Inter-block Interference Research Articles

Experimental full-duplex amplify-and-forward relay scheme for OFDM with power gain control

The fundamental challenges for full-duplex (FD) relay networks are the self-interference cancellation (SIC) and the cooperative transmission design at the relay. This paper presents a practical amplify-and-forward (AF) FD one-way relay scheme for orthogonal frequency division multiplexing (OFDM) transmission with multi-domain SIC. It is found that the residual self-interference (SI) signals at the relay can be regarded as an equivalent multipath model. The effects of the residual SI at the relay are incorporated into the equivalent end-to-end channel model, and the inter-block interference can be removed at the destination by using cyclic prefix (CP) protection. Based on the equivalent multipath model, we present a solution for optimizing the amplification factor on the performance of signal-to-interference-plus-noise ratio (SINR) when the equivalent multipath length is longer than the CP. Furthermore, a practical one way FD relay network with 3 nodes is built and measured. The simulation and measured results verify the superior performance of the proposed scheme.

Design of equalizers for frequency selective MIMO systems with low redundancy

In this paper, we propose several designs of filter bank based decision feedback equalizers (DFE) for frequency selective multi-input multi-output (MIMO) systems. Transmission schemes over frequency selective channels are typically block based, and redundancy in form of guard intervals must be used to avoid inter-block interference (IBI). This paper will focus on the case where spectral efficiency demands low levels of redundancy. Then the remaining IBI will be eliminated by a separate feedback loop. The co-channel interference (CCI) inherited in MIMO channels will be eliminated by different DFE schemes. In another approach, a multi-tap MIMO DFE will be designed to deal with both IBI and CCI. The simulation results show that for quasi-static frequency selective MIMO channels, the multi-tap DFE provides the best performance while for Raleigh frequency selective MIMO channels, the dual loop DFE is a good solution for transmission with low redundancy.

Iterative Channel Estimation and Data Detection of OTSM With Superimposed Pilot Scheme and PAPR Analysis

Orthogonal time sequency multiplexing (OTSM), a novel modulation scheme proposed recently performs superior to orthogonal frequency division multiplexing (OFDM) under doubly dispersive channel. In this Letter, we propose iterative channel estimation and data detection for OTSM systems with superimposed pilot scheme. The channel estimation is done initially considering pilot-data interference (PDI) as noise with high power pilot signal. From the second iteration onwards, PDI is eliminated based on the estimated channel and detected symbols. Block by block symbol detection in the time domain is performed by eliminating another interference known as inter block interference (IBI) between the blocks. The peak to average power ratio (PAPR) is evaluated for OTSM and it is found to increase linearly with the number of sequency bins and not delay bins. The PAPR is reduced with this superimposed pilot scheme compared to conventional zero padded (ZP) pilot scheme. The evaluated error performance of superimposed pilot scheme is close to ZP OTSM scheme with less pilot power.

Improved Detection Schemes for Non-coherent Pulse-Position Modulation.

Energy-detection (ED) pulse-position modulation (PPM) receivers exhibit poor performance and low rates. Coherent receivers do not have such problems but their complexity is unacceptable. We propose two detection schemes to increase the performance of non-coherent PPM receivers. Unlike the ED-PPM receiver, the first proposed receiver cubes the absolute value of the received signal before demodulation and achieves a considerable performance gain. This gain is obtained because the absolute-value cubing (AVC) operation reduces the effect of low-SNR samples and increases the effect of high-SNR samples on the decision statistic. To further increase energy efficiency and rate of the non-coherent PPM receivers at almost the same complexity, we use the weighted-transmitted reference (WTR) system instead of the ED-based receiver. The WTR system has adequate robustness to weight coefficients and integration interval variations. To generalize the AVC concept to the WTR-PPM receiver, the reference pulse is first passed through a polarity-invariant squaring (PIS) operation and then is correlated with the data pulses. In this paper, the performance of different receivers employing the binary PPM (BPPM) is investigated at data rates of 2.08 and 9.1 Mbps over in-vehicle channels in the presence of noise, inter-block interference, inter-pulse interference, and inter-symbol interference (ISI). Simulations show that the proposed AVC-BPPM receiver outperforms the ED-based one in the absence of ISI and offers the same performance in the presence of strong ISI, the WTR-BPPM system considerably outperforms the ED-BPPM system (especially at high rates), and the proposed PIS-based WTR-BPPM system considerably outperforms the conventional WTR-BPPM system.

