Subtractive Interference Cancellation Research Articles

BER performance analysis of Subtractive interference cancellation (SIC) Using Rayleigh channel in DS-CDMA Receiver system

In this paper, we present and analyze the performance of a parallel interference cancellation (PIC) scheme for multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems. In order to mitigate the multi-path interference (MPI) in the DS CDMA system. At each cancellation stage in the proposed PIC scheme, on each subcarrier, a weighted sum of the soft outputs of the other users in the current stage is cancelled from the soft output of the desired user to form the input to the next stage. At the last stage, the interference cancelled outputs from all the subcarriers are maximal ratio combined (MRC) to form the decision statistic. Parallel interference elimination is first proposed in this paper the multi-path interference are evaluated by tentative decision and known user information. Then the performance over Rayleigh fading channel are analyzed and compared to Matched filter, Decorelator, successive interference cancellation (SIC) and conventional parallel interference cancellation (PIC). It is shown that PIC performance can be improved greatly by using this method with simple structure and easy implementation.

PERFORMANCE COMPARISON OF NON-LINEAR INTERFERENCE CANCELLATION TECHNIQUES FOR MULTIUSER DETECTION IN DS-CDMA SYSTEM

Direct sequence code division multiple access (DS-CDMA) is a most popular mobile communication technology. In DS-CDMA communication, signals of all users overlap in time and frequencies and cause multiple access interference (MAI). The conventional DS-CDMA detectors follow the single user matched filter detection technique, in which each user is detected individually without regard for other users and ignores the existence of MAI. Its performance is not very satisfactory and is particularly limited by the near-far problem. Multi-user detection is the efficient technique to use the capabilities of code-division multiple access (CDMA), which is becoming the ubiquitous air-interface in future generation communication systems. The problem of multiple access interference (MAI) is vital for a CDMA system. The different types of multi-user detectors have been proposed to mitigate the MAI. The different sub optimal multi-user detector offers performance in between the single user matched filter and the optimal multiuser detectors with substantially reduced complexity. This paper presents the simulation results regarding performance comparison of some most popular non-linear multi-user detectors i.e., multistage detectors, decision- feedback detectors and subtractive interference cancellation.

Improved Viterbi Decoder Metrics for Two-Stage Detectors in DS-CDMA

Modified branch metrics are proposed for single-user Viterbi decoders in two-stage detectors for convolutionally-encoded code-division multiple-access (CDMA) systems with random spreading sequences. The modifications are based on modeling the residual multiple-access interference (RMAI) after subtractive interference cancellation as conditionally Gaussian with time-dependent variance, where the conditioning is on the time-varying user crosscorrelations. A novel estimate of the variance of the total RMAI is presented, and used in the proposed branch metrics. Significant performance gains are demonstrated over the Euclidean branch metric of the standard Viterbi decoder.

Soft iterative multisensor multiuser detection in coded dispersive CDMA wireless channels

A soft iterative multisensor array receiver for coded multiuser wideband code-division multiple-access wireless uplink channels is proposed, such channels are typically both frequency- and time-selective. A new equivalent discrete-time synchronous representation is used to model asynchronous multiuser dispersive channels that employ, in general, random spreading. The proposed scheme suppresses multiuser interference over a wide range of user signal powers, by iteratively exchanging soft information between a minimum mean-square-error (MMSE) multiuser demodulator and a bank of single-user decoders, feeding back the outputs of the latter to aid in soft multiuser multisensor MMSE-RAKE demodulation and subtractive interference cancellation, in the second and subsequent iterations. It displays near-far resistance since it behaves like a successive interference subtracter across iterations. The array responses are obtained via a multipass estimation scheme that uses both (temporal) pilot symbols and soft estimates fed back from the decoders to effectively provide multisymbol pilot signals and thence successively refined estimates with increasing iterations, while seeking neither to rely on the array geometry nor to estimate the directions from which users' signals arrive at the receiver. Simulation studies indicate that this scheme performs close to the single-user case with a two-sensor receiver array, and perfect channel state information, after four iterations; alternatively, it allows significantly increased user capacity compared with conventional receivers, and suffers only a modest loss with estimated array responses.

