Coarse WDMCDMTDM concept for optical packet transm

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/3240476
Coarse WDM/CDM/TDM concept for optical packet transmission inmetropolitan
and access networks supporting 400 channels at 2.5 Gb/speak rate
ArticleinJournal of Lightwave Technology · January 2001
DOI: 10.1109/50.908792·Source: IEEE Xplore
CITATIONS
33
READS
315
7 authors, including:
Some of the authors of this publication are also working on these related projects:
ACCORDANCE ("A Converged Copper-Optical-Radio OFDMA-based access Network with high Capacity and Flexibility") View project
High-capacity PON View project
Thomas Pfeiffer
Nokia Bell Labs (Stuttgart Germany)
122 PUBLICATIONS1,539 CITATIONS
SEE PROFILE
Jörg-Peter Elbers
ADVA Optical Networking SE
145 PUBLICATIONS1,509 CITATIONS
SEE PROFILE
All content following this page was uploaded by Jörg-Peter Elbers on 11 September 2013.
The user has requested enhancement of the downloaded file.

1928 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 18, NO. 12, DECEMBER 2000
Coarse WDM/CDM/TDM Concept for Optical Packet
Transmission in Metropolitan and Access Networks
Supporting 400 Channels at 2.5 Gb/s Peak Rate
Thomas Pfeiffer, Jens Kissing, Jörg-Peter Elbers, Bernhard Deppisch, Martin Witte, Harald Schmuck, and
Edgar Voges
Abstract—To improve the networking flexibility in the
metropolitan and access area, the granularity in the optical do-
main has to be increased above that in the core network requiring
more channels at lower bit rates. Pure dense wavelength division
multiplexing (DWDM) as it is applied in the core does not seem to
meet this requirement at affordable cost. We propose and analyze
a network based on hybrid optical multiplexing techniques in-
cluding wavelength, code, and time division multiplexing. Applied
to optical packet transmisssion this approach enables several 100
all-optical channels between end users and headend with average
bit rates up to 100 Mb/s per channel while keeping the installation
and maintenance cost at a minimum.
Index Terms—Code division multiplexing, multiaccess commu-
nication,networkreliability, optical crosstalk,opticalnoise,packet
switching, time division multiplexing, wavelength division multi-
plexing.
I. INTRODUCTION
IN metropolitan area networks (MAN) the required number
of independent optical channels is likely to increase to 100
withchannel bit ratesup to theGb/s range withinthe near future.
This fine granularity enables flexible utilization of fiber band-
width and simple network reconfiguration. Network operators
will be able to lease single optical channels or groups of op-
tical channels to service providers on demand. Even in access
networks with multiple optical channels, the increase of total
throughput to the Gb/s range is anticipated [1]. It is, however,
questionable whether the straight forward scaling of today’s
dense wavelength division multiplexing (DWDM) technology,
as it is presently used in the core network, to the specific re-
quirements of MAN and access networks is an affordable op-
tion for the realization of 100 optical channels. Specially for
packet-based services like Internet with average data rates per
user that are up to two orders of magnitude lower than the peak
rate [2] the implementation of DWDM in network parts close
to the end user will not be a realistic option. The spectral effi-
ciency of such networks would be extremely poor making this
approach very expensive.
Manuscript received April 19, 2000. This work was supported in part by the
German Ministry for Research within the Photonik II and KomNet projects.
T. Pfeiffer, B. Deppisch, M. Witte, and H. Schmuck are with Alcatel Corpo-
rate Research Center, D-70499 Stuttgart, Germany
J. Kissing J.-P. Elbers, and E. Voges are with Lehrstuhl für Hochfrequen-
ztechnik, University Dortmund, Germany.
Publisher Item Identifier S 0733-8724(00)11609-3.
With increasing channel number and taking into account the
optical bandwidth of fiber networks that is mostly limited by
the gain bandwidth of optical amplifiers the channel spacing
will rapidly drop below the current 100 GHz and 50 GHz spac-
ings. The technological and economical effort that has to be
spent to realize even smaller optical channel spacings increases
dramatically and is hardly justified by the required channel ca-
pacity. The main issues to be encountered at the optical trans-
mitters and receivers, when the channel spacing drops down to
the 10 GHz range, are selection, tuning and control of the laser
center frequency, the requirement for external modulation even
for low channel bit rates and the fabrication and control of nar-
rowbandoptical filters. Even without considering additional im-
pairments from fiber transmission (e.g., channel crosstalk due to
nonlinear effects) and optical amplification (gain flatness, tran-
sient effects) these systems are expected to become extremely
demanding with respect to component specifications and net-
work management.
