Land Mobile Radio (LMR)

P25 Phase 2 Overview

Technology

• Frequency Division Multiple Access (FDMA) is a method of dividing the spectrum into frequency divisions. TDMA divides the spectrum by time divisions.

  • P25 Phase 1 uses FDMA.
  • P25 Phase 2 uses TDMA.
• Phase 1 FDMA 9600bps control channel retained, with TDMA extensions
• Voice channel: 12kb/ps air link rate

• TDMA voice channel modulation:

  • Outbound: H-DQPSK (Harmonized Differential Quadrature Phase Shift Keying) used in the base station fixed-site equipment and requires linear transmit amplifiers.
  • Inbound: H-CPM (Harmonized Continuous Phase Modulation) used in SUs “to enable use of the same non-linear amplifiers currently employed in P25 Phase 1 FDMA units.”^[1]
    A result of this is that it requires a more complex receiver in the base station, where the complexity is concentrated, rather than the SUs.
• Half rate vocoder: 3600bps with error correction

Interoperability – Trunking

Phase 2 radios and infrastructure must support Phase 1. Current standardization work provides for:

• 9600bps control channel, supporting both FDMA-only (Phase 1) and FDMA+TDMA capable radios,
• Any combination of TDMA-capable and FDMA-only traffic channels at each site in the RF Sub-System (RFSS),
• Traffic involving Phase 1 radios will be handled in Phase 1 FDMA mode,
• Talk-around remains 12.5 kHz Phase 1 FDMA.

Standards Status (as of August 3, 2011)

• 'Phase 2' is not a single document or standard, but a suite of standards.
• Documents that allow vendors to test their designs (conformance, interoperability, and recommended compliance assessment tests) have been published.
• Modifications to several supporting documents (such as the trunking messages) have been published.
• A range of other standards documents have been impacted by Phase 2 (Vocoder, Encryption); ISSI remains to be completed.
• The first drop of Phase 2 standardization, trunked 2-TDMA is reaching completion.
• The protocol definition documents have been published, as have the performance test documents.
• Four test related document remain, they are expected to publish this in 2011 or early 2012.
• The ISSI changes for phase 2 are also outstanding. They should be published in the first half of 2012.

Audio Quality

• Vocoders convert audio streams into compressed digital streams ready for transmission.
• Phase 1 operation uses "full rate" vocoders while Phase 2 operation uses "half rate" vocoders. There are multiple generations of these two vocoder classifications.
• National Institute of Standards and Technology (NIST) has found that enhanced vocoders (both full and half rate) are significantly better than the base line vocoder, with no significant statistical difference between the quality of the full and half rate vocoders.
• Phase 2 radios have a dual rate vocoder installed and they use the correct transmission mode automatically, as standardized in the TIA2-102 suite.
• In addition to the vocoder, the received audio quality depends upon many elements including, for example: the radio mechanical package, the speaker diameter and power rating, the frequency response of all of the audio circuits, signal error correction and network coverage, the electrical and acoustic properties of a radio, the correct set up of audio accessories (correct level/gain settings etc.), coverage interaction.

Migration from P25 Phase 1 to P25 Phase 2

At system level:

• Start adding FDMA and TDMA capable radios to the fleet, initially operating in FDMA
• Add TDMA-capable traffic channels
• TDMA talk groups will be assigned to TDMA capable channels where resources allow and when no FDMA radios involved in the call
• If no TDMA-capable channel available, will be assigned to an FDMA channel
• If FDMA radios involved in the call, will be assigned to an FDMA channel (or a TDMA-capable channel running in FDMA mode)