CDMA Receiver Research Articles

Performance of Iterative Coded CDMA Receivers with APP Feedback: A Use of a Weighted Delay Filter

The prohibitive computational complexity of optimal coded multiuser detection necessitates using suboptimal detectors in practical implementations. The filter is very computationally simple and is also demonstrated to provide faster convergence and superior bit error rate (BER) performance. Further investigation of the weighted delay filter concept produces a second filter—derived via the joint likelihood function. It is analytically demonstrated that extrinsic feedback systems will not benefit from weighted delay filtering. A system model is provided that introduces the notion of feedback ‘residue’, which is shown to be the key difference between a-posterior probability (APP) and extrinsic systems when determining the parallel interference cancellation (PIC) output statistics. It is analytically shown that the weighted delay filter derived via a maximum signal-to-noise ratio (SNR) approach is identical to a weighted delay filter derived via the joint likelihood function. It is analytically shown that when extrinsic feedback is used in a coded-code division multiple access (C-CDMA) system, no benefit will be realised by weighted delay filtering, as soft outputs from previous cycles are a merely scaled, noisy version of the most recent data. The notion of a ‘feedback residue’ for systems with APP feedback is introduced, and it is empirically shown that this residue term is a key consideration when determining the PIC output statistics. Using the ‘residual feedback’ model, it is shown that when APP feedback is utilised, data from previous cycles is not simply “a scaled, noisy version” of the current data. For this reason, benefits may be realised by APP feedback use. The simulation results shows that the residue may be trivial at small loads, the residue builds to the substantial value of nearly 0.4 at a reasonably modest load of K/N=15/10, and continues to grow as the load increases.

M-ARY CDMA SCHEME WITH CCSK MODULATION AND PARALLEL INTERFERENCE CANCELLATION TECHNIQUE

In this paper, we tackle the problem of implementation a parallel interference cancellation scheme for multi-user detection (MUD) in M-ary CCSK-modulated CDMA system. Complexity remains the main obstacle to the practical implementation of MUD and a factor limiting, if not the very possibility, then the feasibility of using high order M-ary modulation formats for applications such as low-probability-of-intercept systems with a large spreading factor or, myriads of small self-powered IoT devices. The design of MUD algorithms for M-ary systems leads to a new round of growth in the receiver complexity comparing with the binary modulation. Demodulator has a major impact on the complexity of CDMA system. Cyclic code-shift keying (CCSK) is a modulation technique which is designed to reduce the complexity of M-ary signaling. In this, each symbol is a circularly shifted version of a single code sequence. Assuming synchronization, the receiver cyclically correlates the input signal plus noise with the base sequence and estimates the position of the correlation peak. The preliminary stage of the proposed scheme is a conventional multi-channel CDMA receiver. The preliminary estimates of the data is multiplied by user codes and amplitude estimates in the spreader, and then fed to the adder to generate the MAI estimates. After the latter are subtracted from the group signal, K-dimensional vector of cleared user signals is passed to the matched filter bank to form the refined estimates of the data. At each subsequent stage, the estimates of the output data of the previous stage are used as input data. The complexity gain is the ratio of the number of computations for finding cyclic convolution and, the amount of computations for finding M linear convolutions, i.e., M/log2M. Since the performance of the algorithm is sensitive to the reliability of the preliminary decisions, there are strict criteria for the selection of spreading codes: the length which must be an integer power of 2, the large family size, good periodic autocorrelation and good cross-correlation properties. To examine the proposed algorithm using computer simulation, we selected minimax periodic and odd-periodic complementary codes, the properties of which are close to the properties of the codes used in the CCSK scheme implemented in Link-16 protocol tactical data networks JTIDS and MIDS. Our study shows that the gain over the conventional receiver increases as the SNR increases, achieving 15 dB for BER 10-5. The system with complementary codes outperform system based on minimax PN-codes, achieving a target bit error rate at a lower SNR.

