ofdmdemod

Demodulate time-domain signal using orthogonal frequency division multiplexing (OFDM)

Syntax

outSym = ofdmdemod(ofdmSig,nfft,cplen)

outSym = ofdmdemod(ofdmSig,nfft,cplen,symOffset)

outSym = ofdmdemod(ofdmSig,nfft,cplen,symOffset,nullidx)

[outSym,pilots] = ofdmdemod(ofdmSig,nfft,cplen,symOffset,nullidx,pilotidx)

Description

outSym = ofdmdemod(ofdmSig,nfft,cplen) performs OFDM demodulation on the input time domain signal specified in ofdmSig, using an FFT size specified by nfft and cyclic prefix length specified by cplen. For information, see OFDM Demodulation.

outSym = ofdmdemod(ofdmSig,nfft,cplen,symOffset) applies the symbol sampling offset, symOffset, for each OFDM symbol before demodulation of the input.

outSym = ofdmdemod(ofdmSig,nfft,cplen,symOffset,nullidx) removes null subcarriers from the locations specified in nullidx. For this syntax, the symbol sampling offset is applied to each OFDM symbol and the number of rows in the output is nfft – length(nullidx), which accounts for the removal of null subcarriers. Use null subcarriers to account for guard bands and DC subcarriers. For information, see Subcarrier Allocation and Guard Bands.

[outSym,pilots] = ofdmdemod(ofdmSig,nfft,cplen,symOffset,nullidx,pilotidx) returns pilot subcarriers for the pilot indices specified in pilotidx. For this syntax, the symbol sampling offset is applied to each OFDM symbol and number of rows in the output is nfft – length(nullidx) – length(pilotidx), which accounts for the removal of null and pilot subcarriers. The function assumes that pilot subcarrier locations are the same across each OFDM symbol and transmit antenna.

Examples

OFDM Demodulation with Different CP Lengths

OFDM-demodulate a signal with different CP lengths for different symbols.

Initialize input parameters defining locations for null and pilot subcarriers. Generate random data and perform OFDM modulation.

nfft = 64;
cplen = [16 32];
nSym = 2;
dataIn = complex(randn(nfft,nSym),randn(nfft,nSym));
y1 = ofdmmod(dataIn,nfft,cplen);

Demodulate the OFDM symbols. Compare the results to the original input data. The difference between the signals is negligible.

x1 = ofdmdemod(y1,nfft,cplen);
max(x1-dataIn)

ans = 1×2 complex
10-15 ×

   0.2220 - 0.7772i   0.2498 - 0.8882i

OFDM Mod-Demod SISO link

Apply OFDM multiplexing to a 16-QAM signal filtered by a SISO link with Rayleigh fading.

Initialize simulation variables, and create Rayleigh fading channel and constellation diagram objects.

s1 = RandStream('mt19937ar','Seed',12345);
nFFT = 64;
cpLen = 16;
nullIdx = [1:6 33 64-4:64].';
numTones = nFFT-length(nullIdx);

k = 4; % bits per symbol
M = 2^k;
constSym = qammod((0:M-1),M,'UnitAveragePower',true); % reference constellation symbols

maxDopp = 1;
pathDelays = [0 4e-3 8e-3];
pathGains = [0 -2 -3];
sRate = 1000;
sampIdx = round(pathDelays/(1/sRate)) + 1;

chan = comm.RayleighChannel('PathGainsOutputPort',true, ...
    'MaximumDopplerShift',maxDopp, ...
    'PathDelays',pathDelays, ...
    'AveragePathGains',pathGains, ...
    'SampleRate',sRate, ...
    'RandomStream','mt19937ar with seed');

cdScope = comm.ConstellationDiagram('ShowReferenceConstellation',true, ...
    'ReferenceConstellation',constSym);

Generate signal data and apply 16-QAM modulation.

data = randi(s1,[0 M-1],numTones,1);
modOut = qammod(data,M,'UnitAveragePower',true);

Apply OFDM modulation and pass the signal through the channel.

y = ofdmmod(modOut,nFFT,cpLen,nullIdx);
[fadSig,pg] = chan(y);

Determine symbol sampling offset.

symOffset = min(max(sampIdx),cpLen)

symOffset = 9

OFDM demodulate the received signal with a time shift. Display the constellation diagram before equalization.

x = ofdmdemod(fadSig,nFFT,cpLen,symOffset,nullIdx);
cdScope(x);

