Summary
International Symposium on Antennas and Propagation
2008
Session Number:3A05
Session:
Number:3A05-2
Relations between Coherence Bandwidth and Average Mode Bandwidth in Reverberation Chamber for Wireless Device measurement
Xiaoming Chen, Per-Simon Kildal,
pp.-
Publication Date:2008/10/27
Online ISSN:2188-5079
Summary:
The reverberation chamber is basically a metal cavity with many excited modes which are stirred to create a statistical isotropic field environment with Rayleigh distributed magnitudes of the field [1]. The reverberation chamber used here is 1.8 m х 1.4 m х 1.2 m in size. The chamber makes use of frequency stirring, platform stirring, and polarization stirring to improve measurement accuracy [1], [2]. It has been used to measure antenna radiation efficiency and diversity gains [2]. For these measurements the intrinsic average mode bandwidth of the chamber is of interest as it determines the averaged power transmission level of the chamber [3]. It may also be sued to control the number of excited modes in the chamber, and thereby also the number of independent samples in order to generate enough independent samples to improve the measurement accuracy [4], [5]. Reverberation chamber can also be used to measure total radiated power and total isotropic sensitivity of active mobile phones [6], [7]. For such measurements the delay spread and coherence bandwidth are of relevance. Coherence bandwidth is defined as the frequency range over which the channel is correlated. When a signal at a certain frequency is transmitted through a reverberation chamber, several cavity modes will be excited. Each of these resonating modes can be characterized by its Q-factor, which is the center frequency over the mode bandwidth. When several modes are excited, averaged mode bandwidth over all the modes excited is usually used. It is evident that that if the reverberation chamber is excited at one frequency, modes at other frequencies around it (within the mode bandwidth) will also be excited. Thus, the transfer functions of the chamber will be correlated within a mode bandwidth. The paper will show that the coherence bandwidth and averaged mode bandwidth are the same when Q is high (loading is small), and that the former will tend to be smaller than the later when Q is low. Coherence bandwidth is used to characterize the channels through an environment for wireless communication. When multipath fading is presented, and the signal bandwidth is much smaller than the coherence bandwidth of the channel, the fading is referred to as being "flat"; on the other hand, if the signal bandwidth is comparable to the coherence bandwidth, or larger than it, the fading will be frequency-selective. Because RMS delay spread is inversely proportional to coherence bandwidth [8], RMS delay spread can also be used to define flat or frequency-selective fading. Many measurements of RMS delay spreads were done for indoor and outdoor environments [8]. In the present paper, measured RSM delay spreads with different loadings of the chamber are presented as well for comparison with those measured in real environments.