Inter-block Interference Research Articles

Distributed Space Time Block Transmission and QRD Based Diversity Detector in Asynchronous Cooperative Communications Systems

Based on the zero prefix (ZP) structure, we propose an efficient block transmission algorithm using distributed space-time codes in the asynchronous cooperative communication systems. To accomplish the new frame, ZP is inserted in front of the data block to combat the relative delay between the asynchronous relays, while zero suffix (ZS) is appended at the end of the block to remove the interblock interference caused by the time dispersing channel; so, we employ this ZP–ZS precoding form to make the coded frames synchronous. It should be noted that single carrier transmission and orthogonal frequency division multiplexing (OFDM) are both compatible with the frame. For single carrier transmission, conventional space-time equalizers can be investigated at the destination node to cancel the multipath interference. To further improve the performance, an improved QR decomposition-based recursive interference cancellation (QRD-RIC) detector is employed, where joint QRD is adopted on the combination of channel matrices from different relays, and then spatial-and-multipath diversity combinations are introduced and improved into the proposed system, where the spatial and multipath diversities are acquired. For OFDM, the fractionally spaced frequency-domain MMSE diversity receiver is employed, since the guard interval in our framing scheme implies oversampling in the frequency domain. Numerical simulations show that the proposed algorithms are effective for the asynchronous transmissions under frequency selective fading channels.

A Compressed Sensing Approach to Block-Iterative Equalizers

The universality of underdetermined systems has nurtured a variety of novel compressed sensing (CS) algorithms that ingeniously exploit data sparsity. Whereas well-studied greedy and iterative threshold-based CS recursions take the form of an adaptive filter followed by a proximal operator, this is no different in spirit from the role of block iterative equalizers , where structure is roughly exploited by the signal constellation slicer. By capitalizing on the intrinsic sparsity of signal modulations, we approach the concept of interblock interference (IBI) more proficiently in light of CS concepts, where the optimal feedback of detected symbols is devised adaptively. This should be contrasted with standard forms of IBI estimation/cancellation commonly seen in block DFEs, where detection is restricted to a contiguous set of entries within the transmitted vector. A significant consequence of the latter is that, while block transceivers commonly employ some form of redundancy that accounts for IBI, a CS algorithm applied to the same transmitted vector may require no redundancy whatsoever, and is capable of retrieving both target symbol and IBI altogether. The CS-based iterative DFE acts as a more efficient re-estimation procedure, proposed under recursive-least-squares based adaptations. Besides maximizing system throughput, the new algorithms exhibit significantly higher performance when compared to existing methods that focus solely on minimized redundancy. Simulations in several scenarios illustrate the merits of the unified approaches.

Hybrid Micro Genetic Algorithm Assisted Optimum Detector for Multi-Carrier Systems

<p>A low-complexity detection scheme, which consists of a Hybrid Micro Genetic Algorithm (Hybrid- µGA), is proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems. In the absence of orthogonality, intercarrier-interference (ICI) occurs because a signal from one subcarrier causes interference to others. In several environment, the OFDM signal reflections from a far obstacle generate inter-block-interference (IBI) due to long time delays. To avoid these unpleasant effects of IBI and ICI in OFDM system, a Hybrid-µGA detection algorithm is proposed. The proposed detector combines the conventional one-Tap equalizer and the Micro Genetic Algorithm (µGA) search engine. The output of one-Tap equalizer is considered as the input to µGA search engine. Therefore, the µGA starts with some knowledge rather than blindly to speed up the search. Theoretical analysis and simulation results show that the proposed detection Hybrid- µGA scheme substantially improves the performance of OFDM systems. Moreover, its complexity is 10 times lower than the conventional GA.</p>

Sparse Channel Estimation of Underwater TDS-OFDM System Using Look-Ahead Backtracking Orthogonal Matching Pursuit

