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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] shared SDR receivers at repeater sites |
Date: | Fri, 5 Feb 2016 11:51:02 +0100 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0 |
Hi Daniel, On 04.02.2016 22:49, Daniel Pocock
wrote:
No, 4MS/s should suffice (if you can live with the filter roll-off at the band edges).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... Still, not wasting too much signal quality: for 8bit samples in I and Q, 4MS/s * 2B/S = 8MB/s = 64Mb/s 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: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 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. 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 2: AX.25 (with repeater callsign) 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?layer 3: IP multicast (UDP packets) 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 .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. 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 |
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