Hi Martin:
Thank you so much for your kindly reply.So kindly of you.I
understand it.
Best regards,
zs
At 2015-02-02 18:24:25, "Martin Braun" <martin.br...@ettus.com> wrote:
>I'll try and keep it short and simple:
>
>First: What's the difference between a cyclic shift and a non-cyclic
>shift? It means that sub-carriers from one end are moved the other end.
>So, if we have 8 subcarriers arranged like this:
>
>0, 1, 2, 3, 4, 5, 6, 7
>
>and do a cyclic shift, we get something like
>
>1, 2, 3, 4, 5, 6, 7, 0
>
>Right?
>
>OK, here's why this would never even be an issue in practice: As you
>know, a subcarrier has the same width as the corresponding FFT bin. But
>you never use all FFT bins. Best example is Wi-Fi, where you have an FFT
>length of 64 in the modulation/demodulation phase, but only use 52
>carriers (outside of the DC carrier). So, all bins at the edge of your
>Nyquist zone are not used. Using the example before, it would be
>something like:
>
>x, x, 0, 1, 2, 3, x, x
>
>Now, if do a shift (cyclic or not) of length 1 (note that in the S&C
>setup, you'd have multiples of 2 for the shift), you get this:
>
>x, 0, 1, 2, 3, x, x, x
>
>Now, in the implementation, I can copy the left-most 'x' to the right,
>but what's the point? There's no information there. So I just do a
>memcpy with an offset. Much simpler, does the same.
>
>So, you might be tempted to say "non-cyclic or cyclic, it doesn't really
>matter". In practice, given what I just discussed, that's kind of
>correct. But, assume you have a really, really big frequency offset.
>Your relevant carriers will be cut off by the resampling filters (or
>even the analog filters) before they even hit the FFT. So, you'd have
>something like this:
>
>1, 2, 3, x, x, x, x, x
>
>No shifting, cyclic or non-cyclic, can save you now.
>
>So where does this cyclic stuff come from?
>
>Well, in the pure discrete domain, when you do frequency shifts by
>multiplying with complex sinusoids, a frequency offset *will* be
>"cyclic". You might even run into this in reality even, because in
>Schmidl & Cox, before you correct the integer FO, you correct the fine
>FO with such a sinusoid multiplication. So, cyclic isn't "wrong", per
>se. But it's not really required.
>
>However, if you take away a single piece of information from this, let
>it be this: In practice, you design your parameters such that it doesn't
>matter if you shift cyclically or non-cyclically. In that case, the
>latter is less computationally heavy.
>
>M
>
>On 02/02/2015 09:09 AM, zs wrote:
>> Hi Martin:
>> Thank you in advance.I have read many papers on the
>> topic "integer cfo of OFDM".They all said it make the subcarriers cyclic
>> shift.
>> For example:
>> "
>> http://nutaq.com/en/blog/brief-overview-frequency-synchronization-ofdm "
>> It said "Integer CFO does not introduce ICI between sub-carriers, but
>> does introduce a cyclic shift of data sub-carriers..."
>> Is it right?And the source code ofdm_chanest_vcvc_impl.cc is right?Maybe
>> I'm wrong.Can you explain it?Thank you.
>> Best regards,
>> zs
>>
>>
>>
>>
>>
>>
>>
>> At 2015-01-25 19:27:30, "Martin Braun" <martin.br...@ettus.com> wrote:
>>>On 01/25/2015 08:10 AM, zs wrote:
>>>> Hi Martin:
>>>> Thank you for your reply.And we know this block do
>>>> "Estimate channel and coarse frequency offset for OFDM from
>>>> preambles".And the coarse frequency offset is a integer.And it make the
>>>> subcarriers cyclic shift.Just illustration as this:
>>>
>>>No cyclic shift. There must be enough space between the out subcarriers
>>>and the Nyquist zone boundaries. Hope this clears things up!
>>>
>>>And please respond to the list.
>>>
>>>M
>>>
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>>
>>
>>
>
>
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