Code Division Multiple Access Receiver Research Articles

PN Code Acquisition in DS-CDMA Wireless Systems Using Smart Antenna and S-CFAR Processor

Code division multiple access (CDMA) is a class of techniques allowing multiple users to transmit simultaneously to meet the demand of the recently unprecedented revolution in wireless services. In CDMA systems, a unique pseudo-random (PN) code is assigned to each user to spread the signal at the transmitter side and facilitate signal’s recovery at the receiver side. In particular, a CDMA receiver initially seeks synchronization or acquisition of PN code. This paper focuses on the development and analysis of a new adaptive serial search acquisition method for direct sequence (DS)-CDMA systems in Rayleigh slowly fading channels. The proposed method makes use of smart antenna and switched constant false alarm (S-CFAR) processor for PN code acquisition. The smart antenna and S-CFAR processor are employed to improve the PN acquisition performance by adjusting a decision threshold according to the environment. The performance of proposed acquisition system has been evaluated in terms of probability of false alarm, probability of detection, and mean acquisition time. Closed form expressions for the probability of false alarm and probability of detection have been derived. Further, numerical and simulation results have been presented to show the performance of proposed PN acquisition system in comparison with other existing CFAR-based PN acquisition schemes.

Low power Multicarrier- Code Division Multiple Access Receiver on Field Programmable Gate Array

Low power Multicarrier- Code Division Multiple Access Receiver on Field Programmable Gate Array

Adaptive Blind CDMA Receivers Based on ICA Filtered Structures

Code division multiple access (CDMA) is a channel access method adopted by various radio technologies worldwide. In particular, CDMA is used as an access method in many mobile standards such as CDMA2000 and WCDMA. We address the problem of blind multiuser equalization in the wideband CDMA systems in the noisy multipath propagation environment. Herein, we propose three new blind receiver schemes based on variations of independent component analysis within several filtering structures. These adaptive blind CDMA receivers do not require knowledge of the propagation parameters or spreading code sequences of the users--they primarily exploit the natural assumption of statistical independence among the symbol signals. We also develop three semi-blind adaptive detectors by incorporating new adaptive methods into the standard Rake receiver structure. Extensive comparative case studies based on bit error rate performance are carried out as a function of (i) the number of users, (ii) the number of symbols per user, and (iii) the signal-to-noise ratio. The conventional detectors include the baseline linear minimum mean squared error detector. The results show that the proposed methods outperform other detectors in estimating the symbol signals from the received mixed CDMA signals. Moreover, the new blind detectors mitigate the multiaccess interference in CDMA.

Despread‐ahead cyclic‐prefix code division multiple access receiver with compressive sensing channel impulse response estimation

The conventional cyclic-prefix code division multiple access (CP-CDMA) system usually requires a chip-rate frequency-domain equaliser (FDE) at the cost of a tremendous discrete Fourier transform (DFT) size. To reduce the required DFT size in the CP-CDMA uplink receiver, this work studies a new symbol-rate equaliser moving the code despreader ahead of the FDE. The authors formulate the composite effect of the chip-rate channel impulse response (CIR) and code despreading as an equivalent symbol-rate channel model together with a small additive noise. Then, an iterative two-stage multiuser interference (MUI) cancellation algorithm can be employed with the decision feedback equalisation method. In addition, a novel compressive sensing (CS)-based interpolation method is proposed for compressible CIR estimation to be used in the MUI cancellation algorithm. With the CS method, the DFT size is dramatically reduced for channel estimation and no high-resolution interpolation kernels are required. The numerical results show that the new receiver is effective on MUI cancellation and only a couple of iterations are required to achieve a performance similar to the chip-rate equaliser.