BER Reduction and Capacity Enhancement with Novel Guard Sequence Selection for Multi-Carrier Communication

Orthogonal frequency division multiplexing (OFDM) is an efficient multicarrier scheme that uses different types of guard intervals such as cyclic prefix (CP) and known symbol padding (KSP) (zero padding (ZP), unique word (UW), etc.) in block formation. Among these guard intervals, CP varies for each block, while other guard intervals remain fixed from block to block. These guard intervals efficiently perform channel estimation, synchronization and remove inter-block interference (IBI); nevertheless, none of the existing schemes develop any relationship between the guard interval (sequence) and the data symbols on different subcarriers of the OFDM block. We present a new idea of selecting the guard interval based on the data symbols of a subset of subcarriers in the block and exploit the high auto-correlation of the selected guard sequence to improve the bit error rate (BER) performance of the system. The results based on a fair comparison show that our enhanced orthogonal frequency division multiplexing (eOFDM) scheme inherits significant improvements in BER and the capacity of a multicarrier system as compared to the existing techniques.

Frequency-Packed Faster-Than-Nyquist Signaling via Symbol-Level Precoding for Multiuser MISO Redundant Transmissions

This work addresses the issue of interference generated by co-channel users in downlink multi-antenna multicarrier systems with frequency-packed faster-than-Nyquist (FTN) signaling. The resulting interference stems from an aggressive strategy for enhancing the throughput via frequency reuse across different users and the squeezing of signals in the time-frequency plane beyond the Nyquist limit. The spectral efficiency is proved to be increasing with the frequency packing and FTN acceleration factors. The lower bound for the FTN sampling period that guarantees information losslesness is derived as a function of the transmitting-filter roll-off factor, the frequency-packing factor, and the number of subcarriers. Space-time-frequency symbol-level precoders (SLPs) that trade off constructive and destructive interblock interference (IBI) at the single-antenna user terminals are proposed. Redundant elements are added as guard interval to cope with vestigial destructive IBI effects. The proposals can handle channels with delay spread longer than the multicarrier-symbol duration. The receiver architecture is simple, for it does not require digital multicarrier demodulation. Simulations indicate that the proposed SLP outperforms zero-forcing precoding and achieves a target balance between spectral and energy efficiencies by controlling the amount of added redundancy from zero (full IBI) to half (destructive IBI-free) the group delay of the equivalent channel.

Sea Experimental for Compressive Sensing-Based Sparse Channel Estimation of Underwater Acoustic TDS-OFDM System

Due to the high spectral efficiency (SE) and fast synchronization, the time-domain synchronization orthogonal frequency division multiplexing (TDS-OFDM) system has gotten much more attraction of researchers as compared to cyclic-prefix (CP) and zero padding (ZP) OFDM in terrestrial as well as underwater acoustic communication. Inter-block interference (IBI) degrades the TDS-OFDM performance due to its long-delay multiple channels. In TDS-OFDM, dual pseudo-random noise (DPN) sequences utilize two PN sequences as a guard interval (GI) after every data block to cope with interference from the OFDM data block to the next PN sequence resulting in compromising the energy efficiency (EE) and spectral efficiency. We have proposed compressed sensing-based technique compressive sensing matching pursuit (CoSaMP), orthogonal matching pursuit (OMP), and look-ahead and backtracking OMP (LABOMP) for TDS-OFDM over the real-time underwater channel in this paper. Moreover, prior to estimating the channel, the received PN sequence is considered in the time domain to compensate for the Doppler shift of the UWA channel. The real-time data experiment has been initially conducted for testing in a water tank in our laboratory. Furthermore, it has been tested on the sea for long communications under the water at the Wuyuan sea area in Xiamen, China. Simulations and experimental results evident that the compressed sensing techniques have better performance over the conventional TDS-OFDM and DPN-TDS-OFDM, even LABOMP outperform OMP and CoSaMP in terms of bit-error-rate (BER), SE, and EE.

