Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Pilot based channel estimation improvement in orthogonal frequency-division multiplexing systems using linear predictive coding

Pilot based least square (LS) channel estimation is a commonly used channel estimation technique in orthogonal frequency-division multiplexing based systems due to its simplicity. However, LS estimation does not handle the noise effect and hence suffers from performance degradation. Since the channel coefficients are correlated in time and hence show a slower variation than the noise, it is possible to encode the channel using linear predictive coding (LPC) without the noise. In this work, the channel is estimated from the pilots using LS estimation and in a second step the channel’s LS estimated is encoded as LPC coefficients to produce an improved channel estimation. The estimation technique is simulated for space-time block coding (STBC) based orthogonal frequency-division multiplexing (OFDM) system and the bit error rate (BER) curves show improvement of the LPC estimation over the LS estimation of the channel

Advanced receiver structures for mobile MIMO multicarrier communication systems

Beyond third generation (3G) and fourth generation (4G) wireless communication systems are targeting far higher data rates, spectral efficiency and mobility requirements than existing 3G networks. By using multiple antennas at the transmitter and the receiver, multiple-input multiple-output (MIMO) technology allows improving both the spectral efficiency (bits/s/Hz), the coverage, and link reliability of the system. Multicarrier modulation such as orthogonal frequency division multiplexing (OFDM) is a powerful technique to handle impairments specific to the wireless radio channel. The combination of multicarrier modulation together with MIMO signaling provides a feasible physical layer technology for future beyond 3G and fourth generation communication systems. The theoretical benefits of MIMO and multicarrier modulation may not be fully achieved because the wireless transmission channels are time and frequency selective. Also, high data rates call for a large bandwidth and high carrier frequencies. As a result, an important Doppler spread is likely to be experienced, leading to variations of the channel over very short period of time. At the same time, transceiver front-end imperfections, mobility and rich scattering environments cause frequency synchronization errors. Unlike their single-carrier counterparts, multi-carrier transmissions are extremely sensitive to carrier frequency offsets (CFO). Therefore, reliable channel estimation and frequency synchronization are necessary to obtain the benefits of MIMO OFDM in mobile systems. These two topics are the main research problems in this thesis. An algorithm for the joint estimation and tracking of channel and CFO parameters in MIMO OFDM is developed in this thesis. A specific state-space model is introduced for MIMO OFDM systems impaired by multiple carrier frequency offsets under time-frequency selective fading. In MIMO systems, multiple frequency offsets are justified by mobility, rich scattering environment and large angle spread, as well as potentially separate radio frequency - intermediate frequency chains. An extended Kalman filter stage tracks channel and CFO parameters. Tracking takes place in time domain, which ensures reduced computational complexity, robustness to estimation errors as well as low estimation variance in comparison to frequency domain processing. The thesis also addresses the problem of blind carrier frequency synchronization in OFDM. Blind techniques exploit statistical or structural properties of the OFDM modulation. Two novel approaches are proposed for blind fine CFO estimation. The first one aims at restoring the orthogonality of the OFDM transmission by exploiting the properties of the received signal covariance matrix. The second approach is a subspace algorithm exploiting the correlation of the channel frequency response among the subcarriers. Both methods achieve reliable estimation of the CFO regardless of multipath fading. The subspace algorithm needs extremely small sample support, which is a key feature in the face of time-selective channels. Finally, the Cramér-Rao (CRB) bound is established for the problem in order to assess the large sample performance of the proposed algorithms.reviewe

Techniques d’Estimation de Canal et de Décalage de Fréquence Porteuse pour Systèmes Sans-fil Multiporteuses en Liaison Montante

