1. Ph.D Theses

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Design and Analysis of Reliable Wireless Optical Communication System for the Underwater Channels
(National Institute of Technology Karnataka, Surathkal, 2021) Naik, Ramavath Prasad.; Acharya, U Shripathi.
Internet of Things (IoT) is a technology that deals with devices and protocols employed to interconnect smart devices that are deployed in benign and hazardous environments to collect or exchange information. In recent years, attempts have been made to interconnect devices deployed in underwater environments. These devices have been employed in underwater environments to monitor physical phenomena such as climate and ocean monitoring and enable activities such as underwater surveillance and ocean exploration. This technology is referred to as the Internet of Underwater Things (IoUT). IoUT is a technology, which requires a high degree of information integrity, high data transfer rates and energy e ciency for e ective deployment. Optical signaling gives the exibility of providing high data-rates than that of acoustic and RF signaling for medium link-ranges. Hence, optical signaling is an excellent candidate to enable high speed IoUT communication link between the underwater devices. The devices that employ optical signaling to enable communication between underwater vehicles and the underwater channel together constitute the Underwater Wireless Optical Communication (UWOC) system. The transmitted optical data experiences beam attenuation, turbulence and pointing errors, all of which can contribute to introduce errors in the received data stream. We initiated the work in this thesis by conducting experimental work to determine various parameters a ecting the propagation of light in an underwater channel and determining the optimum wavelength for UWOC communication. This is followed by a discussion of channel models that are appropriate descriptors of weak and strong turbulence in the underwater medium. Analytic models describing strong and weak turbulence have been derived, and simulation studies (Monte- Carlo simulations) that determine the accuracy of these analytic models have been carried out. The performance of UWOC system is mainly dependent on the underwater turbulence, beam attenuation and pointing errors, to mitigate these e ects we have introduced multiple input multiple output (MIMO), forward error control codes and Space-Time Block Codes (STBCs) to the proposed UWOC system. In many instances, the UWOC link operating under the surface of the water has v to be linked with a RF system operating over the water surface. Such a link is referred to as a cooperative RF-UWOC system. Channel models for the combined RF-UWOC system have been drawn up. It has been recognized that in addition to channel induced impediments, a major cause of link outage is the introduction of pointing errors due to the physical displacement of the transmitter-receiver pair, which causes the Line of Sight (LOS) requirement to be disturbed. Loss of LOS has the potential to severely compromise the working of the UWOC/ RFUWOC system. The performance of the RF-UWOC (co-operative IoT and IoUT) system in the presence of pointing errors has been studied and various outage probabilities have been determined. We have concluded the technical contributions of the thesis by studying underwater image transmission through the turbulent oceanic medium and suggesting various remedial techniques for proper image reception and enhancement. The thesis has been concluded by drawing conclusions from the research work conducted in the thesis and suggesting avenues for further research.
Design Synthesis and Performance Evaluation of Codes with Good Rank Distance Properties for Wireless Communications and Information Storage Systems
(National Institute of Technology Karnataka, Surathkal, 2020) S, Raghavendra M.A.N.; Acharya, U Shripathi.
Rank-metric codes, a class of subspace codes, are error control codes that can be used to correct errors in applications that require two dimensional information transmission. In these applications, errors are confined to certain rows or columns or both. This is due to the nature of perturbations introduced by the channel. When errors are confined to a few columns (error bursts), error control codes possessing burst error correction capability can be employed. However, in scenarios where errors disturb the information transmission (all the columns), such that one or few rows are corrupted, burst error correcting codes by themselves fail to detect and correct all the errors. It has been shown that if error pattern is such that it has disturbed the information transmission uniformly (error matrix having rank less than certain value), then rank metric codes are the best choice for ensuring information integrity. The design and synthesis of rank error correcting codes started with the discovery of maximum rank distance (MRD) codes and maximum rank array codes (MRA) codes. These were mainly designed to overcome rank errors or crisscross errors. The search for codes with good rank distance properties continued and many low rate codes with good rank distance properties were identified within the class of Cyclic and Abelian codes. These were used to construct non-orthogonal Space Time Block codes (STBC). The application of the rank metric codes as Space-Time Block codes for MIMO systems has the potential to improve the performance of MIMO communication systems. In literature, Space- Time Block Code designs have been extracted from (m; 1) MRD codes, MRA codes and Full rank cyclic codes over the Galois fields F qm with rate 1=n. While these full rank codes had good rank-distance properties, they suffered from low spectral efficiencies and the lack of a suitable decoding algorithm. It was then felt that if high rate full rank codes could be synthesized from the family of Cyclic or Abelian codes, and an efficient decoding algorithm could be devised, it could lead to the design of highly efficient STBCs for wireless communication, codes for correcting crisscross errors in both storage media and power line communication. This motivated us to search for the existence of high rate full rank codes from within the families of Quasi-Cyclic, Cyclic and Abelian codes (polynomial codes). We have demonstrated that full rank high rate codes ican be found within the class of polynomial codes by specifying the procedure that can be used to construct (n; k) full rank codes over Fqm. Further, we have stated and proved theorems that allow the determination of the exact rank of these codes. A decoding algorithm based on the parity check matrix representation has been devised. It determines the unique solution if rank of the error vector R q(e) ≤ bm2−1c. The use of Galois Field Fourier Transform (GFFT) description of polynomial allows the specification of a direct relationship between the choice of k free transform components and rank of the corresponding codeword vector. Additionally, the use of GFFT provides an additional degree of freedom in the choice of k− free transform components for a specified rank requirement. This freedom can be employed to construct an index key based communication scheme, which can provide an additional layer of physical layer security. We have demonstrated that the bit error rate (BER) performance of the proposed codes as STBCs in wireless applications is superior to that of codes derived from MRD and MRA constructions. Rank preserving maps such as the Gaussian Integer map or Eisenstein-Jacobi integer map have been employed to synthesize STBC designs. The BER performance of these codes has been determined in power line communication applications also. It is observed that the performance is identical to that of Low Rank Parity Check codes (derived from Gabidulin codes). In addition, the proposed constructions provide flexibility as a large number of full rank codes meeting various needs can be easily synthesized. Thus, the focus of the research work reported in this thesis is the discovery of high rate full rank codes from the families of polynomial codes and assessment of their performance in a variety of applications. The performance of these codes is broadly superior to the state of the art in most cases and comparable in some instances. Hence, we believe that these codes can be gainfully used in many applications to strengthen the process of information transfer, storage and dissemination.
