3- Bipolar [Alternate
Mark Inverted (AMI)] Signaling
This is the
signaling scheme used in PCM these days. A 0 is transmitted by no pulse, and a
1 is transmitted by a pulse p(t) or -p(t) depending on whether
the previous 1 was transmitted by -p(t) or p(t). With consecutive
pulses alternating. Bipolar signaling actually uses three symbols [p(t), 0,
and -p(t)]. and. hence, it is in reality ternary rather than binary
signaling. Bipolar RZ is differ from NRZ in that 1 is represented by a pulse p(t)
or -p(t) for a duration of Tb/2
seconds followed by zero voltage for a duration of Tb /2
seconds
Properties of Bipolar
Signaling
1.Transmission
bandwidth : The essential bandwidth of the signal is Rb (Rb
= 1/Tb), which is half that of polar or on-off signaling and
twice the theoretical minimum bandwidth. Observe that we were able to obtain
the bandwidth Rb, for the polar or on-off case for the full width
pulse. For the bipolar case, the bandwidth is Rb Hz, regardless of whether
the pulse is half-width or full-width. So we can say that the bandwidth of Bipolar
is not excessive.
2. Error
detection and correction capability : It has single-error-detection
capability. This is due to the fact that if a single detection error is made,
it
will cause a
violation of the alternating pulse rule and will be detected immediately.
3. power
spectral density : It has a zero PSD at dc (w = 0)
4. Power
efficiency: Bipolar signal requires twice as much power (3 dB) as a polar
signal. This is because bipolar detection is essentially equivalent to on-off
signaling from the detection point of view.
5. Transparency:
It is not transparent. In practice, various substitution schemes are used to
prevent long strings of logical zeros from allowing the extracted clock signals
to decay away.
6. Adequate timing content : A long
string of 0's causes the absence of a
signal and can create an error in timing extraction.
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