comm.OFDMDemodulator
Demodulate using OFDM method
Description
The OFDMDemodulator
object demodulates
using the orthogonal frequency division demodulation method. The output
is a baseband representation of the modulated signal, which was input
into the OFDMModulator
companion object.
To demodulate an OFDM signal:
Define and set up the OFDM demodulator object. See Construction.
Call
step
to demodulate a signal according to the properties ofcomm.OFDMDemodulator
. The behavior ofstep
is specific to each object in the toolbox.
Note
Starting in R2016b, instead of using the step
method
to perform the operation defined by the System object™, you can
call the object with arguments, as if it were a function. For example, y
= step(obj,x)
and y = obj(x)
perform
equivalent operations.
Construction
H = comm.OFDMDemodulator
creates a demodulator System object, H
,
that demodulates an input signal by using the orthogonal frequency
division demodulation method.
H = comm.OFDMDemodulator(
creates
an OFDM demodulator object, Name
,Value
)H
, with each specified
property set to the specified value. You can specify additional namevalue
pair arguments in any order as (Name1
,Value1
,...,NameN
,ValueN
).
H = comm.OFDMDemodulator(hMod)
creates
an OFDM demodulator object, H
, whose properties
are determined by the corresponding OFDM modulator object, hMod
.
Properties

The length of the FFT, N_{FFT},
is equivalent to the number of subcarriers used in the modulation
process. Specify the number of subcarriers. The default is 

The number of guard band subcarriers allocated to the left and right guard bands. Specify the number of left and right subcarriers as nonnegative
integers in [0,N_{FFT}/ 

A 

A 

If the 

The cyclic prefix length property specifies the length of the
OFDM cyclic prefix. If you specify a scalar, the prefix length is
the same for all symbols through all antennas. If you specify a row
vector of length N_{sym}, the
prefix length can vary across symbols but remains the same length
through all antennas. The default value is 

This property specifies the number of symbols, N_{sym}.
Specify N_{sym} as a positive
integer. The default value is 

This property determines the number of antennas, N_{R},
used to receive the OFDM modulated signal. Specify N_{R} as
a positive integer. The default value is 
Methods
info  Provide dimensioning information for the OFDM method 
showResourceMapping  Show the subcarrier mapping of the OFDM symbols created by the OFDM demodulator System object 
step  Demodulate using OFDM method 
Common to All System Objects  

release  Allow System object property value changes 
reset  Reset internal states of System object 
When using reset
, this method resets the windowed suffix
from the last symbol in the previously processed frame.
Examples
Algorithms
The Orthogonal Frequency Division Modulation (OFDM) Demodulator System object demodulates an OFDM input signal by using an FFT operation that results in N parallel data streams.
The figure shows an OFDM demodulator. It consists of a bank of N correlators with one assigned to each OFDM subcarrier followed by a paralleltoserial conversion.
Guard Bands and Intervals
There are three types of OFDM subcarriers: data, pilot, and null. Data subcarriers are used for transmitting data while pilot subcarriers are used for channel estimation. There is no transmission on null subcarriers, which are used to provide a DC null as well as to provide buffers between OFDM resource blocks. These buffers are referred to as guard bands whose purpose is to prevent intersymbol interference. The allocation of nulls and guard bands varies depending upon the standard, e.g., 802.11n differs from LTE. Consequently, the OFDM modulator object allows the user to assign subcarrier indices as required.
Analogous to the concept of guard bands, the OFDM modulator object supports guard intervals that provide temporal separation between OFDM symbols so that the signal does not lose orthogonality due to timedispersive channels. As long as the guard interval is longer than the delay spread, each symbol does not interfere with other symbols. Guard intervals are created by using cyclic prefixes in which the last part of an OFDM symbol is copied and inserted as the first part of the OFDM symbol. The benefit of cyclic prefix insertion is maintained as long as the span of the time dispersion does not exceed the duration of the cyclic prefix. The OFDM modulator object enables the cyclic prefix length to be set. The drawback in using a cyclic prefix is increased overhead.
Selected Bibliography
[1] Dahlman, E., S. Parkvall, and J. Skold. 4G LTE/LTEAdvanced for Mobile Broadband.London: Elsevier Ltd., 2011.
[2] Andrews, J. G., A. Ghosh, and R. Muhamed, Fundamentals of WiMAX, Upper Saddle River, NJ: Prentice Hall, 2007.
[3] "IEEE Standard 802.16™2009." New York: IEEE, 2009.