![]() ![]() ![]() To allow these various operations, the logical channels transmitted on the beacon are: To access the general parameters of the cell needed for the procedures applied by the MS, or general information concerning the cell, such as its identification, the beacon frequencies of the surrounding cells, or the option supported by the cell (services). This selection is performed on the basis of the received power measurements made on the adjacent cells' beacon channels. To help the mobile in the selection of a cell when it is in idle mode (that is, not in communication, but still synchronized to the system and able to receive an incoming call or to initiate a call). These measurements are used by the network to determine when a handover is necessary, and to which BTS this handover should apply. To help the mobile in estimating the quality of the link during a communication, by measurements on the received signal from the BTS it is transmitting to, and from the other BTSs of the geographical area. The frequency and time synchronization procedures that are performed by the mobile are explained in Section 1.5.7. This synchronization is needed by the MS to access the services of a cell. To allow a synchronization in time and frequency of the MSs to the BTS. ![]() The broadcast channels are transmitted on the beacon carrier frequency presented in Section 1.2.7. An associated control channel is allocated to one mobile, in addition to a dedicated channel, and carries signaling for the operation of this channel. A channel is said to be dedicated if only one MS can transmit or receive in the ARFCN-TN defining this channel, and common if it carries information for several mobiles. A broadcast channel is used by the network (in downlink only) to send general information to the MSs. Among the control channels, according to their functions, four classes are defined: broadcast, dedicated, common, and associated. Two types of logical channels exist, traffic channels and control channels. Logical channels can be seen as pipes, each one used for a different purpose by the higher layers of the system. This signaling information is used for setting up a call, or to adapt the link to rapidly changing radio conditions, or to manage handovers, to give a few examples. On top of the physical channels, logical channels ar mapped to convey the information of voice, data, and signaling. The association of a radio frequency channel and a time slot-the pair ARFCN and TN-uniquely defines a physical channel on both the uplink and the downlink. GSM is based on both FDMA and TDMA techniques (see Figure 1.3). The various multiple-access techniques in use in radio systems are frequency-division multiple access (FDMA), TDMA, and code-division multiple access (CDMA). The multiple-access scheme defines how simultaneous communications share the GSM radio spectrum. There are different ways of sharing the physical resource among all the users in a radio system, and this is called the multiple-access method. Operators may implement networks that operate on a combination of the frequency bands listed above to support multiband mobile terminals. Table 1.1 gives a summary of uplink and downlink frequency bands for the different GSM systems. For instance, in the 900-MHz E-GSM band, the block 880-915 MHz is used for transmission from mobiles to network, and the block 925-960 MHz is used for the transmission from network to mobiles. The system is based on frequency-division duplex (FDD), which means that the uplink (radio link from the mobile to the network-that is, mobile transmit, base receive), and downlink (from the network to the mobile-that is, base transmit, mobile receive) are transmitted on different frequency bands. The GSM-400 is intended for deployment in Scandinavian countries in the band previously used for the analog Nordic Mobile Telephony (NMT) system. The GSM-850 is under development in America. The PCS-1900 is used primarily in North America. It was also introduced to avoid saturation problems with the GSM-900. The DCS-1800 operates in the 1,800-MHz band and is used mainly in Europe, usually to cover urban areas. The GSM-900 is the most common in Europe and the rest of the world. Currently, there are several types of networks in the world using the GSM standard, but at different frequencies. ![]()
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