restrial mạng UMTS là dựa trên các tế bào rất nhỏ (bán kính vài km) cho phép cung cấp tốc độ đỉnh cao (lên đến một vài trăm kbit / s) và đạt được một tần số tái sử dụng lớn. Điều này là có thể nhờ vào công nghệ CDMA là cốt lõi của giao diện vô tuyến UMTS. | 12 Chapter 1 restrial UMTS networks are based on very small cells of a few km radius which allow the provision of high peak rates up to a few hundreds kbit s and attain a large frequency re-use. This is possible thanks to the CDMA technology which is at the core of the UMTS radio interface. However the technical solution becomes very inefficient when the UMTS networks have to be used to transmit the same information to many users . video clips containing highlights of sport events financial information broadcasting of most accessed web pages etc. . Broadcast information may be locally stored in the user terminal cacheing and accessed when required by the user. In this case a satellite broadcast layer with large cell sizes Figure 1-8 on top of T-UMTS will provide a cheaper solution for this kind of services. Satellite systems have a major advantage in broadcasting information since a single satellite can cover regions as large as Europe or USA. Good quality of service can be achieved by means of powerful error correction techniques and by an integrated terrestrial gap filler network. Figure 1-7. Sample multi-beam satellite footprint courtesy of Alenia Spazio Italy . In all cases the efficient use of the very scarce and expensive spectrum resources causes the S-UMTS system to operate under heavy interference conditions. The interference is mainly caused by other satellite beams which 1. Introducing Wireless Communications 13 are re-using the same frequency see Figure 1-9 . In the case of terrestrial gap fillers other co-frequency transmitters also generate further co-channel interference. So the issue of co-channel interference mitigation to increase system capacity is pivotal to the economical development and deployment of S- UMTS. Figure 1-8. Sample broadcast footprint courtesy of Alenia Spazio Italy . Figure 1-9. Interference pattern for a European multi-beam UMTS satellite courtesy of Alenia Spazio Italy . 14 Chapter 1 4. VLSI TECHNOLOGIES FOR WIRELESS .
