDMA systems. It is shown that for the same diversity order, the MC DS/SFH-CDMA systems are superior in reducing multiple-access interference (MAI) while preserving the good capability of narrowband interference suppression, when the system parameters are selected properly.     

A method using optimized combinations based on energy spectrum extension factor to reduce MAI in asynchronous DS-CDMA systems

The principle to suppress multiple access interference (MAI) using double chip waveforms (DCW) in asynchronous DS-CDMA systems is analyzed in the paper. Based on the principle, a new method adopting optimized combinations of chip waveforms (CCW) to reduce MAI is proposed. The energy spectrum extension factor (ESEF) of equivalent chip waveform is introduced to optimally select CCW to reduce MAI, improve the signal to interference plus noise ratio (SINR) and bite error rate (BER) performance of asynchronous DS-CDMA users. The general closed form expression of SINR for asynchronous DS-CDMA users with CCW is obtained. The BER is also derived by improved Gaussian approximation (IGA). The theoretical analysis and numerical simulation results show that the optimized CCW using ESEF can effectively suppress MAI better, achieve higher SINR and BER performance compared with DCW. Moreover, the overlap between the simulation and IGA BER curves verifies the theoretical derivation.     

Adaptive detection in asynchronous code-division multiple-accesssystem in multipath fading channels

In code-division multiple-access systems transmitting data over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. In this paper, we address interference suppression, multipath diversity and processing gain protection for multiuser detection with less noise enhancement by using a parallel cancelling scheme. The proposed detector consists of a RAKE filter, forward filter, and feedback filter with different functions for each filter. The RAKE filter increases the signal-to-noise ratio by taking the advantage of multipath and code diversities. The forward filter is proposed, in combination with the feedback filter, to remove the effects of MAI and ISI by parallel cancellation. In order to avoid performance deterioration due to unreliable initial estimation in the parallel cancellation, a cost function with proper weighting is introduced to improve the performance of the proposed detector. In the proposed design method, a recursive least square algorithm is employed to update the tap-coefficients of all filters for MAI and ISI cancellation. Finally, the performance of the proposed detector is analyzed and compared with other detectors     

Chip-delay locked matched filter for DS-CDMA systems using longsequence spreading

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     

SINR maximizing space-time filtering for asynchronous DS-CDMA

Direct sequence code division multiple access (DS-CDMA) is a core technology for wireless access that is proposed for future generation (3G) mobile and personal communication systems. The use of a base-station antenna array is strongly endorsed in these systems, because of its capabilities of filtering out the interference in the space domain. This paper considers space-time one shot single user receivers for DS-CDMA, based on a multiple input single output time invariant linear filter. The optimization of the filter response is performed jointly in the space and time domains yielding maximal signal-to-interference-plus-noise ratio. Using this framework, a space-time noise whitening matched filter (ST-NWMF) for DS-CDMA demodulation is introduced. It is shown that combined spatio-temporal filtering, taking advantage of the fact that the multiple access interference is colored in the space as well as time domains, can reduce the effects of strong interference components and improves the performance. Simpler suboptimal space-time linear receivers, consisting of a single temporal LT filter following the antenna array, are also considered. It is shown that such a filter approaches the ST-NWMF in the limit, when the M chip waveform has a flat spectrum with no excess bandwidth and N the observation interval is infinite. These schemes, optimal as well as suboptimal, for large observation intervals are suitable also for DS-CDMA systems employing long sequence spreading     

Chip-locked space-time filtering for maximizing SINR in asynchronous DS-CDMA systems

This paper considers a chip-locked space-time (CLST) filtering technique for direct-sequence code-division multiple access (DS-CDMA) systems. CLST filtering exploits the knowledge of the multiple access interference (MAI) chip delays, as well as the fact that the MAI spectrum is colored in the time and space domains. Chip delays of interferers from the same cell as the desired user are available at the base station and can be used to improve performance of single-user receivers. CLST filtering reduces MAI through joint optimization of spatio-temporal filtering and exploitation of chip delays of locked interference, without the need of interferers spreading codes. A significant improvement in signal-to-interference plus noise ratio can be achieved through CLST filtering with respect to the case when the interference is unlocked. The capabilities of CLST filtering to suppress chip-delay-locked interference improves with increasing chip waveform excess bandwidth. Numerical results show that CLST filtering already provides significant performance gains with square-root raised cosine chip pulses of small excess bandwidth. Furthermore, it is also shown that CLST filtering for a long observation interval is suitable for DS-CDMA systems employing long sequence spreading.     

