Ultra-narrowband and BPSK on TI CC chips

15.06.2016 20:56

Ultra-narrowband is a fancy new name for an old thing. The idea is to use a phase modulated carrier to transmit data at a very low bitrate. This saves energy and improves spectral efficiency (bits per second of data throughput per hertz of radio bandwidth). This in turn makes it convenient for battery-powered sensors and 20-billion Internet-connected toasters of tomorrow. For similar reasons, amateur radio operators have been chatting over PSK31, which is essentially the same thing as ultra-narrowband, for almost two decades now.

Currently SIGFOX seems to be the main commercial operator that's pushing this technology. They don't publish protocol details, however they've written a 3GPP proposal for C-UNB standard, which is public. The benefit of ultra-narrowband is that the simple BPSK modulation can be implemented with existing cheap and well tested integrated transceivers. Compare with the original Weightless standard for instance, which required custom silicon for its much more advanced physical layer and seems mostly forgotten these days (although it's not a completely fair comparison, since SIGFOX operates in unlicensed spectrum and Weightless had to deal with complexities of TV whitespaces, but I digress).

CC1101 transceiver on SNE-ISMTV-868

The CC-series of transceivers from Texas Instruments (like CC1101 and CC1120) has a lot of software-configurable modulation blocks built-in, but a BPSK modulator is not among them. However, you can find some references to ultra-narrowband being implemented with these chips which suggests that people are using them for this purpose. The C-UNB proposal also mentions that it can be easily implemented with modified FSK modulation, but doesn't go into more detail. I wanted to implement ultra-narrowband on CC1101 for a project we're doing at the Institute, so I looked into this possibility.

As any introductory course in telecommunications is quick to point out, frequency and phase modulation are basically the same thing. If you take a frequency modulator and feed it a time-derivative of a signal the result is identical to a phase modulator fed with the unmodified signal. In practice however it's not that simple. BPSK requires that the phase changes ±180° for each symbol change. The frequency-shift keying block in CC chips does not have a well-defined relation between frequency deviation and symbol rate. This means that it's hard to define how much signal phase changes during each symbol.

CC1101 does have a minimum-shift keying mode. This is a special form of frequency modulation that has well-defined phase shifts between symbols. Wikipedia says that the carrier phase continuously shifts by ±90° each symbol period, which does not sound useful at first:

Minimum-shift keying illustration.

In this interpretation of phase shifts, the carrier frequency fc is in the middle between frequencies for the two symbols, f0 and f1. This is the usual interpretation for frequency modulation, where you have approximately equal numbers of both symbols in a typical transmission.

However, if you transmit mostly one symbol, say f0, the receiver will consider that to be the carrier f'c. In that case, each occurrence of symbol 1 rotates the phase of the signal compared to f'c by +180°. This is exactly what you need to implement BPSK.

Alternative interpretation of phase in MSK.

BPSK requires that phase shifts are fast compared to symbol rate, so you want to encode each BPSK symbol with many MSK symbols. Ultra-narrowband uses symbol rates on the order of 100 symbols/s while CC1101 supports up to around 1 Msymbol/s. This means that you could have fast phase changes, but 10 MSK symbols per each BPSK symbol seems to suffice.

In the end, bits encoded into MSK symbols look somewhat similar to the theoretical time-derivative I mentioned above. You have an impulse of a single f1 symbol each time you have a transition from bit 0 to 1 or vice versa:

Using multiple MSK symbols as one BPSK symbol.

So far, this has been all theoretic. How well does it work in practice? The most obvious problem is frequency stability. The local oscillator on CC1101 is designed to be re-calibrated often, but you cannot calibrate it while you are transmitting. With such low bitrates, packet transmissions last for several seconds. During that time the frequency can drift quite a lot, especially compared to the very limited bandwidth of these transmissions. This is the usual problem with narrowband transmissions and CC1101 has no mechanism for compensating for it on reception. That is why I doubt a CC1101-to-CC1101 link would work in this way and I haven't tried it.

Transmission from a CC1101 to a specialized receiver however seems to work quite nicely in practice. You just have to use a SDR with a wide-enough channel for reception and compensate for frequency drifts in software. I have some lab measurements to share, but those will have to wait for another post.

Posted by Tomaž | Categories: Digital

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