Phát sóng thông tin có thể được lưu trữ cục bộ trong các thiết bị đầu cuối người dùng (cacheing) và truy cập khi yêu cầu của người sử dụng. Trong trường hợp này, một lớp phát sóng vệ tinh với kích thước tế bào lớn (Hình 1-8) trên đầu trang của T-UMTS sẽ cung cấp một giải pháp rẻ hơn cho các loại hình dịch vụ. | 68 Chapter 2 where pm k pkm is the shorthand notation for cross-correlation coefficient R mck between the two spreading codes of channel m and channel k A is the amplitude of signal on channel i df is the kth data symbol on channel i and v is the ith noise component. The effect of MAI is apparent and it is also clear that it can be potentially destructive. Assume for instance that P1 2 0 codes 1 and 2 are not orthogonal and that A 2 A 1 the MAI term P1 2A 2 d 2 in may overwhelm the useful term A 1 dk1 for data detection of channel 1. This phenomenon is called the near far effect user 2 can be considered as located near the receiver in the BS thus received with a large amplitude whilst user 1 the one we intend to demodulate is the far user and is weaker than user 2. Generalizing to N users equations can be easily cast into a simple matrix form. If we arrange the crosscorrelation coefficients pi k into the correlation matrix R f 1 P24 P1 2 1 Pi N P2 N . p N 1 Pn 2 1 and if we also introduce the diagonal matrix of the user amplitudes A diagIA 1 A 2 . A N I we have z - ARd V where the column vectors z d and V simply collect the respective samples of received signal data and noise. A simple multiuser detector is the decorrelating detector that applies a linear transformation to vector z to provide N soft decision variables relevant to the N data bits to be estimated. Collecting such N decision variables gk into vector g the linear joint transformation on the matched filter output vector z is just g R-1z where R 1 is the decorrelating matrix so that g R 1 ARd V Ad R-1V dlag A1dk1 A2d k . ANdkN v 2. 154 We have thus 2. Basics of CDMA for Wireless Communications 69 g g g . gk where the superscript T denotes transposition gk Aw d vk i and v k is just a noise component. It is apparent that the MAI has been completely cancelled provided that the correlation matrix is invertible or in other words the different channels have been .
