Narrowbanding: FDMA vs. TDMA

Agencies utilizing VHF and UHF bands are met with the Federal Communications Commission (FCC) narrowbanding mandated deadline of January 1, 2013. To avoid immediate cancellation of licenses and possible fines, frequencies and equipment used must be spectrum-efficient, at one user per 12.5 kHz for voice channels or capacity of atleast 9.6 kbps per 12.5 kHz for data channels. The Commission's second-phase plan (with no formal deadline) is to further minimize VHF and UHF land mobile radio channels to 6.25 kHz or equivalent.  Many licensees are choosing solutions compliant with the 6.25 kHz plan, to avoid more upgrade expenses down the road.

Narrowbanding requires agencies to make changes and technology decisions, regarding the use of analog or digital technology, and the multiple access method. Three multiple access techniques are used in land mobile and cellular radio: frequency division multiple access (FDMA) utilizing multiple digital frequencies, time division multiple access (TDMA) which divides signals into seperate time slots, and code division multiple access (CDMA) which uses differing codes to seperate transmissions, but is irrelevant due to FCC's lack of provisioning enough spectrum for this method.

The analog vs. digital debate has been significantly swayed by the inadequacy of analog FM radios on 6.25 kHz channels, and its limitation to only FDMA systems. In order to make a comparison, the Urgent Communication's article on the matter assumes a digital system.

Project 25 Phase II, TETRA, and proprietary systems utilize TDMA method, which actually uses a combination of FDMA and TDMA. Channels are allocated, and then multiple users share the each channel at seperate time slots. FDMA systems for 6.25 kHz channels utilize NXDN and digital private mobile radio (dPMR, standardized by ETSI).

There are multiple issues to examine when agencies choose between these two system types:

• Receiver sensitivity demands a wider intermediate frequency on TDMA systems, meaning FDMA receivers generally have better sensitivity and coverage.
• Adjacent channel interference is more of a problem with narrowband receivers, which makes good frequency planning imperative.
• Battery life may be better with TDMA radios, because time slots allow the radio to be used only 50% of the time. However, for acceptable performance, both TDMA and FDMA receivers require the same amount of power per bit.
• Spectrum planning and licensing is smoothest on TDMA systems. FDMA systems must be licensed for each channel, while TDMA users simply leverage already-licensed channels using time slots. Also, adjacent-channel short spacing issues are more prevalent with FDMA systems.
• Transmitter combiners must be purchased to allow multiple radios to share an antenna. One transmitter port is required for each user on FDMA systems, while TDMA systems require one for each radio channel.
• Peer-to-peer mode is traditionally used by land mobile radio users, but TDMA systems often work through radio repeaters. A two-slot TDMA peer to peer mode is under consideration by the P25 standards committee, which will allow talk-around systems in the near future.

FDMA vendors have created organizations to support their interests, found at nxdn-forum.com and dpmr-mou.org. TDMA advocacy organizations can be found at dmrassociation.org.

If your agency is struggling to meet your narrowbanding needs on time, please contact EMR Consultants to see how we can make a difference.

Find the original article on this issue at Urgent Communications.