iversal Mobile Telecommunications System). Although it uses a new method of modulation, much of the architecture is based on the very successful GSM system. However, it has been optimized to enable Internet data to be handled with ease. In common with other third generation systems, the emphasis is on the flexible delivery of high speed data as well as the normal voice connections and a general improved quality of service. It is also planned to use the exceedingly 276 Newnes Guide to Radio and Communications Technology popular SMS concept. In this way messages will be able to be passed from one network system to another. Obviously one of the main features of W-CDMA or UMTS is that it uses wideband CDMA to enable it to receive many different signals at the same time. GSM uses a mixture of frequency division multiple access (where different users are given different channels or frequencies) and time division multiple access (where users are allocated different timeslots). The W-CDMA system is set up SO that the bandwidth of the signal is 5 MHz wide, but naturally this bandwidth can accommodate many different users. The bandwidth is much wider than that used with cdmaOne, for example, because a higher chip rate is used when encoding the CDMA signal, 3.84 MHz as opposed to 1.2288 MHz for cdmaOne. While most systems allow either FDD or TDD to be used, within the UMTS specification both frequency division and time division duplex are allowed. Where frequency division duplex is used, a separation of 190 5 MHz 5 MHz Frequency 190 MHz (a) Frequency division duplex Transmit time Guard period (b) Time division duplex Figure 10.10 Access methods for WCDMA Cellular telecommunications 277 MHz is used. This type of spectrum allocation is called paired spectrum as there is one band for the uplink and another for the downlink. In some areas of the world where spectrum allocations are limited time division duplex will be used. Here, bursts of data are transmitted in each direction; however, a guard period is required between trans