16-10-2012, 12:42 PM
Adaptive OFDM
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Introduction
The bit error probability of different OFDM subcarriers
transmitted in time dispersive channels depends on the
frequency domain channel transfer function.
The occurrence of bit errors is normally concentrated in a set
of several faded subcarriers, while in the other subcarriers
often no bit errors are observed.
If the subcarriers that will exhibit high bit error probabilities
in the OFDM symbol to be transmitted can be identified and
excluded from data transmission, the overall BER can be
improved in exchange for a slight loss of system throughput.
As the frequency domain fading deteriorates the SNR of
certain subcarriers, but improves other subcarriers above the
average SNR value, the potential loss of throughput due to the
exclusion of faded subcarriers can be mitigated by employing
higher order modulation modes on the subcarriers exhibiting
high SNR values.
Adaptive Techniques
Adaptive modulation is only suitable for duplex
communication between two stations, since the transmission
parameters have to be adapted using some form of two–way
transmission in order to allow channel measurements and
signalling to take place.
Transmission parameter adaptation is a response of the
transmitter to time–varying channel conditions.
Adaptive Techniques - Channel Quality Estimation
For OFDM modems, the bit error probability in each subcarrier is
determined by the fluctuations of the channel’s instantaneous frequency
domain channel transfer function, if no interference is present. The estimate of
the channel transfer function can be acquired by means of pilot–tone based
channel estimation,
More accurate measures of the channel transfer function can be gained by
means of decision–directed or time–domain training sequence based
techniques.
The estimate of the channel transfer function does not take into account effects,
such as co–channel or inter–subcarrier interference.
Alternative channel quality measures including interference effects can be
devised on the basis of the error correction decoder’s soft output information
or by means of decision-feedback local SNR estimations.
The delay between the channel quality estimation and the actual transmission
of the OFDM symbol in relation to the maximal Doppler frequency of the
channel is crucial as regards to the adaptive system’s performance.
Adaptive Techniques- Parameter Adaptation
Low–cost, low power consumption mobile stations can
communicate with a base station that performs the channel estimation
and frequency–domain equalization of the uplink, and uses the
estimated channel transfer function for pre–distorting the down–link
OFDM symbol.
This setup would lead to different overall channel quality on the up–
and downlink, and the superior downlink channel quality could be
exploited by using a computationally less complex channel decoder
having weaker error correction capabilities in the mobile station than
in the base station.