Time division synchronization orthogonal frequency division multiplexing (TDS-OFDM) has been attractive due to its fast synchronization and efficient spectral efficiency over conventional cyclic prefix orthogonal frequency division multiplexing (OFDM) and zero padding OFDM. However, inter-block interference (IBI) affects its performance because of delay over multipath channels. To evade IBI, dual pseudo-random noise (DPN) sequences have been introduced that causes to reduce spectral and energy efficiency. But, DPN is unprepared for underwater acoustic (UWA) communication because of battery-based nature and limited bandwidth. To overcome these issues, this paper exploits compressive sensing theory algorithm for obtaining the time-varying channel state information by utilizing sparse property of UWA channels. In this paper, the IBI free region is utilized to estimate accurate UWA channel impulse response and mitigate its interference. Look-ahead backtracking orthogonal matching pursuit-based sparse channel estimation technique is proposed for underwater TDS-OFDM in a real sparse time-varying multipath channel (channel taps are randomly distributed). Furthermore, Doppler-shift of UWA channel is estimated and compensated by PN sequence in time domain. The performance of the proposed technique is evaluated and demonstrated through numerical computation of bit error rate (BER) and mean square error (MSE) using Monte Carlo iterations. Simulation analysis confirms the superiority of the proposed scheme not only in terms of BER and MSE over the conventional ones but also achieve high energy and spectral efficiency.

Circularly Pulse-Shaped Precoding for OFDM: A New Waveform and Its Optimization Design for 5G New Radio

A new circularly pulse-shaped (CPS) precoding orthogonal frequency division multiplexing (OFDM) waveform, or CPS-OFDM for short, is proposed in this paper. CPS-OFDM, characterized by user-specific precoder flexibility, possesses the advantages of both low out-of-subband emission (OSBE) and low peak-to-average power ratio (PAPR), which are two major desired physical layer signal properties for various scenarios in 5G New Radio (NR), including fragmented spectrum access, new types of user equipments (UEs), and communications at high carrier frequencies. As opposed to most of existing waveform candidates using windowing or filtering techniques, CPS-OFDM prevents block extension that causes extra inter-block interference (IBI) and envelope fluctuation unfriendly to signal reception and power amplifier (PA) efficiency, respectively. An optimization problem of the prototype shaping vector built in the CPS precoder is formulated to minimize the variance of instantaneous power (VIP) with controllable OSBE power (OSBEP) and noise enhancement penalty (NEP). In order to solve the optimization problem involving a quartic objective function, the majorization-minimization (MM) algorithmic framework is exploited. By proving the convexity of the proposed problem, the globally optimal solution invariant of coming data is guaranteed to be attained via numbers of iterations. Simulation results demonstrate the advantages of the proposed scheme in terms of detection reliability and spectral efficiency for practical 5G cases such as asynchronous transmissions and mixed numerologies.

Prefix-Free Frequency Domain Equalization for Underwater Acoustic Single Carrier Transmissions

Prefixes, in forms of cyclic-prefixes or zero-padding, have often been viewed as indispensable in underwater acoustic single-carrier transmissions to avoid inter-block interference (IBI) and to formulate circular convolution. However, the prefix introduces overhead and reduces spectral efficiency. This is especially true in fast time-varying channel conditions. Here, we propose a prefix-free scheme to facilitate frequency-domain equalization (FDE) in an underwater acoustic communication system, based on the time-reversal processing. In the proposed scheme, three main procedures, block partitioning, IBI cancellation, and prefix reconstruction, precede the FDE operation. The block partitioning at the receiver provides flexibility to support channel equalization in different channel fluctuation rates. The utilized prefix reconstruction scheme requires a strong first arrival to minimize the noise enhancement. Our solution is to use the time-reversal processing, because the resultant equivalent impulse response, also known as the $q$ -function, has a stable and compact peak. To further enhance receiver performance, we incorporate two strategies. One is overlapping partitioning. The other is iterative prefix reconstruction. The proposed schemes have been tested using the field measurements obtained from the Gulf of Mexico in August 2016. Communications over four ranges at the carrier frequency of 85 kHz with a symbol rate of 17 kHz have been demonstrated. The overlapping partitioning and iterative prefix reconstruction strategies have been shown to generate improvements in the receiver performance.

Jointly Optimal Pre- and Post-Channel Equalization and Distributed Beamforming in Asynchronous Bidirectional Relay Networks

We consider an asynchronous bi-directional relay network (consisting of two single transceivers and multiple single-antenna relays) where the transceiver-relay paths are subject to different relaying and/or propagation delays. In such a network, the end-to-end link can be viewed as a multipath channel which can cause intersymbol-interference (ISI) in the signals received by the two transceivers. Assuming a block transmission/reception scheme, we consider both pre- and post-channel equalization at both transceivers to combat the interblock-interference (IBI) induced due to ISI. Considering amplify-and-forward relays, we study the problem of optimal design of pre- and post-channel linear equalizers and optimal power loading at the two transceivers as well as optimal network beamforming. To do so, assuming a limited total transmit power budget, we minimize the total mean square error of the linearly estimated signals at both transceivers by optimally obtaining the transceivers’ powers and relay beamforming weights as well as block pre- and post-channel linear equalizers at the two transceivers. We prove rigorously that this minimization leads to a certain relay selection scheme, where only a subset of the relays will be turned on. We also provide semiclosed form solutions to the design parameters.