Joint Iterative Multiuser and Narrowband Interference Suppression inCoded Asynchronous DS-CDMA Channels

Multiuser interference suppression in coded direct sequence code division multiple access (DS-CDMA) uplink channels is significantly impacted by the application of the turbo processing concept. This paradigm essentially involves the iterative exchange of soft information between a multiuser demodulator and a bank of single-user decoders, to their mutual benefit. The present work proposes a joint iterative minimum mean square error (MMSE) multiuser and narrowband interference suppressor for coded asynchronous DS-CDMA channels. Since the parameters of the narrowband interference are unknown a priori, the first iteration in this scheme is effectively just MMSE multiuser interference suppression. The outputs of all users' soft decoders (available at the end of each iteration) are fed back to subtract their (estimated) cumulative contribution from the received signal vector. The residue comprises the narrowband interference embedded in wideband noise, so that the former can be piecewise interpolated over chip intervals of appropriate duration, and then subtracted from the received signal, to provide a narrowband-interference-free input signal for the next iteration. The soft-decoded feedback estimates of the interferers' signals are used in each iteration also to perform soft MMSE multiuser interference suppression, in conjunction with subtractive interference cancellation. This scheme performs well at both low and high received signal powers, and displays the successive cancellation property across iterations, exhibiting good near-far resistance. Introducing multisensor arrays at the receiver relaxes the limitations imposed by the inherent suboptimality of MMSE multiuser demodulation, insofar as it enhances interuser separation, now in the spatial sense. Simulations indicate that the performance of the proposed technique surpasses that of all existing suboptimal algorithms in this context.

A DSP-based DS-CDMA multiuser receiver employing partial parallel interference cancellation

The implementation of advanced DS-CDMA receivers based on multiuser detection principles is becoming a reality thanks to the combination of an improved understanding of the theoretical basis of multiuser detection and advances in digital, mixed-signal, and RF technologies. Due to their lower complexity, subtractive interference cancellation approaches are attractive for the practical implementation of multiuser detection. In a parallel interference cancellation receiver, it is practical to use the soft outputs of a matched filter bank for amplitude estimation. A bias arises in the decision statistics, however, due to imperfect estimation and interference cancellation. In this paper, the source of the bias is explicitly recognized, and a partial interference cancellation scheme that mitigates the negative effects of biased estimation and significantly improves system performance is proposed. A practical real-time algorithm that significantly reduces the implementation complexity of this scheme without sacrificing performance is then derived. To facilitate a software radio implementation, the signal processing complexity of the approach is characterized. The real-time processing algorithm is tested via implementation in software on a floating-point general-purpose DSP. The prototype includes a flexible software-based architecture which performs IF sampling and uses digital downconversion prior to baseband processing. The hardware test setup is described, and the results are presented and compared with simulation and analytical results. The experimental results confirm the simulation and analytical results which show large performance gains over the conventional matched filter.

The error rate exponent of a collision resolution algorithm for underwater acoustic local area networks

There is growing interest in the development of underwater acoustic local area networks for long-term and real-time telemetering of data from ocean-bottom instrumentation. Due to the large propagation delay of the underwater acoustical channel and the finite energy supply of battery powered, ocean-bottom nodes, network protocols designed for radio-based local area networks (LANs) are inappropriate for narrow-band, underwater acoustic LANs and would yield large expected packet delays and low throughput. It has been demonstrated that incorporating interference suppression techniques into protocol design increase throughput and decrease network delay in underwater acoustic local area networks, at the expense of a modest increase in central receiver complexity. These techniques allow simultaneous packet transmissions from cochannel, asynchronous network nodes, and provides the receiver with an algorithm to resolve the packet collisions, often without retransmission. The standard benchmark for comparing such collision resolution techniques is the probability of bit error of a collided packet in the low thermal noise regime. In the additive white Gaussian noise channel with binary phase-shift keying modulation, the logarithm of the bit error rate of the kth packet behaves as −ηkSNRk/2 for sufficiently low background noise, where ηk is the asymptotic multiuser efficiency (AME) of the given collision resolution algorithm, and SNRk is the received signal-to-background-noise ratio from the corresponding transmitter. Thus, ηk is an attenuation of the SNR required for interference cancellation, and provides a simple yet precise means of comparing collision resolution algorithms. To date, this parameter is only known for the following collision resolution techniques in the two-user, asynchronous AWGN channel: maximum likelihood sequence detection, decorrelating detection, linear MMSE detection, and conventional detection. In this talk the error rate exponent for a class of collision resolution algorithms will be presented. This class includes those techniques that estimate and subtract multiple-access interference (MAI) by using tentative data decisions, and includes the two-stage detectors with both conventional or decorrelated tentative decisions. The error rate exponent for this class will be shown to quantify the following conclusions: (1) strong users will suffer from subtractive interference cancellation through error propagation, (2) the AME for a sufficiently weak user rises rapidly to that of the optimum sequence receiver for sufficiently strong interference, but (3) some detectors in this class tend to have low near-far resistance. It will be shown that while the asymptotic multiuser efficiency of two-stage detection with decorrelated tentative decisions dominates that of the decorrelating detector for a wide range of relative energies, the near-far resistances of these detectors exhibit a reverse ordering. More importantly, the asymptotic multiuser efficiencies for this class of detectors clearly indicate regions for which a given user should avoid updating tentative decisions and suggest combinations of the above receivers to improve single-user performance.