While there exist numerous Software Defined Radio (SDR) devices, they are best for receive and not transmit. Many SDR’s, such as the various RTL2832U devices (RTL-SDR, etc.) and AirSpy HF, do not transmit at all, while others (HackRF, USRP, LimeSDR), transmit at rates not much higher than 10mW. As a result, many different SDR’s do not transmit at a power level sufficient to achieve decent longer-range communications. While higher-power amplifiers exist, they often suffer one of several issues with SDRs:
- Many amplifiers require higher input power levels than can be provided from the <10mW inputs that SDR’s provide.
- There isn’t enough output filtering on these broadband SDR’s, so using them at higher power levels leads to excessive out-of-band interference. As a result, output filtering is required.
- Many of these amplifiers are expensive, at least compared to the costs of a lot of the Chinese-brand FM transceivers.
- The amplifiers that do exist, often transmit at only a few watts.
There do exist, however, a large number of affordable Chinese-made FM transceivers that can transmit up to 50W. An example is the TYT TH-9800, which can transmit on 70cm, 2m, 6m, and 10m, costing around $200-$220. Such a device allows for significant power range not only on predominantly Line-of-Sight bands like 2m and 70cm, but also on 6m (popular with sporadic-E DX), and 10m (upper HF).
Currently, the main way to communicate data over FM transceivers is via AFSK, a technique which dates back at least to the 1970s, using audio tones to encode data over FM at rates of up to 1200bps. While this method has been successfully used in e.g. APRS, it’s a non-optimal way to encode data for these usages. Better modulation techniques can allow higher data rates, more sensitivity, or a combination of the two.
David Rowe performed some investigations into this. Through simulation, he observed that he could get better data rates (2400bps) using a Manchester-encoded signal received by an FM demodulator (also see https://www.rowetel.com/?p=4663 and https://www.rowetel.com/?p=5219. His observations:
- DC balance is essential, otherwise the DC block in the audio will block it. He used Manchester encoding to deal with this issue, although whitening might be a more efficient way to achieve the necessary DC balance.
- FM demodulators work best when the frequency deviation is highest. As a result, his experiments with 4-FSK showed worse Eb/N0 than 2-FSK.
- The Manchester-coded 2-FSK generally does work well, with sensitivity performance approaching FM when using codec2 to encode the voice.
I’d like to study this further from the perspective of using the FM radio for transmit, and a low-cost SDR for the receive. In particular, I’d like to look at:
- Can the “idle” FM carrier be used to encode the data (this was mentioned in one of his comments)? What kind of DC-balanced coding schemes are good here?
- Does data whitening work well here as a more-efficient alternative to Manchester Encoding? If yes, then the datarate could be increased to 4800bps.
- Can 4-FSK be made to work better if the decoding is done using an SDR?
- Can slow, narrow-bandwidth FSK modes be implemented somehow that still pass through the low-pass filtering/DC block of the audio input? Perhaps this involves “mirroring” low-amplitude tones between both “sides” of the FM deviation?
- What other sorts of constant-envelope modulations can be done (CSS, MSK, etc.)?
We’ll see. There’s some potentially-interesting uses for this, but time will tell if I ever get around to studying this.