OFDM in flat fading

[Neetukath]

Can any body let me know whether the simulations to determine the BER Vs
SNR for flat fading channels( May be rayleigh or Rician) are valid or not


Thanks,
Neetu

[julius]

On Aug 23, 12:18*pm, "Neetukath" <[email protected]> wrote:
> Can any body let me know whether the simulations to determine the BER Vs.
> SNR for flat fading channels( May be rayleigh or Rician) are valid or not?
>
> Thanks,
> Neetu

Does not validity depend on what the simulation is supposed to
simulate? :-).

Is your channel of interest flat fading or not? :-)

[Steve Pope]

On Aug 23, 12:18*pm, "Neetukath" <[email protected]> wrote:

> Can any body let me know whether the simulations to determine
> the BER Vs. SNR for flat fading channels( May be rayleigh
> or Rician) are valid or not?

"Flat fading" means the fade is across the entire band of
interest, therefore all OFDM tones are attenuated equally,
therefore the simulation is the same for AWGN, just with
a lower SNR corresponding to the amount of fade.

Unless they've invented a new definitin of "flat fading"
recently, which is possible.

The opposite of flat fading is frequency-selective fading.

Steve

[[email protected]]

On Aug 27, 2:40 pm, [email protected] (Steve Pope) wrote:
> On Aug 23, 12:18 pm, "Neetukath" <[email protected]> wrote:
>
> > Can any body let me know whether the simulations to determine
> > the BER Vs. SNR for flat fading channels( May be rayleigh
> > or Rician) are valid or not?
>
> "Flat fading" means the fade is across the entire band of
> interest, therefore all OFDM tones are attenuated equally,
> therefore the simulation is the same for AWGN, just with
> a lower SNR corresponding to the amount of fade.
>
> Unless they've invented a new definitin of "flat fading"
> recently, which is possible.
>
> The opposite of flat fading is frequency-selective fading.
>
> Steve

Not new, just less general. How about slow and fast frequency flat
fading? What you describe above is certainly one simulation approach
for flat slow fading channels but how would the model the inter-
carrier-interference in flat fast fading?

col

[Steve Pope]

<[email protected]> wrote:

>On Aug 27, 2:40 pm, [email protected] (Steve Pope) wrote:

>> "Flat fading" means the fade is across the entire band of
>> interest, therefore all OFDM tones are attenuated equally,
>> therefore the simulation is the same for AWGN, just with
>> a lower SNR corresponding to the amount of fade.

>> Unless they've invented a new definitin of "flat fading"
>> recently, which is possible.

>> The opposite of flat fading is frequency-selective fading.

>Not new, just less general. How about slow and fast frequency flat
>fading? What you describe above is certainly one simulation approach
>for flat slow fading channels but how would the model the inter-
>carrier-interference in flat fast fading?

Let's see. "Medium-fast" flat fading would have fade levels
which vary between OFDM symbols but which are constant within
a symbol. Even faster flat-fading would vary within a symbol.
To a first order the latter effect would just show up as
more noise.

You wouldn't need to ocnsider ICI unless it is frequency-selective.
(Offhand opinion.)

Steve

[Eric Jacobsen]

On Wed, 27 Aug 2008 19:17:51 +0000 (UTC), [email protected]
(Steve Pope) wrote:

><[email protected]> wrote:
>
>>On Aug 27, 2:40 pm, [email protected] (Steve Pope) wrote:
>
>>> "Flat fading" means the fade is across the entire band of
>>> interest, therefore all OFDM tones are attenuated equally,
>>> therefore the simulation is the same for AWGN, just with
>>> a lower SNR corresponding to the amount of fade.
>
>>> Unless they've invented a new definitin of "flat fading"
>>> recently, which is possible.
>
>>> The opposite of flat fading is frequency-selective fading.
>
>>Not new, just less general. How about slow and fast frequency flat
>>fading? What you describe above is certainly one simulation approach
>>for flat slow fading channels but how would the model the inter-
>>carrier-interference in flat fast fading?
>
>Let's see. "Medium-fast" flat fading would have fade levels
>which vary between OFDM symbols but which are constant within
>a symbol. Even faster flat-fading would vary within a symbol.
>To a first order the latter effect would just show up as
>more noise.
>
>You wouldn't need to ocnsider ICI unless it is frequency-selective.
>(Offhand opinion.)
>
>Steve