Efficient Fast-Convolution Based Hybrid Carrier System

The weighted-type fractional Fourier transform (WFRFT)-based hybrid carrier (HC) system has been proposed to bridge the gap between orthogonal frequency division multiplexing (OFDM) system and single-carrier system with frequency domain equalization (SC-FDE). In this paper, we propose a novel HC system based on efficient fast-convolution (FC) without a cyclic prefix to meet the requirements of complex scenarios towards future communications. The proposed system can achieve flexible optimization of system performance, such as minimizing out-of-band emission (OOBE), peak to average power ratio (PAPR) and bit error rate (BER). This efficient FC based hybrid carrier (FCHC) system is among the most competitive waveforms. The closed-form BER expressions are derived and analyzed over additive white Gaussian noise (AWGN) and frequency-selective fading channels. A modified equalizer over frequency-selective channels is also provided to resist the inter-block interference (IBI). Moreover, the banded minimum mean-square error (MMSE) equalization based on FC can be embedded in the proposed system over doubly-selective channels, and then the interference caused by fading channels can be further suppressed through the combination of FC and HC. The simulation results demonstrate the superiority of the proposed system over the conventional HC system in terms of OOBE, PAPR and BER. The significant flexibility and combined performance advantages make the proposed system a potential scheme for future communications.

A Novel Single-Carrier Frequency-Domain Equalization Technique without Prefix or Suffix for Underwater Acoustic Communications

Single-Carrier Frequency-Domain Equalization (SC-FDE) is one of the key techniques to achieve high data-rate transmissions in underwater acoustic communications. However, the periodically inserted prefixes or suffixes, in forms of cyclic-prefixes or guard intervals in transmission sequence, reduce the bandwidth efficiency and data rate. A SC-FDE technique without prefix or suffix is proposed based on the Time Reversal (TR) for underwater acoustic communications. At the transmitter side, the single-carrier signal without prefix or suffix is transmitted. At the receiver side, the suffix is reconstructed in each block before canceling the intersymbol interference with FDE. The suffix reconstruction, which is the core of receiver, contains three main procedures: the TR processing, interference cancelation, and suffix reconstruction. First, the TR processing converts receptions from an array of receiving elements into a single composite signal. The equivalent impulse response (namely the q-function) has a stable peak, which avoids the noise amplification. Next, the interference cancelation is conducted to remove the InterBlock Interference (IBI) from the previous block and the ACausal Interference (ACI) from the current block. Afterward, suffix reconstruction is achieved by calculating the multipath arrivals of each symbol, which are obtained from the causal part of the q-function. Experimental results validate the effectiveness of this technique and its superiority over the SC-FDE technique without prefix or suffix in the current literature.

Faster-Than-Nyquist Signaling with Time-Based Receive Transformation

We propose in this paper the time-based receive transformation (TRT), which can be regarded as the time-based version of the decorrelation based receive transformation proposed in Wang (IEEE Wirel Commun Lett 3(3):305–308, 2014), to suppress the inter-block interference (IBI) in the faster-than-Nyquist (FTN) system. We show that TRT can completely eliminate the effect of IBI in the FTN system so that instead of compromising the performance, FTN with TRT can make the achievable rate reach the maximum rate of the FTN system while having acceptable delay and complexity.

Frequency-Domain Decision Feedback Equalization for Single-Carrier Transmissions in Fast Time-Varying Underwater Acoustic Channels

Frequency-domain equalization (FDE) is a computationally efficient method to recover underwater acoustic single-carrier transmissions in a multipath environment. The FDE receivers often operate in a block-by-block manner, assuming that the channel is time invariant during a block duration. This assumption implies that traditional block-based FDE receivers experience performance degradation in fast time-varying channel conditions, since the interfrequency interference (IFI) is often not considered. Here, we focus on the IFI cancelation (IFIC) in single-carrier transmissions. The IFI formulations are developed for a blockwise FDE receiver, where the time reversal (TR) processing is incorporated to alleviate the interblock interference. Based on the formulations, the IFI is estimated by the time-varying impulse responses, which are obtained from an adaptive basis expansion model. A soft two-step IFIC strategy is then employed to address the IFI. In the first step, the main IFI is removed by the time-varying TR processing and explicit IFI cancelation. The residual IFI is suppressed by the frequency-domain decision feedback equalization (FD-DFE) in the second step. To further enhance the communication performance, we develop a soft-symbol-based iterative receiver, referred to as IFIC-FD-DFE. The impulse responses, explicit IFI, and residual IFI are updated in an iterative fashion, based on the soft symbol estimates. The proposed receiver has been tested using the at-sea measurements collected at the Gulf of Mexico in August 2017. Communication packets at four ranges at the carrier frequency of 160 kHz with a symbol rate of 32 kHz have been decoded. The effectiveness of the IFIC-FD-DFE receiver has been demonstrated via comparisons with several FDE schemes in the current literature.

