1. A Novel Method of Carrier Frequency Offset Estimation for OFDM Systems -Mingqi Li and Wenjun Zhang IEEE Transactions on Consumer 966 Electronics, Vol. 49, No. 4, NOVEMBER 2003 Advisor: Yung-An Kao Student: Chian Young

2. outline Intoduction OFDM system Model Continual-based CFO estimation algorithm Scattered-based CFO estimation algorithm Performance evaluation

3. 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:

4. 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

5. OFDM system model(1/2) The N samples of OFDM symbol l for OFDM demodulation(RX signal): a l,k : tx signal at k-th subcarrier l-th OFDM symbol H l,k : channel freq. response N g : guard interval length N : FFT length N s = N g +N w l,k : complex value of AWGN noise ξ samples of timing offset Integer part of CFO Fractional part of CFO

6. OFDM system model(2/2) The demodulation complex data k -th subcarrier of OFDM symbol l : attenuation: offset by integer CFO

7. 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 x k belonging to the special set k←C+m, integer CFO searching range : m← I, I=[-171,171] By searching the maximum absolute value |y l,m |, we obtain integer CFO at OFDM symbol l: Pilot index data index

8. Continual pilots-Based CFO estimation algorithms -estimation of fractional CFO After compensating the integer CFO, z l,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 :

9. 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

10. 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: S l,n +S l,Q-n+1 is constant for all n : the peak value:

11. 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

12. 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:

13. 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 fading model) OFDM parameters: –DVB-T system 2k mode –1/16 guard interval –16 QAM modulation –Timing offset is 10 samples,but no ISI

14. Integer CFO estimation performance CFO=10.3Δf, 5000 OFDM symbols for the simulation Prob. of failure : Prob. of failure in AWGN and fixed multipath channelProb. of failure in time-varying channel

15. Fractional CFO estimation performance The initial CFO is set to 0.1Δf Tracking performance of fractional CFO In time varying channels

16. 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