26-06-2012, 03:50 PM
Orthogonal Frequency Division Multiplexing (OFDM)
[attachment=25620]
OFDM Advantages
OFDM is a spectrally efficient
modulation technique
It is conveniently implemented using
IFFT and FFT operations
It handles frequency selective channels
well when combined with error
correction coding
Cyclic Prefix Generation
To avoid losing the power from echoes,
a copy of the end is appended to the
beginning of the “useful” part
Cyclic Prefix Appended
The length of the cyclic prefix, also known as
the Guard Interval, D, is supposed to be
longer than the excess delay of the longest
significant echo
Cyclic Prefix Removal
After the symbol has been received and
stored in a buffer, the cyclic prefix is
removed, leaving only the modulated
and faded subcarriers
Forward Error-Correction Coding (FEC)
The information is typically FEC
encoded and interleaved prior to
modulation
The bits carried by faded subcarriers
might be detected in error without the
FEC
With FEC and interleaving, erroneous
bits may be correctable, thereby
providing frequency diversity
Summary
OFDM is a multi-carrier modulation technique
Each subcarrier carries BPSK, QPSK or QAM
The subcarriers are so close, that each
subcarrier sees a flat-faded channel
The Guard Interval ensures that successive
OFDM symbols do not interfere with each
other
FEC and interleaving provide frequency
diversity
OFDM modulation and demodulation is
conveniently performed by fast DSP
operations (FFT and IFFT)
[attachment=25620]
OFDM Advantages
OFDM is a spectrally efficient
modulation technique
It is conveniently implemented using
IFFT and FFT operations
It handles frequency selective channels
well when combined with error
correction coding
Cyclic Prefix Generation
To avoid losing the power from echoes,
a copy of the end is appended to the
beginning of the “useful” part
Cyclic Prefix Appended
The length of the cyclic prefix, also known as
the Guard Interval, D, is supposed to be
longer than the excess delay of the longest
significant echo
Cyclic Prefix Removal
After the symbol has been received and
stored in a buffer, the cyclic prefix is
removed, leaving only the modulated
and faded subcarriers
Forward Error-Correction Coding (FEC)
The information is typically FEC
encoded and interleaved prior to
modulation
The bits carried by faded subcarriers
might be detected in error without the
FEC
With FEC and interleaving, erroneous
bits may be correctable, thereby
providing frequency diversity
Summary
OFDM is a multi-carrier modulation technique
Each subcarrier carries BPSK, QPSK or QAM
The subcarriers are so close, that each
subcarrier sees a flat-faded channel
The Guard Interval ensures that successive
OFDM symbols do not interfere with each
other
FEC and interleaving provide frequency
diversity
OFDM modulation and demodulation is
conveniently performed by fast DSP
operations (FFT and IFFT)