Performance Analysis for Faster-Than-Nyquist Signaling Under Multi-Path Channel

We propose in this letter the time-based receive transformation (TRT) to suppress the inter-block interference (IBI) in the single-input-single-output (SISO) system with faster-than-Nyquist (FTN) signaling under the multi-path channel. For the trailing-zeros based FTN signaling or the OFDM based FTN signaling, the achievable rate benefits from the increased symbol rate but is degraded by the increased number of symbols that multi-path inter-symbol interference (ISI) affects. We show that the TRT based FTN signaling can completely eliminate the effect of IBI so that its achievable rate benefits from both the increased symbol rate and the increased number of symbols that multi-path ISI affects. We also show that the achievable rate for the TRT based FTN signaling is larger than that for the trailing-zeros based FTN signaling and the OFDM based FTN signaling, and this advantage increases with the ratio of the symbol rate to the Nyquist rate.

SVD-Precoded Faster-Than-Nyquist Signaling With Optimal and Truncated Power Allocation

This study proposes a precoded faster-than-Nyquist (FTN) signaling scheme based on singular-value decomposition (SVD) with optimal power allocation. An information-theoretic analysis is conducted on the conventional and proposed SVD-precoded FTN signaling architectures. The associated information rate bound is derived in a closed-form by extending the classic Shannon capacity to support the SVD-precoded FTN signaling schemes. Our analytical performance results demonstrate that the proposed scheme outperforms its conventional Nyquist-criterion-based counterpart and the conventional SVD-precoded FTN signaling scheme, which does not use power allocation. Several important implementation issues, such as inter-block interference, the use of a frequency-selective (dispersive) channel, truncation, and sampling rate, are also considered. Finally, a suboptimal power allocation scheme that overcomes the truncation issue is presented.

Complexity-Reduced Receiver for Universal Filtered Multicarrier Systems

In this letter, a low complexity least squares-based receiver is proposed for universal filtered multicarrier systems. Specifically, the coarse estimate of the transmitted symbol is first obtained by one-tap equalization using the even components of the frequency-domain received signal. The inter-block interference is then removed from the received signal, and the out-of-subband data are punctured. Finally, a complexity-reduced approach is designed to obtain performance improvement based on the overall pre-processed data. A closed-form mean-square error expression and computational complexity analysis are derived to quantify the achievable advantages. Simulation study confirms the effectiveness of the proposed receiver.

Spectrum Sensing for OFDM Cognitive Radio using Matched Filter Detection

The exponentially increasing high data rate applications demand more spectrum. Conventional spectrum allocation schemes failed in providing spectrum to meet the requirement. Cognitive radio introduces dynamic spectrum allocation. Spectrum sensing plays a very important role in cognitive radio. In this paper, OFDM with sensing algorithm based on matched filter detection is presented. An Orthogonal Frequency Division Multiplexing (OFDM) with Cyclic Prefix is used to reduce Inter Block Interference between successive symbols. The Receiver Operating Characteristics (RoC), detection probability with respect to Signal to Noise Ratio (SNR) for a given False Alarm Probability (PFA) obtained and analyzed. Simulation results shows that, this algorithm achieves probability of detection values 0.38 and 0.9 for SNR values of -20dB and -15 dB respectively at PFA=10-2. Chen, Hou-Shin, considered Time Domain Symbol Cross-correlation between two OFDM symbols. This paper mainly focusses on improving the detection probability at low SNR values without considering cross-correlation, which requires more computations.

Compressive sensing-based channel estimation for SC-FDE system

The channel of wireless broadband communication system usually features time domain sparsity. The channel estimation of single carrier frequency domain equalization (SC-FDE) system was modeled as compressive sensing (CS)-based reconstruction of sparse signal. Matching pursuit algorithms based on greedy search were adopted to reconstruct channel information. Under the premise of reduced reference signal overhead, the system reliability can be effectively enhanced. This paper proposes a priori information aided scheme. By inserting the cyclic prefix of the first training sequence to a SC-FDE transmission frame, which is used as guard interval, the priori information including more accurate channel length and sparsity can be obtained with inter-block-interference (IBI)-free training sequence. The proposed scheme shows obvious superiority in slow fading channels. It effectively improves the reconstruction of probability, thereby improving the accuracy of channel estimation, meanwhile sacrificing less spectral efficiency.

