Severe Frequency-selective Fading Research Articles

周波数選択性フェーディング環境下におけるプリコーデッドＯＦＤＭの繰り返し復調方式

We propose a pre-coded OFDM iterative detection scheme. The proposed scheme recovers the pre-coded data symbols that correspond to the carriers whose reception powers are degraded by frequency-selective fading by using the data symbols after decision, and the data symbols are then detected again using the recovered pre-coded data symbols.We confirmed by numerical analysis that better performance under severe frequency-selective fading channels can be achieved by iteratively repeating the proposed scheme.

DS-CDMA HARQ with Overlap FDE

SUMMARY Turbo coded hybrid ARQ (HARQ) is known as one of the promising error control techniques for high speed wireless packet access. However, in a severe frequency-selective fading channel, the HARQ throughput performance significantly degrades for direct sequence code division multiple access (DS-CDMA) system using rake combining. This problem can be overcome by replacing the rake combining by the frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion. In a system with the conventional FDE, the guard interval (GI) is inserted to avoid the inter-block interference (IBI). The insertion of GI reduces the throughput. Recently, overlap FDE that requires no GI insertion was proposed. In this paper, we apply overlap FDE to HARQ and derive the MMSE-FDE weight for packet combining. Then, we evaluate the throughput performance of DS-CDMA HARQ with overlap FDE. We show that overlap FDE provides better throughput performance than both the rake combining and conventional FDE regardless of the degree of the channel frequency-selectivity.

Frequency-Domain Multi-Stage Soft Interference Cancellation for DS-CDMA Uplink Signal Transmission

SUMMARY It is well-known that, in DS-CDMA downlink signal transmission, frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace rake combining to achieve much improved bit error rate (BER) performance in severe frequencyselective fading channel. However, in uplink signal transmission, as each user’s signal goes through a different channel, a severe multi-user interference (MUI) is produced and the uplink BER performance severely degrades compared to the downlink. When a small spreading factor is used, the uplink BER performance further degrades due to inter-chip interference (ICI). In this paper, we propose a frequency-domain multi-stage soft interference cancellation scheme for the DS-CDMA uplink and the achievable BER performance is evaluated by computer simulation. The BER performance comparison of the proposed cancellation technique and the multiuser detection (MUD) is also presented.

Iterative Channel Estimation for Frequency-Domain Equalization of DSSS Signals

As the channel frequency selectivity becomes severer, the bit error rate (BER) performance of direct sequence spread spectrum (DSSS) signal transmission with rake combining degrades due to an increasing inter-path interference (IPI). Frequency-domain equalization (FDE) can replace rake combining with much improved BER performance in a severe frequency-selective fading channel. For FDE, accurate estimation of the channel transfer function is required. In this paper, we propose an iterative channel estimation that uses pilot chips which are time-multiplexed within each chip block for fast Fourier transform (FFT). The pilot acts as a cyclic-prefix of FFT block as well. The achievable BER performance is evaluated by computer simulation. It is shown that the proposed channel estimation has a very good tracking ability against fast fading.

Performance of a Tree-Based Collision Resolution Algorithm in Cellular Systems with Smart Antennas

A smart antenna's ability to simultaneously resolve more than one user on the same channel is exploited to help expedite the process of random access, especially for the reverse channel. However, use of a smart antenna requires frequent array adaptation, a difficult task in a random access environment. In papers H. Jin and Acampora (2005), we proposed and studied the SINR performance of a media access protocol intended for use with smart antennas. In this paper, we study the random access performance of the proposed protocol when a fast collision resolution algorithm is used. Found are the maximum achievable throughput and an upper bound on the expected delay. The impact of the number of antennas on the performance with both flat fading and frequency selective fading is studied. Topical results show significant improvement in throughput for systems with smart antennas. For example, when the fading is flat, the gain in maximum achievable throughput can be as large as 232% and 550% with four and eight antenna elements respectively. Even when severe frequency selective fading is present, there is a gain of 40% and 145% with four and eight antenna elements, respectively. We also show that dynamically adjusting the retransmission probability in the collision resolution process is not worthwhile since this extra complexity produces a gain of only about 6.2% in the best case

Iterative Frequency-Domain Soft Interference Cancellation for Multicode DS- and MC-CDMA Transmissions and Performance Comparison

SUMMARY Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can significantly improve the BER performance of DS- and MC-CDMA systems in a severe frequencyselective fading channel. However, since the frequency-distorted signal cannot be completely equalized, the residual inter-code interference (ICI) limits the BER performance improvement. 4G systems must support much higher variable rate data services. Orthogonal multicode transmission technique has flexibility in offering variable rate services. However, the BER performance degrades as the number of parallel codes increases. In this paper, we propose an iterative frequency-domain soft interference cancellation (IFDSIC) scheme for multicode DS- and MC-CDMA systems and their achievable BER performances are evaluated by computer simulation.

