# Ofdm\_chanest\_vcvc\_impl.cc

**URL:** <https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948>\
**Category:** GNU Radio\
**Created:** [January 24, 2015, 8:44am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948 "2015-01-24T08:44:59Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [January 24, 2015, 8:44am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/1 "2015-01-24T08:44:59Z")

</div>

Hi all:  
Thank you in advance.I have read the source  
code:ofdm\_chanest\_vcvc\_impl.cc.But i have a question.

```
void
ofdm_chanest_vcvc_impl::get_chan_taps(
const gr_complex *sync_sym1,
const gr_complex *sync_sym2,
int carr_offset,
std::vector<gr_complex> &taps)
{
  const gr_complex *sym = ((d_n_sync_syms == 2) ? sync_sym2 : 

```

sync\_sym1);  
std::fill(taps.begin(), taps.end(), gr\_complex(0, 0));  
int loop\_start = 0;  
int loop\_end = d\_fft\_len;  
if (carr\_offset \> 0) {  
loop\_start = carr\_offset;  
} else if (carr\_offset \< 0) {  
loop\_end = d\_fft\_len + carr\_offset;  
}

```
  for (int i = loop_start; i < loop_end; i++) {
if ((d_ref_sym[i-carr_offset] != gr_complex(0, 0))) {
  taps[i-carr_offset] = sym[i] / d_ref_sym[i-carr_offset];
}
  }

```

//Question:  
//For example,when the carr\_offset=2,then:  
//loop\_start =2;  
//for example loop\_end=64  
// for (int i =2; i \< loop\_end; i++) {  
if ((d\_ref\_sym[i-carr\_offset] != gr\_complex(0, 0))) {  
taps[i-carr\_offset] = sym[i] / d\_ref\_sym[i-carr\_offset];  
}  
}  
//  
Now the taps only have values on taps[0]—taps[63-2],and taps[62]  
taps[63] will be equal to zero.It means that the channel estimation have  
value 0.It is wrong?

```
  if (d_interpolate) {
for (int i = d_first_active_carrier + 1; i < d_last_active_carrier; 

```

i += 2) {  
taps[i] = taps[i-1];  
}  
taps[d\_last\_active\_carrier] = taps[d\_last\_active\_carrier-1];  
}

```
  if (d_eq_noise_red_len) {
// TODO
// 1) IFFT
// 2) Set all elements > d_eq_noise_red_len to zero
// 3) FFT
  }
}
        Thank you.

```

Best regards,  
zswx

---

<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [January 25, 2015, 10:57am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/2 "2015-01-25T10:57:05Z")

</div>

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:  
For example,we have 5 subcarriers,cfo=2.  
Right subcarriers:s1,s2,s3,s4,s5  
Wrong subcarriers:s4,s5,s1,s2,s3  
So the channel estimation coefficients will not be zeros.Is it right?  
Best regards,  
zswx

---

<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [January 25, 2015, 10:57am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/3 "2015-01-25T10:57:04Z")

</div>

On 01/24/2015 08:43 AM, zs wrote:

> Now the taps only have values on taps[0]—taps[63-2],and taps[62]  
> taps[63] will be equal to zero.It means that the channel estimation have  
> value 0.It is wrong?

They will be zero, and it’s not wrong. What else should be written here?  
There’s no useful information outside the frame.

M

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<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [February 2, 2015, 9:43am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/4 "2015-02-02T09:43:29Z")

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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](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

---

<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [January 26, 2015, 5:33am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/5 "2015-01-26T05:33:04Z")

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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

---

<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [February 2, 2015, 11:26am UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/6 "2015-02-02T11:26:05Z")

</div>

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

---

<div class="post-metadata">

**Author:** ![zszszs](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zszszs](https://www.ruby-forum.com/u/zszszs)\
**Post date:** [February 2, 2015, 3:03pm UTC](https://www.ruby-forum.com/t/ofdm-chanest-vcvc-impl-cc/233948/7 "2015-02-02T15:03:35Z")

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Hi Martin:  
Thank you so much for your kindly reply.So kindly of  
you.I understand it.  
Best regards,  
zs