Kari Pietikäinen Communications Laboratory / HUT S-72.333 Postgraduate Course in Radio Communications Kari Pietikäinen Communications Laboratory / HUT S-72.333 Postgraduate Course in Radio Communications Orthogonal Frequency Division Multiplexing Kari Pietikäinen [email protected]Postgraduate Course in Radio Communications 30.11.
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Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
• DMT uses bit loading –algorithms– High SNR subchannels carry
more bits• DVB-T can use > 6800
subchannels• WiMAX can divide
subchannels to different users
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsIF
FT
S/P
Map
ping
Inte
rleav
ing
Cod
ing
CP
Coding / Interleaving
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
Coding / Interleaving
• Convolutional and/or Reed-Solomon coding– Adds redundancy to the information– Convolutional coding operates on bit streams– Reed-Solomon coding is block coding– Low implementation cost– OFDM COFDM (Coded OFDM)
• DVB-T uses inner/outer coding and interleaving
• Convolutional coding studied in earlier presentations
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
Coding / Interleaving
• Interleaving– Scatters error bursts – Can be done in time or in
frequency domain
• One of the simplest formis block interleaving– Write row-by-row– Read column-by-column
(or another way around)– Additional matrix
permutation is possible
E E E E E E
E E E
errors
w/o interleaving w/ interleaving
code word
1
0
100
1 10
0
1 1
1 11
111
11
0 00 0
0 0
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsIF
FT
S/P
Map
ping
Inte
rleav
ing
Cod
ing
CP
Mapping
101101011001 110000101111
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
Mapping
• Data on OFDM subcarriers is mapped(modulated) using common digital modulation schemes– IEEE 802.11a/g WLANs
uses BPSK, QPSK, 16-QAM, 64-QAM
• Serial binary data is converted into complex numbers representing constellation points– Constellation mappings
usually Gray-coded
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsIF
FT
S/P
Map
ping
Inte
rleav
ing
Cod
ing
CP
IFFT / FFT
1-7j 5+3j101101011001 110000101111
Pilot insertionZero padding
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
IFFT / FFT
• IFFT / FFT pair is the key factor in OFDM– IFFT: From frequency domain to time domain– FFT: Vice versa
• All signal processing is made in frequency domain• IFFT / FFT low implementation cost
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsIF
FT
S/P
Map
ping
Inte
rleav
ing
Cod
ing
CP
Guard time / Cyclic prefix
D/A converterLNA/HPAAntenna
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
Guard time / Cyclic prefix
• Guard time is inserted between consecutive OFDM symbols– Helps to combat against ISI – Guard time is larger than delay spread– Multipath components fade away before information extraction
• Reduces BW effiency
No ISI
guardtime FFT time
delayspread
OFDM symbol time
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications
Guard time / Cyclic prefix
0 2 4 6 8 10 12−2
−1.5
−1
−0.5
0
0.5
1
1.5
2LOS1. mp2. mpSumprevious 1. mpprevious 2. mp
0 2 4 6 8 10 12−2
−1.5
−1
−0.5
0
0.5
1
1.5
2LOS1. mp2. mpSum1. mp cp2. mp cp
• Implemented with cyclic extension– Part of the signal is copied
to the front of the signal– Orthogonality is maintained
• Every copy of the signal has an integer number of cycles in the FFT window– Same phase signals sums
up
• Phase correction stillneeded
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsIF
FT
S/P
Map
ping
Inte
rleav
ing
Cod
ing
CP
System planning example
Kari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio CommunicationsKari PietikäinenCommunications Laboratory / HUT
S-72.333 Postgraduate Course in Radio Communications