Effects Of Intersymbol Interference Research Articles

Bit error rates for ultrafast APD based optical receivers: exact and large deviation based asymptotic approaches

Exact analysis as well as asymptotic analysis, based on large-deviation theory (LDT), are developed to compute the bit-error rate (BER) for ultrafast avalanche-photodiode (APD) based optical receivers assuming on-off keying and direct detection. The effects of intersymbol interference (ISI), resulting from the APD's stochastic avalanche buildup time, as well as the APD's dead space are both included in the analysis. ISI becomes a limiting factor as the transmission rate approaches the detector's bandwidth, in which case the bit duration becomes comparable to APD's avalanche buildup time. Further, the effect of dead space becomes significant in high-speed APDs that employ thin avalanche multiplication regions. While the exact BER analysis at the generality considered here has not been reported heretofore, the asymptotic analysis is a major generalization of that developed by Letaief and Sadowsky [IEEE Trans. Inform. Theory, vol. 38, 1992], in which the LDT was used to estimate the BER assuming APDs with an instantaneous response (negligible avalanche buildup time) and no dead space. These results are compared with those obtained using the common Gaussian approximation approach showing the inadequacy of the Guassian approximation when ISI noise has strong presence.

Performance Analysis of Decision Feedback and Linear Equalization schemes for Non-Directed Indoor Optical Wireless Systems

Indoor optical wireless systems provide an attractive alternative for realizing next generation Wireless Local Area Networks (WLANs). In this paper, the potential of non-directed, equalized optical wireless systems is theoretically investigated, taking into account the indoor channel impulse response and the characteristics of ambient light noise and thermal noise at the receiver. Three modulation schemes, Pulse-Position-Modulation, On-Off Keying and Pulse Amplitude Modulation, are combined with appropriate equalization methods in order to mitigate the effect of intersymbol interference induced by the infrared chanel. It is shown that the various non-directed configurations can provide data rates of the order of 100Mb/s and beyond, over a medium sized room.

Optimization of InP APDs for High-Speed Lightwave Systems

Calculations based on a rigorous analytical model are carried out to optimize the width of the indium phosphide avalanche region in high-speed direct-detection avalanche photodiode-based optical receivers. The model includes the effects of intersymbol interference (ISI), tunneling current, avalanche noise, and its correlation with the stochastic avalanche duration, as well as dead space. A minimum receiver sensitivity of -28 dBm is predicted at an optimal width of 0.18 mum and an optimal gain of approximately 13, for a 10 Gb/s communication system, assuming a Johnson noise level of 629 noise electrons per bit. The interplay among the factors controlling the optimum sensitivity is confirmed. Results show that for a given transmission speed, as the device width decreases below an optimum value, increased tunneling current outweighs avalanche noise reduction due to dead space, resulting in an increase in receiver sensitivity. As the device width increases above its optimum value, the receiver sensitivity increases as device bandwidth decreases, causing ISI to dominate avalanche noise and tunneling current shot noise.

Bit-error-rate analysis of UWB radio using BPSK modulation over inter-chip radio channels for wireless chip area networks

Wireless chip area networks (WCAN) signify a new development in wireless communications, where wireless interconnections among different cores within a chip (intrachip) or among different chips within a module (inter-chip) can be realized. In this paper, we analyze bit-error rate (BER) of an ultra-wideband (UWB) radio with binary phase-shift keying (BPSK) modulation over an inter-chip wireless radio channel. Specifically, a novel technique of dynamically shifting an integral window is proposed to reduce the effects of inter-symbol interference (ISI). BER expressions are analytically derived and verified by Monte Carlo simulations. Furthermore, by including both thermal and switching noise as the dominant noise sources, it is found that a BER less than 10-6 is feasible for a link distance up to 252 mm at a data rate of up to 650 Mbps. Besides, the link margin analysis shows that a link margin of 28.22 dB could be obtained for the same distance and data rate.

High-Speed Single-Ended Parallel Link Based on Three-Level Differential Encoding

An encoding scheme for high-speed single-ended parallel transceiver system is presented. Compared to the 50% I/O pin utilization of the conventional differential encoding, the proposed system employs 3-level differential coding to increase the utilization to 75% and 93% using a group of four and six conductors, respectively. The proposed coding scheme also reduces the effects of inter-symbol interference (ISI), removes reference ambiguity, and reduces power line fluctuations at the transmitter side. Using simple encoder/decoder, the proposed scheme enables multiple drivers at the transmitter to recycle the same current, reducing power consumption. To validate the proposed system, a parallel link was designed in 0.18 mum CMOS process. The chip implements the coding algorithm over four conductors and achieves a data rate of 4.2 Gb/s/pin while dissipating 17.1 mW/Gb/s.