Independent Code Division Multiple Access in DS-CDMA

An independent component analysis (ICA) based CDMA receiver is proposed, which in addition to the independence of users also exploits the independence of pseudo-codes in user signal separation. The proposed scheme is termed as independent code division multiple access in contrast to orthogonal code division multiple access being used in conventional CDMA receivers. Independence of pseudo-codes has been invoked by introducing the so-called independent Gram-Schmidt process in a deflation based ICA algorithm. The independence via Gram-Schmidt process is claimed by taking the inner product of successive vectors in a nonlinearly transformed Hilbert space, incorporating higher order statistics, instead of that in the original Euclidean space. The proposed receiver has been shown to outperform the existing CDMA receivers, in terms of bit error rate, through simulations. Moreover, the introduction of nonlinear transformation is shown to have no adverse effect on the convergence of the original deflation based ICA algorithm in terms of number of iterations apart from the computational cost of nonlinear transformation at each step. Gold codes are used for the demonstration of the proposed scheme although more work is needed to be done in order to identify the code family more amenable to the independence property.

Contribution to Synchronization and Tracking Modelisation in a CDMA Receiver

We propose and analyze a noncoherent receiver with PN code tracking for direct sequence code division multiple access (DS-CDMA) communication systems. We employ the delay-lock loop (DLL) architectures for the tracking stage. The choice of DLL parameters is studied with special focus on DS-CDMA communication systems and orthogonality conditions. We described the modeling and simulation of the NCO using hardware description language VHDL. Details of the VHDL implementation are shown.

EM-Based Adaptive Frequency Domain Estimation of Doppler Shifts with CRLB Analysis for CDMA Systems

Combating time and frequency selectivity in wireless channels is one of the most challenging tasks in next generation wireless networks. In this paper, we propose an adaptive estimation algorithm to estimate Doppler shifts in a direct sequence code division multiple access (DS-CDMA) radio system with multiple Doppler subpaths. By modeling doubly selective channels using a basis expansion model (BEM), an expectation-maximization (EM) algorithm based adaptive estimation method is developed to extract accurate Doppler shift information. The Cramer-Rao lower bound (CRLB) analysis is conducted to study the performance bound of the proposed estimation algorithm. Based on the estimated Doppler shift results, a frequency domain equalizer (FDE) based receiver architecture is developed to exploit Doppler diversity in the frequency domain. Our analysis and simulation results demonstrate that this receiver architecture features a low complexity while still achieving a good performance compared with traditional CDMA receivers.

Enhancement of Convergence Properties of Adaptive CDMA Receiver over Rayleigh Flat Fading Channel

CDMA system is a promising multiple access technique which extremely increases the channel capacity and supports multiple-access and multimedia transmission. However, the performance of CDMA system degrades in presence of multipath and dense multiple-access environment .Usually an adaptive filter is utilized to compensate the effect of multipath and multiple- access interference and signal fading over the channel. This paper provides a variable step size algorithm to accelerate convergence of the adaptive filter. The optimum performance is achieved by governing the rate of convergence and the amount of steady state excess mean square error (MSE).The performance is optimized in thispaper by two main methods mainly: variable step size parameters and gradient adaptively regularization parameter. The first method is implemented through variable step size least mean square algorithm VSSLMS, modified variable step size MVSS algorithm and variable step size affine projection algorithm VS-APA. On the other hand, the second method isimplemented using generalized normalized gradient descent algorithm GNGD and generalized square-error-regularized LMS algorithm GSER-LMS. This paper concerns with comparative study of the five mentioned adaptive algorithms,(VSSLMS – MVSS – VS-APA – GNGD – GSER-LMS), to overcome the multipath and multiple-access problems of CDMA systems. The mentioned algorithms had the advantages of fast convergence and low steady state MSE, these advantages candidate the mention algorithms to improve the bit error rate (BER) performance of the CDMA system, in the presence of multipath and multiple-access environment. The results indicate that the proposed algorithm provides faster convergence compared with the fixed step size and fixed regularization parameters algorithms.

