SDR: The new wave in amateur radio

December 7th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT


Software defined radio (SDR) is by no means a new technology – not even in amateur radio. It has been widely used in the mobile and other communications industries. AMSAT SA has embarked on a SDR project which is aimed at developing a SDR transponder for a future satellite project.

Anton Janovsky ZR6AIC and Cor Rademeyer ZS6CR demonstrating an SDR repeater using a RTL dongle and a Raspberry Pie at a recent AMSAT SA SDR workshop.

Anton Janovsky ZR6AIC and Cor Rademeyer ZS6CR demonstrating an SDR repeater using a RTL dongle and a Raspberry Pi at a recent AMSAT SA SDR workshop.

But let as take a step back. When AMSAT SA embarked on its “Kletskous” CubeSat programme, the group opted for developing a linear analogue transponder with a bandwidth of 20 kHz so that it could handle both frequency modulated (FM) and single side band (SSB) signals. The first prototype which was demonstrated at the AMSAT SA Space Symposium in 2015 proved highly successfully. It met all the specifications except for one, which turned out to become the biggest headache: it was power-hungry.

Different to the usual CubeSat designs, the Kletskous project supports additional hinged solar panels which, after launch, will (or should) fold out and make more power available. Theoretically speaking the power-hungry transponder should not be such a major issue except if the additional solar panels do not fold out. The various designs tested on the ground have proven to work successfully but they have not yet flown in space. So back to the drawing board to develop another analogue transponder. But why not opt for a SDR transponder?

AMSAT SA seriously considered this, but taking into account the current equipment in use, it would not meet the objectives of encouraging more radio amateurs to become interested in satellite technology and space communication. The redesign of the analogue transponder is back on track and it is expected that a prototype which is less power-intensive will be undergoing tests soon. The first prototype has not be discarded completely and is being relooked at with the objective of finding ways to make it more energy efficient.

While AMSAT SA is at this stage not planning a second CubeSat, there are possibilities of flying on EOSAT-1 planned by the South African National Space Agency. AMSAT SA has made overtures to the Agency and in principle it was agreed that an amateur payload could be considered as part of the experimental payload – the main payload being for Earth observation. If SumbandilaSat is anything to go by, the amateur payload was by far the most popular and gained worldwide recognition for the satellite. SumbandilaSat was in many ways a proof of concept with the main function to collect images of earth.

But what is software defined radio?

A number of definitions can be found to describe SDR, also known as software radio. The SDR Forum, working in collaboration with the Institute of Electrical and Electronic Engineers (IEEE) P1900.1 group, has worked to establish a definition of SDR that provides consistency and a clear overview of the technology and its associated benefits. Simply put, SDR is defined as “radio in which some or all of the physical layer functions are software defined”.

Traditional hardware-based radio devices limit cross-functionality and can only be modified through physical intervention. This results in higher production costs and minimal flexibility in supporting multiple waveform standards. By contrast, SDR technology provides an efficient and comparatively inexpensive solution to this problem, allowing multi-mode, multi-band and/or multi-functional wireless devices that can be enhanced using software upgrades.

The SDR forum has defined it as a collection of hardware and software technologies where some or all of the radio’s operating functions (also referred to as physical layer processing) are implemented through modifiable software or firmware operating on programmable processing technologies. These devices include field programmable gate arrays (FPGA), digital signal processors (DSP), general purpose processors (GPP), programmable system on chip (SoC) or other application specific programmable processors. The use of these technologies allows new wireless features and capabilities to be added to existing radio systems without requiring new hardware.

The SDR Forum’s market and technology studies have shown that cost effective radio frequency technologies supporting the operation of SDRs over a broad spectral range have begun to mature, allowing for the first time the use of SDR as an enabling technology for dynamic spectrum access systems with cognitive or smart radio functionality. This trend is expected to continue over the next several years, allowing SDR to finally achieve the defined vision of reducing costs in providing end-users with access to ubiquitous wireless communications – enabling them to communicate with whomever they need, whenever they need to, and in whatever manner is appropriate.

That radio amateurs are becoming involved in SDR is no surprise. It is the only way to stay relevant as a technology-defined pastime. Maybe the focus is moving away from the soldering iron and changing to software and system integration.

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