For packet-based transmission systems the additional ap-
plication of time and/or code division multiplexing (TDM,
CDM) techniques in the network is a favorable way to increase
the channel number. TDMA (time division multiple access] is
a viable option for increasing the channel number in simple
fiber network structures with limited capacity to several 100
[3]. CDM techniques can also be applied to more complex
network architectures, since some optical CDM approaches
can be realized to support asynchronous operation of channels.
Moreover, with asynchronous CDM a statistical multiplexing
gain can be achieved in packet transmission networks enabling
a large number of channels to be allocated. The bursty nature
of the traffic helps to reduce the performance degradation due
to multiple access interferences. Alternatively, optical TDMA
can be applied to a limited part of the network, typically in
the access part, whereas in the MAN part WDM and CDM
techniques are implemented.
There have been different proposals how to implement op-
tical CDM applying time domain [4], [5] or wavelength domain
[6], [7] coding. Also, the combination of WDM and CDM tech-
niques has been proposed in some papers [8]–[10]. We favor
spectrally encoded CDM due to the fact that, in contrast to most
time domain encoded CDM approaches, the different channels
do not have to be synchronized to each other to achieve full
orthogonality between codes. This guarantees independence of
the optical multiplexing scheme from the network architecture.
Moreover, in time domain CDM the optical pulses would have
0733–8724/00$10.00 © 2000 IEEE
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE

PFEIFFER et al.: OPTICAL PACKET TRANSMISSION SYSTEM 1929
to be much shorter than the bit period, so that the optical com-
ponents must satisfy Gb/s specifications even for Mb/s channel
bit rates.
In this paper, we present a detailed system proposal for the
implementation of CDM enhanced WDM based on previous
experimental and theoretical studies that we have performed on
a special form of spectrally encoded optical CDM. Our CDM
approach applies periodic spectral encoding of broadband
optical sources like light emitting diodes (LEDs) [11]. This
approach has some advantageous features like stability and
insensitivity of system performance with respect to component
drift and specifications making it very attractive for MAN and
access applications. On the networking side the benefits are
asynchronous operation of different optical channels, support
of different signal formats and ease of redistributing electrical
bandwidth among optical channels. After a short review of the
operating principle these aspects will be discussed in the next
section with reference to experiments that have been performed
in multichannel system demonstrators transmitting continuous
data streams. Then the basic system limitations, namely
channel crosstalk and optical intensity noise, will be discussed
yielding some simple rules of thumb to estimate the achievable
channel number and bit rate. A full numerical simulation is
used for system evaluation when using different transmitter
and receiver configurations. From this analysis two network
scenarios for optical multichannel transmission are derived that
also include time domain features like statistical multiplexing
and synchronous TDMA within limited network areas. At the
end impacts from transmission of broadband optical signals are
discussed, specially from chromatic dispersion induced signal
distortions.