Interference Cancellation in Body-Area Networks Using Linear Multiuser Receivers

The problem of interference cancellation may arise in the nearest future for body-area networks (BANs) whenever a certain number of BANs are led to operate in the vicinity of each other under the same spectrum band. In this paper, using the data obtained from the measurements, the performance in terms of bit error rate (BER) of three linear CDMA receiver structures is investigated for BANs: the conventional receiver, the minimum mean square error receiver, and the decorrelator receiver. We show that in synchronous scenario, the three receivers exhibit the same performance regardless of the number of supported users and SNR level. The BER performance of the linear receivers in on-body channels is better than in Rayleigh channel with the belt-chest channel exhibiting the best gain due to the presence of a stronger LOS component. Furthermore, with orthogonal codes, these receivers are shown to be robust to the MAI level increase and do not require a perfect power control.

An efficient receiver scheme for downlink ZP-CDMA

A signal transmitted through a wireless channel may be severely distorted by intersymbol interference (ISI) and multiple access interference (MAI). In this paper, we propose an efficient CDMA receiver based on frequency domain equalization (FDE) with a regularized zero forcing (RZF) equalizer and parallel interference cancellation with a unit clipper decision function (CPIC) to combat both the ISI and the MAI. We call this receiver the FDE-RZF-CPIC receiver. This receiver is suitable for downlink zero padding CDMA cellular systems. The effects of the decision function, the channel estimation, the number of cancelled users, and the user loading on the performance of the proposed receiver are discussed in the paper. The bit error rate (BER) of the data received by the proposed receiver is evaluated by computer simula- tions. The experimental results show that the proposed receiver provides a good performance, even with a large number of interfering users. At a BER of 10 −3 ,t he performance gain of the proposed receiver is about 2 dB over the RAKE receiver with a clipper decision function and PIC in the half-loaded case (eight users) and is much larger in the full-loaded case (16 users).

Low-Complexity Variable Step-Size Mechanism for Code-Constrained Constant Modulus Stochastic Gradient Algorithms Applied to CDMA Interference Suppression

The code-constrained constant modulus algorithm (CCM) implemented with a stochastic gradient (SG) technique is a very effective and efficient blind approach for interference suppression when a communication channel is frequency-selective. In nonstationary wireless environments, users frequently enter and exit the system, making it very difficult for the receiver to compute a predetermined step-size. This suggests the deployment of mechanisms to automatically adjust the step-size in order to ensure good tracking of the interference and the channel. In this paper, the performance of blind CCM adaptive receivers for direct sequence code division multiple access (DS-CDMA) systems that employ stochastic gradient (SG) algorithms with variable step-size mechanisms is investigated. We propose a novel low-complexity variable step-size mechanism for blind CCM CDMA receivers. Convergence and tracking analyses of the proposed adaptation techniques are carried out for multipath channels. Finally, numerical experiments are presented for nonstationary environments, showing that the new mechanism achieves superior performance to previously reported methods at a reduced complexity.

Robust multicarrier CDMA receiver for coded power-line communications

Existing indoor power-line grids are a potential solution to the last mile problem for broadband communication providers. However, power-line communication (PLC) channels suffer from deep frequency notches and severe impulsive noise making powerful signal processing essential if these channels are to be utilized. Broadband communications using Multicarrier Code Division Multiple Access(MC-CDMA) with channel coding is considered. Although resilient to frequency-selective fading, the coding gain from turbo signal processing is substantially compromised in the presence of impulsive noise. To overcome this, we propose a low complexity non-linear matched filter utilizing M-estimation technique. We will show that this matched M-estimate filter is indispensable in providing efficient baseband filtering in impulsive channels.

