Time Domain Channel Impulse Response Research Articles

Joint channel and Doppler shift estimation for underwater acoustic single carrier system

Underwater acoustic channel is a time-varying, strong multipath channel. Due to the low velocity of sound (about 1500 m/s), Doppler shift, occurred by the relative motion between transmitter and receiver, restricts the performance of communication. In order to transmit data in underwater acoustic single carrier communication system with strong stability and high reliability, especially in high mobility scenarios, a joint channel and Doppler shift estimation method is proposed in this paper. We propose a novel data frame structure based on a Gold sequence, so the channel and Doppler shift can be estimated. For different underwater scenarios, we design Gold sequence templates with different Doppler factors. When the Gold sequence from the received data frame is extracted, with good correlation characteristics of the Gold sequence, the maximum correlation value can be calculated, and the Doppler shift corresponding to the maximum is the estimation of Doppler shift. Then Doppler shift will be compensated by resampling the communication data. For channel estimation, the Gold sequence is also used to estimate the time-domain channel impulse response (CIR). Simulation results show that the proposed method can effectively estimate channel and Doppler shift in an environment with multipath and Doppler.

Joint multi-domain channel estimation based on sparse Bayesian learning for OTFS system

Since orthogonal time-frequency space (OTFS) can effectively handle the problems caused by Doppler effect in high-mobility environment, it has gradually become a promising candidate for modulation scheme in the next generation of mobile communication. However, the inter-Doppler interference (IDI) problem caused by fractional Doppler poses great challenges to channel estimation. To avoid this problem, this paper proposes a joint time and delay-Doppler (DD) domain based on sparse Bayesian learning (SBL) channel estimation algorithm. Firstly, we derive the original channel response (OCR) from the time domain channel impulse response (CIR), which can reflect the channel variation during one OTFS symbol. Compare with the traditional channel model, the OCR can avoid the IDI problem. After that, the dimension of OCR is reduced by using the basis expansion model (BEM) and the relationship between the time and DD domain channel model, so that we have turned the underdetermined problem into an overdeter-mined problem. Finally, in terms of sparsity of channel in delay domain, SBL algorithm is used to estimate the basis coefficients in the BEM without any priori information of channel. The simulation results show the effectiveness and superiority of the proposed channel estimation algorithm.

Low-Complexity Semi-Blind Channel Estimation Algorithms for Vehicular Communications Using the IEEE 802.11p Standard

The vehicle-to-vehicle (V2V) communications channel is highly time-varying, making reliable communication difficult. This problem is particularly challenging because the de facto standard for V2V communications, the IEEE 802.11p standard, is based on the IEEE 802.11a standard, which was designed for the indoor and relatively stationary wireless LAN channel. In particular, the frame structure, which allows large packets and has low pilot density makes channel estimation difficult. In this paper, we propose several semi-blind channel estimation and tracking algorithms that are suitable for highly time-varying channels using the 802.11p frame structure. Two of the proposed schemes utilize the finite alphabet property of the transmitted symbols and utilize pilot information. A third scheme is a variant of decision-directed channel estimation that utilizes knowledge of the preamble. All schemes apply time-domain channel impulse response truncation for improved performance. We compare the performance of the proposed schemes using six different V2V channel models. The proposed schemes realize huge performance gains over previously proposed ones, reaching 20 dB in some cases, where previously proposed schemes are unusable. These performance gains are realized for all the V2V channel models, at different vehicle velocities, and for all modulation schemes and packet sizes. Two of the proposed schemes are low-complexity schemes that avoid expensive search operations, yet offer significantly improved performance.

FFT-Based Channel Estimation Scheme in LTE-A Downlink System

In this paper, we propose a channel estimation scheme for a long term evolution-advanced (LTE-A) downlink system. The proposed scheme uses the fast Fourier transform (FFT) interpolation scheme for the user moving at a high speed. The FFT interpolation scheme converts the channel frequency response obtained from the least square or minimum mean square error channel estimation scheme to a time-domain channel impulse response by using the inverse FFT (IFFT). After windowing the channel response in the time domain, the channel frequency response can be obtained using the FFT. We performed a system-level simulation based on the 20 MHz bandwidth of the 3rd Generation Partnership Project LTE-A downlink system. Simulation results show that the proposed channel estimation scheme can improve the signal-to-noise-plus-interference ratio, as well as the throughput and spectral efficiency, of a conventional system.

