Schmidl & Cox OFDM synch.: Difference between revisions

Latest revision as of 17:06, 27 February 2023

Schmidl & Cox synchronisation for OFDM.

Input: complex samples.
Output 0: Fine frequency offset, scaled by the OFDM symbol duration. This is ${\hat {\varphi }}$ 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 not 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 $M(d)={\frac {|P(d)|^{2}}{(R(d))^{2}}}$ , we calculate the normalization as $R(d)={\frac {1}{2}}\sum _{k=0}^{N-1}|r_{k+d}|^{2}$ (N=fft_len),

meaning that we estimate the energy from both 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

This flowgraph can be found at [1].

Source Files

C++ files: [2]

Header files: [3]

Public header files: [4]

Block definition: [5]

@@ Line 1: / Line 1: @@
 [[Category:Block Docs]]
-[[Category:Stub Docs]]
+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.
+; Input: complex samples.
+; Output 0: Fine frequency offset, scaled by the OFDM symbol duration. This is <math>\hat{\varphi}</math>  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 '''''not''''' 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 <math>M(d) = \frac{|P(d)|^2}{(R(d))^2}</math>, we calculate the normalization as <math>R(d) = \frac{1}{2} \sum_{k=0}^{N-1} |r_{k+d}|^2</math> (N=fft_len),
+meaning that we estimate the energy from '''''both''''' 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''): <span class="plainlinks">[https://wiki.gnuradio.org/index.php/GNURadioCompanion#Variable_Controls ''Run-time adjustable'']</span>
-; Param 1 (''R'')
+; 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).
+: FFT Length
-; Param 2
+; Cyclic Prefix Length
-: blah blah blah
+: 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.
+This flowgraph can be found at [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/examples/ofdm/rx_ofdm.grc].
+[[File:Rx_ofdm_fg.png|800px]]
 == Source Files ==
 ; C++ files
-: [https://github.com/gnuradio/gnuradio TODO]
+: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/ofdm_sync_sc_cfb_impl.cc]
 ; Header files
-: [https://github.com/gnuradio/gnuradio TODO]
+: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/ofdm_sync_sc_cfb_impl.h]
 ; Public header files
-: [https://github.com/gnuradio/gnuradio TODO]
+: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/include/gnuradio/digital/ofdm_sync_sc_cfb.h]
 ; Block definition
-: [https://github.com/gnuradio/gnuradio TODO]
+: [https://github.com/gnuradio/gnuradio/blob/master/gr-digital/grc/digital_ofdm_sync_sc_cfb.block.yml]

Schmidl & Cox OFDM synch.: Difference between revisions

Latest revision as of 17:06, 27 February 2023

Parameters

Example Flowgraph

Source Files

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