04-07-2012, 12:55 PM
OFDM: Orthogonal Frequency Division Multiplexing
[attachment=26654]
Basics of OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity.
Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies.
This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own.
How OFDM works
First of all the FDM part - Frequency division multiplexing is a technology that transmits several signals at the same time over a single transmission path, in a medium such as a cable or wireless system.
Each signal is transmitted inside its own unique frequency range (the carrier frequency), which is then modulated by the data that is needing to be transmitted.
Advantages
Makes efficient use of the spectrum by allowing overlap.
By dividing the channel into narrowband flat fading sub channels, OFDM is more resistant to frequency selective fading than single carrier systems are.
Eliminates ISI and IFI through use of a cyclic prefix.
Using adequate channel coding and interleaving one can recover symbols lost due to the frequency selectivity of the channel.
Disadvantages
The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF power amplifiers with a high peak to average power ratio.
It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.
High sensitivity inter-channel interference, ICI
[attachment=26654]
Basics of OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity.
Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies.
This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own.
How OFDM works
First of all the FDM part - Frequency division multiplexing is a technology that transmits several signals at the same time over a single transmission path, in a medium such as a cable or wireless system.
Each signal is transmitted inside its own unique frequency range (the carrier frequency), which is then modulated by the data that is needing to be transmitted.
Advantages
Makes efficient use of the spectrum by allowing overlap.
By dividing the channel into narrowband flat fading sub channels, OFDM is more resistant to frequency selective fading than single carrier systems are.
Eliminates ISI and IFI through use of a cyclic prefix.
Using adequate channel coding and interleaving one can recover symbols lost due to the frequency selectivity of the channel.
Disadvantages
The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF power amplifiers with a high peak to average power ratio.
It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.
High sensitivity inter-channel interference, ICI