tailieunhanh - Next generation wireless systems and networks jul 2006 phần 7

AT liên tục biện pháp các nhà cung cấp dịch vụ kênh để can thiệp (C / I) tỷ lệ, và sau đó yêu cầu tốc độ dữ liệu phù hợp với điều kiện mỗi kênh 1,67 ms. AP nhận được yêu cầu của AT cho tốc độ dữ liệu cụ thể, và | 296 MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS As its name suggests the system is based on OFDM however OFDMA is much more than just a physical layer solution. It is a cross-layer-optimized technology that exploits the unique physical properties of OFDM enabling significant higher layer advantages that contribute to very efficient packet data transmission in a cellular network. Packet-switched air interface The telephone network designed basically for voice is an example of circuit-switched systems. Circuit-switched systems exist only at the physical layer that uses the channel resource to create an end-to-end bit pipe. They are conceptually simple as the bit pipe is a dedicated resource and the pipe does not need to be controlled once it is created some control may be required in setting up or tearing down the pipe . Circuit-switched systems however are very inefficient for burst data traffic. Packet-switched systems on the other hand are very efficient for data traffic but require that the upper layers be controlled in addition to the physical layer that creates the bit pipe. The MAC layer is required for the many data users to share the bit pipe. The data link layer is needed to take the error-prone pipe and create a reliable link for the network layers to pass packet data flows over. The Internet is the best example of a packet-switched network. Because all conventional cellular wireless systems including 3G were fundamentally designed for circuit-switched voice they were designed and optimized primarily at the physical layer. Some people suggested that the choice of CDMA as the physical layer multiple access technology was also dictated by voice requirements. OFDMA on the other hand is a packet-switched scheme designed for data and is optimized across the physical MAC data link and network layers. The choice of OFDM as the multiple access technology is based not only on physical layer consideration but also on the MAC layer data link layer and network layer