Doppler Spread Of Channel Research Articles

Timing estimation of multiple hyperbolic frequency‐modulated signals based on multicarrier underwater acoustic communication

AbstractUnderwater acoustic channel is much more complex than radio channel. Background noise, multipath extension, and Doppler frequency shift are all factors that affect high‐speed and stability for underwater communication. Orthogonal frequency division multiplexing (OFDM) is a modulation technology with high spectrum efficiency and strong anti‐multipath ability, usually used in high‐speed underwater acoustic communication. Signal arrival time detection and timing estimation before receiving data is an essential module in an underwater communication system. Timing estimation error will seriously affect the demodulation of the signal. In order to solve the problem of a complex channel environment in underwater acoustic communication system, a timing estimation method is proposed based on multiple hyperbolic frequency‐modulated (MHFM) signals. In order to improve the performance of underwater acoustic communication systems, this article proposes an iterative method of correlation‐sum‐mean absolute error (MAE) based on hyperbolic frequency‐modulated (HFM). The simulation results show that the proposed method can provide better performance and higher timing estimation accuracy under the conditions of low signal‐to‐noise ratio (SNR) and Doppler effect under oceanic noise channel and Doppler spread channel.

Near-Optimal BEM OTFS Receiver With Low Pilot Overhead for High-Mobility Communications

In this paper, a new receiver design based on basis expansion model (BEM) orthogonal time frequency space (OTFS) is presented for high-mobility communications with Doppler-spread channel. By deriving an analytical BEM OTFS system model, a low-order generalized complex exponential BEM (GCE-BEM) aided rough channel estimation is proposed at the initial stage with low pilot overhead, followed by equalization. Then, the refinement of channel estimation and equalization is conducted iteratively, in which a high-resolution GCE-BEM model with a large BEM order is adopted and the detected data symbols are exploited as pseudo-pilots, leading to higher estimation accuracy. Simulation results show that the proposed BEM OTFS receiver significantly outperforms the existing OTFS receivers in terms of the mean square error (MSE) of channel estimation and bit error rate (BER), while featuring low pilot overhead. Results also show the near-optimal performance of the novel solution, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i.e.</i> , achieved BER is very close to the case of perfect channel estimation. The theoretical lower bound on MSE of channel estimation is derived to verify the effectiveness of the proposed BEM OTFS receiver, which is shown to be close to simulation results.

On the Achievable Rate of Multiple-Input–Multiple-Output Underwater Acoustic Communications

This paper investigates the data gain brought by the multiple-input-multiple-output (MIMO) principle for underwater acoustic (UWA) communication. By considering a class of acoustic MIMO channels where each fading coefficient is modeled by a Rice distribution law with correlation factor depending on channel Doppler spread, we derive the achievable rate of such system that takes into account both channel estimation error and training sequence overhead. Model parameters are then estimated from an experiment campaign in a MIMO shallow water channel conducted in the roadstead of Brest, France. The system achievable rate is evaluated for several MIMO architectures and channel configurations and then compared against the conventional single-input-multiple-output transmission. The achievable rate gain is finally put in perspective with end-to-end data rate performance of a single-carrier MIMO transmission system experimented at sea.

High-Mobility CoMP Massive MIMO Uplink Transmissions: Channel PSD Analysis and Doppler Spread Suppression

Coordinated Multi-Point transmission/reception (CoMP) is one of the promising techniques for high-mobility wireless communications systems to increase cell edge user throughput in both uplink and downlink transmission. In this paper, we consider the high-mobility CoMP uplink transmission, where a high-speed terminal (HST) simultaneously transmits signals to multiple base stations (BSs) with angle-domain Doppler shifts compensation. We derive the exact channel power spectrum density (PSD) for uplink CoMP transmissions. The antenna weighting technique is then proposed to suppress the Doppler spread, where an optimization problem is formulated to minimize the maximum Doppler spread among all the BSs. Numerical results are provided to corroborate both the PSD analysis and the superiority of antenna weighting technique.

Interference Cancellation and Iterative Detection for Orthogonal Time Frequency Space Modulation

The recently proposed orthogonal time–frequency–space (OTFS) modulation technique was shown to provide significant error performance advantages over orthogonal frequency division multiplexing (OFDM) over delay-Doppler channels. In this paper, we first derive the explicit input–output relation describing OTFS modulation and demodulation (mod/demod). We then analyze the cases of: 1) ideal pulse-shaping waveforms that satisfy the bi-orthogonality conditions and 2) rectangular waveforms which do not. We show that while only inter-Doppler interference (IDI) is present in the former case, additional inter-carrier interference (ICI) and inter-symbol interference (ISI) occur in the latter case. We next characterize the interferences and develop a novel low-complexity yet efficient message passing (MP) algorithm for joint interference cancellation (IC) and symbol detection. While ICI and ISI are eliminated through appropriate phase shifting, IDI can be mitigated by adapting the MP algorithm to account for only the largest interference terms. The MP algorithm can effectively compensate for a wide range of channel Doppler spreads. Our results indicate that OTFS using practical rectangular waveforms can achieve the performance of OTFS using ideal but non-realizable pulse-shaping waveforms. Finally, simulation results demonstrate the superior error performance gains of the proposed uncoded OTFS schemes over OFDM under various channel conditions.

