Severe Frequency-selective Fading Research Articles

Anti-Multipath Orthogonal Chirp Division Multiplexing for Underwater Acoustic Communication

Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new anti-multipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of $10^{-6}$ , while the OFDM has a BER floor of $10^{-3}$ . The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

A Geometric Scattering Model for Circularly Polarized Indoor Channels

Wireless systems are being deployed in evermore complex environments, ones that can introduce multipath resulting in not only severe frequency selective fading but also signal depolarization. Polarization diversity is a known technique to mitigate such fades. To date, primarily linearly polarized (LP) systems have been used for indoor communications, but recently circularly polarized (CP) systems have received attention. As such there is motivation to better understand how both linearly and CP systems perform in depolarizing environments. In this article, we present a 3-D geometry-based stochastic channel model (GBSCM) for dual-polarized systems to evaluate circular and LP in cluttered settings. Using a Poisson distribution, the model generates a number of multipath scatterers. Measurements have been conducted at 2.4 GHz to validate the model. Leveraging parameters extracted from measured data, linear and circular polarization conditions are simulated for both line-of-sight (LOS) and non-LOS (NLOS) conditions. cross-polarization discrimination (XPD) and 1% link margins are considered as performance metrics. These metrics applied to both simulated and empirical data are both within 1 dB of each other.

Doppler Scaling Factor Estimation and Receiver Design for Underwater Acoustic Communication

Due to very low propagation speed of acoustic waves, underwater communication via acoustic waves observe a significant Doppler scaling in the received signal. In addition, due to severe frequency selective fading in underwater acoustic channel, the distinct multi-paths may induce different Doppler scaling factor. In this paper, we investigate a transmission strategy in which we append preamble and post-amble in the data frame to estimate the Doppler scaling factor. The frequency selective fading coefficients and additive noise in the system model are statistically characterized by Nakagami and Generalized Gaussian distribution, respectively. Further, the transmit data is assumed to be orthogonal frequency division multiplexing (OFDM) modulated to mitigate the frequency selective nature of underlying channel. In our analysis, we consider two scenarios: (1) multi-paths have common Doppler scaling factor and (2) multi-paths have different Doppler scaling factor. For both the scenarios, we propose receiver algorithms to detect OFDM symbols corrupted by multi-path propagation. We also discuss the improvement in bit error rate performance by exploiting maximal ratio combining scheme. Finally, we validate our approach in the proposed techniques by showing improvement in bit error rate performance via simulation under various signal to noise ratio and fading conditions.

Experimental investigation of multi-band OCT precoding for OFDM-based visible light communications.

In this paper, we propose and experimentally demonstrate a channel-independent multi-band orthogonal circulant matrix transform (MB-OCT) precoding, to efficiently combat the severe frequency-selective fading of visible light communications (VLC). The proposed MB-OCT precoding exhibits an attractive ladder-like signal-to-noise-ratio (SNR) profile, thus can significantly reduce system BER by applying different quadrature amplitude modulation (QAM) level to different sub-bands. The impacts of sub-band number, signal bandwidth, and length of cyclic prefix (CP) on bit error rate (BER) of the VLC system are investigated. We experimentally compare BER performance of the proposed MB-OCT precoding with that of the conventional MB discrete Fourier transform (MB-DFT) precoding and the adaptive-loaded discrete multitone (DMT). The results show that the MB-OCT precoding outperforms the MB-DFT precoding and the single-band case for different data rates. Furthermore, it exhibits reduced implementation complexity and comparable BER performance with the adaptive-loaded DMT. For ~700-Mb/s VLC system with 2-m transmission distance, the BER is reduced from 1.53 × 10-2 to 1.17 × 10-4 by using the proposed MB-OCT precoding.

Time-Domain SI Estimation for SLM Based OFDM Systems Without SI Transmission

Side information (SI) detection is normally needed to achieve successful data reception when selected mapping is implemented for reducing peak-to-average power ratio in orthogonal frequency division multiplexing systems. In severe frequency selective channel fading, existing pilot-assisted SI estimation schemes are ineffective especially when the number of pilots is limited, resulting in performance degradation in the form of increased BER. To address this problem, an alternative pilot-assisted SI estimation method based on a time-domain decision metric is proposed. Simulations show that when compared to a pilot-assisted SI estimation scheme based on frequency domain correlation, the proposed method provides improved SI estimation performance in the form of reduced SI detection error rate.

A Novel Pilot Expansion Approach for MIMO Channel Estimation

A training-based MIMO channel estimation scheme is presented to operate in severe frequency and time selective fading channels. Besides the new pilot bits designed from the ‘Paley-Hadamard’ matrix to exploit its orthogonal and ‘Toeplitz-like’ structures and minimising its pilot length, a novel pilot expansion technique is proposed to estimate the length of the channel impulse response, by flexibly extending its pilot length as required in order to capture the number of multipath existed within the MIMO channel. The pilot expansion can also help to deduce the initial channel variation and its Doppler rate which can be subsequently applied for MIMO channel tracking using decision feedback Kalman filter during the data payload.

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.

Distributed antenna network for gigabit wireless access

For gigabit wireless data services, there are three important technical issues to be addressed: limited bandwidth, severe frequency-selective fading, and limited transmit power. A distributed antenna network (DAN) is a promising solution to the above three technical issues. In DAN, each mobile user is served by using multiple distributed antennas close to it. In this paper, recent advances in various distributed multi-input/multi-output (MIMO) techniques combined with single-carrier (SC) frequency-domain signal processing are presented for DAN. Particular attention is paid to SC frequency-domain MIMO diversity, relay, beamforming, and multiplexing jointly used with frequency-domain equalization (FDE) to significantly improve the signal transmission performance.

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.

