From The RadioReference Wiki
The term "Non-standard Pairs" refers to a UHF (450-470 MHz) 12.5 kHz frequency which has been split in half to create two NXDN frequency pairs, one 3.125 kHz below the original channel center, and one 3.125 kHz above the original channel center. The result essentially creates two talkpaths within the 12.5 kHz channel and achieves the same efficiency in FDMA that DMR systems do with TDMA. Unlike TDMA multiplexing, which has been allowed in a conventional, non-exclusive environment, an FB8 station class (trunking exclusivity) is required.
The practice of doing so was approved by the FCC in its January 5, 2012 letter to the Land Mobile Communications Council (LMCC) in DA 12-10.
The term is loosely chosen and somewhat misleading in that it does not mean that inputs and outputs use a spacing other than 5 MHz.
APPLYING THE NON-STANDARD PAIRS CONCEPT IN PRACTICE
Say a licensee is licensed for 453.3750 MHz and has an emission that conforms within the 12.5 kHz channel space. The licensee can use any emission they are authorized for on their license that is less than 12.5 kHz, to include the common 11K2F3E, 8K10F3E, 7K60FXE, etc. If the licensee chooses to implement NXDN (an FDMA solution), the licensee can put one repeater 3-1/8 kHz lower than the center frequency and one repeater 3-1/8 kHz higher than the center frequency. Each of the newly created frequencies are 6.25 kHz away from each other. They won't interfere with each other because of combiner isolation, and the two talkpaths could be part of an overall trunked system.
Splitting 453.375, it now becomes 453.371875 MHz and 453.378125 MHz, each with 4K00F1D, F1E, F2D, and F7W, as specified. Per DA 12-10, the licensee is required to also list the center frequency that these two fractioned non-standard pairs were derived from.
The license, therefore, would have 453.371875 MHz, 453.3750 MHz, and 453.378125 MHz listed, the first and last frequencies being the "non-standard pairs" and the middle indicating the center frequency they were created from. The center frequency may also contain a wide NXDN 8K30F1* or 8K30F7W emission depending on system configuration. The center frequency and "non-standard pairs" may not be used simultaneously.
SPECTRUM MANAGEMENT ISSUES
The creation of non-standard pairs requires consideration for any use against adjacent channel incumbent Limitation 44 (6.25 kHz) channel center licensees, as, at 3.125 kHz removed with a 4 kHz occupied bandwidth, a portion of energy from the created non-standard pair would encroach into the Limitation 44 channelspace. Frequency coordinators should apply Telecommunication Industry Association (TIA) TR-8 TSB-88.1C or more recent methodologies in evaluating service area reliability degradation. Implementation may necessitate some form of isolation between the proposed system and incumbents.
NON SEQUITUR ISSUES
The concept of two talkpaths within a single 12.5 kHz channelspace is common with TDMA modes which allow conventional (non-trunked), and non-exclusive (FB2, not FB8) operation without frequency band restriction. TDMA systems are now commonplace on VHF, UHF, and higher frequency bands. Achieving multiple talkpaths with FDMA is, in the FCC's DA 12-10 correspondence, restricted to the 450-470 MHz band and requires FB8/MO8 exclusivity, while consuming a relatively similar bandwidth and perhaps equal or less channel occupancy as 2-slot TDMA technologies.