Presence Of Frequency Selective Fading Research Articles

직교주파수분할다중화기반 전력선통신에서 대역 효율적인 전송기법

전력선통신은 스마트그리드 기반의 서비스가 제공될 수 있는 네트워크를 위한 미래지향적 기술이다. 전력선통신 채널의 주파수 선택적 페이딩이 있는 환경에서, 직교주파수분할다중화 기술은 신뢰성 있는 통신을 제공한다. 본 논문에서는 직교주파수분할다중화 기반의 전력선통신 시스템에서 은닉학습신호를 이용한 주파수사용효율이 높은 기법을 제안한다. 은닉학습신호를 사용하면 채널 추정용 주파수를 따로 소모하지 않고도 채널 추정이 가능하고, 이는 데이터와의 간섭을 줄일 수 있는 학습신호에 할당된 파워를 이용해서 해결할 수 있다. 컴퓨터 시뮬레이션을 통해 제안한 기법이 기존의 기법들에 비해 저전압 및 중전압 송전 라인에서 높은 성취 가능한 데이터 율을 보여준다. Powerline communications (PLC) is a promising medium for network access technology where smart grid aided network services can be provided. In the presence of frequency selective fading in the PLC channel, orthogonal frequency division multiplexing (OFDM) is a technique for reliable communications. This paper presents a spectral efficient method using a superimposed hidden pilot for OFDM-based PLC systems. Based on the scheme using a hidden pilot, it is possible to estimate the channel with no consumption of bandwidth, but with utilization of power allocated to the hidden pilot. Computer simulations showed that the proposed scheme provides higher achievable data rate than that of the conventional schemes in low voltage and medium voltage transmission lines.

Integer Frequency Offset Estimation for OFDM Systems With Residual Timing Offset Over Frequency Selective Fading Channels

Accurate integer frequency offset (IFO) estimation is crucial for orthogonal frequency-division multiplexing (OFDM) systems, particularly in the presence of frequency-selective fading and residual timing offset (RTO). For existing algorithms, however, it is still a challenge to obtain a good tradeoff among the estimation performance, complexity, and spectrum overhead. In this paper, we propose a novel cross ambiguity function (CAF)-based IFO estimator using only one training sequence. By designing the training sequence, which has only a single sharp peak on its ambiguity function surface, a highly accurate and full acquisition range estimation of the IFO can be obtained in the presence of frequency-selective fading and RTO. Moreover, the adoption of the CAF expression in terms of time-domain signals ensures that the complexity of the proposed algorithm is relatively low. Simulation results verify its superior accuracy in frequency-selective fading channels and in the presence of RTO.

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.

Precoder for Chip-Interleaved CDMA Using Space-Time Block-Coding

We study the performances of a synchronous chip-interleaved, block spread (CIBS) code division multiple access (CDMA) with space-time block-coding (STBC) in the presence of frequency-selective fading. For providing the space diversity gain due to STBC, we introduce the optimum precoding for the STBC. Zero-forcing and minimum mean square error equalizers for CIBS-CDMA are derived. Simulation results confirm that the proposed precoder is valid under the frequency selective fading.

Performance Analysis of High Data Rate MIMO Systems in Frequency-Selective Fading Channels

The performance of multicode direct-sequence spread-spectrum multiple-input multiple-output (MIMO) systems in the presence of frequency-selective fading is evaluated. We derive the asymptotic distribution of the multiple-antenna interference when the processing gain is sufficiently large. The probability of error is derived for the conventional RAKE receiver, and its performance is compared for various system configurations. We consider system tradeoffs for both fixed rate and fixed diversity. For a fixed total data rate, we demonstrate the advantage of decreasing the number of transmit antennas while increasing the number of codes, and for a fixed total diversity order, we demonstrate the advantage of decreasing the number of RAKE taps while increasing the number of receive antennas.

A new class of interlaced complementary codes based on components with unity peak sidelobes

A new class of biphase complementary code sets is proposed. Individual codes in each set have maximum sidelobes of unity magnitude. The individual codes could have gaps of zeros but they are interlaced together without gaps in the final scheme. A number of such codes, as presented in this paper, use Barker codes as building blocks with additional elements from { + 1 ,- 1 , 0 } . The main drawback of regular complementary codes longer than 4 is that they have peak sidelobe magnitude greater than unity. In the presence of frequency selective fading or other factors, inexact sidelobe cancellation results in non-zero sidelobes at the output. This is minimized in the proposed schemes by using codes that have peak sidelobes of unity magnitude as the individual codes in the sets. The proposed codes are attractive due to their increased resistance to the effect of inexact sidelobe cancellation. A figure of merit is proposed to measure the frequency use efficiency of the proposed codes.

