Quentin H. Spencer

A statistical model for angle of arrival in indoor multipath propagation

Multiple antenna systems are a useful way of overcoming the effects of multipath interference, and can allow more efficient use of spectrum. In order to test the effectiveness of various algorithms such as diversity combining, phased array processing, and adaptive array processing in an indoor environment, a channel model is needed which models both the time and angle of arrival in indoor environments. Some data has been collected indoors and some temporal models have been proposed, but no existing model accounts for both time and angle of arrival. This paper discusses existing models for the time of arrival, experimental data (form the 6.75-7.25 GHz band) that were collected indoors, and a proposed extension of the Saleh-Valenzuela model (1987), which accounts for the angle of arrival. Model parameters measured in two different buildings are compared with the parameters presented in the paper by Saleh and Valenzuela, and some statistical validation of the model is presented.

Wireless indoor channel modeling: statistical agreement of ray tracing simulations and channel sounding measurements

A statistical space-time model for indoor wireless propagation based on empirical measurements is compared with results from the deterministic ray-tracing simulation tool WiSE for the same environment. Excellent agreement is found in terms of the distributions of arrival times and angular spread for both modeling approaches. The WiSE package is also use to synthesize MIMO channel matrices and determine the theoretical capacity, available in the tested environments. It is found that, for narrowband channels, the spatial clustering of the multipaths limits the capacity gains associated with increased array size.

A hybrid approach to spatial multiplexing in multiuser MIMO downlinks

In the downlink of a multiuser multiple-input multiple-output (MIMO) communication system, simultaneous transmission to several users requires joint optimization of the transmitted signals. Allowing all users to have multiple antennas adds an additional degree of complexity to the problem. In this paper, we examine the case where a single base station transmits to multiple users using linear processing (beamforming) at each of the antenna arrays. We propose generalizations of several previous iterative algorithms for multiuser transmit beamforming that allow multiple antennas and multiple data streams for each user, and that take into account imperfect channel estimates at the transmitter. We then present a new hybrid algorithm that is based on coordinated transmit-receive beamforming, and combines the strengths of nonorthogonal iterative solutions with zero-forcing solutions. The problem of distributing power among the subchannels is solved by using standard bit-loading algorithms combined with the subchannel gains resulting from the zero-forcing solution. The result is a significant performance improvement over equal power distribution. At the same time, the number of iterations required to compute the final solution is reduced.

Indoor wideband time/angle of arrival multipath propagation results

Most current indoor propagation experiments measure the time of arrival of characteristics of multipath reflections without regard to the angle of arrival. Because of the increasing number of systems that are used indoors and which use multiple antenna systems to combat multipath interference, a need exists for indoor propagation data which takes the angle of arrival into account. A system is described which was used to collect simultaneous time and angle of arrival data in two indoor environments. A total of 65 data sets were taken in two buildings of different construction. The (7 GHz) data confirmed the temporal model proposed by Saleh and Valenzuela (1987), and showed a unique clustering pattern in angle, which consisted of clusters uniformly distributed in angle, with members of the clusters following a Laplacian distribution.

Downlink transmit beamforming in multi-user MIMO systems

The primary issue in downlink beamforming for wireless communications is how to balance the need for high received signal power for each user against the interference produced by the signal at other points in the network. In this paper, we describe several approaches to this problem: channel inversion, regularized channel inversion, vector modulo pre-coding, channel block diagonalization and coordinated transmit/receive beamforming. While the basic idea behind these algorithms is the same, namely the use of channel information at the transmitter to predict and then counteract the interference produced at each node in the network, each of the algorithms is based on achieving a different performance objective. We compare the various goals of the above algorithms, and detail their respective advantages and disadvantages in terms of computational complexity, required transmit power, network throughput, and assumed receiver capabilities. The results of several simulation studies are presented to quantify these comparisons.

Some results on channel capacity when using multiple antennas

Recent research has demonstrated that the capacity of a multipath-rich channel (such as indoor channel) is dramatically increased when antenna arrays are used at both transmitter and receiver. This is achieved by using the multipath to create parallel sub-channels. This ability to achieve parallelism in the channel is influenced by how much information about the channel is known at the transmitter, the structure of the channel, and the structure of the arrays. This paper discusses the influence of the transmitters knowledge of the channel on capacity, presents closed-form solutions for capacity when there are two paths in the channel, and then presents numerical results based on simulations. For the two-path case, the simulation data compare capacity based on differing assumptions about the transmitter, and compare different array structures. Simulation results are also presented for random channels generated using a statistical model for indoor multipath propagation.

