njit-etd2003-081 - New Jersey Institute of Technology

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deviations are interpreted as A(q) =1 — 0.9960q -1 — 0.0019e 0 and the standard deviation
of "1" is zero (naturally enough, since it is not estimated). The standard deviation of al is
0.0041, a 2 through a9 is 0.0058 and a10 is 0.0041.
Note that leading zeros in the B(q) polynomial indicate the delay. In this case,
Figure 5.56 shows the actual iibi output (derived HRV signal) (blue) and the
estimated model simulated output (green). From this figure, it is cleared that the simple
ARX model does not estimate the amplitude of the HRV output well. However, the
period of the simulated output mimics the heart rate variability period very closely. The
period of the iibi signal is the desired information needed to be obtained from the model
and the amplitude needs not be matched exactly since the iibi of the ECG gives twice the
HRV information. The estimated model provides the period variation information and
this is all one can ask for out of the simple ARX model provided by the System
Identification.