A Conditional <inline-formula> <tex-math notation="LaTeX">${\ell}1$ </tex-math> </inline-formula> Regularized MMSE Channel Estimation Technique for IBI Channels

Inter-block-interference (IBI) caused in the pursuit of spectral efficiency can deteriorate channel estimation performance. For this problem, previously-proposed chained turbo estimation performs IBI cancelation by using the soft replica of the transmitted signal. The IBI cancelation technique can, however, suffer from a mean squared error (MSE) floor problem, since the soft replica is unavailable at the first turbo iteration. The IBI problem can be avoided by using channel impulse response length constraint. Nevertheless, as shown in this paper, the IBI avoidance approach is difficult to perform independently since it requires unbiased second-order statistics. This paper proposes, therefore, a new conditional $\ell 1$ regularized minimum mean square error channel estimation algorithm by jointly utilizing the IBI avoidance/cancelation and subspace techniques. Simulation results verify that the proposed algorithm solves the MSE floor problem, and, hence, improves the bit error rate convergence performance in realistic IBI channels including the effect of pulse shaping filters.

BER Analysis of WFRFT Precoded OFDM and GFDM Waveforms With an Integer Time Offset

In this paper, we investigate bit error rate (BER) performances of weighted-type fractional Fourier transform (WFRFT) precoded orthogonal frequency-division multiplexing (OFDM) and generalized frequency-division multiplexing (GFDM) waveforms with an integer time offset (TO) over additive white Gaussian noise and fading channels. First of all, theoretical BER expressions of hybrid carrier systems with TO are derived according to the linear combination characteristic of WFRFT. In addition, the fusion mechanism is also employed to calculate noise enhancement factor (NEF) of WFRFT precoded GFDM waveforms, and then analytical BER expressions are derived, including the cases with TO. Basically, analytical BER expressions of WFRFT precoded OFDM and GFDM waveforms are under the same framework, and BER of the latter could be simplified to the former when GFDM systems are reduced to OFDM systems. Through WFRFT precoding, the BER performance of GFDM waveforms could be improved by about 2 dB over fading channels with timing errors. Furthermore, using derived NEF of WFRFT precoded GFDM waveforms, theoretical BER expressions could be easily extended to space-time coded systems. Finally, when interblock interference and intercarrier interference are generated due to insufficient cyclic prefix and timing errors, BER performances of WFRFT precoded OFDM and GFDM waveforms also surpass their two special cases, i.e., nonprecoded and discrete Fourier transform precoded ones.

Spread-Spectrum Method Using Multiple Sequences for Underwater Acoustic Communications

Direct-sequence spread-spectrum-based approaches have been adopted in underwater acoustic communications. In this paper, multiple sequences utilizing the periodic correlation characteristics of an original spreading waveform are superimposed, modulating multiple distinct symbols simultaneously within each block at the transmitter. Among those superimposed symbols, one is used as the pilot for channel estimation on chip level and others carry data. Similar to existing superposition schemes, the proposed approach also has low receiver complexity, only requiring matched filtering operations. However, the innovation of the proposed approach is perfect interference suppression, cyclic prefix (CP) is inserted to eliminate the interblock interference totally. Combined CP operation leading to an equivalent cyclic channel matrix with circularly shifted versions of spreading sequences, interferences can be solved considerably. The proposed technique is evaluated with both simulation and experimental data analysis. Data processing gives a brief discussion about low probability of detection and low probability of interception.

Interference Analysis and Mitigation for Time-Asynchronous OFDM CoMP Systems

One of the major challenges, which prevents the coordinated multipoint (CoMP) communications concept from being widely deployed in new cellular systems, is timing synchronization. In order to achieve the gains promised by CoMP systems, the user equipment’s signals in uplink (UL) or the base stations’ signals in downlink should be synchronized, such that the time difference of arrivals do not exceed the cyclic prefix length of the transmitted signals; otherwise, the system suffers from unavoidable integer time offsets. These offsets lead to asynchronous interference in terms of inter-carrier, inter-block, and inter-symbol interference. This limits the percentage of area within a cell which can be covered by cooperation and imposes an upper bound on this area. In this paper, we investigate this problem in the UL, and by using geometrical and semi-analytical approaches, we define this upper bound. Also, by characterizing an accurate mathematical model for the asynchronous interference in Rayleigh fading channels, we were able to employ a standard MMSE-based receiver that mitigates this interference. Furthermore, a typical joint channel and delay estimation block is incorporated into the receiver to examine its performance with estimation errors.