Removal of ICI and IBI in Wireless Heterogeneous Networks With Timing Misalignment

Since a small cell base station (SBS) of an orthogonal frequency division multiple access heterogeneous network (HetNet) does not intend to align arriving macrocell user signals, inter-carrier interference (ICI) and inter-block interference (IBI) inevitably arise in the uplink demodulation at the SBS. This letter proposes a novel precoding scheme based on interference alignment to remove the uplink ICI and IBI in a HetNet completely. The proposed scheme is able to remove the ceiling effect on the small cell uplink rate and increase the degrees of freedom of the system. Simulation results demonstrate the superiority of the scheme.

Analysis of Uplink ICI and IBI in Heterogeneous Cellular Networks With Multiple Macrocells

This letter analyzes the effects of inter-carrier interference (ICI) and inter-block interference (IBI) on uplink reception of small cells (SCs) in orthogonal frequency division multiple access heterogeneous cellular networks. Tight upper and lower bounds of the power of ICI and IBI plus co-channel interference from nearby SCs, average rate, and symbol error rate (SER) per SC subcarrier are derived. Results show that the theoretical analysis is close to simulation. Importantly, the average rate and SER are seriously degraded by the interference at high signal-to-noise ratio.

Mutually Referenced Channel Shortening

This letter proposes a blind channel shortening method for inter-block interference (IBI) suppression in orthogonal frequency division multiplexing (OFDM) systems. In a receiver, there are two OFDM demodulators each with a time-domain equalizer (TEQ) and a frequency-domain equalizer (FEQ). The TEQ parameters are determined such that the outputs of an FEQ are identical to those of another FEQ. It is shown that the proposed method can suppress IBI completely and eliminate the need for channel estimation, which is required by conventional methods.

A Circulated Block Transmission Scheme for FTN Signaling

Fast-Than-Nyquist (FTN) transmission is a promising method to improve the spectrum efficiency for future wireless communication systems. However, this benefit of FTN is at the price of inducing the inter-symbol interference (ISI), which increases the complexity of the receiver. In this paper, a circulated block transmission scheme for FTN signaling, i.e. CB-FTN system is proposed. The detail implementation structure of CB-FTN transceiver is presented, in which the ISI caused by FTN transmission is canceled by the frequency-domain equalization (FDE), and the inter-block interference (IBI) caused by the multi-path channel is overcome by the cyclic-prefix. The postprocessing signal to noise ratio (pSNR) is analyzed for the CB-FTN receiver with zero-forcing FDE in AWGN channel, which is verified by the simulation results. Moreover, the BER performances and computational complexity of CB-FTN system are compared with the existed scheme.

Performance Enhancement of Faster-Than-Nyquist Signaling Based Single-Carrier Frequency-Domain Equalization Systems

This paper provides a method that appends and removes efficiently the cyclic prefix and the cyclic suffix by means of an inter-block interference elimination for a frequency domain equalization (FDE) in faster-than-Nyquist (FTN) signaling schemes. Furthermore, we propose various transmission schemes to integrate such an FTN signaling in the single-carrier FDE system. We propose FTN transmission schemes combined with various FDE techniques such as a post-equalization, a pre-equalization, and a balanced equalization in the FTN-SC-FDE system. We prove that the proposed transmission schemes yield improved performances by deriving the instantaneous bit error rate and error probability. Finally, we evaluate the analytical results and the simulation results of the performance of various FTN-SC-FDE schemes. Our proposed schemes can be applied to enhance the spectral efficiency of the satellite broadcasting systems like DVB-S2/S2X as well as the return channels of interactive broadcasting systems.

New Designs for Reduced-Redundancy Transceivers

Block-based transceivers are widely used in wireless communication systems, mostly due to their well-defined structure and blockwise encoding. Interblock interference caused by superpositions of delayed signal copies is among their main physical-layer challenges, which is commonly eliminated with redundancy between adjacent data-blocks. Besides, channel equalization is also employed to further mitigate multipath channel interferences. The chosen amount of redundancy, however, may be overestimated, which opens an opportunity for reduced-redundancy superfast transceivers, whose key features include high spectral efficiency and low computational cost. Although superfast approaches aim at low complexity, proper equalizer-coefficient design and update procedures are still very intricate tasks, and most approaches are rather computationally cumbersome. This paper addresses such problem by proposing a new design strategy for reduced-redundancy block-based transceivers, which provides semi-blind equalization with data-selective update, in addition to the possibility of using redundant parallel equalizer structures, which are based on the use of fast Fourier transforms and one-tap equalizers. Simulation results indicate that the proposed methodology is effective and also able to achieve a performance comparable to what is obtained with state-of-the-art techniques, depending on the target application.

