Iterative Frequency Domain Equalization Research Articles

Efficient Joint Channel Equalization and Tracking for V2X Communications Using SC-FDE Schemes

Our aim with this paper is to present a solution suitable for vehicle-to-everything (V2X) communications, particularly, when employing single-carrier modulations combined with frequency-domain equalization (SC-FDE). In fact, we consider the V2X channel to be doubly-selective, where the variation of the channel in time is due to the presence of a Doppler term. Accordingly, the equalization procedure is dealt by a low-complexity iterative frequency-domain equalizer based on the iterative block decision-feedback equalization (IB-DFE) while the tracking procedure is conducted employing an extended Kalman filter (EKF). The proposed system is very efficient since it allows a very low density of training symbols, even for fast-varying channels. Furthermore only two training symbols are required to initialize the tracking procedure. Thus, ensuring low latency together with reduced channel estimation overheads.

Low-Complexity SC-FDE Techniques for Massive MIMO Schemes With Low-Resolution ADCs

Massive multiple-input multiple-output (mMIMO) systems are very important for 5G since they are one of the main enablers of the targeted huge capacity gains. On the other hand, the power consumption at the mobile terminals should be as low as possible, making single-carrier waveforms, such as single carrier with frequency-domain equalization (SC-FDE), particularly interesting, since the transmit signals can have a low peak-to-average power ratio (PAPR), allowing a highly efficient power amplification. However, the combination of mMIMO systems with SC-FDE modulations gives rise to implementation difficulties at the receiver side. On one hand, the equalization procedure should avoid matrix inversions, but this might lead to performance degradation. On the other hand, low-resolution analog-to-digital converters (ADCs) should be employed in each receive branch to reduce the implementation complexity, which might lead to severe nonlinear distortion effects. In this work, we study analytically the performance of mMIMO systems based on SC-FDE schemes employing low-complexity, iterative FDE receivers, and low-resolution ADCs. It is shown that the performance degradation associated to low-resolution ADCs can be tolerable if the number of receive antennas is larger than the number of transmit antennas, even when low-complexity FDE receivers are employed.

Multi-user detection for the downlink of NOMA systems with multi-antenna schemes and power-efficient amplifiers

Non-orthogonal multiple access (NOMA) schemes have been recognized as a promising multiple access method for the fifth generation (5G) communication systems. In NOMA, multiple users can share the same physical channel, which allows for a substantial increase in capacity gains when comparing to conventional orthogonal multiple access (OMA) techniques. This paper considers the downlink transmission of NOMA schemes combined with single-carrier (SC) signals. We present a multi-antenna transmission technique that is compatible with highly efficient power amplifiers without requiring any pre-processing. Furthermore, a multi-user detection scheme based on iterative frequency-domain equalization (FDE) is presented. We show that besides the high power-efficiency of the transmission scheme, the system’s performance can be close to the matched filter bound (MFB).

Iterative Frequency Domain Equalization for MIMO-GFDM Systems

This paper proposes a new iterative frequency domain equalization (FDE) algorithm for multiple-input multiple-output (MIMO)-frequency division multiplexing (GFDM) systems. This new FDE scheme is capable of enhancing the system fidelity by considering the complete frequency-domain second order description of the received signal. In addition, a new nulling filter design is also proposed for MIMO-GFDM systems to remove the residual interference, which further improves the system fidelity compared with the traditional scheme. Simulation results are presented to verify the effectiveness and efficiency of the proposed FDE algorithm.

Multi-channel iterative FDE for single carrier block transmission over underwater acoustic channels

Recently, single carrier block transmission (SCBT) has received much attention in high-rate phase-coherent underwater acoustic communication. However, minimum-mean-square-error (MMSE) linear FDE may suffer performance loss in the severely time dispersive underwater acoustic channel. To combat the channel distortion, a novel multi-channel receiver with maximum ratio combining and a low complex T/4 fractional iterative frequency domain equalization (FDE) is investigated to improve diversity gain and the bit error rate (BER) performance. The proposed method has been verified by the real data from a lake underwater acoustic communication test in November 2011. At 1.8 km, the useful data rates are around 1500 and 3000 bits/ s for BPSK and QPSK respectively. The results show the improvements of system performance. Compared with MMSE FDE system, the output SNR improvement is 6.9 dB, and the BER is from 10-3 to no error bits for BPSK. The output SNR improvement is 5.3 dB, and the BER is from 1.91×10-2 to 2.2×10-4 for QPSK.

