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# The error covariance matrix updated sequential fashion

2.6 COMPUTATIONAL CONSIDERATIONS 47

of the derivative matrices described above. This method computes the global scaling matrix Ak by recursively applying the matrix inversion lemma. This procedure provides results that are mathematically identical to conventional matrix inversion procedures, regardless of the degree of decoupling employed. In addition, it can be employed for training of any form of network, static or dynamic, as well as for the multistream procedure. On the other hand, we have found that this method often requires the use of double-precision arithmetic to produce results that are statistically identical to EKF implementations based on explicit matrix inversion methods.

The second class, developed by Plumer [17], treats each output component individually in an iterative procedure. This sequential update procedure accumulates the weight vector update as each output compo-nent is processed, and only applies the weight vector update after all output signals have been processed. The error covariance matrix is updated in a sequential fashion. Plumer’s sequential-update form of EKF turns out to be exactly equivalent to the batch form of GFKF given above in which all output signals are processed simultaneously. However, for decoupled EKF training, it turns out that sequential updates only approximate the updates obtained via the simultaneous DEKF recursion of Eqs. (2.12)–(2.15), though this has been reported to not pose any problems during training.

k;l�1hi
rk;l þP

k;lak;l;
ðhi k;lÞTPi k;l�1hi k;l #�1 ;

48 2

PARAMETER-BASED KALMAN FILTER TRAINING

Note that the scalar rk;l is the lth diagonal element of the measurement covariance matrix Rk in the simultaneous form of DEKF, that the scalar xk;l is the lth error signal, and that the vector hi k;lis the lth column of the augmented derivative matrix Hi k. After all output signals of all training streams have been processed, the weight vectors and error covariance matrices for all weight groups are updated by

2.6.4
2.6.4.1

Without Artificial Process Noise

described the use of square-root filtering as a numerically stable, alternative method to performing the approximate error covariance matrix update given by the Riccati equation (2.6). The square-root filter methods are well known in the signal processing community [19], and were developed so as to guarantee that the positive-definiteness of the matrix is maintained throughout training. However, this insurance is accompanied by increased computational complexity. Below, we summarize the square-root formula-tion for the case of no artificial process noise, with proper treatment of the EKF learning rate as given in Eq. (2.7) (we again assume Sk ¼ IÞ. The square-root covariance filter update is based on the matrix factorization lemma, which states that for any pair of J � K matrices B1 and B2, with J � K, the relation B1B1 T ¼ B2B2 T holds if and only if there exists a unitary matrix Y such that B2 ¼ B1Y. With this in mind, the covariance update equations (2.3) and (2.6) can be written in matrix form as

" R1=2 k " # "
#

A1=2 kHT kPk

# :

ð2:52Þ
0
¼
# "

PkHkA1=2

0

P1=2
kþ1

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