Can you have radio that could change instantly! Say it was receiving the police band or the CB but can instantly change to the broadcast band! That's SDR for you. A radio that can access both cellular as well as the WiFi! What if a radio could detect unused spectrum and set up radio communication! That would be CR or the cognitive radio for you!
What is SDR and Cognitive Radio?
A Software Defined Radio (SDR) is a communication system that implements a number of components of a traditional system such as filters, modulators, amplifiers, etc. in software . These were traditionally implemented in hardware. Once implemented in software they allow for some unique capabilities. First, there can be tunable parameters. Hardware elements are tunable but with SDR these can be done on-the-fly instead of manually. Secondly, because the implementation is software, the implementation itself can be changed (possibly on-the-fly with a certain delay) so that the same radio can function in many different ways (ex: one radio can access Cellular and Wi-Fi ).
Cognitive Radio (CR) is a term used to define an emerging radio standard in which a radio is designed such that it can utilize unused spectrum . For example, in a particular frequency band (such as maritime communications) which is not much used inland, a Cognitive radio (CR) device can use the underutilized spectrum for its communication needs.
Naturally, the requirements for CR are then that:
a) it can detect unused spectrum,
b) create a communication channel to utilize this unused spectrum, and
c) do it in such a way that it does not interfere with native devices.
A native device is one that is licensed to use that spectrum. In our example, it would be the maritime radios. Thus, if a CR was to use maritime frequencies it would require that: a) If done inland, the CR signal is not powerful enough to disrupt maritime signals and therefore, b) that if you are on the coast, then the CR must not use this frequency at all.
Architectures for SDR/CR
Apart from a radio front-end, which would carry out the task of actually sending and receiving radio signals (an antenna etc.), there needs to be a computing system capable of running the required SDR software. A CR implies that there is also a control unit capable of making decisions as to what spectrum to use and in what way. Therefore the computing piece becomes critical as compared to existing radio systems. There are 4 main ways to deal with the computing for SDR/CR:
1. General Purpose Processors: Using a standard microprocessor (ex: X86 family) which has massive computing power and can run almost any type of software. But they also have massive power consumption and this may be over-kill.
2. Application specific processors and ICs: Things such as DSPs and other processors with specialized functionality can be used. Even though they tend to have low power consumption, the trade-off is once you decide on something, you have limited flexibility. Cost can be high unless a large number of devices are produced.
3. Reconfigurable computing: Extremely popular wherever flexibility is desired. Most often FPGAs are used, that have the unique quality of being hardware that can be programmed . They have lower power consumption but are slower.
4. Embedded systems: Systems that have processors that have been specialized for real-time operations. They have the advantage of speed and low power consumption, but they may not be able to handle really complicated stuff.
Where do Embedded Systems fit into all this?
Embedded systems are a natural choice for SDR/CR because of their real-time capabilities. Usually an SDR/CR system has a fairly well-defined set of possible implementations and an embedded system will be capable of handling those. The low power consumption, and the speed of embedded systems make up for a partial lack of flexibility, if any. Since embedded programming is now well-understood, and embedded programs are small, the programming effort needed is less. Real-time OS are such that little time is wasted in the booting process, therefore, an SDR/CR device can, in theory, be reset quickly. In case a rebuild of the OS is required (for updating/reconfiguring the SDR/CR) - this can be done quickly without having to make the device go offline for long. Most importantly, there is a significant cost advantage and the ability to fashion an SDR/CR from off-the-shelf components in very little time.
i.SDR is a term first used by Joseph Mitola in 1991.
ii.Note that this is different from the same device able to do Cellular and Wi-Fi. Even if a current device does that in hardware, it is doing so with two different radios. Example: An 802.11 a/g router or access point uses two different hardware implementations.
iii.Unused spectrum is often called white space. See especially TV white space which aims to use the spectrum being freed by the American TV transition to digital.
iv.Firmware, now a well-known term is often implemented this way.
v. Because of the physical limitation of the RF (radio frequency) front end it can only handle a certain amount of the radio spectrum. So a radio may do Cellular and Wi-Fi because they are close when frequencies are concerned, but not Wi-Fi and TV because they are far apart.