Difference between revisions of "Schmidl & Cox OFDM synch."

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(Created page with "Category:Block Docs Category:Stub Docs This is the template for the "Page-per-block Docs". This first section should describe what the block...")
 
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[[Category:Block Docs]]
 
[[Category:Block Docs]]
[[Category:Stub Docs]]
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Schmidl & Cox synchronisation for OFDM.
This is the template for the [[:Category:Block_Docs|"Page-per-block Docs"]].  This first section should describe what the block does and how to use it, using however many paragraphs necessary.  Note that the title of the wiki page should match the block's name in GRC, i.e. the one defined in the block's .grc file.  Look at the [[FFT]] Block for a good example.
 
  
As this is a basic template, it's also in the [[:Category:Stub_Docs|"Stub Docs category"]]. Please improve it.
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; Input: complex samples. Output 0: Fine frequency offset, scaled by the OFDM symbol duration. This is  in [1]. The normalized frequency offset is then 2.0*output0/fft_len.
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; Output 1: Beginning of the first OFDM symbol after the first (doubled) OFDM symbol. The beginning is marked with a 1 (it's 0 everywhere else).
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The evaluation of the coarse frequency offset is  done in this block. Also, the initial equalizer taps are not calculated here.
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Note that we use a different normalization factor in the timing metric than the authors do in their original work[1]. If the timing metric (8) is \f[ M(d) = \frac{|P(d)|^2}{(R(d))^2}, \f] we calculate the normalization as \f[ R(d) = \frac{1}{2} \sum_{k=0}^{N-1} |r_{k+d}|^2, \f]
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If the timing metric (8) is  we calculate the normalization as  i.e., we estimate the energy from  half-symbols. This avoids spurious detects at the end of a burst, when the energy level suddenly drops.
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[1] Schmidl, T.M. and Cox, D.C., "Robust frequency and timing synchronization for OFDM", Communications, IEEE Transactions on, 1997.
  
 
== Parameters ==
 
== Parameters ==
 
(''R''): <span class="plainlinks">[https://wiki.gnuradio.org/index.php/GNURadioCompanion#Variable_Controls ''Run-time adjustable'']</span>
 
(''R''): <span class="plainlinks">[https://wiki.gnuradio.org/index.php/GNURadioCompanion#Variable_Controls ''Run-time adjustable'']</span>
  
; Param 1 (''R'')
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; FFT Length
: Description of parameter, provide any tips or recommended values.  Note that the name of the parameter above should match the param's label that shows up in grc (e.g. Sample Rate).
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: FFT Length
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; Cyclic Prefix Length
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: Length of the guard interval (cyclic prefix) in samples
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; Preamble Carriers
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: If true, the carriers in the sync preamble are occupied such that the even carriers are used (0, 2, 4, ...). If you use all carriers, that would include the DC carrier, so be careful.
  
; Param 2
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; Threshold (''R'')
: blah blah blah
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: detection threshold. Default is 0.9.
  
 
== Example Flowgraph ==
 
== Example Flowgraph ==
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; C++ files
 
; C++ files
: [https://github.com/gnuradio/gnuradio TODO]
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: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/ofdm_sync_sc_cfb_impl.cc]
  
 
; Header files
 
; Header files
: [https://github.com/gnuradio/gnuradio TODO]
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: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/ofdm_sync_sc_cfb_impl.h]
  
 
; Public header files
 
; Public header files
: [https://github.com/gnuradio/gnuradio TODO]
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: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/include/gnuradio/digital/ofdm_sync_sc_cfb.h]
  
 
; Block definition
 
; Block definition
: [https://github.com/gnuradio/gnuradio TODO]
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: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/grc/digital_ofdm_sync_sc_cfb.block.yml]

Revision as of 14:31, 9 September 2019

Schmidl & Cox synchronisation for OFDM.

Input
complex samples. Output 0: Fine frequency offset, scaled by the OFDM symbol duration. This is in [1]. The normalized frequency offset is then 2.0*output0/fft_len.
Output 1
Beginning of the first OFDM symbol after the first (doubled) OFDM symbol. The beginning is marked with a 1 (it's 0 everywhere else).

The evaluation of the coarse frequency offset is done in this block. Also, the initial equalizer taps are not calculated here.

Note that we use a different normalization factor in the timing metric than the authors do in their original work[1]. If the timing metric (8) is \f[ M(d) = \frac{|P(d)|^2}{(R(d))^2}, \f] we calculate the normalization as \f[ R(d) = \frac{1}{2} \sum_{k=0}^{N-1} |r_{k+d}|^2, \f] If the timing metric (8) is we calculate the normalization as i.e., we estimate the energy from half-symbols. This avoids spurious detects at the end of a burst, when the energy level suddenly drops.

[1] Schmidl, T.M. and Cox, D.C., "Robust frequency and timing synchronization for OFDM", Communications, IEEE Transactions on, 1997.

Parameters

(R): Run-time adjustable

FFT Length
FFT Length
Cyclic Prefix Length
Length of the guard interval (cyclic prefix) in samples
Preamble Carriers
If true, the carriers in the sync preamble are occupied such that the even carriers are used (0, 2, 4, ...). If you use all carriers, that would include the DC carrier, so be careful.
Threshold (R)
detection threshold. Default is 0.9.

Example Flowgraph

Insert description of flowgraph here, then show a screenshot of the flowgraph and the output if there is an interesting GUI. Currently we have no standard method of uploading the actual flowgraph to the wiki or git repo, unfortunately. The plan is to have an example flowgraph showing how the block might be used, for every block, and the flowgraphs will live in the git repo.

Source Files

C++ files
[1]
Header files
[2]
Public header files
[3]
Block definition
[4]