Dec 1, 2007 12:00 PM, By W. Roy McClellan III, P.E.

The Project 25 standards development process — driven by a partnership among Project 25; the Telecommunications Industry Association, or TIA; and the APCO P25 Interface Committee, or APIC — has moved toward creating in the second phase of the standard a two-slot time division multiple access, or TDMA, common air interface standard for 12.5 kHz radio channels.

Previous efforts to create a TDMA air interface for P25 revealed many issues that led to a decision by participants in the process to: renew their efforts; use the positive elements of previous TDMA proposals; and propose a harmonized, industry-consensus two-slot TDMA standard.

The Project 25, TIA and APIC participants also recognized the need for a four-slot TDMA standard for 25 kHz radio channels. Although this article addresses only the current two-slot TDMA efforts, appropriate technical considerations are being incorporated to extend the proposed two-slot TDMA protocol to a four-slot TDMA protocol.

The Project 25 Steering Committee (SC) made two decisions on April 27 that determined the standards development path for P25 Phase 2 TDMA solutions. One concerns the selection of a 12 kb/s air link rate for two-slot TDMA to meet future FCC-mandated spectral efficiencies calling for one voice path in 6.25 kHz of spectrum. (P25 Phase 1 meets the FCC's current requirement of one voice path in 12.5 kHz of spectrum using a 9.6 kb/s air link rate.)

The second decision concerned the selection of the DVSI Enhanced Half Rate IMBE vocoder to provide improved voice quality and the lower vocoder bit rate necessary to enable two voice paths in 12.5 kHz of spectrum using a 12 kb/s air link rate. The half-rate vocoder, when implemented in the 12 kb/s air link data stream, will provide two voice paths in 12.5 kHz of spectrum plus allow for sufficient control and overhead bits to match current P25 Phase 1 control and capacity requirements.

The combination of the 12 kb/s air link rate with the half-rate vocoder provides a two-slot TDMA solution with the enhanced signaling capability needed to improve emergency management of P25 Phase 2 subscriber units (handhelds and mobiles). The 12 kb/s air link rate structure facilitates a transmitter modulation scheme with spectral characteristics similar to Phase 1 at 9.6 kb/s, but it contains overhead (control) capability similar to Phase 1 for management of each voice path (slot) in Phase 2.

The current Project 25 common air interface is a frequency division multiple access (FDMA) scheme. FDMA trunking schemes allow access to unused channels when they are not occupied. When an RF channel is assigned after a push-to-talk (P2T) request, the radio powers up and modulates an RF carrier using the P25 Phase 1 protocol, which contains the digitized voice and embedded control for that one talk spurt. The carrier occupies the entire 12.5 kHz channel for the one call — whether it is a talk group or an individual unit-to-unit call — until the transmission stops. Figure 1 illustrates how three radio channels of 12.5 kHz each are required to carry three simultaneous calls.

The proposed two-slot TDMA common air interface will offer two interlaced slots to accommodate two calls in the same 12.5 kHz radio channel bandwidth, as shown in Figure 1. The placement of two calls into a single radio channel doubles the efficiency of the channel. Thus, a 12.5 kHz radio channel will carry two calls to meet future FCC channel efficiencies of 6.25 kHz for one call.

TDMA accomplishes this by taking the digitized information (voice and control) and parsing the bits into fixed-size packets for each call. A second call is processed in the same way, and then the packets are alternated. The speed at which this occurs is transparent to the caller and listener. Figure 2 illustrates how the packets for each caller are alternated and placed into a single stream.

Want to use this article? Click here for options!
© 2009 Penton Media, Inc.