They can be reconfigured "on-the-fly". It could be a cordless phone one minute, a cell phone the next, a wireless internet gadget the next and a GPS receiver another. They can be quickly and easily upgraded with enhanced features. In fact, the upgrade could be delivered over-the-air. They can talk and listen to multiple channels at the same.
Block Diagram
The ADC
This unit converts the output of the RF front end into digital waveform consisting of 0’s and 1’s. The analog-to-digital converter is the bridge between the physical world of continuous analog signals and the world of discrete digital samples manipulated by software.
RF Front End
Nyquist theorem tells us that, to avoid aliasing when converting from analog to digital, the ADC sampling frequency must be at least twice the maximum frequency of the signal of interest, in order to sustain all the spectrum information accurately and to avoid aliasing. Assuming we're dealing with low pass signals - signals where the bandwidth of interest goes from 0 to fMAX, the Nyquist criterion states that our sampling frequency needs to be at least 2 × f<MAX.
To deal with a large range of frequencies up to hundreds of megahertz, we can use the RF front end. The receive RF front end translates a range of frequencies appearing at its input (RF band) to a lower range at its output (IF band). If we choose a cable modem tuner module as our RF front end, it can translate a 6 MHz chunk of the spectrum centered between about 50 MHz and 800 MHz down to an output range centered at 5.75 MHz. So an ADC with a speed of about 20 MHz can be used conveniently.
Software
The digital signals which finally get into the computer are acted upon by the software. GNU Radio provides a library of signal processing blocks and the glue to tie it all together. In GNU Radio, the programmer builds a radio by creating a graph (as in graph theory) where the vertices are signal processing blocks and the edges represent the data flow between them. The signal processing blocks are implemented in C++. Conceptually, blocks process infinite streams of data flowing from their input ports to their output ports. Blocks' attributes include the number of input and output ports they have as well as the type of data that flows through each.
Some blocks have only output ports or input ports. These serve as data sources and sinks in the graph. There are sources that read from a file or ADC, and sinks that write to a file, digital-to-analog converter (DAC) or graphical display. About 100 blocks come with GNU Radio. As per our requirements we can build and implement new blocks in C. Graphs are constructed and run in Python.
Hardware
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 3 GHz Pentium or Athlon can evaluate 3 billion floating-point FIR taps/s. We now can build, virtually all in software, communication systems unthinkable only a few years ago. A 1 or 2 GHz machine with at least 256 MB of RAM should suffice for the most complex operations which come in implementing the software radio.
We also need some way to connect the analog world to your computer. Low-cost options include built-in sound cards and audiophile quality 96 kHz, 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.
Alternatively, Universal Software Radio Peripheral(USRP) developed by the software radio people can also be used. 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.
ApplicationsA TiVO equivalent of radio capable of recording multiple stations at a time
Radio Astronomy
Software GPS
Distributed measurement of spectrum utilization
Amateur radio tranceiver
RFID detector/reader
GNU Radio comes with a complete HDTV transmitter and receiver, a spectrum analyzer, an oscilloscope, concurrent multichannel receiver and an ever-growing collection of modulators and demodulators.
Source: Exploring GNU Radio by Eric Blossom
