27-08-2012, 12:46 PM
STUDY OF OFDM
1OFDM.pptx (Size: 213.67 KB / Downloads: 78)
What is OFDM?
OFDM is one of the applications of a parallel-data-transmission scheme, which reduces the influence of multipath fading and makes complex equalizers unnecessary.
Why OFDM is used?
To create broadband multimedia mobile communication systems, it is necessary to use high-bit-rate transmission of at least several megabits per second.
If digital data is transmitted at the rate of several megabits per second, the delay time of the delayed waves is greater than 1 symbol time.
Adaptive equalizer at the receiver is one method for equalizing these signals.
There are practical difficulties in operating this equalization at several megabits per second with compact, low-cost hardware.
OFDM reduces the influence of multipath fading and makes complex equalizers unnecessary.
How Equalizers are unnecessary?
OFDM splits high bit rate stream into many low bit rate streams.
Each stream being sent using an independent carrier frequency.
Advantage
Can easily adapt to severe channel conditions without complex time-domain equalization.
Robust against narrow-band co-channel interference.
Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.
Efficient implementation using Fast Fourier Transform (FFT).
Low sensitivity to time synchronization errors.
Tuned sub-channel receiver filters are not required (unlike conventional FDM).
Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.
Disadvantage
Sensitive to Doppler shift.
Sensitive to frequency synchronization problems.
High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
Loss of efficiency caused by cyclic prefix/guard interval.
Interleaving
The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact disc (CD) playback robust.
1OFDM.pptx (Size: 213.67 KB / Downloads: 78)
What is OFDM?
OFDM is one of the applications of a parallel-data-transmission scheme, which reduces the influence of multipath fading and makes complex equalizers unnecessary.
Why OFDM is used?
To create broadband multimedia mobile communication systems, it is necessary to use high-bit-rate transmission of at least several megabits per second.
If digital data is transmitted at the rate of several megabits per second, the delay time of the delayed waves is greater than 1 symbol time.
Adaptive equalizer at the receiver is one method for equalizing these signals.
There are practical difficulties in operating this equalization at several megabits per second with compact, low-cost hardware.
OFDM reduces the influence of multipath fading and makes complex equalizers unnecessary.
How Equalizers are unnecessary?
OFDM splits high bit rate stream into many low bit rate streams.
Each stream being sent using an independent carrier frequency.
Advantage
Can easily adapt to severe channel conditions without complex time-domain equalization.
Robust against narrow-band co-channel interference.
Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.
Efficient implementation using Fast Fourier Transform (FFT).
Low sensitivity to time synchronization errors.
Tuned sub-channel receiver filters are not required (unlike conventional FDM).
Facilitates single frequency networks (SFNs); i.e., transmitter macrodiversity.
Disadvantage
Sensitive to Doppler shift.
Sensitive to frequency synchronization problems.
High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency.
Loss of efficiency caused by cyclic prefix/guard interval.
Interleaving
The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs. A similar design of audio data encoding makes compact disc (CD) playback robust.