Cellular Voice Networks

These essentially represent wireless networks of voice telephony, analog or digital. The analog system like AMPS uses frequency modulation with a peak deviation of 12 kHz. A duplex phone conversation requires one channel for transmitting and another channel for receiving.

The cellular system includes a MTSO (sometimes situated in a CO of PSTN), the cell sites, and the mobile units. The central processor at the MTSO controls the switching equipment needed to interconnect the mobile users with the wireline telephone network (PSTN). It also controls cell-site actions and many of the mobile unit actions through commands relayed to them by the cell sites.

The MTSO is connected to each cell site via wireline PSTN, over which they exchange information required to process the calls. Each cell site has one transreceiver for each of its assigned voice channels and the corresponding transmitting/receiving antennas for those channels.

The equipment at the mobile station contains a control unit, a transreceiver and an antenna, with a duplexer, to segregate the transmitted and received signals. (In space diversity systems, if used to combat fading and interference problems, there will be more than one antenna in operation.)

The control unit has user interfaces, namely, the handset, the dialler and a light-emitting diode (LED) indicator. The transreceiver includes a synthesizer to tune to all the allocated channels. The logic section interprets the user’s actions and various system commands. Subsequently, it also controls the transreceiver and the control units.

A few radio channels are used for call set-up and tear-down procedures. First, they facilitate the necessary exchange of signalling information for call set-up. Whenever the mobile unit is “on”, but not in use, the unit continuously monitors these set-up channels. And selects the strongest one. Thus, the base station/cell and the current location of this mobile unit (linked via that selected channel) are mutually identified.

That is, the mobile unit and the base station are now synchronised through the set-up channel with an identification number (ID) prescribed for the mobile unit. Now, the base station can transfer any call coming for the mobile unit. When the mobile unit is alerted of an incoming call, it again samples the signal-strength of all received signalling/set-up channels and responds through the cell-site offering the strongest signal.

It then indicates its preference for communication through that cell-site/base station, gets an ID and the base station, in turn, responds by supplying the ringing tone so as to alert the user to pick up the phone. Similar sequence of events takes place at the calling subscriber side except in the reverse order. Due to the mobility involved, the system examines the call being received every few seconds at the cell-site.

If necessary, the system “looks” for another site to serve the call. When such a need occurs, the mobile unit retunes itself to the newly found cell-site by receiving a command from the base station. This process of changing the base station or handing over the responsibility of servicing a mobile unit, as mentioned earlier, is known as “hand-off”. It occurs in a brief interrupted period of about 50 ms.

The power output of the mobile unit is about 0.7 to 3W and is controlled by a power up/down “power control signal” from the base station in seven, 1-dB steps. This power control is necessary to reduce the interference with other phones and minimise the overloading of the base-station receiver.

After their inception, analog cellular networks soon exhausted the FDMA channels (with 30 kHz FM bandwidth for each channel) offered to the growing population of subscribers (within the allocated spectrum). Hence, the second generation of digital wireless telephony came into the picture with a channel allocation based on TDMA and/or CDMA schemes.