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= WARPLab 7 Example: SISO OFDM =
File: [source:/ResearchApps/PHY/WARPLAB/WARPLab7/M_Code_Examples/wl_example_siso_ofdm_txrx.m wl_example_siso_ofdm_txrx.m] 

This WARPLab example implements a simple OFDM transmitter and receiver. The script can run with or without WARP hardware in the loop and does not require any extra MATLAB toolboxes.

'''Important:''' this script is not intended as a tutorial of OFDM. If you're not already familiar with the basics of OFDM please start with one of the many good [https://www.google.com/search?q=ofdm+tutorial OFDM tutorials] available online. You will need to know the basics of OFDM before understanding the Tx and Rx processing in this example.

== Transmitter ==
[[Image(wl_ofdm_blkDiag_Tx.png, nolink)]]

The OFDM transmitter code implements the following functions:
 * Random data generation
 * Modulation of random data to complex constellation symbols
 * Mapping of symbols to data-bearing subcarriers
 * Insertion of pilot tones in pilot subcarriers
 * Inverse fast Fourier transform (IFFT)
 * Cyclic prefix insertion
 * Preamble construction and insertion
 * Interpolation by 2x


== Receiver ==
[[Image(wl_ofdm_blkDiag_Rx.png, nolink)]]

The OFDM receiver code implements the following functions:
 * LTS correlation for synchronization
 * CFO estimation and correcting use time-domain estimation from LTS
 * Cyclic prefix removal
 * Fast Fourier transform (FFT)
 * Channel estimation from frequency-domain LTS
 * Residual phase error estimation from frequency-domain pilot tones
 * Equalization of data-bearing subcarriers using channel estimates and phase error estimates
 * Demodulation of complex symbols to data values

== Running the Example ==
The WARPLab OFDM example can be used in simulation only mode and in hardware-in-the-loop mode.

=== Simulation ===
To use the example in simulation mode set the top-level param {{{USE_WARPLAB_TXRX = 0;}}}, then run the m-script. When the script finishes 6 plots will show:
||||||=  '''Simulation Plots'''  =||
||  [[Image(wl_ofdm_plots_sim_txIQ.png,width=250)]][[BR]]Transmitted waveform  ||  [[Image(wl_ofdm_plots_sim_rxIQ.png,width=250)]] [[BR]]Received waveform  ||  [[Image(wl_ofdm_plots_sim_ltsCorr.png,width=250)]] [[BR]]Preamble correlation results[[BR]](peaks indicate locations of preamble LTS)  ||
||  [[Image(wl_ofdm_plots_sim_chanEst.png,width=250)]] [[BR]]Channel estimate per subcarrier[[BR]](I/Q components and complex magnitude)  ||  [[Image(wl_ofdm_plots_sim_phaseError.png,width=250)]] [[BR]]Phase error estimates[[BR]](per OFDM symbol)   ||  [[Image(wl_ofdm_plots_sim_constellations.png,width=250)]] [[BR]]Tx and Rx constellations  ||
||||||=  [[BR]]  =||

By default the simulation script applies AWGN with an arbitrary fixed SNR. You can change the simulated propagation characteristics in the m-code. Search for {{{AWGN}}} and apply whatever Tx->Rx degradations you like.

For example to simulate a perfect (noiseless) link, set the noise power to zero:
{{{
rx_vec_air = tx_vec_air + 0*complex(randn(1,length(tx_vec_air)), randn(1,length(tx_vec_air)));
}}}

The script also prints statistics about the experiment to the MATLAB command prompt:
{{{
Results:
Num Bytes:  4560
Sym Errors:  0 (of 9120 total symbols)
Bit Errors:  0 (of 36480 total bits)
EVM:        3.779%
LTS CFO Est: -0.07 kHz
}}}

=== Hardware in the Loop ===
To use the example with WARP hardware you will need two WARP nodes each running WARPLab 7.3 (or later) and a WARPLab environment setup on your PC. Refer to the [wiki:../../QuickStart WARPLab Quick Start] if you have not yet setup WARPLab.

To target WARP hardware set the top-level param {{{USE_WARPLAB_TXRX = 1;}}}, then run the m-script. When the script finishes 6 plots will show:

||||||=  '''Hardware in the Loop Plots'''  =||
||  [[Image(wl_ofdm_plots_hw_txIQ.png,width=250)]][[BR]]Transmitted waveform  ||  [[Image(wl_ofdm_plots_hw_rxIQ.png,width=250)]] [[BR]]Received waveform  ||  [[Image(wl_ofdm_plots_hw_ltsCorr.png,width=250)]] [[BR]]Preamble correlation results[[BR]](peaks indicate locations of preamble LTS)  ||
||  [[Image(wl_ofdm_plots_hw_chanEst.png,width=250)]] [[BR]]Channel estimate per subcarrier[[BR]](I/Q components and complex magnitude)  ||  [[Image(wl_ofdm_plots_hw_phaseError.png,width=250)]] [[BR]]Phase error estimates[[BR]](per OFDM symbol)  ||  [[Image(wl_ofdm_plots_hw_constellations.png,width=250)]] [[BR]]Tx and Rx constellations  ||
||||||=  [[BR]]  =||

The script also prints statistics about the experiment to the MATLAB command prompt:
{{{
Results:
Num Bytes:  4560
Sym Errors:  0 (of 9120 total symbols)
Bit Errors:  0 (of 36480 total bits)
EVM:        4.541%
LTS CFO Est: 2.64 kHz
}}}


== Limitations ==
This example is intended as a staring point for researchers wishing to use WARPLab to prototype a wireless communications link. This examples does not implement some some blocks common in deployed OFDM systems, such as scrambling, interleaving and error correcting coding. For an example of a real-time OFDM implementation that implements all of these subsystems, please see the PHY in the [wiki:802.11 802.11 Reference Design].