Single Carrier Frequency-Domain Equalization with Transmit Diversity over Mobile Multipath Channels

This paper discusses a cyclic prefixed single carrier frequency-domain equalization (SC-FDE) scheme with two types of transmit diversity. Firstly, we propose a SC-FDE system with space-frequency block coding (SFBC). The transmit sequence of the proposed system is designed to have spatial and frequency diversities, which is equivalent to the SFBC. The corresponding combining receiver is derived under a minimum mean square error (MMSE) criterion. It is shown that the proposed system significantly outperforms the SC-FDE system with space-time block coding (STBC) over fast fading channels, while providing lower computational complexity than orthogonal frequency division multiplexing (OFDM) combined with SFBC. We verify the performance of two-branch transmit diversity systems including the proposed one through bit error rate (BER) analysis. Secondly, as a scheme that combines STBC and SFBC, a space-time-frequency block code (STFBC) SC-FDE system is presented. Computer simulation results show that the proposed STFBC SC-FDE system has better immunity to the distortion caused by both fast fading and severe frequency selective fading, compared to the SC-FDE system with the STBC or the SFBC scheme. Complexity analysis is also conducted to compare their computational loads of the transceiver. It is shown that the proposed STFBC SC-FDE system has lower computational complexity than the STFBC OFDM system.

Low-complexity block coded multi-carrier modulation for line-of-sight microwave systems

The authors present low-complexity block coded multi-carrier modulation (BC-MCM) schemes that more are efficient to overcome the deep frequency-selective fading that appears on high-data-rate transmission line-of-sight digital radio channels. These BC-MCM schemes are based on time-limited orthogonal multi-carrier modulation with waveform shaping (ITLO-MCM) systems associated to an interleaving process and an array of very simple constituent binary block codes. The performance is evaluated in terms of signature areas for fixed BER thresholds, on severe frequency-selective fading channels. Results show that the proposed low-complexity BC-MCM schemes provide significant improvement in performance over equivalent uncoded multi-carrier modulation (MCM) and practical single-carrier TCM systems.

Frequency-Domain Pre-Equalization for Multicode Direct Sequence Spread Spectrum Signal Transmission

Severe frequency-selective fading, encountered in a broadband wireless mobile communication, significantly degrades the bit error rate (BER) performance of direct sequence spread spectrum (DSSS) signal transmission with rake combining. In this paper, frequency-domain pre-equalization transmission, called pre-FDE transmission, is presented for orthogonal multicode DSSS signal transmission. It is confirmed by the computer simulation that pre-FDE transmission can achieve a BER performance almost identical to that attainable by FDE reception.

Robust iterative tree-pruning detection and LDPCC decoding

A novel suboptimal low-complexity equalization and turbo-iterative decoding scheme is proposed in this paper. The scheme is developed for multiple transmit- and multiple receive-antenna systems operating over severe frequency-selective fading intersymbol interference channels. The signal-processing complexity may be of a concern for such systems. The complexity of a full-search equalization grows in a power-law manner O(M/sup NtL/), where M denotes for M-ary channel symbols, N/sub t/ the number of transmit-antennas, and L the number of delay channel taps. A low-complexity solution can be obtained by pruning an equalizer tree. The two main operations include a sphere list detection and a threshold-based tree-search. In the operation of extracting extrinsic messages from the pruned tree, a set of explored paths with different survival lengths poses a fairness problem: a longer-lived path naturally builds a larger discrepancy-metric than a shorter lived path does. A novel survival-length compensation-rule is devised so that all explored paths with different survival lengths are utilized fairly in generating the output message. Simulation results are obtained for multi-input and multi-output systems equipped with four transmit and four receive antennas. They indicate the performance of the receiver is very robust.