Signal processing advances for 3G WCDMA: From rake receivers to blind techniques

As already deployed and proven technology, code-division multiple access has evolved to support competitive high-data-rate low-latency multimedia services over wireless cellular networks. In this article we introduce advanced signal processing techniques to enhance CDMA receiver performance further. In particular, we consider the possibility of optimal joint multiuser detection for long code WCDMA using fast inversion based on a state-space approach with reasonable complexity, and semi-blind channel estimation techniques to realize rate-efficient transmission for the latest 3G standards.

A novel method for increasing the spectral efficiency of optical CDMA

Given K simultaneously active users, an Optical CDMA (OCDMA) receiver needs to discriminate among K codes. Since the number of available codes C is always C Gt K at any given time for any truly asynchronous OCDMA system that employs quasi-orthogonal codes, there is always a set of unused codes. We here propose to exploit these unused codes to increase the spectral efficiency of the system. This can be accomplished by exclusively assigning to each user a set of M codes which represent a log2(M)-tuple of bits so that each user effectively uses a multi-dimensional modulation (multiple information bits per code are conveyed). In this paper, we analyze the performance of such a system and find in closed form an upper bound on the average probability of error. Moreover, we report for the first time that, under lightly loaded, non interference-limited conditions, the multi-dimensional OCDMA scheme here proposed allows us to obtain both higher spectral efficiency and lower bit- error probability (BEP) than a conventional OCDMA system, which employs one code only per user. The analysis here presented allow us to formulate a useful system design criterion that yields truly asynchronous OCDMA systems with very high spectral efficiency.

Performance evaluation of rake receiver of DS-CDMA under AWGN environments

High speed data, particularly, direct sequence code-division multiple access (DS-CDMA) experiences inter symbol interference (ISI) in wireless communications due to delay differences of multipath propagations. Rake receiver is an effective method of recovery of baseband signal for wideband CDMA (WCDMA) under multipath environments. This paper deals with bit error rate (BER) performance of a Rake receiver for multipath wireless CDMA contaminated by additive white Gaussian noise (AWGN), and impact of fingers on reception of fading channel signal.

A Superlinearly and Globally Convergent Algorithm for Power Control and Resource Allocation With General Interference Functions

In wireless networks, users are typically coupled by interference. Hence, resource allocation can strongly depend on receive strategies, such as beamforming, CDMA receivers, etc. We study the problem of minimizing the total transmission power while maintaining individual quality-of-service (QoS) values for all users. This problem can be solved by the fixed-point iteration proposed by Yates (1995) as well as by a recently proposed matrix-based iteration (Schubert and Boche, 2007). It was observed by numerical simulations that the matrix-based iteration has interesting numerical properties, and achieves the global optimum in only a few steps. However, an analytical investigation of the convergence behavior has been an open problem so far. In this paper, we show that the matrix-based iteration can be reformulated as a Newton-type iteration of a convex function, which is not guaranteed to be continuously differentiable. Such a behavior can be caused by ambiguous representations of the interference functions, depending on the choice of the receive strategy. Nevertheless, superlinear convergence can be shown by exploiting the special structure of the problem. Namely, the function is convex, locally Lipschitz continuous, and an invertible directional derivative exists for all points of interest.

Frequency Offset Estimation with Semi-Coherent Accumulation for CDMA Receivers

A semi-coherent technique for low-complexity frequency offset estimation is presented. The proposed estimation is based on discrete-time Fourier transform (DTFT). Since residual frequency offset can be compensated by channel estimation, frequency offset can be estimated only coarsely. We take advantage of the relationship between frequency resolution and accumulation period in DTFT in deriving the coarse estimator. Based on that, a novel method to balance the coherent and the non-coherent accumulation for frequency offset estimation is proposed. The proposed algorithm has low latency and complexity so that it is particularly suitable for packet traffic. The semi-coherent structure of the proposed algorithm is also scalable so that it can be used for both bursty and continuous traffic.