Convert the vector of path gains, pg, to scalar tap gains that correspond to data subcarriers, h_datasubcarr. Use the h_datasubcarr tap gains for equalization during signal recovery.

hImp = complex(zeros(nFFT,1));
hImp(sampIdx) = mean(pg,1);
hall = fftshift(fft(hImp));
dataIdx = setdiff((1:nFFT)',nullIdx);
h_datasubcarr = hall(dataIdx);

Equalize the signal. Display the constellation diagram after equalization.

eqSig = x ./ h_datasubcarr;
cdScope(eqSig);

Demodulate the 16-QAM symbols to recover the signal. Compute the symbol error rate.

rxSym = qamdemod(eqSig,M,'UnitAveragePower',true);
numErr = symerr(data,rxSym);
disp(['Number of symbol errors: ' num2str(numErr) ' out of ' num2str(length(data)) ' symbols.'])

Number of symbol errors: 2 out of 52 symbols.

OFDM Demodulation with Null and Pilot Packing

OFDM-demodulate data input that includes null and pilot packing.

Initialize input parameters, defining locations for null and pilot subcarriers. Generate random data and perform OFDM modulation.

nfft     = 64;
cplen    = 16;
nSym     = 10;
nullIdx  = [1:6 33 64-4:64]';
pilotIdx = [12 26 40 54]';
numDataCarrs = nfft-length(nullIdx)-length(pilotIdx);
dataIn = complex(randn(numDataCarrs,nSym),randn(numDataCarrs,nSym));
pilots = repmat(pskmod((0:3).',4),1,nSym);
y2 = ofdmmod(dataIn,nfft,cplen,nullIdx,pilotIdx,pilots);

Demodulate the OFDM symbols. Compare the results to the original input data to show that there is negligible difference between the demodulated signal and the original data and pilot signals.

symOffset = cplen;
[x2,rxPilots] = ofdmdemod(y2,nfft,cplen,symOffset,nullIdx,pilotIdx);
max(x2-dataIn)

ans = 1×10 complex
10-15 ×

   0.5551 + 0.2220i   0.2220 + 0.4441i   0.4441 - 0.2220i   0.4718 - 0.3331i  -0.1665 - 0.4441i  -0.3331 - 0.8049i   0.6661 - 0.2220i   0.0000 + 0.5829i   0.2220 + 0.4441i   0.3331 + 0.4441i

max(rxPilots-pilots)

ans = 1×10 complex
10-15 ×

   0.0000 + 0.3331i   0.1837 - 0.2220i  -0.4441 - 0.2776i   0.2220 + 0.2220i   0.2220 - 0.1665i   0.0000 - 0.3445i   0.3331 + 0.0441i  -0.4441 - 0.1225i   0.4441 + 0.1943i   0.4441 + 0.3192i

Input Arguments

ofdmSig — Modulated OFDM symbols

2-D array of complex symbols

Modulated OFDM symbols, specified as a 2-D array of complex symbols.

• If cplen is a scalar, the array size is ((nfft + cplen) × N_Sym)-by-N_R.

• If cplen is a row vector, the array size is ((nfft × N_Sym) + sum(cplen))-by-N_R.

  N_Sym is the number of symbols per antenna and N_R is the number of receive antennas.

Data Types: double | single
Complex Number Support: Yes

nfft — FFT length

integer greater than or equal to 8

FFT length, specified as an integer greater than or equal to 8. nfft is equivalent to the number of subcarriers used in the demodulation process.

Data Types: double

cplen — Cyclic prefix length

scalar | row vector of length N_Sym

Cyclic prefix length, specified as a scalar or as a row vector of length N_Sym.

• When you specify cplen as a scalar, the cyclic prefix length is the same for all symbols through all antennas.

• When you specify cplen as a row vector of length N_Sym, the cyclic prefix length can vary across symbols but remains the same length through all antennas.

Data Types: double

symOffset — Symbol sampling offset

cplen (default) | scalar | row vector

Symbol sampling offset, specified as values from 0 to cplen.

• If you do not specify symOffset, the default value is an offset equal to cplen.

• If you specify symOffset as a scalar, the same offset is used for all symbols.

• If you specify symOffset as a row vector, the offset value can be different for each symbol.

For information, see Windowing and Symbol Offset.

Data Types: double

nullidx — Indices of null subcarrier locations

column vector

Indices of null subcarrier locations, specified as a column vector with element values from 1 to nfft. If you specify nullidx, the number of rows in outSym is (nfft-length(nullidx)). For information, see Subcarrier Allocation and Guard Bands.

