RMS Delay Spread


RMS Delay Spread

Section: Effects of Bandwidth on Estimation of UWB Channel Parameters Lesson: RMS delay spread Ultra Wideband Communications. This course is brought to you by Gilad James Mystery School. Learn more at Gilad James.com. Introduction The subject of this lecture is to investigate the effect of bandwidth on a short range indoor UWB channel performance. This research is based on a measurement campaign performed on a wooden desk surface placed in an office room at our faculty building. A vector network analyzer (VNA) is used to sweep the 1 GHz, 2 GHz, 5 GHz and 7.5 GHz bandwidths centered at 6.85 GHz, obtaining the frequency response of the channel. Using the VNA time domain capability the channel impulse response is obtained. This is equivalent to sounding the channel with frequency chirp pulses equal in duration to the inverse of the frequency bandwidth. In other words, a narrower bandwidth results in a wider pulse in time domain. Although the measurements are performed in same environment for all bandwidths, it is expected that an UWB channel itself would not be equally perceived by different pulse widths. A wider system bandwidth results in shorter pulses, which in turn account for finer temporal and spatial resolution. In this way more multipath components can be resolved as the pulses overlap in a lesser extent. The fundamental differences between an UWB channel and a narrowband channel arise from the frequency selectivity of the propagation process (Molisch, 2005). As the UWB signal has a wide frequency spectrum which may extend to several gigahertz, the frequency dependence of diffraction/reflection coefficients and dielectric constants can be significant (Di Benedetto et al., 2006). A number of papers report on the effect of carrier frequency on channel parameters. One such investigation (Cassioli et al., 2004) finds a strong dependence between the path loss model exponent and the carrier frequency, yet states that there is no correlation with the bandwidth. (Ghassemzadeh et al., 2005) presented an extensive measurement campaign at two different bandwidths (1.25 GHz and 6 GHz) centered at 5 GHz, reporting mostly minor differences in parameter values between the two bandwidths. Another paper (Choi et al., 2009) models the path loss exponent variation as a function of frequency. A research project (Chang & Tarng, 2007) investigates the effects of bandwidth on observable multipath clustering and Δ-Kmodel parameters for an indoor UWB wireless channel with signal bandwidths of 0.5, 1 and 2 GHz. However, to authors’ knowledge, so far there have been no attempts to investigate the effects of bandwidth on estimating the path loss, shadowing, mean excess delay and RMS delay spread in short range UWB scenarios, which are the key parameters for assessing the link budget, signal-to-noise ratio and intersymbol interference of the system. The next section of the lecture explains the measurement procedure and equipment and describes the measurement environment. The subsequent sections present the path loss, shadowing, mean excess delay and RMS delay spread parameter values estimated from the measured power delay profiles, respectively. The obtained results are discussed in the seventh section. The lecture is concluded in the eighth section, briefly summarizing all the key findings of this research.
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