Re: [Discuss-gnuradio] Building an RF Front end for DSP FPGA Kits with ADCs

[JP234]

Moeller:  Yes I am trying to solve the near-far problem.  I have used the
USRP radios to test the concept, and it works really well I am getting about
55 - 60 dB suppression which is essentially equal to the practical dynamic
range of the ADCs for the USRP1.  I would insert about 1 MHz of white
Gaussian noise at about 50 dB higher than a NBFM signal on the same
frequency band.  There are still some problems with it though.  I believe
for highly-nonstationary interfering signals (signals that are on and off
constantly with a short duration in between) it might not work. 

However, my solution doesn't fix the problem where the ADC is completely
saturated, which according to wikipedia is the case for CDMA.  Perhaps as I
continue studying, at some point I will extend my project to this level.  

Marcus:  The USRP most likely will not have enough logic cells to handle the
amount of programming I need to do =D.  I have USRP1, and the 12-bit ADC is
also a limitation.  It would also be more expensive for me to go with the
higher USRPs because I need two receive inputs and a transmit port all
working at the same time while as for the Altera FPGA already comes with 2
ADC inputs and 2 DACs, and it just needs an RF frontend, which would be a
nice homebrew project.  ;)



Moeller wrote:
> 
> On 13.01.2011 22:59, sirjanselot wrote:
>> The reason why I do not want to attenuate is because I want to receive a
>> high-powered signal and low-powered signal at the same frequency.  
> 
> Can you be more specific? This is an interesting topic.
> In most cases the signals cannot be separated any more.
> You have certain chances if you want to seperate spread spectrum
> signals or TDMA signals. For TDMA you can operate above the saturation
> level, below the destruction level. For direct sequence CDMA this is
> similar to the near far effect, the high powered signal will create
> interference. Your success will then also depend on the spreading factor
> (correlation lengths).
> 
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