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MIMO Paraunitary Condition

With the above definition for paraconjugation of a MIMO transfer-function matrix, we may generalize the MIMO allpass condition Eq.$ \,$(C.2) to the entire $ z$ plane as follows:


Theorem: Every lossless $ p\times q$ transfer function matrix $ \mathbf{H}(z)$ is paraunitary, i.e.,

$\displaystyle {\tilde{\mathbf{H}}}(z) \mathbf{H}(z) = \mathbf{I}_q $

By construction, every paraunitary matrix transfer function is unitary on the unit circle for all $ \omega$. Away from the unit circle, the paraconjugate $ {\tilde{\mathbf{H}}}(z)$ is the unique analytic continuation of $ \overline{\mathbf{H}^T(e^{j\omega})}$ (the Hermitian transpose of $ \mathbf{H}(e^{j\omega})$). Example: The normalized DFT matrix is an $ N\times N$ order zero paraunitary transformation. This is because the normalized DFT matrix, $ \mathbf{W}=[W_N^{nk}]/\sqrt{N},\,n,k=0,\ldots,N-1$, where $ W_N\isdef e^{-j2\pi/N}$, is a unitary matrix:

$\displaystyle \frac{\mathbf{W}^\ast}{\sqrt{N}} \frac{\mathbf{W}}{\sqrt{N}} = \mathbf{I}_N $

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Properties of Paraunitary Systems
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MIMO Paraconjugate