ican Digital Cellular Both the United States and Canada have implemented digital techniques to increase the capacity and quality of the existing AMPS system. The North American digital cellular (NADC) system implements digital radio Mobile Communications communication in the frequency band of AMPS. It divides the channels of the analog AMPS into six time slots (TDMA). With the help of time division, three or six (half-rate speech mode) users share an analog 30-kHz AMPS chan- nel. The terminals with dual-mode capability use a digital system when it is available; otherwise, the analog AMPS service is used [1]. Because of this prin- ciple the NADC system is also known as dual-mode AMPS (D-AMPS). The NADC network system is able to provide service for even the old- est analog AMPS terminal. The common control channels of analog AMPS and NADC are compatible and a mobile station, analog or dual mode, first searches the forward (downlink) control channel (FOCC) that occupies one FDMA channel. Then the terminal informs the network with a signaling message that contains the information about its capabilities. There are three enhanced modes of operation in addition to the original analog AMPS: nar- rowband AMPS (NAMPS) (an analog enhancement designed by Motorola, which increases the capacity of the system), CDMA, and NADC. Upon recognizing the mobile's enhanced digital capabilities, the net- work will assign a digital traffic channel (DTC) to the mobile for a call. If a DTC is not available, then an analog channel is assigned instead. When a channel is assigned a channel number (frequency), a time slot number, a tim- ing advance value, and a mobile power setting are given to the MS. A timing advance value is needed in all TDMA systems and it defines the transmission time of the MS. A distant MS has to transmit earlier than an MS close to the BS, otherwise subsequent transmissions from mobile stations would overlap in the BS's receiver. The channel is maintained until disconnect time with the help of the continuous quality m