Enhancement of Data Throughput in the AMC-Employed DS-CDMA Systems through Suppression of Channel Frequency Selectivity by a MTMR Antenna System

SUMMARY In this paper, a new algorithm for MTMR adaptive array antenna (AAA) system combined with analog-type transmit power control (TPC) is proposed for DS-CDMA systems in order to employ high level modulation schemes like 64 QAM in wireless multimedia services. A conventional AAA system considering the strongest path as a target path cannot work effectively when angular dispersion between the strongest path and other delayed paths is large, that is, beam selectivity is so small due to severe frequency selective multipath fading. So, in order to solve such a beam selectivity problem, a beam directivity control scheme using a path manipulation technique is introduced for the BS and MS AAA combining in this paper, along with analog-type TPC. It utilizes virtual delay profiles which are modified from the measured complex delay profile and selects a desired path giving the maximum DUR with an optimized weight vector for BS and MS beamforming. We will show through computer simulation that the proposed scheme is very effective in enhancing the data throughput at the downlink of wideband DS-CDMA systems as compared with the conventional system.

Frequency-Domain Adaptive Prediction Iterative Channel Estimation for OFDM Signal Reception

In this letter, pilot-assisted adaptive prediction iterative channel estimation in frequency-domain is presented for the antenna diversity reception of orthogonal frequency division multiplexing (OFDM) signals. A frequency-domain adaptive prediction filtering is applied to iterative channel estimation for improving the tracking capability against frequency-domain variations in a severe frequency-selective fading channel. Also, in order to track the changing fading environment, the tap weights of frequency-domain prediction filter are updated using the simple NLMS algorithm. Updating of tap weights is incorporated into the iterative channel estimation loop to achieve faster convergence rate. The average bit error rate (BER) performance in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is confirmed that the frequency-domain adaptive prediction iterative channel estimation provides better BER performance than the conventional iterative channel estimation schemes.

送信ダイバー・シティとサブキャリヤ制御を用いたデータ拡散ＯＦＤＭに関する一検討

It is difficult to improve the performance of OFDM transmission by equalizing the channel characteristic under severe frequency selective fading conditions. Transmission diversity and data symbol spreading in the frequency domain are effective schemes for improving BER characteristics, and more improvement can be achieved by combining them. Moreover, further improvement should be achieved by controlling the sub-carrier power and phase of the signal transmitted by each antenna, especially under severe frequency selective fading conditions. In this paper, data spread OFDM using transmission diversity and a sub-carrier control scheme is proposed. Computer simulation of the proposed scheme demonstrated that it performs is well.

Application of space–time transmit diversity to single-carrier transmission with frequency-domain equalisation and receive antenna diversity in a frequency-selective fading channel

In a frequency-selective fading channel, the performance of non-spread-spectrum single-carrier (SC) transmission degrades significantly due to severe intersymbol interference (ISI). The joint use of space–time transmit diversity (STTD), receive antenna diversity and frequency-domain minimum mean square error (MMSE) equalisation in non-spread SC transmission is studied. Space–time encoding and decoding for frequency-domain MMSE equalisation and receive antenna diversity is presented. The achievable bit error rate (BER) performance of non-spread SC transmission is evaluated by computer simulation. It is found that joint use of STTD, receive antenna diversity and frequency-domain MMSE equalisation can significantly improve the BER performance in a severe frequency-selective fading channel.

Detection of digital signals transmitted over a known time-varying channel

A technique has recently been developed whereby it is possible, without undue complexity and without any adaptive linear prefiltering, to achieve near-maximum-likelihood detection of a sampled digital signal, where there is intersymbol interference extending over several samples of the signal. It is, however, important here that the channel impulse response does not undergo large changes with time, since, when it does, a considerable increase in the complexity of the system may be required to maintain correct operation. The paper describes a development of the detection process whereby correct operation is achieved with a relatively simple system, even when the channel introduces severe frequency-selective fading of the type sometimes experienced over HF radio links. Results of computer-simulation tests are presented, showing the tolerance of a synchronous serial data-transmission system to additive white Gaussian noise, when a 4-point quadrature amplitude modulated signal is transmitted at 2400 bit/s over a model of an HF radio link, with two independent Rayleigh fading sky waves and frequency spreads of 0.5, 1 and 2Hz, and when the novel detection process is used at the receiver. Correct estimation of the channel is assumed throughout.