ormation. More powerful smartphones and laptops are ing more popular nowadays, demanding advanced multimedia capabilities. This has resulted in an explosion of wireless mobile devices and services. The EMO pointed out that there has been a 92 percent growth in mobile broadband per year since 2006 [2]. It has been predicted by the Wireless World Research Forum (WWRF) that 7 trillion wireless devices will serve 7 billion people by 2017; that is, the number work-connected wireless devicesРwill reach 1000 times the world’s population [4]. As more and more devices go wireless, many research challenges need to be addressed.РOne of the most crucial challenges is theРphysical scarcity of radio frequency (RF) spectra allocated for munications. Cellular frequencies use ultra-high-frequency bands for
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