Co-channel Interference Suppression Research Articles

Space-Time Interference Cancellation in MIMO-OFDM Systems

In this paper, a two stage hybrid interference cancellation and equalization framework is proposed for interference cancellation in the uplink of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The first stage uses time domain equalization to suppress cochannel interference, mitigate asynchronism, and shorten the postequalization channel response to be no longer than the length of the cyclic prefix. The second stage performs low-complexity single tap equalization and detection in the frequency domain. The framework is developed specifically for spatial multiplexing and is applied to multiuser MIMO-OFDM systems with asynchronism between users as well as to single-user MIMO-OFDM systems. Various equalizer design methods are proposed that determine the coefficients directly from the training data and are compared with methods based on channel estimates. The equalizer coefficients and postequalization channel response are found by solving a joint optimization that maximizes the signal to interference-plus-noise ratio (SINR) in the frequency domain. Simulations compare various training-based methods and show the proposed methods provide good bit error rate (BER) and SINR performance in a variety of interference scenarios.

Spatio-Temporal Equalization for Space-Time Block Coded Transmission over Frequency Selective Fading Channel with Co-channel Interference

In this paper, we propose a spatio-temporal equalizer for the space-time block coded transmission over the frequency selective fading channels with the presence of co-channel interference (CCI). The proposed equalizer, based on the tapped delay line adaptive array (TDLAA), performs signal equalization and CCI suppression simultaneously using the minimum mean square error (MMSE) method. It is to show that our scheme outperforms the previous two-stage combined adaptive antenna and delayed decision feedback sequence estimator (DDFSE) approach. We also show that performance can be further improved if the synchronization between the preceding and delayed paths is achieved.

Adaptive cochannel interference suppression based on subarrays for HFSWR

The paper analyzes the characteristics of cochannel interference (CCI) in the high-frequency (HF) surface wave radar (HFSWR), which adopts the linear frequency modulated interrupted continuous wave (FMICW). CCI will influence all the range bins, including all the positive and negative frequencies, and the negative frequency range bins contain only the external interference information. Based on the above characteristics, we introduce a new adaptive coherent side-lobe cancellation (CSLC) algorithm based on subarrays that use the negative frequency range bin samples to estimate the interference covariance matrix and correlation vector. Experimental results confirm that the general and robust algorithm can achieve effective CCI suppression using the data recorded by the Ocean State Monitor and Analysis Radar (OSMAR2003, manufactured in 2003), located near Zhoushan in Zhejiang, China.

Spectral Efficiency of Dynamic Time Division Duplex Fixed Wireless Cellular System for Asymmetric and Dynamic Multimedia Traffic

We analyze spectral efficiency of dynamic time division duplex in a fixed wireless cellular network. Conventionally, spectral efficiency has been analyzed for static and fully loaded systems. In this paper, we investigate how asymmetric and dynamic traffic affect the spectral efficiency of Time-Division-Duplex (TDD) systems. Recently, dynamic TDD (D-TDD) has gained much attention as an efficient duplex scheme for high-speed data communications, because adaptive switching ability enables the system to obtain statistical multiplexing gain by exploiting dynamic and asymmetric data traffic. However, it has been noted that a rather strong co-channel interference can be present due to adaptive switching in a cellular network that uses frequency reuse. Thus, benefits of dynamic TDD may not be justifiable unless a proper countermeasure is employed. To suppress the effect of strong co-channel interference, we employ time slot allocation (TSA) strategy along with sector antenna layouts, as proposed in our previous work Jeong and Kavehrad (IEEE and Transactions on communication, Vol. 50, no.10 pp. 1627–1636, 2002). We note that higher spectral efficiency is obtained in D-TDD systems by employing TSA strategy. We also evaluated spectral efficiency of D-TDD system employing adaptive modulation, assuming that traffic is delay tolerant. It is observed that five times higher spectral efficiency can be obtained by employing adaptive modulation. The effect of variance of ratio of offered load on uplink and downlink is also evaluated. Our computer simulation results show that spectral efficiency of D-TDD system with time slot allocation algorithm is more than that of static TDD (S-TDD) over a large range of traffic variation. In conclusion, D-TDD system can take advantage of statistical multiplexing gain of dynamic traffic by adaptively positioning the boundary to the varying traffic bandwidth in its two-way transmissions when TSA strategy is employed for suppression of strong co-channel interference.

Spatial–Temporal Processing for Wireless Systems With Intersymbol and Cochannel Interference

In this paper, spatial-temporal receive filters for multiple receiver systems with intersymbol interference (ISI), cochannel interference (CCI), and additive white Gaussian noise (AWGN) are investigated. An optimal spatial-temporal filter that is optimal for any following baud-rate processor with any reasonable criterion of goodness is found. Therefore, it allows CCI suppression and equalization to be treated separately. Furthermore, it is shown in this paper that, under certain conditions, the baud-rate samples of the optimal spatial-temporal filter output form a set of sufficient statistics for the following baud-rate signal detector for channels with ISI, CCI, and AWGN, which is very similar to the traditional matched filter for AWGN channels with only ISI. Thus, the optimal spatial-temporal filter is also called an analogical spatial-temporal matched filter.

