Propagation Issues for Cognitive Radio WhiletheCRsystem designercandeterminethetransmit power,it is thepropagation channel that determines how much of that power arrives as interference to primary and other CR users.

Cognitive radios are expected to work in bands below about 3.5 GHz and may be used for a variety of appli- cations, e.g., broadband fixed wireless access, mobile and nomadic access, etc. Cognitive radio system designers must have access to a wide range of channel models covering a wide span of operating frequencies, carrier bandwidths, deployment conditions, and environments. This paper provides a compre- hensive overview of the propagation channel models that will be used for the design of cognitive radio systems. We start with classical models for signal loss versus distance and discuss their dependence on the physical properties of the environ- ment and operating frequency. Here we also introduce the concept of log-normal shadowing resulting from signal block- age by man-made and natural features. Next, we discuss the time-varying nature of the wireless channel, introduced as a result of the motion of objects in the channel. This is followed by a discussion on the dispersion of the signal caused by various effects of propagation, especially in the time and frequency domains. Angular dispersion, which is discussed next, is important because cognitive radios may be based on modems that exploit the spatial domain. Lastly, we summarize channel models that have been standardized for fixed and mobile systems.

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