Multicarrier modulation is the common feature of high-data rate mobile wireless systems. In that case, two phenomena disturb the symbol detection. Firstly, due to the relative transmitter-receiver motion and a difference between the local oscillator (LO) frequency at the transmitter and the receiver, a carrier frequency offset (CFO) affects the received signal. This leads to an intercarrier interference (ICI). Secondly, several versions of the transmitted signal are received due to the wireless propagation channel. These unwanted phenomena must be taken into account when designing a receiver. As estimating the multipath channel and the CFO is essential, this PhD deals with several CFO and channel estimation methods based on optimal filtering. Firstly, as the estimation issue is nonlinear, we suggest using the extended Kalman filter (EKF). It is based on a local linearization of the equations around the last state estimate. However, this approach requires a linearization based on calculations of Jacobians and Hessians matrices and may not be a sufficient description of the nonlinearity. For these reasons, we can consider the sigma-point Kalman filter (SPKF), namely the unscented Kalman Filter (UKF) and the central difference Kalman filter (CDKF). The UKF is based on the unscented transformation whereas the CDKF is based on the second order Sterling polynomial interpolation formula. Nevertheless, the above methods require an exact and accurate a priori system model as well as perfect knowledge of the additive measurementnoise statistics. Therefore, we propose to use the H∞ filtering, which is known to be more robust to uncertainties than Kalman filtering. As the state-space representation of the system is non-linear, we first evaluate the “extended H∞ filter”, which is based on a linearization of the state-space equations like the EKF. As an alternative, the “unscented H∞ filter”, which has been recently proposed in the literature, is implemented by embedding the unscented transformation into the “extended H∞ filter” and carrying out the filtering by using the statistical linear error propagation approach.Multicarrier modulation is the common feature of high-data rate mobile wireless systems. In that case, two phenomena disturb the symbol detection. Firstly, due to the relative transmitter-receiver motion and a difference between the local oscillator (LO) frequency at the transmitter and the receiver, a carrier frequency offset (CFO) affects the received signal. This leads to an intercarrier interference (ICI). Secondly, several versions of the transmitted signal are received due to the wireless propagation channel. These unwanted phenomena must be taken into account when designing a receiver. As estimating the multipath channel and the CFO is essential, this PhD deals with several CFO and channel estimation methods based on optimal filtering. Firstly, as the estimation issue is nonlinear, we suggest using the extended Kalman filter (EKF). It is based on a local linearization of the equations around the last state estimate. However, this approach requires a linearization based on calculations of Jacobians and Hessians matrices and may not be a sufficient description of the nonlinearity. For these reasons, we can consider the sigma-point Kalman filter (SPKF), namely the unscented Kalman Filter (UKF) and the central difference Kalman filter (CDKF). The UKF is based on the unscented transformation whereas the CDKF is based on the second order Sterling polynomial interpolation formula. Nevertheless, the above methods require an exact and accurate a priori system model as well as perfect knowledge of the additive measurementnoise statistics. Therefore, we propose to use the H∞ filtering, which is known to be more robust to uncertainties than Kalman filtering. As the state-space representation of the system is non-linear, we first evaluate the “extended H∞ filter”, which is based on a linearization of the state-space equations like the EKF. As an alternative, the “unscented H∞ filter”, which has been recently proposed in the literature, is implemented by embedding the unscented transformation into the “extended H∞ filter” and carrying out the filtering by using the statistical linear error propagation approach

Convergence of millimeter-wave and photonic interconnect systems for very-high-throughput digital communication applications

In the past, radio-frequency signals were commonly used for low-speed wireless electronic systems, and optical signals were used for multi-gigabit wired communication systems. However, as the emergence of new millimeter-wave technology introduces multi-gigabit transmission over a wireless radio-frequency channel, the borderline between radio-frequency and optical systems becomes blurred. As a result, there come ample opportunities to design and develop next-generation broadband systems to combine the advantages of these two technologies to overcome inherent limitations of various broadband end-to-end interconnect systems in signal generation, recovery, synchronization, and so on. For the transmission distances of a few centimeters to thousands of kilometers, the convergence of radio-frequency electronics and optics to build radio-over-fiber systems ushers in a new era of research for the upcoming very-high-throughput broadband services. Radio-over-fiber systems are believed to be the most promising solution to the backhaul transmission of the millimeter-wave wireless access networks, especially for the license-free, very-high-throughput 60-GHz band. Adopting radio-over-fiber systems in access or in-building networks can greatly extend the 60-GHz signal reach by using ultra-low loss optical fibers. However, such high frequency is difficult to generate in a straightforward way. In this dissertation, the novel techniques of homodyne and heterodyne optical-carrier suppressions for radio-over-fiber systems are investigated and various system architectures are designed to overcome these limitations of 60-GHz wireless access networks, bringing the popularization of multi-gigabit wireless networks to become closer to the reality. In addition to the advantages for the access networks, extremely high spectral efficiency, which is the most important parameter for long-haul networks, can be achieved by radio-over-fiber signal generation. As a result, the transmission performance of spectrally efficient radio-over-fiber signaling, including orthogonal frequency division multiplexing and orthogonal wavelength division multiplexing, is broadly and deeply investigated. On the other hand, radio-over-fiber is also used for the frequency synchronization that can resolve the performance limitation of wireless interconnect systems. A novel wireless interconnects assisted by radio-over-fiber subsystems is proposed in this dissertation. In conclusion, multiple advantageous facets of radio-over-fiber systems can be found in various levels of end-to-end interconnect systems. The rapid development of radio-over-fiber systems will quickly change the conventional appearance of modern communications.PhDCommittee Chair: Gee-Kung Chang; Committee Member: Bernard Kippelen; Committee Member: Shyh-Chiang Shen; Committee Member: Thomas K. Gaylord; Committee Member: Umakishore Ramachandra