A spectral domain approach to design spreading sequences for DS-CDMA systems in frequency selective fading channels

In this paper, we propose a frequency domain approach to design a set of spreading sequences for a DS-CDMA system in the presence of frequency selective fading. We show that the number of parameters involved in the design of optimal spreading sequences can be made smaller when they are analyzed in the frequency domain, than when they are analyzed in the time domain. Further, it is shown that for multipath channels, designing a set of spreading sequences in the frequency domain reduces to assigning a spreading sequence for each user with a spectrum whose spectral lines are located at the strongest spectral components of the user's channel's spectrum while keeping the cross correlation at a very low value for spreading sequences with overlapping spectra. We show that by designing spreading sequences in the frequency domain, the performance of some existing algorithms for the design of spreading sequences can be improved and/or the computational complexities can be reduced. We further show that the proposed idea can be extended to the design of spreading sequences where the chips of the spreading sequences are derived from a constant amplitude alphabet

MUI-free receiver for a synchronous DS-CDMA system based on block spreading in the presence of frequency-selective fading

We discuss a synchronous direct-sequence code division multiple-access (DS-CDMA) system based on block spreading in the presence of frequency-selective fading. Note that block spreading, which is also known as chip interleaving, refers to a spreading of a data block sequence, which is obtained by dividing a data symbol sequence into consecutive blocks. For such a system, we develop a simple new receiver that completely removes the multiuser interference (MUI) without using any channel information. The MUI-free operation is obtained by the use of a shift-orthogonal set of code sequences on which this receiver is based. Within the framework of the MUI-free receiver, we further present a subspace deterministic blind single-user channel estimation algorithm. As a benchmark for the MUI-free receiver and the corresponding subspace deterministic blind single-user channel estimation algorithm, we consider the linear multiuser equalizer and the corresponding subspace deterministic blind multiuser channel estimation algorithm developed by Liu and Xu (1996) for a standard synchronous DS-CDMA system in the presence of frequency-selective fading. We show that the complexity of the MUI-free receiver using the corresponding subspace deterministic blind single-user channel estimation algorithm is much smaller than the complexity of the linear multiuser equalizer using the corresponding subspace deterministic blind multiuser channel estimation algorithm. We further show that the performance of the MUI-free receiver is comparable with the performance of the linear multiuser equalizer. This is for the case in which the channels are known as well as for the case in which the channels are estimated with the corresponding subspace deterministic blind channel estimation algorithm.

An adaptive soft-output Viterbi equalizer for fast time-varying frequency-selective fading

In this paper, an adaptive soft-output Viterbi equalizer (SOVE) is proposed for fast time-varying frequency-selective fading. The demodulator can make a symbol-by-symbol or bit-by-bit soft-decision based on a posteriori probabilities. The proposed adaptive SOVE has the following advantages: (1) it has a superior tracking ability for fast time-varying frequency-selective fading, (2) it works in blind if the coding rules are suitably designed, and (3) a good BER is obtained at the output of the decoder by introducing interleaves. In this paper, these advantages are confirmed by computer simulations. In particular, in the presence of frequency-selective fading, we can show that the proposed method is superior to adaptive maximum-likelihood sequence estimation which carries equalizing and decoding simultaneously. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(4): 31–39, 1998

An adaptive soft‐output Viterbi equalizer for fast time‐varying frequency‐selective fading

In this paper, an adaptive soft-output Viterbi equalizer (SOVE) is proposed for fast time-varying frequency-selective fading. The demodulator can make a symbol-by-symbol or bit-by-bit soft-decision based on a posteriori probabilities. The proposed adaptive SOVE has the following advantages: (1) it has a superior tracking ability for fast time-varying frequency-selective fading, (2) it works in blind if the coding rules are suitably designed, and (3) a good BER is obtained at the output of the decoder by introducing interleaves. In this paper, these advantages are confirmed by computer simulations. In particular, in the presence of frequency-selective fading, we can show that the proposed method is superior to adaptive maximum-likelihood sequence estimation which carries equalizing and decoding simultaneously. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(4): 31–39, 1998

A characterization of the frequency selective fading of the mobile radio channel

An experiment has been designed to determine the frequency selective behavior of a mobile radio channel for a medium bandwidth (maximum of 128 kHz). The measurement technique uses a multitone approach. The transmitted signal consists of several tones (five) with fixed phase and amplitude relationships. Coherent demodulation and separation of each tone is performed at the receiving end. Quantification of the mobile radio channel is conveniently described by its frequency correlation function. Measurements for several types of environments (rural, suburban, urban) were taken and analyzed. The results characterize the amplitude and phase behaviors of a signal for each type of environment. Finally, the computations of the probability of error for a differential phase shift keying (DPSK) modulation scheme in the presence of frequency selective fading show the dominance of this factor as the signal-to-noise ratio (SNR) increases.

