ormation that is intended for that particular subscriber. Security must also be ensured. Since a mobile station does not have the decoding key for other mobile stations, it will not be able to decode information to other terminals. When the number of users of the same carrier increases, the more difficult it becomes for a mobile station to decode its own information. The decoding is enhanced by a wide bandwidth when using CDMA solutions. This leads to the term WCDMA (wide- band CDMA). Another way to understand the concept of CDMA is to use the analogy of an airport international arrivals lounge where everyone is talking in different languages at the same time. If a French-speaking person arrives one may hear a French conversation across the room. See Figure 13.20. Similarly if an English person arrives, one may hear an English conversation. For all these conversations to be intelligible by listeners, each speaker's volume must not exceed that of the others, which 'explains' why power control is vital to WCDMA networks, since each transmitter is an interference source to all others. The type of WCDMA that will be used in most cases, at least in the beginning, is called frequency division duplex (FDD). This means that UMTS will use different bands for uplink and downlink just like GSM. The duplex distance is 190 MHz. 13.11 SYSTEM EVOLUTION In 1999, the International Telecommunication Union (ITU) approved an industry standard for third-generation wireless networks. This standard, called International Mobile Tele- communications-2000 (IMT-2000), is composed of three standards - commonly referred to as WCDMA, CDMA2000 and TD-SCDMA - based on CDMA technology. Access Network In general, mobile systems develop from voice carriers to IP-based multimedia carriers in a number of phases, called releases by 3GPP (for WCDMA/UMTS type systems) and 3GPP2 (for CDMA type systems). The evolution towards increasing bandwidth for GSM and cdmaOne type systems was shown in Figure 13.10. 13.11.1 GSM The second generation of cellular sys