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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 correction 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
 * SFO estimation and correction 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 ===

'''Requirements: 2 WARP v3 kits, 1 antenna each.'''

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
}}}

== CFO, Phase Error, and SFO ==

At the [https://warpproject.org/trac/browser/ResearchApps/PHY/WARPLAB/WARPLab7/M_Code_Examples/wl_example_siso_ofdm_txrx.m#L27 top of the script], there are three control variables that toggle three different recovery systems: {{{DO_APPLY_CFO_CORRECTION}}}, {{{DO_APPLY_PHASE_ERR_CORRECTION}}}, {{{DO_APPLY_SFO_CORRECTION}}}. In this section, we will explore what happens when different combinations of these recovery systems are enabled or disabled. For reference, here is an example output from the design when all three recovery systems are activated:

=== CFO Recovery: Enabled, Phase Error Recovery: Enabled, SFO Recovery: Enabled ===

||  [[Image(Cy_PEy_Sy_phaseError.png,width=250)]][[BR]]Phase Error Estimates and SFO Recovery Matrix  ||  [[Image(Cy_PEy_Sy_constellations.png,width=250)]] [[BR]]Rx Constellation  ||  [[Image(Cy_PEy_Sy_evm.png,width=250)]] [[BR]]Error Vector Magnitude (EVM)[[BR]] and effective SNR  ||

== Limitations ==
This example is intended as a starting 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].