Multiuser Code Division Multiple Access Research Articles

Effect of Coupling Discretization on Coherent-Ising-Machine-Implemented Hopfield Model

The coherent Ising machine (CIM) is being developed for solving combinatorial optimization problems scalably at high speed. The actual equipment of the measurement-feedback type of CIM (MFB-CIM) uses a field-programmable gate array (FPGA) to calculate the local field. Because the product of the coupling strength matrix and the measured amplitude vector of the optical parametric oscillator pulses must be calculated in real-time to achieve feedback, the coupling strength must have a low-bit discrete representation in the FPGA. By reducing the number of bits of coupling strength, the amount of memory needed to store the coupling matrix can be reduced and the calculation speed increased so that the MFB-CIM can implement more spins. Therefore, it is necessary to reduce the number of bits of coupling strength as much as possible while maintaining the performance in solving combinatorial optimization problems. In this paper, to evaluate the effect of discretization of the coupling strength in the MFB-CIM, we quantitatively evaluate the performance of the Hopfield model with discrete coupling when it is implemented in the CIM. The Hopfield model, which is a canonical model of Ising computation in information statistical mechanics, is defined in a coupling manner called the correlation-type interaction, in common with other models such as Sourlas code, a code division multiple access multiuser detector, and L0 regularization-based compressed sensing. We derived the macroscopic equations (MEs) of the Hopfield model with discrete coupling when it is implemented in the CIM by using the non-equilibrium statistical mechanics method we previously proposed. Moreover, we clarified the relationship between the number of bits of the coupling strength and the critical memory capacity for various pump rates.

Mean field analysis of reverse annealing for code-division multiple-access multiuser detection

Code-division multiple-access (CDMA) multiuser detection is a kind of signal recovery problem. The main problem of CDMA multiuser detection is to estimate the original signal from the degraded information. In CDMA multiuser detection, the first-order phase transition happens. The first-order phase transition degrades the estimation performance. To avoid or mitigate the first-order phase transition, we apply adiabatic reverse annealing (ARA) to CDMA multiuser detection. In ARA, we introduce the initial Hamiltonian, which corresponds to the prior information of the original signal into quantum annealing (QA) formulation. The ground state of the initial Hamiltonian is the initial candidate solution. By using the prior information of the original signal, ARA enhances the performance of QA for CDMA multiuser detection. We evaluate the typical ARA performance of CDMA multiuser detection by means of statistical mechanics using the replica method. At first, we consider the oracle cases where the initial candidate solution is randomly generated with a fixed fraction of the original signal in the initial state. In the oracle cases, the first-order phase transition can be avoided or mitigated by ARA if we prepare for the proper initial candidate solution. We validate our theoretical analysis with quantum Monte Carlo simulations. The theoretical results to avoid the first-order phase transition are consistent with the numerical results. Next, we consider the practical cases where we prepare for the initial candidate solution obtained by commonly used algorithms. We show that the practical algorithms can exceed the threshold to avoid the first-order phase transition. Finally, we test the performance of ARA with the initial candidate solution obtained by the practical algorithm. In this case, ARA cannot avoid the first-order phase transition even if the initial candidate solution exceeds the threshold to avoid the first-order phase transition.

Statistical mechanics of CDMA multiuser detector implemented in coherent Ising machine

The scalability and computational ability of the coherent Ising machine (CIM) have made it attract attention as one of the most effective Ising computing architectures for solving large-scale optimization problems. However, the theory and techniques of equilibrium classical thermodynamics cannot be directly applied to the CIM because it is an open-dissipative system. In an attempt to adapt the theory and techniques to the CIM, we have used a canonical frustrated model of Ising computation called the CDMA (Code Division Multiple Access) multiuser detector. Here, we derive a macroscopic equation to estimate the critical value of the spreading factor for achieving a low bit error rate as a function of the normalized pump rate in the CIM-implemented CDMA multiuser detector.

