Mary Ann Ingram

Broadband MIMO-OFDM wireless communications

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.

Six time- and frequency- selective empirical channel models for vehicular wireless LANs

Three vehicle-to-vehicle (V2V) models and three roadside-to-vehicle (RTV) models, each suitable for RF channel emulation and based on measurements at 5.9 GHz, are presented. Each model captures the joint Doppler-delay characteristics of a different environment. The packet error rate (PER) for each model, measured with an emulator and an 802.11p wireless access in vehicular environments (WAVE) prototype, is presented.

Medium access control in ad hoc networks with MIMO links: optimization considerations and algorithms

we present a medium access control (MAC) protocol for ad hoc networks with multiple input multiple output (MIMO) links. MIMO links provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links coupled with several key optimization considerations, necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm called stream-controlled medium access (SCMA) that has the key optimization considerations incorporated in its design. Finally, we present a distributed SCMA protocol that approximates the centralized algorithm and compare its performance against that of baseline protocols that are CSMA/CA variants.

Measurements of small-scale fading and path loss for long range RF tags

RF modulated backscatter (RFMB), also known as modulated radar cross section or sigma modulation, is a RF transmission technique useful for short-range, low-data-rate applications, such as nonstop toll collection, electronic shelf tags, freight container identification and chassis identification in automobile manufacturing, that are constrained to have extremely low power requirements. The small-scale fading observed on the backscattered signal has deeper fades than the signal from a traditional one-way link of the same range in the same environment because the fading on the backscattered signal is the product of the fading on the off-board-generated carrier times the fading on the reflected signal. This paper considers the continuous wave (CW) type of RFMB, in which the interrogator transmitter and receiver antennas are different. This two-way link also doubles the path loss exponent of the one-way link. This paper presents the cumulative distribution functions for the measured small-scale fading and the measured path loss for short ranges in an indoor environment at 2.4 GHz over this type of link.

A fair medium access control protocol for ad-hoc networks with MIMO links

We present a new medium access control (MAC) protocol for ad-hoc networks with multiple input multiple output (MIMO) links. Links that use multiple element arrays (MEAs) at both ends are referred to as MIMO links. MIMO links are known to provide extremely high spectral efficiencies in multipath channels by simultaneously transmitting multiple independent data streams in the same channel. MAC protocols have been proposed in related work for ad-hoc networks with other classes of smart antennas such as switched beam antennas. However, as we substantiate in the paper, the unique characteristics of MIMO links necessitate an entirely new MAC protocol. We identify several advantages of MIMO links, and discuss key optimization considerations that can help in realizing an effective MAC protocol for such an environment. We present a centralized algorithm that has the optimization considerations incorporated in its design. Finally, we present a distributed protocol that approximates the centralized algorithm, and compare its performance against that of baseline protocols that are variants of the CSMA/CA protocol.

Pattern synthesis for arbitrary arrays using an adaptive array method

This paper presents a new pattern synthesis algorithm for arbitrary arrays based on adaptive array theory. With this algorithm, the designer can efficiently control both mainlobe shaping and sidelobe levels. The element weights optimize a weighted L/sub 2/ norm between desired and achieved patterns. The values of the weighting function in the L/sub 2/ norm, interpreted as imaginary jammers as in Olen and Comptons (1990) method, are iterated to minimize exceedance of the desired sidelobe levels and minimize the absolute difference between desired and achieved mainlobe patterns. The sidelobe control can be achieved by iteration only on sidelobe peaks. In comparison to Olen and Comptons method, the new algorithm provides a great improvement in mainlobe shaping control. Example simulations, including both nonuniform linear and planar arrays, are shown to illustrate the effectiveness of this algorithm.

Measured joint Doppler-delay power profiles for vehicle-to-vehicle communications at 2.4 GHz

Measured per-tap Doppler spectra are presented for a frequency selective vehicle-to-vehicle or mobile-to-mobile wireless communications link in various multipath environments in Atlanta, Georgia. The measurements were taken using the direct sequence spread spectrum (DSSS) technique at 2.45 GHz. The environments, chosen for their exceptionally long delay spreads, include an expressway, an urban T-intersection, and an exit ramp. The different environments produced quite different spectra. Also, for a given channel, the spectra corresponding to different delays were different, implying a non-separable channel model.

Six Time- and Frequency-Selective Empirical Channel Models for Vehicular Wireless LANs

Three vehicle-to-vehicle (VTV) models and three roadside-to-vehicle (RTV) models, each suitable for RF channel emulation and based on measurements at 5.9 GHz, are presented. Each model captures the joint Doppler-delay characteristics of a different environment. The packet error rate (PER) for each model, measured with an emulator and a 802.11p Wireless Access in Vehicular Environments (WAVE) prototype, is presented.

Power-controlled capacity for interfering MIMO links

An iterative algorithm is presented to jointly optimize the capacities for a number of interfering array-to-array or MIMO flat-fading links. All signals are co-channel. At each iteration for a given link, water-filling is used to calculate the optimum transmitter signal vector correlation matrices and receiver transformations under a constraint of total transmitted power for that link. Thus, by controlling the relative transmit powers at each of the interfering links the relative capacities can be controlled. A Monte-Carlo simulation was performed for an example three-link geometry. Several statistics for the three capacities are given for various power control conditions.

Spherical-wave model for short-range MIMO

The plane-wave assumption has been used extensively in array signal processing, parameter estimation, and wireless channel modeling to simplify analysis. It is suitable for single-input single-output and single-input multiple-output systems, because the rank of the channel matrix is one. However, for short-range multiple-input multiple-output (MIMO) channels with a line-of-sight (LOS) component, the plane-wave assumption affects the rank and singular value distribution of the MIMO channel matrix, and results in the underestimation of the channel capacity, especially for element spacings exceeding half a wavelength. The short-range geometry could apply to many indoor wireless local area network applications. To avoid this underestimation problem, the received signal phases must depend precisely on the distances between transmit and receive antenna elements. With this correction, the capacity of short-range LOS MIMO channels grows steadily as the element spacing exceeds half a wavelength, as confirmed by measurements at 5.8 GHz. In contrast, the capacity growth with element spacing diminishes significantly under the plane-wave assumption. Using empirical fitting, we provide a threshold distance below which the spherical-wave model is required for accurate performance estimation in ray tracing.