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).

- 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.

- 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?

Kaiserstra?e 12

Building 05.01

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www.cel.kit.edu http://www.cel.kit.edu/

KIT – University of the State of Baden-W?rttemberg and

National Laboratory of the Helmholtz Association

–

*---------------------------------------------------*

*Mohammed Hassan Karmoose*

*Teaching Assistant, Electrical Engineering Dept.*

*Faculty of Engineering, Alexandria University*

*Al-Horeya Rd, El-Hadara,* *Alexandria, Egypt - 21544*

*Tel: **(++203)592-1852* | *Fax: **(++203)592-1853*

*Email: m [email protected]_h_karmoose@a [email protected]
lexu.edu.eg*