老師：高永安 學生：蔡育修

1. Channel Estimation and Equalization for High Speed Mobile OFDM SystemsHeejung Yu, Myung-Soon Kim, and Sok-Kyu Lee2003 IEEE 老師：高永安 學生：蔡育修

2. Outline • Introduction • System Model • Conventional Methods • Proposed Channel Estimation and Equali- zation Method • Simulation results

3. Introduction • Motivation： There is ICI due to the channel variation within one OFDM symbol in high speed mobile conditions. higher the mobile speed ↑，ICI↑. • The performance degradation due to the ICI becomes si- gnificant as the carrier frequency, OFDM symbol duratio- n and vehicle velocity increase.

4. At first, we suggest a channel model for the time selecti- ve fading channel. • Coherent time is very smaller than an observation interv- al, we can assume that the fading channel is varying in a linear fashion. • At receiver, we compose a channel matrix with these tap coefficients and slopes, and perform equalization by mul- tiplexing with the inverse channel matrix.

5. System Model

6. Provided that the Doppler frequency is enough small, we are able to neglect the channel variation within one OFDM symbol. Received OFDM symbol can be discribed as：

7. The frequency domain channel matrix , WHWH, is a diagonal matrix. And then, there is no ICI. • The Doppler frequency is high, there exists the channel variation even in one OFDM symbol duration. • The variation would be linear in certain condition. In detail, if the relative Doppler frequency (fdT, where fd is the Doppler frequency and T is the observation interval) is smaller than 0.1, the above statement is correct. Strange!!與introduction矛盾!!!

8. Carrier frequency 5GHz, observation interval 800μsec use the Jakes fading channel model. fdT = 0.7407

9. The time-selective fading channel within one OFDM symbol cab be modeled as following：

10. In this case, the frequency domain channel matrix, WVWH, is not a diagonal one and there exist ICI, induced from non-diagonal terms. • To avoid such ICI, we have to estimate the elements, hi and εi.

11. WVWH in 200km/h mobile environment： • Nondiagonal terms are getting larger with mobile speed. ICI

12. Conventional Methods • Using block type of pilot, the channel estimate from one pilot block is held until the next pilot block. • When we use the comb-type pilot, the channel estimate is updated by every new estimation result like the LMS algorithm.

13. LMS algorithm as following： In fast fading channel, the second one can achieve better performance than the first one. 感覺比較像time average!!!

14. The 1st order linear interpolator is used in estimating the channel between two block-type pilot symbols. Show better performance under the assumption that the channel variation is linear during the two pilot block interval.

15. Even though we use above two schemes, we cannot avoid ICI due to channel variation in one OFDM symbol. • For that reason, there is an error floor in high SNR. • To estimate and cancel the ICI, time domain channel estimation, such as channel coefficients and slopes, is necessary.

16. We perform equalization by multiplying with an inverse matrix of channel estimate to cancel ICI effect. In this paper, we use a block-type pilot to estimate initial channel coefficients and their slopes. Proposed Channel Estimation and Equalization Method

17. At first, we estimate slopes, elements of E. From the above relation, εi are estimated by the following equation because p is the known pilot vector.

18. Next, we estimate the initial channel taps hi. So we can apply the same method that used in the slope estimation.

19. The channel coefficient estimate is given by The channel compensation is performed with the inverse of this channel matrix. For an instance, the channel matrix of k-th OFDM symbol from the previous pilot block is

20. Simulation results

21. Simulation under 4 different mobile speed,50,100,150 and 200km/h.

22. The propose scheme is compared with conventional methods. comb-type：16 pilots subcarriers. block-type；the same pilot