Remove Intersymbol Interference Research Articles

A 15 Gb/s Non-Return-to-Zero Transmitter With 1-Tap Pre-Emphasis Feed-Forward Equalizer for Low-Power Ground Terminated Memory Interfaces

This brief presents a 1-tap pre-emphasis transmitter (TX) for a single-ended ground-terminated memory interface with 28 nm complementary metal-oxide-semiconductor (CMOS) technology. By employing a charge pump scheme, a voltage level below ground was used to remove inter-symbol interference (ISI). Encoded with the unit interval (UI) delayed data, the proposed equalization technique increases the vertical voltage margin for the receiver (RX) compared to conventional feed-forward equalization (FFE). In addition, the short current after equalization was removed, and impedance matching for reflection was facilitated. The data rate of the proposed work is 15 Gb/s, and the data path power dissipation for the entire design is 20.3 mW. The measured energy efficiency is 1.35 pJ/bit, and the total area occupation is 0.008 mm².

A 6-Gb/s Inductively-Powered Non-Contact Connector With Rotatable Transmission Line Coupler and Interface Bridge IC

A 6 Gb/s 9.8 pJ/b rotatable non-contact connector applicable to industrial robot arms is developed. The proposed non-contact connector enables a high-speed and highly reliable wireless data connection between three-dimensional (3-D) cameras and an edge computer in an autonomously controlled robot. A rotatable transmission line coupler (RTLC) is proposed to accommodate the rotational operation of the robot arm. Since the proposed doughnut-shaped RTLC has a constant overlapped area at any rotation angle, the RTLC has a constant coupling gain at all rotation angles. To remove inter-symbol interference (ISI) occurring at a specific rotation angle, a pulse-equalizing receiver is also developed. An interface bridge IC is proposed to transfer a wide range of interface signals from a slow-control signal such as the control area network (CAN) to a high-speed video interface signal. A signal protocol conversion technique applying on-off keying (OOK) modulation is also developed in order to transfer slow legacy interfaces used in the robot arms that include a dc component. Experiments with a test chip fabricated in a 40 nm CMOS process confirmed that bit error rate (BER) was lower than 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-12}$ </tex-math></inline-formula> at any rotation angle in a communication distance of 3 mm. The proposed system improves energy efficiency by a factor of 3.7, area efficiency by a factor of 1.8.

Widely-Linear Nyquist Criteria for DFT-Spread OFDM of Constellation-Rotated PAM Symbols

In this paper, the Nyquist criteria for ordinary linear modulation are generalized to the discrete-Fourier transform-spread (DFT-spread) orthogonal frequency-division multiplexing (OFDM) of constellation-rotated PAM symbols. Unlike the conventional Nyquist criteria with a linear receiver, the new criteria are to remove intersymbol interference in the output of a widely-linear (WL) receiver. It is shown that not only the folded spectrum, but also the shifted and reverse-folded spectrum is crucial in the criteria, where the shift offset is determined by the rotation angle. As a design example, combinations of a constellation rotation angle and a square-root raised cosine pulse are proposed. It is shown that the proposed design can achieve better performance both in spectral efficiency and in peak-to-average power ratio than the conventional DFT-spread OFDM of π/2-BPSK symbols. The optimal one-tap frequency-domain equalizer is also derived under the WL zero-forcing (ZF) constraint. It is shown that, when the rotation angle is properly chosen, the proposed one-tap WL-ZF equalizer significantly outperforms the one-tap linear ZF equalizer in output signal-to-noise ratio. The proposed equalizer also outperforms the conventional equalizer in error vector magnitude measured at the output of a nonlinear power amplifier.

