The angle spread causes space-selective fading, which manifests itself as variation of signal amplitude according to the location of antennas. The significance of delay spread depends on the time-width of the signal relative to that of the channel hence a quantitative characterization of the severances of channel delay-spread is necessary. This phenomenon is referred to as delay spread. Consequently, when a “narrow” pulse is transmitted over a multipath propagation channel, distorted replicas of the transmitted pulse arrive at the receiver at various different times, making the received signal “wider” in time than the transmitted signal. In a multipath propagation environment the received signal consists of a large number of components having different delays. The small-scale variations include signal fading, delay-spread and Doppler-spread, which we believe contribute to transmission errors in a digital environment. Our research focuses on mitigating the effects of the small-scale variations using digital signal processing techniques. These variations are referred to as large-scale variations because they are obtained from measurement data taken at various locations in a large area. ![]() In the second step, variations of the large-scale statistics are determined from measurements taken in different multipath environments. The statistics thus collected are referred to as small-scale variations, because they are usually obtained from measurement data obtained at various locations in a small area. In the first step, it is assumed that the multipath environment is fixed, and variations of the received signal are measured for the given multipath environment. Since the properties of the received signal are clearly a strong function of the multipath environment, statistical characterization of the received signal is often done in a two-step process. The statistics of this random process can be collected from extensive field measurements in selected operation environments. One common approach is to treat the received signal as a spatial-temporal random process. Multipath propagation is a rather complicated phenomenon that is cumbersome to characterize. The receiver antenna output is the sum of the multiple signal copies weighted by the antenna gain pattern. Since the different versions of the signal propagate through different paths, they will in general have different attenuation, phase shifts, time delays, and angles of arrival. The receiver antenna will therefore receive multiple copies of the transmitted signal. The much stronger proton signals allow the coil to be less optimized for these signals and still produce adequate images. Often, dual-tuned coils are used so that proton images can be obtained using the same coil. The coil should also be uniformly sensitive throughout the VOL Surface coils are often employed because the coil can be positioned closer to the signal-generating nuclei, and because the localized reception profile means that less noise will be collected from regions of the object (usually the dominant source of noise) remote from the coil. The coil should be tuned to the object to maximize the efficiency of signal collection. Because of the acute SNR limitations to MRS measurements, additional care needs to be taken with respect to coil efficiency and other areas of potential signal loss. ![]() Measurement at different resonance frequencies typically means that different RF transmitter and receiver antenna coils must be designed and used. Sanders, in Functional Brain Imaging, 1995 Coils. In addition, the dual-antenna terminals could potentially handle almost 10 dB more interference 79 101. ![]() Analysis shows that a dual-antenna solution in GSM terminals can give a substantial coverage improvement of up to 6 dB. While multiple antennas have an implementation impact for the terminal, there is no impact on the base-station hardware or software.įor GSM/EDGE, a dual-antenna solution called Mobile Station Receive Diversity (MSRD) is standardized. There are also possibilities for interference cancellation through multiple antennas. ![]() There will thus be improvements for both interference-limited and noise-limited scenarios. Johan Sköld, in 4G LTE/LTE-Advanced for Mobile Broadband, 2011 19.2.2 Dual-Antenna TerminalsĪs was shown in Chapter 5, multiple receiver antennas are an effective means against multipath fading and to provide an improved signal-to-noise ratio through “energy gain” when combining the antenna signals.
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