In present – day mobile cellular systems, transmission of a digital voice in a multipart fading environment is a challenging job. The major considerations in implementing   digital voice in cellular mobile systems are discussed below, along with a tentatively recommended transmission rate for the cellular mobile system.

The criterion for judging a good digital voice through a wire line is employed in three existing digital voice schemes.

In a continuously variable step delta (CVSD) modulation scheme, the present transmission rate is 16 kHz. This is not toll – quality voice transmission and is commonly used by the military. In a LPC scheme, the present transmission rate of 2.4 kHz provides a synthetic quality voice, but a rate of 4.8 kHz using vector quantization may provide a communication quality voice a rate of 16 kbps can provide a toll quality voice.

In a pulse code modulation (PCM) scheme, the present transmission rates of 32 kbps and 64 kbps are commonly used; 32 kbps is used by the military while 64 kbps is used commercially of the three schemes, LPC seems most attractive because of its low transmission    rate. However, LPC is more vulnerable in terms of distortion to the mobile fading environment. Digital voice has to be processed in real time, which imposes constrains on the digital processing time. This adversely affects LPC but no CVSD.  When sending a digital stream (voice) through a radio channel in a fading environment, in general an LPC scheme needs more code   protection n than CVSD scheme does because LPC is not implemented in continuous waveform in either the frequency domain or the time domain which CVSD is implemented in a continuous waveform in the time domain.  Because the mobile unit is moving, sometimes rapidly, sometimes slowly, insertion of extra synchronization bits is needed in the normal digital stream.

The major difference between AMPS and the digital advanced mobile phone systems (DAMPS) is the AMPS used analog radio signals and DAMPS uses digital radio signals. DAMPS is a technique  that  places multiple calls over one radio frequency using pulse code  modulation (PCM)     and time division multiplexing (TDM). The PCM signal is not the standard PCM signal found in the PSTN, which converts an analog signal into 64,000 bps. DAMPS technology converts the analog signal into 16,000 bps. DAMPS were introduced in 1992 for the existing AMPS, and DAMPS allows cellular operators to carry four times as many calls as a regular AMPS system. DAMPS are backward compatible with analog AMPS. The system will handle either a regular analog call per channel or multiple calls per channel.   Manufactures of mobile   telephone sets provide a switch on the set that allows the user to select between AMPS and DAMPS. Cellular service providers could gradually change their   systems from AMPS to DAMPS. This technology was used in the 825 – to 895 MHz frequency band for AMPS. Cellular service providers also use, DAMPS in the higher – frequency band of 1850 to 1990 MHz to provide PCS.

THE BEGINININGS OF MOBILE TELEPHONE SYSTEMS

Mobile telephone systems got under way in 1946 and were called MTSs. These systems used the radio frequencies between 35 and 45 MHz. although MTS usually stands for “Mobile Telephone system,” this acronym also    meant “manual telephone systems” all calls had to be handled by an operator. These early systems used one frequency for the mobile phone and the base station     the handset had a push button.   The mobile party used a push to talk protocol. When the button was depressed, the transmitter of the mobile unit was activated. This would   cause a signal to be transmitted to the base station. On receiving the signal, the base station    would light a light above a jack at the operator’s position.  The operator would answer the call by plugging   a patch cord from her position into     the jack connected to the base station transmitter. She would ask the calling mobile party for information on whom they wished to call. The operator would use the other end of the patch cord to connect the mobile telephone caller to the desired PSTN destination.

The PSTN telephone was connected to the base station’s transmitter through the patch cord at the operator’s position. A conversation could now take place, with the caller and called party taking turns. The MTS systems usually had only one   radio frequency for both the base and mobile phones. Therefore, only one   phone call at a time could be placed over the MTS. Because all mobile phones used the same frequency, the MTS was like a party line. All MTS mobile phones could hear caller would reach the operator by pressing the push – to – talk button the person would then ask the operator to ring the other mobile phone. Once the operator rang the mobile phone, she was no longer   needed. The base station was not needed for a mobile- to – mobile call, because the transmitters and receivers of both phones were tuned to the same frequency.

The early MTS phones were connected to a transmitter/ receiver located in the trunk of the car. This transceiver was a tube – type device and placed a heavy drain on a car’s battery. The transceiver was about 24 in. High the receiver of the MTS mobile unit contained a mechanical decoder device. The tube- type MTS mobile units were replacing d by transistor- type mobile units around 1970. Solid state logic circuits in these units served as the decoder circuit. When the operator placed a call to a mobile phone, the dial pulses from her dial were converted into pulses of a signaling tone. Let’s assume that a 40 MHz signal a being used by a MTS. If a 5 was dialed, the base station would modulate the 40 MHz Carrier signal 5 times with a tone of 2400 Hz.