Why TDMA makes sense for P25

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.

As shown in Figure 2, the number of talkers has doubled from three to six for the same three 12.5 kHz channels. It also is shown for talkers 5 and 6 (T5 and T6) that they only use their assigned slot as necessary to support when that particular user initiates a P2T transmission. Thus, each of the two slots on each 12.5 kHz channel functions independently as an individual talk channel.

TDMA implementations keep track of the location of each slot using timing and control signaling from the radio base station. All registered and authenticated mobiles are synchronized to a control channel and monitor that control channel until a P2T user requests a voice path. The radio base station system, upon receipt of a P2T request, checks to see if there is a free time slot on one of the 12.5 kHz channels in service and assigns the mobile to move to the free time slot on a particular 12.5 kHz channel for voice communications.

Once on the voice channel and associated time slot, the mobile receives periodic signaling and synchronization updates to keep it synchronized to the radio base station. Once the voice communication is completed, the mobile returns to the control channel and waits there until another P2T request initiates a move to another free time slot on an available voice channel.

It is well-known that public safety and other land mobile system operators in the frequency bands below 512 MHz are moving to 12.5 kHz (and 15 kHz) channels from their current 25 kHz (and 30 kHz) channels. The FCC mandates that when a channel user moves to narrowband operations, the same channel center frequency is maintained. This means the operator that decides to move to 12.5 kHz does not receive any benefit from being more spectrally efficient. Only when the owner of the adjacent channel moves to 12.5 kHz (from 25 kHz) is the FCC able to recover the newly available 12.5 kHz of spectrum that now exists between the two original channels.

For example, two users may have narrowbanded their operations to 12.5 kHz using P25 Phase 1 operation, but they still would operate one voice path in 12.5 kHz (as compared with 25 kHz prior to narrowbanding). In the future, as those users require — or desire — increased capacity of their networks, they would have to negotiate for additional channels freed by other non-adjacent operators to obtain this capacity. Additionally, the FCC soon will require manufacturers and operators to move to 6.25 kHz efficiencies of one voice path (or 4800 b/s) in 6.25 kHz of spectrum. This means an operator could face another migration, using the same center frequency, to 6.25 kHz channels and again would have to negotiate for additional channels to meet future capacity needs.

When system operators move to 6.25 kHz narrowband channel configurations (12.5 to 6.25 kHz), they will maintain their original center frequency. One reason why the original center frequency is preserved is because it minimizes frequency coordination issues for the operator, which is not able to use the newly freed spectrum on each side of its original frequency due to channel coordination issues when the adjacent channels are located on the same radio base station and antenna site.

A two-slot TDMA scheme in 12.5 kHz provides major benefits to the land mobile radio system operator by doubling capacity and improving spectral utilization, while meeting FCC requirements for 6.25 kHz (and 4800 b/s) channel efficiency. Converting to two-slot TDMA in 12.5 kHz allows a current operator of P25 Phase 1 in 12.5 kHz channels to preserve its original channel center frequency, maintain adjacent channel coordination performance, and double voice channel capacity in trunking systems by upgrading the base station repeater and subscriber equipment.

Other benefits are realized by maintaining existing antennas, couplers, combiners, filters, and antenna feeders. Further, a P25 Phase 1 system can be migrated to Phase 2, as the Phase 2 solution uses a Phase 1 control channel. Thus Phase 1 FDMA and Phase 2 TDMA users can be supported on the same site.

The proposed two-slot TDMA standard for P25 Phase 2 will use a 12 kb/s air link rate in 12.5 kHz of spectrum to provide two voice paths to meet the FCC requirements for 6.25 kHz equivalent channel efficiency. The proposed standard uses an enhanced half-rate vocoder and a robust signaling scheme equivalent to Phase 1, while improving emergency management over Phase 1 with the ability to signal and shut down a talker. To facilitate interoperability and/or migration from Phase 1, a P25 Phase 1 FDMA control channel with extensions for TDMA slot assignment is specified.

The key parameters of the proposed TDMA air interface are:

• 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.

Because two-slot TDMA improves channel efficiency and meets FCC requirements, migration to two-slot TDMA provides a significant benefit to system operators by minimizing the migration from 12.5 kHz channel efficiency to 6.25 kHz equivalent channel efficiencies. In addition, the proposed two-slot P25 Phase 2 solution meets future FCC channel efficiency requirements. A future article will describe in greater detail the proposed two-slot P25 Phase 2 solution being debated in TIA TR8.

W. Roy McClellan III, P.E., is the director of standards and regulatory for EADS Secure Networks North America. He also is the chairman of APCO’s Project 25 TDMA and ISSI Task Groups.