Radio Channel Impulse Response Research Articles

Study on precoding optimization algorithms in massive MIMO system with multi-antenna users

The paper studies the multi-user precoding problem as a non-convex optimization problem for wireless multiple inputs and multiple outputs (MIMO) systems. In our work, we approximate the target Spectral Efficiency function with a novel computationally simpler function. Then, we reduce the precoding problem to an unconstrained optimization task using a special differential projection method and solve it by the Quasi-Newton L-BFGS iterative procedure to achieve gains in capacity. We are testing the proposed approach in several scenarios generated using Quadriga-open-source software for generating realistic radio channel impulse response. Our method shows monotonic improvement over heuristic methods with reasonable computation time. The proposed L-BFGS optimization scheme is novel in this area and shows a significant advantage over the standard approaches. The proposed method has simple implementation and can be a good reference for other heuristic algorithms in this field.

System Error Calibration in Large Datasets of Wireless Channel Sounding for Industrial Applications.

In industrial applications, the large comprehensive wireless channel impulse response (CIR) reference dataset, measured by National Institute of Standards and Technology (NIST), has been a useful tool for understanding propagation within factory environments. The NIST CIR reference dataset is obtained using a precision channel sounder instrument where transmitter and receiver are time-synchronized by two rubidium clocks. While the accuracy of the NIST CIRs is much higher than the CIRs measured by general commercial digital receiver, two types of system errors have been discovered within the dataset from the perspective of signal processing. These errors are significant for wireless localization, physical layer security, and related applications. To calibrate the CIR, two channel sounder error calibration methods (CSEC) is proposed: the CSEC based on phase compensation and carrier frequency offset recovery. Our results reveal that the CSEC method can improve the accuracy of the CIR to the accuracy that precise instruments cannot achieve. To demonstrate the consequence of these systemic errors, a case study involving physical layer authentication is investigated showing a marked improvement in authentication accuracy after the systemic errors in the dataset are removed. Moreover, the CSEC method may be used to correct other CIR datasets with similar systemic errors.

Performance Limits for Fingerprinting-Based Indoor Optical Communication Positioning Systems Exploiting Multipath Reflections

In this paper, we introduce an uplink optical wireless positioning system for indoor applications. This technique uses fingerprints based on the indoor optical wireless channel impulse response for localization. Exploiting the line of sight peak power (LOS), the second power peak (SPP) of the impulse response, and the delay between the LOS and SPP, we present a proof of concept design and theoretical analysis for localization employing a single fixed reference point, i.e., a photodetector (PD) on the ceiling. Adding more PDs leads to more accurate transmitter position estimation. As a benchmark, we present analytical expressions of the Cramer-Rao lower bound (CRLB) for different numbers of PDs and features. We further present closed form analytical approximations for the chosen features of the channel impulse response. Simulation results show a root mean square (RMS) positioning accuracy of 25\,cm and 5\,cm for one and four PDs, respectively, for a typical indoor room at high SNR. Numerical results verify that the derived analytic approximations closely match the simulations.

Improvement of FER using Differential Space-Time Block Code

Over recent years, various modulation and coding techniques have been proposed in MIMO wireless communication systems. A MIMO system uses the concept of spatial diversity which is very successful and promising technique. When a coherent transmission system is considered, the estimation of radio channel impulse response is done precisely. In MIMO systems, the radio channel is estimated among every transmitting and receiving antennas such that the complexity can be increased. For this reason, in MIMO systems differential modulation schemes are estimated. A Differential Space-Time Block Code (DSTBC) is useful in the Raleigh fading channel as they do not require channel estimates. Space-time coding with MIMO antennas at transmitting and receiving reduces the consequences of fading in multiple paths and therefore the performance of digital transmission throughout wireless radio channel can be improved. So it has been presumed that perfect CSI is available at the receiver and coherent detection is employed. This paper presents improvement of Frame Error Rate (FER) for differential space-time block code using various Doppler spectra. When the channels estimates are not available the DSTBC system noticed that at SNR of 10 dB, for two transmitting and four receiving antennas the FER is 0.0067 for rounded Doppler spectrum. The differential schemes attains full transmit diversity owing to orthogonal designs. However, the receiver or the transmitter needs the channel state information so these differential schemes are 3 dB worse than the STBC with coherent detection.

