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 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 Standards APCO 25D-StarAX.25 Packet Modulation Type Digital Voice/Data Data Modulation Scheme CQPSK C4FM QPSK 4FSK GMSK NBFM Bit Rate9600 bit/sec4800 bit/sec 128 kbit/sec 10 MBit/sec 1200 bit/sec 9600 bit/sec Bandwidth6.25 KHz 12.5 KHz 6.25 KHz 150 KHz 10.5 MHz 15 KHz VocoderIMBE vocoder, 4400 bits/s bits/s FEC AMBE vocoder, 2400 bit/s bit/s FEC None DigipeatingNoYes Call sign Addressing NoYes EncryptionYesNo Legacy System Compatible No Yes Open StandardNoYes

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 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.