GNU Radio: Tools for Exploring the Radio Frequency Spectrum
Graphical interfaces for GNU Radio applications are built in Python. Interfaces may be built using any toolkit you can access from Python; we recommend wxPython to maximize cross-platform portability. GNU Radio provides primitives that use interprocess communication to transfer chunks of data from the real-time C++ flow graph to Python-land.
GNU Radio is reasonably hardware-independent. Today's commodity multi-gigahertz, super-scalar CPUs with single-cycle floating-point units mean that serious digital signal processing is possible on the desktop. A 2GHz Pentium or Athlon can evaluate 2 billion floating-point FIR taps/s. We now can build, virtually all in software, communication systems unthinkable only a few years ago.
Your computational requirements depend on what you're trying to do, but generally speaking, a 1 or 2GHz machine with at least 256MB of RAM should suffice. You also need some way to connect the analog world to your computer. Low-cost options include built-in sound cards and audiophile quality 96kHz, 24-bit, add-in cards. With either of these options, you are limited to processing relatively narrow band signals and need to use some kind of narrow-band RF front end.
Another possible solution is an off-the-shelf, high-speed PCI analog-to-digital board. These are available in the 20M sample/sec range, but they are expensive, about the cost of a complete PC. For these high-speed boards, cable modem tuners make reasonable RF front ends.
The preferred solution is the Universal Software Radio Peripheral (USRP). Figure 2 shows the block diagram of the USRP. The brainchild of Matt Ettus, the USRP is an extremely flexible USB device that connects your PC to the RF world. The USRP consists of a small motherboard containing up to four 12-bit 64M sample/sec ADCs, four 14-bit, 128M sample/sec DACs, a million gate-field programmable gate array (FPGA) and a programmable USB 2.0 controller. Each fully populated USRP motherboard supports four daughterboards, two for receive and two for transmit. RF front ends are implemented on the daughterboards. A variety of daughterboards is available to handle different frequency bands. For amateur radio use, low-power daughterboards are available that receive and transmit in the 440MHz band and the 1.24GHz band. A receive-only daughterboard based on a cable modem tuner is available that covers the range from 50MHz to 800MHz. Daughterboards are designed to be easy to prototype by hand in order to facilitate experimentation.
The flexibility of the USRP comes from the two programmable components on the board and their interaction with the host-side library. To get a feel for the USRP, let's look at its boot sequence. The USRP itself contains no ROM-based firmware, merely a few bytes that specify the vendor ID (VID), product ID (PID) and revision. When the USRP is plugged in to the USB for the first time, the host-side library sees an unconfigured USRP. It can tell it's unconfigured by reading the VID, PID and revision. The first thing the library code does is download the 8051 code that defines the behavior of the USB peripheral controller. When this code boots, the USRP simulates a USB disconnect and reconnect. When it reconnects, the host sees a different device: the VID, PID and revision are different. The firmware now running defines the USB endpoints, interfaces and command handlers. One of the commands the USB controller now understands is load the FPGA. The library code, after seeing the USRP reconnect as the new device, goes to the next stage of the boot process and downloads the FPGA configuration bitstream.
FPGAs are generic hardware chips whose behavior is determined by the configuration bitstream that's loaded into them. You can think of the bitstream as object code. The bitstream is the output of compiling a high-level description of the design. In our case, the design is coded in the Verilog hardware description language. This is source code and, like the rest of the code in GNU Radio, is licensed under the GNU General Public License.
An FPGA is like a small, massively parallel computer that you design to do exactly what you want. Programming the FPGA takes a bit of skill, and mistakes can fry the board permanently. That said, we provide a standard configuration that is useful for a wide variety of applications.
Using a good USB host controller, the USRP can sustain 32MB/sec across the USB. The USB is half-duplex. Based on your needs, you partition the 32MB/sec between the transmit and the receive directions. In the receive direction, the standard configuration allows you to select the part or parts of the digitized spectrum you're interested in, translate them to baseband and decimate as required. This is exactly equivalent to what's happening in the RF front end, only now we're doing it on digitized samples. The block of code that performs this function is called a digital down converter (Figure 3). One advantage of performing this function in the digital domain is we can change the center frequency instantaneously, which is handy for frequency hopping spread spectrum systems.
In the transmit direction, the exact inverse is performed. The FPGA contains multiple instances of the digital up and down converters. These instances can be connected to the same or different ADCs, depending on your needs. We don't have room here to cover all the theory behind them; see the GNU Radio Wiki for more information.