Sampling Quantization for GPS or GNSS RPS Systems
When talking about Record and Playback Systems (RPS), more bits mean more resolution which has got to be better right? Indeed if familiar with the classic graphic used to describe the operation of an ADC you may be asking yourself how GNSS signals over a 20 or 30dB range can possibly be captured by only one or two bits at all. After all the well-known rule of thumb is 6dB per bit and so 30dB dynamic range will need at least 5 bits, right?
Well wrong actually and the reason is that the classical graphic represents signals clear of noise but with GNSS the signal is right down deep buried in the noise. For all practical purposes the signal is not there until the receiver’s correlators dig it out. It’s too small to see and falls well below the quantisation steps in the DAC.
What is important is that the noise is captured (and replayed) as faithfully as possible. The tiny phase shifts around the transition and distribution of levels will be available to allow the correlators to do their thing. But nothing comes for free and the more that the “noise” is changed during the process the harder it is for the GNSS signal to be subsequently recovered. Think of it as reducing the processing gain from a starting point of 43dB. This will be seen as a lower C/N0. So the apparent effect of the reduced resolution in the ADC – storage – DAC process is a drop in GNSS signal level and the amount of the drop relates directly to the resolution. 1-bit systems “add” around 3dB noise, 2-bit systems add about 1.3dB noise whilst a 4-bit system adds around 0.6dB noise. So going from 1 to 2 bit provides a performance improvement of around 1.7dB whilst going from 2 to 4 bits only lifts performance another 0.7dB. In other words diminishing returns in performance in exchange for vastly more storage requirements.
At bit depths much beyond 2-bits the system performance is dominated by the device under test RF noise figure and so further increasing the bit depth has little impact on performance.
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