Use of channel estimation in OFDM

Hello,

I have a confusion with the use of channel estimation in the context of
OFDM.
Suppose subcarrier 'k' has a channel gain of H(k) and this subcarrier is
used to transmit data to some user U(k).

(1) If the required data to send is, for example, X(k), the transmitter
multiplies X(k) by 1/H(k) to compensate for the channel attenuation.

Then at the receiver, after FFT, the received signal at subcarrier 'k'
is:

Y(k) = H(k)*[X(k)/H(k)] + W(k)
Y(k) = X(k) + W(k)

So the desired data is available.
Does that mean that receiver does not need to know the channel gain?
provided the channel gain remained constant.

I read that OFDM needs channel information at the receiver for coherent
modulation. Can somebody tell me where this channel information is used in
the receiver? is it used in the FFT module? Does the FFT need to know the
phase change?

(2) Supposed transmitter send the data with an outdated channel gain,
H1(k). and the new channel gain is H2(k). Assume that the receiver knows
the current and outdated channel gains.

The received signal is:

Y(k) = H2(k)*[X(k)/H1(k)] + W(k)

This is supposedly the output from the FFT.

By the knowledge of H1(k) and H2(k), if the receiver perform the
following:

[H1(k)/H2(k)]*Y(k) = X(k) + W'(k)

Shouldnt this method work? 
But what if H2(k)<H1(k)? in this case the SNR will be lower and might
increase BER. But how to determine this?

If somebody could shed some light into this I would greatly appreciate.
Even a text discussing would be nice.

Thanks a lot.

On Dec 7, 5:19 pm, "m26k9" <maduranga.liyan...@gmail.com> wrote:
> Hello,
>
> I have a confusion with the use of channel estimation in the context of
> OFDM.
> Suppose subcarrier 'k' has a channel gain of H(k) and this subcarrier is
> used to transmit data to some user U(k).
>
> (1) If the required data to send is, for example, X(k), the transmitter
> multiplies X(k) by 1/H(k) to compensate for the channel attenuation.
>
hmm... typically, the transmitter will not know H(k).

> Then at the receiver, after FFT, the received signal at subcarrier 'k'
> is:
>
> Y(k) = H(k)*[X(k)/H(k)] + W(k)
> Y(k) = X(k) + W(k)
>

> So the desired data is available.
> Does that mean that receiver does not need to know the channel gain?
> provided the channel gain remained constant.

hmm... as I said prior, the transmitter does not know H(k). The
receiver will estimate H(k) using a
known preamble, then use that H^(k) for demodulation.

>
> I read that OFDM needs channel information at the receiver for coherent
> modulation. Can somebody tell me where this channel information is used in
> the receiver? is it used in the FFT module? Does the FFT need to know the
> phase change?
>

Typically, for OFDM, channel estimation happens in frequency domain.
The channel estimation provides
information about phase and amplitude.

> Even a text discussing would be nice.

The book 'OFDM Wireless LANs: A Theoretical and Practical Guide
(Kaleidoscope) (Paperback)
by Juha Heiskala (Author), John Terry (Author)' should be a very nice
read to get going.

HTH,
Krishna
http://www.dsplog.com

>On Dec 7, 5:19 pm, "m26k9" <maduranga.liyan...@gmail.com> wrote:
>> Hello,
>>
>> I have a confusion with the use of channel estimation in the context
of
>> OFDM.
>> Suppose subcarrier 'k' has a channel gain of H(k) and this subcarrier
is
>> used to transmit data to some user U(k).
>>
>> (1) If the required data to send is, for example, X(k), the
transmitter
>> multiplies X(k) by 1/H(k) to compensate for the channel attenuation.
>>
>hmm... typically, the transmitter will not know H(k).
>
>> Then at the receiver, after FFT, the received signal at subcarrier 'k'
>> is:
>>
>> Y(k) = H(k)*[X(k)/H(k)] + W(k)
>> Y(k) = X(k) + W(k)
>>
>
>> So the desired data is available.
>> Does that mean that receiver does not need to know the channel gain?
>> provided the channel gain remained constant.
>
>hmm... as I said prior, the transmitter does not know H(k). The
>receiver will estimate H(k) using a
>known preamble, then use that H^(k) for demodulation.
>
>>
>> I read that OFDM needs channel information at the receiver for
coherent
>> modulation. Can somebody tell me where this channel information is used
in
>> the receiver? is it used in the FFT module? Does the FFT need to know
the
>> phase change?
>>
>
>Typically, for OFDM, channel estimation happens in frequency domain.
>The channel estimation provides
>information about phase and amplitude.
>
>> Even a text discussing would be nice.
>
>The book 'OFDM Wireless LANs: A Theoretical and Practical Guide
>(Kaleidoscope) (Paperback)
>by Juha Heiskala (Author), John Terry (Author)' should be a very nice
>read to get going.
>
>HTH,
>Krishna
>http://www.dsplog.com
>
>
>

Thanks a lot Krishna.
I will try to get hold of that book.
>