|Subject:||Re: [Discuss-gnuradio] Bit rate of source and output of PSK demod|
|Date:||Fri, 11 Dec 2015 13:38:48 +0100|
|User-agent:||Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0|
On 11.12.2015 10:21, Haaris wrote:
We don't. Bit rate has no meaning for these sources!
GNU Radio doesn't care or even know what the things you put in your samples are. For GNU Radio, it's just a sequence of things ("items" we call them, to make clear that we don't even know whether they are numbers, symbols, samples, bits...) that you exchange between blocks.
So, "sample" is the only relation between GNU Radio and signals. Hence, a sampled signal doesn't have something like a bandwidth in GNU Radio, and a random source doesn't have a bit rate -- it just produces random numbers, one each item.
Take the signal source: It "claims" to have a "frequency" and a "sample_rate" field, but in reality, this is only used to calculate the relative frequency of the generated signal, ie. the number of samples per period. That doesn't have anything to do with real world frequencies until you give it that meaning. So, if you set the frequency of a signal source to 1 and the sampling rate to 20, then the generated signal will have a period of 20 samples. If you set the frequency to 2e6 and the sampling rate to 40e6, exactly the same signal will be generated: A signal with a period of 20 samples.
The only difference that you can make is if you configure your graphical sink to act as if your signal was sampled at a given rate. So for example, take that freq=1/sampling rate=20 signal, and connect it to the scope plot, setting that scope plot's "sampling rate" setting to 20 -- then, your signal will be displayed as having a period of 1s. But it's really really just a setting for display, and that's what the display does: you tell it "assume this signal was sampled at x Hz", and the sink displays it as if it was actually sampled at that rate.
Especially, it doesn't say anything about how fast the samples are being processed. If you look at how many samples will go through a simple signal source->scope display flowgraphs, it will be in Megasamples/second, and again, that's totally independent of what you set your signal source or your visualization to. "sample rate" is really just a concept of how to interpret the numbers.
From the documentation tab in GRC:
The input is a complex modulated signal at baseband.So, you get out bytes (hence the single-byte output type), and a byte is either 0 or not, representing a received bit. The order is MSB first.
Packing is what you should do, here.
How does your PSK demod know where to start? How do you account for random phase?
Timing/phase synchronization as well as start-of-payload detection are non-optional in a receiver.
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