Analytical study on the effect of cochannel interference on partially coherent diversity systems

In this paper, we study the effect of cochannel interference (CCI) on the performance of partially coherent BPSK and QPSK in uncorrelated L-branch equal-gain combining systems. We consider a generalized propagation model wherein the desired and interfering signals undergo Nakagami-m or Rician fading with different amounts of fading severity. Further, the interfering signals are assumed to be asynchronous symbol timing with the desired signal, so that the effect of cross-signal intersymbol interference (ISI) is taken into account. Using a convergent Fourier series method, we derive extensive analytical results for the average bit error probability and the SNR gain penalty caused by the interference signals for different signal to-interference ratio levels. The numerical results presented in this paper demonstrate the system performance under very realistic propagation and detection conditions including CCI, carrier phase error recovery, cross-signal ISI, generalized fading channels, and AWGN. Hence our results are expected to be of significant practical use for such scenarios.

Transmitter pre-filtering for the downlink of a time-division-duplex/direct sequence-code division multiple access system over time-varying multipath fading channels

In this paper, we investigate an optimised transmitter pre-filtering technique for downlink time-division-duplex (TDD) code division multiple access (CDMA) communications, which employs the conventional matched filter (MF) detector at the mobile receivers. The proposed pre-filtering technique eliminates the multiple-access interference and intersymbol interference (MAI/ISI) effects by applying a very simple transmission scheme that combines a signal transformation with a cyclic prefix strategy under a power constraint condition. Two constrained pre-filtering transformations are suggested depending on the information required at the mobile unit. An open-loop transmitter pre-filtering is first formulated; however, this solution does not consider the properties of the noise at the mobile receiver. A second solution is then presented via a closed-loop transmitter pre-filtering that includes an optimum gain for a given transmit and noise power. Some associated issues such as system efficiency, computational complexity and channel estimation errors are also addressed. Simulation results show that the proposed transmitter pre-filtering scheme can be used to increase the system performance and capacity. In addition, its performance is compared with another similar transmit pre-processing scheme in order to evaluate the performance improvement by the proposed algorithm.

Performance Analysis of Optimum Diversity Combining for Partially Coherent Frequency-Selective Fading Channel With Intersymbol Interference

The performance of coherent BPSK and QPSK systems in the presence of frequency-selective fading and carrier phase errors is analyzed. Frequency-selective fading channels are modeled as multipath Rayleigh, Nakagami- m, or Rician two-ray fading models wherein the presence of a delayed ray causes intersymbol interference (ISI). A diversity system with L-branches is used for signal detection using an optimum equal-gain-combining receiver. The effect of ISI on system performance is assessed using a truncated pulse train of the ISI signal. For Rayleigh-faded channels, the simulation result that demonstrates the influence of Doppler spread on system performance was also included. The following two performance criteria are considered: the average bit error probability and the signal-to-noise-ratio (SNR) gain penalty due to ISI.

Controlled Intersymbol Interference Design Techniques of Conventional Interconnect Systems for Data Rates Beyond 20 Gbps

This paper presents a new design technique of high-speed interconnects with controlled intersymbol interference (ISI) to create efficient signaling over a band-limited channel. Performance of high-speed electrical links is limited by conductor loss, dielectric dispersion, and reflections in the board, package, and connector. These nonidealities result in significant ISI. In current systems, the effect of ISI is either mitigated through complex equalization, signal conditioning, and coding techniques, or through costly impedance control and manufacturing processes. In the proposed approach, instead of eliminating ISI, we shape the response of the channel into a set of channel characteristics with controlled ISI using simple passive structures in the board and the package. The resulting controlled ISI is exploited at the transmitter and receiver to simplify the architecture of the system and to achieve high data rates. The techniques to design interconnects with controlled ISI are reasonably simple to implement in conventional interconnect technologies. Simulation examples are given to demonstrate the validity and advantages of the design technique using duobinary and analog multitone (AMT) signaling methods.

On Equalization for Direct Sequence-Ultra Wideband System Using Received Response Code Sequence

This paper proposes a combination of novel Received Response (RR) sequence at the transmitter and Matched Filter-Equalizer-RAKE (MF-EQZ-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. When binary code sequence such as M sequence is used, there is a possibility of generating extra Inter-Symbol Interference (ISI) in the UWB system. Therefore, it is quite a challenging task to collect the energy efficiently although RAKE reception method is applied at the receiver. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by improving the RAKE reception performance using RR sequence and suppressing the ISI effect with the equalizer. Simulation results verify that significant improvement can be obtained by the proposed system especially in UWB multipath channel models such as channel CM4 that suffered severe ISI effect.