Spectral efficiency enhancement using an antenna‐based orthogonal frequency division multiaccess technique

An approach to enhance antenna spectral efficiency is proposed based on combining spacetime electromagnetic (EM) models of Tx/Rx antennas with orthogonal frequency division multiaccess (OFDM), leading to the EM-OFDM, a technology capable of removing intersymbol interference (ISI) in high-data rate communication links caused by the EM-induced distortion antenna effects. The proposed approach differs from traditional OFDM in wireless communication in several aspects. First, the technique suggests a new decoupling approach by treating each given antenna transreceive device pair as a “stand-alone channel” with its own distortion mechanisms considered separately from the propagation channel. Moreover, the deterministic distortion caused by the nonflat pure antenna EM filtration effects is exploited to carefully design a specialized OFDM transmission techniques based on the antenna parameters, not the multipath fading channels often invoked in conventional uses of OFDM methods. (The EM-OFDM, however, can be combined with a traditional OFDM later if fading channels are present.) In this manner, a more efficient implementation of the wireless link equalization strategy may be enacted since the EM antenna origin of ISI is very different from the traditional propagation channel one. As a proof of concept, the proposed EM-OFDM method is implemented for a single-input-single-output link comprised of half-wavelength linear wire antennas. A careful use of finite difference time-domain to provide EM data allowed the construction of 64 decoupled “pure antenna OFDM subchannels.” Simulation results suggests that the antenna-based OFDM system is capable of completely neutralizing all ISI effects caused by the limited antenna matching bandwidth of the transreceive wires, therefore, supporting considerably higher data rates with low symbol error rate (SER). A concrete evaluation of the SER using quadrature phase-shift keying (QPSK) digital carrier modulation resulted in an increase of the effective antenna digital communication spectral efficiency by ratios up to 300%. Moreover, the EM-OFDM error rate was found to be close to the ideal QPSK level or the maximum possible theoretical limit. Thus, combining detailed EM knowledge with standard signal processing methods can lead to considerable improvement in system design without modifying the antenna physical layout. The proposed approach is expected to play a role in the forthcoming 5G/6G and millimetre wave technology systems currently under development where there is a trend toward integration of EM and digital signal processing at the physical layer level.

OTFS-NOMA: An Efficient Approach for Exploiting Heterogenous User Mobility Profiles

This paper considers a challenging communication scenario, in which users have heterogenous mobility profiles, e.g., some users are moving at high speeds and some users are static. A new non-orthogonal multiple-access (NOMA) transmission protocol that incorporates orthogonal time frequency space (OTFS) modulation is proposed. Thereby, users with different mobility profiles are grouped together for the implementation of NOMA. The proposed OTFS-NOMA protocol is shown to be applicable to both uplink and downlink transmission, where sophisticated transmit and receive strategies are developed to remove inter-symbol interference and harvest both multi-path and multi-user diversity. Analytical results demonstrate that both the high-mobility and the low-mobility users benefit from the application of OTFS-NOMA. In particular, the use of NOMA allows the spreading of the high-mobility users’ signals over a large amount of time-frequency resources, which enhances the OTFS resolution and improves the detection reliability. In addition, OTFS-NOMA ensures that low-mobility users have access to bandwidth resources which in conventional OTFS-orthogonal multiple access (OTFS-OMA) would be solely occupied by the high-mobility users. Thus, OTFS-NOMA improves the spectral efficiency and reduces latency.

A PSO-Based Hybrid Adaptive Equalization Algorithm for Asynchronous Cooperative Communications

This paper proposes an adaptive equalization algorithm for asynchronous cooperative communications in ad hoc networks, where amplify-and-forward relays are adopted, each of which is equipped with single antenna. Adaptive equalization technique is carried out at the destination node in this paper to remove inter-symbol interference, which is caused by the retransmissions of the asynchronous relays. Least mean squares (LMS) has been regarded as an effective adaptive method, but it has difficulty in obtaining the optimal solution. In this paper, we present a hybrid adaptive scheme by combining particle swarm optimization (PSO) with conventional LMS algorithm, where PSO is utilized to search the optimal solution during the iterative process, and LMS is employed to avoid the local convergence, which is usually caused in PSO. Numerical simulation results show that, the proposed scheme outperforms conventional LMS algorithm in convergence performance over Rayleigh flat fading channel, and meanwhile, a signal–noise-ratio gain of 6 dB or so is obtained when BER is 10−3.

