Software-defined radio at the OpenRadio miniconf

The first two days of linux.conf.au (LCA) are reserved for a series of miniconfs proposed and run by the conference delegates. This year, one of Monday's miniconfs centered around free-software and amateur (ham) radio, especially where it involves software-defined radio (SDR). The ham and free-software fields have a great deal in common—especially at the philosophical level—but, practically speaking, using free software with SDR can be a bit clunky. The OpenRadio miniconf attempted to kickstart interest in the topic, both by including relevant presentations and by giving attendees the opportunity to construct their own device: a new SDR peripheral that is more flexible and more open than most of the competition.

Sessions

The miniconf day started and ended with plenary talks. Don Wallace, who is the Overseas Liaison Officer for the New Zealand Association of Radio Transmitters (NZART, New Zealand's amateur radio association) gave an overview of the radio-spectrum allocation process and how New Zealand's allocation rules fit in with the multi-tier regional and global system of radio regulation. He also discussed the current landscape of ham radio allocations. Notably, he pointed out that almost all existing frequency allocations are at risk, as more and more spectrum is being allocated for cellular telephone systems every year.

He also explained the background of the unlicensed industrial, scientific, and medical (ISM) bands, where the miniconf's SDR peripheral is designed to operate. The 2.5GHz and 5GHz ISM bands were initially designed for use in operations like welding, microwave oven heating, and medical devices. It was only co-opted for use in wireless networking when Cisco and other networking hardware manufacturers balked at the costs that would accompany a new spectrum allocation. This strange pairing continues to be a problem for ham radio and for wireless networking as new devices come to market: the cumulative RF noise generated by the radio sources in compact-fluorescent and LED bulbs is rapidly reaching a problematic level. During a recent city-wide blackout, he said, the RF noise floor dropped by 20dB. It can be hard to work in such a noisy radio band, he said, and ham radio operators are often perceived as the least important group when national and international regulations are being re-examined.

Next, Paul Warren gave a talk (the slides from which are available online [ODP]) comparing and contrasting the current crop of "open" ham radio transceivers. There were a great many options covered, from stand-alone hardware devices that take microphone input and produce audio output (which makes those devices the most similar to traditional ham radio gear) to peripherals that require a computer for their functionality. The sense in which each device is "open" varies considerably as well, he explained. Some of the devices include schematics and printed circuit board (PCB) designs—which makes them "open hardware"—but they might use firmware that is not released under a free software license (if its source is published at all). Others might be the reverse: fully free software, but without plans or schematics. In selecting which devices qualified for inclusion in the talk, Warren sad, he just made his best guess for which projects "were attempting to do the right thing."

A thorough examination of the options is important for anyone considering a purchase. Warren identified a particular favorite device at the moment in the PortableSDR, a powerful but handheld-sized device that recently won third prize in a project-building competition held by the Hackaday web site. He also looked at several devices he called "SDR exciters" rather than true transceivers. These include well-publicized projects like the HackRF and BladeRF. Some of them are quite nice, he said, but they belong in a different class because they come with serious limitations: very low transmission power and a lack of filtering on input signals. Due to these limitations, most are good for experimentation, but would require modification to do any long-range or sensitive communication.

Last but certainly not least, Paul Campbell gave an overview of his work designing "smaller, cheaper, open wireless" for Internet-connected devices. The project is still in the experimental stage, but more details can be found at his web site Moonbase Otago. The project in question is a type of extremely low-power mesh-networking device. The devices communicate with the IEEE 802.15.4 low-power wireless Personal Area Networking (WPAN) networking layer. 802.15.4 is best known for ZigBee products, although ZigBee communication is actually implemented in a higher-level protocol on top of 802.15.4.

Moonbase Otago's cheap RF devices are built around a tiny CC2533 CPU and very few physical parts: just a handful of inexpensive resistors and capacitors. The CC2533 provides serial communication, I2C, and several GPIOs; just 32K of storage is available, plus 4K of RAM and five timers—but there is a AES encryption chip. Campbell's team has developed a tiny operating system (occupying 6K) that provides threading and task queueing for applications, but also incorporates several interesting features. Using the AES chip, all communication is encrypted automatically: paired devices receive messages from each other, and drop any that use a different key ID in the message packet. The devices can also update application software over the air, propagating updates from device to device automatically.

The project is still in active development. Because GCC does not support the CC2533 CPU, the team has been using the free-software SDCC compiler, and has had to reverse-engineer a number of features of the device where the manufacturer's sample code does not work. Writing a GUI development tool is underway as well, and the boards will support a variety of Arduino-compatible macros to assist new developers.

Hardware

The majority of the OpenRadio miniconf schedule, however, was devoted to the construction of the OpenRadio SDR device. The board is a radio transceiver, designed by Mark Jessop. The basic architecture is modeled on the open-hardware SoftRock SDR board, with some modifications intended to reduce cost and a few enhancements to its functionality. Kits were manufactured ahead of LCA, and attendees purchased them online to pick up at the beginning of the workshop session.

An Arduino Nano is used to control the board's local oscillator, setting the receive frequency, over I2C. The Nano also does phase-shift-keying (PSK) modulation and generates transmitter output. Incoming signals are mixed with the local oscillator signal to produce a baseband audio signal that can be fed into a host computer via any sound card with a stereo input.

The board is designed to run on the 27MHz ISM radio band, although it can be used with other bands. Another of the distinctions between the OpenRadio and the SoftRock design on which it is built is that the OpenRadio hardware includes a prototyping area: a set of large pads onto which the user can construct a set of filters to change the operating frequency. The SoftRock can also be constructed to tune to one of several specific radio bands, but the OpenRadio has a pluggable option, too: the frequency filters can be built on removable daughterboards (instead of being soldered directly to the prototyping area) so users can make several filters and swap them out as desired.

The Arduino code that runs on the Nano is available on GitHub. Several applications are available to run on the host computer, to read samples from the stereo input and process the signal. Both the Python-based Quisk and fldigi were in use around the room once testing began. Both programs can demodulate the radio signals received by the board, decoding a wide variety of signals modes—from single side-band (SSB) voice to digital data packets of various varieties.

Unfortunately, even given the simplicity of the kits, time did run fairly short by the end of the day. Not everyone was able to complete the assembly process, although quite a few did. Afterward, OpenRadio miniconf organizer Kim Hawtin said that he hopes the kits will be just the first step, and that the project will be able to build on the initial design at subsequent events.

Ham radio has long been a popular hobby among free-software fans, so it can come as an understandable surprise to hear supporters of both movements say that there is important work left to be done in a popular area like SDR. As the LCA OpenRadio miniconf demonstrated, though, even when there is considerable interest, the hardware is far from standardized (even on items like licensing) and the free-software tools for working with SDR tend to be of the "toolkit" variety rather than the full-blown application variety. But the miniconf sold out in advance (there were a fixed number of spaces available in the workshop); one would hope that this and the increasing number of projects available are indicators that the community is actively maturing and good things are still to come.

[The author would like to thank LCA 2015 for travel assistance to Auckland.]