How Project 25 two-slot TDMA works

Jan 1, 2008 12:00 PM, By W. Roy McClellan III, P.E.

Last month's article, “Why TDMA makes sense for P25,” addressed the decisions by the Project 25 Steering Committee that led to the acceptance and adoption of a proposed technical path to an industry harmonized Project 25 Phase 2 two-slot TDMA solution. Highlighted were the benefits to operators of public safety systems, which include capacity improvement, reuse of existing 12.5 kHz frequency plans, the ability to meet future FCC narrowbanding requirements, and support for the migration from Phase 1 to Phase 2.

This month's article provides a technical overview of the proposed solution currently being debated in the APCO Project 25 Interface Committee (APIC), which develops standards proposals for consideration by the Telecommunications Industry Association (TIA) TR8 Land Mobile Radio (LMR) Committee.

The P25 Phase 2 TDMA air interface uses two different schemes to modulate the 12 kb/s data stream for over-the-air transmission in a 12.5 kHz channel. The first scheme, called harmonized continuous phase modulation (H-CPM), is used in the subscriber units (mobiles and handhelds) to enable use of the same non-linear amplifiers currently employed in P25 Phase 1 FDMA units. This decision by the TDMA task group (TG) lets existing manufacturers of mobiles and handhelds reuse existing RF front-end technology and thus leverage existing investments.

The second scheme, called harmonized differential quadrature phase shift keyed modulation (H-DQPSK), is used in the base station fixed-site equipment and requires linear transmit amplifiers. As many LMR manufacturers already use linear amplifiers in their base station equipment for improved simulcast solutions, the TDMA TG was able to select this scheme with minimal impact to existing fixed infrastructure manufacturers. The term “harmonized” is applied to both schemes, as the final parameters employed by H-CPM and H-DQPSK are the consensus result of technical discussions by all active participants of the TDMA TG.

The following summarizes the proposed TDMA air interface key parameters:

12 kb/s air link rate for two-slot TDMA.
Modulation:
- H-DQPSK for downlink (outbound);
- H-CPM for uplink (inbound).
Dual-rate vocoder.
30 msec slot size.
Concentrated signaling:
- Dedicated signaling burst per superframe;
- Inverted position to allow subscriber units to switch and listen when transmitting (bidirectional).
FDMA control channel (Phase 1 CCH base) with TDMA extensions for migration and compatibility with Phase 1.
TDMA voice channels that are not synchronized to the FDMA control channel.

Let's now examine each of these components individually.

12 kb/s air link rate for two-slot TDMA

Project 25 Phase 2 TDMA uses a 12 kb/s rate on the air interface, where Phase 1 uses 9.6 kb/s. The increase in the bit rate is needed to accommodate the signaling and slot assignment required for two voice paths and the enhanced half-rate IMBE vocoder.

The 9.6 kb/s bit rate for Phase 1 is the sum of 7.2 kb/s for the full rate IMBE vocoder (with error correction) plus 2.4 kb/s for the link management and in-channel signaling.

The 12 kb/s bit rate for Phase 2 is the sum of two 3.6 kb/s streams for the two enhanced half-rate IMBE vocoders (2 × 3.6 = 7.2) plus the 4.8 kb/s associated link management and in-channel signaling to support two voice paths in the channel.