One major semiconductor company is so bold as to call its wideband digital down- and upconverters "the only true SDR ICs." Those components may be necessary pieces of the puzzle, but they're far from sufficient. And an antenna with a bandwidth wide enough to handle several cellular frequencies can be called an "SDR antenna," though no SDR system exists.

So, what is SDR? Well, there's no official definition, so we'll offer one: an adaptable radio transceiver system that learns and adapts to transmission and channel environments with software.

In cellular, the goal is to dynamically adapt to any of several air interfaces, such as GSM, GPRS, W-CDMA, cdma2000 and so on, through software detection and dynamic reconfigurability. In concept, it's like today's trimode cell phones, but those devices simply "switch" receiver and transmitter circuits for the predefined air interfaces, rather than employ a single, common RF front end.

I'll reiterate that no SDR system yet exists commercially, although some of our three-letter agencies and the military have long employed very wideband receivers that accept most of the radio spectrum and analyze the signals found through massive DSP computation. Such electronic intelligence systems have been continually improved over the years, but we aren't aware of any implementations that also adapt a transmitter to match the detected signal, as true SDR must do.

For the future cellular SDR environment, a number of questions remain. Should basestations be the only dynamically adaptable system elements or should cell phones be adaptable? If both are SDR-capable, would the cell phone and the basestations continually hunt for a common air interface? How much DSP horsepower will be needed? And should the DSP engine employ reconfigurable architecture or is context switching sufficient?

The key question is, "What features could an SDR cellular system offer that aren't available with today's 2.5G and 3G technology?" Until the last question is answered-with an indication of additional profit for the operators-there is no impetus to implement SDR any time soon.

Will Strauss is President of Forward Concepts (www.forwardconcepts.com).