https://urgentcomm.com/wp-content/themes/ucm_child/assets/images/logo/footer-logo.png
  • Home
  • News
  • Multimedia
    • Back
    • Multimedia
    • Video
    • Podcasts
    • Galleries
  • Commentary
    • Back
    • Commentary
    • Urgent Matters
    • View From The Top
    • All Things IWCE
    • Legal Matters
  • Resources
    • Back
    • Resources
    • Events
    • Webinars
    • White Papers
    • Reprints & Reuse
  • IWCE
    • Back
    • IWCE
    • Conference
    • Special Events
    • Exhibitor Listings
    • Premier Partners
    • Floor Plan
    • Exhibiting Information
    • Register for IWCE
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Terms of Service
    • Privacy Statement
    • Cookies Policy
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • Mission Critical Technologies
    • Microwave/RF
    • T&D World
    • TU-Auto
  • In the field
    • Back
    • In the field
    • Broadband Push-to-X
    • Internet of Things
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Call Center/Command
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Network Tech
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Operations
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Regulations
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • Organizations
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
Urgent Communications
  • NEWSLETTER
  • Home
  • News
  • Multimedia
    • Back
    • Video
    • Podcasts
    • Galleries
  • Commentary
    • Back
    • All Things IWCE
    • Urgent Matters
    • View From The Top
    • Legal Matters
  • Resources
    • Back
    • Events
    • Webinars
    • White Papers
    • Reprints & Reuse
    • UC eZines
  • IWCE
    • Back
    • Conference
    • WHY ATTEND
    • Exhibitor Listings
    • Floor Plan
    • Exhibiting Information
    • Registration Opens April 2019-Join Our Mailing List
  • About Us
    • Back
    • About Us
    • Contact Us
    • Advertise
    • Terms of Service
    • Privacy Statement
    • Cookies Policy
  • Related Sites
    • Back
    • American City & County
    • IWCE
    • Light Reading
    • IOT World Today
    • TU-Auto
  • newsletter
  • In the field
    • Back
    • Internet of Things
    • Broadband Push-to-X
    • Project 25
    • Public-Safety Broadband/FirstNet
    • Virtual/Augmented Reality
    • Land Mobile Radio
    • Long Term Evolution (LTE)
    • Applications
    • Drones/Robots
    • IoT/Smart X
    • Software
    • Subscriber Devices
    • Video
  • Call Center/Command
    • Back
    • Artificial Intelligence
    • NG911
    • Alerting Systems
    • Analytics
    • Dispatch/Call-taking
    • Incident Command/Situational Awareness
    • Tracking, Monitoring & Control
  • Network Tech
    • Back
    • Cybersecurity
    • Interoperability
    • LMR 100
    • LMR 200
    • Backhaul
    • Deployables
    • Power
    • Tower & Site
    • Wireless Networks
    • Coverage/Interference
    • Security
    • System Design
    • System Installation
    • System Operation
    • Test & Measurement
  • Operations
    • Back
    • Critical Infrastructure
    • Enterprise
    • Federal Government/Military
    • Public Safety
    • State & Local Government
    • Training
  • Regulations
    • Back
    • Narrowbanding
    • T-Band
    • Rebanding
    • TV White Spaces
    • None
    • Funding
    • Policy
    • Regional Coordination
    • Standards
  • Organizations
    • Back
    • AASHTO
    • APCO
    • DHS
    • DMR Association
    • ETA
    • EWA
    • FCC
    • IWCE
    • NASEMSO
    • NATE
    • NXDN Forum
    • NENA
    • NIST/PSCR
    • NPSTC
    • NTIA/FirstNet
    • P25 TIG
    • TETRA + CCA
    • UTC
acc.com

Project 25


How Project 25 two-slot TDMA works

How Project 25 two-slot TDMA works

Benefits include enhanced spectral efficiency and doubling of channel capacity
  • Written by Urgent Communications Administrator
  • 1st January 2008

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.

