Asynchronous multiplexing schemes are efficient than synchronous schemes, in a
bursty traffic environment of multiple access local area network (LAN), as fixed
bandwidth is not allocated among the users and there is no access delay. Fiber-
Optic Code-Division Multiple Access (FO-CDMA) is one such asynchronous multiplexing
scheme suitable for high speed LAN networks. While FO-CDMA offers
potential benefits it also faces challenges in three diverse areas which are
1) coding algorithms and schemes
2) advanced encoding and decoding hardware and
3) network architecture.
In this thesis, as a solution to the first challenge, we propose the design and
construction of a new family of codes, wavelength/time multiple-pulses-per-row
(W/T MPR) codes. These codes have good cardinality, spectral efficiency and
minimal cross-correlation values. Performance analysis of the W/T MPR codes is
carried out and found to be superior to other codes.
In unipolar 1-D Optical Orthogonal Codes (OOCs) proposed by Salehi et al.,
the ratio of code length/code weight grows rapidly as the number of users is increased
for a reasonable weight. Hence, for a given pulse width, the data rate decreases
or in other words for a given data rate very narrow pulses have to be used,
because of which dispersion effects will be dominant. To overcome the drawbacks
of non-linear effects in large spread sequences of 1-D unipolar codes in FO-CDMA
networks, several two-dimensional codes have been proposed. Wavelength-time
(W/T) encoding of the two-dimensional codes is practical in FO-CDMA networks.
W/T codes reported so far can be classified mainly into two types:
1) hybrid sequences, where one type of sequence is crossed with another to improve
the cardinality and correlation properties and
2) matrix codes, 1-D sequences converted to 2-D codes or 2-D codes by construc-
tion, to reduce the ’time’ spread of the sequences/codes.
Prime-hop and eqc/prime W/T hybrid codes have been proposed where one type
of sequence is crossed with another to improve the cardinality and correlation
properties. Other constructions deal with conversion of 1-D sequences to 2-D
codes either by using Chinese remainder theorem or folding GoLomb rulers. W/T
single-pulse-per-row (W/T SPR) codes are 2-D codes constructed using algebraic
method Addition Modulo Group operation.
Motivation for this work: To design a family of 2-D codes which have
the design choice of length of one dimension over the other, and also have better
cardinality, spectral efficiency and also low cross-correlation values (thereby have
low BER) than that of the reported unipolar 2-D codes.
In this thesis, we describe the design principles of W/T MPR codes, for in-
coherent FO-CDMA networks, which have good cardinality, spectral efficiency
and minimal cross-correlation values. Another feature of the W/T MPR codes is
that the aspect ratio can be varied by trade off between wavelength and temporal
lengths. We lay down the necessary conditions to be satisfied by W/T MPR codes
to have minimal correlation values of unity. We analytically prove the correlation
results and also verify by simulation (of the codes) using Matlab software tool. We
also discuss the physical implementation of the W/T MPR FO-CDMA network
with optical encoding and decoding.
We show analytically that when distinct 1-D OOCs of a family are used as the row vectors of a W/T MPR code, it will have off-peak autocorrelation equal to ‘1’. An expression for the upper bound on the cardinality of W/T MPR codes is derived. We also show that 1-D OOCs and W/T SPR codes are the limiting cases of W/T MPR codes.
Starting with distinct 1-D OOCs, of a family, as row vectors, we propose a
greedy algorithm, for the construction of W/T MPR codes and present the repre-
sentations of the results. An entire W/T MPR code family, generated using greedy
algorithm, is simulated for various number of interfering users.
Performance analysis of the W/T MPR codes and their limiting cases is carried
out for various parameter variations such as the dimensions of wavelength, time
and weight of the code. We evaluate the performance in terms of BER, capacities
of the networks, temporal lengths needed (to achieve a given BER). Multiple access
interference (MAI) signal can be reduced, by using a bistable optical hard-limiter
device in the W/T MPR code receiver, by eliminating those signal levels which
exceed a certain preset level. Performance analysis of the W/T MPR codes and
their limiting cases is studied for various parameter variations.
For given wavelength × time dimensions, we compare various W/T codes, whose
cardinalities are known, and show that W/T MPR family of codes have better
cardinality and spectral efficiency than the other (reported) W/T codes.
As W/T MPR codes are superior to other W/T codes in terms of cardinality,
spectral efficiency, low peak cross-correlation values and at the same time have
good performance, makes it a suitable coding scheme for incoherent FO-CDMA
access networks.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.ncsi.iisc.ernet.in:2005/323 |
| Date | 07 1900 |
| Creators | Shivaleela, E S |
| Contributors | Srinivas, T |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Rights | I grant Indian Institute of Science the right to archive and to make available my thesis or dissertation in whole or in part in all forms of media, now hereafter known. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. |
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