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Cellular Networking Perspectives

TDMA Digital Cellular and PCS


TDMA digital systems get their name, Time Division Multiple Access, by dividing a single channel into a number of timeslots. Each user gets to use only one out of every few slots. First, a small amount of the voice conversation of one user is transmitted, and then the second, and so on until all users on one channel have transmitted when the cycle repeats. Obviously, it is necessary to compress voice conversations with TDMA systems. Consider a TDMA system with 5 users in each frequency band, alternating every 10 milliseconds. Each user only gets 2 milliseconds (less, considering overhead) for each 10 milliseconds of conversation.

The first implementation of AMPS digital cellular used TDMA, in the TIA IS-54 standard (often known as D-AMPS). It provided 3 TDMA voice channels in the space of one 30 khz analog channel. IS-136 was the next generation which extended the use of TDMA to the control channel. This technology is now recognized by ANSI, and published as the TIA/EIA-136 series of standards.

Some of the challenges related to TDMA systems are voice quality (which has improved over time), interference with other electronic devices, such as hearing aids (due to regular fluctuations in signal strength) and the area covered (it becomes harder to keep three mobiles synchronized as their distance from the cellsite increases).

TDMA, as defined in TIA IS-54, IS-136, and TIA/EIA-136 theoretically triples the capacity of cellular frequencies (the actual gain is between 2 and 3 times over analog), by dividing a 30 khz cellular channel into 3–8 kbps timeslots, which supports 3 users in strict alternation. There were plans to use half-rate voice coders, which would allow 6 users in one 30 khz channel. Hughes Network Systems at one time promoted the concept of E-TDMA, which uses dynamic timeslot allocation to avoid the waste of timeslots when one side of the conversation is silent. Among polite company (i.e. people that don't talk over each other), this technique can almost double the spectral efficiency of TDMA once again, bringing it to about 10:1 over analog. The lack of implementation of such high capacity TDMA systems is probably due to the low bit rates available for voice coding (4kbps), resulting in unacceptable voice quality.

Another well known TDMA system is GSM, most intensely implemented in Europe, but with a growing presence in US and Canadian PCS systems, in the 1800 MHz band. GSM is digital, but barely more efficient in spectrum utilization than AMPS analog cellular (25 khz per user for GSM versus 30 khz per user for AMPS). GSM uses 8 timeslots in a 200 kHz frequency band, as compared to 3 timeslots in 30 kHz used by US TDMA.

The IS-136 TDMA digital cellular and PCS standard was described in the August and September, 1995 issues of Cellular Networking Perspectives. An update on the network impact of new Digital Control Channel (DCCH) features was published in the October and November, 1997 issues. Updates on the status of TDMA standards are published regularly (check the most recent March or September issue).

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© Copyright Thu, Jan 8, 2004: Cellular Networking Perspectives Ltd.