Wideband Multiple-input Multiple-output Channel Research Articles

A WINNER+ Based 3-D Non-Stationary Wideband MIMO Channel Model

In this paper, a three-dimensional (3D) non-stationary wideband multiple-input multiple-output (MIMO) channel model based on the WINNER+ channel model is proposed. The angular distributions of clusters in both the horizontal and vertical planes are jointly considered. The receiver and clusters can be moving, which makes the model more general. Parameters, including number of clusters, powers, delays, azimuth angles of departure (AAoDs), azimuth angles of arrival (AAoAs), elevation angles of departure (EAoDs), and elevation angles of arrival (EAoAs) are time-variant. The cluster time evolution is modeled using a birth-death process. Statistical properties, including spatial cross-correlation function (CCF), temporal autocorrelation function (ACF), Doppler power spectrum density (PSD), level-crossing rate (LCR), average fading duration (AFD), and stationary interval are investigated and analyzed. The LCR, AFD, and stationary interval of the proposed channel model are validated against the measurement data. Numerical and simulation results show that the proposed channel model has the ability to reproduce the main properties of real non-stationary channels. Furthermore, the proposed channel model can be adapted to various communication scenarios by adjusting different parameter values.

Wideband MIMO Channel Capacity Analysis in Multiprobe Anechoic Chamber Setups

This paper discusses over-the-air (OTA) testing for multiple-input–multiple-output (MIMO) capable terminals with emphasis on wideband MIMO channel capacity analysis in a multiprobe anechoic chamber setup. In the literature, the spatial correlation simulation accuracy at the receiver (Rx) side has been used to determine the test area size for a limited number of probes. However, it is desirable that the test area size is defined in terms of data rate deviation of the simulated channel in the laboratory from that of the target channel model. This paper reports MIMO capacity analysis results for wideband spatiotemporal channel models, with emphasis on the impact of the spatial correlation at the transmit (Tx) side, the channel model, and the spatial correlation at the Rx side on the capacity simulation accuracy. Simulation results show that the number of probes is irrelevant to capacity simulation accuracy when the spatial correlation at the Tx side is in the high region (e.g., $\rho > \mbox{0.7}$ ). Furthermore, when correlation at the Tx side is low, the spatial correlation accuracy is less critical with small correlation at the Rx side. The simulation results are further supported by measurements in a practical multiprobe anechoic chamber setup. The capacity simulation accuracy is shown to be a valid measure to determine the test area size.

Modeling of Time-varying Ultra Wideband Multiple-input Multiple-output Channel

Ultra wideband (UWB) communication is a promising technology for achieving high data rate wireless communications. UWB transmission can be operated in the range of 3.1 to 10.6 GHz with low transmission power to prevent interference with other wireless systems. However, the low transmission power of the UWB signal limits the coverage distance of the UWB system. By using diversity techniques such as the multiple-input multiple-output (MIMO) scheme, enhancement in the communication range can be obtained without compromising data rate. For the purposes of system and network design, it is always important to understand the behavior of the MIMO - UWB channel through modeling. In this paper, we propose a method to derive the synthesis equation for the time-varying MIMO-UWB channel in the time scale-domain. The channel modeling is based on the MIMO correlative modeling approach. The channel operator is represented as a fourth order tensor and the synthesis equation is derived and illustrated. The UWB correlation matrix was plotted for the stationary situation. The results show the importance of the off-diagonal channel correlation elements on the MIMO-UWB channel.

A General Coupling-Based Model Framework for Wideband MIMO Channels

A general coupling-based model framework for wideband multiple-input multiple-output (MIMO) channels is presented in this paper. Under this framework, the channel state information (CSI) tensor can be expressed by the product of a coupling tensor, a complex Gaussian tensor and three unitary matrices. The unitary matrices can be either eigenbases or steering matrices in different domains. The coupling tensor reflects the relationship between the column vectors of these unitary matrices. The complex Gaussian tensor is used to describe the small-scale fading. Several realizations of this framework are introduced, including the wideband Kronecker-based (WKB) model, the wideband eigenvalue-decomposition-based (WEB) model, the wideband virtual presentation (WVP) model and the wideband hybrid (WHY) model. To evaluate the performance of these models, channel measurements were carried out in different indoor scenarios both at Tsinghua University and Lund University. The results show that these models have good agreement with the measured data. Furthermore, we can see that the WHY model can provide a tradeoff between complexity and accuracy in channel synthesis.

