Hi Ekko,
> i connect the usrp“tx and rx pin whit a wire to imitate that no
> error,so i think the ber should be zero.
If you still think that, you have understood neither mine nor Marcus
Leech's email; I'm afraid I can only ask you to read them again.
> my problem is that if i want to use uhd_sink and source to test this
> grc,is there some other work i need to do compare with the connect of
> virtual sink +source.
Again: Virtual sink/source are not doing *anything* to the signal,
whereas the USRP does.
I feel like I shouldn't be simplifying as much, but consider this:
digital complex baseband is */in theory /*///equivalent/ to a real RF
bandpass signal, but it's not the same.
If you directly connect digital things (which is what virtual
sink/source does) then it would be like taking a cake recipe, and giving
it to your neighbor, to read it. Your neighbor will have no problems
reading it.
Transmitting something with a real piece of SDR hardware and receiving
it is more like:
1. writing down that recipe (generating the digital signal)
2. baking that cake, (digital-to-analog conversion, yielding a physical
baseband signal)
3. putting that in a box, (mixing it up to the carrier frequency)
4. bringing that box to a post office, (transporting it through a
coax/microwave connection to your channel)
5. sending that box to your neighbor, (sending the RF signal through
that channel, i.e. your direct cable)
6. your neighbor getting that box, (your receiver getting that signal)
7. taking the cake out of the box, (receiver mixes down to I/Q baseband),
8. and decomposing the cake to finally (analog-to-digital conversion)
9. get an estimate for the original recipe (SDR algorithm)
You will *never* get a zero BER (which means of *infinitely *many
recipes you transmit with the bake-cake-box-post-box-cake-decompose
method, *every single one *is perfectly estimated), but you can get one
that is *sufficiently small *for a given application if you can get your
SNR to be good enough.
Best regards,
Marcus
On 11/05/2015 02:37 PM, chai E wrote:
> hello marcus
> i connect the usrp“tx and rx pin whit a wire to imitate that no
> error,so i think the ber should be zero.
> my problem is that if i want to use uhd_sink and source to test this
> grc,is there some other work i need to do compare with the connect of
> virtual sink +source.
> this is my question : with the connect of virtual sink+source the ber
> is zero ,i think with the connect of uhd_sink+source the ber should be
> 0.01 or other number clear to zero,but when i use uhd_sink +source the
> ber is clear to 1.
> thank for you reply
>
> --Ekko
>
> 2015-11-03 22:49 GMT+08:00 Marcus Müller <marcus.muel...@ettus.com
> <mailto:marcus.muel...@ettus.com>>:
>
> Hi Ekko,
>
> >difference between uhd_sink+uhd_source with
> virtual_sink+virtual_source
>
> uhd_sink / _source talk to radio *hardware*, virtual sink and
> source are just "invisible" GRC connections, and simply talk to
> the block that is attached to them.
>
> > in this grc when i use virtual_sink+virtual_source,the ber is zero
>
> obviously, because it's simply a direct connection, no noise,
> hence, no errors.
>
> > but when i use uhd_sink+uhd_source, and i connect the B200's tx
> and rx port with a RF wire. but the ber is not zero.
>
> Real world radios have noise (and other imperfections), and hence
> there's bit errors. That's the core problem of digital
> communications, and has been for the last couple of decades.
>
> > how can i use uhd_sink+uh_source just like
> virtual_sink+virtual_source
>
> You can't. One reproduces perfectly, the other is the real RF
> world. Getting information across the wireless channel without
> error is what science has been working on, and since Shannon we
> know that you can not achieve a transmission with arbitrarily low
> BER over a channel with limited SNR at arbitrary speed. Every real
> channel has limited SNR.
>
> What you do is apply your signal processing and communication
> theory skills to counteract the effects of the channel, check
> correctness and correct errors (if possible), possibly make the
> sender retransmit etc; but you will never break Shannon's limits.
>
> Best regards,
> Marcus
>
>
> On 11/03/2015 01:58 PM, chai E wrote:
>> hello all
>> i have a question about the difference between
>> uhd_sink+uhd_source with virtual_sink+virtual_source
>> the accessory is the grc file i am running.
>> in this grc when i use virtual_sink+virtual_source,the ber is
>> zero ,in other words,the sorce file is same with the dest file
>> but when i use uhd_sink+uhd_source, and i connect the B200's tx
>> and rx port with a RF wire. but the ber is not zero.
>> i want to konw what is the difference ,and how can i use
>> uhd_sink+uh_source just like virtual_sink+virtual_source
>>
>>
>> --Ekko
>>
>>
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