Implementation of CDMA receiver using Recursive Digital Matched Filter

Code Division Multiple Access (CDMA) is a multiple access technique widely used in cellular and military communication systems. In Direct Sequence Spread Spectrum (DSSS) communications, data samples are embedded inside Pseudo Noise code (PN code) prior to transmission. Synchronization of this unique code with the incoming data samples forms an important step in data recovery. Single Dwell Serial Search (SDSS) and Matched Filtering are some of types of the CDMA code synchronization techniques. This paper presents design and implementation of Recursive Digital Matched Filter (RDMF) for code synchronization in CDMA. From design summary, it is observed that with RDMF, lesser hardware is utilized compared to the conventional methods. And also, a modification in despreading steps is proposed for achieving reduced Bit Error Rate (BER) performance

CDMA Transmitter and Receiver Implementation Using FPGA

Code Division Multiple Access (CDMA) is a spread spectrum technique that uses neither frequency channels nor time slots. With CDMA, the narrow band message (typically digitized voice data) is multiplied by a large bandwidth signal that is a pseudo random noise code (PN code). All users in a CDMA system use the same frequency band and transmit simultaneously. The transmitted signal is recovered by correlating the received signal with the PN code used by the transmitter. The DS - CDMA is expected to be the major medium access technology in the future mobile systems owing to its potential capacity enhancement and the robustness against noise. The CDMA is uniquely featured by its spectrum-spreading randomization process employing a pseudo-noise (PN) sequence, thus is often called the spread spectrum multiple access (SSMA). As different CDMA users take different PN sequences, each CDMA receiver can discriminate and detect its own signal, by regarding the signals transmitted by other users as noise- like interferences. In this project direct sequence principle based CDMA transmitter and receiver is implemented in VHDL for FPGA. Modelsim 6.2(MXE) tool will be used for functional and logic verification at each block. The Xilinx synthesis technology (XST) of Xilinx ISE 9.2i tool will be used for synthesis of transmitter and receiver on FPGA Spartan 3E.

Iterative Multiuser Detection for Underwater Acoustic Channels

Two different synchronous downlink multiple-access schemes, interleave-division multiple access (IDMA) and code-division multiple access (CDMA), are considered for pragmatic underwater communications channels exhibiting extended multipath spread and time variability. Two single-element DFE-IDMA and DFE-CDMA receivers are proposed that utilize chip-level adaptive decision feedback equalization (DFE) and carrier phase tracking along with iterative interference cancellation (IC) and channel coding. The receiver equations describing the detection algorithms are derived and their performance is investigated and compared. To track and compensate for the channel effects, the DFE and carrier phase tracking units are jointly optimized based on the mean square error (MSE) criterion and adapted iteratively by exchanging soft information in terms of log-likelihood ratio (LLR) estimates with the turbo processing stage. The detection is implemented by using soft chip cancellation to remove multiple-access interference (MAI) effects between users. The performance of the proposed receiver structures is investigated and compared in short-range shallow-water acoustic channels using offline processing of signals acquired during sea trials in the North Sea. Results for synchronous multiuser scenarios, with two and four users at an effective rate of 439.5 b/s per user, demonstrate that the proposed DFE-based IDMA and CDMA receivers can provide bit error rate (BER) performances of approximately 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> at an average signal-to-interference-and-noise ratio (SINR) of 11 dB . The experimental results also demonstrate that these direct adaptive receivers have better performance and significantly mitigate the bit errors associated with the channel estimation (CE)-based IDMA and CDMA receivers with Rake reception while maintaining lower complexity. Furthermore, in some cases, the receivers with partial knowledge of the interleavers' patterns or codes can still achieve performance comparable to those with full knowledge.

Area-Efficient Prefilter Architecture for a CDMA Receiver

This brief proposes an area-efficient memory-based prefilter delay line structure. A prefilter-rake chip-level equalizer is used in a code-division multiple-access receiver to deal with multiple-access interference. A prefilter, which functions as an adaptive filter, has a sparsity property, where the number of taps with nonzero coefficients is much smaller than the number of whole taps. On the basis of the sparsity property, this brief shows how a memory device can be a reasonable candidate for a prefilter delay line. After proposing a scheme to reduce the area of the memory-based delay line, this brief shows that the proposed structure provides less area than a conventional register-based structure in typical industrial cases.

Performance Analyses of OFDM–CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM-CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM-CDMA receivers in multipath fading channels: 1) MC-CDMA and 2) MC-direct sequence (DS)-CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC-CDMA and MC-DS-CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC-DS-CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC-DS-CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations.

Minimum Cross Correlation Spreading Codes

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.