Modulation

Project 25 Phase 2 TDMA uses two different modulation schemes for the downlink (outbound) and uplink (inbound) radio links.

H-DQPSK is a non-coherent modulation technique that splits the information stream into two channels, delays one channel by 90° in phase (quadrature) and then recombines the two phase shift keyed channels using differential coding (encoding the difference of the current data word applied to the transmitter with its delayed output). Combining two channels in quadrature (again, 90° out of phase with each other) lowers the transmitted baud rate, improving the transmitted spectral characteristics. H-DQPSK modulation requires linear amplifiers in the radio base station.

H-CPM (uplink/inbound) is a common constant-envelope modulation that preserves the ability of manufacturers to use existing P25 Phase 1 RF technology in handsets and mobiles.

Dual-rate vocoder

A dual-rate vocoder is specified in the Phase 2 TDMA solution. The dual-rate vocoder will consist of the existing Phase 1 full-rate IMBE vocoder (7.2 kb/s) and extensions for the enhanced half-rate vocoder (3.6 kb/s). The TDMA solution will use the enhanced half-rate IMBE vocoder for voice operations.

To achieve interoperable communications, the P25 Phase 2 TDMA solution uses a Phase 1 mode (full-rate IMBE) for compatibility with Phase 1 users in cross-patched and emergency operations. The Phase 1 mode also is used for direct or talk-around operations. TDMA systems typically require timing from the radio base station equipment via the system control channel and traffic channels for slot synchronization. This makes direct-mode operations challenging for TDMA when no master timing reference from a base station is present. While there are some mechanisms — such as TDMA vehicular repeaters and master/slaving of portables and mobiles — to facilitate direct-mode TDMA operations, such communications will require further study.

An issue for TDMA systems that interoperate with a Phase 1 system concerns the loss of radio channel capacity in the Phase 2 system when a radio channel is switched from two-voice-path (Phase 2) operation to single-voice-path (Phase 1) operation. Future work for P25 Phase 1 may address use of the dual-rate vocoder in Phase 1 systems and subscriber units. This would allow Phase 1 units so equipped to switch to the enhanced half-rate vocoder for communications with Phase 2 units with no loss of capacity in Phase 2 or Phase 1 systems.

30 millisecond slot size

The 30 msec slot size defined for the two-slot TDMA solution provides for the following formats:

  • One slot represents 360 bits and 30 msec.
  • 12 slots are contained in a 360 msec superframe. (Phase 1 uses a 360 msec superframe.)
  • Four voice frames in a slot.
  • Two voice frames plus some signaling (encryption sync) in a slot.
  • Dedicated signaling slot (P25 link control words) plus synchronization. The signaling conveys call setup, call teardown and in-call progress signaling messages.

Concentrated signaling

Phase 2 systems will use a concentrated signaling scheme with a dedicated signaling slot in each superframe. This slot will occur in an inverted position, meaning the signaling slot for Slot 1 will appear in the Slot 2 stream and vice versa. This inverted slot scheme ensures a transmitting subscriber unit has adequate time to momentarily switch from transmit to receive, and to look at its signaling slot from the base station without impairing the uplink voice traffic.

The signaling slot location and switching scheme enables bidirectional signaling when a subscriber unit is talking, a significant improvement over Phase 1 systems. Bidirectional signaling means a talking subscriber can be alerted, and if necessary, shut down for emergency situations. Current Phase 1 implementations can take over a channel and allow communications to the talk group with the exception of the talker, who is not aware that the talk group has been pre-empted.

The other benefit of bidirectional signaling is transmitter power control. A talking unit can receive power control updates to mitigate interference.