Mitigation of Uplink ICI and IBI in OFDMA Two-Tier Networks

For two-tier networks using orthogonal frequency-division multiple access (OFDMA) over frequency-selective channels, macrocell-user (MU) signals generally arrive at small-cell base stations (SBSs) with different delays and can hardly be synchronized in time, causing intercarrier interference (ICI) and interblock interference (IBI) in OFDM demodulation. The effects on small-cell uplink capacity by ICI and IBI and their mitigation have not been well addressed in the literature. In this paper, the ICI and IBI of OFDMA two-tier networks with fractional power control (FPC) in multipath fading channels are formulated, and closed-form expressions of their covariance matrices at the SBS with random MU locations are derived. Simulation results verify the validity of the theoretical analysis. It is shown that small-cell uplink capacity is substantially degraded in the presence of ICI and IBI. A precoding method for small cells for mitigating the effects of ICI and IBI on uplink capacity is proposed. In the method, the received signals over small-cell subcarriers at the SBS are decorrelated jointly by small-cell user (SU) precoders and an equalizer at the SBS. Closed-form design procedures for precoding of different subcarrier sizes to maximize uplink capacity under different information feedback are developed. Simulation results demonstrate that the proposed method for using all the subcarriers for precoding has a large capacity gain over that of no small-cell precoding. Moreover, the method can perform very well, even if MU locations are the only information fed back from the macrocell.

Pre-Channel Equalization and Distributed Beamforming in Asynchronous Single-Carrier Bi-Directional Relay Networks

We consider a single-carrier asynchronous two-way amplify-and-forward relay network, where two single-antenna transceivers exchange information with the help of several single-antenna relay nodes. We assume that the propagation delay of each relaying path, originating from one transceiver, going through a certain relay, and ending at the other transceiver, can be different from those of the other relaying paths. This assumption turns the end-to-end link into a multi-path channel, which produces inter-symbol-interference at the transceivers. In a block transmission/reception scheme, ISI results in inter-block-interference (IBI) between successive transmitted blocks. To combat IBI, cyclic prefix insertion and deletion as well as pre-channel block equalization are used at the two transceivers. Assuming a limited total transmit power budget, we minimize the total mean squared error between the transmitted and received signals at both transceivers by optimally obtaining the transceivers’ transmit powers and the relay beamforming weight vector as well as the pre-channel block equalizers at the transceivers. We prove that this optimization problem leads to a relay selection scheme, where only the relays contributing to one tap of the end-to-end channel impulse response, are turned on and the remaining relays are switched off. We present an efficient method to obtain the optimal values of the design parameters. Our simulation results show that compared to post-channel equalization, the proposed pre-channel equalization technique has the same performance as the post-channel equalization approach does, when the total available power is relatively low compared to the noise power at the transceivers, while offering receiver simplicity.

Iterative Identification of Sparse Mobile Channels for TDS-OFDM Systems

Spectral efficiency improvement is essential for mobile systems development in the light of growing demand for high data transmission rate. The Time-domain synchronous OFDM transmission system increases bandwidth utilization by time-domain channel estimation without any support from frequency-domain pilot signals. Channel identification methods use a training sequence, added at the beginning of every OFDM symbol. Sparse nature of the channel impulse response, which manifests itself during wireless broadband transmission, enables one to implement compressed sensing techniques for channel identification. Channel sparsity reduces the number of measurement samples required in channel reconstruction procedure, but they should be taken from the inter-block interference (IBI) free part of the received training sequence. If maximum delay spread of the channel is comparable with the length of the training sequence, the number of measurements within the IBI-free region may be insufficient for accurate channel identification. This paper discusses a new iterative approach to sparse channels estimation, which uses the measurements from outside the IBI-free region. The proposed solution is examined for simulated transmission over randomly generated doubly-selective channels when compared with the conventional CS technique and perfect channel state information condition. The experiments reveal an enhancement of the transmission quality in terms of SER for slow-time selective channels when both delays and gains of the channel impulse response are estimated with the orthogonal matching pursuit algorithm. Further improvement, also for transmission over moderate varying channels, is observed when the path delays are found in advance. A suitable frame structure is proposed to do so.