A Combination of Adaptive Equalizer and LMS-RAKE Combining Scheme for DS-UWB System

This paper proposes a combination of adaptive equalizer and Least Mean Square-RAKE (LMS-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by mitigating the multipath effect using LMS-RAKE receiver and suppressing the ISI effect with the adaptive equalizer. Simulation results verify that significant equalization gain can be obtained by the proposed system especially in UWB multipath channel models such as channel CM3 and channel CM4 that suffered severe ISI effect.

Receiver simplification in synchronous communication systems: simulation study

This paper presents a simulation study for code-division multiplex system over time varying channels. The scheme investigated here uses a precoding technique in the transmitter depending on the channel information, so that no need for any signal processing in the receiver rather than testing against a certain threshold. This coding technique minimizes the effect of intersymbol interference (ISI). The data itself is transmitted in the form of blocks of data symbols separated by blocks of no signal. The appropriate length of each data block depends on the channel parameters expected and the allowed complexity. The impulse response is required to be known at the transmitter which is the requirement for all systems that employ coding at the transmitter.

Error propagation analysis for underwater cooperative multi-hop communications

The potential gains of cooperative communication and multi-hopping in underwater acoustic communication channels is examined. In particular, performance of such systems is compared to a comparable single hop system (direct transmission) with a common transmission distance. The effects of error propagation with decode and forward at each relay are explicitly treated and it is shown that strong gains can be achieved by multi-hopping (an effective SNR gain) as well as cooperation, which contributes to a diversity gain. We observe that cooperative diversity gains are retained even when considering error propagation. The analysis is done via a Markov chain analysis for both regular linear and grid networks. Our initial analysis is for single path channels; the effects of inter-symbol interference as well as multi-user interference are examined. It is found that due to the strong decay of signal power as a function of transmission distance, multi-user interference is not as significant as inter-symbol interference. In both cases, cooperative and multi-hopping gains are observed.

Interference Mitigation in Cooperative SFBC-OFDM

We consider cooperative space-frequency block-coded OFDM (SFBC-OFDM) networks with amplify-and-forward (AF) and decode-and-forward (DF) protocols at the relays. In cooperative SFBC-OFDM networks that employ DF protocol, (i), intersymbol interference (ISI) occurs at the destination due to violation of the "quasistatic" assumption because of the frequency selectivity of the relay-to-destination channels, and (ii) intercarrier interference (ICI) occurs due to imperfect carrier synchronization between the relay nodes and the destination, both of which result in error-floors in the bit-error performance at the destination. We propose an interference cancellation algorithm for this system at the destination node, and show that the proposed algorithm effectively mitigates the ISI and ICI effects. In the case of AF protocol in cooperative networks (without SFBC-OFDM), in an earlier work, we have shown that full diversity can be achieved at the destination if phase compensation is carried out at the relays. In cooperative networks using SFBC-OFDM, however, this full-diversity attribute of the phase-compensated AF protocol is lost due to frequency selectivity and imperfect carrier synchronization on the relay-to-destination channels. We propose an interference cancellation algorithm at the destination which alleviates this loss in performance.

Evolution analysis of low-cost iterative equalization in coded linear systems with cyclic prefixes

This paper is concerned with the low-cost iterative equalization/detection principles for coded linear systems with cyclic prefixes. Turbo frequency-domain-equalization (FDE) is applied to systems that may contain the joint effect of multiple-access interference (MAI), cross-antenna interference (CAI) and inter-symbol interference (ISI). We develop an SNR-variance evolution technique for the performance evaluation of the proposed systems. Numerical results in various channel environments demonstrate excellent agreement between the predicted and simulated system performance.

