With code division multiple access (CDMA) systems being the prominent multiple access scheme for the air interface for 3G cellular systems, most standardisation bodies have based their terrestrial cellular systems on DS-CDMA (W-CDMA, UMTS, cdma2000). With 4G systems fast approaching, bringing with them improved services and quality of service standards, there is growing interest in further investigating and developing more efficient multiple access techniques such as multicarrier CDMA (MC-CDMA) systems. MC-CDMA combines multicarrier modulation (MCM), namely OFDM, with CDMA profiting from the benefits of both multiplexing techniques; as such, MC-CDMA is emerging as a possible candidate for the air interface multiple access scheme for 4G cellular systems. Multiple access interference (MAI) is a limiting factor of CDMA systems in terms of system capacity as orthogonally designed spreading sequences lose their orthogonality in the presence of timing misalignments amongst mobile subscribers in a cell; such is the case over the uplink channel. Ensuring orthogonal code properties minimises the MAI over synchronous environments, however, it is when the users are allowed to transmit asynchronously, as is the case over the uplink channel, that MAI inflicts significant performance degradation. In CDMA systems, all subscribers are active on the same frequency band simultaneously and signal separation is facilitated upon reception via the properties of the assigned spreading codes. Under asynchronous conditions the code properties alone do not provide the necessary separation and an additive MAI term remains in the detection process. In addition to the separation abilities of the spreading codes, a further method of deciphering the desired subscriber signal from the interfering subscriber signals is sought. In this thesis we propose a statistical model for both the probability density function (pdf) of the total MAI power and the corresponding bit-error rate (BER) observed during asynchronous CDMA transmission. The modelling offers the full statistic the MAI power and resulting BER, not just the first and second order statistics. In addition to statistically quantifying the MAI power, the thesis also proposes a technique for the successful reduction of MAI caused by asynchronous transmission. This interference reduction technique is derived from an ambiguity domain analysis of the asynchronous CDMA detection problem and its application to both the DS-CDMA and MC-CDMA multiplexing techniques is presented and the results show significant MAI reduction, and thus an improved the BER. A methodology for the approximation of the total MAI power pdf and the resulting BER pdf is proposed for the asynchronous DS-CDMA and MC-CDMA techniques. This methodology is derived for the use of Walsh-Hadamard (WH) and Gold spreading sequences, however, it is applicable to any given set of deterministic spreading sequences. The total MAI power pdfs of both systems are statistically modelled as being Nakagamim distributed and the corresponding BER modelling is derived from the Nakagami-m formulation offering the full statistic of both the incurred MAI power and the achievable BER. The proposed pdf acquisition methodology and statistical models can be used as analysis tools to assess the relative performances of the DS-CDMA and MC-CDMA techniques for a variety of communications environments. Here the asynchronous uplink channel is considered in the absence of fading and the results show a clear distinction between the BER performances of the MC-CDMA and DS-CDMA systems, for which the MC-CDMA system offers a superior performance for the purely asynchronous channel considered. The results suggest a higher resistance to MAI in the MC-CDMA technique in comparison to the DS-CDMA system for the considered transmission scenario. Following ambiguity function analysis of the asynchronous CDMA detection problem, the concept of dual-frequency switching is introduced to the existing DS-CDMA and MC-CDMA techniques giving rise to the proposed dual-frequency DS-CDMA (DF/DSCDMA) and dual-frequency MC-CDMA (DF/MC-CDMA) schemes. Periodically switching the carrier frequency between dual frequency bands at consecutive symbol boundaries facilitates partial CDMA signal separation upon asynchronous reception. Such switching of the carrier frequency induces a separation in frequency between offset interference signals and the reference signal; this is equivalent to shifting the energy concentration of the interference signals away form the ambiguity domain origin (representing the decision variable of the matched filter). Further MAI reduction is demonstrated through careful design of the dual carrier frequencies. The newly proposed DF systems clearly outperform the standard DS-CDMA and MC-CDMA systems when adopting equivalent spreading factors. The DF/DS-CDMA technique in particular achieves the most MAI reduction and in doing so, surpasses all other considered techniques to offer the best BER performance for the purely asynchronous channel considered. In terms of bandwidth usage, the DF/DS-CDMA band width is 1.5 times that of the DF/MC-CDMA system and from the BER results presented, one may argue that DF/MC-CDMA offers the better BER given the bandwidth usage. The multicarrier systems presented, MC-CDMA and DF/MC-CDMA, offer attractive BER performances for the bandwidth used and it is concluded that MC-CDMA is a genuine candidate for the uplink air interface multiple access scheme for future mobile cellular technologies.

