a novel method of carrier frequency offset estimation for ofdm systems -mingqi li and wenjun zhang...
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A Novel Method of Carrier Frequency Offset Estimationfor OFDM Systems
-Mingqi Li and Wenjun ZhangIEEE Transactions on Consumer 966 Electronics, Vol. 49, No. 4, NOVEMBER 2003
Advisor: Yung-An KaoStudent: Chian Young
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outline
• Intoduction
• OFDM system Model
• Continual-based CFO estimation algorithm
• Scattered-based CFO estimation algorithm
• Performance evaluation
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Introduction(1/2)
• This paper proposed a method using scattered pilots to estimate integer and fractional CFO, which are usually estimated by continual pilots.
• Ex: DVB-t pilot structure:
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Introduction(2/2)
• The advantage of using scattered pilots to estimate CFO:– Scattered pilot are usually use to estimate
channel freq. response (CFR), using them at the same time to estimate freq. offset.
will not bring about additional cost for the system
Increasing spectral efficiency by saving the use of continual pilots
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OFDM system model(1/2)
• The N samples of OFDM symbol l for OFDM demodulation(RX signal):
• al,k: tx signal at k-th subcarrier l-th OFDM symbol
Hl,k: channel freq. response
Ng: guard interval length N: FFT length
Ns= Ng +N
wl,k: complex value of AWGN noise
ξ samples of timing offset
Integer part of CFO
Fractional part of CFO
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OFDM system model(2/2)
• The demodulation complex data k-th subcarrier of OFDM symbol l:
• attenuation:
offset by integer CFO
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Continual-based CFO estimation algorithm-estimation of integer CFO
• The algorithm estimates integer CFO according to the offset of the correlating peak of continual pilots in two consecutive OFDM symbol:
• Accumulating all correlation samples xk belonging to the special set
k←C+m, integer CFO searching range :m← I, I=[-171,171]
• By searching the maximum absolute value |yl,m|, we obtain integer CFO at OFDM symbol l:
Pilot index
data index
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Continual pilots-Based CFO estimation algorithms-estimation of fractional CFO
• After compensating the integer CFO, zl,k becomes:
• correlating continual pilots in demodulation output of two consecutive OFDM symbols l −1 and l (assume the channel and timing offset is constant during two OFDM symbol):
• the fractional CFO estimate at OFDM symbol l :
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Scattered-based CFO estimation algorithm-estimation of integer CFO(1/3)
• The scattered pilots are known in the receiver. The four scattered
pilots vector:
• Moving correlating respectively with demodulation output:
• According to the maximum peak, it can be decided that which kind of pilot can be transmitted:
• Then according to the indices offset of the peak value, integer CFO estimate at OFDM symbol l :
Q: total number of scattered pilots
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Scattered-based CFO estimation algorithm-estimation of integer CFO(2/3)
• In the matched case, the peak value is:
to eliminate timing offset
the peak value:
Sl,n+Sl,Q-n+1 is constant for all n:
the peak value:
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Scattered-based CFO estimation algorithm-estimation of integer CFO(3/3)
To reduce the effect of CFR
the peak value:
The sum item is less effect by CFR
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Scattered pilots-Based CFO estimation algorithms-estimation of fractional CFO
• assume that receiver is correct sampling and the received two consecutive OFDM symbols have same timing offset
• correlating scattered pilots in demodulation output of two consecutive OFDM symbols l −1 and l:
m=(lmod4),l=0,1…∞, ⊕denotes addition module 4
• the fractional CFO estimate at OFDM symbol l:
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Simulation environment and parameters
• Channel:– AWGN channel– Fixed multipath Rayleigh fading channel with 20 paths– Time varying multipath channel (GSM TU with six taps Jacks fa
ding model)
• OFDM parameters:– DVB-T system 2k mode– 1/16 guard interval– 16 QAM modulation– Timing offset is 10 samples ,but no ISI
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Integer CFO estimation performance
• CFO=10.3Δf, 5000 OFDM symbols for the simulation• Prob. of failure :
Prob. of failure in AWGN and fixed multipath channel Prob. of failure in time-varying channel
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Fractional CFO estimation performance
• The initial CFO is set to 0.1Δf
Tracking performance of fractional CFO In time varying channels
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Fractional CFO estimation performance
BER performance of fractional CFO In time varying channels with initial CFO=0.1Δf
BER performance of fractional CFO In time varying channels with initial CFO=0