Optimization of CDMA Systems with Respect to Transmission Probability, Part II: Signal to Noise Plus Interference Ratio Optimization

Code division multiple access (CDMA) systems commonly use power control mechanism to reduce the amount of interference between the users. In part I of this paper, we introduced the concept of probability control that optimizes system performance with respect to the users' transmission probabilities. The importance of probability control was demonstrated by proving that probability control alone optimizes the mutual information rates over a frequency-flat slow-fading multiple access channel. In this part we extend this result to the optimization of the average signal-to-noise plus interference ratio (SIR) in a CDMA system over a frequency-selective slow- fading channel model and any network topology. We prove that there is a group of CDMA receivers, in which the average SIR (ASIR) of all users is maximized using probability control (while all users transmit their maximal allowed power). This receivers group includes, among others, the common matched filter (MF) RAKE receiver, as well as the sophisticated minimal mean square error (MMSE) RAKE receiver. Simulations demonstrate for a 2- users scenario that both users achieve significantly higher ASIR using probability-control instead of power-control. Results are applicable both for CDMA and impulse radio (IR) systems.

Efficient parameter determination of code acquisition detector for CDMA wireless communication

The optimal procedure minimising the mean acquisition time is proposed to determine the parameter values of the code acquisition detector in code division multiple access (CDMA) wireless communication. The optimal relationships among the threshold (vth), the number of correlation samples (N) and the number of noncoherent accumulations (L) are obtained for the code acquisition detector. A suboptimal solution is presented to obtain the integer solution and is shown to reduce the code synchronisation time and the computational complexity of the CDMA receiver.

SVM-based CDMA receiver with incremental active learning

Recently, the feasibility of using support vector machines (SVMs) for multiuser detection in code division multiple access (CDMA) systems has been investigated. Previous results show that SVMs perform well with short training sequences but suffer from two drawbacks that are highly undesirable in real-time applications: the run-time complexity and the block-based learning. To deal with these problems, here we propose a sample-by-sample adaptive algorithm for CDMA systems based on incremental SVMs, incorporating an active learning strategy aimed to reduce the complexity of both the training phase and the final classifier.

The Exit Problem in a Nonlinear System Driven by 1/f Noise: The Delay Locked Loop

The frequency generated by high frequency oscillators contains a small but significant noise component known as phase noise, also known as oscillator noise or phase jitter. The phase noise belongs to the family of stochastic processes with spectra $1/f^\alpha$, which exhibits scaleinvariance (or self-similarity) and a long-term correlation structure that decays polynomially in time. Both the phase and thermal noises cause errors in receivers that contain the oscillators. In particular, they cause losses of lock in phase tracking systems such as the phase locked loop in coherent systems, which include cellular phones, global positioning systems (GPS), and radar (e.g., synthetic aperture radar (SAR)), and in the delay locked loop (DLL), which is an important component of code division multiple access receivers and interface to modern memory modules, such as double data rate synchronous dynamic random access memory. The mean time to lose lock (MTLL) is well known to be an important design objective for various tracking loops. The evaluation of the MTLL is known in the mathematical literature as the exit problem for a dynamical system driven by noise, which is the problem of calculating the mean time for the noisy trajectories to reach the boundary of the domain of attraction of a stable point of the noiseless dynamics. In this paper we develop an analytic approach to the evaluation of the leading order term for MTLL of a second order DLL, due to both the non-Markovian $1/f^\alpha$ noise and to thermal white noise. The method is applicable to more general systems driven by a wide class of phase noises. The keys to the solution of this exit problem are the construction of a series of higher order Markovian processes that converge to the non-Markovian $1/f^\alpha$ noise and the asymptotic solution to a multidimensional elliptic boundary value problem that the mean first passage time (MFPT) satisfies.

A time-interleaved recursive loop band-pass delta-sigma modulator for a digital IF CDMA receiver

A bandpass delta-sigma modulator (BPDSM) is a key building block to implement a digital intermediate frequency (IF) receiver in a wireless communication system. This paper proposes a time-interleaved (TI) recursive loop BPDSM architecture that consists of five-stage TI blocks for a code-division multiple-access (CDMA) receiver. The proposed TI BPDSM provides reduction in the clock frequency requirement by a factor of 5 and relaxes the settling time requirement to one-fourth of the conventional approach. The test chip was designed and fabricated for a 30-MHz IF system with a 0.35-/spl mu/m CMOS process. The measured peak SNR for a 1.25-MHz bandwidth is 48 dB while dissipating 75 mW from a 3.3-V supply and occupying 1.3 mm/sup 2/.

