Re: Channel estimation/equalization in OFDM

GNU Radio
1

Martin,

Thank you for your reply. Please, check my replies below.

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
sub-carrier spacing. Interpolation is done in frequency direction (the
Schmidl & Cox sync algo requires double sub-carrier spacing on the sync
symbol).

  1. 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.

I checked, and it was not 1. I can’t seem to find a pattern in the
obtained
numbers.

  1. 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.

The main idea was to perform cooperative beamforming in order to null
out
transmission at certain locations. Based on the acquired channel
estimates,
a precoding matrix can be obtained to cancel the sum of transmitted
signals
at certain receiving nodes. However, a precise estimate of the channel
coefficients (magnitude and phase) should be obtained. Therefore, I
cannot
simply discard the phase part. Is there some sort of uncompensated phase
ambiguity in the hardware of the used USRP nodes?

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Mohammed Hassan Karmoose
Teaching Assistant, Electrical Engineering Dept.
Faculty of Engineering, Alexandria University
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