在正交分頻多工系統進行通道估計時，導引信號可以使用疏狀或塊狀分佈。我們
可以用塊狀分布的導引信號沿著時間用多項式估計出通道響應，而疏狀分佈則是沿著子通道使用通道脈衝響應模組去估計通道響應。假如取樣不在通道延遲時間點上時，通道脈衝響應模組會不再準確。在這種情況，我們需要在通道估測前先估計出多路徑延遲。

在此篇論文中，我們提出了在正交分頻多工通道下估計多路徑延遲的改良方法。
我們先用多項式配合塊狀分布的導引信號估計出通道響應，再使用估計出的通道響應去估計多路徑延遲，最後把估計出的多路徑延遲套回通道脈衝響應模組去進行通道估計。與僅使用導引信號來估計多路徑延遲的傳統方法相比，此方法使用更多通道響應來估計多路徑延遲，模擬結果顯示此方法可獲得更好的效能。

For the channel estimation of the orthogonal frequency-division multiplexing (OFDM)system, the pilot symbols can be arranged in the comb type or the block type scheme. One can model the channel responses (CR) along time slots by a polynomial of time indices for the block type pilot arrangement, while model the CRs along sub-channels by the channel impulse response (CIR) for the comb type pilot arrangement. For the scenario that the multipath delays are not on the sampling time instants exactly, the CIR model is no longer accurate. In this scenario, one needs to estimate the multipath delays before the channel estimation.

In this thesis, we propose an improved scheme for the estimation of multipath delays in the OFDM channel estimation. We can first use the block type pilot arrangement and apply the polynomial model of CR variation. Then we use these estimated CRs to estimate the multipath delays. With the estimated multipath delays, we can apply the CIR model with the comb type pilot arrangement. Comparing with traditional schemes that use only pilot symbols to estimate the multipath delays, our scheme uses more estimated CRs to estimate the multipath delays, and the simulation results show that our scheme obtains better performance.

[1] R. V. Nee and R. Prasad, OFDM for wireless multimedia communications, Artech House Publishers, 1999.
[2] M. X. Chang,“A new derivation of least-squares-fitting principle for ofdm channel estimation, ”IEEE Trans. Wireless Commun., vol. 5, no. 4, pp. 726-731, Apr. 2006.
[3] K. C. Hung and D. W. Lin,“Pilot-based lmmse channel estimation for ofdm systems with power-delay profile approximation, ”IEEE Trans. Veh. Technol., vol. 59, no. 1, pp. 150-159, Jan. 2010.
[4] R. Roy and T. Kailath,“Esprit-estimation of signal parameters via rotational invariance techniques, ”IEEE Trans. Acoust., Speech, Signal Process, vol. 12,no. 7, pp. 984-995, Jul. 1989.
[5] C. L. Wei,“A low-complexity channel estimation algorithm for ofdm systems based on parametric channel modeling, ”Master's thesis, National Tsing Hua University,R.O.C, July 2010.
[6] Y. Li and X. Hung,“The simulation of independent rayleigh fader, ”IEEE Trans. Commun, vol. 50, no. 9, pp. 1503-1514, Sept. 2002.
[7] M. X. Chang and Y. T. Su,“Performance analytic of equalized ofdm system in rayleigh fading, ”IEEE Trans. Commun, vol. 1, no. 4, pp. 721-732, Oct 2002.
[8] S. Coleri, M. Ergen, A. Puri, and A. Bahai,“Channel estimation techniques based on pilot arrangement in ofdm systems, ”IEEE Trans. Broadcast, vol. 48, no. 3,pp. 223-229, Sept. 2002.
[9] M.X. Chang and Y. T. Su,“Model-basedased channel estimation for ofdm signals in rayleigh fading, ”IEEE Trans. Commun, vol. 50, no. 4, pp. 540-544, Apr. 2002.
[10] M. R. Raghavendra, S. Bhashyam, and K. Giridhar,“Exploiting hopping pilots for parametric channel estimation in ofdm systems, ”IEEE Signal Process Lett.,vol. 12, no. 11, pp. 737-740, Nov. 2005.
[11] B. Yang, K. B. Letaief, R. S. Cheng, and Z. Cao, “Channel estimation for ofdm transmission in multipath fading channels based on parametric channel modeling, ”IEEE Trans. Commun, vol. 49, no. 3, pp. 467-478, Mar. 2001.