Narrowband interference mitigation in BICM OFDM systems (original) (raw)
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Narrowband interference mitigation in OFDM systems
2008
Orthogonal frequency division multiplexing (OFDM) is noted for its resistance to narrowband interference when equipped with forward error correction. This technique along with erasure insertion is adequate when the the signal-to-interference ratio (SIR) is moderate, around 0 dB, however, when the interference is severe, i.e. SIR = -20 dB, a non-orthogonal interference corrupts a large number of the data subcarriers due to spectral leakage. This situation requires a large number of erasures that compromise the code's error correction capability. The prediction-error lter (PEF) is proposed as an erasure insertion mechanism that localizes the erasures to the tones surrounding the interference, while not affecting the remaining tones. The simulation results indicate excellent performance, for the bit-interleaved coded modulated (BICM) OFDM system using the PEF and BPSK modulation when the SIR = -20 dB in a frequencyselective fading channel.
2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006
We consider the decoding of bit interleaved coded modulation (BICM) applied to multiband OFDM for practical scenarios where only a noisy (possibly very bad) estimate of the channel is available at the receiver. First, a decoding metric based on the channel a posteriori probability density, conditioned on the channel estimate is derived and used for decoding BICM multiband OFDM. Then, we characterize the limits of reliable information rates in terms of the maximal achievable outage rates associated to the proposed metric. We also compare our results with the outage rates of a system using a theoretical decoder. Our results are useful for designing a communication system where a prescribed quality of service (QoS), in terms of achievable target rates with small error probability, must be satisfied even in the presence of imperfect channel estimation. Numerical results over both realistic UWB and theoretical Rayleigh fading channels show that the proposed method provides significant gain in terms of BER and outage rates compared to the classical mismatched detector, without introducing any additional complexity.
European Transactions on Telecommunications, 2008
This paper proposes a new method, which combines interference cancellation with channel estimation for an orthogonal frequency division multiplexing (OFDM) system in the case of insufficient guard interval (GI) length. To avoid the problem of causing additional errors of conventional cancellers in the presence of strong additive noise, the proposed cancellation scheme uses the channel state information (CSI) and the so-called remodulation technique to reproduce the interference distortions. Then, these distortions in both the received pilot symbols and the data symbols are iteratively cancelled. Consequently, the channel estimation as well as the data equalisation performances are iteratively enhanced. In the range of high signal-to-noise ratio (SNR), the proposed scheme improves significantly the performance of the system. It does not cause any additional errors to the received signal, when the system suffers from very low SNR. In comparison to the case of sufficient GI length, the required SNR can be reduced by 0.5 dB to achieve the same symbol error rate (SER). Moreover, the spectral efficiency of the system can be increased, because the GI length is allowed to be insufficient. The proposed method is applicable to future OFDM-based wireless local area networks (WLANs).
Orthogonal Frequency Division Multiplexing (OFDM) is an emerging multi-carrier modulation scheme, which has been adopted for several wireless standards such as IEEE 802.11a and HiperLAN2. A well-known problem of OFDM is its sensitivity to frequency offset between the transmitted and received carrier frequencies. This frequency offset introduces inter-carrier interference (ICI) in the OFDM symbol. This project investigates three methods for combating the effects of ICI: ICI self-cancellation (SC), maximum likelihood (ML) estimation, and extended Kalman filter (EKF) method. These three methods are compared in terms of bit error rate performance, bandwidth efficiency, and computational complexity. Through simulations, it is shown that the three techniques are effective in mitigating the effects of ICI. For high values of the frequency offset and for higher order modulation schemes, the ML and EKF methods perform better than the SC method.
A spectral approach to inter-carrier interference mitigation in OFDM systems
2014
In this paper, we propose a new method for inter-carrier interference (ICI) mitigation in orthogonal frequency-division multiplexing (OFDM) systems. The proposed approach views the signal reconstruction problem at the receiver end as an integer least squares (ILS) problem, and uses a recently developed spectral approach called sequential probabilistic ILS (SPILS) to solve it. The proposed approach outperforms other state-of-the-art approaches while having the same computational complexity. In addition, we present a novel extension to the SPILS scheme that allows the generation of soft decisions (for communication systems which use soft-decision decoding). The use of soft-decision decoding (naturally) brings significant improvement in the detection reliability, and we show that the proposed method again outperforms other state-of-the art approaches. In order to better address the tradeoff between performance and complexity, we first suggest a novel method to reduce the number of matrix inversions required and hence, to reduce the implementation complexity without any degradation in performance. We also introduce a novel low complexity scheme termed Quick SPILS (QSPILS) in which we lose a little in detection reliability, but significantly reduce the implementation complexity.
Choose your subcarriers wisely: Active interference cancellation for cognitive OFDM
2013
A novel low complexity active interference cancellation (AIC) scheme for primary user (PU) protection is presented for application to cognitive OFDM systems, in which out-ofband radiation spilling over the PU protected band is to be minimized. A set of carefully selected cancellation subcarriers are modulated by appropriate linear combinations of the remaining data subcarriers. The combination weights are fixed so that, in contrast with previous AIC approaches, they need not be recomputed on a symbol-by-symbol basis. Weight optimization can thus be carried out offline, drastically reducing the online computational cost. In addition, it is shown that by carefully selecting the location of cancellation subcarriers, significant performance improvements can be achieved. Given that finding the optimal location is an intractable combinatorial problem, an heuristic approach is proposed, based on a greedy search which provides a good tradeoff. The proposed scheme is shown to outperform current AIC solutions both in terms of performance and computational cost, obtaining significant improvements in terms of notch depth, with almost 50 dB depth in typical settings, to protect a narrowband PU inside the secondary user OFDM band. Further, experimental measurements from the implementation of the proposed scheme on both professional and off-the-shelf hardware platforms validate its effectiveness.
