Time-domain Equalizer Research Articles

Performance Analysis of Rake Receivers in IR–UWB System

Suppression of interference in time domain equalizers is attempted for high data rate impulse radio (IR) ultra wideband communication system. The narrow band systems may cause interference with UWB devices as it is having very low transmission power and the large bandwidth. SRAKE receiver improves system performance by equalizing signals from different paths. This enables the use of SRAKE receiver techniques in IR UWB systems A semi analytical approach is used to investigate the BER performance of SRAKE receiver on IEEE 802.15.3a UWB channel models. Study on non-line of sight indoor channel models (both CM3 and CM4) illustrates that bit error rate performance of SRake receiver with NBI performs better than that of Rake receiver without NBI

PN-based single carrier block transmission with iterative frequency domain equalization over underwater acoustic channels

Single carrier modulation with time-domain equalization (SC-TDE) in underwater acoustic channel is sensitive to receiver parameters and its computational complexity is very high. Orthogonal frequency division multiplexing (OFDM) signal has high peak-to-average power ratio (PAPR) and is sensitive to Doppler shift. Aiming at these problems, this paper proposes the pseudo-noise (PN)-based single carrier block transmissions through underwater acoustic channels and corresponding receiver algorithms. The receiver employs PN signals for residual Doppler shift estimation, and channel estimation. A low complexity T/4 fractional iterative frequency domain equalizer is introduced to improve the system performance. One underwater acoustic communication system has been designed and tested in a lake in November 2011. At a distance of 1.8 km under a complex channel condition, the useful data rates of around 1500 and 3000 bps are achieved with un-coded bit error rates 10-2–10-4 in the lake experiment.

Linear and Nonlinear Time Domain Block Equalizers on MIMO Frequency Selective Channels

Single-Carrier (SC) transmission with the same bandwidth as Multi-Carrier (MC) transmission (such as OFDM) may have far shorter symbol duration and is considered to be more robust against time selective fading. In this paper, we proposed the novel equalization and signal separation schemes in time domain for short block length transmission, i.e., Block Linear Equalization (BLE) and Block Nonlinear Equalization (BNLE) on MIMO frequency selective fading channels. The proposed BLE uses the MMSE based inverse matrix in time domain and the BNLE utilizes the QRD-M (QR Decomposition with M algorithm) with appropriate receiver complexity. We compared the computational complexity among the conventional SC-FDE (Frequency Domain Equalization) scheme and the proposed equalizers. We also used the Low-Density Parity Check (LDPC) decoder concatenated to the proposed BLE and BNLE.

Adaptive Frequency-Domain Equalization for Mode-Division Multiplexed Transmission

We propose single-carrier adaptive frequency-domain equalization (SC-FDE) for mode-division multiplexed transmission. A two-mode fiber long haul transmission system is simulated. The performances of both FDE and time-domain equalization (TDE) are verified and compared. The FDE approach reduces the computational complexity significantly compared to the TDE while maintaining the same performance. For a two-mode transmission of 2000 km, FDE decreases the complexity by a factor of as much as 77 compared with TDE at the expense of a large memory length of 2048 that may require higher hardware complexity. The dynamic response of the adaptive equalizer is simulated by using a mode scrambler. FDE achieves the same tracking speed as TDE.

Evaluation of O-OFDM communication systems using LMS-TEQ and DF-TEQ by efficiency and complexity

In this paper, optical orthogonal frequency division multiplexing (O-OFDM) using least mean square time domain equalizer (LMS-TEQ) and decision feedback time domain equalizer (DF-TEQ) to reduce the cyclic prefix (CP) length is evaluated. The evaluation is done based on the trade-off between O-OFDM system efficiency and LMS-TEQ/DF-TEQ complexity.

Time-Domain Block and Per-Tone Equalization for MIMO–OFDM in Shallow Underwater Acoustic Communication

Shallow underwater acoustic (UWA) channel exhibits rapid temporal variations, extensive multipath spreads, and severe frequency-dependent attenuations. So, high data rate communication with high spectral efficiency in this challenging medium requires efficient system design. Multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO---OFDM) is a promising solution for reliable transmission over highly dispersive channels. In this paper, we study the equalization of shallow UWA channels when a MIMO---OFDM transmission scheme is used. We address simultaneously the long multipath spread and rapid temporal variations of the channel. These features lead to interblock interference (IBI) along with intercarrier interference (ICI), thereby degrading the system performance. We describe the underwater channel using a general basis expansion model (BEM), and propose time-domain block equalization techniques to jointly eliminate the IBI and ICI. The block equalizers are derived based on minimum mean-square error and zero-forcing criteria. We also develop a novel approach to design two time-domain per-tone equalizers, which minimize bit error rate or mean-square error in each subcarrier. We simulate a typical shallow UWA channel to demonstrate the desirable performance of the proposed equalization techniques in Rayleigh and Rician fading channels.

