4www.intechopen.com x k/t of the state x k for a fixed k based on the observations {Y t , t = k + 1, k + 2, ···}. Algorithms for computingx k/t , t = k + 1, k + 2, ··· , recursively are called fixed-point smoothers. For the second problem, the fixed-interval smoothing, we investigate the optimal estimatex t/N of the state x t at all times t = 0, 1, ··· , N based on the observation Y N of all the outputs {y 0 , y 1 , ··· , y N }. Fixed-interval smoothers are algorithms for computingx t/N , t = 0, 1, ··· , N recursively. The third problem, the fixed-lag smoothing, is to investigate the optimal estimatê x t−L/t of the state x t−L based on the observation Y t for a given L ≥ 1. Fixed-lag smoothers are algorithms for computingx t−L/t , t = L + 1, L + 2, ··· , recursively. See the references such as Anderson & Moore (1979), Bryson & Ho (1969), Kailath (1975) and Meditch (1973) for early research works on smoothers. More recent papers have been published based on different approaches such as stochastic realization theory (e.g., Badawi;Lindquist & Pavon (1979) and Faurre; Clerget & Germain (1979)), the complementary models (e.g., Ackner & Kailath (1989a), Ackner & Kailath (1989b), Bello;Willsky & Levy (1989), Bello; Willsky; Levy & Castanon (1986) Desai; Weinert & Yasypchuk (1983) and Weinert & Desai (1981)) and others. Nice surveys can be found in Kailath; Sayed & Hassibi (2000) and Katayama (2000). When stochastic systems contain unknown inputs explicitly, Tanikawa (Tanikawa (2006)) obtained a fixed-point smoother for the first problem. The second and the third problems were discussed in Tanikawa (2008). In this chapter, all three problems are discussed in a comrehensive and self-contained manner as much as possible. Namely, after some preliminary results in Section 2, we derive the fixed-point smoothing algorithm given in Tanikawa (2006) in Section 3 for the system with unknown inputs explicitly by applying the optimal filter with disturbance decoupling property obtained in Tanikawa & Sawada (2003). In Section 4, we construct the fixed-interval smoother given in Tanikawa (2008) from the fixed-point smoother obtained in Section 3. In Section 5, we construct the fixed-lag smoother given in Tanikawa (2008) from the optimal filter in Tanikawa & Sawada (2003). Finally, the new feature and advantages of the obtained results are summarized here. To the best of our knowledge, no attempt has been made to investigate optimal fixed-interval and fixed-lag smoothers for systems with unknown inputs explicitly (see the stochastic system given by (1)-(2)) before Tanikawa (2006) and Tanikawa (2008). Our smoothing algorithms have similar recursive forms to the standard optimal filter (i.e., the Kalman filter) and smoothers. Moreover, our algorithms reduce to those known smoothers derived from the Kalman filter (see e.g., Katayama (2000)) when the unknown inputs disappear. Thus, our algorithms are consistent with the known smoothing algorithms for systems without unknown inputs.
PreliminariesConsider the following discrete-time linear...