The part played by the USRP in advancing the state of the art in open source SDR cannot be overstated, but even an entry level system is priced out of the range of most casual experimenters. More affordable purpose-designed SDR hardware does exist, although it tends to be specified for amateur radio use – there, the focus is mainly on signals under 30MHz and sample rates of up to 192kHz, which rules out use with the majority of modern wireless communications systems.
There had long been talk in the GNU Radio community of repurposing PC DVB tuner hardware for SDR. This resulted in very little actual success, until in early 2012, when a Linux kernel developer reported that receivers using the Realtek RTL2832U chip could be configured to provide raw samples instead of a demodulated signal.
The rtl-sdr project provides tools for using RTL2832U-based USB receivers with SDR and claims that reliable sample rates of up to 2.8MS/s have been achieved, with many devices able to tune over a frequency range of 64-1700MHz (with a gap around 1100-1250 MHz), or even as wide as 50MHz-2.2GHz when operating out-of-spec. This is incredible for commodity hardware that can be had for under £10.
The RTL2832U discovery was an important breakthrough that has enabled many to get their first taste of reasonably powerful SDR, with low cost technology being used to receive wireless sensor, GSM, GPS and aircraft Mode S signals, among others.
Although RTL2832U hardware is great value for money, and the uses to which it may be put are impressive, it does have significant limitations. These include an inability to pick out weak signals and support for sample rates of up to only 2.8 MS/s – this is adequate for many applications but far too low for use with wireless broadband data networks, for example. Also, crucially, it has no transmit capability.
Connecting everything wireless
If SDR is to deliver on its promises it will require very high performance digital radio hardware to be in the hands of a much wider development community, with support for transmit and receive, high sample rates, wideband frequency coverage and the ability to pick weaker signals out from noise.
Hardware start-up, Per Vices, has risen to this challenge and went public with its first offering in April 2012 at Y Combinator. Dubbed Phi, this was a GNU-Radio-compatible PCI Express card that packed two 125 MS/s ADCs and a dual-channel 250 MS/s DAC; this is capable of working with a hefty 200MHz chunk of wireless spectrum anywhere from 100kHz to 4GHz.
Per Vices recently announced its second generation board, Noctar, with support for an increased bandwidth of 250MHz and numerous improvements that make it more suitable for real world use. These include additional RF shielding, GPIO pins for interfacing with external hardware, and support for high performance synchronisation as required by certain telecoms applications.
When asked how its hardware compares with the USRP, Per Vices CEO, Victor Wollesen, says: "A large part of it is cost, bandwidth and overall performance; our platform is integrated and we provide superior bandwidth and lower latency." He goes on to explain: "low latency is important for a lot of different connections, especially when it gets to Wi-Fi or some of the wireless protocols."
The use of a four-lane PCIe card means that the Per Vices hardware is able to transfer up to a staggering 8Gbp/s between it and the host computer. Wollesen sees this as key, explaining that it "opens the door to some really interesting applications of the technology. Specifically when it comes to handling independent signals in the same band or concurrent streaming applications."
Asked if Noctar could simultaneously support multiple wireless systems, Wollesen enthuses: "That's the wonderful ideal promoted by SDR and in principle it's possible. The actual implementation of that relates to both the RF bandwidth that you're processing at any one time but also the computational back-end. So, if you get a new Intel processor or a new graphics card you can find a substantial increase in the performance and capabilities you're able to exercise using our device."
Like the USRP, the Per Vices boards include an FPGA for certain compute-intensive operations, but at present the code for this is only available under NDA, with the Linux device driver also closed source. The licensing position may be about to change as Wollesen explains that they are "now looking at open sourcing both the firmware and the drivers, under a dual licence."
Few hardware alternatives at present
The USRP can be configured for wideband frequency coverage similar to the Per Vices card, but this pushes the price up. Also, Noctar still has a clear lead when it comes to bandwidth – the amount of radio spectrum it can process in one go – and latency.
At present, the hardware options capable of providing simultaneous support for multiple broadband wireless systems are limited, and it's not uncommon to pay five times the price of Noctar for commercial equipment with a little over a tenth of the bandwidth.