Analysis of Intercarrier Interference Cancellation Scheme in OFDM Systems
Multipleks Pembagi Frekuensi Ortogonal (OFDM) adalah skema modulasi multipembawa yang tengah berkembang, diadopsi untuk beberapa standar nirkabel seperti IEEE 802.11a dan HiperLAN2. Dalam sistem OFDM, performanya sangat peka terhadap kesalahan frekuensi subpembawa (offset). Makalah ini menunjukkan analisis dan derivasi dari penguatan kompleks interferensi antar pembawa (ICI) yang digunakan dalam skema pembatalan diri dan ketergantungannya pada offset frekuensi subpembawa. Simulasi menunjukkan bahwa perbaikan dalam kinerja yang lebih baik dicapai untuk sistem yang menggunakan skema pembatalan. Selain itu, analisis dan simulasi menunjukkan bahwa teori rasio pembawa terhadap gangguan (CIR) untuk OFDM dengan skema pembatalan lebih besar dari konvensional dengan lebih dari 14dB.
The wireless communications with high data rates entails a substantial need for additional spectral resources through more flexible and efficient use of the available spectrum. Orthogonal frequency division multiplexing (OFDM) has been a prime candidate for spectrum pooling-based wireless transmission systems since subcarriers in the vicinity of the licensed users can be deactivated in order to minimize interference from the unlicensed user. Orthogonal frequency division multiplexing (OFDM) is a recognized transmission technique for Cognitive radio (CR) systems but one of the major drawbacks of OFDM scheme is its out-of-band (OOB) leakage due to high spectral sidelobes. The major challenge that OFDM has to deal with is high spectral sidelobes. Active interference cancellation (AIC) and subcarrier weighting (SW) are recognized techniques for sidelobe suppression. This paper proposes a joint SW-AIC scheme which has the ability to produce much deeper notch as compared to individual AIC and SW and system parameters can be adjusted depending on user's requirements. By using both the technique we will suppress the sidelobes. In AIC Technique, instead of turning off a large number of tones, we define two special tones at the edge of the interference band and these two tones can sufficiently cancel the interference in the band. The tone values can be arbitrarily determined without affecting the information tones due to the orthogonality relationship. We call these special tones Active Interference Cancellation (AIC) tones. We will discuss how to compute the AIC tones, and how to create the notch using the minimum number of tones, or, equivalently, how to maxime the spectrum efficiency. The Subcarrier Weighting (SW) Technique is based on the multiplication of the used subcarriers with subcarrier weights. The subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized according to an optimization algorithm which allows several optimization constraints. As a result, sidelobe suppression by subcarrier weighting reduces OFDM sidelobes by more than 10 dB in the average without requiring the transmission of any side information. Active Interference Cancellation (AIC) scheme is suitable for the transmissions which can reduce throughput but will increase BER degradation, for e.g. important data losses and performance of software. On the other hand SW scheme is best suited for the systems which can reduce BER degradation but also reduce throughput of the system such as live video/audio streaming. But for a dynamic system where requirements such as BER and throughput of a cognitive user are changing with time, we need to have a scheme which can dynamically change its parameters to best serve the user needs. Motivated by this, we propose a SW-AIC scheme which provides us with various knobs for adjusting system parameters. Apart from this, interference reduction capability of SW-AIC scheme is far better than individual AIC or SW.
Demodulation of OFDM Signals in the Presence of Deep Fading Channels and Signal Clipping
2012
: In this thesis, an optimal estimation algorithm, based on the Kalman Filter, is introduced for data recovery of orthogonal frequency-division multiplexed (OFDM) signals transmitted over fading channels. We show that the use of a zero prefix (ZP) along with a fast Fourier transform (FFT) operation zero padded to twice the data length allows for the recovery of subcarriers located next to a deep faded (at low signal-to-noise ratio [SNR]) values, exploiting all other subcarriers with higher SNR. The same approach is also shown to improve demodulation in the presence of signal clipping due to high peak to average power ratio (PAPR), as is often seen in OFDM signals. The proposed method assumes prior knowledge of the channel, usually estimated using the preamble. Testing was conducted for random channels with zero frequency response at a random frequency omega 0 and a signal in additive white Gaussian noise for various conditions. Further testing was done with typical Stanford Universi...
Successive narrowband interference cancellation for OFDM systems
IEEE Communications Letters, 2000
We consider the problem of mitigating narrowband interference (NBI) in conventional orthogonal frequency-division multiplexing (OFDM) systems, wherein the presence of the NBI dramatically degrades the performances of both the linear zeroforcing and minimum mean-square error equalizers. A new receiver based on successive NBI cancellation is synthesized, which is capable of assuring good NBI rejection, without requiring insertion of additional redundancy in the OFDM signal.