A General Approach for Image and Co-Channel Interference Suppression in Diversity Wireless Receivers Employing ICA

In this contribution, a general approach to suppress image signal and co-channel interference in a wireless receiver is presented. The technique employs diversity reception, and the algorithm is based on complex-valued independent component analysis (ICA). The proposed ICA method increases the channel capacity through greater frequency reuse, and it simplifies the receiver’s front end by eliminating the need for analog image filtering. Also, the new technique is computationally efficient and has attractive implementation features in terms of the requirement for A/D converters’ speed and bandpass filters’ selectivity. Simulation results show that the performance is robust over a wide range of input signal-to-interference ratios, and the algorithm has fast convergence.

A blind adaptive MSINR filter for co-channel interference suppression in DS/CDMA systems

In the blind adaptive filter, the Minimum Output Energy (MOE) filter is well known, but the algorithm for this filter has the problem that the convergence performance is dropped under the strong Near-Far Problem (NFP). In this paper, we propose and analyze a blind adaptive filter based on the Maximum Signal to Interference and Noise Ratio (MSINR) criterion, called the blind adaptive MSINR filter. In this filter, the despreading vector is so renewed as to maximize the Signal to Interference and Noise Ratio (SINR) by minimizing the despread interference and noise power under the condition that the despread desired signal power remains constant with a decision directed data instead of using any a priori training sequence. We analyze the loci of the despreading vectors by several computer simulations, and calculate the despread interference and noise powers for both filters to derive the theoretical SINR convergence performances. Also, we conduct other simulations in order to show the difference between the proposed filter and the conventional one in terms of SINR. As a result, we confirm that the adaptive filter based on the MSINR criterion achieves significant progress in terms of SINR performance. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 88(3): 42–53, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20073

Modulation diversity for frequency-selective fading channels

In this article, modulation diversity (MD) for frequency-selective fading channels is proposed. The achievable performance with MD is analyzed and a simple design criterion for MD codes for Rayleigh-fading channels is deduced from an upper bound on the pairwise error probability (PEP) for single-symbol transmission. This design rule is similar to the well-known design rule for MD codes for flat fading and does not depend on the power-delay profile of the fading channel. Several examples for MD codes with prescribed properties are given and compared. Besides the computationally costly optimum receiver, efficient low-complexity linear equalization (LE) and decision-feedback equalization (DFE) schemes for MD codes are also introduced. Simulations for the widely accepted COST fading models show that performance gains of several decibels can be achieved by MD combined with LE or DFE at bit-error rates (BERs) of practical interest. In addition, MD also enables the suppression of cochannel interference.

An adaptive multiuser detector based on radial basis function network and linear adaptive filter for DS/CDMA communications

AbstractIn this paper, we propose a radial basis function–linear adaptive filter (RBF‐LAF) detector that uses a radial basis function (RBF) network with an MMSE (minimum mean squared error) detector based on a linear adaptive filter (LAF) to suppress co‐channel interference in DS‐CDMA communications. The objective of the proposed RBF‐LAF detector is to achieve an excellent bit error rate while limiting the computational complexity by calculating the received signal power or the signal‐to‐interference ratio (SIR) of each user and assigning an RBF detector to users receiving strong co‐channel interference and an MMSE‐LAF detector to users experiencing little interference. By appropriately assigning the users, an excellent bit error rate can be obtained with a relatively low computational complexity compared to conventional RBF detectors. And by using an adaptive algorithm, changes in the communication environment can be handled. Finally, the effectiveness of the proposed method is demonstrated by comparing the proposed RBF‐LAF detector and the conventional detectors in terms of the bit error rate and the computational complexity in numerical examples. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 86(10): 19–35, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.1154

Adaptive cochannel interference cancellation in space-time coded communication systems

Space-time coding is a powerful scheme that combines channel coding, modulation, and multiple transmit antennas to achieve higher data rates and combat fading in wireless systems. In this letter, we propose a multiple-input multiple-output minimum mean-square error spatial-filtering-based adaptive antenna arrays method to suppress cochannel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time structure, thereby significantly improving the system's interference suppression ability without significant bit error rate performance degradation.

Space-time iterative receivers for narrowband multichannel networks

The iterative MMSE multiuser detection paradigm is applied to the suppression of cochannel interference in the coded narrowband (multicell) uplink. The equivalent of multiple chips per bit (necessary for MMSE multiuser demodulation) is generated via multisensor reception, the array responses serving as user signatures. This receiver's robustness to overloading allows its sensor count to be much lower than the typical number of other-cell cochannel interferers. A permutation transmit diversity technique that exploits channel time-selectivity is proposed in order to enhance the interuser separation afforded by multisensor reception.

Adaptive co-channel interference cancellation in space-time coded communication systems

A multi-input-multi-output, minimum mean squared error spatial filtering-based adaptive antenna arrays method is proposed to suppress co-channel interference (CCI) in space-time coded systems. It is shown that the proposed method can effectively suppress CCI while preserving the space-time structure, thereby significantly improving the system's interference suppression ability.

