Adaptive reception of wireless CDMA signals using empirical detection

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

This paper carries comprehensive comparison of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems in terms of performance (BER) and behavior to different number of users. Performance analysis of two systems whose transmitter and receiver structures are almost same are studied under Additive White Gaussian Noise (AWGN) and performance differences are illustrated by simulation results. Although Multiple Access Interference (MAI) and Intersymbol Interference (ISI) can affect the performance of the CDMA system, simulation results illustrates that IDMA system model achieves this type of problem using their interleavers. Results also show that large number of users can easily be adapted by the IDMA scheme much easier than CDMA scheme.

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method.

Improvement of the capacity of the CDMA (code division multiple access) system is a major objective. The capacity of the system is mainly limited by the signal to noise ratio, where the noise comes from the channel background and from MAI (multiple access interference). Usually, the way to improve the capacity is to decrease the MAI; this is why orthogonal codes are adopted by the IS-95 standard. One problem is that the number of orthogonal codes is constrained by the dimensionality of the signal space. So in IS-95, there are only 64 orthogonal codes available when the number chips for each sequence is 64. To solve this problem, a non-orthogonal code called WBE (Welch bound equality) sequences (Massey and Mittelholzer 1991) is used in this paper The criterion to construct the code is to minimize the MAI. Two methods are used to construct the WBE sequences: from a linear cyclic code or from a Hadamard matrix. In order to improve the performance of the WBE sequence set, an iteration receiver (Sari et al. 1999) is applied. The conclusion obtained is that when the number of users is slightly greater than the signal space dimensionality, after the first iteration, the performance is only marginally worse than the orthogonal CDMA system.

One of the major advantages of a code division multiple access (CDMA) system is the flexibility in frequency assignment, which allows all the users to share the same frequency band and, possibly, to partially overlay the spectrum allocation of different services. However, before synchronism to the desired sequence is achieved, the presence of multiple access interference (MAI) is a major impairment which determines the performance of the code acquisition sub-system. It has been argued that the acquisition performance in the presence of MAI may pose limits to the capacity of a CDMA system which are more stringent than those coming from bit error probability specifications. We introduce a general definition for acquisition-based capacity of a CDMA system. This quantity is evaluated for an acquisition sub-system with non-coherent correlator/detector and straight serial search in the time uncertainty region. The effect of imperfect power control is also analysed, showing that the capacity may be substantially decreased when the residual standard deviation of the received power level is above 1 dB. >

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each processing window. The asymptotic convergence of the mean of the iterative estimate to the actual channel is also shown. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. Finally, the proposed iterative channel estimation technique is extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimate with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.

Introduction: Whether exposure to the 848.5 MHz code division multiple access (CDMA) signal affects adult neurogenesis is unclear.Materials and methods: An animal experiment was performed with a reverberation chamber designed as a whole-body CDMA exposure system. Male Sprague-Dawley rats were assigned to three groups (n = 6 per group): Cage-control, sham-exposed, and CDMA-exposed groups. Rats in the CDMA-exposed group were exposed to the CDMA signal at a 2 W/kg whole-body specific absorption rate (SAR) for 1 or 8 h daily, 5 days per week, for 2 weeks. Rats received a single intraperitoneal injection of Bromodeoxyuridine (BrdU) to label proliferative cells daily for the last five consecutive days of CDMA signal exposure. An unbiased stereological method was used to estimate the number of BrdU+ cells in the subventricular zone (SVZ) and dentate gyrus (DG).Results: We found no significant changes in the number of BrdU+ cells in the SVZ or DG in the CDMA-exposed rats, compared with rats in the cage-control and sham-exposed groups (p > 0.05).Conclusion: Our results suggest that exposure to the CDMA signal does not affect neurogenesis in the adult rat brain, at least under our experimental conditions.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

We experimentally demonstrate what we believe to be a novel detection scheme for interfacing asynchronous optical code division multiple access (CDMA) signals with an electronic clock and data recovery system that operates only at the baseband bandwidth. This allows using a large optical bandwidth expansion factor in which the optical chip rate is much larger than the bandwidth of the optoelectronic receiver. The received optical CDMA signal is launched into a four-wave-mixing-based wavelength-aware all-optical front end that rejects multiaccess interference, followed by an amplitude-noise suppression stage comprised of a semiconductor optical amplifier. The clean signal is then converted into a non-return-to-zero-like signal by a baseband receiver. Using the proposed detection scheme, asynchronous transmission and detection of optical CDMA signals is implemented. With the novel detection scheme, the classic CDMA near-far problem is mitigated, and error-free detection is easily obtained.