Signal hole repair strategy based on sensor deployment density for mobile crowd network

In order to reduce the signal holes in wireless sensor networks, we proposed a sensor deployment density-aware signal hole repair strategy of mobile crowd network. On the one hand, based on the multi-dimensional connected graph, the density continuous jitter problem of the sensor network is solved effectively. Sensor density prediction method improves the efficiency of random deployment of sensor nodes. Based on the above schemes, the sensor deployment density sensing model is proposed. On the other hand, according to the regional geometry, diversity of the sensor network channel and density can detect the time domain channel impulse response signal successfully. Finally, the network signal is detected by the signal detection of the mobile crowd network. The signal holes can be detected and repaired based on coverage density and crowd. Experimental results show that the proposed algorithm has outstanding performance in terms of signal strength and signal void ratio compared with the energy-aware repair algorithm.

Time domain equalization method for TS-OFDM signal under higher mobile environments

In higher time-varying fading channels, the signal quality of orthogonal frequency division multiplexing (OFDM) technique would be degraded relatively due to the occurrence of inter-carrier interference (ICI). To solve this problem, this paper firstly proposes a new design of time domain training sequence (TS) in the estimation of channel impulse response (CIR) for the TS-OFDM signal which can reduce the leakage of power spectrum density (PSD) at the outside of OFDM allocated frequency bandwidth with keeping higher CIR estimation accuracy. Secondly, this paper proposes a time domain equalization (TDE) method which can achieve better bit error rate (BER) performance with keeping lower computation complexity even in higher time-varying fading channels. The salient feature of the proposed TDE method is to employ a partial differentiation for the time domain CIR matrix for solving the maximum likelihood (ML) equation in which the time domain CIR matrix becomes a symmetric banded matrix. From this feature, a low-complexity parallel block inverse matrix algorithm can be employed in the calculation of inverse matrix in keeping the same accuracy as that of the direct inverse matrix calculation. This paper presents various computer simulation results to demonstrate the effectiveness of the proposed TDE method for the TS-OFDM signal as compared with conventional frequency domain equalization (FDE) methods under higher mobile environments.

Per-antenna signal to leakage noise ratio precoding for MU-MIMO-OFDM system with reduced feedback

In this work, multi-user multiple input multiple outputs orthogonal frequency division multiplexing (MU-MIMO-OFDM) precoding is introduced. The proposed method makes use of per-antenna signal to leakage plus noise ratio (PA-SLNR) as optimisation criteria. This optimisation criterion takes both the inter-user and inter-antennas interference cancellation into account. Unlike the conventional proposed methods in which simple frequency flat fading is assumed, in the proposed method we generalised the precoding problem to more realistic frequency selective MU-MIMOOFDM Broadcast channel where OFDM is used to mitigate inter-symbol-interference (ISI) and the MU-MIMO is used for spatial multiplexing. Conventionally, each individual user has to feedback the channel frequency response of each subcarrier to the central node base station for precoding matrix design which leads to high traffic load in the feedback channel. In this work we also introduce a simple frequency response channel factorisation such that each user is required to feedback only one subcarrier time domain channel impulse response which dramatically reduces the amount of the feedback load.

