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Multi carrier and spread spectrum systems phần 9

Điều này dẫn đến một tổng số (K + 16) × 8 + 6] / R bit. Cuối cùng, các bit thủng nối tiếp song song chuyển đổi và trình mapper biểu tượng. Nếu BER trước khi giải mã RS là đảm bảo được khoảng 2 - 04/10, sau đó đan xen đầy đủ (ví dụ, 8 từ mã RS) cho các giá trị SNR cùng được đưa ra trong Bảng 4-3, một lỗi gần như miễn phí (tức là, BER coding, the systematic structure of the RS code allows one to shorten the code, i.e., remove the filled 239 −. | 222 Applications DVB-T base station tranceiver Subscribers Uplink return channel Figure 5-21 DVB-RCT network architecture Figure 5-22 Overview of the DVB-RCT standard parameters of the DVB-RCT specification is to employ the existing infrastructure used for broadcast DVB-T services. As shown in Figure 5-22 the interactive downlink path is embedded in the broadcast channel exploiting the existing DVB-T infrastructure 7 . The access for the uplink interactive channels carrying the return interaction path data is based on a combination of OFDMA and TDMA type of multiple access scheme 6 . The downlink interactive information data is made up of MPEG-2 transport stream packets with a specific header that carries the medium access control MAC management Interaction Channel for DVB-T DVB-RCT 223 data. The MAC messages control the access of the subscribers i.e. terminal stations to the shared medium. These embedded MPEG-2 transport stream packets are carried in the DVB-T broadcast channel see Figure 5-22 . The uplink interactive information is mainly made up of ATM cells mapped onto physical bursts. ATM cells include application data messages and MAC management data. To allow access by multiple users the VHF UHF radio frequency return channel is partitioned both in the frequency and time domain using frequency and time division. Each subscriber can transmit his data for a given period of time on a given sub-carrier resulting in a combination of OFDMA and TDMA multiple access. A global synchronization signal required for the correct operation of the uplink demodulator at the base station is transmitted to all users via global DVB-T timing signals. Time synchronization signals are conveyed to all users through the broadcast channel either within the MPEG2 transport stream or via global DVB-T timing signals. In other words the DVB-RCT frequency synchronization is derived from the broadcast DVB-T signal whilst the time synchronization results from the use of MAC management .