Optical Coding Schemes 81
WDM MUX is used to collect incoming wavelengths that match those wavelengths
in its assigned (address) spectral codeword, and the bottom WDM MUX collects the
rest of the wavelengths arriving at the receiver. For the caseof1-Dspectralampli-
tude coding, the “+1” elements of a bipolar codeword, such as the maximal-length
sequences, are conveyed by wavelengths, the “−1” elements are emulated through
subtraction at the balanced photodetectors. This is done by routing the wavelengths
corresponding to the “−1” elements of the address codeword to the bottom WDM
MUX by properly setting th e 1 ×2opticalswitches.Thetunabledelay-linesareall
set to gi ve zero time-delay. Wavelength tunability is achie ved by the 1 ×M optical
router, which routes incoming 1-D spectral-amplitude codewords to one of the in-
put ports of the M ×M AWG device for the proper amount of wavelength rotations.
Optical samplers ar e used to improve the signal-to-interf erence ratio and reduce the
bandwidth requirement of the balanced photodetectors. If one of the arriving code-
words matches the address codeword of the receiver, an autocorrelation peak results
and is then passed to an electronic hard-limiter/regenerator for data-bit-1 recovery.
For other bipolar codes, such as Walsh codes and modified maximal-length se-
quences, tunability is achieved via the use of 1 ×2opticalswitchestoroutethe
wavelengths corresponding to the “+1” and “−1” elements of the address codeword
to the true and conjugated decoders, respectively. Also, thetunabledelay-linesare
all set to give zero time-delay. For 2- D spectral-temporal amplitude coding, time
despreading is performed by the tunable delay-lines. The un used wavelengths are
routed to the bottom WDM MUX and then ignored by disabling the lower optical
sampler.
2.11 POTENTIAL APPLICATIONS
As hardware technologies mature, optical coding has been gathering attention and
proposed for various applications [33, 34, 36, 37, 39, 60, 92–104]. Experimental O-
CDMA testbeds at 10 Gbit/s ha v e been successfully demonstrated. For example, Br´es
et al. [36] demon strated an incoherent, tunable, wavelength-time-coding testbed sup-
porting 8 simultaneous users by means of four-code keying with the carrier-hopping
prime codes in Section 5.1. Hernandez et al. [35] demonstrated a coherent, spectral-
phase-coding testbed, supporting 32 simultaneous users by transmitting 8 Walsh
codewords in two time slots and two polarizations with forward error correction.
Both coherent and incoherent (or synchronous versus asynchron ous) coding have
their pros and cons. Which one to use depends on the application and operation en-
vironment. In fact, they can be used together to complement their deficiencies. For
example, coding-based passive optical networks have been recently proposed with
the downlink traffic transported by coherent (or synchronous) codes and the uplink
traffic carried by incoherent (or asynchronous) codes [60, 92].
Desirable features of o ptical c oding, in particular 2-D spe ctral-temporal amp li-
tude coding, include dynamic bandwidth assignment, efficient in bursty traffic, asyn-
chronous and un co ordinated statistical multiple-user access, high scalability for sup-
porting more users by simply adding codewords, flexible code-cardinality enlarge-
ment by means of increasing the nu mber of wavelengths and/or time slots indepen-
82 Optical Coding Theory with Prime
dently, performance degradation gracefully under heavy traffic, p o ten tial data o b-
scurity, and the support of multiple bit rates, variable QoS,multimediaservices.It
is predictable that optical coding, in particular O-CDMA, isslowlyreplacingO-
TDMA and WDMA as means of co ntention resolution in optical systems, and grad-
ual upgrade strategies from O-TDMA or WDMA to O-CDMA are beingsought.
For example, O-CDMA has been proposed to incorporate into various network
applications, such as local and metropolitan area networks,burst-modeswitching,
ring networks, passive optical networks, optical interconnects, optical wireless, op-
tical interconnects, optical microarea networks, and O-CDMA-to-WDM gateways
for long-haul WDM backbone [3 3, 34, 3 6, 39, 92, 93 , 95–97, 99, 101]. In addition,
optical coding finds other applications, such as IP-routing,in-servicemonitoring,
and fiber fault surveillance in optical networks and sensor identification in fiber-
sensor systems, which use optical codes for the purpose of address or user identifi-
cation [94, 98, 100, 104].
Last but not least, optical coding theory has unexpectedly found application in
the area of preventing four-wave-mixing crosstalk in high-capacity, long- h aul, re-
peaterless, WDM lightwave systems due to fiber nonlinearities. While Forghieri
et al. [105, 106] prop o sed the use o f unequal channel spacingsinordertoprevent
four-wave-mixing crosstalk from falling in wavelength channels and solved for the
channel spacings b y mean s of integer linear programming withcomputerexhaustive
search, Kwong et al. [107, 108] formulated optimal solutionsalgebraicallybyrec-
ognizing that the problem was identical to the construction 1-D temporal-amplitude
codes with ideal autocorrelation sidelobes.
2.12 SUMMARY
In this chapter, various coding techniques and enabling hardware technologies of
the seven major categories of optical coding schemes were investigated. Support-
ing the transmission of multirate, multimedia services by means of mu ltiple-length
codes, and incr easing bit rate by means of m ulticode and shifted-code keying were
also r eviewed. AWG- an d FBG-based programmable encoder/decoder designs were
investigated. Finally, the potential applications of optical coding were discussed.
Among these major op tical codin g schemes, 2- D spectral-temporal amplitude (or
wavelength-time) coding is found to be the most advantageousbecauseof1)sim-
plicity: asynchronous access (no need of global clock), little scheduling, supporting
bursty traffic, gradual performance degradation under heavyload,andlesssensitive
to fiber nonlinearities; 2) ease of implementation: tunable and supporting dy n am ic
services, such as variable QoS and multiple data rates; 3) larger code size and flexi-
ble coding in wavelength and time independently; 4) more functionalities: support-
ing multiple bit rates and QoS by varying code length and weight, better obscurity
by means of hopping in wavelength and time, and supporting multicode and shifted-
code keying for better spectral efficiency and code obscurity; and 5) better scalability
and compatibility: trading bandwidth for scalability, com patible with WDM technol-
ogy, and supporting overlay and gradual upgrade strategies.
Optical Coding Schemes 83
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