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What are the minimum and maximum values for txGain, pktdetthresh and csmathresh? txGain should have a range of 0-63 I think. What about the others?
In the warpphy_init() function I found : txGain = 0x3f, pktdetthresh = 9000 and csmathresh = 4000 as initial values, but then:
ofdm_pktDetector_mimo_WriteReg_csma_avgThresh(PKTDET_BASEADDR, 16000);
Why is it set to 16000 and not to csmathresh? Isn't it quite a high value and does it make sense to choose a higher csmathresh than pktdetthresh?
Thanks,
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txGain is a six-bit value in [0,63], with 63 corresponding to maximum gain.
pktdetthresh and csmathresh are thresholds relative to the packet detector's sum of RSSI values. By default, the core sums the 16 most recent samples, so a threshold of 9000 corresponds to an instantaneous RSSI value of 562 (9000/16). Setting the CSMA threshold to 16000 disables CSMA (the summed RSSI never exceeds that value).
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Thank you so far. You are helping me very much with your answers.
Is the following mapping between txGain and txPower right: 0 -> -35dBm; 63 -> -5dBm. I took these values from the Max2829 data sheet on page 21.
The PA applies +30dB gain and can be either turned on or off. Turned on the mapping should be : 0->-5dBm; 63 -> 25dBm. Is this correct?
In some posts I read that the PA applies 18dBm output power and in some others, that it applies +30dB gain. Do +18dBm correspond to +30dB gain? Or does this mean, that it applies 30dB gain, but its maximum output power is 18dBm?
I appreciate each comment.
Thanks,
Last edited by domenique (2009-Jan-13 17:23:14)
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dB is a unit of gain; dBm is a unit of power, calculated as dB relative to 1mW (where 0dBm is 1mW). See this Wikipedia article for a complete explanation.
The WARP radio board Tx PA applies 30dB gain. It is rated for good EVM performance up to ~18dBm output power. This is approximately the Tx power of the OFDM reference design when the other Tx gains are set to maximum.
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If you have an output power of X dbm and you apply a Y dB gain then your final output power will be X+Y dBm. Maybe I'm wrong, but this is how I learned it. See for example this article : Adding / Subtracting dB and dBm.
That would mean, according to the Max2829 data sheet on page 21:
0 -> -35dBm .... 63 -> -5dBm
+ PA
0 -> -5dBm .... 48 -> 18dBm .... 63 -> 18dBm
But it seems not logical that the output power is constant at +18dBm in the range from txGain = 48 to 63. ?
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I agree that plot is inconsistent with our actual hardware implementation. The discrepancy is that the datasheet doesn't specify what the input I/Q signal was for that test. In our case, the maximum input signal (full range output from the DAC) will produce an output around 19dBm.
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The 18dbm output, is it peak or the average power? Also, have you looked at more powerful PAs, possibly for outdoor usage?
Thanks.
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It's definitely peak. The average power will depend on the waveform. Our OFDM design, for example, backs off at least a few dB for PAPR reasons.
A more powerful amplifier would be great, but it would require redesigning a big chunk of the radio board. We may do this someday but not anytime soon.
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