An Experimental Approach to CDMA and Interference Mitigation phần 2 potx

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 (e.g., 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 ser￾vice 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).