Near-Optimal Detection of Monobit Digitized UWB Signals in the Presence of Noise and Strong Intersymbol Interference

Ultra-wideband (UWB) channels include many multipath components and thereby introduce a strong intersymbol interference (ISI) for high-rate communications. Hence, in such cases, a high-performance low-complexity detection is required to cancel ISI. Since monobit analog-to-digital converters (ADCs) are simple and consume less energy, they are increasingly used in UWB communication systems. However, since monobit digitization is a nonlinear operation, ISI removal after monobit digitization becomes a difficult task. In this paper, a near-optimal two-stage algorithm is proposed to iteratively detect the noise-and-ISI-contaminated monobit digitized UWB signals. Moreover, a novel theoretical framework for bit-error-rate (BER) evaluation is developed which guarantees a real-valued BER and doubles the speed of the numerical calculations by using the conjugate (Hermitian) symmetric property of the characteristic function. Computer simulations have been employed to validate the theoretical analysis and the approximations used therein. Both simulation and theoretical results show that the monobit receiver employing the proposed algorithm only incurs about 0.9 dB performance loss compared to the optimal monobit receiver. Moreover, the resistance of the proposed algorithm to error propagation is validated.

Flip Left-to-Right Approach Based Inverse Tree Interleavers for Unconventional Integrated OFDM-IDMA and SCFDMA-IDMA Systems

Several interleavers have been proposed for conventional interleave division multiple access (CIDMA) systems which provides a mean to control burst errors and to reduce multi user and multiple access interferences. However, CIDMA alone is incapable of completely removing inter-symbol interference and inter-carrier interference problems even in presence of such interleaver. In this paper, a recently developed novel interleaver i.e. ‘flip left–right approach based inverse-tree interleaver’ (FLRITI) or simply ‘inverse tree interleaver’, has been explored for two unconventional integrated interleave division multiple access techniques i.e. single carrier frequency division multiple access cum interleave division multiple access (SCFDMA-IDMA) and orthogonal frequency division multiplexing based interleave division multiple access (OFDM-IDMA). The results and analysis reveal that this unconventional integration of CIDMA with SCFDMA and OFDM techniques in presence of FLRITI improves the overall system performance in terms of bit-error rate, memory footprint and computation complexity. Therefore, it validates the worthiness of FLRITI as a competent interleaver for the communication systems to be used even beyond fourth generation.

Reducing Phase Offset Using Novel Adaptive Cyclic Prefix Based Adaptive Sub Block Correlation for BWA Networks

In this paper, a generic approach has been developed for the reduction of phase offset in BWA networks considering a novel Adaptive Cyclic Prefix (ACP) and Adaptive Sub Block (ASB) correlation computation for both WiMAX and LTE networks. The successive design steps along with practical assumptions and mathematical derivations are incorporated in developing the ACPASB algorithm. In comparison to the conventional Fixed Sub Block (FSB) mechanism, the ACPASB algorithm improves the correlation among the subcarriers by drastic reduction of phase offset, leading to maximum removal of Inter Symbol Interference (ISI). The index for reduction of phase offset is evaluated in terms of increased estimated correlation factor (η) for which the correlation threshold (ηth) has been determined using Pearson Product Moment formula and specific degree of freedom (df). Estimated ηth serves as an indicator of maximum ISI free region. The estimation of phase offset is accomplished by the measurement of Error Vector Magnitude (EVM) and Relative Constellation Error (RCE) with the help of Vector Signal Analyzer (VSA) incorporated in the model of WiMAX/LTE transceiver. The data transmission with such minimum ISI, guarantees the selection of highest order modulation and coding scheme (64-QAM (3/4)) even under severe degradation of the channel in a typical BWA network. It has alsobeen observed that even under severely fading channel, the selection of dynamic sub-block length inherently delivers a substantially good Raw Data Rate (RDR) under 64-QAM (3/4) modulation. Complexity analysis and simulation results show that ACPASB scheme outperforms ACPFSB thereby saving design turnaround time significantly.

