Interference Suppression Filter Research Articles

Tunable microwave photonic filter for noise and interference suppression in UMTS base stations

A design of a tunable microwave photonic filter for noise suppression and channel interference mitigation in the front-end stage after the receiving antenna of an UMTS base station is presented. The filter provides a tunable and very high selective performance (5 MHz along the 1935-1977 MHz band) with 14 dB suppression in the stopband using 30 fibre Bragg gratings and a reconfigurable dispersive delay line.

Data-Aided and Blind Stochastic Gradient Algorithms for Widely Linear MMSE MAI Suppression for DS-CDMA

In this paper, three novel stochastic gradient algorithms for adjustment of the widely linear (WL) minimum mean-squared error (MMSE) filter for multiple access interference (MAI) suppression for direct-sequence code-division multiple access (DS-CDMA) are introduced and analyzed. In particular, we derive a data-aided WL least-mean-square (LMS) algorithm, a blind WL minimum-output-energy (MOE) algorithm, and a WL blind LMS (BLMS) algorithm. We give analytical expressions for the steady-state signal-to-interference-plus-noise ratios (SINRs) of the proposed WL algorithms, and we also investigate their speed of convergence. Wherever possible, comparisons with the corresponding linear adaptive algorithms are made. Both analytical considerations and simulations show, in good agreement, the superiority of the novel WL adaptive algorithms. Nevertheless, all proposed WL algorithms require a slightly lower computational complexity than their linear counterparts.

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

High-resolution photonics-based interference suppression filter with wide passband

A new topology for a photonic signal processor, which overcomes the basic recursive frequency response problem that limits the passband range, is presented. The structure is based on a new multiple-wavelength offset-cavity structure that is cascaded with a series of unbalanced delay line structures. This not only can synthesize a very narrow notch response with good shape factor but also permits a multifold extension of the free spectral range (FSR) and passband width. Results on the interference mitigation filter demonstrate a stopband of 1% of center frequency and a fourfold increase in the FSR and passband width, while also having a very small shape factor, in excellent agreement with predictions.

Comparison of MOE and blind LMS

In this letter, we provide a rigorous analytical comparison of two blind adaptive algorithms for adjustment of the minimum mean-squared error (MMSE) filter for multiple access interference (MAI) suppression for direct-sequence code-division multiple access (DS-CDMA). In particular, we compare the popular minimum-output-energy (MOE) algorithm and the blind least-mean-square algorithm (BLMS) in terms of complexity, transient behavior, convergence, stability, and steady-state performance. We show analytically that the MOE algorithm enjoys a faster speed of convergence and has a superior steady-state performance, while the BLMS algorithm is computationally less complex.

Concatenated convolutional and differential codes in interference-suppressed DS-CDMA systems

A serial concatenated convolutional and differential coding scheme is employed in a multi-user direct-sequence code-division multiple-access (DS-CDMA) system. The system consists of single-user detectors (SUDs), which are used to suppress multiple-access interference (MAI) with no requirement of other users' spreading codes, timing, or phase information. The differential code, treated as a convolutional code of code rate 1 and memory 1, does not sacrifice the coding efficiency and has the least number of states. The iterative process exchanges information between the differential decoder and the convolutional decoder. Both component decoders adopt the a posteriori probability (APP) algorithm. Numerical results in additive white Gaussian noise (AWGN) channels show that this concatenated coding scheme provides better performance and more flexibility than conventional convolutional codes in DS-CDMA systems, even in the sense of similar complexity. Further study shows that the performance of this coding scheme applied to DS-CDMA systems with SUDs improves by increasing the processing gain or the number of taps of the interference suppression filter, and degrades for higher near–far interfering power or additional near–far interfering users.

A novel iterative multiuser detector for complex modulation schemes

A novel multiuser detector for direct sequence code division multiple access is proposed. The receiver performs iterated soft decision interference cancellation (ISDIC) based on multiuser interference suppression filters designed for minimization of the mean-square error. Assuming a complex modulation format, we show that the multiuser interference becomes rotationally variant in the course of the iterations. Regarding this rotational variance in the design of the multiuser interference suppression filter, the presented iterative multiuser detector achieves significant performance gains compared with conventional ISDIC employing a standard minimum mean-squared error filter which is optimum only for rotationally invariant multiuser interference.