The interconnection of molecular machines with different functionalities to form molecular communication systems can increase the number of design possibilities and overcome the limited reliability of the individual molecular machines. Artificial information exchange using molecular signals would also expand the capabilities of single engineered cell populations by providing them a way to cooperate across heterogeneous cell populations for the applications of synthetic biology and lab-on-a-chip systems. The realization of molecular communication systems necessitates analysis and design of the communication channel, where the information carrying molecular signal is transported from the transmitter to the receiver. In this thesis, significant progress towards the use of microfluidic channels to interconnect molecular transmitter and receiver pairs is presented. System-theoretic analysis of the microfluidic channels are performed, and a finite-impulse response filter is designed using microfluidic channels. The spectral density of the propagation noise is studied and the additive white Gaussian noise channel model is developed. Memory due to inter-diffusion of the transmitted molecular signals is also modeled. Furthermore, the interference modeling is performed for multiple transmitters and its impact on the communication capacity is shown. Finally, the efficient sampling of the signal transduction by engineered bacterial receivers connected to a microfluidic channel is investigated for the detection of the pulse-amplitude modulated molecular signals. This work lays the foundation for molecular communication over microfluidic channels that will enable interconnection of engineered molecular machines.

Cette thèse est dédié à l’étude de la problématique des interférences multi utilisateurs dans le système Argos et à la proposition des diverses techniques pour réduire les effets de ces interférences. Le système Argos est un système mondial de localisation et de collecte de données géo positionnées par satellite. Il permet à l’échelle mondiale de collecter et de traiter les données émises par des émetteurs installés sur la surface de terre. Ces émetteurs sont connus sous le nom de balises. Ces balises sont installées sur des voiliers, des stations météo, des bouées, ainsi que sur quelques animaux (phoques, penguins, etc.…). Le système Argos a été créé en 1978 par le Centre National des Études spatiales (CNES), l’agence spatiale américaine (NASA) et l’agence américaine d’étude de l’atmosphère et de l’océan (NOAA). Depuis sa création, le nombre de balises Argos n’a cessé d’augmenter afin de couvrir au mieux la couverture mondiale. Nous sommes orientés ainsi à la saturation de la bande d’émission et à la présence des interférences multi utilisateurs (MUI) provenant de la réception simultanée de plusieurs signaux émis par les balises. Cette MUI limite la capacité du système Argos et dégrade les performances en termes de Taux Erreur Bit (TEB). Actuellement, le système Argos n’est capable de traiter qu’un seul signal reçu à un instant donné. D’où, l’intérêt d’implanter des techniques au niveau du récepteur capable de réduire les effets des interférences et de traiter les signaux émis par toutes les balises. Plusieurs techniques de détection multi utilisateurs (MUD) ont été développées dans le cadre de cette problématique. Ces techniques sont principalement implantées dans les systèmes CDMA où des codes d’étalement sont utilisés afin de différencier entre les différents signaux. Ceci n’est pas le cas du système Argos où les signaux ne présentent pas des séquences d’étalement et que les bandes de fréquences pour ces différents signaux ne sont pas disjointes à cause de l’effet Doppler et donc, un recouvrement spectral au niveau du récepteur est très probable. Dans ce contexte, l’objectif du travail présenté dans cette thèse est d’étudier différentes techniques MUD appliquées au système Argos et d’évaluer ces techniques au niveau des performances en termes de TEB et de complexité d’implantation. Dans ce travail, nous présentons les différentes composantes du système Argos ainsi que son mode de fonctionnement. Ensuite, nous présentons la problématique dans le système Argos ainsi que les différentes solutions proposées. Parmi ces solutions, nous montrons celle basant sur l’implantation des techniques MUD au niveau du récepteur. Ces différentes techniques MUD sont alors présentées ainsi que les avantages et les inconvénients de chacune d’elles. Parmi les techniques possédant un bon compromis entre les performances d’une part et la complexité d’autre part, nous notons la technique d’annulation par série d’interférence (SIC). Dans cette technique, les signaux sont démodulés successivement suivant l’ordre décroissant des puissances. Cette technique nécessite une étape d’estimation des paramètres des signaux à chaque étape. L’impact d’une estimation imparfaite des différents paramètres est aussi étudié. Après l’étude des impacts des erreurs d’estimation, nous proposons des estimateurs adaptables au système Argos. Les performances de ces estimateurs sont obtenues en comparant les variances de leurs erreurs aux bornes de Cramer Rao (CRB). Enfin, nous terminons le travail par une conclusion générale des résultats obtenus et nous envisageons les perspectives des prochains travaux. / In our thesis, we investigate the application of multi user detection techniques to a Low Polar Orbit (LPO) satellite used in the Argos system. Argos is a global satellite-based location and data collection system dedicated for studying and protecting the environment. User platforms, each equipped with a Platform Transmitter Terminal (PTT), transmit data messages to a 850 km LPO satellite. An ARGOS satellite receives, decodes, and forwards the signals to ground stations. All PTTs transmit at random times in a 100 kHz bandwidth using different carrier frequencies. The central carrier frequency f0 is 401.65 MHz. Due to the relative motion between the satellite and the platforms, signals transmitted by PTTs are affected by both a different Doppler shift and a different propagation delay. Thus, the Argos satellite receives