Smart antennas for wireless communications beyond the third generation

Smart antennas are essential to increase the spectral efficiency of wireless communication systems. They can be realized by an antenna array at the base station and sophisticated base-band signal processing. Thereby, adaptive directional reception is achieved on the uplink and adaptive directional transmission on the downlink if channel knowledge is available at the transmitter. Hence, an increased antenna gain and an increased diversity gain are realized towards the desired user. At the same time, less interference is received from the other directions on the uplink or transmitted in the other directions on the downlink if channel knowledge is available at the transmitter. Therefore, more users can be accommodated by the system and a corresponding increase of the spectral efficiency is achieved. Even higher spectral efficiencies can be obtained if antenna arrays are not only used at the base station but also at the mobile. The theoretical capacity of such multiple-input-multiple-output systems has been shown to grow linearly with the size of the antenna arrays in sufficiently rich multi-path environments. Open problems in this new area of wireless communications include better and more realistic propagation models as well as improved algorithms for transmission and reception which approach the theoretical limits of the channel.

Performance of MIMO spatial multiplexing algorithms using indoor channel measurements and models

Department of Electrical and Computer Engineering, Brigham Young University, Provo, UT 84602, U.S.A.SummarySeveral algorithms have recently been proposed for multiplexing multiple users in multiple input multiple output(MIMO) downlinks. The ability of a transmitter to accomplish this using spatial methods is generally dependent onwhether the users’ channels are correlated. Up to this point, most of the multiplexing algorithms have been tested onuncorrelated Gaussian channels, a best-case scenario. In this paper, we examine the performance of multiplexingmultiple users under more realistic channel conditions by using indoor channel measurements and a statisticalmodel for test cases. We use a block zero-forcing algorithm to test performance at various user separation distances,optimizing for both maximum throughput under a power constraint and minimum power under a rate constraint.The results show that for the measured indoor environment (rich scattering, non-line-of-sight), a separation of fivewavelengths is enough to achieve close to the maximum available performance for two users. Since many spatialmultiplexing algorithmsrequirechannel state information(CSI) at the transmitter, we alsoexamine the performanceloss due to CSI error. The results show that a user can move up to one-half wavelength before the original channelmeasurement becomes unusable. Copyright # 2004 John Wiley & Sons, Ltd.KEY WORDS: MIMO systems; multi-user downlink beamforming; multi-user MIMO systems; MIMO channelmodels; MIMO channel measurement; indoor channel models; indoor channel measurement1. IntroductionMultiple input multiple output (MIMO) wireless sys-tems offer the possibility of spatially multiplexingmultiple users, creating an additional dimension formultiple access beyond time-, frequency- and code-division approaches. The uplink of a multi-userMIMO system follows a model for which numerousalgorithms already exist; although non-trivial, thisproblem has been studied (either directly or in-directly) by numerous researchers. Less understoodis the MIMO downlink or ‘multicast’ channel, where amultiple antenna transmitter (e.g. a basestation) at-tempts to send different messages to multiple users,each of which may be equipped with a multipleantenna receiver. If one assumes that all users haveonly one antenna, the problem is somewhat simplified,and optimal transmitter beamforming solutions exist[1,2], as well as sub-optimal, but simpler solutions [3–5]. When the users have multiple antennas, achieving

On the performance of multicarrier CDMA using multiple transmitters

Recent research has demonstrated the existence of transmission schemes and codes which provide diversity gain by using multiple transmitting antennas. This paper applies the concepts of multicarrier DS-CDMA systems to the multiple transmitter case. The multicarrier CDMA system discussed here was originally based on the assumption that the frequency-selective fading was uncorrelated from one subcarrier to the next. We derive the performance for a system whose subcarriers are subject to correlated fading, and the performance of a system partially or completely decorrelated by distributing the subcarriers across transmit antennas.

Performance of MIMO spatial multiplexing algorithms using indoor channel measurements and models: Research Articles

This paper introduces a general concept for the design of efficient multi-input multi-output (MIMO) narrowband fading channel simulators. The proposed procedure is applied on a reference model for the geometrical one-ring scattering model consisting of an infinite number of local scatterers laying on a ring around the mobile station (MS). The statistical properties of both the reference model and the simulation model are studied. Closed-form solutions are presented, for example for the space-time cross-correlation function (CCF), the time autocorrelation function (ACF) and the two-dimensional (2-D) space CCF. It will be shown, how the parameters of the MIMO channel simulator can be determined for any given space-time CCF describing the reference model. The excellent performance of the proposed space-time channel simulator is demonstrated by comparing its temporal and spatial correlation properties with those of the reference model. Moreover, it is demonstrated how the channel simulator can be used to analyze the statistics of the capacity of MIMO channels from time-domain simulations. Copyright