Frequency-Domain Receiver Design for Transmission Through Multipath Channels with Strong Doppler Effects

Single-carrier modulation (SC) with frequency-domain equalization (FDE) combined with iterative (turbo) FDE schemes has excellent performance in severely time-dispersive channels, making it a promising candidate for future broadband wireless systems. To avoid significant performance degradation due to strong Doppler effects, SC-FDE schemes employ frequency-domain receivers that require an invariant channel within the block duration. In this paper we propose iterative receivers for SC-FDE schemes able to attenuate the impact of strong Doppler effects. These receivers can be considered as modified turbo equalizers implemented in the frequency-domain, which are able to compensate the Doppler effects associated to different groups of multipath components while performing the equalization operation. The performance results show that the proposed receivers have excellent performance, even in the presence of significant Doppler spread between the different groups of multipath components. Therefore, our receivers are suitable for SC-FDE scheme based broadband transmission in the presence of fast-varying channels.

Joint Transceiver Design for Iterative FDE

This paper considers the wideband single-carrier transmission through frequency-selective wireless channels with a quality-of-service (QoS) requirement. Power allocation at the transmitter and iterative frequency domain equalization (FDE) at the receiver are jointly designed to achieve the target QoS with minimum transmission power consumption. The joint transceiver design is based on a novel evolutional analysis framework for iterative FDE without the aid of decoding during the iterative processing. It introduces a set of QoS and quality-of-convergence (QoC) constraints, which recast the design problem into a convex and structured joint QoS/QoC constrained power allocation problem. Thanks to the underlying sparsity in the mathematical structure of the problem, a specific interior-point method (IPM) solver is developed, which significantly reduces the computational complexity, as compared to the generic solvers. A duality gap analysis is then applied to show that the performance gap between the numerical algorithms and the theoretical optimum can be controlled by adjusting its key parameters, which, in turn, determines the convergence behavior of the iterative solver. Extensive simulation results are given to support the mathematical development of the problem.

Iterative Frequency Domain Equalization With Generalized Approximate Message Passing

An iterative frequency domain equalization approach for coded single-carrier block transmissions over frequency selective channels is developed by using the recently proposed generalized approximate message passing (GAMP) algorithm. Compared with the low-complexity iterative frequency domain linear minimum mean square error (FD-LMMSE) equalization, the proposed approach can achieve significant performance gain with slight complexity increase.

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.

Single-carrier frequency domain equalisation with hierarchical constellations: an efficient transmission technique for broadcast and multicast systems

Orthogonal frequency division multiplexing (OFDM) schemes are the choice modulation for broadband wireless broadcast and multicast systems. However, OFDM schemes have important limitations such as high-envelope fluctuation of the transmitted signals and its sensibility to carrier frequency errors. When these limitations prove critical the authors consider single-carrier frequency-domain equalisation (SC-FDE) schemes, that allow much higher power efficiency because of lower envelope fluctuations. The overall performance can be further improved if the conventional linear FDE is replaced by an iterative FDE such as an iterative block decision feedback equaliser (IB-DFE). Conventional IB-DFE are usually designed for a quadrature phase shift keying constellation. However, it is strongly recommended that broadcast and multicast systems employ hierarchical constellations with several classes of bits with different error protection. In this study the authors consider the use of SC-FDE schemes combined with IB-DFE receivers in broadband wireless broadcast and multicast systems. The authors emphasise the advantages of these schemes and the authors present IB-DFE designs suitable for hierarchical constellations with several classes of bits with different error protection.

SC-FDE for Offset Modulations: An Efficient Transmission Technique for Broadband Wireless Systems

It is widely accepted that SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an excellent candidate for broadband wireless systems, especially when an efficient power amplification is intended. If grossly nonlinear power amplifiers are employed, conventional QPSK (Quaternary Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) modulations should be replaced by offset modulations such as OQPSK (Offset QPSK) and OQAM (Offset QAM). In fact, offset signals have much lower dynamic range than non-offset signals, with OQPSK signals being able to have an almost constant envelope. This paper considers frequency-domain receiver design for OQPSK and OQAM schemes. It is shown that FDE (Frequency Domain Equalization) designed for non-offset modulations are not suitable for offset modulations due to the residual IQI (In-phase/Quadrature Interference), i.e. the interference between in-phase (I) and quadrature (Q) components at the sampling instants. Therefore, we propose several FDE designs where there is no IQI in the sampling instants as well as iterative FDE receivers with IQI cancellation. Our receivers have excellent performance, with the linear designs significantly outperforming linear FDE for non-offset modulations and the iterative designs with performance close to the MFB (Matched Filter Bound).

