FPGA Central

Hi,

I am working on a uncode 16-QAM OFDM simulation. The simulation works fin
when used in AWGN channel. I have a couple of questions now when i a
simulating performance in multipath channels.

1) What I want to understand through simulation is the effect of cyclic
prefix length on performance in a multipath channel.

For example consider that the channel impulse response is [0.7071 0 0 0 0
0 0 0 0 0.7071], which is a channel with impulse response length 10 (
static paths with no fading component).
Now if we consider four cases
1)cp=0,
2)cp=5,
3) cp=11
4) cp =15

The first two cases, based on theory should result in ISI. My questio
is:
when i perform the simulations i do not see much of a difference. Wil
the
difference be seen only at high SNR's ?


2) Presently, I transmit a known OFDM symbol at the beginning of the
simulation, At the receiver after FFT operation I divide the output with
the known OFDM symbol values and store them as channel state information
for the remaining simulation.
For all subsequent transmissions after the FFT operation i divide the
output with the stored channel state information ( Zero forcing in
frequency domain).
My question is even for cyclic prefix lengths greater than the impulse
response, don't we still need to do equalization operation at th
receiver
( something trivial as i am doing now).
Also can anyone suggest me a reference to other frequency domain
equalization approaches.

Thanks

>
>Hi,
>
>I am working on a uncode 16-QAM OFDM simulation. The simulation work
fine
>when used in AWGN channel. I have a couple of questions now when i am
>simulating performance in multipath channels.
>
>1) What I want to understand through simulation is the effect of cyclic
>prefix length on performance in a multipath channel.
>
>For example consider that the channel impulse response is [0.7071 0 0
0
>0 0 0 0 0.7071], which is a channel with impulse response length 10 (
>static paths with no fading component).
>Now if we consider four cases
>1)cp=0,
>2)cp=5,
>3) cp=11
>4) cp =15
>
>The first two cases, based on theory should result in ISI. My question
>is:
>when i perform the simulations i do not see much of a difference. Will
>the
>difference be seen only at high SNR's ?
>

Having a cyclic prefix that is too small will cause one OFDM symbol t
interfere with another, which causes inter-carrier interference (ICI) i
the frequency domain.

If the cyclic-prefix is sufficiently long, you will mitigate ICI, bu
within a frequency bin, you will still have ISI.

Both of these effects will defocus the constellations.

When you say you don't see much of a difference, what criterion are yo
using to compare the two scenarios?

>
>2) Presently, I transmit a known OFDM symbol at the beginning of the
>simulation, At the receiver after FFT operation I divide the output with
>the known OFDM symbol values and store them as channel state information
>for the remaining simulation.
>For all subsequent transmissions after the FFT operation i divide the
>output with the stored channel state information ( Zero forcing in
>frequency domain).
>My question is even for cyclic prefix lengths greater than the impulse
>response, don't we still need to do equalization operation at the
>receiver
>( something trivial as i am doing now).
>Also can anyone suggest me a reference to other frequency domain
>equalization approaches.
>
>Thanks
>
>

>>
>>Hi,
>>
>>I am working on a uncode 16-QAM OFDM simulation. The simulation works
>fine
>>when used in AWGN channel. I have a couple of questions now when i am
>>simulating performance in multipath channels.
>>
>>1) What I want to understand through simulation is the effect of cyclic
>>prefix length on performance in a multipath channel.
>>
>>For example consider that the channel impulse response is [0.7071 0 0 0
>0
>>0 0 0 0 0.7071], which is a channel with impulse response length 10 (
>>static paths with no fading component).
>>Now if we consider four cases
>>1)cp=0,
>>2)cp=5,
>>3) cp=11
>>4) cp =15
>>
>>The first two cases, based on theory should result in ISI. My question
>>is:
>>when i perform the simulations i do not see much of a difference. Will
>>the
>>difference be seen only at high SNR's ?
>>
>
>Having a cyclic prefix that is too small will cause one OFDM symbol to
>interfere with another, which causes inter-carrier interference (ICI) in
>the frequency domain.
>
>If the cyclic-prefix is sufficiently long, you will mitigate ICI, but
>within a frequency bin, you will still have ISI.
>
>Both of these effects will defocus the constellations.
>
>When you say you don't see much of a difference, what criterion are you
>using to compare the two scenarios?
>
>>
>>2) Presently, I transmit a known OFDM symbol at the beginning of the
>>simulation, At the receiver after FFT operation I divide the outpu
with
>>the known OFDM symbol values and store them as channel stat
information
>>for the remaining simulation.
>>For all subsequent transmissions after the FFT operation i divide the
>>output with the stored channel state information ( Zero forcing in
>>frequency domain).
>>My question is even for cyclic prefix lengths greater than the impulse
>>response, don't we still need to do equalization operation at the
>>receiver
>>( something trivial as i am doing now).
>>Also can anyone suggest me a reference to other frequency domain
>>equalization approaches.
>>
>>Thanks
>>
>>
>
>
>

Hi Mike,

I am comparing performance in terms of bit error probability vs signal t
noise ratio.