Low probability of detection underwater acoustic communications

Low probability of detection (LPD) underwater acoustic communications are an essential requirement for command and control of autonomous underwater vehicles (AUV) or submarines performing covert missions, avoiding their detection while communicating. Based on low power signals, these covert communications may also extend the autonomy of battery-operated AUVs, and contribute to reducing the impacts of the environmental noise level on marine life. The present thesis aims to develop LPD communications based on modeled and real data from three shallow water experiments. Thus, a superimposed training method has been proposed. A bitstream is created superimposing a long probe to the message before transmission. Computationally simple, the algorithm explores temporal diversity to increase the processing gain and uses a Wiener filter for equalization. Experimental results presented bit-error rates (BER) < 10−2 for signal-tonoise ratios (SNR) < −8 dB. To understand the effects of coastal upwelling phenomena over low SNR communications, a study compares the acoustic propagation for different sound speed profiles using a propagation model and analyzes data from the BioCom’19 experiment, performed off Cabo Frio Island (Brazil). Temporal and spatial coherence of low power signals propagating in this harsh environment are estimated, and both a criterion for multichannel combining and a double Wiener filter to improve equalization are presented. Passive time-reversal (pTR) techniques have been widely employed for communications. To address the pTR channel mismatch due to the environmental variability between the probe and the data transmissions, this work proposes a superimposed training pTR (STpTR) approach for single and multichannel systems. Despite the high noise levels, varying from -5 to +6 dB, the STpTR combined with a Wiener filter achieved BER < 10−2, for bit rates up to 220 bps. To improve covert communications for AUVs, this work also presents a study about vector sensor multichannel combining. Using the STpTR approach, results from an experiment on the coast of Algarve/Portugal indicate that combining the pressure and particle velocity channels of a vector sensor may provide an average SNR and mean squared-error gain of up to 9.4 and 3.1 dB, respectively, compared to the pressure channel. Therefore, a better understanding of the environment combined with the superimposed training pTR using a vector sensor may improve the LPD communication system’s performance and robustness while keeping covertness.No ´últimos anos, os trabalhos de investigação sobre comunicações acústicas submarinas com baixa probabilidade de deteção (BPD) tem sido incentivados pela indústria, pelos governos, e pela própria academia em razão de suas m´múltiplas aplicações. Na ´área militar, as comunicações BPD permitem que submarinos e veículos autónomos possam se comunicar sem serem detectados. Na ´área civil, permitem a economia de energia de sensores alimentados por baterias, aumentando o tempo de funcionamento, bem como contribui para reduzir os impactos sobre a vida marinha causados pelos altos n´ıveis de ru´ıdo submarino, entre outras aplica¸c˜oes. Neste contexto, esta tese pretende desenvolver comunica¸c˜oes BPD utilizando um modelo de propaga¸c˜ao ac´ustica e dados obtidos a partir de trˆes experimentos em ´aguas rasas. Este trabalho apresenta um m´etodo de treinamento superposto para comunica¸c˜oes submarinas em um ambiente com baixa rela¸c˜ao sinal/ru´ıdo, e demonstra sua aplica¸c˜ao para comunica¸c˜oes BPD. Computacionalmente simples, o m´etodo foi desenvolvido para funcionar com um ´único projetor acústico, transmitindo com baixa potência, e um hidrofone, sem o ganho de um arranjo de sensores