Explanation of IEEE 802.3, Clause 68 TWDP
Norman L. Swenson, Paul Voois, Tom Lindsay, Steve Zeng
ClariPhy Communications, Inc.
5 January 2006
This paper describes the TWDP test methodology used by the test procedure and algorithm specified in
IEEE 802.3 Clause 68. The penalty is defined as the difference (in dB) between a reference signal to
noise ratio (SNR) and the equivalent SNR at the slicer input of a reference decision feedback equalizer
(DFE) receiver for the measured waveform after propagation through a simulated fiber channel.
Reference SNR
The reference SNR is the SNR of an ideal channel with a matched filter receiver. For the ideal channel,
rectangular on-off keyed pulses are transmitted, and the transmitter and fiber are assumed perfect. The
received pulse is a rectangular pulse with an amplitude measured in OMA of OMA
RCV
and time duration
of one unit interval T. The receiver has a perfect matched filter front end matched to the rectangular
receive pulse. The output of the matched filter is sampled once per unit interval (without timing error)
and presented to the decision element (binary slicer).
The signal to noise at the slicer input is given by
( )
RCV 0
SNR OMA / 2
T N
=
(1)
where N
0
is the one-sided power spectral density of the additive white Gaussian noise assumed for the
receiver. The bit error rate is given by
BER (SNR)
Q
=
(2)
where Q( ) is the Gaussian error probability function
2
2
1
( )
2
x
y
Q y e dx
π
∞
−
=
∫
(3)
6.5 dB has been assigned as budget to allow for optical power penalties related to dispersion. Therefore,
the reference SNR, SNR
REF
, is set 6.5 dB optical (13 dB electrical) above the level required to give a
BER of 10
-12
for the ideal channel. Hence