Impact of Cooperative Space-Time/Frequency Diversity in OFDM Based Wireless Sensor Systems over Mobile Multipath Channels.

Cooperative linear dispersion coding (LDC) can support arbitrary configurations of source nodes and destination nodes in virtual multi-input multi-output (MIMO) systems. In this paper, we investigate two spatial diversity applications of cooperative LDC for orthogonal frequency division multiplexing (OFDM) based wireless sensor systems in order to achieve space-time/frequency (ST/SF) diversity gains when transmitting over time-/frequency-selective fading channels. Cooperative LDC-aided ST/SF-OFDM is flexible in configuring various numbers of cooperative source nodes and time-slots or frequency-tones. Our results show that the ST-OFDM scheme is sensitive to exploiting diversity gains, subject to the impact of varying channel Doppler spreads; while the performance of SF-OFDM is mainly subject to delay spread. Specifically, when the system involves more than two cooperative nodes, the cooperative LDC-aided ST/SF-OFDM outperforms the cooperative orthogonal block codes (e.g. Tarokh's codes) aided ST/SF-OFDM, when communicating over a higher Doppler/delay spread.

Parametric Replay-Based Simulation of Underwater Acoustic Communication Channels

This paper presents an underwater acoustic channel simulation methodology that combines parametric modeling with stochastic replay of at-sea measured channel impulse responses. The motivation behind this approach is to extend the scope of use of replay-based methods by allowing some parameterization of the channel properties while complying with some level of realism. Such an approach is beneficial for extensive testing of communication links. The key idea is to deliberately distort the statistics of the experimental channel in order to meet some user-specified constraints. Our approach is based on a relative entropy minimization between the original time-varying channel impulse response and the simulated one. A particular attention is given to constraints on the channel Doppler spread and on the level of covariance between channel taps. The testing capabilities provided by parametric replay-based simulations are illustrated with real data collected in the bay of Brest, France.

Modulation network coding

In this study, we propose a four-dimensional modulation, referred to as modulation network coding (MNC), to address the problem of decoding a signal from multiple source node transmissions in mobile fast fading channels. The MNC scheme judiciously mixes a two-dimensional (2D) pilot symbol with a 2D information symbol, and makes use of a π/4 rotated M-pulse-amplitude modulation (PAM) constellation to guaranty an effective decoding of all the interfering symbols even in the case of a smaller channel Doppler spread compared with the period of the MNC symbol. In addition, because the MNC scheme makes use of an additional dimension introduced through orthogonal pulse waveform, the use of the pilot symbols does not reduce the effective information rate. The analytical and simulation results show that it is possible to achieve a low symbol error probability with a good signal-to-noise ratio when controlling a few parameters impacting the performance of the system such as the synchronisation in time and in frequency of the source nodes.

On Discrete-Time Modeling of Time-Varying WSSUS Fading Channels

The general property of noncommutativity of linear time-varying (LTV) systems has to be considered when a digital transmission system composed of LTV channel and transmit/receive filtering shall be represented in the equivalent discrete-time domain. In this correspondence, we analyze the implications of serially concatenated LTV systems with respect to the discrete-time representation of wide-sense stationary uncorrelated scattering (WSSUS) fading channels. We provide a detailed discussion of the largest tolerable channel Doppler spread (in relation to the system bandwidth) for which the concatenation is commutative in good approximation. In this case, the discrete-time description facilitates an efficient simulation model. The approximation accuracy is quantified by a detailed error analysis and further verified by numerical simulation adopting Universal Mobile Telecommunications System (UMTS) parameters.

Robotic Mobile System's Performance-Based MIMO-OFDM Technology

In this paper, a predistortion neural network (PDNN) architecture has been imposed to the Sniffer Mobile Robot (SNFRbot) that is based on spatial multiplexed wireless Orthogonal Frequency Division Multiplexing (OFDM) transmission technology. This proposal is used to improve the system performance by combating one of the main drawbacks that is encountered by OFDM technology; Peak-to-Average Power Ratio (PAPR). Simulation results show that using PDNN resulted in better PAPR performance than the previously published work that is based on linear coding, such as Low Density Parity Check (LDPC) codes and turbo encoding whether using flat fading channel or a Doppler spread channel.