Spatial characteristics and capacity investigation of indoor hotspot channel based on wideband MIMO measurement at 4.9 GHz

In this article, results obtained from wideband multiple-input multiple-output channel measurements at 4.9 GHz are presented. Measurements were performed in an indoor hotspot scenario with both line-of-sight and non-line-of-sight propagation conditions. Vertical polarized uniform linear arrays (ULAs) with different antenna spacing and orientation were utilized for investigating the impact of antenna configurations on the system performance. Based on measured channel impulse responses, the spatial correlation of ULA is analyzed and compared with the theoretical values calculated from the power azimuth spectrum. Following that, the eigenvalue statistics and capacity with respect to antenna spacing and orientation are given and compared. The diversity-multiplexing tradeoff (DMT) of measured channels at operational signal-to-noise ratio (SNR) is also presented. The reason why the DMT of measured channels is much lower than that of i.i.d. Rayleigh channels is specified.

Frequency-domain bit-flipping equalizer for wideband MIMO channels

We propose a low-complexity equalizer whose performance approaches that of the optimal maximum-likelihood estimators in wideband multiple-input multiple-output (MIMO) channels. The proposed algorithm makes use of a bit-flipping refinement procedure preceded by a frequency-domain equalizer and is based on local-optima searching algorithms. Through performance evaluations, it is demonstrated that the proposed equalizer can perform well when a large number of diversity branches are available in severely dispersive fading channels.

Wideband MIMO channel characterization in TV studios and inside buildings in the 2.2–2.5 GHz frequency band

Multiple input multiple output (MIMO) measurements were performed with a chirp sounder with a switched antenna array at the transmitter and eight parallel receiver channels with 96‐MHz bandwidth at 2.25 GHz in TV studios and 240‐MHz bandwidth at 2.38 GHz inside buildings. MIMO capacity was estimated from the data with linear and circular arrays with array sizes 2 by 2, 4 by 4, and 6 by 8. Mean capacity estimates for 30‐dB signal‐to‐noise ratio for both line of sight and obstructed line of sight were on the order of 11–31 b/s/Hz for the 2 by 2 to 4 by 4 antenna configurations and for the 6 by 8 directional antenna measurements a median capacity of 50 b/s/Hz. This is an increase of 20 b/s/Hz from the 4 by 4 omnidirectional antenna array indicating diversity gain as well as an increase due to the number of antennas.

Physical limits to the capacity of wide-band gaussian MIMO channels

In this letter a physical limit to the information capacity of a multiple-input multiple-output (MIMO) Gaussian channel is presented exploiting the theory of non-redundant sampling of scattered fields. For a MIMO narrow-band system of arbitrarily large spatial extension, the information capacity limit is the same as the one of a single-input single-output (SISO) ultra-wide band (UWB) system. For MIMO systems of finite size, transmitting over a range of frequencies, space and frequency diversities can be optimally combined by allocating the signal power uniformly across space, and increasing linearly across frequency.

Spatial correlation and capacity measurements for wideband MIMO channels in indoor office environment

This paper describes a broadband multiple input, multiple output (MIMO) channel characterization platform and capacity measurement results in indoor office environment. The MIMO testbed has been designed for broadband MIMO channel sounding, capacity measurements and for characterizing the directional-multipaths of the radio propagation channel. The MIMO channel data have been collected in 5 GHz band inside a modern office environment. Capacity results from these experiments are discussed for different propagation conditions, including non line-of-sight (NLOS) and LOS propagations with various spacings between array elements. Spatial correlations are analyzed from the measured data and a frequency selective MIMO channel model based on the correlation statistics is validated. Post-processing of the measured data with a sequential ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm is utilized to extract the directions of departures and arrivals of multipath components at the transmitting and receiving arrays, respectively. A method of obtaining broadband MIMO capacity, indirectly, from a single directional measurement of MIMO propagation channel is also proposed and verified from the measurement results.

