Key Differences between FDMA vs. TDMA Technology

FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) technologies are used in P25 and in business and industrial digital radios (P25 Phase I & NXDN™ for FDMA; P25 Phase II & DMR for TDMA). Understanding the differences between these two technologies is crucial for selecting the right communication solution for your needs.

Channel Definition Differences

The basic difference between FDMA and TDMA lies in how they define and utilize channels for communication.

FDMA Technology

In FDMA, a particular bandwidth (e.g. 6.25 kHz) at a particular frequency (e.g. 150.000 MHz) is used to define a channel. This is the way channels have been allocated in analog land mobile radios (LMR) for decades. All information is contained in the channel – compressed to the smallest frequency footprint.

FDMA divides the frequency spectrum into discrete channels, each carrying a separate communication.

Analog radio bandwidth has recently shrunk from 25 kHz to 12.5 kHz, which is about the limit for analog technology without seriously degrading radio voice quality. With digital technology, channel bandwidth can be compressed to a spectrum-efficient 6.25 kHz by using vocoders and error correction.

TDMA Technology

In regard to TDMA and digital technology, the 12.5 kHz channel bandwidth is maintained. The RF spectrum efficiency is achieved when two voice channels share time to create a 6.25 kHz equivalency. TDMA technology is possible only by using intelligent infrastructure to make and control the time slots.

TDMA divides time into slots on a single frequency channel, allowing multiple conversations on the same frequency.

Benefits of FDMA Technology

FDMA offers several advantages over TDMA in certain applications and environments:

Simplex Operation

FDMA allows subscriber radios to keep the efficiency of true 6.25 kHz at all times, as it is native to the basic operation of the radio. FDMA technology allows radios to continue operating with all channels available.

Spectrum Efficiency

An FDMA system is a "true" 6.25 kHz channel system; TDMA is simply a 6.25 kHz channel equivalent. If 12.5 kHz is the standard for narrowband channel spacing, then FDMA achieves maximum spectral efficiency – regardless of application and with or without a repeater.

NXDN (or Icom's IDAS™) is an FDMA technology and the only protocol to be licensed on the newly created 6.25 kHz channels assigned by the Federal Communications Commission (FCC). TDMA radios cannot be licensed for the FCC's 6.25 channels, thus limiting the areas where this technology can be deployed.

Audio Quality

With the use of the AMBE+2 vocoder in FDMA, IDAS achieves exceptional audio quality while always operating in a true 6.25 kHz channel. Although it takes some time to get used to the digital voice, the benefits are worth it: the vocoder eliminates the background noise and static associated with analog radio signals.

Coverage

When transmitted at the same output power in identical conditions, an FDMA system's 6.25 kHz narrower channel width should allow the signal to achieve better coverage than a 12.5 kHz TDMA system. Noise floor of any receiver is proportional to the filter bandwidth; therefore, the smaller the bandwidth, the smaller the signals that can be received.

Conclusion

Both FDMA and TDMA technologies have their place in modern digital radio communications. FDMA offers true 6.25 kHz efficiency in all operating modes, excellent audio quality, and potentially better coverage. TDMA provides spectrum efficiency through time division multiplexing but requires infrastructure support to achieve its full potential.

The choice between FDMA and TDMA depends on your specific communication needs, infrastructure availability, and spectrum licensing situation. In urban areas where RF spectrum licenses are difficult to locate, FDMA technology may be advantageous since 6.25 kHz channel licenses are easier to find than 12.5 kHz channels.