Efficiency droop effects of GaN-based light-emitting diodes on the performance of code division multiple access visible-light communication system

The physical mechanism in efficiency droop of GaN-based light-emitting diodes (LEDs) was investigated using a modified rate equation model considering inhomogeneous carrier distribution and was compared with the measured result. On this basis, the efficiency droop effect on the performance of a code division multiple access (CDMA) visible-light communication (VLC) system using GaN-based LEDs was also analyzed. The results reveal that the obvious transmitted signal error under the effect of LED efficiency droop leads to performance deterioration of multiuser CDMA VLC systems. Also, the performance of CDMA VLC systems is reduced with the user number increase due to LED efficiency droop. The bit error rate of a CDMA VLC system was further calculated for different branch signal levels and bias currents. It is demonstrated that the efficiency droop effect on the performance of CDMA VLC systems can be alleviated by adjusting the branch signal level and the bias current.

Multi-user Detection in DS-CDMA System Using Biogeography Based Optimization

To attain large information rates, code division multiple access (CDMA) is a well-built aspirant for the downlink of cell phone communications. However, while transmitting any signal over fading channel, the performance of CDMA system is highly affected. For that reason, Multiuser detection (MUD) and channel estimation play a significant function for overcoming the interference and characterizing the channel. In this paper, biogeography base optimization (BBO) algorithm introduced for multiuser detection CDMA system. The objective of BBO algorithm is used to minimize the error rate of the user transmitting signal. According to immigration rate and emigration rate, the optimal solution of the detection problem is decided. Therefore, the user detection complexity and the interference of user transmitting signal are resolved. Proposed multiuser detection algorithm is appropriate for run time user identification process; since the detection complication and the required time are concentrated. The proposed approach is implemented in MATLAB working platform and the performance measures are examined. The error rate of proposed algorithm is compared with harmony search algorithm, enhanced harmony search algorithm, and without multiuser detection procedure.

Code-division multiple-access multiuser demodulator by using quantum fluctuations.

We examine the average-case performance of a code-division multiple-access (CDMA) multiuser demodulator in which quantum fluctuations are utilized to demodulate the original message within the context of Bayesian inference. The quantum fluctuations are built into the system as a transverse field in the infinite-range Ising spin glass model. We evaluate the performance measurements by using statistical mechanics. We confirm that the CDMA multiuser modulator using quantum fluctuations achieve roughly the same performance as the conventional CDMA multiuser modulator through thermal fluctuations on average. We also find that the relationship between the quality of the original information retrieval and the amplitude of the transverse field is somehow a "universal feature" in typical probabilistic information processing, viz., in image restoration, error-correcting codes, and CDMA multiuser demodulation.

Decoder-Assisted Timing Synchronization in Multiuser CDMA Systems

The problem of synchronizing weak user signals in multiuser code-division multiple access systems is addressed. We integrate timing synchronization into an iterative multiuser detection (MUD) framework for the improved synchronization performance of weak users. First, the effect of correctly detected users on the remaining undetected users is suppressed via soft parallel interference cancellation. Then, the time offset (TO) of each undetected user is estimated. To speed up the MUD, interference cancellation is performed after only a few decoder iterations of the detected users, and the TO estimator is designed taking into account the residual multiple access interference. A low-complexity non-data-aided TO estimator that estimates the TO of each undetected user separately is proposed. The proposed technique significantly improves the synchronization performance of weak users under high near-far conditions compared to the full interference case. Assuming Gaussian distributed log-likelihood ratios at the decoder outputs, we directly relate the missed detection probability of a desired user to the root-mean-squared error of the detected user's data sequences. The extrinsic information transfer chart of the decoder is used to analyze the acquisition performance of the desired user with the number of decoder iterations of the detected users. An upper bound for the missed detection probability of the proposed estimator is derived.

Belief propagation-based joint iterative algorithm for detection and decoding in asynchronous CDMA satellite systems

As the geostationary orbit (GEO) is congested in the commercial Ku band, the adjacent satellites will receive the interference from small aperture terminals which have a wide field of view. To reduce the interference toward the adjacent satellites, code division multiple access (CDMA) with spread spectrum needs to be adopted in the satellite communications on-the-move system. However, the mutual interference between CDMA users leads to the performance loss, which is not acceptable for the power-sensitive small aperture terminals, and the computation complexity is still an intractable problem when the conventional iterative minimum mean square error (MMSE) scheme is employed to suppress the multiple access interference (MAI) in satellite systems especially that is coded by low-density parity-check (LDPC) code. Moreover, the long transmission delays of different users caused by the long transmission distance require the robustness of the interference suppression for the asynchronous case. In this paper, a joint chip-level algorithm for CDMA multiuser detection and LDPC decoding with a new and simple iterative structure based on belief propagation (BP) for satellite systems is developed, which integrates the iterations of multiuser detection and the iterations of LDPC decoding with factor graphs. Through analysis and simulations, it is verified that the proposed algorithm is robust to the asynchronous CDMA satellite system, and with the similar iterative complexity, the performance of the proposed algorithm outperforms that of the conventional iterative MMSE scheme in the asynchronous CDMA satellite system.

