Re: Use of channel estimation in OFDM

On Dec 7, 5:19 pm, "m26k9" <maduranga.liyan...@xxxxxxxxx> 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.

.

Relevant Pages

  • Channel Correlatoin Function & Noise Varaince
    ... In OFDM Channel Estimation, we firstly get a raw channel estimate from the ... pilots and then perform interpolation for the remaining subcarriers. ... If the channel of each subcarrier is already known, ...
    (comp.dsp)
  • LS channel estimation for LTE
    ... I am developing a least square channel estimation in matlab. ... Once the frequency domain estimates are found we will perform FD ... interpolation it should perfrom better. ...
    (comp.dsp)
  • Re: eliminating a doppler shift in a rayleigh fading channel
    ... the amount of doppler shift in the channel is a one which is related to the channel estimation but there is no way to eliminate it since it is already happened. ...
    (comp.dsp)
  • Re: Use of channel estimation in OFDM
    ... Suppose subcarrier 'k' has a channel gain of Hand this subcarrier is ... I read that OFDM needs channel information at the receiver for coherent ... Typically, for OFDM, channel estimation happens in frequency domain. ...
    (comp.dsp)
  • Re: about Space time coding
    ... "How the real channel is Quasi static for the length of whole space ... the channel coherence time ... then channel estimation schemes would be scrambling to keep up. ... the beginning of the packet, regardless of the length of the packet, ...
    (comp.dsp)