overlapping signals in both frequency and time domains inducing Multiple Access Interference (MAI). One common approach to mitigate the MAI problem is to implement Multi User Detection (MUD) techniques at the receiver. To tackle this problem, several MUD techniques have been proposed for the reception of synchronous and asynchronous users. In particular, the Successive Interference Cancelation (SIC) detector has been shown to offer a good optimality-complexity trade-off compared to other common approaches such as the Maximum Likelihood (ML) receiver. In an Argos SIC receiver, users are decoded in a successive manner, and the signals of successfully decoded users are subtracted from the waveform before decoding the next user. This procedure involves a parameter estimation step and the impact of erroneous parameter estimates on the performance of Argos SIC receiver has been studied. Argos SIC receiver has been shown to be both robust to imperfect amplitude and phase estimation and sensitive to imperfect time delay estimation. The last part of our work focuses on the implementation of digital estimators for the Argos system. In particular, we propose a time delay estimator, a frequency estimator, a phase estimator and an amplitude estimator. These estimators are derived from the ML principle and they have been already derived for the single user transmission. In our work, we adapt successfully these estimators for the multi user detector case. These estimators use the Non Data Aided (NDA) cases in which no a priori information for the transmitted bits is required. The performance of these different estimators are compared to the Cramer Rao Bound (CRB) values. Finally, we conclude in our work by showing the different results obtained during this dissertation. Also, we give some perspectives for future work on Argos system.

Aiming to achieve the better efficiency in the use of bandwidth (spectral efficiency), as well as a compromise with the energy utilized in the process (power efficiency), the service operators are obliged to change the way in which the information is conveyed. The simplicity of the OOK modulation format has always been the main reason for using it in optical communications. Hence, once the coherent technologies had reached the maturity from a commercial point of view, they have become real alternatives to extend the capacity of networks. In this thesis, the performance of single and multirate FFH-OCDMA networks based on coherent modulation formats is addressed. The following modulation formats are assumed: binary and quadrature phase shift keying (BPSK and QPSK). For single rate networks, BPSK or QPSK are used as the modulation format for all users. For multirate networks, on the other hand, BPSK and QPSK modulation formats are employed for low and high rate transmission users, respectively. For both networks, new bit error rate (BER) formalisms regarding the main deleterious source in OCDMA are derived, namely multipleaccess interference (MAI). The mathematical formalism developed here accounts for any kind of users\' signal waveforms (which are accurately accounted for at the desired user\'s receiver). As a result, it can be successfully used to carry out performance analysis of any OCDMA network. Moreover, it is shown that both low and high rate users can surprisingly achieve similar BER levels. In addition, it is demonstrated that incoherent OOK-based networks are not good candidates for future single and multirate FFH-OCDMA implementations in this interference limited scenario. On the other hand, our results pave the way for BPSK- and QPSK -based networks, which are seen as potential candidates for next generation passive optical networks (NG-PON2), where higher data rates and a vast amount of users will be major network requirements. / No intuito de alcançar uma melhor eficiência no uso da largura de banda (eficiência espectral), bem como um compromisso com a energia utilizada no processo de transmissão de informações (eficiência em potência), as operadoras de serviços estão sendo obrigadas a alterar o modo em que a informações estão sendo transmitidas. Neste sentido, a simplicidade do formato de modulação OOK sempre foi a principal razão para a sua utilização em sistemas de comunicações ópticas. Assim, uma vez que a tecnologia coerente tem alcançado a maturidade do ponto de vista comercial, estas são reais alternativas para aumentar a capacidade das redes de comunicação ópticas. Nesta tese, são avaliados os desempenhos de redes FFH-OCDMA convencionais e de múltiplas taxas baseadas em formatos de modulação coerentes. Os seguintes formatos de modulação assumidos são: modulação por chaveamento de fase binário e por quadratura (BPSK e QPSK). Para redes convencionais, BPSK e QPSK são utilizados como formatos de modulação para todos os usuários. Por outro lado, para redes de múltiplas taxas de transmissão, os formatos de modulação BPSK e QPSK são empregados para baixas e altas taxas, respectivamente. Para ambas as redes, foram derivados novos formalismos matemáticos considerando a principal fonte deletéria de desempenho em OCDMA, comumente chamada de MAI, no cálculo da BER. Os formalismos matemáticos desenvolvidos são generalizados para quaisquer tipos de formas de onda empregada nos sinais dos usuários (as quais são contabilizadas precisamente no receptor do usuário de interesse). Como resultados, o formalismo matemático pode ser utilizado com sucesso para realizar a análise de desempenho de qualquer rede OCDMA. Adicionalmente, é demonstrado que ambos os usuários em baixa ou alta taxa de transmissão podem surpreendentemente alcançar níveis similares de BER. Concomitantemente, é observado que redes incoerentes baseadas em OOK não são boas candidatas para implementações convencionais e de múltiplas taxas de transmissão considerando esse cenário limitado por interferência. Por outro lado, os resultados trilham o caminho para redes baseadas em BPSK e QPSK, as quais são potenciais candidatas para a próxima geração de redes ópticas passivas (NG-PON2), onde os maiores requisitos são as altas taxas de transmissão e grande capacidade de inserção simultânea de usuários.

Aiming to achieve the better efficiency in the use of bandwidth (spectral efficiency), as well as a compromise with the energy utilized in the process (power efficiency), the service operators are obliged to change the way in which the information is conveyed. The simplicity of the OOK modulation format has always been the main reason for using it in optical communications. Hence, once the coherent technologies had reached the maturity from a commercial point of view, they have become real alternatives to extend the capacity of networks. In this thesis, the performance of single and multirate FFH-OCDMA networks based on coherent modulation formats is addressed. The following modulation formats are assumed: binary and quadrature phase shift keying (BPSK and QPSK). For single rate networks, BPSK or QPSK are used as the modulation format for all users. For multirate networks, on the other hand, BPSK and QPSK modulation formats are employed for low and high rate transmission users, respectively. For both networks, new bit error rate (BER) formalisms regarding the main deleterious source in OCDMA are derived, namely multipleaccess interference (MAI). The mathematical formalism developed here accounts for any kind of users\' signal waveforms (which are accurately accounted for at the desired user\'s receiver). As a result, it can be successfully used to carry out performance analysis of any OCDMA network. Moreover, it is shown that both low and high rate users can surprisingly achieve similar BER levels. In addition, it is demonstrated that incoherent OOK-based networks are not good candidates for future single and multirate FFH-OCDMA implementations in this interference limited scenario. On the other hand, our results pave the way for BPSK- and QPSK -based networks, which are seen as potential candidates for next generation passive optical networks (NG-PON2), where higher data rates and a vast amount of users will be major network requirements. / No intuito de alcançar uma melhor eficiência no uso da largura de banda (eficiência espectral), bem como um compromisso com a energia utilizada no processo de transmissão de informações (eficiência em potência), as operadoras de serviços estão sendo obrigadas a alterar o modo em que a informações estão sendo transmitidas. Neste sentido, a simplicidade do formato de modulação OOK sempre foi a principal razão para a sua utilização em sistemas de comunicações ópticas. Assim, uma vez que a tecnologia coerente tem alcançado a maturidade do ponto de vista comercial, estas são reais alternativas para aumentar a capacidade das redes de comunicação ópticas. Nesta tese, são avaliados os desempenhos de redes FFH-OCDMA convencionais e de múltiplas taxas baseadas em formatos de modulação coerentes. Os seguintes formatos de modulação assumidos são: modulação por chaveamento de fase binário e por quadratura (BPSK e QPSK). Para redes convencionais, BPSK e QPSK são utilizados como formatos de modulação para todos os usuários. Por outro lado, para redes de múltiplas taxas de transmissão, os formatos de modulação BPSK e QPSK são empregados para baixas e altas taxas, respectivamente. Para ambas as redes, foram derivados novos formalismos matemáticos considerando a principal fonte deletéria de desempenho em OCDMA, comumente chamada de MAI, no cálculo da BER. Os formalismos matemáticos desenvolvidos são generalizados para quaisquer tipos de formas de onda empregada nos sinais dos usuários (as quais são contabilizadas precisamente no receptor do usuário de interesse). Como resultados, o formalismo matemático pode ser utilizado com sucesso para realizar a análise de desempenho de qualquer rede OCDMA. Adicionalmente, é demonstrado que ambos os usuários em baixa ou alta taxa de transmissão podem surpreendentemente alcançar níveis similares de BER. Concomitantemente, é observado que redes incoerentes baseadas em OOK não são boas candidatas para implementações convencionais e de múltiplas taxas de transmissão considerando esse cenário limitado por interferência. Por outro lado, os resultados trilham o caminho para redes baseadas em BPSK e QPSK, as quais são potenciais candidatas para a próxima geração de redes ópticas passivas (NG-PON2), onde os maiores requisitos são as altas taxas de transmissão e grande capacidade de inserção simultânea de usuários.