Investigation of Rank Distance Properties of Cyclic, Abelian Codes and Study of their Applicability for Error Correction in Wireless Devices
(National Institute of Technology Karnataka, Surathkal, 2021) S, Godkhindi Shrutkirthi.; Acharya, U Shripathi.
The useofmultiple-antennaconfigurationscoupledwithsuitablesignalprocessing techniques isoneofthemostpromisingtechniquetoachievehighdataratesandim- provedqualityofserviceinwirelesscommunicationsystems.Overtheyearsmany Multiple-Input Multiple-Output(MIMO)techniqueshavebeenproposedtoachieve high spectralefficiencies,higherdataratesandimprovedreliabilityofcommunica- tion. MIMOtechniqueshavebeenbroadlydividedintotwobroadcategories;Spatial Multiplexingtechniques(SMX)[Foschini(1996)],whichisemployedtoenhancethe capacity ofsystemandSpaceTimeCodingTechniques(STC)[Tarokhetal.(1998)], which havebeenemployedtoimprovethereliabilityofcommunications.InSMX,in- dependent streamsofinformationareradiatedfrommultipletransmitantennas.SMX achieveshighermultiplexinggain,butthistechniqueisnotsuitableforlargescale MIMO, duetoincreaseddecodingcomplexityatthereceiver.SpaceTimeBlockCodes (STBCs) arechannelcodeswhichmaximizespatialdiversity.STBCsarecapableof providingimprovedcodinggainforsamespectralefficiencycomparedwithequivalent diversityschemes.Thisinturnenhancestheoverallreliabilityofthewirelesscom- munication system.STBCscombatchannelimperfectionsandimprovetheintegrityof data transferbycombiningcodinggainwithdiversitygain[Alamouti(1998)]. Space Time(ST)techniquesemployingmultipleantennassufferfromexcessenergy consumption, duetotheactivationofallantennasandtheirpoweramplifierchains.A energyefficientMIMOtechniquenamely,SpatialModulation(SM)wasproposedby Mesleh etal.in[Meslehetal.(2008)].Thistechniqueachievesincreasedenergyeffi- ciencybyactivatingonlyasingleantennaatatime.Spectralefficiencyisfurtherim- provedbyconveyinginformationthroughthemediumofactiveantennaindices[Mesleh et al.(2008)].ThusSMisanenergyconservingtechniquethatutilizesthespatialdi- mension ofmultipleantennaconfigurationtoconveyadditionalinformation.Thecon- cept ofSMwasfurtherimprovedbyappendingaSTBCwiththeSMscheme.This i arrangement wasintroducedby[Basaretal.(2011)]andisknownasSpaceTimeBlock Coded SpatialModulation(STBC-SM).Mostoftheliteratureonthistopichasbeen devotedtothestudyanddesignofOrthogonalSTBC-SMs.However,ithasbeenob- servedthatseveralNon-orthogonalSTBCsprovideperformance(bothintermsofBit Error Rate(BER)andSpectralefficiency)thanconventionalOrthogonal-STBCs.This has leadustostudytheperformanceofseveralfamiliesofNon-OrthogonalSTBCs when employedoverdifferenttypesofchannelsinthisthesis. The useoferrorcontrolcodeenhancestheintegrityofdatatransfer,especiallywhen the communicationsystemisoperatinginanenvironmentperturbedbychannelinduced distortion andnoise.Thisisdonebyintroducingcontrollednumberofredundantbits into theinformationbitstream.Thedisciplineoferrorcontrolcodeshasevolvedfrom designing binarycodesforbinarysymmetricchannelstodesigningcomplexsymbol oriented codesforvarioustypesofchannelsincludingsemiconductor,opticalandmag- netic memories[Moon(2005)].Acomprehensivestudyoftheliteraturehasinspired us toexploretheuseoffullrankcodesoverSTBCs,STBC-SMsystemsandinother evolvingapplications. In thisthesiswehavedesignedaclassoffullrankNonorthogonalSpaceTime Block Codes(NSTBC)whicharederivedfromfullrankCyclicandAbeliancodes.It is observedthattheseNSTBCsoutperformconventionalSTBCswhenemployedover a varietyofchannelfadingconditions.Thedescriptionofresearchworkconducted commences inChapter3afterabriefdescriptionofthestateofart(literaturesurvey) giveninChapter2.InChapter3,wehaveproposedaclassofSpatiallyModulated