Performance of vector antennas with power control in CDMA under multipath Rayleigh fading channel

Receivers currently employ single- or dual-polarized antennas, which measure one or two components of the received electromagnetic (EM) signal. In rich scattering environments, however, it may be possible to improve performance by using “vector antennas” as they can detect up to six independent components of the EM field. In this paper, theoretical and simulation results are obtained. We consider the design and analysis of vector-antenna receiver for code division multiple access (CDMA) signals. We consider a closed loop power control (CLPC) for a beamformer-Rake receiver for wireless CDMA multiuser system. We will investigate the performance enhancements in Bit Error Rate (BER) using various vector antennas that can respond to two polarization components combined in a frequency selective multipath fading channel.

A Fast Soft Bit Error Rate Estimation Method

We have suggested in a previous publication a method to estimate the Bit Error Rate (BER) of a digital communications system instead of using the famous Monte Carlo (MC) simulation. This method was based on the estimation of the probability density function (pdf) of soft observed samples. The kernel method was used for the pdf estimation. In this paper, we suggest to use a Gaussian Mixture (GM) model. The Expectation Maximisation algorithm is used to estimate the parameters of this mixture. The optimal number of Gaussians is computed by using Mutual Information Theory. The analytical expression of the BER is therefore simply given by using the different estimated parameters of the Gaussian Mixture. Simulation results are presented to compare the three mentioned methods: Monte Carlo, Kernel and Gaussian Mixture. We analyze the performance of the proposed BER estimator in the framework of a multiuser code division multiple access system and show that attractive performance is achieved compared with conventional MC or Kernel aided techniques. The results show that the GM method can drastically reduce the needed number of samples to estimate the BER in order to reduce the required simulation run-time, even at very low BER.

Performance of a space-time block coded code division multiple access system over Nakagami-m fading channels

By introducing an orthogonal space-time coding scheme, multiuser code division multiple access (CDMA) systems with different space time codes are given, and corresponding system performance is investigated over a Nakagami-m fading channel. A low-complexity multiuser receiver scheme is developed for space-time block coded CDMA (STBC-CDMA) systems. The scheme can make full use of the complex orthogonality of space-time block coding to simplify the high decoding complexity of the existing scheme. Compared to the existing scheme with exponential decoding complexity, it has linear decoding complexity. Based on the performance analysis and mathematical calculation, the average bit error rate (BER) of the system is derived in detail for integer m and non-integer m, respectively. As a result, a tight closed-form BER expression is obtained for STBC-CDMA with an orthogonal spreading code, and an approximate closed-form BER expression is attained for STBC-CDMA with a quasi-orthogonal spreading code. Simulation results show that the proposed scheme can achieve almost the same performance as the existing scheme with low complexity. Moreover, the simulation results for average BER are consistent with the theoretical analysis.

Blind identification and synchronization of unequal power users in CDMA systems based on eigenvalues variations in slow flat fading channels

In this paper a new blind number of users estimation and sequences synchronization method in multi-user code division multiple access (CDMA) context with unequal power signals has been proposed. Instead of the traditional approach, based on the FROBENIUS square norm behavior (FSNB) and successive detection using serial interference canceler, which was proposed previously in Ghavami and Vakili [Blind SNR estimation on WCDMA systems with unequal power signals and without any prior knowledge. In: IEEE-IST2007; July 2007. p. 1–5; Joint blind users identification and synchronization in non-cooperative CDMA systems in slow flat fading channels. In: ICEE2008; May 2008], we develop a new blind method for joint identification and synchronization. This new method is based on the adaptive threshold and eigenvalues variations (EV) in terms of processing window shifts, number of active users estimation is performed using an adaptive threshold. Theoretical analysis shows that the EV-based criterion avoids the successive detection and non-overlapping synchronized peaks and hence improves the synchronization performance, and proves that it is a powerful tool for blind synchronization especially in unequal power scenario, which we face in eavesdropping case. We show that the improvement is mainly due to the suppression of delay and error propagation that occur with the successive detection in the previous method [Ghavami S, Vakili VT. Blind SNR estimation on WCDMA systems with unequal power signals and without any prior knowledge. In: IEEE-IST2007; July 2007. p. 1–5; Ghavami S, Vakili VT. Joint blind users identification and synchronization in non-cooperative CDMA systems in slow flat fading channels. In: ICEE2008; May 2008]. Simulation results confirm the performance of the identification and synchronization process using this new criterion, allows achieving very good performance at the receiver side in terms of chip error rate (CER) and bit error rate (BER), even at very low signal to noise ratios (SNRs) in unequal power scenarios.