A Class of Signal-Processing Schemes for Reducing the Envelope Fluctuations of CDMA Signals

In this paper, we present a class of nonlinear signal-processing schemes to reduce the envelope fluctuations in the downlink transmission within code-division multiple-access (CDMA) systems. These schemes are compatible with conventional CDMA receivers since they just require some modifications at the transmitter side. The proposed transmitter structure includes a nonlinear operation in the time-domain followed by a linear frequency-domain filtering operation. We also include a semi-analytical statistical characterization of the transmitted signals, which takes advantage of the Gaussian-like nature of CDMA signals with a high number of users. This characterization is used for an analytical performance evaluation of the proposed techniques. A set of performance results is presented showing that we can reduce significantly the envelope fluctuations of the transmitted signals, while maintaining the spectral occupation of the corresponding conventional CDMA signals. Moreover, the performance degradation due to the nonlinear distortion effects on the transmitted signals can be kept relatively low.

Modified derivative superposition method for linearizing FET low-noise amplifiers

Intermodulation distortion in field-effect transistors (FETs) at RF frequencies is analyzed using the Volterra-series analysis. The degrading effect of the circuit reactances on the maximum IIP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in the conventional derivative-superposition (DS) method is explained. The noise performance of this method is also analyzed and the effect of the subthreshold biasing of one of the FETs on the noise figure (NF) is shown. A modified DS method is proposed to increase the maximum IIP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> at RF. It was used in a 0.25-mum Si CMOS low-noise amplifier (LNA) designed for cellular code-division multiple-access receivers. The LNA achieved +22-dBm IIP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with 15.5-dB gain, 1.65-dB NF, and 9.3 mA@2.6-V power consumption

A Low-Complexity Enhancement to Suboptimal CDMA Receivers

Conventional matched-filter detectors for code-division multiple-access (CDMA) systems suffer from multiple-access interference (MAI) caused by nonzero correlation between spreading codes at the receiver. A host of advanced detector structures have been proposed to reduce the effect of MAI and, hence, improve performance. However, most multiuser detectors suffer from their relatively complex implementations. A simple method is proposed to improve the performance of the conventional detector by detecting and correcting decision errors at its output without the use of forward error correcting (FEC) codes. The proposed post-detection error control method is shown to substantially improve the performance of the conventional detector, but has a much lower complexity than most other multiuser detectors.

Effects of Imperfect Blind Channel Estimation on Performance of Linear CDMA Receivers

In a code division multiple access (CDMA) system, signal detection under multipath distortion typically requires estimation of unknown channel parameters first. In such a scenario, performance of receivers highly relies on the accuracy of channel estimates. In this paper, effects of channel estimation errors on the performance of linear CDMA receivers due to finite data samples are studied when channel parameters are estimated blindly by a recently proposed covariance-matching technique. Those receivers include zero-forcing (ZF), direct matrix-inversion (DMI) minimum mean-square-error (MMSE), subspace MMSE, and RAKE receivers. Their output signal-to-interference-plus-noise ratios (SINRs) and bit-error-rates (BERs) are adopted for performance measures. Expressions for performance indicators under such an imperfect condition are derived from a perturbation perspective and verified by simulation examples.

Multicarrier CDMA Overlay for Ultra-Wideband Communications

In this letter, the performance of multicarrier code-division multiple-access (CDMA) systems is studied in the presence of narrowband interference for future ultra-wideband (UWB) communications. A Nakagami fading channel is assumed, and notch filters along with diversity techniques are used in the multicarrier CDMA receiver. A complete performance analysis of error probability is given. It is shown that when the number of subcarriers jammed by narrowband interference is small, the multicarrier receiver without notch filters can work well, due to the gain of frequency diversity from nonjammed subcarriers. On the other hand, when the number of subcarriers jammed by the narrowband interference is large, using notch filters can improve the multicarrier system performance significantly.