Performance of an array receiver with a Kalman channel predictor for fast Rayleigh flat fading environments

We develop an approach for using an antenna array for tracking fast Rayleigh flat fading channels and suppressing cochannel interference. The fast flat fading process is assumed to be a general autoregressive (AR) process in order to characterize temporal variation of channels and evaluate its effect on the receiver structure and performance. The optimal array receiver structure that minimizes the probability of error for BPSK signals is derived, which includes a Kalman filter to predict the fading channels. A simple integral expression for the probability of error is also derived for the optimal receiver. In particular, we analyze the case with identical shaping filters. An irreducible probability of error is shown to exist due to the prediction error of multiple channels. Another interesting observation from the study is that the diversity gain with m antenna elements in the presence of k interferences is usually greater than (m-k), even in the presence of channel prediction error. Simulations are carried out to verify the theoretical analysis.

Minimum-error-probability single-user detection for ISI-impaired narrow-band multiuser systems

The goal of this contribution is to give some insight into the cochannel interference (CCI) suppression capability of novel symbol-by-symbol-type maximum a posteriori probability single-user receivers for nonspread-spectrum multiuser narrow-band systems impaired by intersymbol interference and CCI. The presented multiuser receivers minimize the error probability in the detection of the desired user message on a per-symbol basis. Practical application environments for the developed detectors can be constituted by Ethernet-type LANs and DSL-based high-throughput links connecting central offices to subscribers.

Cochannel interference suppression with successive cancellation in narrow-band systems

Successive cancellation of cochannel signals is described in the context of TDMA mobile radio systems employing coherent receivers. Both symbol aligned and mis-aligned cochannel signals are studied. Signal separability, a major problem in successive cancellation of cochannel signals, is obtained using the relative timing delay between cochannel signals. Both hard and soft subtractions are examined. Soft subtraction, along with initial signal separation, leads to significant C/I gains.

Joint equalization and interference suppression for high data rate wireless systems

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles.

Fuzzy techniques for adaptive nonlinear equalization

This paper presents a review of the application of fuzzy filters to nonlinear channel equalization problem. The paper presents the development of fuzzy filters for channel equalization applications first. The close relationship between the fuzzy equalizers and the equalizer based on maximum a posteriori probability principle (MAP) is derived. The MAP equalizers are also called Bayesian equalizers since their decision function is based on Bayes’ decision theory. It is shown that the fuzzy implementation of the MAP equalizer provides major computational advantages. The structure of this fuzzy equalizer also provides scope for co-channel interference (CCI) suppression in the presence of inter symbol interference (ISI) and additive white Gaussian noise (AWGN) with the help of a pre-processor. The advantages associated with the fuzzy equalizers can make them a candidate for mobile communication applications.

Cochannel interference suppression through time/space diversity

Wireless systems are subject to a time-varying and unknown a priori combination of cochannel interference, fading, and Gaussian noise. It is well known that multiple antennas can provide diversity in space that allows system tradeoffs between interference suppression and mitigation of fading. This paper describes how to achieve these same tradeoffs through diversity in time provided by channel coding. The mathematical description of time diversity is identical to that of space diversity, and what emerges is a unified framework for signal processing. Decoding algorithms are provided for repetition codes, rate 1/n convolutional codes, first-order Reed-Muller codes, and a new class of linear combination codes that provide cochannel interference suppression. In all cases it is possible to trade performance for complexity by choosing between joint estimation and a novel low-complexity linear canceler structure that treats interference as noise. This means that a single code can be used in a variety of system environments just by changing the processing in the receiver.

Super-resolution broad null beamforming for cochannel interference cancellation in mobile radio networks

An innovative beamforming-type approach for suppressing cochannel interference in mobile radio networks is introduced. This approach combines a super-resolution direction and power profile finding algorithm, together with a signal subspace-type broad null beamformer which can steer both sharp and controlled broad nulls in the appropriate directions. The latter property makes the beamformer ideal for mobile communication systems where the angular spread of the incoming rays from a cochannel interfering mobile station is often too large to be eliminated by a sharp null, resulting in a significant degradation in the performance of a conventional system.

A sectorized beamspace adaptive diversity combiner for multipath environments

The beam diversity technique is effective in combating multipath fading in wireless communications. In a beam diversity system, multiple receiving branches are formed with multiple antenna beams with distinctive patterns. These beams are synthesized in such a fashion that the fading phenomena observed at different branches are nearly uncorrelated. The disadvantage of such a system is the lack of adaptivity for cochannel interference (CCI) suppression. In this paper, an adaptive beam diversity combiner is proposed for sectorized signal reception. The diversity branches are formed with several adaptive beamformers whose response patterns encompass an angular sector in the field-of-view of the receiver. With a set of judiciously chosen weight vectors, effective diversity combining can be achieved inside the sector, and out-of-sector CCI can be suppressed via nulling. Simulation results confirm the efficacy of the proposed scheme.