TAILIEUCHUNG - Next Generation Mobile Systems 3G and Beyond phần 3

Tín hiệu vô tuyến từ các máy phát tán thông qua con đường khác nhau, kết quả là tín hiệu đa nhận với mức công suất khác nhau và Đặc điểm tuyên truyền trong các môi trường di động Người nhận sử dụng các tính năng và chức năng sau đây: • thích ứng va chạm cửa sổ | RADIO ACCESS TECHNOLOGIES IN CELLULAR NETWORKS 59 Background of Radio Access Technologies This section briefly reviews the propagation characteristics in a mobile communication environment basic multiple access methods and discusses why CDMA is the best scheme for cellular communications. Propagation Characteristics in Mobile Environments In a territory mobile communication system the path between antennas of base station and mobile station is usually non-line-of-sight. The propagation characteristics vary in time because of the mobility of the station itself and to changes in the surrounding physical environments. A major objective in mobile communications is to overcome the degradation in communication quality caused by the channel variation. Figure illustrates the multipath propagation in a mobile environment. Radio signals from the same transmitter propagate via different paths resulting in multipath signals at the receiver with various power level and arrival times. Figure depicts propagation characteristics in a mobile environment. Long-term variation is due to the geographic path-loss law short-term variation is caused by shadowing fading and instantaneous variation is due to the change of the surrounding physical environment at the receiver. Raleigh distribution is most often used for describing the instantaneous variation of a multipath channel. MS BS Figure Multipath propagation in mobile communication environment 60 RADIO ACCESS TECHNOLOGIES IN CELLULAR NETWORKS Figure Propagation characteristics in a mobile environment The frequency characteristics of propagation depend on the delay spread of propagation. The longer the delay spread the larger the impact on the frequency characteristics. Therefore fading related to frequency characteristics can be classified as frequency-flat fading and frequency-selective fading. A frequency-flat fading channel is composed of long-term and short-term variations. A frequency-selective fading .

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