Re: [Discuss-gnuradio] Using two USRPs - how to synchronise a receiver (scan a range of frequencies) looking for a transmitter which is transmitting on a specific frequency

[Marcus Müller]

Hi Neil,

1) in this case, the method of choice would be to simply generate a 1MHz wide baseband signal and only fill the channel you're interested in with payload signal – the "Rotator", or the "Frequency Xlating FIR" or OFDM or something like a PFB synthesizer would be viable.

2) well, that very much depends on your channel, noise and signal models!!! Whatever you pick, you will probably need to write down a mathematical justification, a false alarm rate and a detection rate, soooo it might be worth reading into the theory of estimators and detectors. In any case, this sounds pretty simple to me: Use a PFB channelizer (the Doxygen explains how they work, and Tom Rondeau's blog has some info, too) to divide your 1MHz reception into 5 channels, and do independent detection concurrently on each of those. Detection can be done in a lot of ways. For your own signal, you'd probably look for something like a preamble after a matched filter (hint: you can use a matched filter as prototype filter for your PFB). For the question whether a channel is occupied by something else or not: uh, you'll need a signal model for that. That signal model would lead to something like an average signal power in each band of the same PFB as above, to something like an estimate for positions of indepent sinusoids in the whole 1 MHz passband, to cyclostationary estimation of unknown patterns in the signal to... whatever your signal model indicates might be a good idea!

3) Well, that sounds like a back channel! In other words, you're designing a Media Access Control method. A common thing to do is dividing the channel into time slots. Sender A selects channel, says "Hello, I use this channel now" on that channel, then switches to reception mode on the same channel. B hears the "Hello", detects it, and answers within a limited time window with a "Hey, yeah, sure, let's use this channel", which A listens for. As soon as it got that, it switches back to transmission and sends the data it wants to send.

But that's just one out of millions of imaginable schemes for this. If you think your channel will be seldom used at all, you might want to omit the handshake completely (assuming your detection probability is high enough, of course), as it just wastes time (if you just send whenever you please, that media access scheme would be called ALOHA. Understanding ALOHA should definitely be in the syllabus for every comms theory student!).

Best regards,

Marcus

On 07/20/2017 09:04 PM, neil shelley wrote:

Hi all,

We are currently doing a project, which aims to look for white space over a chosen frequency range,  automatically select the best channel (considering a 1 Mhz span with 5 channels, each of 200kHz). The transmitter would tune to the centre frequency of this best channel, and the receiver should be able to detect this transmission from the 1Mhz range, and perform a handshake with the transmitter so the transmitter knows the receiver has identified the chosen frequency.  The transmitter should then transmit the information (i.e a text file).

For the sending a text file part, we have been able to do this using QPSK and GMSK with partial success (however the text file is missing information at the end), the main part we have not been able to resolve is how the USRP receiver could detect the chosen frequency, and then let the transmitter know it has identified this.

Can anyone suggest some ways in which this can be done, I have broken it into a few stages:

1) How to get the transmitter to continuously transmit at the specific frequency  (i.e either 710.1, 710.3, 710.5, 710.7, 710.9 (Mhz) - what should be transmitted, e.g. a type of continuous code that could be identified by the receiver?

2) What is a good way to get the receiver to scan the 5 frequencies, and at each frequency, how could it identify that this is a transmission from the correct device, i.e using an access code, or a tag of some sort?, if it does not identify this as the required transmission then it will move on to the next frequency.

3) Once the receiver has detected the correct frequency, how can it inform the transmitter that it has identified this frequency is to be used and tell it to start transmitting the real information (the text file).

I know this is probably a lot to ask, but any help on any part of it, or some ideas that we can start to look at would be appreciated.

Thanks,

Neil.

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