Wireless systems and interference avoidance

Motivated by the emergence of programmable radios, we seek to understand a new class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference ratio as a metric and a general signal space approach, we present a class of iterative distributed algorithms for synchronous systems which results in an ensemble of optimal waveforms for multiple users connected to a common receiver (or colocated independent receivers). That is, the waveform ensemble meets the Welch (1974) bound with equality and, therefore, achieves minimum average interference over the ensemble of signature waveforms. We derive fixed points for a number of scenarios, provide examples, look at ensemble stability under user addition and deletion as well as provide a simplistic comparison to synchronous code-division multiple-access. We close with suggestions for future work     

A new construction of signature waveforms for synchronous CDMA systems

We consider synchronous code-division multiple access (CDMA) systems over an additive white Gaussian noise (AWGN) channel, where all users are divided into groups of small size. The signature waveforms for users in each group are constructed from the same signature sequence but with different chip waveforms. To minimize the multiple access interference (MAI) at the output of the correlators, Welch-bound-equality (WBE) sequences and chip waveforms having optimal correlation property are employed. The main idea behind the proposed construction is to suppress the inter-group interference from users in different groups as much as possible (even to remove it completely) at the expense of introducing the intra-group interference among the users in the same group. The intra-group interference, however, can be easily handled by a low-complexity, optimal (or suboptimal) multiuser detector(s) if the group size is kept small enough. As special cases, the proposed constructions correspond to the optimal design of the signature waveforms and the conventional system that uses a single chip waveform, respectively. Thus the proposed construction offers a flexibility to trade performance for complexity. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even a small amount of overload, the proposed system with two users per group can have up to 100% overload with an excellent error performance.     

Performance of successive interference cancellation inconvolutionally coded multicarrier DS/CDMA systems

This article presents a successive interference cancellation (SIC) scheme for a multicarrier (MC) asynchronous DS/CDMA system, wherein the output of a convolutional encoder modulates bandlimited spreading waveforms at different subcarrier frequencies. In every subband, the SIC receiver successively detects the interferers' signals and subtracts them from that of the user-of-interest. The SIC receiver employs maximal-ratio combining (SIC-MRC) for detection of the desired user, and feeds a soft decision Viterbi decoder. A comparison is made among SIC-MRC, matched filter detection with MRC (MF-MRC), and N-tap minimum mean-squared error (MMSE) receivers with optimal tap coefficients, assuming a slowly varying, frequency selective, Rayleigh fading channel, where N is the processing gain. Analysis and simulation results show that the SIC-MRC can obtain performance close to that of N-tap MMSE receivers, and both of them have better ability to suppress multiple-access interference (MAI) than does MF-MRC. Finally, with timing or phase tracking errors, the results show that SIC-MRC can still retain a performance advantage over MF-MRC     

An adaptive CMMSE receiver for space-time block-coded CDMA systems in frequency-selective fading channels

A technique that can suppress multiple-access interference (MAI) in space-time block-coded (STBC) multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) systems is developed. The proposed scheme, called a constrained minimum mean square error (CMMSE) receiver, is an extension of the CMMSE receiver for a single-input-single-output system to MIMO systems. It is shown that the complexity of the proposed CMMSE receiver is almost independent of the number of transmitter antennas. The advantage of the proposed receiver over the existing receivers for STBC CDMA systems is demonstrated by comparing the closed-form expressions of the signal-to-interference plus noise ratio and simulated bit error rates. The results indicate that the proposed CMMSE receiver can provide a significant performance improvement over the conventional receivers and that the gain achieved by suppressing the MAI can be larger than that from increasing the transmitter or receiver diversity.