II. CONCEPT AND FEATURES OF PERIODIC SPECTRAL
ENCODING
The concept of periodic spectral encoding as proposed by
Möller [11] is a generalization of coherence multiplexing
[6] in the sense that all kind of optical filters having peri-
odic power transmission characteristics are considered for en-
coding and decoding. The optical output power of a thermal
light source, like an LED, is intensity modulated by the
electrical data. After emission the broadband spectrum is op-
tically shaped using passive filters with periodic transmission
functions like Mach–Zehnder (MZ) or Fabry–Perot (FP) fil-
ters (Fig. 1). The periodicity of the transmission function
for these type of filters is given in terms of the free spec-
tral range (FSR) which is defined by the filter round trip
time via . The round trip time is determined
by the filter geometry and given as for
MZ filters and for FP filters, where
and are the differential delay and the cavity length of the
filters, respectively, is the group index of the cavity ma-
terial and is the vacuum speed of light. In the network
different FSR are allocated to different optical transmitters
and define the codes in the system. At the receiver an op-
tical filter with periodic transmission function is tuned by
matching its FSR to the desired channel. The filter types
at transmitter and receiver do not have to be identical, only
the FSR or the round trip times and , respectively,
must be matched (Fig. 2). To achieve nearly perfect orthogo-
nality between channel codes a differential receiver set-up is
used that removes the offset of the received signal power in
Fig. 2 at . In the theoretical analysis of this
paper, (cf. Section III) it will be assumed that the common
mode rejection ratio (CMRR) of the differential receiver is
infinite, i.e., the signal level and group delay for the unde-
sired channels are assumed to be identical on the signal arm
and on the reference arm of the receiver. In our 155 Mb/s
differential receiver the CMRR was better than 40 dB for
frequencies up to 200 MHz [12]. The optical source spectra
are assumed to be much broader than the FSR of the trans-
mitter filters, or in other words, the source coherence time
defined by the source optical power density spectrum
as [13] be much smaller than
the filter round trip time . Then the relative positioning
of the source spectrum center frequency and the filter trans-
mission function is irrelevant which leads to the first benefit
of this approach: the selection of the source spectra and the
filter transmission is not critical as long as and
the difference of the round trip times for every
pair of transmitters and is in the order of the source
coherence time or larger. [In practice the gain spectra of
the optical amplifiers define the coherence time, since usually
they are narrower ( 30 nm) than the source spectra ( 50
nm).] This has been experimentally verified in an 8 155
Mb/s system where the transmitter LEDs were taken from two
different suppliers (MRV and Anritsu) with different spectral
width [FWHM (full width at half maximum) 53–68 nm]
and center wavelengths (1539–1551 nm) [12]. The filter FSR
in this system (MZ at the transmitters, FP with finesse 4 at
the receiver) were chosen in the range 10–20 GHz, the power
extinction of the transmitter filters was 10–13 dB and slightly
polarization dependent, the FP receiver filter extinction ratio
was only 9 dB. Nevertheless the system operated with bit
error rate (BER) . The second benefit of pe-
riodic encoding is that only the difference in filter roundtrip
times for any two transmitters needs to be larger
than some picoseconds. The absolute value of the is not
important as long as they are much larger than . From that
it follows that even temperature changes of more than 100
C do not affect the system performance as proven by exper-
iment [14]. The reason for this unsensitivity to temperature
changes is the low temperature coefficient of the refractive
index of glass (about 10 C for fused silica [15]). Our
MZ encoding filters were made from standard 3 dB fiber
couplers, so that for the above temper-
ature range. The fine tuning of the receiver filter to drifting
transmitter filter FSR was accomplished by applying a local
low-frequency dithering technique in the receiver [16], but
no control of the transmitter filter FSR was required to keep
[14].
The robustness of the system with respect to component spec-
ifications and drift as demonstrated above is unique to the ap-
proach applying periodic spectral encoding with broad source
spectra and small filter FSR. Other code multiplex systems like
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE

1930 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 18, NO. 12, DECEMBER 2000
Fig. 1. Principle set-up of CDM system applying periodic spectral encoding.
Fig. 2. Principle of periodic spectral encoding and decoding (left). Received optical power with single-ended detection around point of optimum tuning (right).
Encoding and decoding filter type have been exchanged with respect to Fig. 1 for more instructive visualization. Gaussian source spectrum is assumed with
.
-sequence encoded systems [7] are not expected to show sim-
ilar robustness due to the requirement of exact matching of the
aperiodic spectra at transmitter and receiver.
With systems as shown above, where the individual CDM
channels were modulated with independent continuous data
streams, different transmission experiments have been per-
formed. Transmission of 8 155 Mb/s over 111 km field
installed standard single-mode fiber (SMF) was demon-
strated [17] as well as over 204 km Alcatel TeraLight™ fiber
(ps/nm km @ 1550 nm) with for
each channel (Fig. 3). Transmission and splitting losses were
compensated by three cascaded erbium-doped fiber amplifiers
(EDFAs) and the chromatic dispersion of the fiber link was
compensated by using matched lengths of dispersion com-
pensating fiber (DCF). In more complex networks including
parallel groups of transmitters in a tree-like configuration the
capability of the chosen CDM approach to support different
channel bit rates was experimentally demonstrated [12]. In
these experiments, it was also found that the length tolerance
of the dispersion compensation using DCF was at least 4 km
SMF at this channel bit rate (cf. Section V). Even transmission
of quasi-analog electrical signals (16-QAM subcarriers) over
optical CDM parallel to baseband signals within the same
network was experimentally demonstrated [14]. These last
features cannot easily be realized with CDM systems based on
time domain coding.