FDMA control channel (Phase 1 CCH base)

The Phase 2 TDMA solution uses the existing P25 Phase 1 FDMA control channel with extensions added to allow determination of subscriber unit TDMA capabilities (i.e., two-slot or future four-slot), vocoder capabilities (full-rate or half-rate IMBE), and initial FDMA/TDMA frame synchronization. Additionally, the control channel enables outbound messages to advise subscriber units of TDMA channel capability on the base station. Phase 1 FDMA subscriber units will ignore any messages pertinent to TDMA subscriber units.

The use of the existing Project 25 Phase 1 control channel provides an easy migration path for existing Phase 1 systems, as the same Phase 1 FDMA control channel can control and manage both Phase 1 FDMA and Phase 2 TDMA subscriber units on the same radio site. Interoperability is facilitated because Phase 1 subscriber units can recognize and operate on a Phase 2 site as long as some traffic channels are available for Phase 1 voice operations.

TDMA voice channels

To ease the migration from Phase 1 to Phase 2, the initial TDMA proposal provides the ability for a Phase 1 control channel (CCH) to control Phase 2 traffic channel (TCH) assignments without synchronizing the CCH to the TCH. Traditional TDMA implementations require synchronization of the TCHs to the CCH. The Phase 2 air interface protocol has been developed to allow an acceptable low latency, CCH-to-TCH switching without requiring a synchronization scheme.

Additionally, TDMA synchronization with the FDMA control channel would require a complete redesign of the Phase 1 FDMA control channel to accommodate both Phase 1 and Phase 2 subscriber units. Phase 2 TDMA subscriber units, when monitoring the Phase 1 FDMA control channel, can use the current Phase 1 synchronization broadcast message (with minor clarifications for TDMA operations) to determine timing alignment with the inbound (uplink) TDMA traffic channel.

A two-slot TDMA solution for P25 Phase 2 benefits users by meeting FCC spectral-efficiency mandates, creating a migration path from Phase 1 to Phase 2, nearly doubling system capacity, improving emergency management, and delivering control channel capacity equivalent to Project 25 Phase 1.

The APIC and TR8 subcommittees are working hard to vet the proposed technical solution, while the P25 Steering Committee and the User Needs Subcommittee are providing critical user input and guidance to ensure the solution meets the needs and requirements of public safety users and operators.


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.

ANTICIPATED TWO-SLOT TDMA BENEFITS

Improved channel efficiency:

  • Increases the number of voice paths in 12.5 kHz radio channels (2:1 for two-slot TDMA).
  • Minimizes frequency plan churn when migrating from 12.5 kHz FDMA to 12.5 kHz two-slot TDMA.
  • Meets future FCC spectral efficiency requirements.

Ease of migration from Phase 1 to Phase 2:

  • Phase 1 FDMA control channel with extensions for TDMA.
  • Compatible IMBE vocoder family.

Improved emergency management for users:

  • Bidirectional signaling allows emergency alerting and informed preemption of talkers.

Subscriber unit power uplink (inbound) control for interference mitigation:

  • Via bidirectional signaling.

Slotted/burst transmission of subscriber units improves subscriber unit battery life:

  • TDMA subscriber units transmit approximately 50% of the time vs. FDMA.

Signaling and control channel capacity equivalent to Phase 1.