THE EFFECT OF PARTIAL BAND INTERFERENCE ON MULTI-CARRIER DS-CDMA SYSTEMS

ABSTRACTIn this paper, a multi-carrier signaling technique is applied to a direct sequence code division multiple access (DS-CDMA) system. This type of signaling has the desirable properties of lower chip rate, exhibiting a narrow band interference suppression effect, along with robustness to fading, without the need of either an explicit RAKE receiver or an interference suppression filter. Band limited spreading waveforms are used to prevent self-interference. The system performance over a frequency selective Rayleigh channel is evaluated analytically and through computer simulation, in the presence of partial band interference.

Adaptive nonlinear filters for narrow-band interference suppression in spread-spectrum CDMA systems

This paper presents a novel nonlinear filter and parameter estimator for narrow band interference suppression in code division multiple access spread-spectrum systems. As in the article by Rusch and Poor (1994), the received sampled signal is modeled as the sum of the spread-spectrum signal (modeled as a finite state independently identically distributed (i.i.d.) process-here we generalize to a finite state Markov chain), narrow-band interference (modeled as a Gaussian autoregressive process), and observation noise (modeled as a zero-mean white Gaussian process). The proposed algorithm combines a recursive hidden Markov model (HMM) estimator, Kalman filter (KF), and the recursive expectation maximization algorithm. The nonlinear filtering techniques for narrow-band interference suppression presented in Rusch and Poor and our proposed HMM-KF algorithm have the same computational cost. Detailed simulation studies show that the HMM-KF algorithm outperforms the filtering techniques in Rusch and Poor. In particular, significant improvements in the bit error rate and signal-to-noise ratio (SNR) enhancement are obtained in low to medium SNR. Furthermore, in simulation studies we investigate the effect on the performance of the HMM-KF and the approximate conditional mean (ACM) filter in the paper by Rusch and Poor, when the observation noise variance is increased. As expected, the performance of the HMM-KF and ACM algorithms worsen with increasing observation noise and number of users. However, HMM-KF significantly outperforms ACM in medium to high observation noise.

New nonlinear algorithms for narrowband interference suppression in CDMA spread-spectrum systems

The narrow-band interference suppression capability of spread-spectrum systems can be further enhanced by employing interference suppression filters. This paper proposes a number of new nonlinear algorithms for narrow-band interference suppression in code division multiple access spread-spectrum systems. Our research consists of two parts. (1) We propose a multiuser decision-directed Kalman (MDK) filter, which has the same performance as the nonlinear approximate conditional mean (ACM) filter but a much simpler algorithm. (2) We use the nonlinear function in the ACM and the MDK filters to develop nonlinear adaptive least mean square filters with significantly improved performance. Simulation results indicate that our nonlinear algorithms outperform conventional linear ones.

Convolutionally coded multicarrier DS-CDMA systems in a multipath fading channel. II. Narrow-band interference suppression

For pt.I see ibid., vol.47, p.1570-92 (1999). In Rowitch and Milstein (1999), a convolutionally encoded multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system was proposed and compared to a classical single-carrier DS-CDMA system employing a RAKE receiver and the same convolutional code, in the presence of additive white Gaussian noise and multiple-access interference. This paper considers the additional impact of various forms of narrow-band interference on the performance of these two systems and the ability of the multicarrier system to effectively suppress the interference using the innate structure of its receiver. At roughly equivalent receiver complexity, results demonstrate superior performance of the multicarrier system in the presence of such interference, without requiring the addition of a front-end interference suppression filter.

BPSK Interference Rejection in DS SS Systems Using IIR Notch Filter

The use of FIR filters in anti-jam and interference rejection of spread spectrum systems has been studied extensively. However, little work have been donefor cases of IIR filters. This paper employs a simple second order IIR filter for interference suppression in a direct sequence spread spectrum system which is overlaid on a narrow-band BPSK system. The bit error rate expression of such a DS SS system is derived and numerical results are presented.