Coded Single-Sideband QPSK and Its Turbo Detection for Mobile Communication Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes a novel single-sideband quadrature phase shift keying (SSB QPSK) modulation scheme that utilizes a single-reference carrier frequency and transforms both baseband I and Q components into the same radio frequency band, whose width is exactly half that of QPSK. The modulation method is first derived and followed by the analysis of the matched filter demodulation. The analysis shows that there are performance degradations due to intersymbol interference (ISI) in the received signal and correlation in the received noise. To suppress the degradations, this paper employs a turbo equalizer for demodulation, in which the effects of ISI and noise correlation can be suppressed by a soft canceler and a noise whitening filter in the minimum mean square error (MMSE) linear equalizer, respectively. The computer simulations are then used to verify the proposed modulation scheme. At the transmitter, the root-raised cosine rolloff filter is employed for pulse shaping and Hilbert-transformed pulse generation. The truncation of the signal pulse is performed by the window function. The results show that the proposed modulation scheme outperforms 16-quadratic-amplitude modulation (16-QAM) with the turbo code in the Rayleigh-fading environment. </para>

REGULARIZED OPTIMUM BEAMFORMING FOR DOWNLINK CDMA SYSTEMS

In this paper, we propose an efficient receiver scheme to mitigate the effects of multiple access interference (MAI) and intersymbol interference (ISI) in downlink CDMA systems. This scheme comprises beamforming at the base station and a regularized zero forcing equalizer at the mobile unit. Beamforming is used to reduce the effect of the MAI. Then, a regularized zero forcing equalizer is used to reduce the effect of ISI and provide a better estimate of the data of interest. The performance of the proposed scheme is studied and compared with other traditional schemes. The simulation results show that the proposed scheme has a better performance than the other traditional schemes with a low degree of complexity at the mobile unit.

Low‐complexity fuzzy filter‐assisted turbo equalization for dispersive Rayleigh‐fading channels

AbstractTurbo equalisation (TEQ) is an iterative detection and decoding technique that has been shown to be successful in mitigating the effects of intersymbol interference (ISI) introduced by partial response modems and by dispersive channels. In this paper, we propose a fuzzy filter‐assisted decision‐feedback equaliser (DFE)‐based TEQ scheme that not only is comparable in performance to the Jacobian radial basis function (RBF) DFE‐based TEQ but also is low‐complexity. The per iteration computation of both the fuzzy filter DFE TEQ using product inference and maximum defuzzification and the fuzzy filter DFE TEQ using minimum inference and maximum defuzzification is much lower than that of the Jacobian RBF DFE TEQ for binary phase‐shift keying (BPSK) modulation, 4 quadrature amplitude modulation (QAM), and 16 QAM. Notably, the computational complexity of the Jacobian RBF DFE TEQ is exponentially related to the feedforward order, but the fuzzy filter DFE TEQ using minimum inference and maximum defuzzification is independent of that. Previous TEQ research has shown that the Jacobian RBF DFE TEQ considerably reduces the computational complexity compared to the logarithmic maximum a posteriori (Log‐MAP) TEQ with similar performance. In this present study, the proposed fuzzy filter DFE TEQ further reduces the computational complexity with slight performance degradation in contrast to the Jacobian RBF DFE TEQ. With this, the materialisation of the TEQ scheme becomes more feasible. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Performance Analysis of Multiband OFDM UWB Systems With Imperfect Synchronization and Intersymbol Interference

This paper provides performance analysis of MB-OFDM UWB systems that not only captures the characteristics of realistic ultra-wideband (UWB) channels, but also takes into consideration of the imperfection of the frequency and timing synchronizations and the effect of intersymbol interference. The systems are considered in multipath fading channels, the IEEE 802.15.3a channel standard, with four channel models: CM1, CM2, CM3, and CM4. These channel models are characterized by cluster arrival rate, ray arrival rate within clusters, and cluster and ray decay factors. First, an average signal-to-noise ratio with imperfect synchronizations and intersymbol interference is derived. Then a closed-form bit error rate performance formulation that provides insightful understanding of the system performance in various synchronization conditions is obtained under Rayleigh fading assumption. This analytical performance formulation serves as an upper bound on the bit error rate performance of UWB systems. Finally, simulation results under various channel and synchronization conditions are provided to validate the theoretical analysis.

Novel approach for reducing the probability of error in the presence of intersymbol interference and Gaussian noise

A novel method to reduce the probability of error in the presence of inter-symbol interference (ISI) and additive Gaussian noise is introduced. The proposed technique is based on modifying the characteristic function of the ISI variable used in the Beaulieu series method to evaluate the probability of error for binary symmetric channels, such that some pulses are omitted to reduce the effect of ISI. This is accomplished by mapping words of certain bit lengths to prescribed series of pulses where some of the pulses are omitted (zeros). The probability of error for the proposed method is evaluated for different word lengths using Gaussian pulses. Test results show that certain pulse series of lengths 12 or greater, with free distances of two or more and consisting of 30% zeros or more, would achieve lower probability of error in the presence of ISI and Gaussian noise.