수중통신에서 Zero Forcing기법을 이용한 최적의 시공간 부호화 알고리즘

수중통신에서는 다중경로로 인해 심벌간의 상호간섭이 발생하여 시스템의 성능을 저하시킨다. 최근 수중 통신시스템에서는 이러한 열악한 채널환경을 극복하고 전송률을 향상시키기 위하여 등화기 및 채널 부호화와 시공간 부호화 방식의 효율적인 연접 구조가 수중통신에서 MIMO시스템으로 응용되어 연구되고 있다. 수중 MIMO 환경에서 시공간 부호화 방식 중에서는 시공간 블록 부호화 방식과 시공간 격자 부호화 방식이 있으며, 시공간 격자 부호화 방식이 등화기 및 채널 부호화 방식에 최적이다. 따라서 본 논문에서는 다중경로 채널 환경에서 효율적인 전송을 위해 터보 Pi 부호화기를 외부부호로 사용하고, MIMO시스템에서 전송 다이버시티와 부호화이득을 동시에 가지는 시공간 격자부호를 내부부호로 구성한다. 그리고 심볼간 상호 간섭 제거를 위해 Zero Forcing 등화기법을 사용하여 수중채널에서의 시뮬레이션을 통해 성능을 확인하였다. In the underwater communication, the performance of system is reduced because of the inter-symbol interference occur by the multi-path. In the recent years, to deal with poor channel environment and improve the throughput, the efficient concatenated structure of equalization, channel codes and Space Time Codes has been studied as MIMO system in the underwater communication. Space Time Codes include Space Time Block Codes and Space Time Trellis Codes in underwater communication. Space Time Trellis Codes are optimum for equalization and channel codes among the Space Time Codes to apply in the MIMO environment. Therefore, in this paper, turbo pi codes are used for the outer code to efficiently transmit in the multi-path channel environment. The inner codes consist of Space Time Trellis Codes with transmission diversity and coding gain in the MIMO system. And Zero Forcing method is used to remove inter-symbol interference. Finally, the performance of this model is simulated in the underwater channel.

Near‐optimum detection scheme with relay selection technique for asynchronous cooperative relay networks

A new near-optimum detection scheme for asynchronous wireless relay networks is proposed to cancel the interference component at the destination node caused by timing misalignment from the relay nodes. The detection complexity at the destination node as compared with a previous sub-optimum detection scheme is reduced. Closed-loop extended orthogonal space time block coding and outer convolutive coding are utilised to maximise end-to-end performance. A relay selection technique is also proposed in this study for two dual-antenna relay nodes to enhance the system performance by selecting the best links and the smallest time delay error among the relay nodes. Simulation results confirm that the proposed near-optimum detection scheme with relay selection is very effective at removing intersymbol interference at the destination node and achieving full cooperative diversity with unity data transmission rate between the relay nodes and the destination node.

Precursor inter-symbol interference removal by block transmission-based time-reversed equalization

Single-carrier transmission is considered in the general finite impulse response inter-symbol interference (ISI) channel. In an ISI channel with a matched filter, the folded spectrum of the received pulse can be factored into a minimum phase causal part and a maximum phase anticausal part corresponding to the postcursor and precursor ISI, respectively. In this paper, zero-forcing ISI cancellation is considered. In a direct implementation, the precursor equalization is carried out based on truncating and delaying the ideal anticausal precursor equalizer impulse response. In the proposed scheme, a block transmission is adopted, and the precursor equalization is carried out by a time reversal within each block and using a practical minimum phase filter. We show that the ISI can be removed perfectly using the proposed scheme. By means of a numerical example, it is shown that the proposed scheme achieves improved performance compared to the truncate- and delay-based equalizer in terms of transmission rate, delay, and implementation complexity.