Off-Body Radio Channel Impulse Response Model Under Hospital Environment: Measurement and Modeling

A novel frequency domain autoregressive (AR) model for off-body channel at 6–8.5-GHz band is proposed. It can be employed to characterize the channel impulse responses of an off-body wireless channel under hospital environments. In this model, the magnitude and phase of the poles of the AR transfer function are normally distributed and uniformly distributed random variables, and the variance of the input signal follows the normal distribution. By comparing the average power delay profile and the root mean square delay spread distribution with the conventional tapped delay line (TDL) model, the proposed model is verified to have higher accuracy and lower complexity. The proposed model is expected to be useful for the physical layer implementation of wireless communication systems for future smart healthcare applications.

Joint Channel Estimation and Data Detection in MIMO-OFDM Systems: A Sparse Bayesian Learning Approach

The impulse response of wireless channels between the N-t transmit and N-r receive antennas of a MIMO-OFDM system are group approximately sparse (ga-sparse), i.e., NtNt the channels have a small number of significant paths relative to the channel delay spread and the time-lags of the significant paths between transmit and receive antenna pairs coincide. Often, wireless channels are also group approximately cluster-sparse (gac-sparse), i.e., every ga-sparse channel consists of clusters, where a few clusters have all strong components while most clusters have all weak components. In this paper, we cast the problem of estimating the ga-sparse and gac-sparse block-fading and time-varying channels in the sparse Bayesian learning (SBL) framework and propose a bouquet of novel algorithms for pilot-based channel estimation, and joint channel estimation and data detection, in MIMO-OFDM systems. The proposed algorithms are capable of estimating the sparse wireless channels even when the measurement matrix is only partially known. Further, we employ a first-order autoregressive modeling of the temporal variation of the ga-sparse and gac-sparse channels and propose a recursive Kalman filtering and smoothing (KFS) technique for joint channel estimation, tracking, and data detection. We also propose novel, parallel-implementation based, low-complexity techniques for estimating gac-sparse channels. Monte Carlo simulations illustrate the benefit of exploiting the gac-sparse structure in the wireless channel in terms of the mean square error (MSE) and coded bit error rate (BER) performance.

Accurate OFDM Sparse Channel Estimation by Optimal Deterministic Pilot Pattern

ABSTRACTIn orthogonal frequency division multiplexing (OFDM) systems the wireless channel impulse response can be modelled as a sparse vector. Recently emerged compressed sensing (CS) theory can be used for reconstruction and estimation of unknown non-zero channel taps. In CS-based channel estimation, the number of pilot sub-carriers can be decreased, so the spectral efficiency of the system can be increased. The efficiency and performance of sparse channel estimation (SCE) highly depend on the measurement matrix, which in turn depends on the pilot pattern. In this paper, based on minimization of the correlation between all columns of measurement matrix, a new criterion is proposed. In addition, a deterministic pilot pattern optimization algorithm for OFDM SCE is introduced. Greedy CS recovery methods, such as orthogonal matching pursuit (OMP) and regularized OMP are used for estimation of the channel taps. The simulation results confirm the efficiency and accuracy of proposed criterion and optimization algorithm in terms of mean square error.