Per-antenna signal to leakage noise ratio precoding for MU-MIMO-OFDM system with reduced feedback

In this work, multi-user multiple input multiple outputs orthogonal frequency division multiplexing (MU-MIMO-OFDM) precoding is introduced. The proposed method makes use of per-antenna signal to leakage plus noise ratio (PA-SLNR) as optimisation criteria. This optimisation criterion takes both the inter-user and inter-antennas interference cancellation into account. Unlike the conventional proposed methods in which simple frequency flat fading is assumed, in the proposed method we generalised the precoding problem to more realistic frequency selective MU-MIMOOFDM Broadcast channel where OFDM is used to mitigate inter-symbol-interference (ISI) and the MU-MIMO is used for spatial multiplexing. Conventionally, each individual user has to feedback the channel frequency response of each subcarrier to the central node base station for precoding matrix design which leads to high traffic load in the feedback channel. In this work we also introduce a simple frequency response channel factorisation such that each user is required to feedback only one subcarrier time domain channel impulse response which dramatically reduces the amount of the feedback load.

Analytic Comparison for Channel Response Estimation Based on Time- and Frequency-Domain Pilot Signals

For the orthogonal frequency-division multiplexing (OFDM) system, one can estimate the channel responses (CRs) based on frequency-domain (FD) pilot symbols. One can also estimate the time-domain (TD) channel impulse response (CIR) by inserting a unit pulse ahead of each OFDM block. However, there is no analytic comparison between the CR estimation based on FD pilot symbols and TD unit pulses. In this paper, we first consider the 1-D blocks of CRs and give the analytic comparison of mean-squares error (MSE) for the CR estimation based on TD and FD pilot signals, respectively. For the linear minimum MSE (LMMSE) algorithm, which attains the minimum MSE for the CR estimation based on FD pilots, we derive an MSE lower bound among all possible distributions of pilot symbols. This MSE lower bound coincides with the MSE attained by the optimal CR estimation based on TD unit pulses, under the condition that both FD and TD pilot signals have the same power. Our analysis also indicates that when the number of FD pilots is smaller than the rank of correlation matrix of CRs, it results in an error floor. We further extend our analysis to 2-D blocks of CRs and show that the MSE formulas for CR estimation based on FD and TD pilot signals share the same forms and that they have close numerical values.

Time Domain Channel Estimation for OQAM-OFDM Systems: Algorithms and Performance Bounds

In this paper, we first present a general time domain model for the channel estimation in the orthogonal frequency division multiplexing system with offset quadrature amplitude modulation (OQAM-OFDM), and utilize the frequency domain pilots to estimate the time domain channel impulse responses. Different form the conventional methods, there is no specific requirement for the length of the symbol interval compared to the the maximum channel delay spread in the proposed scheme. Furthermore, with the proposed time domain model, the channel statistic information could be utilized to improve the performance of the channel estimation. Then, we propose two channel estimation schemes, i.e., linear minimum mean square error (LMMSE) and weighted least square (WLS), and we also derive their corresponding Bayesian Cramér-Rao Bound (BCRB) and Cramér-Rao Bound (CRB) bounds, respectively. Simulation results demonstrate that the BCRB and CRB bounds could be achieved by the proposed LMMSE and WLS methods, respectively. Moreover, simulation results show that the proposed methods are much robust to the time synchronization error compared to the conventional frequency domain methods, and imply that the pulse shaping filter with waveforms concentrated in the time domain could be employed in OQAM-OFDM systems to improve the channel estimation performance and spectral efficiency.

Time-Domain Compression Based Analog Feedback for MIMO-OFDM Systems

We investigate the analog feedback of channel state information (CSI) in multiple-input-multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) based wideband wireless communication systems. In MIMO-OFDM systems, the amount of CSI to be fed back grows prohibitively with the number of subcarriers. A straightforward way of reducing the overhead is to feed back a sampled version of the CSI. But sampling based feedback suffers from the aliasing effect in some scenarios. We proved that the aliasing effect can not be eliminated unless the system feeds back more subcarrier channel matrices than the length of the time-domain channel impulse response. To improve the feedback quality and cope with the aliasing problem, we make use of the sparsity of the time-domain channel impulse response and propose a time-domain compression based analog feedback scheme. The proposed scheme overcomes the the aliasing problem, and achieves good performance.

