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ICI Mitigation for Pilot-Aided OFDM Mobile Systems Yasamin Mostofi, Member, IEEE and Donald C. Cox, Fellow, IEEE IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO.2, MARCH 2005. 老師:高永安 學生:蔡育修. Outline. Introduction System model Piece-Wise Linear Approximation Method I

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slide1
ICI Mitigation for Pilot-Aided OFDM Mobile SystemsYasamin Mostofi, Member, IEEE and Donald C. Cox, Fellow, IEEEIEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO.2, MARCH 2005

老師:高永安

學生:蔡育修

outline
Outline
  • Introduction
  • System model
  • Piece-Wise Linear Approximation

Method I

Method II

  • Mathematical Analysis and Simulation Result
  • Noise/Interference Reduction
  • Simulation Results and Conclusion
introduction
Introduction
  • Transmission in a mobile communication environment is impaired by both delay and Doppler spread.
  • As delay spread increases, symbol duration should also increase.

reasons---1.near-constant channel in each frequency subband. 2.prevent ISI.

  • OFDM system become more susceptible to time-variations as symbol length increases. Time-variations introduce ICI. be mitigated to improve the performance.
slide4
We introduce two new methods to mitigate ICI. Both methods use a piece-wise linear model to approximate channel time-variations.
slide11
In the absence of mobility, L pilots would have been enough to estimate the channel.
  • However, in the presence of Doppler, due to the ICI term,

using them for data detection results in poor perfor-mance.

  • This motivates the need to mitigate the resultant ICI.
piece wise linear approximation
Piece-Wise Linear Approximation
  • We approximate channel time-variations with a piece-wise linear model with a constant slope over the time duration T.
slide13
For normalized Doppler of up to 20%, linear approxi-

mation is a good estimate of channel time-variations.

We will derive the frequency domain relationship.

Therefore, we approximate

slide14
Then,

we will have

slide17
To solve for X, both Hmid and Hslope should be estimated.
  • Matrix C is fixed matrix and Hmid is readily available.
  • So we show how to estimate Hslope with our two methods.
slide20
Equations (9) and (11) provide enough information to solve for X.
  • We use a simpler iterative approach to solve for X.
method ii ici mitigation utilizing adjacent symbols
Method II:ICI Mitigation Utilizing Adjacent Symbols
  • This can be done by utilizing either the previous symbol

or both adjacent symbols.

  • A constant slope is assumed over the time duration of

T+(N/2)*Ts for the former and T for the latter.

slide26
Method I and Method II can handle considerably higher

delay and Doppler spread at the price of higher compu-

tation complexity.

mathematical analysis and simulation result
Mathematical Analysis and Simulation Result
  • We define SIRave as the ratio of average signal power

to the average interference power.

  • Our goal is to calculate SIRave when ICI is mitigated and

compare it to the that of the “no mitigation” case.

noise interference reduction
Estimated channel taps are compared with a Threshold.

Let MAV represent the tap with maximum absolute value.

All the estimated taps with absolute values smaller than

MAV/γ for some γ>=1 will be zeros.

Noise/Interference Reduction
simulation results
Simulation Results
  • System parameters
slide30
The power-delay profile of channel#1 has two main taps

that are separated by 20μs.

  • The power-delay profile of channel#2 has two main clus-

ters with total delay of 36.5μs.

slide31
Each channel tap is generated as Jakes model.
  • To see how ICI mitigation methods reduce the error floor.

in the absence of

noise for both channels.

slide33
To see how ICI mitigation methods reduce the required

received SNR for achieving a Pb = 0.2.

conclusion
Conclusion
  • Both methods used a piece-wise linear approximation to

estimate channel time-variations in each OFDM symbol.

  • These methods would reduce average Pb or the required

received SNR to a value close to that of the case with no

Doppler.

  • The power savings become considerable as fd,norm incre-

ases.

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