Structured compressive sensing‐based non‐orthogonal time‐domain training channel state information acquisition for multiple input multiple output systems

In practical multiple input multiple output (MIMO) systems, accurate knowledge of the channel state information (CSI) is a prerequisite to guarantee the system performance. The conventional CSI acquisition methods for MIMO system usually rely on the orthogonal (either time- or frequency-domain) training sequences (TSs) to estimate the channel associated with each transmit–receive antenna pair, which is not spectrally efficient. This study proposes a non-orthogonal time-domain training-based CSI acquisition approach for MIMO systems under the framework of structured compressive sensing. By exploiting the spatial–temporal correlations of the sparse MIMO channels, a spatially–temporally spARsity-adaptivE-simultaneous orthogonal matching pursuit algorithm is proposed, which could use the inter-block interference free region of very small dimension within the received TS to recover the multiple channels. Furthermore, the proposed algorithm could utilise the priori channel partial common support to improve the recovery probability and reduce the complexity. Simulation results show that the proposed scheme has better performance and higher spectral efficiency than the conventional MIMO schemes, which might be an appealing solution for the future wireless communications.

BER-Optimized Precoders for OFDM Systems With Insufficient Cyclic Prefix

This letter considers precoder design for orthogonal frequency division multiplexing (OFDM) systems with insufficient cyclic prefix (CP). Interference alignment is utilized to formulate the precoder design problem as a bit error rate (BER) minimization problem subject to a total power constraint. Subsequently, a novel closed-form solution is derived for the optimal precoder by exploiting Schur convexity and interference alignment-based interblock interference (IBI) suppression. The resulting precoder is also shown to be mean-square-error (MSE) optimal, and generalizes the existing designs. Closed-form expressions are also developed for quantifying the signal-to-noise ratio (SNR) gain of the proposed design over existing designs for practical channels with a decaying power-delay profile. Simulation results corroborate the performance improvements over existing MSE optimal designs considering the availability of both full and partial channel state information at the transmitter (CSIT) for various power delay profiles.

Achievable Rate Region for Energy Harvesting Asynchronous Two-Way Relay Networks

In this paper, we consider an asynchronous two-way relay network where two single-antenna transceivers are using a single carrier communication scheme to exchange information and incur different delays. It is assumed that the relay nodes can harvest energy from their surrounding environment and utilize this harvested energy to forward their received messages using harvest-then-forward protocol. Since the transceiver-relay paths are subject to different propagation delays, the end-to-end channel can be viewed as a multi-path channel, which can cause inter-block-interference (IBI) in the signal blocks received by the two transceivers. In order to avoid such an IBI, we deploy a relay selection scheme where only those relay nodes are selected that contribute to a single tap end-to-end channel. Our goal is to maximize the sum rate of such a network subject to individual and total power constraints as well as energy harvesting profiles of the relays in order to obtain the maximum achievable rate region. Numerical results show that for a relay selection scheme and considering specific energy harvesting profiles for the relay nodes and under the condition that the total transmit budget of the transceivers are limited, the so obtained achievable rate region is the union of the rate region for each individual tap of the end-to-end channel.

FBMC System: An Insight into Doubly Dispersive Channel Impact

It has been claimed that filter bank multicarrier (FBMC) systems suffer from negligible performance loss caused by moderate dispersive channels in the absence of guard time protection between symbols. However, a theoretical and systematic explanation/analysis for the statement is missing in the literature to date. In this paper, based on one-tap minimum mean square error (MMSE) and zero-forcing (ZF) channel equalizations, the impact of doubly dispersive channel on the performance of FBMC systems is analyzed in terms of mean square error of received symbols. Based on this analytical framework, we prove that the circular convolution property between symbols and the corresponding channel coefficients in the frequency domain holds loosely with a set of inaccuracies. To facilitate analysis, we first model the FBMC system in a vector/matrix form and derive the estimated symbols as a sum of desired signal, noise, intersymbol interference (ISI), intercarrier interference (ICI), interblock interference (IBI), and estimation bias in the MMSE equalizer. Those terms are derived one-by-one and expressed as a function of channel parameters. The numerical results reveal that under harsh channel conditions, e.g., with large Doppler spread or channel delay spread, the FBMC system performance may be severely deteriorated and error floor will occur.