火曜日, 1月 17, 2006

Notes on dsp for software radio

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“A software radio poses an even greater challenge for the RF designer because it is very difficult to create an RF front-end that is applicable to a variety of signals with disparate parameters, such as bandwidth and center frequency…” ( software radio, by Jeffrey Reed)

This is exactly what Jean Tourrilhes pointed out in his essay on the anatomy of a radio LAN,
“Most implementations of common Wireless LANs use fixed analog components in the modem, so are not suitable. So, a digital radio needs to digitize (with a fast AtoD) the whole bandwidth and to feed that to a fast super DSP or EPLD (Electric Programmable Logic Device, like a Xilinx or Altera) and to work entirely in the digital domain to demodulate (and modulate) the signal. Unfortunately this is not really cost effective and doesn't work that well at the frequency we are talking about (GHz).”

Note that he doesn’t say it’s impossible only that it’s not really cost effective…
(link to interesting comment on digital signal processing from one blogger)

So Reed goes on to point out that in the RF front end, dealing with the receiver “The key is to reject undesired signals and condition the desired signal for digital signal processing by taking the signal from the antenna, filtering it to remove undesired signals, converting the signal…”

Yichuang Sun in his book, ‘Wireless Communications systems and Circuits’ says this “processing only the real part of the complex IF signal after the mixer, it avoids the need for a complex ADC, thereby resulting in a very much simpler and more adaptable ADC circuit design. Advancing the position of the ADC in a low-IF receiver already gives improved adaptability by moving the channel filtering into the digital domain.

"Dynamic range is the key design challenge in building an RF front-end since it provides a measure of the highest- and lowest-level signals that can be simultaneously accommodated by the radio. Furthermore, there is a strong tie between battery consumption and dynamic range, which is a particularly important trade-off for mobile systems."

"Downstream digital signal processing can sometimes improve the dynamic range of the system, but the improvement provides minimal compensation for non-ideal characteristics of the RF front-end"

"Dynamic range is limited at the bottom of the range by noise that enters the system through thermal effects of the components or through non-idealities of the ADC, such as quantization noise or sampling aperture jitter"

"Dynamic range is limited at the high-end by interference."

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