Adaptive frequency-domain equalization for the transmission of the fundamental mode in a few-mode fiber

We propose and experimentally demonstrate single-carrier adaptive frequency-domain equalization (SC-FDE) to mitigate multipath interference (MPI) for the transmission of the fundamental mode in a few-mode fiber. The FDE approach reduces computational complexity significantly compared to the time-domain equalization (TDE) approach while maintaining the same performance. Both FDE and TDE methods are evaluated by simulating long-haul fundamental-mode transmission using a few-mode fiber. For the fundamental mode operation, the required tap length of the equalizer depends on the differential mode group delay (DMGD) of a single span rather than DMGD of the entire link.

Polarization-Time Code and 4$\,\times\,$4 Equalizer-Decoder for Coherent Optical Transmission

A linear polarization-time code in conjunction with an adaptive multi-tap 4 4 joint equalizer-decoder is proposed to average the effects of polarization-dependent impairments on two polarizations. For the first time in the context of polarization-time coding, there is no assumption of polarization-dependent loss (PDL) of knowledge at the receiver. This method compensates the performance loss penalty due to polarization-dependent impairments in polarization division multiplexed coherent optical single-carrier systems. Using the proposed method, a 1.25-dB optical signal-to-noise ratio gain is achieved in the presence of 6-dB PDL. The proposed 44 joint equalizer-decoder has twice the complexity of conventional time-domain equalizers. Further changes at the receiver including the demodulator are not required. Since the outputs of the equalizer are noisy quadrature phase shift keying signals, adoption of the method to work with both hard and soft forward error corrections is straightforward.

OMP 알고리즘을 이용한 OFDM 시스템의 시간 영역 등화기

본 논문에서는 OFDM (Orthogonal Frequency Division Multiplexing) 시스템에서 OMP (Orthogonal Matching Pursuit) 알고리즘을 이용한 시간 영역 등화를 제안한다. OFDM 시스템은 보호구간을 삽입하여 다중 경로 페이딩에 강인한 특성을 보이지만, 도플러 환경에서는 부반송파간의 직교성이 유지되지 않아 채널간의 간섭이 발생한다. 대표적인 시간 영역 등화 방법으로 LS (Least Squares) 알고리즘이 있지만 채널 길이가 길어질수록 잡음에 의한 오차가 커지는 단점이 있다. 다중 경로 페이딩은 서로 다른 지연을 갖는 신호의 합으로 볼 수 있고, 시간 영역에서 성긴 (sparse) 특성이 있다고 할 수 있다. CS (Compressive Sensing) 알고리즘 중 하나인 OMP (Orthogonal Matching Pursuit) 알고리즘은 성긴 채널에서 중요한 요소만을 선택하여 채널을 복원함으로서 잡음의 영향을 줄일 수 있다. 모의실험에서는 여러 가지 채널 환경에서 OMP 알고리즘의 채널 등화 성능을 다른 등화 방법과 비교하여 평가하였다.

Performance Evaluation of DWT-OFDM and FFT-OFDM for Multicarrier Communications Systems using Time Domain Zero Forcing Equalization

Other than the conventional fast Fourier transform (FFT) for multicarrier modulation, a new approach for multicarrier modulation (MCM) has been known. Meanwhile, multicarrier modulation involves dividing the broadband channel into many orthogonal but overlapping narrowband carriers. In an OFDM modulation based multicarrier system using the FFT, a cyclic prefix (CP) is inserted after each symbol frame to combat the effects of inter symbol interference (ISI). By inserting the CP, which results in spectral inefficiency OFDM schemes trades up to 25% of the transmit bandwidth. A new MCM approach that is void of the expense is the wavelet transform-based systems. These systems also have very suppressed side-lobes and exhibit improved BER performance. In wavelet based systems, the latest challenge in its implementation is in the channel estimation. In this work we have studied the performance of the FFT based OFDM system against wavelet transform (WT) based multicarrier system using a simple zero forcing (ZF) equalization in time domain. The studied system shows some improved BER performance.