Spatial diversity in multichannel processing for underwater acoustic communications

This paper discusses spatial diversity for underwater communications and presents a passive-phase conjugation (PPC) based multichannel equalizer. Multichannel processing involves taking advantage of multiple receivers distributed in space to take advantage of spatial diversity. The multichannel decision feedback equalizer (DFE) uses multiple nonlinear adaptive filters to remove intersymbol interference (ISI), and its complexity increases with time spread plus the number of receiving channels. Passive time reversal realized by the passive-phase conjugation (PPC) approach refocuses time delayed arrivals at the receiving array, where spatial diversity is obtained by the multichannel combining. The property of refocusing can be used to mitigate ISI for underwater communications, and only one single channel equalizer is required to remove residual ISI. PPC achieves pulse compression for time delayed arrivals, where the number of taps is significantly reduced for a PPC based equalizer. Based on temporal diversity obtained by pulse compression, the proposed structure improves the performance of time reversal communications by adaptive diversity combining. Three receiver structures are evaluated by processing real data collected in an experiment, which was conducted in a range-dependent acoustic channel over a range of 4 km. Results have demonstrated superior performance of the proposed receiver structure.

Experimental assessment of a multicarrier underwater acoustic communication system

This paper presents a multicarrier communication system which transmits information on independent subcarriers to achieve an increased data rate. For this system, a passive-phase conjugation (PPC) based receiver structure is assessed by processing data collected in sea trials. Based on temporal diversity (pulse compression) exploited by PPC processing, an adaptive multichannel decision feedback equalizer is used to remove intersymbol interference, where spatial diversity is exploited by adaptive multichannel combination. The digital phase-locked loop (DPLL) technique is implemented for carrier-phase tracking. In the scenario of low input signal-to-noise ratios (SNRs), the receiver structure achieved superior performance using a common DPLL. In a depth-fluctuated environment, two sea experiments were conducted over ranges of 2 km and 4 km, respectively, and this communication system was assessed with a data rate of 4 kbps. In terms of mean square error, output SNR and bit error rate, this receiver structure has demonstrated its performance for the multicarrier communication system.

Blind equalization of MIMO systems based on orthogonal Constant Modulus Algorithm

This paper investigates adaptive blind source separation and equalization for Multiple Input Multiple Output (MIMO) systems. To effectively recover input signals, remove Inter-Symbol Interference (ISI) and suppress Inter-User Interference (IUI), the array input is first transformed into the signal subspace, then with the derived orthogonality between weight vectors of different input signals, a new orthogonal Constant Modulus Algorithm (CMA) is proposed. Computer simulation results illustrate the promising performance of the proposed method. Without channel identification, the proposed method can recover all the system inputs simultaneously and can be adaptive to channel changes without prior knowledge about signals.

A 6.25-Gb/s binary transceiver in 0.13-/spl mu/m CMOS for serial data transmission across high loss legacy backplane channels

A transceiver capable of 6.25-Gb/s data transmission across legacy communications equipment backplanes is described. To achieve a bit error rate (BER) <10/sup -15/, transmit and receive equalization that can compensate up to 20 dB of channel loss is employed to remove intersymbol interference (ISI) resulting from finite channel bandwidth and reflections. The transmit feed-forward equalizer (FFE) uses a four-tap symbol-spaced programmable finite impulse response (FIR) filter followed by a 4-bit digital-to-analog converter (DAC) that drives a 50-/spl Omega/ transmission line. The receiver uses a half-baud-rate adaptive decision feedback equalizer (DFE) that cancels the first four symbol-spaced taps of postcursor ISI without use of speculative techniques. Both the transmitter and receiver use an LC-oscillator-based phase-locked loop (PLL) to provide low jitter clocks. Techniques to minimize the complexity of the FIR and DFE implementations are described. The transceiver is designed to be integrated in a standard ASIC flow in a 0.13-/spl mu/m digital CMOS technology. System measurements indicate the ability to transmit and recover data eyes that have been fully closed due to crosstalk and signal loss.

