In binary frequency shift keying (FSK), the given signal waveforms are

$u_{0}(t)=5\, cos(20000\pi t)\, ;0\leq t\leq T$, and

$u_{1}(t)=5\, cos(22000\pi t)\, ;0\leq t\leq T$

where $T$ is the bit-duration interval and $t$ is in seconds.Both $u_{0}(t)$ and $u_{1}(t)$ are zero outside the interval  $0\leq t\leq T$. With a matched filter (correlator) based receiver, the smallest positive value of $T$ (in milliseconds) required to have  $u_{0}(t)$ and $u_{1}(t)$ uncorrelated is

1. 0.25 ms
2. 0.5 ms
3. 0.75 ms
4. 1.0 ms

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