Thomas P. Krauss

An experimental OFDM system for broadband mobile communications

We present an overview of an experimental 20 MHz 2/spl times/2 MIMO-OFDM system for operation at 3.65 GHz. The transmitted signal characteristics, data collection setup, measurement analysis, and timing acquisition algorithms are described. Field trials from a base station with two transmit antennas to a two-antenna mobile receiver are planned in a coverage area having a two mile radius. The first phase of the development process of this experimental system is complete, and initial laboratory results in the area of MIMO channel estimation and demodulation are presented for an indoor pedestrian MIMO channel.

Delay spread measurements on a wideband MIMO channel at 3.7 GHz

This paper summarizes the delay spread characteristics of a 20 MHz MIMO channel at a carrier frequency of 3.676 GHz. The signals are transmitted from a two-antenna base to a two-antenna mobile receiver, and the experiments were conducted in a suburban setting outside Chicago, Illinois. The results include a statistical description of the delay spread environment. In addition, a case study of the temporal behavior of the channel is presented along with a one-bounce analysis to locate macro scatterers.

Wideband MIMO mobile impulse response measurements at 3.7 GHz

This paper summarizes the propagation characteristics of a 20 MHz channel at a carrier frequency of 3.676 GHz transmitted from a two-antenna base to a two-antenna mobile receiver. The experiments were conducted in a moderate density suburban setting outside Chicago, Illinois. The transmit and receive systems are described, as are the data collection procedures and data analysis. The results include path loss measurements and various metrics of the MIMO channel, such as channel impulse responses, rms delay spread, and other spatial characteristics.

An Experimental Adaptive Beamforming System for the IEEE 802.16e-2005 OFDMA Downlink

Transmit adaptive beamforming can be applied on the subcarriers of an OFDM system to obtain a coherent processing gain at the receiver. The IEEE 802.16e-2005 standard for mobile broadband wireless access contains signaling strategies that facilitate the use of downlink adaptive beamforming which leverages time-division duplex (TDD) channel reciprocity. Using these strategies, Motorola Labs has developed an experimental mobile multi-antenna OFDM system that implements a real-time, adaptive beamformer. This paper describes key components and parameters of this experimental system and describes the results of initial field experiments that show the performance gains realized by this platform

Seasonal variations in path loss in the 3.7 GHz band

This work presents path loss measurements through a 20 MHz wireless channel at a carrier frequency of 3.676 GHz in a suburban setting outside Chicago, Illinois. The data collection system is described and the measurement environment is characterized. Results are presented for two seasons, winter and summer, demonstrating that tree foliage creates an average excess path loss of between 3 and 7 dB.

CHAMPS: a near-ML joint Doppler frequency/TOA search for channel characterization

The paper introduces a frequency-domain near-maximum likelihood (ML) joint Doppler frequency (DF)/time of arrival (TOA) search for channel characterization. The search is iterative and operates by finding one or more DF/TOA pairs at each step that minimize the channel estimation mean squared error. Because of its excellent ability to estimate frequency-domain channels as well as identify channel parameters (i.e., DFs and TOAs), we call the algorithm CHAMPS (CHannel estimation via an Approximate ML Parameter Search). CHAMPS is used to characterize the channels measured from an experimental 18.775 MHz OFDM mobile system operating at 3.676 GHz. The experimental system was used to collect data in Schaumburg, Illinois, an environment characterized by both industrial and residential areas. Differential delays up to 24 /spl mu/sec were detected as well as Doppler frequencies that are three times the expected Doppler frequency due to reflections off vehicles traveling in the opposite direction as the test van.

Direction of arrival and capacity characteristics of an experimental broadband mobile MIMO-OFDM system

This paper describes a series of field experiments that were performed in order to evaluate direction-of-arrival (DOA) and capacity characteristics of a 18.8 MHz 2/spl times/2 MIMO-OFDM channel at 3.676 GHz. These experiments were conducted in a suburban setting outside Chicago, Illinois with an experimental mobile broadband MIMO-OFDM system. We present multipath DOA statistics calculated based on a synthetic aperture array technique. This technique operates on data snapshots collected while the receiver was moving and makes use of the velocity and direction information provided by a GPS receiver. We also present MIMO capacity statistics that are computed from the frequency-domain matrix channel response measured by the experimental OFDM system. To explore the effect of frequency and receive angular diversity on the MIMO capacity distribution, we present cumulative distribution functions of the capacity gain of a 2/spl times/2 link (i.e., gain relative to 1/spl times/1 link) for different measured delay spread ranges as well as different DOA spread ranges.

An experimental evaluation of the subscriber antenna pattern effect in a MIMO-OFDM system

The antenna pattern impact on a 2/spl times/2 MIMO-OFDM broadband system is studied by evaluating radiation efficiency, received power, antenna pattern correlation, and the spatial characteristics of various antennas and pairs of antennas. Four different multiple antenna portable devices are described and analyzed. Three of the devices were developed with current Motorola cell-phone plastics where the antennas were mounted. The fourth device was developed on a laptop. Antennas and pairs of antennas for the different devices are compared, first apart from the environment through analysis of anechoic chamber measurements of the antenna patterns, and then in indoor environments through field measurements. Spatial characteristics in an outdoor environment are predicted by incorporating the measured portable antenna patterns into the suburban propagation environment estimated in previous data collection efforts with vehicle rooftop antennas.

Impact of Channel Variation on a Code Multiplexed Pilot in Multicarrier Systems

In spread multi-carrier systems with a code-multiplexed pilot channel, variations of the channel within the spreading block impact channel estimation and data demodulation through the loss of orthogonality of the spreading codes. In this paper the mean-squared error (MSE) between the true channel and a channel estimate obtained through simple despreading of the pilot code is derived. The closed-form MSE expression is a function of the channel fading statistics, the currently employed spreading codes, and the powers of the different users. By evaluating the MSE for a time-spread orthogonal frequency-division multiplexed (OFDM) system, it is observed that while spreading in time will have little impact in a pico-cell system scenario, a wide-area cell system experiences significant degradation in channel estimation performance at moderate Doppler frequencies. The impact on data demodulation is evaluated through simulations with the data users employing a turbo code.