It is done by a receiver monitoring its received power and sending a control signal to the transmitter to control its power transmission as required. Sometimes a separate pilot signal is used for this purpose.
Power control reduces the near–far problem in CDMA systems and helps to minimize the interference near the cell boundaries when used in forward-link [32, 33].
1.3 First-Generation Systems
These systems use analog frequency modulation for speech transmission and frequency shift keying (FSK) for signaling, and employ FDMA to share the allocated spectrum. Some of the popular standards developed around the world include Advanced Mobile Phone Service (AMPS), Total Access Communication System (TACS), Nordic Mobile Telephone (NMT), Nippon Telephone and Telegraph (NTT) and C450. These systems use two separate frequency channels, one for base to mobile and the other for mobile to base for full duplex transmission [1].
1.3.1 Characteristics of Advanced Mobile Phone Service
AMPS system uses bands of 824 to 849 MHz for uplink and 869 to 894 MHz for downlink transmission. This spectrum is divided into channels of 30-kHz bandwidth. In a two-way connection two of these channels are used. A pair of channels in a connection is selected such that channels used for uplink and downlink transmission are separated by 45 MHz. This separation was chosen so that inexpensive but highly selective duplexers could be utilized. A typical frequency-reuse plan in this system either uses clusters of 12 cells with omnidirectional antennas or 7-cell clusters with three sectors per cell.
There are a total of 832 duplex channels. Of these, 42 are used as control channels and the remaining 790 channels are used as voice channels. The control channels used for downlink and uplink transmission are referred to as forward control channels (FCC) and reverse control channels (RCC), respectively. Similarly, voice channels are referred to as forward voice channels (FVC) and reverse voice channels (RVC).
Each base continuously broadcasts FSK data on FCC and receives on RCC. A mobile scans all FCCs and locks on an FCC with the strongest signal. Each mobile needs to be locked on an FCC signal to receive and send a call. Base stations monitor their RCCs for transmission from mobiles that are locked on the matching FCCs.
1.3.2 Call Processing
When a mobile places a call, it transmits a message on RCC consisting of destination phone number, its mobile identification number, and other authorization information. The base station monitoring an RCC receives this information and sends it to the MSC. The MSC in turn checks the authentication of the mobile; assigns a pair of FVC and RVC, a supervisory audio tone (SAT), and a voice mobile attenuation code (VMAC); and connects the call to a public switched telephone network (PSTN). The mobile switches itself to the assigned channels. The SAT is used to ensure the reliable voice communication and the VMAC is used for power control.
The SAT is an analog tone of 5970, 6000, or 6030 Hz. It is transmitted during a call on both FVC and RVC. It is superimposed on voice signal and is barely audible to the user. It helps the mobile and the base to distinguish each other from co-channel users located in nearby cells. It also serves as a handshake between the base station and the mobile. The base transmits it on FVC and the mobile retransmits it on RVC after detection. If the SAT is not detected within 1 s, both the mobile and the base stop transmission.
A call to a mobile originating at a PSTN is processed by the MSC in a similar fashion. When a call arrives at an MSC, a paging message with mobile identification number (MIN) is sent out on FCCs of every base station controlled by the MSC. A mobile terminal recognizes its MIN and responds on RCC. The MSC assigns a pair of FVC and RVC, SAT and VMAC. The mobile switches itself to the assigned channel.
© 2002 by CRC Press LLC