Vinko Erceg

A fourth-generation MIMO-OFDM broadband wireless system: design, performance, and field trial results

Increasing demand for high-performance 4G broadband wireless is enabled by the use of multiple antennas at both base station and subscriber ends. Multiple antenna technologies enable high capacities suited for Internet and multimedia services, and also dramatically increase range and reliability. In this article we describe a multiple-input multiple-output OFDM wireless communication system, lab test results, and field test results obtained in San Jose, California. These are the first MIMO system field tests to establish the performance of MIMO communication systems. Increased capacity, coverage, and reliability are clearly evident from the test results presented in this article.

Adaptive modulation and MIMO coding for broadband wireless data networks

Link adaptation techniques, where the modulation, coding rate, and/or other signal transmission parameters are dynamically adapted to the changing channel conditions, have emerged as powerful tools for increasing the data rate and spectral efficiency of wireless data-centric networks. While there has been significant progress on understanding the theoretical aspects of time adaptation in LA protocols, new challenges surface when dynamic transmission techniques are employed in broadband wireless networks with multiple signaling dimensions. Those additional dimensions are mainly frequency, especially in multicarrier systems, and space in multiple-antenna systems, particularly multiarray multiple-input multiple-output communication systems. We give an overview of the challenges and promises of link adaptation in future broadband wireless networks. We suggest guidelines to help in the design of robust, complexity/cost-effective algorithms for these future wireless networks.

Moment-method estimation of the Ricean K-factor

In many radio propagation environments, the time-varying envelope of the received signal can be statistically described by a Ricean distribution. Traditional methods for estimating the Ricean K-factor from measured power versus time are relatively cumbersome and time consuming. We describe a simple and rapid approach wherein the K-factor is an exact function of moments estimated from time-series data. Comparisons with empirical distributions for fixed wireless paths validate the method.

A new path-gain/delay-spread propagation model for digital cellular channels

We derive a statistical model for the distribution of RMS delay spread (/spl tau//sub rms/) within a cellular environment, including the effects of base-to-mobile distance, environment type (urban, suburban, rural, and mountainous areas), and the correlation between delay spread and shadow fading. We begin with intuitive arguments that /spl tau//sub rms/ should be lognormally distributed at any given distance d; that the median of this distribution should grow as some (weak) power of d and that the variation about the median should be negatively correlated with shadow fading gain. We then present empirical evidence, drawn from a wide array of published reports, which gives strong support to these conjectures. Finally, we combine our findings with the widely used model for path gain in a cellular environment. The result is a compact statistical model for the joint distribution of path gain and delay spread. The model lends itself readily to Monte Carlo simulation and is useful for performance studies of cellular systems with bandwidths up to tens of kilohertz.

On the feasibility of a CDMA overlay for personal communications networks

Because of the continually increasing demand for mobile communications, it has been suggested that personal communication networks (PCNs) be established in the 1850-1990 MHz range. However, that band of frequencies is currently occupied by various microwave signals transmitted by users ranging from utility companies to state and local agencies. In order to allow both sets of users to occupy these frequencies as well as improve the spectral efficiency of this band, a spread-spectrum overlay is proposed, whereby a code-division multiple-access (CDMA) PCN would share the spectral band with the existing narrowband microwave traffic. The results of several field tests which have been designed to demonstrate the feasibility of an overlay of this type are discussed. >

Performance of multiantenna signaling techniques in the presence of polarization diversity

Multiple-input multiple-output (MIMO) antenna systems employ spatial multiplexing to increase spectral efficiency or transmit diversity to improve link reliability. The performance of these signaling strategies is highly dependent on MIMO channel characteristics, which, in turn, depend on antenna height and spacing and richness of scattering. In practice, large antenna spacings are often required to achieve significant multiplexing or diversity gain. The use of dual-polarized antennas (polarization diversity) is a promising cost- and space-effective alternative, where two spatially separated uni-polarized antennas are replaced by a single antenna structure employing orthogonal polarizations. This paper investigates the performance of spatial multiplexing and transmit diversity (Alamouti (see IEEE J. Select. Areas Commun., vol.16, p.1451-58, Oct. 1998) scheme) in MIMO wireless systems employing dual-polarized antennas. In particular, we derive estimates for the uncoded average symbol error rate of spatial multiplexing and transmit diversity and identify channel conditions where the use of polarization diversity yields performance improvements. We show that while improvements in terms of symbol error rate of up to an order of magnitude are possible in the case of spatial multiplexing, the presence of polarization diversity generally incurs a performance loss for transmit diversity techniques. Finally, we provide simulation results to demonstrate that our estimates closely match the actual symbol error rates.

