137 research outputs found
Applications of perfect difference codes in fiber-optics and wireless optical code-division multiplexing/multiple-access systems
After establishing itself in the radio domain, Spread spectrum code-division
multiplexing/multiple-access (CDMA) has seen a recent upsurge in optical
domain as well. Due to its fairness, flexibility, service differentiation and
increased inherent security, CDMA is proved to be more suitable for the bursty
nature of local area networks than synchronous multiplexing techniques like
Frequency/Wavelength Division Multiplexing (F/WDM) and Time Division
Multiplexing (TDM). In optical domain, CDMA techniques are commonly known
as Optical-CDMA (O-CDMA). All optical CDMA systems are plagued with the
problem of multiple-access interference (MAI). Spectral amplitude coding (SAC)
is one of the techniques used in the literature to deal with the problem of MAI.
The choice of spreading code in any CDMA system is another way to ensure the
successful recovery of data at the receiving end by minimizing the effect of MAI
and it also dictates the hardware design of the encoder and decoder.
This thesis focuses on the efficient design of encoding and decoding hardware.
Perfect difference codes (PDC) are chosen as spreading sequences due to their
good correlation properties. In most of the literature, evaluation of error
probability is based on the assumptions of ideal conditions. Such assumptions
ignore major physical impairments such as power splitting losses at the
multiplexers of transmitters and receivers, and gain losses at the receivers, which
may in practice be an overestimate or underestimate of the actual probability of
error.
This thesis aims to investigate thoroughly with the consideration of practical
impairments the applications of PDCs and other spreading sequences in optical
communications systems based on spectral-amplitude coding and utilizing codedivision
as multiplexing/multiple-access technique. This work begins with a
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general review of optical CDMA systems. An open-ended practical approach has
been used to evaluate the actual error probabilities of OCDM/A systems under
study. It has been concluded from results that mismatches in the gains of
photodetectors, namely avalanche photodiode (APDs), used at the receiver side
and uniformity loss in the optical splitters results in the inaccurate calculation of
threshold level used to detect the data and can seriously degrade the system bit
error rate (BER) performance. This variation in the threshold level can be
compensated by employing techniques which maintain a constant interference
level so that the decoding architecture does not have to estimate MAI every time
to make a data bit decision or by the use of balanced sequences.
In this thesis, as a solution to the above problem, a novel encoding and decoding
architecture is presented for perfect difference codes based on common zero code
technique which maintains a constant interference level at all instants in CDM
system and thus relieves the need of estimating interference. The proposed
architecture only uses single multiplexer at the transmitters for all users in the
system and a simple correlation based receiver for each user. The proposed
configuration not only preserves the ability of MAI in Spectral-Amplitude Coding
SAC-OCDM system, but also results in a low cost system with reduced
complexity. The results show that by using PDCs in such system, the influence of
MAI caused by other users can be reduced, and the number of active users can be
increased significantly.
Also a family of novel spreading sequences are constructed called Manchestercoded
Modified Legendre codes (MCMLCs) suitable for SAC based OCDM
systems. MCMLCs are designed to be used for both single-rate and Multirate
systems. First the construction of MCMLCs is presented and then the bit error rate
performance is analyzed.
Finally the proposed encoding/decoding architecture utilizing perfect difference
codes is applied in wireless infrared environment and the performance is found to
be superior to other codes
Hybrid Free-Space Optical and Visible Light Communication Link
V souÄastnosti bezdrĂĄtovĂ© optickĂ© komunikace (optical wireless communication, OWC) zĂskĂĄvajĂ ĆĄirokou pozornost jako vhodnĂœ doplnÄk ke komunikaÄnĂm pĆenosĆŻm v rĂĄdiovĂ©m pĂĄsmu. OWC nabĂzejĂ nÄkolik vĂœhod vÄetnÄ vÄtĆĄĂ ĆĄĂĆky pĆenosovĂ©ho pĂĄsma, neregulovanĂ©ho frekvenÄnĂho pĂĄsma Äi odolnosti vĆŻÄi elektromagnetickĂ©mu ruĆĄenĂ. Tato prĂĄce se zabĂœvĂĄ nĂĄvrhem OWC systĂ©mĆŻ pro pĆipojenĂ koncovĂœch uĆŸivatelĆŻ. SamotnĂĄ realizace spojenĂ mĆŻĆŸe bĂœt provedena za pomoci rĆŻznĂœch variant bezdrĂĄtovĂœch technologiĂ, napĆĂklad pomocĂ OWC, kombinacĂ rĆŻznĂœch OWC technologiĂ nebo hybridnĂm rĂĄdio-optickĂœm spojem. Za ĂșÄelem propojenĂ tzv. poslednĂ mĂle je analyzovĂĄn optickĂœ bezvlĂĄknovĂœ spoj (free space optics, FSO). Tato prĂĄce se dĂĄle zabĂœvĂĄ analĂœzou pĆenosovĂœch vlastnostĂ celo-optickĂ©ho vĂce skokovĂ©ho spoje s dĆŻrazem na vliv atmosfĂ©rickĂœch podmĂnek. V dneĆĄnĂ dobÄ mnoho uĆŸivatelĆŻ trĂĄvĂ Äas ve vnitĆnĂch prostorech kancelĂĄĆĂ Äi doma, kde komunikace ve viditelnĂ©m spektru (visible light communication, VLC) poskytuje lepĆĄĂ pĆenosovĂ© parametry pokrytĂ neĆŸ Ășzce smÄrovĂ© FSO. V rĂĄmci tĂ©to prĂĄce byla odvozena a experimentĂĄlnÄ ovÄĆena zĂĄvislost pro bitovou chybovost pĆesmÄrovanĂ©ho (relaying) spoje ve VLC. Pro propojenĂ poskytovatele datavĂœch sluĆŸeb s koncovĂœm uĆŸivatelem mĆŻĆŸe bĂœt vĂœhodnĂ© zkombinovat vĂce pĆenosovĂœch technologiĂ. Proto je navrĆŸen a analyzovĂĄm systĂ©m pro pĆekonĂĄnĂ tzv. problĂ©mu poslednĂ mĂle a poslednĂho metru kombinujĂcĂ hybridnĂ FSO a VLC technologie.The field of optical wireless communications (OWC) has recently attracted significant attention as a complementary technology to radio frequency (RF). OWC systems offer several advantages including higher bandwidth, an unregulated spectrum, resistance to electromagnetic