Cellular Network Non-Line-of-Sight Reflector Localization Based on Synthetic Aperture Radar Methods

The dependence of radio signal propagation on the environment is well known, and both statistical and deterministic methods have been presented in the literature. Such methods are either based on randomised or actual reflectors of radio signals. In this work, we instead aim at estimating the location of the reflectors based on geo-localized radio channel impulse response measurements using methods from synthetic aperture radar (SAR). Radio channel measurements from 3GPP E-UTRAN have been used to verify the usefulness of the proposed approach. The obtained images show that the estimated reflectors are well-correlated with the aerial map of the environment. Also, trajectory segment contributions to different reflectors have been estimated with promising results.

Analysis and mitigation of antenna effects on wideband wireless channel

Provided is an alternative physical interpretation of the wireless channel impulse response using the knowledge of the antenna impulse responses. As a result, it can be shown that the clusters in the popular Saleh-Valenzuela model actually represent one multipath component only, not several rays as is often assumed. This knowledge has a significant impact on receiver architecture since the number of fingers in a rake receiver can be reduced significantly, for some systems by a factor of up to eight. The drawback of the method is an increase in processing complexity in each finger, but it is anticipated that the overall processing complexity in the receiver will be reduced.

Optimal Approximation of the Impulse Response of Wireless Channels by Stochastic Differential Equations

Wireless communication networks are characterized by nodes and scatters mobility which make the propagation environment time-varying and subject to fading. These variations are captured by random time-varying impulse responses. The latter are fairly general finite energy functions of both time and space that cannot be specified by a finite number of parameters. In this letter, we show that the impulse responses can be approximated in a mean square sense as close as desired by impulse responses that can be realized by stochastic differential equations (SDEs). The behaviors of the SDEs are characterized by small finite dimensional parameter sets.

Analysis of inter-tap correlation for measured mobile radio channels at 1.9 GHz

Tapped-delay line representations are analysed by presenting an empirical model relating the inter-tap correlation to the tap powers obtained from pedestrian radio channel impulse responses measured at 1.9 GHz. The impact of the filtering bandwidth on the parameters of this model is also investigated.

A Study of Shadowing on Indoor Visible-Light Wireless Communication Utilizing Plural White LED Lightings

White LED is considered as a strong candidate for the future lighting technology. We have proposed an optical wireless communication system that employs white LEDs for indoor wireless networks. In this system, LED is used not only as a lighting device, but also as a communication device. The transmitter has large optical power and large emission characteristics to function as lighting device. And the system has specific wireless channel impulse response differing from infrared wireless communication. In this paper, we discuss about shadowing effect on the system utilizing plural LED lightings including the performance of ISI based on the impulse response. We consider the downlink transmission based on TDMA and evaluate the shadowing effect caused by pedestrians with computer simulation. When the shadowing often occurs at 800 Mb/s, the performance of outage call duration rate and blocking rate are improved by using 3 LED lightings compared with 1 or 2 LED lightings. And, we show that the system with the optimal number of the LED lighting is robust against shadowing and can accommodate more calls.

Relation between multipath and wave propagation attenuation

A theoretical multi-slope wave propagation model is proposed based on ultra-wideband radio channel impulse response measurements in a dispersive channel. The theory, which applies to radiowave and acoustic propagation, relates multipath delay spread, propagation attenuation exponent, and the maximum possible rake gain for the multipath propagation channel.

Identification of time-variant directional mobile radio channels

For the identification of the time-variant, directional structure of the mobile radio channel impulse response (CIR), a broadband vector channel sounder is described. The measurement procedure relies on periodic multifrequency excitation signals, correlation processing, and joint delay-azimuth super-resolution based on the two-dimensional (2-D) unitary ESPRIT algorithm. Problems of imperfect receiver and antenna performance as well as antenna array calibration methods are discussed. Correlation analysis of the directional impulse response records is performed in the time-frequency-spatial domain and the corresponding Doppler-delay-angular domain. Results from measurements in the 5.2 GHz frequency range in an industrial environment are presented.