Even if it were frequency selective ICI isn't a problem. That's the
benefit of "orthogonality" between subcarriers. Frequency
selectivity by itself doesn't disturb the orthogonality.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php

[julius]

On Aug 27, 4:54*pm, Eric Jacobsen <[email protected]> wrote:
>
> Even if it were frequency selective ICI isn't a problem. * That's the
> benefit of "orthogonality" between subcarriers. * Frequency
> selectivity by itself doesn't disturb the orthogonality.
>

I think this is not quite correct. The orthogonalization comes from
the FFT operator, which assumes periodic convolution. So if the
channel length is larger than the cyclic prefix length, aka frequency
selectivity is too severe, then ICI will happen, too, since the effect
of convolving with the channel can no longer be modeled by periodic
convolution.

[Eric Jacobsen]

On Wed, 27 Aug 2008 15:33:00 -0700 (PDT), julius <[email protected]>
wrote:

>On Aug 27, 4:54*pm, Eric Jacobsen <[email protected]> wrote:
>>
>> Even if it were frequency selective ICI isn't a problem. * That's the
>> benefit of "orthogonality" between subcarriers. * Frequency
>> selectivity by itself doesn't disturb the orthogonality.
>>
>
>I think this is not quite correct. The orthogonalization comes from
>the FFT operator, which assumes periodic convolution. So if the
>channel length is larger than the cyclic prefix length, aka frequency
>selectivity is too severe, then ICI will happen, too, since the effect
>of convolving with the channel can no longer be modeled by periodic
>convolution.

Yup. I was assuming the OFDM system was properly designed for the
channel, i.e., there is adequate CP length, in which case there's no
ICI due to the frequency selectivity.

But frequency synchronization is never perfect, so there's always ICI
in practice.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php

[Steve Pope]

julius <[email protected]> wrote:

>On Aug 27, 4:54*pm, Eric Jacobsen <[email protected]> wrote:

>> Even if it were frequency selective ICI isn't a problem. * That's the
>> benefit of "orthogonality" between subcarriers. * Frequency
>> selectivity by itself doesn't disturb the orthogonality.

>I think this is not quite correct. The orthogonalization comes from
>the FFT operator, which assumes periodic convolution. So if the
>channel length is larger than the cyclic prefix length, aka frequency
>selectivity is too severe, then ICI will happen, too, since the effect
>of convolving with the channel can no longer be modeled by periodic
>convolution.

Well, here's my thinking, which is orthogonal to either of the above.

If the channel is frequency-selective, then you have the possible
scenario of energy from stronger tones spilling over onto the weaker
tones. This is going to be more of a noticeable impairment than
in the flat-fading case. You still will get ICI in the latter
case (from non-linearities or from different FFT timebases if you fail
to correct for this), but you won't get disproportionate
ICI on weaker tones.

Steve

[Ravi Srikantiah]

On Aug 28, 5:02*am, [email protected] (Steve Pope) wrote:
> julius *<[email protected]> wrote:
> >On Aug 27, 4:54*pm, Eric Jacobsen <[email protected]> wrote:
> >> Even if it were frequency selective ICI isn't a problem. * That's the
> >> benefit of "orthogonality" between subcarriers. * Frequency
> >> selectivity by itself doesn't disturb the orthogonality.
> >I think this is not quite correct. *The orthogonalization comes from
> >the FFT operator, which assumes periodic convolution. *So if the
> >channel length is larger than the cyclic prefix length, aka frequency
> >selectivity is too severe, then ICI will happen, too, since the effect
> >of convolving with the channel can no longer be modeled by periodic
> >convolution.
>
> Well, here's my thinking, which is orthogonal to either of the above.
>
> If the channel is frequency-selective, then you have the possible
> scenario of energy from stronger tones spilling over onto the weaker
> tones. *This is going to be more of a noticeable impairment than
> in the flat-fading case. *You still will get ICI in the latter
> case (from non-linearities or from different FFT timebases if you fail
> to correct for this), but you won't get disproportionate
> ICI on weaker tones.
>
> Steve

