Will I need new TDMA Accessories?

By Sander Colenbrander, Senior R&D Technician, Tait Communications.

Six things you should know about radio accessories for your new network.

TDMA Accessories - firefighterWhen you are upgrading your network to P25 Phase 2 or to DMR, the TDMA (Time Division Multiple Access) signals can cause significant interference on your audio accessories. Probably your existing accessories were designed for FDMA (Frequency Division Multiple Access) radios, which did not encounter this issue, so will not operate properly on a new network. Network operators need to understand the issues and budget for accessory upgrades.

  1. What causes the problem? TDMA signals are pulsed on and off so that, during the “off” phase, a second radio can then transmit, effectively creating two channels on the bandwidth formerly used by a single channel. It is this pulsing (at around 16Hz) that interferes with the audio on the radios.
  2. What accessories are affected? This applies to audio accessories with an electret condenser microphone. These ECM modules contain a Field Effect Transistor (FET) for pre-amplification. It is this device that reacts to the TDMA pulse, introducing interference on the audio path. The pulsed signal causes “chuffing” through the audio. You will also need to check any legacy options boards you may be planning to reuse. In susceptible digital circuits, the pulse can cause false logic conditions.
  3. What about batteries? Generally, batteries will not be affected. However, under very rare circumstances (radio is receiving at maximum volume and the transmitting mic is covered) a small amount of interference may be detected. Operators need to make the call on whether or not this is an issue that requires new batteries.
  4. How can I prevent it? Unfortunately, there is no technical solution to prevent the interference. It is not related to individual product design, and is an issue for all vendors.This is simply a matter of physics. The low frequency of the interference makes it extremely difficult to filter from the audio path. Your TDMA network will need TDMA-compliant audio accessories.
  5. Will all vendors resolve it in the same way? The open standards for P25 Phase 2 or DMR do not specify a solution, so individual vendors may approach it differently. The Tait approach is to prevent the RF energy interference from entering the ECM in the first place, providing high levels of TDMA RF immunity. This is done by filtering and/or shielding to keep the energy out.
  6. What do I need to look out for? Before you purchase new accessories, you need to be confident that they will operate with your new radios. Check that rigorous and formalized testing has been carried out between the manufacturers of the radios and the accessories.Compatibility with your legacy fleet is also critical, to be sure the new accessories will work with your legacy fleet as well. This is important during migration, and if you intend to operate a mixed fleet. Check that the accessory connectors are compatible, and that there are no other technical issues that inhibit correct functioning.

Check to see if you need to upgrade your accessories when you upgrade your system:

Your legacy system is… Your new system is… Upgrade or retain? Backward compatible?
Analog/MPT DMR Upgrade Yes
Analog/MPT P25 Phase 1 Retain Check Vendor
Analog/MPT P25 Phase 2 Upgrade Yes

Tait Connection MagazineThis article is taken from Connection Magazine, Edition 3. Connection is a collection of educational and thought-leading articles focusing on critical communications, wireless and radio technology.

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