Time Division Multiple Access or TDMA was initially defined by the IS-54 standard and is now specified in the IS-13x series2.50 of specifications of the EIA/TIA. Because of its heritage as the original North American digital standard it is sometimes called digital AMPS or D-AMPS.2.51
TDMA services were initially deployed during 1992 by McCaw, Southwest Bell, Bell South and others. Although initial customer adoption was slow, there were an estimated half million TDMA subscribers by early 1995. This number is expected to grow dramatically in coming years, especially with new generation vocoders (which improve the perceived voice quality). Because TDMA physical channels are the same as the physical channels of AMPS, TDMA can be easily migrated into and coexist with AMPS systems in a dual mode manner.
TDMA subdivides each of the 30 kHz AMPS channels into 3 full-rate TDMA channels, each of which is capable of supporting a single voice call.2.52 In the future, each of these full-rate channels will be further subdividable into two half-rate channels, each of which-with the necessary coding and compression-could also support a voice call. Thus, TDMA could provide 3 to 6 times the capacity of AMPS traffic channels, with a corresponding gain in trunking efficiency. A similar calculation to that of previous sections yields an estimate of 3.5 to 6.3 times the capacity of an AMPS system [FALC95].
Like AMPS, some of the digital channels are designated as control channels, called digital control channels or DCCH. These control channels serve the same purpose as in AMPS-paging and call control. Three forward-direction call setup control channels are used. The "A-stream" is used to page mobiles with even-numbered MINs. The "B-stream" is used to page mobiles with odd-numbered MINs. The "B/I-stream" indicates the busy/idle status of the reverse control channel, control of which is contested by mobiles wishing to originate calls.
Because of its time-division nature, by offsetting corresponding forward- and reverse-direction time slots, TDMA allows half-duplex phones to be used. This has the benefit of reducing cost and power consumption (i.e., battery size) of the mobile station, but with an increase in complexity due to the variable power envelope. It also allows the monitoring of control channels for out-of-band signalling during a call. Finally, the half-duplex operation allows mobiles to monitor the quality of channels used in neighboring cells in order to assist handoffs.
Originally, TDMA used parametric coding voice digitization, which is based on mathematical models of human vocal sounds. This prohibited the use of analog facsimile and modems due to the resultant distortion of modem signals (which are unlike human voice). Due to complaints of voice quality, the vocoders specified for TDMA have been upgraded with the 1995 standards revision.
TDMA traffic channels use p/4-DQPSK modulation at a 24.3-kbaud channel rate. This results in an effective 48.6 kbps data rate across the six time slots comprising one frame in the 30-kHz physical channel. TDMA standards specify RS-232 and AT-command set-capable mobile units which can use the system at a full-rate data speed of 9.6 kbps, which can be effectively doubled with V.42bis data compression. A triple-rate data speed of 28.8 uncompressed (57.6 kbps compressed) is also specified. Gateways for facsimile and landline modems can be installed at MSCs by TDMA service providers.
A capability called short messaging service (SMS) has been specified in IS-136 to use the DCCH for short messages. This two-way service can deliver messages of up to 256 characters to the display on a subscriber's phone. Similar services are also specified for CDMA and N-AMPS2.53 systems.
A very recent packet data initiative has been underway under the auspices of the TDMA Forum, the trade association for TDMA technology participants. The approach favored by the committee working on packet data services uses a dynamic time slot assignment with reservation algorithm which melds directly into the existing TDMA standard to provide CDPD-type services over TDMA channels.
In this proposed standard, all of the usual capabilities are supported in addition to variable bandwidth, which is potentially very large if enough TDMA channels are momentarily available for this purpose. Also specified is an efficient MAC layer ARQ mechanism plus the capability for a mobile unit to monitor both voice and data services simultaneously [CHAN96].