Non OrthogonalSpaceTimeBlockCodes(SM-NSTBC)designedfrom n􀀀length non- binary Cycliccodesover GF(qm) where q is aprimeand m is orderoftheextensionfield (m n). TheproposedSM-NSTBCoutperformsconventionalSpatiallyModulated Orthogonal SpaceTimeBlockCodes(SM-OSTBC),STBC-SM,SMschemesandtheir variantsintermsofaveragebiterrorrate(ABER)performance. In chapter4,theconceptofreceivespatialmodulationhasbeenexplored.Thetech- nique ofPrecodedSM-NSTBCisproposedtoattainreceivespatialmodulation.Inthis scheme apartofinformationisconveyedbyreceiveantennaindicesinadditiontothe ii conventionallyradiatedNSTBCsymbols.Thisisachievedbyadoptingsuitableprecod- ing techniqueatthetransmitter.TheperformanceofproposedPrecodedSM-NSTBC scheme issynthesizedforspatiallycorrelatedandspatiallyuncorrelatedRayleighfad- ing environments. The designofacooperativecommunicationsystemwithAmplifyandForwardre- laying forSpatiallyModulatedNon-orthogonalSpaceTimeBlockCode(SM-NSTBC- AF) isdiscussedinChapter5.Asinglerelaywithmultipleantennaelementsisassumed to existbetweensourceanddestination.Theinformationtransmittedbythesourceis receivedbytherelayinfirsttimeslot,inthesecondtimeslotthisinformationisam- plified andforwardedtothedestination.Intheproposedscheme,adirectlinkbetween source anddestinationaswellasthecooperativelink(linkbetweensourceanddesti- nation througharelay)areassumedtoexistandoperateintandem.Itisobservedthat this approachprovidesimprovedABERperformanceoverconventionalschemes.This scheme hasbeencomparedwithequivalentschemesemployingamplifyandforward relaying, namelyCooperativeSTBC-SMschemesandCooperativeSMschemes. In Chapter6,thesystemandchannelmodelofHighAltitudePlatforms(HAPs)has been studied.TheeffectofImperfectChannelStateInformation(Imp-CSI)isinvesti- gatedtoobtaininsightintotheperformanceofHAP-MIMOlinkswhendeployedinreal life situations.TheperformanceoffullrankSM-NSTBC,STBC-SMandSM-OSTBC schemes overaHAP-MIMOenvironmentisanalyzed.ItisobservedthatproposedSM- NSTBC outperformscompetingschemesundertheconditionsofperfect-CSIavailabil- ity aswellasimperfect-CSI. In thelastpartofthisthesiswehaveinvestigatedthedesignofNSTBCsderived from non-binaryAbeliancodes.First,adescriptionofthetransformdomainproper- ties andrank-distancepropertiesofAbeliancodeshasbeenprovided.Thisisfollowed by thedesignandsynthesisofSpatiallyModulatedNon-orthogonalSpaceTimeBlock Codes derivedfrom n-length fullrankAbeliancodesover GF(qm). Itisobservedthat the proposedAbelianSM-NSTBCprovidesaperformanceimprovementwhencom- pared withCyclicSM-NSTBCandotherconventionalSTBCs. In summary,inthisthesiswehaveutilizedtherankdistancecharacterizationof iii Cyclic andAbeliancodes.ThecodewordsderivedfromCyclicandAbeliancodeswhen viewedas (m n) matrices possessfullrank.Byemployingsuitablerankpreserving maps wehavesynthesizedfullrankNon-orthogonalSTBCsfromthesefullrankcodes. SM-NSTBC schemeswhichachievehighreliability,highspectralefficiencywithmod- erate decodingcomplexityhavebeendesigned.Theperformanceofproposedschemes havebeenanalyzedunderRayleighfadingenvironments.Further,wehaveexplored the conceptofreceivespatialmodulationandhavesynthesizedschemeswhichemploy full rankNon-orthogonalSTBCs.Thisisfollowedbythederivationofatechnique which employscooperativeSM-NSTBCwithamplifyandforwardrelayingtoimprove the overalllinkperformance.WehavealsoproposedaMIMOschemewhichcanbe employedonaHighAltitudePlatformcommunicationsystem.Theperformanceofthis scheme overHAP-MIMOenvironmenthasbeendetermined.Thedesignofallsystems has beendonekeepinginviewofthe3GPPcommunicationstandards. It isexpectedthattheresultscontainedinthisthesiswillbeusefulforthedesigns of moderncommunicationsystemssuchas5Gandbeyond-5G.