SC-FDE for Offset Modulations: An Efficient Transmission Technique for Broadband Wireless Systems

It is widely accepted that SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an excellent candidate for broadband wireless systems, especially when an efficient power amplification is intended. If grossly nonlinear power amplifiers are employed, conventional QPSK (Quaternary Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) modulations should be replaced by offset modulations such as OQPSK (Offset QPSK) and OQAM (Offset QAM). In fact, offset signals have much lower dynamic range than non-offset signals, with OQPSK signals being able to have an almost constant envelope. This paper considers frequency-domain receiver design for OQPSK and OQAM schemes. It is shown that FDE (Frequency Domain Equalization) designed for non-offset modulations are not suitable for offset modulations due to the residual IQI (In-phase/Quadrature Interference), i.e. the interference between in-phase (I) and quadrature (Q) components at the sampling instants. Therefore, we propose several FDE designs where there is no IQI in the sampling instants as well as iterative FDE receivers with IQI cancellation. Our receivers have excellent performance, with the linear designs significantly outperforming linear FDE for non-offset modulations and the iterative designs with performance close to the MFB (Matched Filter Bound).

Estimation of the Feedback Reliability for IB-DFE Receivers

single-carrier (SC) modulation with frequency-domain equalization (FDE) combined with iterative (turbo) FDE schemes has been shown to be suitable for cyclic-prefix (CP)-assisted block transmission within broadband wireless systems. Its excellent performance in severely time-dispersive channels, makes it a promising candidate for future broadband wireless systems. In this paper we investigated the impact of the correlation factor estimation in the performance of iterative block decision feedback equalization (IB-DFE) receivers. Since the correlation factor represents a key parameter to ensure the good performance of these receivers, reliable estimates of this factor are needed in the feedback loop. We present several methods to estimate the correlation coefficient. We also propose a technique to compensate the inaccuracy of the estimation.

Efficient Iterative Frequency Domain Equalization for Single Carrier System with Insufficient Cyclic Prefix

Efficient Iterative Frequency Domain Equalization for Single Carrier System with Insufficient Cyclic Prefix

Joint Tx/iterative Rx FDE for broadband direct‐sequence code division multiple access

AbstractDirect‐sequence code division multiple access (DS‐CDMA) is a promising uplink access technology for broadband cellular mobile communication systems. In this paper, we propose a joint transmit (Tx)/iterative receive (Rx) frequency‐domain equalization (FDE) for DS‐CDMA. In the proposed scheme, simple one‐tap FDE at the transmitter and iterative one‐tap FDE at the receiver are jointly performed using the common knowledge of channel state information (CSI). Before transmitting the signal, the transmitter predicts the degree of residual inter‐chip interference (ICI) power after the iterations of Rx FDE to determine the Tx FDE weight while the receiver performs the iterative Rx FDE. In practice, it is almost impossible to share the perfect knowledge of CSI. In this paper, a Gaussian approximation of channel estimation (CE) errors is introduced. We theoretically derive a set of Tx and iterative Rx FDE weights based on the minimum mean square error (MMSE) criterion in the presence of the CE error. We show by computer simulation that the proposed scheme provides better bit error rate (BER) performance than the conventional scheme which uses iterative Rx FDE only (i.e., without Tx FDE) even in the presence of CE error. Copyright © 2010 John Wiley & Sons, Ltd.

