TABLE OF CONTENTS In Chapter 5, we discussed some pilot-assisted DFT window timing and frequency offset estimation methods and a pilot-assisted subcarrier recovery method. In this chapter, we discuss a blind or pilotless joint DFT window timing, frequency offset, and DFT window width estimation method based on a maximum likelihood criterion.
Several blind parameter estimation methods have been proposed for OFDM systems [1, 2]. As compared with a pilot-assisted approach, in general, a blind approach requires longer observation symbols to give accurate estimates for parameters we want to estimate. On the other hand, as shown in Chapters 3 and 4, to maintain orthogonality among subcarriers even in multipath fading channels, each OFDM symbol is cyclically extended with a guard interval, whose waveform is exactly the same as the tail of the symbol itself [see Figure 3.8(a)]. In other words, an OFDM transmitter transmits the same waveform twice in each symbol period that is still unknown, but we can deal with it as "an unknown pilot signal," unlike a normal pilot signal whose waveform we know. This waveform structure introduces the cyclostationary property of an OFDM signal [3]. Making effective use of that very property, we can obtain a good estimate within shorter observation symbols [4, 5].
This chapter is organized as follows. A system model is presented in Section 6.2. Section 6.3 analyzes the maximum likelihood parameter estimation for cyclostationary signal. Numerical results and discussions are summarized in Section 6.4. Finally, conclusions are given in Section 6.5.
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