# Help with OFDM transmitted over multipath channel

Started by August 1, 2009
```Hi

I want to simulate the transmission of a set of OFDM symbols over a
multipath channel. The steps I intend to implement are

1) Generate a 64 subcarrier OFDM system as follows. Take binary data and
modulate it using a standard modulation scheme, say 64 QAM. Pass these
modulated symbols through a 64 IFFT block. Before passing it through the
IFFT block insert 16 pilots at 16 subcarriers. The output of the IFFT block
is the OFDM symbol. Let the number of samples of each OFDM symbol be N.(I
am not sure if there is any relation between the number of samples of an
OFDM symbol and the number of subcarriers, if there is any please
indicate).

2) Generate multipath channel coefficients using a standard multipath
channel model(Rayleigh fading). The number of samples of the multipath
channel generates is let's say Nc < N.

3) Apply this multipath channel to the OFDM symbol. This is nothing but a
filter whose impulse response is the channel coefficients generated.

I realize that the above steps are too generic and I may have missed some
important parameters. I would really appreciate if someone can confirm that
the above steps are in the right direction. Once I fix these steps, I will

Thanks
```
```sasuke <a.ssjgoku@gmail.com> wrote:

>I want to simulate the transmission of a set of OFDM symbols over a
>multipath channel. The steps I intend to implement are
>
>1) Generate a 64 subcarrier OFDM system as follows. Take binary data and
>modulate it using a standard modulation scheme, say 64 QAM. Pass these
>modulated symbols through a 64 IFFT block. Before passing it through the
>IFFT block insert 16 pilots at 16 subcarriers. The output of the IFFT block
>is the OFDM symbol. Let the number of samples of each OFDM symbol be N.(I
>am not sure if there is any relation between the number of samples of an
>OFDM symbol and the number of subcarriers, if there is any please
>indicate).

>2) Generate multipath channel coefficients using a standard multipath
>channel model(Rayleigh fading). The number of samples of the multipath
>channel generates is let's say Nc < N.

>3) Apply this multipath channel to the OFDM symbol. This is nothing but a
>filter whose impulse response is the channel coefficients generated.
>
>I realize that the above steps are too generic and I may have missed some
>important parameters.

One thing you are missing is any concept of a cyclc prefix.
This may not completely invalidate your simulation, but it
means you'll see both ISI and frequency-selective fading
degrading your signal, without being able to separate these
two effects.

Steve
```