Effect of adjacent channel interference and frequency-selective fading on outage of digital radio

Digital radio is affected by frequency-selective fading (FSF), which causes intersymbol interference (ISI), and by adjacent channel interference (ACI), which in its turn is also affected by the same FSF. Using a two-ray model for the FSF, the transfer function of the channel is H(f) = a[1 − b exp ( − j2πτ(f − f0))]. Outage in digital radio occurs when the bit error probability exceeds a critical value of 10−3, or when an equivalent mean-square error exceeds a critical value σ2εc, which depends on the modulation used. In the paper we compute the critical mean-square error for M-QAM and M-PSK modulation where M = 4, 8, 16, 64 is the number of symbols. In the absence of FSF, the system response for the signal in the main channel is a raised cosine with excess bandwidth β, and the receiver filter is matched to the transmitter filter. The signal in the adjacent channel has the same modulation as the signal in the main channel, although they differ in carrier frequency, amplitude, phase and symbol timing. We derive an expression for the mean-square error in the presence of FSF and ACI. The outage probability is a product of two terms, one of which is independent of the modulation methods, while the second is proportional to the area under a signature. The signature is a plot of the minimum value of 1 – bc as a function of f0, where bc is a critical value of b such that, if b ≥ bc, the critical mean-square error is exceeded. We compute the signatures and the area under the signatures for M-QAM and M-PSK (M = 4, 8, 16, 64) with a bit rate of 140 Mbit/s and frequency separation between channels of 40 MHz. We conclude that, for low values of ACI, 4-QAM is the best system, while for large values of ACI, 64-QAM is the best system. 16-QAM is always better than 8-PSK.

The Effect of Frequency Selective Fading on Intermodulation Distortion and Subcarrier Phase Stability in Frequency Modulation Systems

Time varying multipath channels produce a form of fading known as frequency selective fading. The presence of frequency selective fading on the transmission channel causes intermodulation distortion to appear at the base-band output of frequency modulation systems. In addition this frequency selective fading causes phase and amplitude fluctuations to appear on demodulated subcarriers. This paper derives for the first time probability density functions and distribution functions for intermodulation distortion and subcarrier phase fluctuations as a function of the delay power density spectrum of the channel. The derivations involve the use of a canonical channel model called the power series model which is particularly useful when the transmitted signal bandwidth is less than the correlation bandwidth of the channel. Another novel feature of the analysis is the inclusion of the effects of diversity combining.

The Effect of Frequency Selective Fading on the Binary Error Probabilities of Incoherent and Differentially Coherent Matched Filter Receivers

The presence of frequency-selective fading in a communication channel limits the maximum data rate capability of conventional communication systems. Surprisingly little analysis has been carried out to determine the effect of frequency-selective fading on digital (or analog) communication systems. This paper considers the effect of frequency-selective fading on the binary error probabilities of incoherent and differentially coherent matched filter receivers employing postdetection diversity combining. In the analysis it is assumed that the fading is slow enough so that over a few bits at least the channel transfer function remains constant. In addition, it is assumed that the amplitude and phase fluctuations on a received carrier have the same statistical character as those of narrow-band Gaussian noise. The general analytical results are specialized to the cases of frequency-shift keying using incoherent detection, and phase-shift keying using differentially coherent detection for the case of a Gaussian frequency autocorrelation function. For these special cases, signal-to-noise degradation curves are given as a function of the ratio of the binary data rate to the correlation (or coherence) bandwidth. Two types of FSK are considered, phase-continuous and phase-discontinuous. In phase-continuous FSK there is no discontinuity in the phase of the transmitted waveform at the markspace or space-mark epochs. Such an FSK system results when the mark and space frequencies are obtained by frequency modulating an oscillator. In phase-discontinuous FSK phase discontinuities exist at the transition epochs. Such an FSK system results, for example, when the mark and space frequencies are derived by switching between two independent oscillators. An interesting result of the analysis is that the differentially coherent PSK system and the phase-discontinuous FSK system degrade considerably more rapidly with increasing (normalized) data rate than the phasecontinuous FSK system. The existence of an irreducible error probability is demonstrated for the incoherent and differentially coherent matched filter receivers. Thus, in general, an increase in transmitted signal power cannot reduce the error probability below a certain value depending upon the ratio of data rate to correlation bandwidth and order of diversity. Theoretical curves of irreducible error probability are given for the incoherent FSK and differentially coherent PSK systems.