Data Types: double

pilotidx — Indices of pilot subcarrier locations

column vector

Indices of pilot subcarrier locations, specified as a column vector with element values from 1 to nfft. If you specify pilotidx, the number of rows in outSym is (nfft-length(nullidx)-length(pilotidx)).For information, see Subcarrier Allocation and Guard Bands.

Data Types: double

Output Arguments

outSym — Output demodulated symbols

3-D array

Output demodulated symbols, returned as an N_D-by-N_Sym-by-N_R array of symbols. N_D must equal nfft – length(nullidx) – length(pilotidx). N_Sym is the number of OFDM symbols per antenna. N_R is the number of receive antennas. For information, see OFDM Demodulation.

pilots — Pilot subcarriers

3-D array

Pilot subcarriers, returned as an N_Pilot-by-N_Sym-by-N_R array of symbols. N_Pilot must equal the length of pilotidx. N_Sym is the number of OFDM symbols per antenna. N_R is the number of receive antennas. The function assumes that the pilot subcarrier locations are the same across each OFDM symbol and transmit antenna. Use the comm.OFDMDemodulator to vary pilot subcarrier locations across OFDM symbols or antennas.

More About

OFDM Demodulation

An OFDM demodulator demultiplexes a multi-subcarrier time-domain signal using orthogonal frequency division modulation.

The OFDM demodulation uses an FFT operation that results in N parallel data streams. An OFDM demodulator consists of a bank of N correlators, with one correlator assigned to each OFDM subcarrier, followed by a parallel-to-serial conversion.

Subcarrier Allocation and Guard Bands

Individual OFDM subcarriers are allocated as data, pilot, or null subcarriers.

As shown here, subcarriers are designated as data, DC, pilot, or guard band subcarriers.

• Data subcarriers transmit user data.

• Pilot subcarriers are used for channel estimation.

• Null subcarriers transmit no data. Subcarriers with no data are used to provide a DC null and serve as buffers between OFDM resource blocks.

  • The null DC subcarrier is the center of the frequency band with an index value of (nfft/2 + 1) if nfft is even, or ((nfft + 1) / 2) if nfft is odd.

  • The guard bands provide buffers between consecutive OFDM symbols to protect the integrity of transmitted signals by reducing intersymbol interference.

Null subcarriers enable you to model guard bands and DC subcarrier locations for specific standards, such as the various 802.11 formats, LTE, WiMAX, or for custom allocations. You can allocate the location of nulls by assigning a vector of null subcarrier indices.

Similar to guard bands, guard intervals are used in OFDM to protect the integrity of transmitted signals by reducing intersymbol interference.

Assignment of guard intervals is analogous to the assignment of guard bands. You can model guard intervals to provide temporal separation between OFDM symbols. The guard intervals help preserve intersymbol orthogonality after the signal passes through time-dispersive channels. Guard intervals are created by using cyclic prefixes. Cyclic prefix insertion copies the last part of an OFDM symbol as the first part of the OFDM symbol.

As long as the span of the time dispersion does not exceed the duration of the cyclic prefix, the benefit of cyclic prefix insertion is maintained.

Inserting a cyclic prefix results in a fractional reduction of user data throughput because the cyclic prefix occupies bandwidth that could be used for data transmission.

Windowing and Symbol Offset

To reduce intersymbol interference (ISI) introduced by signal windowing applied at the transmitter, the function applies a fractional symbol offset before demodulation of each OFDM symbol. Signal windowing is often applied to transmitted OFDM symbols to smooth the discontinuity between consecutive OFDM symbols. Windowing reduces intersymbol out-of-band emissions but increases ISI.

The windowed OFDM symbol consists of the cyclic prefix (CP), ODFM symbol data, plus windowing regions at the beginning and end of the symbol. The leading and trailing windowing shoulders have tails as shown in the figure.

To reduce ISI, you can align signal sample timing by specifying a symbol sampling offset that gets applied before OFDM symbol demodulation.

Specify the symbol sampling offset as a value from 0 to L_CP.

• When the symbol sampling offset is a scalar from 0 to L_CP, the FFT window begins at the X+1 sample of the CP length.

• When the symbol sampling offset is zero, no offset is applied and the FFT window starts at the first sample of the symbol.

• When the symbol sampling offset is the cyclic prefix length, L_CP, the FFT window begins after the last CP sample. This offset is the default setting if symbol sampling offset is not specified.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.