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

The use of multiple antennas at the transmitter or receiver provides a powerful means, of increasing the capacity of wireless systems. The goal of this paper is to illustrate ways in which analogies with multiuser code division multiple access (CDMA) systems may be used to provide design insights for space-time communication systems, for both single and multiple users. Three examples are considered: (a) noncoherent space-time codes for transmit antenna diversity against Rayleigh fading, constructed using an analogy with a CDMA downlink; (b) optimal multiuser transmit beamforming on a cellular downlink, where the algorithm for computing beamforming weights is obtained by transforming the problem into a CDMA uplink with MMSE interference suppression and power control; (c) design of high bandwidth efficiency systems exploiting both transmit and receive antenna arrays, using analogies with CDMA systems with random and slowly time-varying spreading waveforms.

In this treatise, the performance of Code division multiple access (CDMA) system is investigated using multi-user transmitter pre-processing (MUTP). The MUTP is promising signal processing technique which is carried out at the transmitter side to reduce the burden of detector and hence low complexity receiver is made feasible at mobile station (MS) in the context of downlink (DL) communication. Further, the unique signature sequence is considered to support more user population. In particular, the performance of MMSE-MUTP assisted CDMA system is investigated for realistic channel specifications based on Stanford University Interim (SUI) channel model. It has been demonstrated that the MMSE-MUTP assisted CDMA system provides achievable bit error rate with low complexity of receiver for DL transmission.

The W-CDMA system plays an important role in the 3G cellular systems because of its compatible networking architecture to the present popular Global System for Mobile Communications (GSM) systems as well as the salient features of a Code Division Multiple Access (CDMA) system, including multi-path fading tolerance, high system capacity, low power consumption, good performance and coverage. In CDMA system, a procedure used by a Mobile Station (MS) to search for the best cell site and achieve code, time, and frequency synchronization with it is referred as cell search. Fast cell search is essential to reduce switch on delay (initial cell search), increase stand by time (idle mode search) and maintain good link quality (active-mode search). This is particularly true for the Wideband Code Division Multiple Access-Frequency Division Duplex (W-CDMA-FDD) system since it employs nonsynchronous base stations instead of synchronous ones of other CDMA systems to extend coverage from outdoor to indoor. Cell search is critical for achieving code and time synchronization. The main process of achieving cell search is divided into three stages followed by code verification, tracking and carrier frequency adjustment: 1) slot synchronization, 2) frame synchronization and code group identification, 3) primary scrambling code identification. This paper addresses slot synchronization in downlink as cell search algorithm in W-CDMA. In the three step search, the receiver searches for the slot timing by correlating the received signal with the Primary Synchronization (PSCH) code using Matched Filter (MF). This paper propose a design for a hierarchical matched filter for Primary Synchronization Code (PSC). The aim is to reduce the hardware complexity using Field Programmable Gate Array (FPGA) logic resources required to realize the filter.

In recent years, more and more wireless communications systems are required to provide also a positioning measurement. In code division multiple access (CDMA) communication systems, the positioning accuracy is significantly degraded by the multiple access interference (MAI) caused by other users in the system. This MAI is commonly managed by a power control mechanism, and yet, MAI has a major effect on positioning accuracy. Probability control is a recently introduced interference management mechanism. In this mechanism, a user with excess power chooses not to transmit some of its symbols. The information in the nontransmitted symbols is recovered by an error-correcting code (ECC), while all other users receive a more reliable data during these quiet periods. Previous research had shown that the implementation of a probability control mechanism can significantly reduce the MAI. In this paper, we show that probability control also improves the positioning accuracy. We focus on time-of-arrival (TOA) based positioning systems. We analyze the TOA estimation performance in a generalized CDMA system, in which the probability control mechanism is employed, where the transmitted signal is noncontinuous with a symbol transmission probability smaller than 1. The accuracy of the TOA estimation is determined using appropriate modifications of the Cramer-Rao bound on the delay estimation. Keeping the average transmission power constant, we show that the TOA accuracy of each user does not depend on its transmission probability, while being a nondecreasing function of the transmission probability of any other user. Therefore, a generalized, noncontinuous CDMA system with a probability control mechanism can always achieve better positioning performance, for all users in the network, than a conventional, continuous, CDMA system.

In multi-user wireless communication system, there is huge demand for higher data rates. One promising solution to achieve it is through the amalgamation of Code Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) technologies which results in Multi Carrier-CDMA (MC-CDMA) system. In CDMA system, the multiple access interference (MAI) can be reduced through proper selection of spreading codes. Bit error rate (BER) is an effective measure to evaluate the interference performance of a multi-user communication system. In this paper the BER and Peak to Average Power Ratio (PAPR) of MC-CDMA system is analyzed for different orthogonal spreading codes such as Walsh codes, complementary Golay codes and Orthogonal Gold codes in Rayleigh multipath fading channel employing Low Density Parity Check (LDPC) channel coding technique.