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Next generation wireless systems and networks phần 4

Nhiều giấy tờ [44-48] đã được công bố như là một kết quả của nỗ lực rất lớn của cả hai học viện và ngành công nghiệp. Do không gian hạn chế, chúng tôi sẽ không sống nhiều hơn trên lý thuyết của các kênh thông tin liên lạc vô tuyến điện | 140 3G MOBILE CELLULAR TECHNOLOGIES Link adaptive modulation The link adaptive modulation scheme is another important feature introduced in the CDMA2000 1xEV system. In fact the 1xEV forward link offers a range of different data rates. The data rates match the range of channel conditions experienced in a typical cellular or PCS networks. QPSK modulation is used to achieve 38.4 kbps through 1228.8 kbps data rates with the exception of 921.6 kbps 8PSK for 921.6 kbps and 1843.2 kbps and 16QAM for 1228.8 kbps and 2457.6 kbps. Table 3.12 shows the correspondence of adaptive modulation schemes code rate transmission rate number of slots and so on in the CDMA2000 1xEV Forward Link. The CDMA2000 1xEV forward link supports dynamic data transmission rates. The AT constantly measures the channel carrier to the interference C I ratio and then requests the appropriate data rate for the channel conditions every 1.67 ms. The AP receives the AT s request for a particular data rate and encodes the forward link data at exactly the highest rate that the wireless channel can support at the requested instant. Just enough margin is included to allow the AT to decode the data with a low erasure rate. In this way as the subscriber s application needs and channel conditions change the optimum data rate is determined and served dynamically to the user. In summary the following steps are performed Accurate and rapid measurement of the received C I ratio from the set of best serving sectors Selection of the best serving sector Request of transmission at the highest possible data rate that can be received with high reliability given the measured C I Transmission from the selected sector and only from the selected sector at the requested data rate. The AT continuously updates the AP on the DRC channel indicating a specified data rate to be used on the forward link. The DRC is sent with a Walsh Cover which indicates which sector should transmit. CDMA2000 1xEV combines the functions of the .