An improved method to process measured radio-channel impulse responses

Impulse responses (IRs) in a corridor with and without visibility were measured in UHF. The effects of noise and windowing were considered. A new combined method to filter noise away from the IRs was developed. The effect of different windows in the IR was compared. Windows with lower sidelobe level (SLLs) yield larger rms delays. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 158–162, 2000.

Performance of Adaptive Antennas in FH-GSM Using Conventional Beamforming

This paper presents the performance of adaptive antennas in a 1/3 reuse frequency hopping GSM network using conventional beamforming. It mainly focuses on C/I improvement for the purpose of capacity enhancement. The performance evaluation has been conducted by means of network computer simulations, where measured time-space radio channel impulse responses are applied for the desired user in the network. Measurements with an M = 8 element uniform linear array were conducted in the cities of Aarhus, Denmark, and Stockholm, Sweden. The simulated C/I improvement shows an almost 10 a log10(M) behavior for low azimuth spread values. For large values of azimuth spread (relative to the antenna beamwidth), the performance gain is reduced significantly. For an azimuth spread of 10°–12°, which has been measured in urban macro-cellular environments, the C/I gain for M = 8 is reduced to approx. 5.5–7.5 dB (which should be compared to the theoretical value of 9 dB for a point source). The designed DoA algorithm is very robust to co-channel interference and only a small degradation in performance is observed for single element C/I down to approx. –8 dB. We conclude that the designed beamforming implementation facilitates a potential capacity gain of ×3 in a 1/3 reuse FH-GSM network for an array size of M = 4–6.

Excess delay estimation from time-varying mobile radio channel impulse response measurements

This letter proposes a method for estimating the instantaneous excess delay in time-varying mobile radio channel impulse response measurement data using image processing techniques. The method is shown by example to produce more accurate mean power delay profiles than those produced assuming either constant or linearly varying excess delay.

Characterization of in-building UHF wireless radio communication channels using spectral energy measurements

A simple, cost-effective means is developed to estimate the time-invariant wireless radio channel impulse response h(t) using only the magnitude of the channel transfer function, H(jw). The Hilbert transform is used to calculate the phase of H(jw) from its magnitude. Inverse discrete Fourier transformation (IDFT) of H(jw) yields h(t). The Hilbert transform relation is applicable provided H(jw) is a minimum phase transfer function. An experimental in-building wireless channel testbed was established, for which h(t) was determined over the 1000-2500 MHz range. Both line of sight (LOS) and non-LOS transmission was investigated. Good agreement was observed between values of h(t) calculated from measured values of H(jw) and from those based only on [H(jw)] and its Hilbert transform. Even when the minimum phase condition is violated, h(t) as calculated from [H(jw)] and its Hilbert transform provides a useful lower bound on the time-spread of h(t). The measurement of [H(jw)] is easily implemented using a signal source, receiving antenna, and spectrum analyzer. A personal computer and software are required to calculate the phase of H(jw) and its IDTF. Existing frequency-domain measurement schemes utilize a vector network analyzer to measure H(jw) (magnitude and phase angle). Such equipment is expensive, subject to transmitter-receiver crosstalk, and restrictive as to the relative locations of the transmitting and receiving antenna.

Computer simulation of radio channels using a harmonic decomposition technique

Based on a harmonic decomposition of stochastic processes, a new approach for approximating the time variant radio channel impulse response, given an arbitrary Doppler power density spectrum is presented. The basic idea is to use a Fourier expansion with uncorrelated coefficients to approximate the actual radio fading process. An upper bound on the mean square error between the actual and approximated fading as a function of the number of terms in the expansion is also given. The proposed method is particularly adapted for simulating broadband radio channels. >

A comparison of two radio propagation channel impulse response determination techniques

A comparison of two radio propagation channel impulse response determination techniques is described. Presented are typical impulse response and transfer functions obtained from each measurement system. Also included for comparison are average impulse response envelopes and cumulative probability distributions for the RMS delay spread of static indoor radio channels calculated from 120 measurements using each system. The comparisons show good agreement between results. >