Receiver Design for Uplink Multiuser Code Division Multiple Access Communication System Based on Neural Network

In this paper, we consider the receiver design problem for the uplink multiuser code division multiple access (CDMA) communication system based on the neural network technique. The uplink multiuser CDMA communication system model is described in the form of space---time domain through antenna array and multipath fading expression. Novel suitable neural network technique is proposed as an effective signal processing method for the receiver of such an uplink multiuser CDMA system. By the appropriate choice of the channel state information for the neural network parameters, the neural network can collectively resolve the effects of both the inter-symbol interference due to the multipath fading channel and the multiple access interference in the receiver of the uplink multiuser CDMA communication system. The dynamics of the proposed neural network receiver for the uplink multiuser CDMA communication system is also studied.

On Antijamming in General CDMA Systems-Part II: Antijamming Performance of Coded Multicarrier Frequency-Hopping Spread Spectrum Systems

In the first part of the paper, the capacity of a general multiuser code division multiple access (CDMA) jamming channel was analyzed for noncooperative and cooperative users in uplink and downlink static and Nakagami fading channels when the receiver has or lacks jammer state information. The results were applied to a unified channel model encompassing a variety of multiuser spread spectrum systems contaminated by jamming. It was found that the jammer should spread its energy evenly over all degrees of freedom in order to minimize the average capacity. In the second part of the paper, using a standard orthogonal frequency division multiplexing (OFDM) model, the performance of a coded version of a multicarrier frequency-hopping (MC-FH) CDMA system in static and Rayleigh fading jamming uplink channels is analyzed. The MC-FH system under study is a hybrid of the OFDM and frequency hopping concepts, and is also a practical example of the model developed in the first part. It is demonstrated that in the cases where the receiver knows the jammer state and the receiver lacks the jammer state, spreading and contracting the jamming power over the system bandwidth, respectively, will give rise to the worst performance for the communicators. Optimal decorrelator weights for the receiver soft outputs in different channels, valuable for practical system design, are also obtained.

Fast CDMA multiuser detection by orthogonal projection onto an interference null space

This paper presents a method for asynchronous code-division multiple-access (CDMA) multiuser detection that has low computational complexity and that provides fast convergence. The proposed method efficiently finds an interference subspace from data samples and projects received signals onto its orthogonal complement to suppress multiple access interferences. Its performance has been analytically investigated in terms of the signal-to-interference-plus-noise ratio (SINR). The steady-state SINR is substantially the same as the optimum when the interferences are strong compared with noise. As to the transient response, the orthogonal-projection-based method converges quickly outperforming other methods such as the stochastic-gradient (SG) multistage Wiener filter (MSWF) and the affine projection method, which have similar computational complexity. Simulation results validate its effectiveness.

Robust CDMA Multiuser Detectors: Probability-Constrained Versus the Worst-Case-Based Design

In this letter, a robust code-division multiple-access (CDMA) multiuser detection technique which is based on the probability-constrained optimization approach is developed, and its relationship to the popular worst-case-based robust CDMA multiuser detection technique is established. The important advantage of the proposed probability-constrained optimization-based approach with respect to the worst-case-based design is that in the former approach, the parameter of the uncertainty region of the worst-case-based design is quantified in terms of the outage probability and second-order statistics of the user signature estimation error. A simulation example demonstrates the advantages of such statistically motivated choice of the parameter of the uncertainty region.