Editors' opening address

Wireless Communications Systems are at the centre of a new and passionate era for telecommunications characterised, in industrial terms, by the convergence of systems and technologies, a transition towards all-IP networks, and the development of technologies with a user-centric focus. On the other hand, in terms of Research and Development, researchers are already considering other, more futuristic, aspects beyond those just mentioned, and these days hot topics include things like the Internet of the Future, and all the Post-IP perspectives that come with it. In both grounds, that is the more industrial related one and the one nearer an academic perspective, there is still a long way to go, before many of the ideas being discussed today can see the light of day as a commercial product. In this context, the International Symposium on Wireless Communication Systems (ISWCS) has positioned itself as a dynamic forum for researchers and technologists to present and discuss original ideas and contributions in all fields related to mobile and wireless communication systems. ISWSC'06, the 3rd edition, held in Valencia, Spain, on September 2006, brought together leading and enthusiastic researchers and technologists, willing to identify and discuss the future technical challenges and business opportunities arising in the path towards the realisation of ambient wireless communication networks. Besides this, it was a good forum for Ph.D. students to present their work, and exchange their ideas. The idea of setting up a special issue of a journal with the very best papers of the conference was brought up by the excellent quality of some papers presented at the conference. The objective of a special issue of this kind is always to put together, beyond the conference proceedings, the best papers in an enhanced and revised version, enabling the journal to publish good papers with timely updated information in a very short time. This special issue of Wireless Communications and Mobile Computing contains the very best papers presented at ISWCS'06. An invitation was sent to a restricted number of papers, and an additional review procedure was held, beyond the conference one. At the end, this set of six papers was accepted. The papers in this issue address some of the hot topics being investigated these days, ranging from localisation and frequency sharing to multi carrier-code division multiple access (MC-CDMA) and multiple input multiple output (MIMO). This shows that the physical layer still attracts a lot of attention, and that it still has a lot of topics to be explored. The first two papers address MIMO channels, in modelling or measurement perspectives. Matthias Pätzold and Bjørn Olav Hogstad present a model for a wideband MIMO channel based on the geometric elliptical scattering one, which can be used as a starting point for the derivation of a frequency-selective space–time channel simulator, being also useful for studying MIMO channel capacity under various propagation conditions. Michail Matthaiou et al. bring a paper on dual frequency MIMO measurements around the ISM 2.4 GHz band, in indoor environments, with initial results being promising enough, as the joint spectra seem to be remarkably similar, implying that the underlying propagation mechanisms at the considered bands are also similar. Farrukh Rashid and Athanassios Manikas present one of the papers dealing with MC-CDMA, namely on arrayed reception in space-time diffused multipath vector channels; results show that the use of a diffused channel modelling in arrayed receivers yields better bit error rate (BER) and signal to noise plus interference ratio (SNIR) performance than receivers that ignore the presence of spatial and/or temporal diffusion. The other paper on MC-CDMA, from Nguyen Tran Khoa et al., addresses frequency sharing in hotspot communications for cellular networks; it proposes an overlap in the uplink of an OFDM hotspot system to an MC-CDMA cellular one, in the same frequency band, beam-forming being applied to eliminate inter-system interference. The paper for Barbara Mavì Masini et al. also deals with the coexistence of different systems, in particular on the benefits of diversity schemes for Bluetooth coverage extension in the presence of IEEE 802.11g interference, analysing packet error and coverage probabilities, among other parameters. Finally, Joseph Shmuel Picard and Anthony Joseph Weiss present a paper addressing localisation of networks using various ranging bias models, presenting a set of iterative algorithms that cope successfully with the various bias models and provide Maximum Likelihood position estimates.

A Novel Wideband MIMO Channel Model and Experimental Validation

We present a novel wideband multiple-input, multiple-output (MIMO) channel model, which we refer to as the structured model. The structured model is based on the eigenvalue decomposition (EVD) of the wideband channel correlation matrix. It does not assume the scatterers at the receiver fade independently of those at the transmitter. It also models correlation between delay bins in the power delay profile (PDP). With preliminary data gathered using McMaster's wideband MIMO software defined radio (the WMSDR) in fixed outdoor locations, and Brigham Young University's (BYU) wideband channel sounder in fixed indoor locations, we show good agreement between modeled and measured data. The two platforms used and the environments in which the data were collected were very different. The proposed model performed equally well with both data sets, demonstrating its robustness.

Correlation-based double-directional stochastic channel model for multiple-antenna ultra-wideband systems

A correlation-based double-directional stochastic channel model for indoor multiple-input multiple-output (MIMO) ultra-wideband (UWB) propagation channels is proposed. The proposed model extends the IEEE 802.15.3a standard model to spatially correlated MIMO channels. Both angular and temporal statistics are taken into account in the modelling procedure. Spatial correlation is introduced into the multipath amplitude and time-of-arrival (ToA) matrices of the channel model. Each amplitude matrix consists of entries of correlated lognormal random variables, whereas each ToA matrix is obtained as the sum of a reference matrix and a difference matrix. The frequency-dependent spatial correlation function is derived to give an insight of model properties. Model parameters are determined based on well-known measurement campaigns. In addition, simulation-based analysis indicates that this model has desirable spatial correlation properties in both the time and frequency domains. The ToA correlation matrix was also found to have dominant effects on the correlation characteristics. This suggests that future research into spatial correlation properties of MIMO-UWB channels should focus on ToA correlation characteristics, rather than amplitude correlation characteristics, which are the current focus of narrowband and wideband MIMO channel research.

