Re: [Discuss-gnuradio] shared SDR receivers at repeater sites

[Marcus Müller]

Wait, is this discussion still about forwarding spectrum from band A over band B, or is it already also incorporating IP-based access over "classical" internet access technology? In the latter case, I'd really say that that not having multicast is not really a problem, here - the way to go would be to extract the channels of interest at the receiver, decode them, and forward them only occupying the actual data rate - which for the typical 2m channel is ridiculously low compared to what your average ISP can offer you as uplink.

Bonus realization is that decode and forward (i.e. decoding, and then encoding the decoded signal in the same manner as the original transmitter for relaying it, be it digital or analog voice FM or whatever), under the assumption that your relay has a decoder that isn't worse than what you had done locally with the same digital samples, always leads to a better overall SNR then what  forwarding the raw input signal would have had. (Think in terms of noise figures - in decode and forward, your overall noise figure is basically is close to that of the worse receiver, be it that of your relay or final receiver, whereas in a "bent digital pipe and upmix" system, it would be the product of all noise figures.)

Now, if we talk about forwarding the raw spectrum from band A over band B, maybe because the relay itself can't know how/what to decode, and need to add additional data (e.g. call signs of the relay), it's mandatory that we occupy more bandwidth*power in B than we receive in A, lest we lose information. Luckily, with increasing carrier frequency, bandwidth usually gets easier to acquire; sadly, the same relation exists for carrier frequency and attenuation...

Now, sampling signal with bit depth B, of which only E<B bits are actually effective, putting these bits into symbols, and transmitting those including pulse shaping, channel coding/forward error correction and framing as packets is bound to increase the bandwidth a lot, unless you can use a modulation that has a very high spectral efficiency, i.e. bit/s/Hz, which in turn means that the receiver of these packets needs to have a much more impressive SNR than what can get reconstructed as signal contained in the packets. That doesn't sound too tempting!

Best regards,
Marcus

Am 4. Februar 2016 19:02:56 MEZ, schrieb Daniel Pocock <address@hidden>:

On 04/02/16 18:56, Martijn Moeling wrote:
 that is why websdr is the way to go, the software is free and receivers can be cheap (like one or more rtl dongles)
 


Not quite... while WebSDR could be accessed over amateur bands using
packet modes, I didn't see any evidence that it supports IP multicasting
the whole spectrum from the radio.  Therefore, if several people try to
access it simultaneously, it will send a separate IP flow to each of
them, which is not scalable.

Regards,

Daniel


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