A Study of Pilot-Assisted OFDM Channel Estimation Methods With Improvements for DVB-T2

We provide a comparative study of channel estimation methods for pilot-assisted orthogonal frequency-division multiplexing (OFDM) communication systems in time-varying frequency-selective fading channels. We also propose an improved low-complexity least-squares (LS) estimation method based on the domain-transform process, which estimates time-domain channel impulse response (CIR) using the LS estimate of pilot subchannels. Our method is particularly suited to commercial OFDM systems such as the second generation of digital terrestrial television broadcasting systems (DVB-T2) because it has the following properties: 1) It uses only one OFDM symbol; 2) it does not require knowledge of channel statistics; 3) it works for any pilot pattern; and 4) the size of the estimation matrix for obtaining CIR only depends on the number of pilot subcarriers, but not on the size of OFDM symbol. Because of the first three properties, our method is robust to high Doppler frequency shifts, timing synchronization errors, and pilot patterns in commercial systems. Moreover, through computer simulations, we verify that our method provides competitive bit error rate and channel estimation error performance, compared with some well-known methods, which suffer from either a much higher computational load or less robustness to Doppler shifts or timing errors. For irregular pilot patterns, efficient yet simple regularization methods are suggested to solve the ill-conditioned problem of the estimation matrix. An upper bound on the mean squared error of the estimated CIR is also provided.

QRD-based precoded MIMO-OFDM systems with reduced feedback

QR decomposition (QRD)-based precoded MIMO-OFDM systems with reduced feedback are proposed to convert the MIMO-OFDM channel into layered subchannels. QRD-M is further combined with either singular value (SVD) or geometric mean decomposition (GMD) of the time-domain channel impulse response matrix. As a result, the receiver in the proposed systems only needs to feed back information describing one precoding matrix for all carriers. Simulation results confirm the bit-error-rate (BER) and throughput performance superiority of the proposed systems compared to conventional SVD per-carrier precoding schemes.

Estimation of Channel Transfer Function and Carrier Frequency Offset for OFDM Systems With Phase Noise

The joint estimation of carrier frequency offset (CFO) and channel transfer function (CTF) for orthogonal frequency-division multiplexing (OFDM) systems with phase noise is discussed in this paper. A CFO estimation algorithm is developed by exploring the time-frequency structure of specially designed training symbols, and it provides a very accurate estimation of the CFO in the presence of both unknown frequency-selective fading and phase noise. Based on the estimated CFO, phase noise and frequency-selective fading are jointly estimated by employing the maximum <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a posteriori</i> (MAP) criterion. Specifically, the fading channel is estimated in the form of the frequency-domain CTF. The estimation of the CTF eliminates the requirement of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> knowledge of channel length, and it is simpler compared with the time-domain channel impulse response (CIR) estimation methods used in the literature. Theoretical analysis with the Cramer-Rao lower bound (CRLB) demonstrates that the proposed CFO and CTF estimation algorithms can achieve near-optimum performance.

Adaptive raised-cosine channel interpolation for pilot-aided OFDM systems

In this paper, we first show equivalence of OFDM channel estimation using time-domain windowing and using frequency-domain interpolation. Based on this equivalence, a new frequency-domain channel interpolator featuring the advantage of time-domain channel impulse response windowing is proposed. Furthermore, the proposed raised-cosine channel interpolator can adaptively adjust its coefficients to accommodate various channel power delay profiles. Both theoretical and simulation results verify that the proposed interpolator achieves more accurate channel estimation performance than other existing solutions.

Stability of Narrowband Dynamic Body Area Channel

The stability of a dynamic narrowband on-body area channel is characterized based on real-time measurements of the time domain channel impulse response (CIR) at frequencies near the 900- and 2400-MHz industrial, scientific, and medical (ISM) bands. A new parameter, channel variation factor, characterizes channel coherence time. Body movement is considered at various transmit-receive pair locations on the human body. Movement has considerable impact on the stability of the channel, a reasonable assumption for coherence time is approximately 10 ms, and there is greater temporal stability at the lower frequency.

