Re: [Discuss-gnuradio] uhd_sink+uhd_source VS virtual_sink+virtual_source

[chai E]

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 <address@hidden>:

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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