Severe Frequency-selective Fading Channel Research Articles

Multicarrier Spread Spectrum Communication Scheme for Cruising Sensor Network in Confined Underwater Space

A cruising sensor network works as an online monitoring system for industrial liquid environments such as oil tanks and nuclear storage ponds. The cruising sensor network consists of a few submerged nodes which can actuate themselves to execute tasks like “cruising.” This paper presents a multicarrier spread spectrum scheme, which is designed for communications between the nodes and the control station. Although underwater acoustic communication has been widely researched due to the growing demands from ocean development and marine research, communication in confined underwater space is an unexplored domain as most research focuses on communication in spacious water areas. The occasions that the cruising sensor network works in are confined. The main distinction in confined underwater spaces is the existence of strong multipath arrivals reflected by the boundaries, which can cause more severe intersymbol interference (ISI). We propose a communication scheme which applies spread spectrum in orthogonal frequency-division multiplexing (OFDM) to address severe frequency selective fading channels. This scheme can be robust in confined underwater channels when coupled with turbo code. The simulation and experimental results prove the feasibility and reliability of this scheme. It is demonstrated that significantly better performance is achieved than that of the conventional OFDM method.

Efficient OFDM Symbol Timing Estimator Using Power Difference Measurements

This paper presents an efficient blind symbol timing estimation scheme for orthogonal frequency-division multiplexing (OFDM) systems with constant modulus constellation. The proposed technique is designed to estimate symbol timing offsets by minimizing the power difference between subcarriers with similar indices over two consecutive OFDM symbols based on the assumption that the channel slowly changes over time. The proposed power difference estimator (PDE) is totally blind because it requires no prior information about the channel or the transmitted data. Monte Carlo simulation is used to assess the PDE performance in terms of the probability of correct timing estimate Plock-in. Moreover, we propose a new performance metric denoted as the deviation from safe region (DSR). Simulation results have demonstrated that the PDE performs well in severe frequency-selective fading channels and outperforms the other considered estimators. The complexity of the PDE can be significantly reduced by incorporating a low-cost estimator to provide initial coarse timing information. The proposed PDE is realized using feedforward and early-late gate (ELG) configurations. The new PDE-ELG does not suffer from the self-noise problem inherent in other ELG estimators reported in the literature.

Joint Iterative Transmit/Receive FDE & FDIC for Single-Carrier Block Transmissions

In this paper, we propose a novel iterative transmit/receive equalization technique for single-carrier (SC) block transmission in a severe frequency-selective fading channel. Iterative frequency-domain inter-symbol interference (ISI) cancellation (FDIC) is introduced to the previously proposed joint iterative transmit/receive frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion. 1-tap FDE is employed at the transmitter. At the receiver, a 1-tap FDE and FDIC are jointly used and they are updated in an iterative manner. The transmit FDE weight is derived based on the MMSE criterion by taking into account the reduction of residual ISI in the receiver. To derive the weight, the transmitter assumes that the receiver can partially reduce the residual ISI after the FDIC. We conduct a computer simulation to investigate the achievable bit error rate (BER) performance to confirm the effectiveness of our proposed technique.

Spectrum Sensing in Wideband OFDM Cognitive Radios

In this paper, spectrum sensing of an orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) is addressed. The goal is to identify the portions of the spectrum that are unused by primary user systems and other CR systems, called existing user (EU) systems altogether, with the emphasis on conquering the challenge imposed by multipath fading channel. The sensing of EU systems consists of two steps. In the first step, the maximum likelihood (ML) estimates of the frequency bands of EU systems are calculated; in the second step, detection is performed at each suspected band to decide whether an EU system is truly in operation. The idea is that an EU system appears at a segment of continuous subcarriers. This fact can be exploited by employing measurements at a continual subcarriers and executing the sensing along the frequency domain. An autoregressive (AR) model is adopted to track the variation of the received EU signal strength along frequencies. It is shown by simulations that the proposed spectrum sensing algorithm is robust in a severe frequency-selective fading channel.

Pilot-Assisted Channel Estimation for Orthogonal Multi-Carrier DS-CDMA with Frequency-Domain Equalization

Orthogonal multi-carrier direct sequence code division multiple access (orthogonal MC DS-CDMA) is a combination of time-domain spreading and orthogonal frequency division multiplexing (OFDM). In orthogonal MC DS-CDMA, the frequency diversity gain can be obtained by applying frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion to a block of OFDM symbols and can improve the bit error rate (BER) performance in a severe frequency-selective fading channel. FDE requires an accurate estimate of the channel gain. The channel gain can be estimated by removing the pilot modulation in the frequency domain. In this paper, we propose a pilot-assisted channel estimation suitable for orthogonal MC DS-CDMA with FDE and evaluate, by computer simulation, the BER performance in a frequency-selective Rayleigh fading channel.

Pilot-Assisted Adaptive Channel Estimation for Coded MC-CDMA with ICI Cancellation

One of the promising wireless access techniques for the next generation mobile communications systems is multi-carrier code division multiple access (MC-CDMA). MC-CDMA can provide good transmission performance owing to the frequency diversity effect in a severe frequency-selective fading channel. However, the bit error rate (BER) performance of coded MC-CDMA is inferior to that of orthogonal frequency division multiplexing (OFDM) due to the residual inter-code interference (ICI) after frequency-domain equalization (FDE). Recently, we proposed a frequency-domain soft interference cancellation (FDSIC) to reduce the residual ICI and confirmed by computer simulation that the MC-CDMA with FDSIC provides better BER performance than OFDM. However, ideal channel estimation was assumed. In this paper, we propose adaptive decision-feedback channel estimation (ADFCE) and evaluate by computer simulation the average BER and throughput performances of turbo-coded MC-CDMA with FDSIC. We show that even if a practical channel estimation is used, MC-CDMA with FDSIC can still provide better performance than OFDM.

Frequency-Domain Eigenbeam-SDM and Equalization for Single-Carrier Transmissions

In mobile communications, the channel consists of many resolvable paths with different time delays, resulting in a severely frequency-selective fading channel. The frequency-domain equalization (FDE) can take advantage of the channel selectivity and improve the bit error rate (BER) performance of the single-carrier (SC) transmission. Recently, multi-input multi-output (MIMO) multiplexing is gaining much attention for achieving very high speed data transmissions with the limited bandwidth. Eigenbeam space division multiplexing (E-SDM) is known as one of MIMO multiplexing techniques. In this paper, we propose frequency-domain SC E-SDM for SC transmission. In frequency-domain SC E-SDM, the orthogonal transmission channels to transmit different data in parallel are constructed at each orthogonal frequency. At a receiver, FDE is used to suppress the inter-symbol interference (ISI). In this paper, the transmit power allocation and adaptive modulation based on the equivalent channel gains after performing FDE are applied. The BER performance of the frequency-domain SC E-SDM in a severe frequency-selective Rayleigh fading channel is evaluated by computer simulation.

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.

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.

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 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.

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.