>>
>>Hi,
>>
>>I am working on a uncode 16-QAM OFDM simulation. The simulation works
>fine
>>when used in AWGN channel. I have a couple of questions now when i am
>>simulating performance in multipath channels.
>>
>>1) What I want to understand through simulation is the effect of cyclic
>>prefix length on performance in a multipath channel.
>>
>>For example consider that the channel impulse response is [0.7071 0 0 0
>0
>>0 0 0 0 0.7071], which is a channel with impulse response length 10 (
>>static paths with no fading component).
>>Now if we consider four cases
>>1)cp=0,
>>2)cp=5,
>>3) cp=11
>>4) cp =15
>>
>>The first two cases, based on theory should result in ISI. My question
>>is:
>>when i perform the simulations i do not see much of a difference. Will
>>the
>>difference be seen only at high SNR's ?
>>
>
>Having a cyclic prefix that is too small will cause one OFDM symbol to
>interfere with another, which causes inter-carrier interference (ICI) in
>the frequency domain.
>
>If the cyclic-prefix is sufficiently long, you will mitigate ICI, but
>within a frequency bin, you will still have ISI.
>
>Both of these effects will defocus the constellations.
>
>When you say you don't see much of a difference, what criterion are you
>using to compare the two scenarios?
>
>>
>>2) Presently, I transmit a known OFDM symbol at the beginning of the
>>simulation, At the receiver after FFT operation I divide the outpu
with
>>the known OFDM symbol values and store them as channel stat
information
>>for the remaining simulation.
>>For all subsequent transmissions after the FFT operation i divide the
>>output with the stored channel state information ( Zero forcing in
>>frequency domain).
>>My question is even for cyclic prefix lengths greater than the impulse
>>response, don't we still need to do equalization operation at the
>>receiver
>>( something trivial as i am doing now).
>>Also can anyone suggest me a reference to other frequency domain
>>equalization approaches.
>>
>>Thanks
>>
>>
>
>
>

Hi Mike,

I am comparing performance in terms of bit error probability vs signal t
noise ratio.

Hello,

In order to check the effect of the CP, you take two values of CP one
less than the max delay of your muli-path channel and othe other CP is
greater then this delay...

When the CP is less than the max delay of the channel, ICI will occur..
when will consecueltly increase the BER at the same SNR value "compared
with the other case of CP < MAx delay"..

Hope that helps
Thanks

Hany



ramsay wrote:
> >>
> >>Hi,
> >>
> >>I am working on a uncode 16-QAM OFDM simulation. The simulation works
> >fine
> >>when used in AWGN channel. I have a couple of questions now when i am
> >>simulating performance in multipath channels.
> >>
> >>1) What I want to understand through simulation is the effect of cyclic
> >>prefix length on performance in a multipath channel.
> >>
> >>For example consider that the channel impulse response is [0.7071 0 0 0
> >0
> >>0 0 0 0 0.7071], which is a channel with impulse response length 10 (
> >>static paths with no fading component).
> >>Now if we consider four cases
> >>1)cp=0,
> >>2)cp=5,
> >>3) cp=11
> >>4) cp =15
> >>
> >>The first two cases, based on theory should result in ISI. My question
> >>is:
> >>when i perform the simulations i do not see much of a difference. Will
> >>the
> >>difference be seen only at high SNR's ?
> >>
> >
> >Having a cyclic prefix that is too small will cause one OFDM symbol to
> >interfere with another, which causes inter-carrier interference (ICI) in
> >the frequency domain.
> >
> >If the cyclic-prefix is sufficiently long, you will mitigate ICI, but
> >within a frequency bin, you will still have ISI.
> >
> >Both of these effects will defocus the constellations.
> >
> >When you say you don't see much of a difference, what criterion are you
> >using to compare the two scenarios?
> >
> >>
> >>2) Presently, I transmit a known OFDM symbol at the beginning of the
> >>simulation, At the receiver after FFT operation I divide the output
> with
> >>the known OFDM symbol values and store them as channel state
> information
> >>for the remaining simulation.
> >>For all subsequent transmissions after the FFT operation i divide the
> >>output with the stored channel state information ( Zero forcing in
> >>frequency domain).
> >>My question is even for cyclic prefix lengths greater than the impulse
> >>response, don't we still need to do equalization operation at the
> >>receiver
> >>( something trivial as i am doing now).
> >>Also can anyone suggest me a reference to other frequency domain
> >>equalization approaches.
> >>
> >>Thanks
> >>
> >>
> >
> >
> >
>
> Hi Mike,
>
> I am comparing performance in terms of bit error probability vs signal to
> noise ratio.