Generalized projection algorithm for blind interference suppression in DS/CDMA communications

New blind adaptive interference suppression algorithms for DS/CDMA communication systems are presented. A generalized projection algorithm is presented to update a multiple access interference (MAI) suppression filter in a static communication environment. The algorithm adaptively adjusts the filter coefficients by iteratively projecting them onto constraint sets. The constraint sets consist of data sets and property sets, which are obtained from the received signal and prior knowledge of the desired signal, respectively. A space alternating generalized projection algorithm with approximate EM mapping is also presented to suppress the MAI in a dynamic communication environment. This algorithm jointly estimates the filter and desired signal amplitude using a stochastic approximation of the EM algorithm. The performance characteristics of the proposed algorithms are shown by output signal-to-interference ratio and bit-error-rate simulation results. The results show that the proposed algorithms significantly outperform the existing adaptive algorithms in steady state.

A coded VSB-QPSK transmission scheme for digital satellite communications in co-channel interference environments

A coded VSB (vestigial side band)-QPSK (quadrature phase-shift keying) transmission scheme is proposed to realize interference-resistant digital satellite communication system. It uses an interference suppression filter to suppress the interference power in the receiving side. The suppressed power can be assigned to the transmission signals in advance by also using a same characteristic band limitation filter in the transmission side. Moreover, high coding-gain forward error correction (FEC) is employed to improve the degradation due to I and Q channel crosstalk by interference suppression. When a D/U (desired and undesired signal power ratio) equals 3 dB and a /spl Delta/f/f/sub N/ (/spl Delta/f: interference frequency offset, f/sub N/: one-half the Nyquist bandwidth) equals 1.05, the proposed scheme achieves an Eb/No improvement of 7.3 dB in a CCI (co-channel interference) environment.

Performance of multicarrier DS CDMA systems

In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system.

Cellular CDMA overlay systems

The CDMA network is considered to share a common spectrum with a strong narrowband microwave user. An interference suppression filter at each CDMA receiver is employed to reject the narrowband waveform. Gaussian approximation is assumed for the total interference, which includes fading interference, self interference, Gaussian noise, narrowband interference, multipath and multiple access interference.

The performance of a direct-sequence spread-spectrum acquisition system using a linear prediction filter for narrowband interference suppression

The coarse acquisition performance of a direct sequence spread-spectrum receiver is analyzed when a linear prediction filter is used for narrowband interference suppression. We show that once an appropriate matching strategy is identified, the linear prediction filter can provide favorable performance when narrowband interference is present over a considerable range of both interference power and bandwidth. In addition, the presence of the filter dramatically improves the performance over the case where there is no filter, except when the interference bandwidth and the power are both small (i.e., when the processing gain provides sufficient interference immunity without the filter). If long spreading sequences are used with moderately sized observation windows, the acquisition performance can be severely degraded when a parallel acquisition scheme is used due to the linear predication filter. We show, however, that a slower serial receiver will provide reliable performance.

Interference suppression for CDMA overlay of a narrowbandwaveform

The performance of CDMA networks which overlay a narrowband signal to increase spectral efficiency is presented. Such systems employ interference suppression filters in the CDMA receivers to minimise the interference caused by the narrowband waveform.

CDMA overlay situations for microcellular mobile communications

Direct sequence code division multiple access communications is a promising approach to cellular mobile communications, which operates in an environment characterized by multipath Rician fading. In this paper, the CDMA network is assumed to share common spectrum with a narrowband microwave user. Because of the presence of the narrowband waveform, an interference suppression filter at each CDMA receiver is employed to reject the narrowband interference. The problem of interference from adjacent cells is also considered. Average power control is assumed to combat the near/far problem, and multipath diversity, in conjunction with simple interleaved channel coding, is considered for improving the performance of the CDMA system. >

Interference suppression to aid acquisition in direct-sequence spread-spectrum communications

The acquisition and tracking systems of a spread-spectrum receiver are probably the most critical components of the receiver, since if they fail to function properly, it is doubtful that the desired signal can be successfully detected. This means that the affect of interference (such as jamming) on the receiver while it is attempting to learn the correct phase position of the incoming code might be especially harmful, since the interference might not allow the receiver to acquire the signal. To address this problem, a narrow-band interference suppression filter is used to enhance the performance of a serial search acquisition scheme for a direct-sequence spread-spectrum receiver. Analytical expressions for the probabilities of error in both the search and lock modes are derived, and numerical results are used to illustrate the sensitivity of the receiver to various system parameters. It is shown that the presence of the rejection filter can be significantly improves the performance of the acquisition system. >