Joint Channel and Carrier Offset Estimation for Synchronous Uplink CDMA Systems

This paper addresses the problem on blindly estimating the channel impulse response (CIR) and the carrier frequency offset (CFO) in the uplink transmissions of multiuser code division multiple access system. Two blind subspace-based CIR and CFO estimation methods, namely, the exact determinant minimization method and the approximate determinant minimization method, are proposed. The performance of the proposed methods is compared with the available solution based on the generalized eigenvalue problem method (GEVPM). The computational complexity of the proposed methods is compared with that of GEVPM and the estimator proposed by Li and Liu. Simulation results are given to show that the proposed methods give better performance than the GEVPM and have a wide CFO acquisition range.

An End-to-End Embedded Approach for Multicast/Broadcast of Scalable Video over Multiuser CDMA Wireless Networks

We investigate an embedded multicast/broadcast approach for transport of digital video over spread-spectrum code-division multiple access (CDMA) cellular networks. Previous work has shown that the incorporation of a scalable source coding scheme with multiresolution modulation provides a promising design paradigm for the practical realization of the information-theoretic performance predictions originally developed by Cover, which demonstrated that optimal multicast/broadcast performance could be achieved by an embedded transmission scheme. Hence, the major technical challenge associated with the design of an end-to-end embedded multicast/broadcast system is how to match an embedded modulation constellation with a scalable source coding scheme. In this work, taking into consideration both the interference-limited and bandwidth-limited characteristics of a CDMA system, we provide a cross-layer approach incorporating adaptive power allocation and channel coding strategies and effectively match a discrete cosine transform based scalable motion-compensated video encoder to an embedded multiresolution modulation scheme to simultaneously deliver a basic quality-of-service (QoS) to less capable receivers while maximizing both the QoS for more capable receivers and the system capacity. We demonstrate the efficacy of this approach using the ITU-T H.263+ video scalable hybrid coder, although the approach is generally extensible to other scalable coding schemes as well

A System Level Energy Model and Energy-Quality Evaluation for Integrated Transceiver Front-Ends

As CMOS technology scales down, digital supply voltage and digital power consumption goes down. However, the supply voltage and power consumption of the RF front-end and analog sections do not scale in a similar fashion. In fact, in many state-of-the-art communication transceivers, RF and analog sections can consume more energy compared to the digital part. In this paper, first, a system level energy model for all the components in the RF and analog front-end is presented. Next, the RF and analog front-end energy consumption and communication quality of three representative systems are analyzed: a single user point-to-point wireless data communication system, a multi-user code division multiple access (CDMA)-based system and a receive-only video distribution system. For the single user system, the effect of occupied signal bandwidth, peak-to-average ratio (PAR), symbol rate, constellation size, and pulse-shaping filter roll-off factor is analyzed; for the CDMA-based multi-user system, the effect of the number of users in the cell and multiple access interference (MAI) along with the PAR and filter roll-off factor is studied; for the receive-only system, the effect of 1/f noise for direct-conversion receiver and the effect of IF frequency for low-IF architecture on the RF front-end power consumption is analyzed. For a given communication quality specification, it is shown that the energy consumption of a wireless communication front-end can be scaled down by adjusting parameters such as the pulse shaping filter roll-off factor, constellation size, symbol rate, number of users in the cell, and signal center frequency

Concatenated fibre-wireless channel identification in a multiuser CDMA environment

Radio-over-fibre (ROF) has received increasing attention for its ability to enable broadband wireless access. This fibre-based wireless access scheme meets the demand for broadband service by integrating the high capacity of optical networks with the flexibility of radio networks (the optical and wireless channels are concatenated with one another). There are, however, impairments that come with this appealing technology. The nonlinear distortion of the optical link and the multipath dispersion of the wireless channel are two of the major factors. In order to limit the effects of these distortions, estimation, and subsequently equalisation, of the concatenated fibre-wireless channel needs to be done. An estimation algorithm for the fibre-wireless uplink in a multiuser code division multiple access (CDMA) environment is presented using pseudonoise training sequences. It has already been shown by Fernando et al. (2001) that identification of the fibre-wireless uplink is possible in a single user CDMA environment. However, the more difficult task of identification in a multiuser spread spectrum environment, which is more realistic, is shown. In the multiuser case, the cumulative effect of multiuser interference, multipath dispersion, nonlinear distortion and noise should all be handled together which makes it more challenging. Numerical evaluations of the developed algorithm show that a good estimation of both the linear and nonlinear systems is possible in the presence of 16 independent users and an signal-to-noise ratio (SNR) of 22 dB. The estimation accuracy increases with the length of the PN sequence.