Performance analysis of an adaptive two-stage PIC CDMA receiver in AWGN channels

Parallel interference cancellation (PIC) is considered a simple yet effective multiuser detector for direct-sequence code-division multiple-access (DS-CDMA) systems. However, its performance may deteriorate due to unreliable interference cancellation in the early stages. Thus, a partial PIC detector, in which partial cancellation factors (PCFs) are introduced to control the interference cancellation level, has been developed as a remedy. Recently, an interesting adaptive multistage PIC algorithm was proposed. In this scheme, coefficients combining the channel responses and optimal PCFs are blindly trained with the least mean square (LMS) algorithm. The algorithm is simple to implement, inherently applicable to time-varying environments, and superior to the non-adaptive type of partial PICs. Despite its various advantages, its performance has not been theoretically analyzed yet. The contribution of this paper is to fill the gap by analyzing an adaptive two-stage PIC in AWGN channels. We explicitly derive the analytical results for optimal weights, weight-error means, and weight-error variances. Based on these results, we finally derive the output bit error rate (BER) for each user. Simulation results indicate that our analytical results highly agree with empirical ones.

On the performance of O 3BPSK LDD with diversity combining techniques over fading channels

Zheng and Barton developed a convenient signaling scheme (orthogonal on-off BPSK (O 3BPSK)) with a simple one-shot linear decorrelating detector (LDD) for near–far resistant detection in asynchronous DS/CDMA systems. This paper studies the performance of the O 3BPSK LDD system over a frequency-nonselective and slowly fading channel. Three types of channel model are considered, namely Rayleigh fading, Nakagami- m fading, and Ricean fading. The analytical results show that channel fading causes a significant degradation in the performance of the O 3BPSK LDD receiver. Nevertheless, the receiver still demonstrates a better near–far resistant property than a conventional CDMA receiver over fading channels. This study also analyzes the effect of two well-known diversity combining techniques on the O 3BPSK LDD performance over Rayleigh fading channels. The results indicate that diversity combining techniques significantly improve the performance of the O 3BPSK LDD scheme over fading channels.

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.

An Efficient Multipath Detection Scheme for CDMA Systems

In conventional CDMA receivers, the detection of multipath components and RAKE finger management is normally based on the received signal energy per path. These schemes essentially overlook the interference component contaminating the total received power. Consequently, they exhibit poor multipath detection capability especially at low signal-to-interference-plus-noise ratio (SINR). In this paper, we present a new scheme for multipath detection that takes into consideration the interference level in each resolved path individually. Specifically, the proposed scheme is devised to estimate and cancel the interference per path before detection. To account for the hardware limitations of the receiver, we propose a low complexity version of the above scheme which can be easily incorporated into the receiver structure. Our results show that the proposed scheme provides significant improvements in the detection probability of multipath components over the energy-based schemes.

Asymptotic Analysis of Reduced-Rank Chip-Level MMSE Equalizers in the Downlink of CDMA Systems

In this paper, we address the performance of downlink CDMA receivers that consist in a reduced rank Wiener chip-rate equalizer followed by a despreading operation. In particular, we tackle the question of whether a performance close to the optimum can be achieved for small values of the rank. To answer this question, it is standard to consider the output signal-to-interference plus noise ratio (SINR), and to study its convergence speed versus the rank of the receiver. Unfortunately, this task is difficult due to the fact that the SINR expressions depend on the spreading codes allocated to the various users in a rather complicated way. To be able to obtain positive results, we assume that the spreading factor and the number of users tend to infinity while their ratio remains finite. As in the third-generation UMTS system, the spreading codes we consider coincide with orthogonal Walsh-Hadamard codes scrambled by an independent identically distributed sequence. In this context, we show that the SINR of each reduced-rank receiver converges toward a deterministic limit which depends only of the rank of the receiver, and not of the spreading codes given to the various users. Using some previous results, we prove that the asymptotic SINRs converge exponentially to the SINR of the plain Wiener receiver when the rank of the receiver increases. We obtain the corresponding convergence rate, and exhibit the parameters that influence the convergence speed. We finally compare our asymptotic performance expressions with the results of numerical simulations. We observe a good agreement for spreading factors as low as 16