Digital Signal Processing Reference
In-Depth Information
Tabl e 1
Typical parameter values for single mode fiber used in long haul commu-
nications
Parameter
Symbol
Value
Wavelength
λ
1
,
550 nm
2
.
997925
×
10
8
Propagation velocity
c
m/s
0
.
2
×
10
−
3
dB/m
Fiber attenuation
α
Dispersion
D
17 ps/nm-km
β
2
=
−
Dc
(
10
−
6
)
/
(
2π
f
c
)
20
×
10
−
27
s
2
Second order dispersion
/
m
1
.
1
×
10
−
40
s
3
Dispersion slope
β
3
/
m
10
12
.
×
Center frequency
f
c
193
1
Hz
10
−
20
m
2
W
−
1
Core refractive index
n
2
2
.
6
×
10
−
12
m
2
×
Core area
A
90
10
−
3
m
−
1
W
−
1
Nonlinearity
γ
=
2
π
n
2
/
λ
A
5
×
N
m
−
1
2
where
0
|
h
k
|
=
1,
N
m
is the number of propagating modes, an amount equal to
∑
k
=
2
of the incident power is coupled into the
k
th mode,
|
h
k
|
τ
k
is the differential modal
delay with respect to the overall baseline delay of
τ
0
induced by the wavelength of
the source,
is the attenuation, and
L
is the fiber length. In the presence of modal
coupling, the terms
h
k
would become time-varying and can be modelled as Rayleigh
α
2.3
Receiver Models
The receiver processing in both back-plane and optical links begins with a filter that
removes out-of-band noise. In back-plane systems, the receive filter is an electrical
component, which also serves as an amplifier, and an equalizer. In an optical link,
the optical signal is optically filtered and then passed on to a device for optical-to-
electrical (O/E) conversion, which generates an electrical current signal. We refer
to this device as the O/E detector in order to distinguish it from statistical detectors,
such as slicers, that appear later in the receive chain. A transimpedance amplifier
(TIA) which converts the detector current into a voltage signal for subsequent
processing.
Both links have synchronization sub-systems referred to as a clock-recovery unit
(CRU), which recovers a baud-rate clock from the receiver signal. The output clock
of a CRU is employed to sample the output of the receive filter (or the TIA in case
of an optical link), using a decision-making latch or slicer. The combination of the
CRU and slicer is referred to as a clock-data recovery (CDR). A CDR, including the
front-end filter (TIA in case of optical links), implements the bulk of processing
in optical and electrical receivers. However, a CDR employs little or no signal
processing and thus fails to achieve the requisite
BER
in the presence of ISI. As
a result, modern day receiver for both link types, include additional components
such as equalizers (both digital and analog), ADCs, in addition to a CDR. In the
optical context, such signal processing enhanced links are called EDC-based links.