A wideband MIMO channel model derived from the geometric elliptical scattering model

AbstractIn this paper, we present a reference model for a wideband multiple‐input multiple‐output (MIMO) channel based on the geometric elliptical scattering model. The model takes into account the exact relationship between the angle of departure (AOD) and the angle of arrival (AOA). Based on this relationship, the statistical properties of the reference model are studied. Analytical solutions are presented for the three‐dimensional (3D) space‐time cross‐correlation function (CCF), the temporal autocorrelation function (ACF), the 2D space CCF, and finally the frequency correlation function (FCF). The correlation properties are studied and visualized under the assumption of isotropic as well as non‐isotropic scattering conditions. The proposed reference model can be used as a starting point for the derivation of a frequency‐selective space‐time MIMO channel simulator enabling the performance evaluation of multi‐antenna wideband communication systems. The reference model is also quite useful for studying the MIMO channel capacity under various propagation conditions imposed by the geometry of the underlying scattering model. Copyright © 2007 John Wiley & Sons, Ltd.

A new approach to diversity and multiplexing gains for wideband MIMO channels

In this paper, a new approach to improving reliability and bandwidth efficiency in communications over frequency-selective channels using multiple antennas is proposed. In the heart of the new approach is a novel space-time orthogonal frequency division multiplexing (OFDM) scheme. The proposed space-time OFDM modulator translates a multiple-input multiple-output (MIMO) channel into a single-input multiple-output (SIMO) channel without the loss of system freedom (the available diversity gain). This translation simplifies code design as compared to that in the conventional MIMO OFDM approach. Instead of more complicated space-time codes, codes that are designed for single-input fading channels can be used with the proposed space-time modulation. For bandwidth-efficient applications, a channel multiplexing scheme is developed to work with the space-time modulator. Unlike the conventional spatial multiplexing schemes, an arbitrary number of data streams can be created and each layer occupies all the transmit antennas all the time. As a result, all the available degrees of freedom are preserved for each layer and a full range of optimal tradeoffs between data rate and reliability is possible. Several examples are given to demonstrate the advantages of the proposed approach over the conventional MIMO OFDM approach

Wideband MIMO measurements of outdoor NLOS channels

AbstractThe measurement results of the wideband wireless multiple‐input multiple‐output (MIMO) channels in outdoor environments are presented. Our wideband wireless MIMO channel sounder consists of four transmitters and eight receivers, and operates at a carrier frequency of 1.8 GHz with a bandwidth of 2.5 MHz. After obtaining the multipath delay profiles from the data collected in non‐line‐of‐sight (NLOS) environments, realistic wideband MIMO channel capacities are computed and compared with the ideal channel simulation. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 216–218, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21309

Capacity scaling and spectral efficiency in wide-band correlated mimo channels

The dramatic linear increase in ergodic capacity with the number of antennas promised by multiple-input multiple-output (MIMO) wireless communication systems is based on idealized channel models representing a rich scattering environment. Is such scaling sustainable in realistic scattering scenarios? Existing physical models, although realistic, are intractable for addressing this problem analytically due to their complicated nonlinear dependence on propagation path parameters, such as the angles of arrival and delays. In this paper, we leverage a recently introduced virtual representation of physical models that is essentially a Fourier series representation of wide-band MIMO channels in terms of fixed virtual angles and delays. Motivated by physical considerations, we propose a D-connected model for correlated channels defined by a virtual spatial channel matrix consisting of D nonvanishing diagonals with independent and identically distributed (i.i.d.) Gaussian entries. The parameter D provides a meaningful and tractable measure of the richness of scattering. We derive general bounds for the coherent ergodic capacity and investigate capacity scaling with the number of antennas and bandwidth. In the large antenna regime, we show that linear capacity scaling is possible if D scales linearly with the number of antennas. This, in turn, is possible if the number of resolvable paths grows quadratically with the number of antennas. The capacity saturates for linear growth in the number of paths (fixed D). The ergodic capacity does not depend on frequency selectivity of the channel in the wide-band case. Increasing bandwidth tightens the bounds and hastens the convergence of scaling behavior. For large bandwidth, the capacity scales linearly with the signal-to-noise ratio (SNR) as well. We also provide an explicit characterization of the wide-band slope recently proposed by Verdu. Numerical results are presented to illustrate the key theoretical results.