Adaptive Frequency-Domain Equalization with Narrowband Interference Suppression

Frequency-domain equalization (FDE) is an effective technique that exhibits the property of relatively low complexity which grows with increasing the number of symbols of dispersion in multipath propagation environments for broadband wireless communications compared with the conventional time-domain equalization. However, in practical broadband wireless communications, there exists not only multipath but also narrowband interference (NBI). The conventional FDE methods do not consider NBI and their performance degrades obviously in such case. In this paper, we propose a new optimization criterion which can effectively suppress NBI to obtain the maximum decision signal-to-noise ratio. The proposed scheme employs a conventional adaptive algorithm such as least-mean-square or recursive-least-square and operates in the spatial-frequency domain, which is concerned with the use of FDE and space diversity within block transmission schemes jointly. The simulation results show that the proposed schemes have better error-rate performance with low complexity and can be used even in the presence of strong NBI, compared to other existing adaptive FDE algorithms.

Virtual Receive Antenna for Overloaded MIMO Layered Space-Time System

Layered space time processing in spatial multiplexing systems requires the number of receive antennas to be equal to or larger than the number of transmit antennas. However, it is impractical for small-sized mobile units to accommodate a large number of antennas. To loosen this stringent requirement, a novel concept of virtual receive antennas (VRA) for overloaded MIMO system is presented. The VRA system architecture consists of two major parts: fractional timing offset in the transmitter and oversampling at the receive matched filter. This procedure expands the received signal dimension and thus creates virtual receive antennas. Due to these created virtual receive antennas, the minimum number of physical receive antennas could be reduced. In order to explore the potential of VRA, this paper evaluates its system capacity and performance bound. Results show that the VRA system can achieve higher ergodic capacity and outage rate than conventional overloaded MIMO system. Performance analysis also suggests a potential performance gain for the VRA system. To eliminate the created inter-symbol interference in the VRA, time domain and frequency domain equalization are utilized and evaluated.

Low complexity adaptation for SISO channel shortening equalizers

Since the days of Digital Subscriber Links (DSL), time domain equalizers (TEQ's) have been used to combat time dispersive channels in Multicarrier Systems. In this paper, we propose computationally inexpensive techniques to recompute TEQ weights in the presence of changes in the channel, especially over fast fading channels. The techniques use no extra information except the perturbation to the channel itself, and provide excellent approximations to the new TEQ weights. Adaptation methods for two existing Channel shortening algorithms are proposed and their performance over randomly varying, randomly perturbed channels is studied. The proposed adaptation techniques are shown to perform admirably well for small changes in channels for OFDM systems.

Comparison of LMS-TEQ and DF-TEQ to reduce cyclic prefix length in direct detection optical OFDM system

This paper presents the comparison of using least mean square time-domain equalizer (LMS-TEQ) and decision feedback time-domain equalizer (DF-TEQ) to reduce cyclic prefix (CP) length for direct-detection of optical orthogonal frequency division multiplexing (O-OFDM) over 6960km of single mode fiber (SMF). Both TEQs are used immediately after the channel. Numerical modeling results show that they can cancel the residual inter symbol interference (ISI) and inter carrier interference (ICI) caused by both the group velocity dispersion (GVD) and the CP length being shorter than the channel impulse response (CIR). Using these TEQs allow the reduction of CP length, and consequently leading to system performance improvement. On the other hand, each of TEQs adds complexity to the system. Therefore, the aim of this paper is to analyze and compare the performance of LMS-TEQ and DF-TEQ while considering different CP length and complexity.

Channel shortening techniques for differential encoded OFDM

In OFDM systems, the Inter-Symbol Interference (ISI) and the Inter-Block Interference (IBI) are mitigated by using Null-Tones (NTs) and the Guard Interval (GI) redundancy of length higher than the channel size. To preserve a high bit rate, channel shortening is required to reduce the GI length. We propose here a new adaptive method for the blind estimation of the Time domain EQualizer (TEQ) for time varying communication channels. Our contribution is three-fold. First, we exploit the knowledge of the first emitted symbol in a differential encoded OFDM system to derive the initial estimate of the TEQ. Then, we update the TEQ coefficients according to the channel variation by optimizing a criterion exploiting both the GI and NT redundancies and using a fast Generalized Eigen Value (GEV) tracking algorithm. Finally, the optimization of the previous criterion is achieved in such a way we control the Target Impulse Response (TIR) quality and we improve the overall system performance. Simulation results are provided to illustrate the performance of our method and assess our theoretical derivations.

New equalisation approach for maximising data rate in discrete multi tone systems

This article presents a new time domain equaliser (TEQ), which can be used in discrete multi-tone (DMT) systems to maximise the bit rate. The proposed TEQ is tested in the fast Fourier transform based discrete multi-tone (FFT–DMT) system, and in a proposed discrete cosine transform based discrete multi-tone (DCT–DMT) system. The objective of the proposed DCT–DMT system is to make use of the energy compaction property of the DCT to reduce the channel effects on the transmitted signals. A mathematical model of the proposed TEQ is presented. Simulation experiments have been carried out to test the effect of the proposed TEQ with the FFT–DMT system and the proposed DCT–DMT system. The results show that the performance of the DCT–DMT system with the proposed TEQ is better than the FFT–DMT system with this TEQ over the eight standard carrier serving area (CSA). The results also show that employing the proposed TEQ in the DCT–DMT system can achieve a high bit rate, ranging from 2.899 Mbps to 5.369 Mbps.