A Novel Method for Performance Analysis of OFDM Polarization Diversity System in Ricean Fading Environment

OFDM (Orthogonal Frequency Division Multiplexing) is proven to be a very effective modulation and multiple access technique that enables high data rate transmission. Due to its good performance it is already implemented in several standardized technologies, and it is very promising technique for the next generation wireless communication systems. Still, further system performance improvements under severe frequency selective fading conditions are necessary, and they can be obtained implementing diversity, at either transmit or/and receive end of a wireless link. Since polarization diversity can be realized using only one compact, dual polarized antenna, it can be considered as an attractive, space and cost effective solution. Analysis presented in this paper shows that implementation of dual polarized antenna at the receiver can lead to significant performance improvement, under certain propagation conditions. In order to calculate BER (Bit Error Rate) for the considered OFDM polarization diversity system with a certain level of the received signals correlation, we propose a novel analytical method. The obtained results are compared with the ones attained by simulation.

Low Complexity Iterative Receiver Design for Shallow Water Acoustic Channels

An adaptive iterative receiver structure for the shallow underwater acoustic channel (UAC) is proposed using a decision feedback equalizer (DFE) and employing bit-interleaved coded modulation with iterative decoding (BICM-ID) in conjunction with adaptive Doppler compensation. Experimental results obtained from a sea trial demonstrate that the proposed receiver not only reduces inherent problem of error propagation in the DFE but also improves its convergence, carrier phase tracking, and Doppler estimation. Furthermore, simulation results are carried out on UAC, modelled by utilizing geometrical modelling of the water column that exhibits Rician statistics and a long multipath spread resulting in severe frequency selective fading and intersymbol interference (ISI). It has been demonstrated that there is a practical limit on the number of feedback taps that can be employed in the DFE and data recovery is possible even in cases where the channel impulse response (CIR) is longer than the span of the DFE. The performance of the proposed receiver is approximately within 1 dB of a similar system employing DFE and turbo code, however, at a significantly reduced computational complexity and memory requirements, making our system attractive for real-time implementation.

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.

Joint transmit/receive one-tap minimum mean square error frequency-domain equalisation for broadband multicode direct-sequence code division multiple access

Multicode direct-sequence code division multiple access (DS-CDMA) can flexibly support multimedia services with various data rates simply by changing the code multiplexing order. The use of simple one-tap frequency-domain equalisation (FDE) at a receiver is known to improve the bit error rate (BER) performance of multicode DS-CDMA in a severe frequency-selective fading environment. However, the BER performance improvement is limited due to the presence of the residual inter-chip interference (ICI). The authors propose a joint transmit/receive minimum mean square error (MMSE) FDE, which carries out one-tap transmit FDE and one-tap receive FDE jointly based on the MMSE criterion. The authors theoretically derive a suboptimal set of transmit and receive FDE weights and investigate the BER performance improvement by computer simulation in a freqeuncy-selective Rayleigh fading channel. The proposed scheme improves the received signal-to-interference plus noise ratio after despreading, and consequently, the BER performance can be significantly improved compared to the conventioal receive MMSE-FDE by (a) making the variations in the equivalent channel gain shallower to reduce the residual ICI for large code multiplexing order U and (b) allocating the transmit power to the frequencies having good condition to improve the received signal-to-noise ratio for a small U. This is conformed by the computer simualtion.

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.

Interference Suppression for Multiuser Downlink Transmission in Frequency-Selective Fading Channels

This paper presents an interference suppression scheme for multiuser downlink transmission in severe frequency-selective fading environments. Frequency-domain transmit beamforming and pre-equalization based on single-carrier frequency-domain equalization (SC-FDE) technique are employed to cancel co-channel interference (CCI) and to suppress intersymbol interference (ISI) with relatively low complexity. The proposed scheme effectively suppresses the ISI with a zero-forcing constraint on the CCI, and provides a viable solution to an otherwise analytically complex, if not unsolvable, problem. This paper also shows that, in frequency-selective fading channels, a multiuser system with K users and Mt base transceiver station antennas can offer lower bit error rate than a single-user system with Mt - K + 1 BTS antennas, if the proposed frequency-domain multiuser beamforming is employed.

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.

Adaptive Method of Slot and Bit Allocation for OFDMA Transmission Systems

This paper proposes the Orthogonal Frequency Division Multiple Access (OFDMA) transmission scheme with an adaptive slot and bit allocation method which can provide the broadband wireless LAN (WLAN) systems. From the fact that the radio links between different users and a base station experience the independent multi-path fading, this paper proposes a priority-based adaptive slot and bit allocation method in conjunction with an allocation adjustment method. The proposed method can achieve the higher efficient usage of frequency resource than those for the conventional fixed slot assignment with adaptive bit allocation method and the fixed bit assignment with adaptive slot allocation method. This paper presents various computer simulation results to evaluate the proposed method and demonstrates that the proposed method can achieve the higher frequency resource utilization with less computation complexity in the severe frequency selective multi-path fading channels.

Adaptive Method of Slot and Bit Allocation for OFDMA Transmission Systems

This paper proposes the Orthogonal Frequency Division Multiple Access (OFDMA) transmission scheme with an adaptive slot and bit allocation method which can provide the broadband wireless LAN (WLAN) systems. From the fact that the radio links between different users and a base station experience the independent multi-path fading, this paper proposes a priority-based adaptive slot and bit allocation method in conjunction with an allocation adjustment method. The proposed method can achieve the higher efficient usage of frequency resource than those for the conventional fixed slot assignment with adaptive bit allocation method and the fixed bit assignment with adaptive slot allocation method. This paper presents various computer simulation results to evaluate the proposed method and demonstrates that the proposed method can achieve the higher frequency resource utilization with less computation complexity in the severe frequency selective multi-path fading channels.