TDMA

The idea of TDMA is to take wider band channels, for example, 25 kHz, 30 kHz, or
200 kHz RF channels and time-multiplex a number of users simultaneously onto the
channel. Time slots are organized within a frame structure (frames, multiframes,
superframes, hyperframes) to allow multiple users to be multiplexed together in an
organized way. The objective is to improve channel utilization but at the same time
relax the RF performance requirements (filtering and frequency stability) and reduce
RF component costs in the handset and base station.
An example of TDMA used in two-way radio is the European Trans European
Trunked Radio Access (TETRA) standard. A25 kHz channel is split into four time slots
each of 14.17 ms, so that up to 4 users can be modulated simultaneously onto the same
25 kHz RF carrier.
TETRA is presently implementing a fairly simple bandwidth-on-demand protocol
where a single user can be given one, two, three, or four time slots within a frame. This
means that one relatively high rate user per RF channel or four relatively low rate users
or any combination in between can be supported. Asimilar format is used by Motorola
in their proprietary iDEN air interface (six slots in a 990 ms frame length).

In the United States, the AMPS 30 kHz analog channels were subdivided during the
1990s using either TDMA or CDMA. The time-division multiplex uses a three-slot
structure (three users per 30 kHz RF channel), which can optionally be implemented as
a six-slot structure.
A similar time-division multiplex was implemented in the Japanese Personal Digital
Cellular networks but using a 25 kHz rather than 30 kHz RF channel spacing. In
Europe, an eight-slot time multiplex was implemented for GSM using a 200 kHz RF
channel, as shown in Figure 1.4.
One specific objective of the air interface was to reduce RF component cost by relaxing
the RF channel spacing, from 25 kHz to 200 kHz. In common with all other TDMA
interfaces, additional duplex separation is achieved by introducing a time offset. In
GSM, transmit and receive are both on the same time slot�"for example, time slot 2 but
with a three-slot frame offset. This helps to keep transmit power (+30 dBm) out of the
receiver front end (having to detect signals at �"102 dBm or below). The combination of
RF and time-division duplexing helps to deliver good sensitivity and provides the
option to reduce RF component costs by dispensing with the duplex filter in some
GSM phone designs.
Another route to reducing component costs is to use the air interface to provide synchronization
and frequency correction as part of the handset registration procedure�"
an S burst to synchronize, an F burst to provide a frequency fix.
A long, simple burst on the forward control channel aligns the handset, in time, to
the downlink time slots. In the frequency domain, the modulation is given a unidirectional
π/2 phase shift for similar successive bits, giving a demodulated output of a sine
wave at 1625/24 kHz higher than the center carrier frequency. This means that the
F burst aligns the handset, in frequency, to the downlink RF carrier.