Time-domain equalizer for multicarrier systems in impulsive noise

In multicarrier communication systems, a time-domain equalizer (TEQ) can be applied to shorten the channel impulse response and to eliminate inter-symbol interference (ISI). However, the presence of impulsive noise in the channel may paralyze the operation of TEQs and subsequently lead to poor error performance. In this paper, a multicarrier receiver that incorporates a constant false alarm rate algorithm and an iterative estimation technique (CFAR-IET) in conjunction with a TEQ is proposed to increase the robustness of the receiver against impulsive noise. Furthermore, an improved version of the CFAR-IET-TEQ scheme, which uses the buffering, sorting, removing and amplitude averaging (BSRA) processes, is presented. Performance comparisons of the proposed schemes with the existing Gaussian-optimized schemes are made. Simulation results show that the BSRA-IET-TEQ scheme is an effective approach to reduce symbol error rate (SER) in impulsive channels while performing satisfactorily in Gaussian channels. Copyright © 2011 John Wiley & Sons, Ltd.

A Signal Subspace Detection Technique for Single Carrier Block Transmission with Unique Words

For frequency selective channels, conventional equalizations based on a whole vector space enhance the noise power at randomly distributed spectral nulls. A signal subspace detection technique is proposed for single carrier (SC) block transmission with unique words (UW). Different from conventional Fourier basis, a new orthogonal basis to orderly separate the space into signal subspace and noise subspace is adopted. In this way, known symbols of UW can compensate the noise subspace and facilitate data recovery by a maximum likelihood (ML) detector in the signal subspace, which is robust to the ill-conditioned channel matrix. The proposed technique does not require any feedback from the receiver to the transmitter. A near optimal performance is achieved with its computational complexity similar to the time domain equalization.

No-guard-interval coherent optical OFDM with self-tuning receiver

We propose and experimentally demonstrate a no-guard-interval (No-GI) coherent optical orthogonal frequency division multiplexed (CO-OFDM) system that uses Fractionally-Spaced Time-Domain Equalizers (FS-TDE) to simultaneously demultiplex and equalize each subcarrier. Least-mean-squares algorithms (LMS) tune each FS-TDE to a subcarrier. A short unique training sequence is transmitted on each subcarrier, allowing each FS-TDE to lock onto its subcarrier. After the initial training, the adaptive blind equalizers remain tuned to their respective subcarriers. Unlike previous systems, this system does not require digital filtering or mixing of each subcarrier to baseband, so is more computationally efficient. Error-free transmission was measured over 800 km of fiber with a three-subcarrier 30 Gb/s system and a five-subcarrier 33.33 Gb/s system. The required OSNRs for a BER of 10(-3) were 8.6 dB and 9.3 dB respectively, which are within 1.5 dB of the theoretical limit for coherent systems.

Cyclic prefix reduction for 20.48 Gb/s direct‐detection optical OFDM transmission over 2560 km of SSMF

AbstractOptical Orthogonal Frequency Division Multiplexing (O‐OFDM) provides major advantages in mitigating Group‐Velocity Dispersion (GVD) in Single Mode Fiber (SMF). Unfortunately, when the uncompensated long‐haul transmission ranges become very large, substantial dispersion is accumulated. Owing to the large accumulated dispersion, the Cyclic Prefix (CP) duration will occupy a substantial fraction of the OFDM frame. This effect sets some limitations on the overall throughput and the spectral efficiency. Moreover, the transmission is inefficient because of the energy wastage contained within the CP. In the case where the CP length is shorter than the Channel Impulse Response (CIR), energy wastage is reduced but the system performance is limited by the Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). To overcome this problem, a Time Domain Equalizer (TEQ) immediately after the channel is proposed. It can cancel the residual ISI and ICI caused by both the GVD and the CP length being shorter than the CIR. The simulation results show that, using BER of 10−3 as a reference, the system performance improves by 2.69thinspacedB while considering 6‐feedforward/5‐feedback‐weight Decision Feedback TEQ (DF‐TEQ) and 2.03 dB while considering 10‐tap Least Mean Square TEQ (LMS‐TEQ) even though the CP length is shorter than the CIR. This method reduces size of the CP, and consequently the performance of the system will be improved. Copyright © 2011 John Wiley & Sons, Ltd.