TDMA Time Slots (Multiplexing Time Division)

Raveon’s M7-GX AVL transceiver uses a  Time Division Multiple Access (TDMA) protocol to transmit position, status, and data over the air.  TDMA protocols greatly increase the available channel bandwidth but they require more system planning than conventional carrier-sense methods. 

With a TDMA system, each radio in the system is assigned a time slot to use for sending its data (data being GPS postion, status, or ASCII data).  Because only one radio transmits in any single timeslot, there is never any interference between radios.  Hundreds of GPS tracked vehicles or assets may be all located in the same place, and still communicate without interference – an impossible task with conventional radio systems.

The M7 radio’s TDMA timing may be configured by the system installer to optimize it for a particular application.  The TDMA slot width and the number of slots maybe configured to achieve the maximum system performance.  Each TDMA slot needs to be long enough so that the complete GPS position/status message can be sent in it. 

  Table 1 below shows the recommended slot times for various over-the-air data rates.

Over-the-air rate

Radio Bandwidth

Number of Modulation Levels

Position /Status Transmission Duration

Recommended TDMA SLOTTIME in mS

4800bps  (ATR2=3)

12.5kHz

2

70mS

100  (200 with repeaters)

8000bps  (ATR2=4)

12.5kHz

4

50mS

50 (150 with repeaters)

9600bps  (ATR2=5)

25kHz

2

45mS

50 (100 with repeaters)

14400bps (ATR2=10)

25kHz

4

30mS

50 (100 with repeaters)

Use the ATR2 x command to set the over the air rate.  Use the SLOTTIME xx to set the TDMA slot time.  The SLOTTIME must be wide enough to accommodate a Position/Status transmission from the GPS.  Refer to the Table 1 above to determine how long a position/status transmission will take.  The factory default for a stock RV-M7-UC-GX radio is 200mS slots, 4800 baud over-the-air.  This is an excellent compromise between speed and communication range.

The TDMA Frame Length

The TDMATIME is the length of one TDMA frame/epoch.  A TDMA frame contains small time slots where each radio is allowed to transmit in.  The slots are sequentially numbered.  For example, in the configuration below, the TDMA frame is configured to have 20 slots.

Figure 1

TDMA Time Slots / Multiplexing Time Division

Radio number 1 transmits in Slot 1. Radio number 2 in slot 2… 

TDMA Frames have a pre-set number of slots, and once the TDMA Frame Time has passed, the frame restarts at slot 0 again.  Slot 0 is reserved for future base-station control signals.  The length of a TDMA Frame is set with the TDMATIME xx command. 

 Figure 2

TDMA Time Slots / Multiplexing Time Division versus CDMA

In the M7 GX radio, the TDMA Slot Time is programmable, in 50mS increments.  The SLOTTIME xx command is used to set the slot width.  Typically it is set at 50, 100, 150, or 200mS.  The factory default is 200mS. 

For example, with a TDMATIME of 4 second, and a SLOTTIME of 200mS, there are enough slots to support 19 radios reporting every 4 seconds.  The Frame will repeat every 4 seconds so every 4 seconds, each of the 19 radios has 200mS of air-time available.   Figure 3 depicts this configuration.

 Figure 3

 tdma4

Configuration 1 below shows the slot timing with a 1200 baud M7 GX radio using 750mS time slots.

 Configuration 1

1200baud with repeaters
TDMA Frame time, 20 seconds
750mS time slots
  Slot Number Time (S) into frame Radio ID
20-second TDMA Frame for all mobile units in the system 0 0 -
1 0.75 1
2 1.5 2
3 2.25 3
4 3 4
5 3.75 5
6 4.5 6
7 5.25 7
8 6 8
9 6.75 9
10 7.5 10
11 8.25 11
12 9 12
13 9.75 13
14 10.5 14
15 11.25 15
16 12 16
17 12.75 17
18 13.5 18
19 14.25 19
20 15 20
21 15.75 21
22 16.5 22
23 17.25 23
24 18 24
25 18.75 25
26 19.5 26
     

Configuration 2 below shows the slot timing with a 4800 baud M7 GX radio using 200mS time slots.

 Configuration 2

4800baud with repeaters
TDMA Frame time, 10 seconds
200mS time slots
  Slot Number Time (S) into frame Radio ID
10-second TDMA Frame for all mobile units in the system 0 0 -
1 0.2 1
2 0.4 2
3 0.6 3
4 0.8 4
5 1 5
6 1.2 6
7 1.4 7
8 1.6 8
9 1.8 9
10 2 10
11 2.2 11
12 2.4 12
13 2.6 13
14 2.8 14
15 3 15
16 3.2 16
17 3.4 17
18 3.6 18
19 3.8 19
20 4 20
21 4.2 21
22 4.4 22
23 4.6 23
24 4.8 24
25 5 25
26 5.2 26
27 5.4 27
28 5.6 28
29 5.8 29
30 6 30
31 6.2 31
32 6.4 32
33 6.6 33
34 6.8 34
35 7 35
36 7.2 36
37 7.4 37
38 7.6 38
39 7.8 39
40 8 40
41 8.2 41
42 8.4 42
43 8.6 43
44 8.8 44
45 9 45
46 9.2 46
47 9.4 47
48 9.6 48
49 9.8 49