That still wraps back to the adequate CP-length argument. If there's
so large a variation in the channel from one tone to the other as to
cause disproportionate increase in interference on a neighbouring
tone, the sudden channel variation in frequency would result in the
impulse response spreading out over a large time duration exceeding
the designed CP. There's a limit to how much the CP can be increased,
and that depends on the symbol duration, which in turn depends on the
carrier spacing. A smaller carrier spacing produces an intermediate
carrier which is not as disproportionately affected by a sudden
channel fade, and allows for a larger CP.

[Steve Pope]

Ravi Srikantiah <[email protected]> wrote:

>On Aug 28, 5:02*am, [email protected] (Steve Pope) wrote:

>> Well, here's my thinking, which is orthogonal to either of the above.

>> If the channel is frequency-selective, then you have the possible
>> scenario of energy from stronger tones spilling over onto the weaker
>> tones. *This is going to be more of a noticeable impairment than
>> in the flat-fading case. *You still will get ICI in the latter
>> case (from non-linearities or from different FFT timebases if you fail
>> to correct for this), but you won't get disproportionate
>> ICI on weaker tones.

>That still wraps back to the adequate CP-length argument. If there's
>so large a variation in the channel from one tone to the other as to
>cause disproportionate increase in interference on a neighbouring
>tone, the sudden channel variation in frequency would result in the
>impulse response spreading out over a large time duration exceeding
>the designed CP.

Okay you're lost me. What is meant by "sudden channel variation
in frequency"? Frequency response, or frequency offset, or
something else? Does "sudden" mean time-varying?

And, a channel can be frequency selective and stationary, and
can have ICI and be stationary. The ICI can originate from
nonlinearity/distortion. In that case, the CP doesn't necessarily
enter into it.

Steve

[Ravi Srikantiah]

On Aug 29, 10:06*pm, [email protected] (Steve Pope) wrote:
> Ravi Srikantiah *<[email protected]> wrote:
>
> >On Aug 28, 5:02*am, [email protected] (Steve Pope) wrote:
> >> Well, here's my thinking, which is orthogonal to either of the above.
> >> If the channel is frequency-selective, then you have the possible
> >> scenario of energy from stronger tones spilling over onto the weaker
> >> tones. *This is going to be more of a noticeable impairment than
> >> in the flat-fading case. *You still will get ICI in the latter
> >> case (from non-linearities or from different FFT timebases if you fail
> >> to correct for this), but you won't get disproportionate
> >> ICI on weaker tones.
> >That still wraps back to the adequate CP-length argument. If there's
> >so large a variation in the channel from one tone to the other as to
> >cause disproportionate increase in interference on a neighbouring
> >tone, the sudden channel variation in frequency would result in the
> >impulse response spreading out over a large time duration exceeding
> >the designed CP.
>
> Okay you're lost me. *What is meant by "sudden channel variation
> in frequency"? *Frequency response, or frequency offset, or
> something else? *Does "sudden" mean time-varying?

Wrong choice of words. I was referring exclusively to the frequency
domain - no change in time. By "sudden" I was referring to a large
rate of change of the channel attenuation across frequency.

>
> And, a channel can be frequency selective and stationary, and
> can have ICI and be stationary. *The ICI can originate from
> nonlinearity/distortion. *In that case, the CP doesn't necessarily
> enter into it.
>
> Steve

[Steve Pope]

Ravi Srikantiah <[email protected]> wrote:

>On Aug 29, 10:06*pm, [email protected] (Steve Pope) wrote:

>> Okay you're lost me. *What is meant by "sudden channel variation
>> in frequency"? *Frequency response, or frequency offset, or
>> something else? *Does "sudden" mean time-varying?

>Wrong choice of words. I was referring exclusively to the frequency
>domain - no change in time. By "sudden" I was referring to a large
>rate of change of the channel attenuation across frequency.

Thanks. Yes, I agree that a large variation in magnitude
of channel response over a short range of frequency implies
a long impulse response, perhaps longer than the CP in some systems.

Steve