OFDM¶

Orthogonal Frequency Devision Multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. Within HermesPy, OFDM can be modeled by a flexible propgramming interface allowing for the definition of custom time-freqeuency grids.

Considering a simplex-link scenario of two modems communicating over a 3GPP 5G TDL channel

# Initialize a simulation with two dedicated devices for transmission and reception
carrier_frequency = 3.7e9
simulation = Simulation()
tx_device = simulation.new_device(carrier_frequency=carrier_frequency)
rx_device = simulation.new_device(carrier_frequency=carrier_frequency)

# Assume a 5G TDL channel model
channel = TDL(TDLType.A, 1e-7, doppler_frequency=10)
simulation.set_channel(tx_device, rx_device, channel)

# Link the devices
link = SimplexLink(tx_device, rx_device)

configuring an OFDM waveform requires the specification of the resource-time grid onto which the transmitted data and pilot symbols are placed:

# Configure an orthogonal waveform featuring 128 subcarriers
grid_resources = [
    GridResource(16, PrefixType.CYCLIC, .1, [GridElement(ElementType.DATA, 7), GridElement(ElementType.REFERENCE, 1)]),
    GridResource(128, PrefixType.CYCLIC, .1, [GridElement(ElementType.DATA, 1)]),
]
grid_structure = [
    SymbolSection(64, [0, 1])
]
waveform = OFDMWaveform(
    grid_resources=grid_resources,
    grid_structure=grid_structure,
    num_subcarriers=128,
    subcarrier_spacing=1e3,
)
link.waveform = waveform

The grid considers \(128\) orthogonal subcarriers each modulated with a unique symbol, with 128 repetitions in time-domain, so that overall \(16384\) symbols are transmitted per frame. The grid alternates between two types of symbol sections, one carrying a reference element on every \(8\)-th subcarrier and one consisting only of data symbols.

Additionally, post-processing routines for channel estimation and channel equalization may be specified on the waveform level

# Configure channel estimation and equalization
waveform.channel_estimation = OrthogonalLeastSquaresChannelEstimation()
waveform.channel_equalization = ZeroForcingChannelEqualization()

# Configure frame synchronization
waveform.pilot_section = PilotSection()
waveform.synchronization = CorrelationSynchronization()