Iterative Frequency Domain Equalization and Carrier Synchronization for Multi-Resolution Constellations

Broadband broadcast and multicast wireless systems usually employ OFDM modulations (Orthogonal Frequency Division Multiplexing) combined with non-uniform hierarchical constellations. However, these schemes are very prone to nonlinear distortion effects and have high carrier synchronization requirements. SC-FDE (Single-Carrier with Frequency-Domain Equalization) is an attractive alternative for OFDM, especially when an efficient power amplification is intended. In this paper we consider the use of SC-FDE schemes combined with non-uniform hierarchical constellations in broadband broadcast and multicast wireless systems. We study the impact of residual CFO (Carrier Frequency Offset) on the performance of multi-resolution schemes and we propose iterative frequency domain receivers with joint detection and carrier synchronization to cope with residual CFO estimation errors (a coarse CFO estimation and compensation is assumed before the equalization procedure). Our results show that while a very high carrier synchronization accuracy is required for the least protected bits, the most protected bits are relatively robust to the CFO. By employing the proposed receiver we increase significantly the robustness to residual CFO estimation errors.

An efficient iterative frequency domain equalization for ATSC DTV receiver

In this paper, we present an efficient single carrier frequency domain equalization for Advanced Television Systems Committee (ATSC) digital television receiver. The proposed scheme employs iterative cyclic prefix reconstruction (CPR) combined with block-overlapping process to reduce inter-carrier interference (ICI) component caused by the absence of cyclic prefix in multipath fading channel. In addition, trellis decoder aided iterative decision feedback frequency domain equalizer is derived to further mitigate inter-symbol interference (ISI). Finally, low-complex iterative frequency domain channel estimation is proposed to predict and track the time-varying multipath fading channel coefficients. The computer simulation results reveal that the proposed scheme has a significant performance improvement in multipath fading channel with respect to the previous time domain equalization (TDE) and frequency domain equalization (FDE).

Decision-directed phase noise compensation for millimeter-wave single carrier systems with iterative frequency-domain equalization

This paper proposes a receiver that repeats iterative frequency-domain equalization (FDE) and decision-directed phase noise compensation (DD-PNC) to alleviate degradation due to the phase noise for millimeter-wave single carrier (SC) systems. High bit-rate SC-FDE transceivers based on the single-chip Si RF-CMOS IC technology in the 60-GHz millimeter-wave band have been extensively studied for wireless personal area network (WPAN) systems, and the relatively large phase noise in a phase-locked loop (PLL) synthesizer severely degrades transmission performance. In an initial processing of the proposed receiver, a cyclic prefix (CP)-based phase noise compensator (CP-PNC) removes the phase noise from a time-domain received signal by using CP, which is known to the receiver, and the channel is equalized by the iterative FDE using the conventional minimum mean-square-error (MMSE) weight. In an iterative processing, DD-PNC estimates the phase noise each symbol by exploiting an output of a channel decoder, and then compensates the time-domain received signal for the phase noise by using the estimate. In order to equalize the compensated received signal, the iterative FDE performs both the MMSE filtering and residual inter-symbol interference cancelation using the decoder output. Computer simulations following the 60-GHz WPAN standard demonstrate that in the 64QAM with the coding rate of 3/4, the proposed receiver with three iterations can drastically remove the phase noise of −85 dBc/Hz at 1 MHz offset, and that it can achieve excellent transmission performance.

Bit-interleaved coded multilevel modulation for single-carrier frequency-domain equalization

This paper proposes a bit-interleaved coded multilevel modulation for single-carrier frequency-domain equalization (SC-FDE). In the proposed system, multiple layers are encoded and interleaved together, using one encoder and one interleaver, and then they are mapped by hierarchical symbol mapping. A corresponding receiver structure is presented, where multiple layers can be decoupled by iterative frequency-domain equalization with improved soft information. Simulation results show that the proposed system has better performance than conventional modulation schemes such as bit-interleaved coded modulation (BICM) and multilevel BICM (MLBICM).

GMP-Based Channel Estimation for Single-Carrier Transmissions over Doubly Selective Channels

We present a graph-based channel estimation approach for SC-IFDE (single-carrier transmissions with iterative frequency domain equalization) without CP (cyclic prefix) over doubly selective channels using the recently developed Gaussian message passing (GMP) technique. A direct application of the GMP updating rules in the FFG (Forney-style factor graph) of the SC-IFDE system model incurs high complexity. Approximate updating rules are therefore developed to overcome this problem. The proposed GMP-based channel estimation approach has similar complexity as the low-complexity Kalman-filtering based frequency domain channel estimation approach in the literature, but significantly outperforms the latter due to its enhanced capability in capturing the time correlation information of doubly selective channels through bidirectional processing.