Ultra Wideband Indoor Radio Channel Measurements

This paper explains the ultra wideband (UWB) indoor radio channel measurement procedure performed at the main building at the University of Oulu, Finland. The measurement campaign included rooms with various sizes and layouts. The frequency response of the radio channel was recorded using a network analyzer in frequency sweep mode. The impulse response of the radio channel was calculated by taking inverse Fourier transform of the recorded radio channel frequency response. Channel characterization and, finally, the statistical radio channel models, will be generated from the impulse responses. UWB transmission techniques produce extremely wide bandwidth for the propagating radio signal. This allows one to utilize UWB technology in different kinds of applications that demand precise temporal or spatial resolution. Knowledge of the signal propagation mechanism in the channel is vital for the radio system design and performance analysis. However, currently published radio channel models do not offer spatial resolution high enough for the UWB applications and real channel measurements are needed. 1. UWB RADIO CHANNEL MEASUREMENTS To gain knowledge of the UWB radio channel, there are several possible techniques to perform the radio channel sounding; technique based on frequency sweeping or the one based on impulse transmission. In the former case, a wide frequency band is swept by a set of narrowband signals to form a frequency response using a network analyzer. This corresponds to a S21-parameter measurement setup, where the device under test (DUT) is the radio channel. In the latter case, a narrow pulse is sent to the channel and the channel impulse response is measured using a digital sampling oscilloscope. Impulse train can also be generated using direct sequence spread spectrum signal having very fast chip rate. Theoretically, having a static measurement environment and occupying an unlimited bandwidth, all the techniques end up in the same result. In our study, the physical indoor environment during the sounding was kept as static as possible, but no effort was made to control the radio interference from other RF sources. 1.1 Measurement Setup The radio channel measurement system consists of a vector network analyzer (Agilent 8720ES), a wideband amplifier (Agilent 83017A), a conical antenna pair and a control computer with LabVIEW software. The network analyzer is operated in response measurement mode, where PORT1 is a transmitter port (TX) and PORT2 is a receiver port (RX). An external amplifier is connected to PORT1 to increase the TX power level. The antennas are CMA-118/A conical antennas by Antenna Research Associates, Inc [1]. The chosen antenna has typically an omni-directional radiation pattern, with a constant phase centre. The sweep time of the analyzer depends on the bandwidth, and on the number of frequency points within the sweeping band, and it is automatically adjusted by the analyzer. Table 1 lists the main parameters of the measurements. The antenna gain is specified by the manufacturer. The used measurement setup is presented in Fig.1. The figure introduces also the post-processing procedure. Table 1. Measurement setup parameters.