Then we should now understand as to what are the difficulties in the wireless mobile communications: a major difficulty is the limited spectrum. You have a very small spectrum, maybe in the first generation it was only 25 MHz. You have limited power, you cannot carry a large amount of power with you in the mobile terminal. Fading because of the urban and the suburban environment creates lots of difficulties. Interference from the co-users and other users because of the frequency reuse is another problem, and mobility and portability are most important service requirements; then the last is the large number of users.

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