Write short notes on, (a) TDMA structure (b) Frame length (c) Frame offset (d) Modulation timing

7. Write short notes on,
(a) TDMA structure
(b) Frame length
(c) Frame offset
(d) Modulation timing.

(a) TDMA Structure (Digital Channels):

In NA-TDMA, the set-up channels are analog channels shared with the AMPS system. One digital channel (a 30-kHz TDMA channel) contains 25 frames per second. Each frame is 40-ms long and has 6 time slots. Each time slot is 6.66-ms long. One frame contains 1944 bits (972 symbols), as shown in Fig. 7. Each slot contains 324 bits (162 symbols) and the duration between bits is 20.57 µs. Therefore, one radio channel is transmitted at 48.6 kbps but only 24,000 symbols per second over the radio path. Each frame consists of 6 time slots. The maximum effect on the signal for a forward time slot is one-half full symbol period and for a reverse time slot is 6 symbol periods (Fig. 7b).

TDMA frame and slot (a) TDMA frame structure; (b) overall length in each slot

Frame Length: There are two frame lengths, full rate and half rate. Each full-rate traffic channel shall use two equally spaced time slots of the frame. The overall length in each slot is shown in Fig. 7b.

Channel 1 uses time slots 1 and 4

Channel 2 uses time slots 2 and 5

Channel 3 uses time slots 3 and 6

Each half-rate traffic channel shall use one time slot of the frame:

Channel 1 uses time slot 1

Channel 2 uses time slot 2

Channel 3 uses time slot 3

Channel 4 uses time slot 4

Channel 5 uses time slot 5

Channel 6 uses time slot 6

Frame Offset: At the mobile station, the offset between the reverse and forward frame timing (without time advanced applied), is

Forward frame = reverse frame + (1 time slot + 44 symbols)

                 = reverse frame + 206 symbols

The time slot (TS) 1 of frame N (in forward link) occurs 206 symbol periods after TS 1 of frame N in the reverse link.

Modulation Timing:

Modulation timing within a forward time slot: The first modulated symbol (the first symbol of the sync word) used by the mobile unit shall have maximum effect on the signal (156 symbols) transmitted from the base antenna, one-half symbol (1 bit) period after beginning the time slot.

Modulation timing within a reverse time slot: The first modulated symbol has a maximum effect on the signal transmitted at the mobile unit 6 symbol periods after the beginning of the reverse time slot.