 periodically access this set of TSs throughout the duration of the frame. Spreading codes described in Chapter 4 separate user signals within one or more slots. Hence, in the TDD mode we define a physical channel by a code, one TS, and one frequency, where each TS can be assigned to either the uplink or the downlink depending on the demand. Users may obtain flexible transmission rates by occupying several TSs of a frame as illustrated in Figure 2.2, without additional proc- essing resources from the transceiver hardware. On the other hand, when more than one frequency channel gets occupied, utilization of transceiver resources will increase if the wide-band transmission cannot prevent it. We achieve variable data rates through either multi-code transmission with fixed spreading or through single code with variable spreading. In the 1st case, a single user or users may get multiple spreading codes within the same TS; while in the 2nd case, the physical channel spreading factor may vary according to the data rate. The UMTS Network and Radio Access Technology 2.1.2 Signal Processing Aspects In the following, we review Signal Processing characteristics for the WCDMA as well as TD/CDMA as a base to describe key functions of the UTRA FDD and TDD modes. These include spreading aspects and modulation and coding. 2.1.2.1 The Spread Spectrum Concept Digital designs of communications systems aim to maximise capacity utilization. We can for example increase channel capacity by increasing channel bandwidth, and/or transmitted power. In this context, CDMA operates at much lower SIN ratios as a result of the extra channel bandwidth used to achieve good performance at low signal-to-noise ratio. From Shannon's channel capacity principle [22] expressed as: (2.1) where B is the bandwidth (Hz), C is the channel capacity (bits/s), S is the signal power, and N is the noise power; we can find a simple definition of the bandwidth as: (2.2) Thus, for a particular SIN ratio, we can achieve a low information error rate by increas- ing the