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Adios Symbol Rate Limit
30 October 2023 | by 
The FCC will be voting on and will likely approve a Report and Order that eliminates the symbol rate restriction on HF data transmissions, replacing it with a bandwidth limit of 2.8 kHz. See FCC To Vote on Removing Symbol Rate Restrictions. The symbol rate limit of 300-baud is an obsolete way of limiting the signal bandwidth, created back when the data transmissions were predominately Frequency Shift Keying (FSK). It was a simple, practical way to regulate the bandwidth at that time but technology has moved on. The use of digital signal processing and efficient wireless encoding techniques require a better approach to bandwidth regulation.
A practical impact of this change is to allow higher speed protocols such as PACTOR-4 having a bandwidth of 2.4 kHz. I suspect we will see other protocols emerge that squeeze the best data rate out of the 2.8 kHz bandwidth.
Living in a Narrowband World
The FCC proposal implements a 2.8 kHz bandwidth limit on data emissions on the HF bands. Some folks have suggested a narrower bandwidth while others argue that wider bandwidth signals should be allowed. And some even think we should have no bandwidth limit at all.
The problem is that the amateur HF bands are not very wide. For example, the popular 20m band is 14.0 to 14.350 MHz, providing only 350 kHz of spectrum. Common practice on this and the other HF bands is to use modulation types that have bandwidths of 3 kHz or less. (Yeah, AM signals are twice that wide, at 6 KHz, a topic for another day.) Of course, CW and some of the data modes are much narrower than 3 kHz. But the general approach to regulating HF is to allow many narrowband signals on the band. Limiting HF data transmissions to 2.8 kHz bandwidth is consistent with existing practice while still allowing for innovation and experimentation.
VHF/UHF Bandwidth Limits
The FCC also plans to issue a Further Notice of Proposed Rulemaking (FNPR) that:
- Proposes to remove the baud rate limitation in the 2200 meter and 630 meter bands, which the Commission allocated for amateur radio use after it released the Notice of Proposed Rulemaking in 2016.
- Proposes to remove the baud rate limitation in the VHF and UHF bands.
- Seeks comment on the appropriate bandwidth limitation for the 2200 meter band, the 630 meter band, and the VHF/UHF bands.
I won’t comment on the 2200 meter and 630 meter bands. The FCC proposes to remove the symbol rate limit on the VHF and UHF bands and asks what bandwidth limit is appropriate. The current bandwidth limits are 20 kHz for the 6m and 2m bands, 100 kHz for the 1.25m and 70 cm bands, and the FCC seems fine leaving these the same. Authorized emission types are listed in FCC Part 97.305.
With 4 MHz of spectrum, the 2m band is much wider than any of the HF bands. It might be tempting to conclude that there is plenty of room for wideband signals on this band. Many hams think 2 meters is just used for FM simplex and repeaters but a closer look reveals that it supports many diverse modes: weak-signal SSB/CW, meteor scatter, EME, FM simplex, FM repeaters, digital voice modes (D-STAR, DMR, Fusion), satellites, and more. The 20-kHz limit seems appropriate, as it roughly matches the bandwidth of the most common (FM) voice signals on that band. It is not an appropriate band for trying out wider bandwidth signals.
The 6m band should probably keep the same 20-kHz limit. (I don’t think there is a compelling reason to change it.) The 1.25m band already allows 100-kHz bandwidth data signals, which some radio amateurs have used for higher-speed data links (still not what I would call wideband).
The 70 cm band is much bigger (420 MHz to 450 MHz) and could accommodate some wider bandwidth signals. Perhaps the existing 100-kHz limit should be increased? Keep in mind that fast-scan ATV is allowed on this band with a bandwidth of 6 MHz. Maybe we can make some room for a few larger bandwidth data channels, to encourage innovation and experimentation.
The bands above 70 cm have no bandwidth limit other than the signal must stay within the designated ham band. It has been this way for a long time, without causing any issues (that I know of).
Conclusion
The FCC’s proposal makes a lot of sense and it is long overdue. Frankly, it is a bit of an embarrassment that it has taken so long.
Better late than never.
73 Bob K0NR
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4 Responses to “Adios Symbol Rate Limit”
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Interesting, and I think a good move on FCC’s part. It does leave an open question on digitised voice such as Icom’s D-Star HF-DV mode which occupies just slightly more than 6 kHz. Is DV to be considered a voice mode or a digital mode under the proposed regulation?
Any transmission that is contained within the Amateur radio approved bandwidths should be acceptable, in the absence of serious testable parameters.
@Hugo, VE3KTN… The FCC classifies modes on how they are used. Thus Digital voice = voice, nevermind the fact that its 1’s and 0’s (data). This is a bigger issue that I feel needs to be addressed.
Apparently the ARRL position is to eliminate both the baud rate AND the band width limit on all bands above 50 MHz. The obvious impact on 2 meters is a costly problem. Lots of repeaters, and even satellite repeater systems that cannot easily be moved to accommodate a 1 MHz New Packet Radio signal. Band occupancy can be debated on other bands, but 2 meters often comes into play for local emergencies and disaster relief. Digital modes occupying 1 MHz and up need to be located on higher frequency bands where it will not displace essential services. Also, the first thing a new (Technician) ham probably acquires is a 2 meter hand held battery powered voice transceiver, often with the goal of participating in emergency communications. Imagine their disappointment in not being able to communicate over a band covered with squawks and buzzes. Moreover, the advocates of unlimited bandwidth emissions disregard the FCC rules regarding spread spectrum being limited to 10 watts PEP. 97.313 (j) No station may transmit with a transmitter output exceeding 10 W PEP when the station is transmitting a SS emission type. The 2 meter allowed transmit band width should remain at the current limit.