III. BASIC LIMITS ON SYSTEM PERFORMANCE
After having reviewed the basic features of the optical CDM
approach applying periodic spectral encoding we will now dis-
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE

PFEIFFER et al.: OPTICAL PACKET TRANSMISSION SYSTEM 1931
Fig.3. Transmissionof8 155 Mb/s opticalCDM signals over204 km Alactel
TeraLight™ fiber.
cuss the main factors that pose an upper limit to the achievable
system capacity.
The BER is evaluated using the -factor approximation
, where erfc denotes the complemen-
tary error function and is defined as
with the current and for the “1” and “0” signal and the
associated standard deviations and . The noise on the “1”
and on the “0” is assumed to be identical and is set to 0, so
that with the differential receiver current and its
standard deviation . The main contributions to originate
from (thermal) receiver noise, shot noise, optical intensity
noise, and crosstalk. The variance of the latter two disturbances
increases linearly with electrical signal power and are respon-
sible for the potential occurrence of a BER floor, hence they
decide on the feasibility of a system configuration. They will
now shortly be discussed, under simplifying assumptions, how
they are influenced by the width of the optical source spectra
and by the optical filters. We first focus the discussion on
Gaussian source spectra and on combinations of MZ and FP
filters. The results are characteristic of the discussed system
and are only slightly modified if other spectra or filters are
used. In the next section of this paper realizations are presented
that are more useful in systems with many channels.
When in the following we talk about source spectra, it is as-
sumedthattheyarenotmodifiedduring transmission duetoe.g.,
optical amplifiers. Otherwise they have to be replaced in the for-
mulae by the respective spectra at the receiver. Some basic con-
siderations about crosstalk have been discussed in [18] (note:
the symbols used here slightly differ from those in [18]). Here,
we derive the number of optical channels that could be allocated
in a system, if crosstalk was the only limiting factor taking into
account the data statistics. For broad source spectra (as
compared to the filter FSR) with total power the
photocurrent after the receiver filter is given by the transmitter
and receiver filter transmission functions and
as where denotes the average over the
optical frequency . The photodetector response is omitted here
for simplicity. With a differential receiver the signal current
is given by , where
is the difference between the receiver transmission func-
tion in the filtered arm and the reference arm with
andwith theperiodicity .
When is detuned from the transmitter , the signal
varies in an oscillatory manner (Fig. 2) the details of which de-
pend on the specific choice of filters and source spectra. Spe-
cially with a FP–FP combination for transmitter and receiver
filtertheenvelopeof thetuning curveyieldsmaximaalsoaround
points, where is a rational number. This is in contrast
to systems, where, for example, the receiver filter is an MZ filter
yielding maxima only around points, where is an in-
teger. So in an FP–FP system the allocation of transmitter FSR
is preferably done by choosing as the ratio of large
prime numbers, whereas in an –MZ system ( being either
MZ or FP) the can be allocated to the transmitters
equidistantly within one octave with
or larger [18]. To simplify the discussion the latter case is as-
sumed with identical Gaussian source spectra for all transmit-
tersdescribedby .
Then the envelope of the tuning curve for an MZ receiver filter
with is described as function of
as [18]
(1)
with FWHM given by . The
parameter in (1) is the second coefficient of the transmitter
filter cos-series with the
value for MZ or for FP with
finesse . In the following, it is assumed that all transmitters
continuously send NRZ signals (non return to zero) with equal
probability for “0” and “1.” This results in a
binomial probability distribution of “1” signals at the receiver
which for many interfering transmitters is approximated by
a Gaussian distribution of the crosstalk current
with the mean value , where
is calculated from (1) for all transmitters with
. The standard deviation is given
by , where for channels
transmitting synchronously to each other and in the
asynchronous case [19]. With these results the crosstalk-limited
-factor for the channel with ( denotes the
integer part of its argument) can now be expressed as
(2)
with source FWHM and .
The number of addressable codes is very large (cf. Fig. 5)
if transmitter or receiver filter or both are MZ like filters. In
case of an FP/FP combination the crosstalk around points
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE