DESIGN AND SYSTEM IMPLEMENTATION OF PULSE POSITION MODULATION (PPM) BASED CODING SYSTEMS

The physical layer or modulation scheme plays a key role in a communication system, where performance features like Bit-Error Rate (BER), bandwidth efficiency and sensitivity are all dependent on the type of modulation scheme used. Currently, there are numerous modulation schemes for any given communication system, requiring the designer to decide which modulation scheme to apply. Many researchers propose different modulation schemes as the optimal for a given system with the aid of mathematical models and equations. However, there is minimal evidence on the practical implementation and testing of difference schemes to fully justify the selection. The scope of this research is to practically analyse, compare and validate the performance of different Pulse Position Modulation (PPM) schemes, where PPM is preferred for modulating and demodulating the signal in optical communications. This work presents, for the first time, practical analysis and comparison of different PPM techniques including Digital PPM (DPPM), Multipulse PPM (MPPM), Offset PPM (OPPM), Dicode (DiPPM) and Duobinary PPM (DuoPPM). The system implementation of these PPM techniques was carried under identical operational system conditions using Hardware-in-loop (HIL) approach, to validate the performance. A visible light communication (VLC) system incorporating a high power commercial 20 W LED was used for the implementation of PPM schemes. An FPGA was used to encode the message into PPM formats and to transmit it over the LED. A comprehensive comparison was performed between several PPM schemes in terms of BER, power estimation and bandwidth utilisation. Additionally, a new modified form of MPPM, called modified MPPM (MPPM64), was proposed in this study, which improves the bandwidth utilisation of the communication system by 14.28%. Furthermore, a new error correction method for OPPM, called Priority Decoding, was proposed to improve the BER of OPPM. Experimentation revealed the improved performance of OPPM by achieving 10 times fewer errors with BER of less than 10-8. A testbench was developed, which enables the user to apply any PPM from the given PPM schemes to a communication system. This testbench can be used to evaluate the performance of the communication system and to find the suitable PPM scheme which will deliver the best performance. Error correction techniques including Parity check and Cyclic Redundancy Check (CRC) were implemented to improve system performance. MPPM64 was implemented with Parity check and CRC achieving 0.4 m in transmission distance at identical transmission speeds when compared with the original scheme. Maximum Likelihood Detection (MLSD) was implemented with DiPPM and DuoPPM, showing 40% and 37.4% theoretical improved performance, respectively. In the practical implementation of MLSD, a 10 times achievement was recorded in BER at 1.5 m, outperforming original DiPPM and DuoPPM schemes. Determining the most appropriate PPM scheme for a VLC or any given system is not straight forward as it depends on many system parameters. However, this work enables the user to identify the most appropriate scheme for any given VLC system. This work enables different system parameters such as BER, transmission distance, power estimation and bandwidth utilization to be taken into account when determining the most appropriate setup. A detailed comparison is shown to guide modulation scheme selection in optical applications based on different parameter limitations

Analysis of Offset Pulse Position Modulation

This work presents the performance analysis of the offset pulse position modulation (PPM) scheme using graded-index plastic optical fibre with a Gaussian impulse response. The aim of this analysis is to predict how sensitivity, error, number of required photons, threshold voltage, and the effect of inter-symbol interference will change with the change in the number of data bits encoded at a rate of 1 Gbit/s. An information theory analysis is presented in detail and also the band-utilization efficiency is determined. Results are compared to equivalent digital PPM and multiple PPM schemes and it is also shown that offset PPM gives an advantage over on-off keying (OOK). Bit error rate (BER) analysis has been presented numerically. The errors due to different coding techniques are compared. It has also been shown that offset pulse position modulation is more power efficient than multiple pulse position modulation. The spectral analysis of offset pulse position modulation coding scheme has been carried out. For an offset PPM sequence the spectral characteristics is presented both theoretically and numerically. The results show strong frequency components at the frame rate and, if return-to-zero pulses are used, the slot rate. Slot synchronisation has been taken into consideration for the first time as offset PPM spectrum exhibits discrete slot rate component. The effect of pulse shaping and modulating index on the spectrum has been shown. The dependency of slot component on the pulse shape is examined. The results show that the frame synchronisation is possible for offset PPM as this coding exhibits a strong frame rate component. A comparison of spectral characteristics has been presented considering digital, multiple and shortened PPM. For ease of implementation an offset PPM coder has been designed. In this work an efficient clock recovery topology is presented for offset PPM data sequence at the receiver end. For clock recovery, a phase locked loop is designed. Data recovery has also been presented. It is shown that a frame clock can be extracted from the data sequence that yields the possibility of frame synchronization. A detailed noise analysis has been performed for random offset PPM input. It has been shown that the proposed clock recovery system is also effective for extracting other data sequence. To elucidate, a multiple Pulse Position Modulation (MPPM) data sequence is considered. The MPPM data sequence has also been synchronised with the recovered clock. A noise analysis is carried out for multiple PPM

Implementation of a VLC HDTV Distribution System for Consumer Premises

A unidirectional, visible light communication (VLC) system intended for the distribution of Digital Video Broadcasting (DVB), high-definition television (HDTV) content to DVB compatible TVs within consumer premises is presented. The system receives off-air HDTV content through a consumer grade DVB-T/T2 terrestrial set-top-box (STB) and re-encodes its Moving Picture Experts Group (MPEG) transport stream (TS) using a pulse position modulation (PPM) scheme called inversion offset PPM (IOPPM). The re-encoded TS is used to intensity modulate (IM) a blue light-emitting diode (LED) operating at a wavelength of 470 nm. Directed line-of-sight (DLOS) transmission is used over a free-space optical (FSO) channel exhibiting a Gaussian impulse response. A direct-detection (DD) receiver is used to detect the transmitted IOPPM stream, which is then decoded to recover the original MPEG TS. A STB supporting a high-definition multimedia interface (HDMI) is used to decode the MPEG TS and enable connectivity to an HD monitor. The system is presented as a complementary or an alternative distribution system to existing Wi-Fi and power-line technologies. VLC connectivity is promoted as a safer, securer, unlicensed and unregulated approach. The system is intended to enable TV manufacturers to reduce costs by, firstly, relocating the TV’s region specific radio frequency (RF) tuner and demodulator blocks to an external STB capable of supporting DVB reception standards, and, secondly, by eliminating all input and output connectors interfaces from the TV. Given the current trend for consumers to wall-mount TVs, the elimination of all connector interfaces, except the power cable, makes mounting simpler and easier. The operation of the final system was verified using real-world, off-air broadcast DVB-T/T2 channels supporting HDTV content. A serial optical transmission at a frequency of 66 MHz was achieved. The system also achieved 60 Mbit/s, error free transmission over a distance of 1.2 m without using error correction techniques. The methodology used to realise the system was a top-down, modular approach. Results were obtained from electrical modelling, simulation and experimental techniques, and using time-domain and FFT based measurements and analysis. The modular approach was adopted to enable design, development and testing of the subsystems independently of the overall system

Zur breitbandigen Infrarot-Indoorkommunikation

In der vorliegenden Arbeit wurden die wichtigsten physikalischen Aspekte der drahtlosen IR- Übertragung analysiert; wesentliche Ergebnisse flossen in den Entwurf und den Aufbau einer experimentellen IR-Schnittstelle ein, die bei 16-Mbit/s arbeitet. Der Experimentalaufbau zeigt: Unter der Prämisse sehr einfacher optischer Komponenten ist eine Bitrate von 16-Mbit/s realisierbar, wenn LOS-Verbindungen und vergleichsweise hohe Sendeleistungen akzeptiert werden. Der Aufbau bestätigt damit zuvor gewonnene theoretische Erkenntnisse. Der Entwurf der Schnittstelle wurde nachvollziehbar dargestellt, er verdeutlicht am praktischen Beispiel die wesentlichen Probleme der ungerichteten optischen Übertragung. Aus Sicht des Nutzers hat die flexible Plazierbarkeit eines Endgeräts eine hohe Bedeutung: Im Vergleich zu LOSKonfigurationen treten bei difusen Verbindungen aber neben erhöhten Einbußen durch Mehrwegedispersion auch deutlich höhere Ausbreitungsverluste auf. Dabei ist der Ausbreitungsverlust nicht die eigentliche Schwachstelle der optischen Übertragung, wenn als Referenz die Funkübertragung dient. Denn die difuse Reaktivität vieler Umgebungsmaterialien ist hoch, das haben die eigenen Messungen bestätigt. Das wesentliche Problem der IR- Übertragung ist die schlechte Empfängerempfindlichkeit; im Beispiel wies der optische Detektor einen im Vergleich zum Funkempfänger 46-56 dB niedrigeren Wert auf - letztlich muß dieser Parameter durch einen möglichst geringen Pfadverlust kompensiert werden. Da mit einer difusen Reflexion immer eine hohe Ausdünnung der Signalintensität korrespondiert, sind mit der heutigen Empfängertechnologie keine ökonomischen Lösungen für difuse oder quasidifuse Verbindungen möglich, wenn die Detektorfläche auf wenige cm2 begrenzt bleiben soll. Gerade bei zunehmend kleiner werdenden Endgeräten bzw. der wachsenden Verbreitung von Handhelds ist aber die Einbaugröße von hoher Bedeutung. "Nicht ökonomisch" bezieht sich dabei sowohl auf die notwendigen Herstellungskosten als auch auf die Sendeleistung. Einem leistungseffizienten Modulationsverfahren kommt in Anbetracht der limitierten Empfängerempfindlichkeit eine hohe Bedeutung zu. In diesem Zusammenhang wurde der theoretisch erreichbare Gewinn durch Binärcodierung dargestellt. Es wurde gezeigt, daß sich MPPM-Varianten mit großer Symbollänge dieser theoretischen Grenze immerhin bis auf etwa 3-4 dB nähern. Die tatsächliche Eignung eines Modulations- bzw. Codierverfahrens kann allerdings erst in Zusammenhang mit dessen Eigenschaften bei Übertragung in realen (dispersiven) Kanälen beurteilt werden: Für die experimentelle Schnittstelle zeigte "konventionelles" 4-PPM das ausgewogenste Verhalten. PPM höherer Ordnung verspricht bei ungerichteter Übertragung mit nicht-sektorisierten Komponenten aufgrund einer erhöhten Anfälligkeit gegenüber Mehrwegedispersion keine Vorteile bzgl. der Signalleistung. Optischen Mehrträgerverfahren kommt im Zusammenhang mit der aktuellen Empfängertechnologie dagegen keine relevante praktische Bedeutung zu: Letztlich kann die optische Übertragung aufgrund der großen spektralen Breite der Detektoren nur in geringem Maße von der eigentlichen Größe des optischen Spektrums proftieren. Die Parameter der optischen bzw. optoelektronischen Empfängerkomponenten haben wesentlichen Einfluß auf Gütekriterien wie die mögliche Bitrate (in diesem Zusammenhang wurde gezeigt, wie der Delay-Spread durch den FOV beeinflußt wird) und die schon angesprochene Empfängerempfindlichkeit. Technologische Aspekte zum Empfänger wurden ausführlich diskutiert. Es wurde eine neue Realisierungsmöglichkeit für ein optisches Filter vorgestellt, das in seiner einfachsten Variante einen Leistungsvorteil von ca. 1.5 dB gegenüber einem Farbglasfilter verspricht - bei ähnlichen Kosten. Wird von einer Silizium-Photodiode abgesehen, sind Gewinne von 5-6.5 dB realistisch. Ausführlich wurde auch die Photodiode behandelt, aus deren (quadratischer) Wandlungscharakteristik die prinzipiellen Unterschiede zum Funk resultieren. Avalanche-Photodioden können bei angepaßtem Verstärkerdesign (auch dieser Aspekt wurde sowohl theoretisch als auch am praktischen Beispiel diskutiert) keine Vorteile gegenüber PIN-Photodioden erreichen, weil die Empfängerempüdlichkeit selbst bei Imaging-Receivern durch das Schrotrauschen des Photostroms bestimmt wird. Silizium-Alternativmaterialien für größere Betriebswellenlängen scheitern dagegen zumindest bei großflächigen Dioden an einer zu hohen Kapazität. Überhaupt sind die Parameter einer Photodiode, die für drahtlose Anwendungen eingesetzt werden soll, nicht unmittelbar mit denen einer Diode vergleichbar, die für den faseroptischen Einsatz konzipiert wurde: Durch die große Fläche der Diode muß die Kapazitätä durch eine vergleichsweise hohe Schichtdicke kompensiert werden, die wiederum die Ladungsträgerlaufzeiten negativ beeinflußt. Tatsächlich muß eine Photodiode dem konkreten Anwendungsfall angepaßt werden - das zeigte sich gerade beim Aufbau der Experimentalschnittstelle anhand der zu geringen Grenzfrequenz einer eingesetzten Photodiode. Viele internationale Aktivitäten gelten heute der Untersuchung sektorisierter Sender und Empfänger. Soll die Datenrate der vorliegenden IR-Schnittstelle deutlich erhöht werden, wären zumindest auf der Seite der Basisstation sektorisierte Komponenten angebracht, wenn der abzudeckende Raumbereich beibehalten werden soll. Dabei muß allerdings eine genaue Kosten-Nutzen-Abwägung durchgeführt werden: Nach Ansicht des Autors müssen IR-Produkte letztlich deutlich preiswerter als Funklösungen sein, weil der Nutzer auch einen erheblichen Kompromiß hinsichtlich der Plazierbarkeit und der Störanfälligkeit eingehen muß. An dieser Tatsache wird auch eine im Vergleich zum Funk höhere Datenrate nur wenig ändern. Nach Ansicht des Autors können Verbesserungen vornehmlich auf technologischem Gebiet erreicht werden. Gerade für hoch-sektorisierte Sender und Empfänger sind einfachere Lösungen gefragt. Bei LOS-Verbindungen erspricht vor allem die Sektorisierung der Sender hohe Gewinne bzgl. der Signalleistung | allerdings benotigt ein solcher Sender auch eine Information über die Richtung des anderen Endgeräts. Auch hier fehlen einfache Lösungen. Ein besonders kritischer Punkt ist das optische Filter; ob ein Gewinn von 5-6.5 dB allerdings die Anwendung direkter Mischhalbleiter als Photodiodenmaterial rechtfertigt, kann vom Autor als Nicht-Technologen derzeit nicht mit Klarheit beantwortet werden. Nach Ansicht des Autors wird IR im WLAN-Bereich eine Nischenlösung bleiben. Das Potential liegt vornehmlich in sehr preiswerten Schnittstellen, die dem Nutzer über geringe Entfernungen einen besonders schnellen Datenabgleich zwischen Endgeräten ermöglichen. Evtl. bieten auch Doppellösungen von Funk und IR in einem Gerät Vorteile: Funk garantiert eine hohe Flexibilität, IR eine hohe Datenrate. Dazu muß die IR-Komponente aber klein und preiswert ausfallen

Quaternary pulse position modulation electronics for free-space laser communications

The development of a high data-rate communications electronic subsystem for future application in free-space, direct-detection laser communications is described. The dual channel subsystem uses quaternary pulse position modulation (QPPM) and operates at a throughput of 650 megabits per second. Transmitting functions described include source data multiplexing, channel data multiplexing, and QPPM symbol encoding. Implementation of a prototype version in discrete gallium arsenide logic, radiofrequency components, and microstrip circuitry is presented

Microwave vs optical crosslink study

The intersatellite links (ISL's) at geostationary orbit is currently a missing link in commercial satellite services. Prior studies have found that potential application of ISL's to domestic, regional, and global satellites will provide more cost-effective services than the non-ISL's systems (i.e., multiple-hop systems). In addition, ISL's can improve and expand the existing satellite services in several aspects. For example, ISL's can conserve the scarce spectrum allocated for fixed satellite services (FSS) by avoiding multiple hopping of the relay stations. ISL's can also conserve prime orbit slot by effectively expanding the geostationary arc. As a result of the coverage extension by using ISL's more users will have direct access to the satellite network, thus providing reduced signal propagation delay and improved signal quality. Given the potential benefits of ISL's system, it is of interest to determine the appropriate implementations for some potential ISL architectures. Summary of the selected ISL network architecture as supplied by NASA are listed. The projected high data rate requirements (greater than 400 Mbps) suggest that high frequency RF or optical implementations are natural approaches. Both RF and optical systems have their own merits and weaknesses which make the choice between them dependent on the specific application. Due to its relatively mature technology base, the implementation risk associated with RF (at least 32 GHz) is lower than that of the optical ISL's. However, the relatively large antenna size required by RF ISL's payload may cause real-estate problems on the host spacecraft. In addition, because of the frequency sharing (for duplex multiple channels communications) within the limited bandwidth allocated, RF ISL's are more susceptible to inter-system and inter-channel interferences. On the other hand, optical ISL's can offer interference-free transmission and compact sized payload. However, the extremely narrow beam widths (on the order of 10 micro-rad) associated with optical ISL's impose very stringent pointing, acquisition, and tracking requirements on the system. Even if the RF and optical systems are considered separately, questions still remain as to selection of RF frequency, direct versus coherent optical detection, etc. in implementing an ISL for a particular network architecture. These and other issues are studied

Comparison of direct and heterodyne detection optical intersatellite communication links

The performance of direct and heterodyne detection optical intersatellite communication links are evaluated and compared. It is shown that the performance of optical links is very sensitive to the pointing and tracking errors at the transmitter and receiver. In the presence of random pointing and tracking errors, optimal antenna gains exist that will minimize the required transmitter power. In addition to limiting the antenna gains, random pointing and tracking errors also impose a power penalty in the link budget. This power penalty is between 1.6 to 3 dB for a direct detection QPPM link, and 3 to 5 dB for a heterodyne QFSK system. For the heterodyne systems, the carrier phase noise presents another major factor of performance degradation that must be considered. In contrast, the loss due to synchronization error is small. The link budgets for direct and heterodyne detection systems are evaluated. It is shown that, for systems with large pointing and tracking errors, the link budget is dominated by the spatial tracking error, and the direct detection system shows a superior performance because it is less sensitive to the spatial tracking error. On the other hand, for systems with small pointing and tracking jitters, the antenna gains are in general limited by the launch cost, and suboptimal antenna gains are often used in practice. In which case, the heterodyne system has a slightly higher power margin because of higher receiver sensitivity

