VHF/UHF Digital Modes and SCHEARTS

by Marc C. Tarplee Ph.D.

N4UFP

ARRL Technical Coordinator

SC Section

History/Background

VHF Packet

• Developed in Canada in 1978.• Uses AX.25 protocol at 1200 or 9600 bit/sec• Popularity of packet increased throughout the

1980’s, then declined in the late 1990’s with the advent of the WWW.

• AX.25 supports digital repeating (digipeating)• Packet modulation scheme (NBFM) is

compatible with existing repeaters.

APCO Project 25• Commercial standard for digital voice/data

communications for public service agencies• Developed in 1996 by APCO (Association of

Public Safety Communications Officials) and equipment manufacturers

• Supports digital voice/data transmission at 9600 bit/sec

• Supports encryption and talk groups• Modulation schemes for P-25 (CQPSK, C4FM)

are not compatible with existing repeaters

D-Star• Developed in Japan in 2001• Result of partnership between Japanese

government, JARL, and equipment manufacturers

• Supports digital voice transmission at 4800 bit/sec• Supports data transfer at 128 kbit/sec or

10 Mbit/sec• Modulation schemes for D-Star (GMSK, QPSK,

4FSK) are not compatible with existing repeaters• Supports digital repeating (digipeating)

Comparison

Technical StandardsAPCO 25 D-Star AX.25 Packet

Modulation Type

Digital Voice/Data Digital Voice/Data Data

Modulation Scheme

CQPSKC4FM

QPSK4FSK GMSK

NBFM

Bit Rate 9600 bit/sec 4800 bit/sec128 kbit/sec10 MBit/sec

1200 bit/sec9600 bit/sec

Bandwidth 6.25 KHz12.5 KHz

6.25 KHz150 KHz10.5 MHz

15 KHz

Vocoder IMBE vocoder, 4400 bits/s + 2800 bits/s FEC

AMBE vocoder, 2400 bit/s + 3600 bit/s FEC

None

Digipeating No Yes Yes

Call sign Addressing

No Yes Yes

Encryption Yes No No

Legacy System Compatible

No No Yes

Open Standard No Yes Yes

Analysis• Packet’s severe limitations (low speed, no DV capability)

offset its compatibility advantage.

• P25 implementation in SC will be limited to frequencies above 700 MHz, so equipment that could be modified for use on amateur bands will not be available.

• P25 does not support call sign addressing and encryption capability violates Part 97 Rules, and it is a closed standard

• D-Star offers DV, low-speed and high-speed data transfer, call sign addressing and digipeating. It is also an open standard

• D-Star’s incompatibility with legacy equipment limits its deployment to new systems.

Recommendation• Implement D-Star within SCHEARTS.• This implementation would not affect existing FM

repeater networks.• D-Star would add additional functionality, rather

than replace existing systems• High-speed D-Star systems operating on 23cm

would relieve crowding on 2m and 70cm bands• Low-speed D-Star systems can be interleaved

between existing NBFM on 2m and 70cm to improve spectrum usage

Interleaving

• D-Star signals are placed in guard bands between NBFM signals

• Modulation schemes are incompatible, so NBFM receiver rejects D-Star signals and vice versa.

Applications of D-Star

D-Star across the US

• Alabama Linked Repeater Network is implementing D-Star on 23cm to provide statewide data transfer

• Emergency Minnesota 145.67 MHz Wide Area Amateur Packet Radio Network is adding 23cm D-Star capability to “to support 128kbps digital data and web software based public service applications on Amateur Radio frequencies”.

D-Star in Williamson County, TN

• 23cm D-Star links used to provide RF data link between ARES computers at various sites

• For more details, see Oct 2005 QST p 81

Application to SCHEARTS• Link existing local repeater networks through wide

area D-Star repeaters.– This would allow statewide voice and data traffic

– This would provide an RF backup for web-based applications

• Use low-speed D-Star to augment existing local repeater networks– D-Star allows simultaneous transmission of DV and low-speed data

(1200 bit/sec) – GPS/APRS info can be transmitted simultaneously with DV

– D-Star’s narrow bandwidth promotes better spectrum utilization

– D-Star’s DV addressing capabilities facilitate directed communications

Closing Comments

Food For Thought

• Implementation of new digital technologies does not have to make legacy technologies obsolete

• Digital technologies can coexist with existing technologies, delivering services such as file sharing, access to the WWW, and instant messaging that cannot be otherwise provided

• Implementation of P25 in the public service arena means that the agencies we serve will become comfortable with and reliant on digital communications technologies and will expect amateurs to provide similar services.