January 31, 2005, 10:14 AM — I've mentioned software-defined radio (SDR) several times over the past few years, but I was surprised recently to discover I've never written a column on it. This is perhaps a minor oversight - SDR is already playing a role in many radio designs, but it's not yet mainstream, for reasons that I'll cover below. But SDR is regardless one of the most important directions in wireless today, and one that will be more than influential in the products you'll be buying in just a few years.
What is SDR? Well, imagine a very-high-performance computer designed to run software that emulates (becomes) a radio. Imagine an antenna on, for example, the back of your PC, and a little hardware inside to convert the analog signals of radio into corresponding digital signals. Then we process this digital representation of the airwaves with software that performs all of the major functions formerly done by analog hardware in the radio. That's about it. And, in reality, many radio chipsets today, including those in your cell phone and wireless LAN, already do something a lot like this - almost all have digital cores that run firmware (software that's burned into the chip and can't be changed) performing key radio functions.
But let's suppose for the moment that all we have is analog processing - radio waves are, after all, an analog phenomenon, since they belong to the "real world." Keep in mind here that all this "digital" stuff you hear about is just another way of representing that analog information. Analog is typified by continuous waves; digital represents a sampling of the amplitude (loudness) of these waves. The more samples we take per unit of time, and the more bits of resolution in each sample, the better the result - up to a point of diminishing returns. In general, sampling at twice the highest frequency in the analog signal is sufficient, but "oversampling" is now popular in many applications (particularly digital audio).