interference and a high order of reusability. The thesis focuses on the deployment and analyses of end-user interconnections using the OWC systems. Interconnection can be established by many wireless technologies, for instance, by a single OWC technology, a combination of OWC technologies, or by hybrid OWC/RF links. In order to establish last mile outdoor interconnection, a free-space optical (FSO) has to be investigated. In this thesis, the performance of all-optical multi-hop scenarios is analyzed under atmospheric conditions. However, nowadays, many end users spend much time in indoor environments where visible light communication (VLC) technology can provide better transmission parameters and, significantly, better coverage. An analytical description of bit error rate for relaying VLC schemes is derived and experimentally verified. Nonetheless, for the last mile, interconnection of a provider and end users (joint outdoor and indoor connection) can be advantageous when combining multiple technologies. Therefore, a hybrid FSO/VLC system is proposed and analyzed for the interconnection of the last mile and last meter bottleneck
Synchronising coherent networked radar using low-cost GPS-disciplined oscillators
This text evaluates the feasibility of synchronising coherent, pulsed-Doppler, networked, radars with carrier frequencies of a few gigahertz and moderate bandwidths of tens of megahertz across short baselines of a few kilometres using low-cost quartz GPSDOs based on one-way GPS time transfer. It further assesses the use of line-of-sight (LOS) phase compensation, where the direct sidelobe breakthrough is used as the phase reference, to improve the GPS-disciplined oscillator (GPSDO) synchronised bistatic Doppler performance. Coherent bistatic, multistatic, and networked radars require accurate time, frequency, and phase synchronisation. Global positioning system (GPS) synchronisation is precise, low-cost, passive and covert, and appears well-suited to synchronise networked radar. However, very few published examples exist. An imperfectly synchronised bistatic transmitter-receiver is modelled. Measures and plots are developed enabling the rapid selection of appropriate synchronisation technologies. Three low-cost, open, versatile, and extensible, quartz-based GPSDOs are designed and calibrated at zero-baselines. These GPSDOs are uniquely capable of acquiring phase-lock four times faster than conventional phase-locked loops (PLLs) and a new time synchronisation mechanism enables low-jitter sub-10 ns oneway GPS time synchronisation. In collaboration with University College London, UK, the 2.4 GHz coherent pulsed-Doppler networked radar, called NetRAD, is synchronised using the University of Cape Town developed GPSDOs. This resulted in the first published example of pulsed-Doppler phase synchronisation using GPS. A tri-static experiment is set up in Simonâs Bay, South Africa, with a maximum baseline of 2.3 km. The Roman Rock lighthouse was used as a static target to simultaneously assess the range, frequency, phase, and Doppler performance of the monostatic, bistatic, and LOS phase corrected bistatic returns. The real-world results compare well to that predicted by the earlier developed bistatic model and zero-baseline calibrations. GPS timing limits the radar bandwidth to less than 37.5 MHz when it is required to synchronise to within the range resolution. Low-cost quartz GPSDOs offer adequate frequency synchronisation to ensure a target radial velocity accuracy of better than 1 km/h and frequency drift of less than the Doppler resolution over integration periods of one second or less. LOS phase compensation, when used in combination with low-cost GPSDOs, results in near monostatic pulsed-Doppler performance with a subclutter visibility improvement of about 30 dB
Study of modulation techniques for multiple access satellite communications
Multiple access communication utilizing small ground stations for satellite communication modulation - multiplexing technique
Space station tracking requirements feasibility study, volume 2
The objective of this feasibility study is to determine analytically the accuracies of various sensors being considered as candidates for Space Station use. Specifically, the studies were performed whether or not the candidate sensors are capable of providing the required accuracy, or if alternate sensor approaches should be investigated. Other topics related to operation in the Space Station environment were considered as directed by NASA-JSC. The following topics are addressed: (1) Space Station GPS; (2) Space Station Radar; (3) Docking Sensors; (4) Space Station Link Analysis; (5) Antenna Switching, Power Control, and AGC Functions for Multiple Access; (6) Multichannel Modems; (7) FTS/EVA Emergency Shutdown; (8) Space Station Information Systems Coding; (9) Wanderer Study; and (10) Optical Communications System Analysis. Brief overviews of the abovementioned topics are given. Wherever applicable, the appropriate appendices provide detailed technical analysis. The report is presented in two volumes. This is Volume 2, containing Appendices K through U
A note on the R sub 0-parameter for discrete memoryless channels
An explicit class of discrete memoryless channels (q-ary erasure channels) is exhibited. Practical and explicit coded systems of rate R with R/R sub o as large as desired can be designed for this class
The telecommunications and data acquisition
Radio astronomy and radio interferometry at microwave frequencies are discussed. Other topics concerning the Deep Space Network include program planning, planetary and interplanetary mission support, tracking and ground based navigation, communications, and station control and system technology
Cumulative index to NASA Tech Briefs, 1963-1965
Annotated bibliography of NASA technical briefs on electrical, energy sources, materials, life sciences, and mechanical informatio
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