TDMA Transmission Overview
TDMA, or Time-Division-Multiple-Access is a very effective way of allowing a lot of radios to share one radio channel. Used extensively in GSM cellular and APCO public-safety systems, TDMA excels at allowing quick and reliable access to radio channels. Raveon’s M7 series of GPS tracking radios and all narrow-band GPS tracking radios use TDMA to send GPS position information, status, and data. It allows 2-10 times more radios to share a radio channel than conventional carrier-sense methods. This allows 2-10 times more tracking radios on one channel, as compared to radios that do not have TDMA capability.
The following diagram illustrates how it works.
When a GPS tracking Data Radio modem wants to report its position and status, it waits until its assigned time-slot, and then transmits its data. By default, TDMA time slot positions are assigned by unit-ID, so RV-M7 GX with ID 1 uses the first slot, and ID 2 uses the second slot, and so on.
A TDMA “Frame” time is the time it takes all units to transmit once. This is configured with the TDMATIME xx command. The factory default is 10 seconds, so every 10 seconds, each RV-M7 GX may transmit. The TDMA frame must be set long enough for all units to transmit. For example, if you have 50 RV-M7s, and use 200mS TDMA slots, then the TDMATIME should be set to 10 seconds. The simplest way to set it the TDMATIME is to make it equal to the TXRATE, the rate you wish to report position
The duration of a TDMA time slot is programmed into the RV-M7 GX with the SLOTTIME command. If SLOTTIME is set to 200 milliseconds (factory default), then every 10 seconds, the RV-M7 will have a 200mS window to report its position in.
All TDMA frames are synchronized automatically in all RV-M7 GX Transponders to the top of the minute. Slot 0, frame 0 is at the top of each minute. They use the internal GPS receiver to determine the current time, and calculate when their are supposed to transmit their position and status information.
A unit may be allocated additional time slots. The SLOTQTY command sets the number of slots each unit receives. It is normally set to 1.