Multiple-input multiple-output fixed wireless radio channel measurements and modeling using dual-polarized antennas at 2.5 GHz

This paper presents outdoor propagation measurements together with derivative analysis, modeling, and simulation of the 2/spl times/2 fixed wireless multiple-input multiple-output (MIMO) channel. Experimental data were collected in the suburban residential areas of San Jose, CA, at 2.48 GHz by using dual-polarized antennas. Measurement results include the estimation of path loss, Rician K-factor, cross-polarization discrimination (CPD), correlation coefficients, and the MIMO channel capacity. An elaborate K-factor model that assumes variation over location, time, and frequency is developed. Distance-dependent CPD models of the variable and constant signal components are proposed. A generalized 2/spl times/2 MIMO channel model is then derived based on the correlation among the path loss, the copolarized K-factor, and the CPDs distribution of the constant and scattered signal components. Finally, the MIMO channel response is simulated using the newly developed model, and results are found to be well in agreement with measurements.

Analysis and modeling of multiple-input multiple-output (MIMO) radio channel based on outdoor measurements conducted at 2.5 GHz for fixed BWA applications

This paper summarizes our 2/spl times/2 multiple-input multiple-output (MIMO) fixed wireless outdoor propagation measurements at 2.48 GHz conducted in the suburban residential areas of San Jose, California. We report on various channel characteristics such as path loss, Ricean K-factor, cross-polarization-discrimination (XPD) and channel capacity. We present simple models for these characteristics, focusing on excess loss dependency and, derived from that, the variation with distance. Also, we introduce an idea for a generalized MIMO channel model based on these modeled channel characteristics and the correlation properties between them. Path loss results show that blockage due to buildings or foliage causes an excess loss of 35.45 dB compared to free space propagation at a distance of 1 km. The narrowband K-factor distribution matches previously reported results. The XPD of the total received signal varies from -10 to 15 dB at various locations. The K-factor and XPD were found to be very much dependent on excess loss due to blockage conditions at various distances.

Propagation modeling of MIMO multipolarized fixed wireless channels

This paper addresses the extension of a stochastic geometry-based scattering model to multipolarized transmissions. The initial approach is based on a geometrical distribution of obstacles derived from known power-delay profiles. Each scattering process is statistically described by a matrix reflection coefficient corresponding to dual-polarization states. Ultimately, the model allows us to simulate the effects of the range on K-factor, delay-spread, Doppler spectrum, channel correlations and capacity, branch power ratio, and cross-polar discrimination. Simulation results are compared with existing measurements at 2.5 GHz. The proposed model is then used to investigate various dual-polarization 2 /spl times/ 2 multiple-input-multiple-output (MIMO) schemes such as 0/spl deg//90/spl deg/ or /spl plusmn/45/spl deg/, as well as to optimize the design of multipolarized MIMO schemes.

Diffraction around corners and its effects on the microcell coverage area in urban and suburban environments at 900 MHz, 2 GHz, and 4 GHz

For rectilinear streets in urban and suburban environments, a theoretical model has been developed to characterize signal propagation around corners. Ray theory and the uniform geometrical theory of diffraction (UTD) were combined to predict the spatial average of signal strength. The model was compared with data measured at 900 MHz, 2 GHz, and 6 GHz. The results show excellent agreement between theory and measurements for different bands and different locations. This indicates that accurate prediction of signal and interference levels is possible through simulation rather than costly field measurements. Using building and street databases, the model can be used to predict the signal coverage and interference in large cities, thus allowing system designers to determine cell layouts, reuse factors, capacity, etc. Furthermore, theoretical microcell coverage areas were determined and compared to existing diamond-shape models at 900 MHz and 2 GHz with the base-station antenna located in the intersection. The theoretical and empirical results were in very good agreement. >