Analysis of the Multimodulus Blind Equalization Algorithm in QAM Communication Systems

The constant modulus algorithm (CMA) for blind equalization requires a separate carrier-recovery system for phase recovery. This letter mathematically analyzes a modified CMA, called the multimodulus algorithm (MMA), which may perform joint blind equalization and carrier recovery without the need for a separate carrier-recovery system for quadrature amplitude modulation signal constellations. The analyses focus on five aspects of the MMA: 1) derivation of the general formulation of the MMA cost function; 2) stationary points of the MMA; 3) desired global minima of the MMA; 4) unstable equilibria (saddle points) of the MMA; and 5) analytic verification of a discrete-time first-order phase-locked loop hidden inside the MMA, which is absent from the conventional CMA. Analysis results indicate that the MMA alone may be able to remove intersymbol interference and simultaneously correct the phase error, because it implicitly incorporates a phase-tracking loop, which automatically recovers carrier phase.

Frequency-domain interference cancellation and nonlinear equalization for CDMA systems

In wireless broadband communications using code division multiple access (CDMA), interference cancellation (IC) techniques have been proposed to reduce multiuser access interference (MAI); however, their performance is limited by complexity constraints that still favor the use of rake receivers. In this paper, we propose an IC architecture that makes use of a block decision feedback equalizer (DFE) to remove intersymbol interference (ISI) and whose overall complexity is much lower than equivalent (in terms of performance) existing structures. In fact, by making use of a particular block data transmission format, all filters are implemented in the frequency domain (FD), and in particular, the DFE is iteratively designed according to the reliability of the detected data in the previous iteration. Due to the strong interaction of the FD-IC and the block DFE, error propagation in the receiver is reduced by simply interleaving (INT) chips before transmission. Simulations performed for an uplink communication on a wireless multipath channel show that the combination of FD-IC, block DFE, and chip INT provides an efficient solution with good performance for CDMA systems in dispersive channels.

Carrier phase synchronization for GMSK signals

AbstractSeveral carrier phase recovery schemes for Gaussian minimum shift keying (GMSK) modulated signals are derived and analysed in terms of squaring loss and acquisition time.Two bandwidths are considered for the GMSK signal: BTb=0.5 and 0.25, following the recommendations of the Consultative Committee for Space Data Systems. Pre‐encoding is assumed to remove the inherent differential encoding of the GMSK modulations.The carrier phase recovery schemes are derived from the approximate expression of the GMSK signal as an OQPSK signal or as the superposition of two OQPSK signals maximum a posteriori/maximum likelihood phase estimation is considered, with perfect clock reference; both non‐data‐aided and decision–directed schemes are analysed. Moreover, in the case of GMSK with BTb=0.25, an equalizer is also included in the synchronizer loop to remove inter‐symbol interference. High and low signal to noise ratio (SNR) approximations are considered to simplify the structure of the synchronizers.It is shown that, for the considered BTb values, the complex synchronizers initially derived do not provide any improvements over their simplified versions; the introduction of the equalizer for GMSK with BTb=0.25 leads to a better performance. Moreover, it is shown that GMSK with BTb=0.5 has a better performance than unfiltered OQPSK, which is better than GMSK with BTb=0.25. The SNR values above which the high SNR approximation turns out to be convenient with respect to the low SNR approximation are also given. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Complementary filter design for testing of IS-95 code division multiple access wireless communication systems

In this paper, a simple design of a complementary filter is presented for performance measurement of an IS-95 code division multiple access wireless communication system (1995). The complementary filter is required to meet the design requirements, which are specified in IS-97 document (1994). The purpose of employing the complementary filter in test equipment is to remove intersymbol interference introduced by the base station transmit-filter and the transmit-phase equalizer. Five window functions, which do not affect the desirable response at the proper sampling times, are applied to the desirable output. The output of a window Is set to be the output of the complementary filter. Then, the complementary filter is obtained by taking a deconvolution of the output response of the window with the impulse response resulted by cascading the base station transmit-filter and the transmit-equalizer. It is found that the output of the complementary filter with the Kaiser window function meets all requirements specified in IS-97. In addition, it is observed that the Kaiser window has the smallest number of the filter coefficients among all windows considered in this paper. Therefore, the complementary filter with the Kaiser window is recommended for implementation of the IS-95 system measurement.