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

Sorry, I had brain rottage.
So, 4 dBW is a little more than 2W (unlike 40dBW, which is 10kW, indeed). Also, small errors in the SNR formula (suddenly dropped a -1, but that doesn't really hurt much, there).

On 05.02.2016 11:51, Marcus Müller wrote:

Hi Daniel,

On 04.02.2016 22:49, Daniel Pocock wrote:

To give a more specific example:    a) SDR device sampling the 2 meter band (144 - 148 MHz), this input  range is locked and can't be changed by users    b) using something like the USRP B200  - it can do 61 Million samples/sec, 12 bit samples, 732 Mbit/sec  - but maybe that sample rate is not needed for a band that is 4 MHz wide...

No, 4MS/s should suffice (if you can live with the filter roll-off at the band edges).
Still, not wasting too much signal quality: for 8bit samples in I and Q, 4MS/s * 2B/S = 8MB/s = 64Mb/s

c) an instance of GNU Radio taking all the samples and encapsulating  them into packets    d) transmitting to local users  layer 1/physical: 23cm or 13cm, using 8 - 10 MHz bandwidth

So, since AX.25 doesn't specify a modulation (and if we used the AX.25 that seem to be dominant, we'd end up with a data rate whopping three to four orders of magnitude too low), let's look at the data rate here to determine a minimal modulation order and SNR:

Shannon Channel Capacity says that our bitrate is bound, if we want to achieve transmission with arbitrarily low bit error rate over a channel of bandwidth and given :




I'd say, wow, for a wide-range 10MHz link, that's a pretty good minimum SNR!

Now, for the modulation:
; i.e. our modulation would have to have at least that many bits per symbol, which means at least 85 different states.

Effectively, this calls for something like a 128 QAM, or a 256 QAM (from a gut feeling, this makes sense if SNR is in fact quite a bit higher than 19.2 dB) ; more likely the latter, because it's a square number, making the constellation easier to implement, and also, because we'll definitely want some bitrate headroom to add redundancy for channel coding/forward error correction, and , which is pretty handy for code implementation.
Whether to send those symbols in time-domain or over a set of OFDM or filterbank carriers would be up for discussion; from an equalization point of view, using multiple carriers seems to make a lot of sense; those 10MHz will probably not be nicely flat.

layer 2: AX.25 (with repeater callsign)

As calculated above, not that much room in those 10 MHz for framing overhead, the relatively ineffective CRC32 and the 5-bit-stuffing, to be honest... I don't think AX.25 is the optimum choice here. I'd rather go for something that has a usuable preamble for equalization, and a more compressed header, and complements the FEC used more nicely.

layer 3: IP multicast (UDP packets)

Why that? If we're going to be fully utilizing the link with sample packets, anyway, it's not really necessary to have different logical endpoints, right?

e) Receiving stations would receive the UDP multicast packets and feed  them as input to a flow graph in a local instance of GNU Radio    I can imagine there may be risks with packet loss and the receiving  users may need directional antennas.  As it would be a licensed amateur  repeater, it would be able to legally put out more watts than a wifi  router though.

The point is that I lack knowledge about typical SNRs for the 13cm (2.4GHz) or the 23cm (1.3GHz) bands; problematic for me sounds that free space loss for 23cm over a distance of 10km would be around .
So with an minimum SNR of and a thermal noise floor of  , and assuming a relatively nice receiver with a noise figure :

You'd need a transmit power of
.

So, you can't reliably talk to someone further away than 10km with a 10kW TX, assuming you have no antenna gains. Sure, a very nicely aligned dish with low losses can achieve almost 30dB, but that effectively only means that 1kW is enough for 10km.

Best regards,
Marcus

Regards,    Daniel