Tags: content Project 25

Related


  • Florida SLERS renewal could wait until May as SLERS-2 procurement begins
    Florida agencies that depend on the State Law Enforcement Radio System (SLERS) for mission-critical voice communications will be able to maintain access to the system, but the longstanding maintenance agreement is expected to end when the current contract with L3Harris expires at the end of June, state officials said today. Patrick Gillespie, deputy secretary of […]
  • Spectrum coordinators initiate submittal of new T-Band applications
    Spectrum coordinators last week submitted applications to expand land-mobile-radio (LMR) systems operating on T-Band spectrum (470-512 MHz) into a queue, with applications expected to be submitted formally to the FCC beginning next month and continuing through June, according to people familiar with the process. Both public-safety and enterprise incumbent licensees operating T-Band radio systems were […]
  • Enterprises prepare T-Band applications to alter systems after lengthy FCC freeze
    Many business enterprises currently licensed to operate land-mobile-radio (LMR) networks on T-Band spectrum (470-512 MHz) are taking the first step this week toward making the first significant updates to their systems in almost nine years, when the FCC froze the band in anticipation of an auction that recently was scrapped. Mark Crosby, president and CEO […]
  • Motorola Solutions looks to be part of Florida statewide system, exec says
    Motorola Solutions failed to reach a contract agreement with the state of Florida to build a statewide P25 network a year ago, but the vendor plans to participate in the upcoming Statewide Law Enforcement Radio System (SLERS-2) procurement, according to a company executive. Jack Molloy, Motorola Solutions’ executive vice president of products and worldwide sales, […]

Leave a comment Cancel reply

To leave a comment login with your Urgent Comms account:

Log in with your Urgent Comms account

Or alternatively provide your name, email address below:

Your email address will not be published. Required fields are marked *

Related Content

  • LMR licensing activity again dips to new all-time lows in 2020
  • T-Band auction officially repealed as spending package becomes law
  • Congress repeals T-Band auction as part of massive funding bill with COVID-19 relief
  • SBC: John Foley details in-building organization's 'No Noise' initiative

Commentary


Ransomware? Let’s call it what it really is: extortionware

21st February 2021

Redefining communications for today’s mobile workforces

18th February 2021

Hi-tech sewer can help safeguard public health, environment and economies

18th February 2021
view all

Events


UC Ezines


IWCE 2019 Wrap Up

13th May 2019
view all

Twitter


UrgentComm

Hytera, Motorola Solutions DMR royalty dispute to be decided by federal judge dlvr.it/Rthqp3

1st March 2021
UrgentComm

Using data to improve emergency response resources in healthcare arena dlvr.it/RtYfFJ

26th February 2021
UrgentComm

3 security flaws in devices and IoT that need fixing dlvr.it/RtYRxm

26th February 2021
UrgentComm

Newscan: America’s creaky payment infrastructure is showing cracks dlvr.it/RtTzBD

25th February 2021
UrgentComm

California PD: ‘Game-changing’ Live911 streaming of emergency calls accelerates responses dlvr.it/RtPgXS

24th February 2021
UrgentComm

Why Tuscon is building its own 4G network dlvr.it/RtPDG5

24th February 2021
UrgentComm

SolarWinds attackers lurked for ‘several months’ in FireEye’s network dlvr.it/RtP07s

24th February 2021
UrgentComm

Buffalo’s 48 hours to navigate a mission-critical transition to remote work dlvr.it/RtNwfl

24th February 2021

Newsletter

Sign up for UrgentComm’s newsletters to receive regular news and information updates about Communications and Technology.

Expert Commentary

Learn from experts about the latest technology in automation, machine-learning, big data and cybersecurity.

Business Media

Find the latest videos and media from the market leaders.

Media Kit and Advertising

Want to reach our digital and print audiences? Learn more here.

DISCOVER MORE FROM INFORMA TECH

  • American City & County
  • IWCE
  • Light Reading
  • IOT World Today
  • Mission Critical Technologies
  • Microwave/RF
  • T&D World
  • TU-Auto

WORKING WITH US

  • About Us
  • Contact Us
  • Events
  • Careers

FOLLOW Urgent Comms ON SOCIAL

  • Privacy
  • CCPA: “Do Not Sell My Data”
  • Cookies Policy
  • Terms
Copyright © 2021 Informa PLC. Informa PLC is registered in England and Wales with company number 8860726 whose registered and Head office is 5 Howick Place, London, SW1P 1WG.
This website uses cookies, including third party ones, to allow for analysis of how people use our website in order to improve your experience and our services. By continuing to use our website, you agree to the use of such cookies. Click here for more information on our Cookie Policy and Privacy Policy.
X