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Coded Orthogonal Frequency Division Multiplexing (COFDM) has captured the imagination of the engineers who are defining the current generation of digital broadcast systems. The technology is used in wireless LANs, ADSL, DAB, DVB-T, DRM, Sirius, XM, and IBOC just to mention a few well known applications. COFDM acquires its ‘magical’ properties as follows:
++ Frequency Division Multiplexing (FDM) means that a large number of tightly defined frequency bands are used that each carries a portion of the overall data stream. In practice each frequency band will have a single carrier that is modulated in amplitude and phase using schemes such as Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK).
++ Orthogonal where there is an inverse relationship between the carrier spacing and the carrier duration. Say that the FDM carriers are spaced apart by F Hz and have a duration of Tu seconds. The carriers are said to be orthogonal if the following mathematical relationship exists:
Coded in some way so that the data payload is protected and can be recovered by the receiver using checksum and other algorithms that allows the lost data to be restored. The OFDM spectrum can be visualised as a grid with time along one axis and narrow frequency bands along the other. The ’width’ of the box is in units of time and is Tu plus the guard time. The ‘height’ of the box is in Hertz and is F Hz wide. Each box is called an OFDM cell and contains an OFDM symbol that comprises a carrier with a defined amplitude and phase. The cells are positioned according to a pre-determined pattern where some are data cells and others have reference, timing and control functions.
The other concept to be aware of is that of a guard time or guard band. This is a well known concept and is used in OFDM to ensure that the receiver is not sampling erroneous data generated by propagation or other effects that serve to extend a carrier’s duration beyond its normal lifetime. Any carriers that appear during this guard time are simply ignored by the receiver.
Below are some common OFDM deployments with data rates varying from 20 kbits/sec up to 32 Mbits/sec. OFDM is also used by IBOC, XM and SIRIUS systems.
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System
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Bandwidth
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No. carriers
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Carrier Spacing
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DVB-T (DTT)
|
8 MHz
|
1705
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4.4 kHz
|
|
7 MHz
|
1705
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3.9 kHz
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|
6 MHz
|
1705
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3.34 kHz
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|
8 MHz
|
6817
|
1.1 kHz
|
|
7 MHz
|
6817
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0.97 kHz
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|
6 MHz
|
6817
|
0.84 kHz
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DAB Mode IV
|
1.536 MHz
|
192
|
8 kHz
|
|
Mode III
|
1.536 MHz
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384
|
4 kHz
|
|
Mode II
|
1.536 MHz
|
768
|
2 kHz
|
|
Mode I
|
1.536 MHz
|
1536
|
1 kHz
|
|
DRM Mode A
|
10 kHz
|
228
|
412/3 Hz
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Mode B
|
10 kHz
|
206
|
467/8 Hz
|
|
Mode C
|
10 kHz
|
138
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682/11 Hz
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Mode D
|
10 kHz
|
88
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1071/7 Hz
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