Implementation of Offset Pulse Position Modulation

Optical fibre systems have played a key role in making possible the extraordinary growth in world-wide communications that has occurred in the last 25 years, and are vital in enabling the proliferating use of the Internet. Its high bandwidth capabilities, low attenuation characteristics, low cost, and immunity from the many disturbances that can afflict electrical wires and wireless communication links make it ideal for gigabit transmission and a major building block in the telecommunication infrastructure. The main concern of this thesis is a full and detailed investigation and implementation of the Offset Pulse Position Modulation (Offset PPM) communication system. Novel work is carried out for applying Offset PPM over an optical communication channel theoretically and experimentally to examine the system performance. An Offset PPM encoder and decoder were implemented to code Pulse Code Modulation (PCM) format into Offset PPM format and to decode back the Offset PPM to PCM. The first paradigm of implementation was implemented using electronic components. A further investigation took place on the Offset PPM associated output. Computer programming and simulation using the VHSIC Hardware Description Language (VHDL) of this PPM code was considered and comparison with previous theoretical results presented. The received Offset PPM signal returned back to its original input PCM form without errors. Successful VHDL and Field Programmable Gate Array (FPGA) implementation using Altera Quartus II of Offset PPM encoder and decoder as a single system has been presented in the study. An FPGA embedded Bit Error Rate (BER) test device has also been implemented for sensitivity measurements purposes and all the designs have been tested successfully with back-to-back testing. Results show that Offset PPM is an advantageous PPM code for optic communication. Furthermore, the system has achieved a very high data rate of 50 Mb/s without an optical communication set. An optical communication system (transmitter/receiver) over POF was developed and the Offset PPM scheme was investigated through this optical channel. Results show that the Offset PPM sequence transferred through the optic system without being altered. In addition, this implementation is optimised PPM coding; the system is working perfectly with up to 10 Mb/s with 10-12 BER based on the limitations of the optical communication set. All the results and analyses indicate that Offset PPM is an ideal alternative to be considered for highly dispersive optical channels, and performance evaluation for higher bandwidths also favourably compares to existing coding schemes

Theoretical Analyses and Practical Implementation of Duobinary Pulse Position Modulation Using Mathcad, VHDL, FPGA and Purpose-built Transceiver

Duobinary pulse-position modulation (PPM), a novel channel coding scheme, has been proposed in this thesis as an alternative method of improving bandwidth utilisation efficiency and sensitivity over existing coding schemes such as digital PPM, dicode PPM, multiple PPM and offset PPM while operating over slightly or highly dispersive graded-index (GI) plastic optical fibre (POF) channels of limited bandwidth. Theoretical investigation based on simulations of mathematical models with maximum likelihood sequence detection (MLSD) at 1 Gbps on-off keying (OOK) data shows that duobinary PPM significantly outperforms optimised digital PPM at low fibre bandwidths by 8.7 dB while only operating at twice the original pulse code modulation (PCM) data rate. It has also been shown at high fibre bandwidth duobinary PPM gives a sensitivity of -42.2 dBm which is favourably comparable to digital PPM seven-level coding sensitivity of -44.1 dBm. Results presented in the thesis also demonstrate that at very low normalised fibre bandwidths (below 1 and down to 0.43) duobinary PPM outperforms dicode PPM by 1.2 dB requiring 27 x 103 photons per pulse compared to 40.3 x 103 required by Dicode PPM. Due to the use of MLSD at low bandwidths, wrong-slot errors are completely eliminated, and the effect of erasure and false-alarm errors are significantly reduced thus resulting in significantly improved sensitivity. Successful VHSIC hardware description language (VHDL) and field programmable gate array (FPGA) implementation of duobinary PPM coder, decoder and MLSD as a single system has been presented in the thesis. An FPGA embedded bit error rate (BER) test device has also been implemented for sensitivity measurements purposes and all the designs have been tested successfully with back-to-back testing. A purpose-built VCSEL 850 nm wavelength based transceiver system has been designed and successful functional tests have been carried out. Maximum operational data rate of the transceiver is currently 622 Mbps to match the maximum operating frequency of the FPGA, however, it has the capability to operate up to 3.2 Gbps. Further work on receiver characterisation and slot and frame synchronisation of duobinary PPM is required. All the results and analyses indicate that duobinary PPM is an ideal alternative to be considered for highly dispersive optical channels, and performance evaluation for higher bandwidths also favourably compares to existing coding schemes with only twice the expansion of original PCM data rate