Novel Frequency and Time Domain Method for Efficient Uplink Ranging in Cellular OFDMA

We address initial ranging process in OFDMA system such as IEEE 802.16e. The proposed ranging method includes two main tasks multi-user ranging code detection and timing estimation. The multi-user ranging code is detected based on a time domain channel (impulse response) detection corresponding to each ranging user through a correlation process that can be performed efficiently in the frequency domain. For initial ranging users, the timing offset also needs to be extracted, which is straightforward after channel is estimated in the process. The simplicity and the efficiency of the proposed method are demonstrated through computer simulation studies.

Wideband channel sounder with measurements and model for the 60 GHz indoor radio channel

A wideband channel sounder and measurement results for the short range indoor 60 GHz channel are presented. The channel sounder is based on a 1 gigasamples/s dual channel arbitrary waveform generator and A/D converter/software demodulator, which synthesize and detect a baseband PN sequence with 500 MHz bandwidth. A heterodyne transmitter and receiver translate the baseband PN sequence to and from the 60 GHz band. Ten channel measurements taken across the 59 GHz to 64 GHz range are concatenated to provide a continuous channel measurement covering 5 GHz of bandwidth, resulting in 0.2 ns time domain channel impulse response resolution. The dynamic range and maximum sensitivity performance of the channel sounder are discussed in detail. Comparisons of results with a vector network analyzer based system are shown to verify the accuracy of the sounder. In an extensive measurement campaign with vertically polarized omnidirectional antennas, several different rooms (offices, labs, conference rooms and others) in four different buildings have been investigated. Over 700 channel measurements are the basis for a comprehensive characterization of the short range 60 GHz indoor radio channel with omnidirectional antennas. Finally, a simple stochastic static multipath channel model is derived from the measurement results.

Iterative Based Time Domain Equalization Method for DFTS-OFD in Highly Mobile Environments

In highly mobile environments, signal quality of discrete Fourier transform spreading-orthogonal frequency division multiplexing (DFTS-OFDM) with a frequency domain equalization method would be degraded due to the occurrence of inter-channel interference (ICI). To solve this problem, this paper proposes an iterative based time domain equalization (TDE) method with a time domain channel impulse response (CIR) estimation for DFTS-OFDM signals. The salient features of the proposed method are the use of a time domain training sequence (TS) in the estimation of CIR instead the conventional pilot subcarriers and to employ the TDE method with a maximum likelihood (ML) estimation instead of the conventional one-tap minimum mean square error frequency domain equalization (One-Tap MMSE- FDE) method. This paper also proposes a low complexity iterative based TDE method using the properties of a symmetric banded CIR transfer matrix for solving the simultaneous equations instead of direct calculation of an inverse matrix. It also presents various simulation results in highly mobile environments to demonstrate the effectiveness of the proposed iterative based TDE with CIR estimation for the TS inserted DFTS-OFDM signals compared to the conventional One-Tap MMSE-FDE method.

Time Domain Equalization Method for DFTS-OFDM Signal without GI under Highly Mobile Environments

In highly time-varying fading channel, the Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiplexing (DFTS-OFDM) signal would be damaged significantly by the inter-channel interference (ICI) due to the loss of orthogonality among subcarriers which leads a fatal degradation of bit error rate (BER) performance. To solve this problem, this paper proposes a time domain equalization (TDE) technique in conjunction with a time domain channel impulse response (CIR) estimation method for the DFTS-OFDM signal without using a guard interval (GI). The features of proposed method is to employ a time domain training sequence (TS) both for the estimation of time domain CIR at every sampling time and for removing the inter-symbol interference (ISI) incurred in the multipath fading channel. The proposed method also employs the TDE with a maximum likelihood (ML) estimation method in the demodulation of received time domain signal at every symbol instead of using the conventional Minimum Mean Square Error-Frequency Domain Equalization (MMSE-FDE) method. This paper presents various simulation results to demonstrate the effectiveness of proposed demodulation method for the DFTSOFDM signal without GI as comparing with the conventional MMSE-FDE and TDE methods both of usingGI under highly mobile environments.