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Re: [Discuss-gnuradio] Channel estimation/equalization in OFDM


From: Martin Braun (CEL)
Subject: Re: [Discuss-gnuradio] Channel estimation/equalization in OFDM
Date: Wed, 21 Aug 2013 11:58:53 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Mohammed,

you should also check the new OFDM implementation (see
examples/ofdm/rx_ofdm.grc and python/digital/ofdm_txrx.py). Much more
modular.

On Wed, Aug 21, 2013 at 11:33:26AM +0200, Mohammed Karmoose wrote:
> However, there are two differences in the implementation in relation to this
> equation: 1) the block computes the inverse of the channel coefficient
> (d_known_symbol/symbol), and 2) there is a frequency compensation term
> (coasre_freq_comp) which basically rotates the complex samples by phase
> corresponding to the frequency deviation and obtained via the preceding sync
> block. One final note is that channel coefficient is obtained in every other
> tab, and the coefficient in the inter-taps are obtained via linear
> interpolation of the acquired channels.
> 
> 
> Now, my questions are as follows:
> 
> 
> 1) Does this explanation seems correct?

Hm... coarse freq compensation deals with freq. offsets larger than one
sub-carrier spacing. Interpolation is done in frequency direction (the
Schmidl & Cox sync algo requires double sub-carrier spacing on the sync
symbol).

> 2) By modifying the VERBOSE variable to be equal to 1 in
> digital_ofdm_frame_acquisition.cc, the block also plots the estimated channel
> coefficients in the following order: transmitted symbol --> known symbol -->
> estimated channel inverse --> output). I noticed that when using a file 
> source/
> sink to store/receive packets from OFDM benchmark transmitter and receiver, 
> the
> channel coefficients are still not equal to 1, despite the fact that no 
> receive
> noise nor wireless fading occurs. What do these coefficients represent?

Most likely phase rotation due to cyclic prefix and timing errors. Have
you checked the magnitude? It's probably 1.

> 3) Are the obtained coefficients eligible to be used in further precoding of
> transmitted packets, assuming that the channel between Tx/Rx is reciprocal, 
> and
> that a receiver can switch roles with the transmitter?

That's more of a signal processing question, and as such the (annoying)
answer is: Depends on your application. Are you attempting some kind of
waterfilling algorithm? In any case, discard the phase before you do
so, and make sure you have some kind of limiter.

MB

-- 
Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

Kaiserstra├če 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071
www.cel.kit.edu

KIT -- University of the State of Baden-W├╝rttemberg and
National Laboratory of the Helmholtz Association

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