Mobile Vehicle-to-Vehicle Narrow-Band Channel Measurement and Characterization of the 5.9 GHz Dedicated Short Range Communication (DSRC) Frequency Band

This study presents narrow-band measurements of the mobile vehicle-to-vehicle propagation channel at 5.9 GHz, under realistic suburban driving conditions in Pittsburgh, Pennsylvania. Our system includes differential Global Positioning System (DGPS) receivers, thereby enabling dynamic measurements of how large-scale path loss, Doppler spectrum, and coherence time depend on vehicle location and separation. A Nakagami distribution is used for describing the fading statistics. The speed-separation diagram is introduced as a new tool for analyzing and understanding the vehicle-to-vehicle propagation environment. We show that this diagram can be used to model and predict channel Doppler spread and coherence time using vehicle speed and separation.

Estimation of Rapidly Time-Varying Sparse Channels

The estimation of sparse shallow-water acoustic communication channels and the impact of estimation performance on the equalization of phase coherent communication signals are investigated. Given sufficiently wide transmission bandwidth, the impulse response of the shallow-water acoustic channel is often sparse as the multipath arrivals become resolvable. In the presence of significant surface waves, the multipath arrivals associated with surface scattering fluctuate rapidly over time, in the sense that the complex gain, the arrival time, and the Dopplers of each arrival all change dynamically. A sparse channel estimation technique is developed based on the delay-Doppler-spread function representation of the channel. The delay-Doppler-spread function may be considered as a first-order approximation to the rapidly time-varying channel in which each channel component is associated with Doppler shifts that are assumed constant over an averaging interval. The sparse structure of the delay-Doppler-spread function is then exploited by sequentially choosing the dominant components that minimize a least squares error. The advantage of this approach is that it captures both the channel structure as well as its dynamics without the need of explicit dynamic channel modeling. As the symbols are populated with the sample Dopplers, the increase in complexity depends on the channel Doppler spread and can be significant for a severely Doppler-spread channel. Comparison is made between nonsparse recursive least squares (RLS) channel estimation, sparse channel impulse response estimation, and estimation using the proposed approach. The results are demonstrated using experimental data. In training mode, the proposed approach shows a 3-dB reduction in signal prediction error. In decision-directed mode, it improves significantly the robustness of the performance of the channel-estimate-based equalizer against rapid channel fluctuations.

QoS guarantee and provisioning for realtime digital video over mobile ad hoc cdma networks with cross-layer design

In this article we investigate the trade-offs and the constraints for multimedia over mobile ad hoc CDMA networks, and propose a cross-layer distributed power control and scheduling protocol to resolve those trade-offs and constraints in order to provide high-quality video over wireless ad hoc CDMA networks. In particular, a distributed power control and scheduling protocol is proposed to control the incurred delay of video streaming over multihop wireless ad hoc networks, as well as the multiple access interference (MAI). We also investigate the impacts of Doppler spread and noisy channel estimates upon the end-to-end video quality, and provide a relatively robust system which employs a combination of power control and coding/interleaving to combat the effects of Doppler spread by exploiting the increased time diversity when the Doppler spread becomes large. Thus, more robust end-to-end video quality can be achieved over a wide range of channel conditions

Analysis of PCC-OFDM systems for general time-varying channel

In this letter, we derive the general expression of ICI power for 1/2 rate PCC-OFDM system in terms of the correlation function of the channel. We have also developed the upper and lower bound of 1/2 rate PCC OFDM system and found that the ICI power reduction due to coding is about 11.9 dB. To examine the validity of the general expressions, we consider the example of carrier frequency offset (CFO), 2-path, Jakes' and uniform Doppler spread channel models and show that the results are consistent with simulations.

ＯＦＤＭ移動受信におけるＭＭＳＥ型ＩＣＩキャンセラに関する一検討

In OFDM transmission, a loss of sub-carrier orthogonality due to Doppler-spread channels leads to inter-carrier interference (ICI). This problem is especially severe in mobile reception environments. ICI causes significant degradation in bit error rate characteristics and its influence increases as the carrier frequency or velocity of the receiver increases. We propose a method to reduce ICI caused by the Doppler-spread channel. The transmission channel is modelled by a combination of multiple Doppler-shifted propagation paths and their parameters, such as attenuation, relative delay, and Doppler-shift, and are then estimated using scattered pilot symbols. The ICI is cancelled by multiplying the inverse matrix of the estimated channel matrix by the received OFDM symbol vector. However, the ICI canceller emphasizes a noise component, so it does not improve the bit error rate characteristics sufficiently. In this paper, we propose an ICI canceller based on MMSE that does not emphasize a noise component. Using computer simulation, we confirmed that ICI cancellers based on MMSE can reduce the bit error rate more than ICI cancellers based on zero-forcing under the Doppler-spread channel.

Optimal training design for MIMO OFDM systems in mobile wireless channels

This paper describes a least squares (LS) channel estimation scheme for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems based on pilot tones. We first compute the mean square error (MSE) of the LS channel estimate. We then derive optimal pilot sequences and optimal placement of the pilot tones with respect to this MSE. It is shown that the optimal pilot sequences are equipowered, equispaced, and phase shift orthogonal. To reduce the training overhead, an LS channel estimation scheme over multiple OFDM symbols is also discussed. Moreover, to enhance channel estimation, a recursive LS (RLS) algorithm is proposed, for which we derive the optimal forgetting or tracking factor. This factor is found to be a function of both the noise variance and the channel Doppler spread. Through simulations, it is shown that the optimal pilot sequences derived in this paper outperform both the orthogonal and random pilot sequences. It is also shown that a considerable gain in signal-to-noise ratio (SNR) can be obtained by using the RLS algorithm, especially in slowly time-varying channels.

デジタル放送・伝送方式 ＯＦＤＭ受信における遅延プロファイルとドップラープロファイルの推定によるキャリヤ間干渉の除去

During an OFDM transmission, a loss of sub-carrier orthogonality due to Doppler-spread of the channel leads to inter-carrier interference (ICI). This problem is especially bad in mobile reception environments. especially, under mobile reception environments. ICI causes significant degradation of the bit error rate characteristics and its influence becomes larger as the carrier frequency or velocity of the receiver increases. This paper proposes a method to reduce the ICI caused by the Doppler spread of the channel. The transmission channel is modeled by a combination of multiple Doppler shifted propagation paths, and their parameters such as attenuation, relative delay and Doppler-shift are estimated using scattered pilot symbols. The ICI is canceled by multiplying the inverse matrix of the estimated channel matrix with the received OFDM symbol vector. Computer simulation confirmed that the proposed method can reduce the bit error rate of a Doppler spread channel.

Terrestrial digital video broadcasting for mobile reception using OFDM

Performance of a system design for digital video broadcasting is examined with emphasis on mobile reception. Orthogonal frequency division multiplexing (OFDM) is used to achieve good bandwidth efficiency and to mitigate the intersymbol interference resulting from the channel delay spread. The resulting equivalent channel including OFDM can be modeled as a flat Rayleigh fading channel plus an interchannel interference (ICI) term due to the channel Doppler spread. This ICI term is analyzed and shown to result in an error floor. Performance improvements due to antenna diversity and trellis-coded modulation (TCM) are given. Finally, multiresolution modulation is discussed as a means of achieving graceful degradation and giving degrees of freedom for further performance improvement.

Discrete realization for receivers detecting signals over random dispersive channels. Part II: Doppler-spread channel

We propose a new suboptimal receiver for detecting signals transmitted over random doppler-spread channels. We discuss in particular the first order channel and a transmitted signal of a constant envelope. The proposed receiver is composed of an integrator, a sampler and a quadratic filter. We give numerical results concerning the performances of this receiver. In particular, we give the evolution of error probability versus the number of samples (or sampling rate). We show the important role of the ratio of signal duration to channel coherence time and we suggest how to choose the transmitted signal duration and the sampling rate for a given signal to noise ratio.

Atmosphere Propagation and Communication Channel Model for Laser Wavelengths

The advent of the laser has generated much interest in its use for communication. Consideration is given in this paper to obtaining a communication channel model of the atmosphere for laser frequencies. For this purpose a fairly exhaustive, tutorial, and partly critical review of a major part of the extensive literature pertaining to theoretical and experimental results on the propagation of laser signals in the atmosphere is given. Some of the basic characteristics of the channel for Which detailed results are given are 1) channel time spread (or, equivalently, channel coherence bandwidth) due to multipathing and dispersion, 2) channel gain, 3) channel doppler spread and coherence time, 4) channel spatial coherence and amplitude covariance functions. Consideration is given to both point receivers and area receivers, heterodyne and direct detection receivers, nonplanar (i.e., spherical and beam) wave propagation, and infinite plane-wave propagation. The results are given for clear weather conditions. Brief attention is given to additive noise. In reviewing the extensive literature dealing with the propagation of optical signals through a turbulent medium, an attempt is made to put this varied, diverse, and in some instances controversial material into proper perspective; to note where theoretical results derived for one set of conditions were applied to experimental results obtained for another set of conditions; to correct some invalid conclusions drawn from some theoretical results as far as signal degradation effects and parameter variations; and to indicate where out-of-date vertical profile models were used with updated models being given, and where possible the new theoretical predictions based on these models provided. The paper should, as a result, be of help to those familiar with the literature as Well as those who are not. For those not acquainted with the literature, in addition to providing a fairly in-depth knowledge of the characteristics of propagation through a turbulent medium and an indication of the status of our understanding of this subject, the paper should provide a stepping stone for obtaining a more thorough understanding of the subject and for doing work in the field.