distribuídos no espaço. Antes da transmissão, uma longa sequência de comprimento m´máximo ´e somada `a mensagem para efeitos de equalização e sincronismo. Os dois sinais são binários, modulados em fase e possuem 2047 bits. Porém, possuem amplitudes diferentes. Na realidade, a amplitude do sinal de treinamento ´e ligeiramente superior `a da mensagem. Em um ambiente com baixa rela¸c˜ao sinal ru´ıdo, um sinal de treinamento mais forte permite ocultar a mensagem a ser transmitida, bem como melhorar o ganho para a estima¸c˜ao da resposta impulsiva e para a sincroniza¸c˜ao do sistema. A mensagem ´e composta por 3 bits nulos e 4 sequˆencias de 511 bits. Delimitados por uma curta sequˆencia de comprimento m´aximo de 31 bits, para dupla sincroniza¸c˜ao, os pacotes de dados possuem 480 bits e transportam o seguinte pangrama: (The Quick Brown Fox Jumps Over the Lazy Dog 0123456789!@#$). O m´etodo explora a diversidade temporal do canal, transmitindo a mesma sequˆencia diversas vezes para aumentar o ganho de processamento do sinal e implementar a corre¸c˜ao de erros atrav´es da soma coerente dos sinais. A resposta impulsiva do canal ´e estimada pela transformada r´apida de Hadamard, e a equaliza¸c˜ao do sinal ´e feita por um filtro de Wiener. A remo¸c˜ao da interferˆencia causada pelo sinal de treinamento ´e realizada pelo m´etodo “hyperslice cancellation by coordinate zeroing (HCC0)”, e a seguir a mensagem ´e decodificada. Resultados obtidos a partir de um experimento em ´aguas rasas, utilizando uma fonte e um ´único hidrofone, apresentaram taxas de erro de bit menores que 10−2, para relações sinal/ruído inferiores a −8 dB. A ressurgência costeira ´e um fenómeno oceanográfico dinâmico que modifica, profundamente, a estratificação de temperatura do oceano, influenciando diretamente na propagação acústica. Por outro lado, os crescentes n´níveis de ruído antropogénico não apenas reduzem o desempenho dos sistema de comunicação, corrompendo a informação transmitida, mas tamb´em afetam a vida marinha. Para compreender os efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes com baixa rela¸c˜ao sinal/ru´ıdo, um estudo analisa os dados do experimento BioCom’19, realizado nas proximidades da Ilha do Cabo Frio, Rio de Janeiro (Brasil). As respostas impulsivas do canal e a propaga¸c˜ao ac´ustica, para diferentes perfis de velocidade do som, foram estimadas usando o modelo de propaga¸c˜ao ac´ustica “Monterey-Miami Parabolic Equation model (MMPE)”. Al´em disso, o desempenho do sistema de comunica¸c˜oes foi correlacionado com os perfis de temperatura durante o experimento. Os resultados obtidos indicam uma significativa redu¸c˜ao da energia ac´ustica nos receptores durante a ressurgˆencia, degradando o desempenho do sistema. A coerˆencia temporal e espacial dos sinais de baixa potˆencia transmitidos no experimento BioCom’19 foi estimada, e um crit´erio para combina¸c˜ao dos sinais, provenientes dos m´ultiplos hidrofones, foi proposto. Utilizando dados de um arranjo piramidal e um arranjo vertical linear, de 4 hidrofones cada, a coerˆencia foi estimada antes e depois do filtro de Wiener para mostrar o impacto do multicaminhamento sobre a taxa de erro de bit. Os resultados mostram que a coˆerencia temporal pode ser utilizada como crit´erio para combinar sinais consecutivos em um mesmo canal, enquanto a diversidade espacial permite a combina¸c˜ao de m´ultiplos canais do arranjo de sensores. Sequˆencias cuja coerˆencia temporal esteja acima de um limite pr´e-definido s˜ao somadas. A coerˆencia espacial entre canais foi estimada e comparada em termos da taxa de erro de bit. Para diferentes taxas de transmiss˜ao, as taxas de erro de bit est˜ao em concordˆancia com a evoluc˜ao da coerˆencia espacial. Quanto mais elevada a coerˆencia, melhor o desempenho e menor a taxa de erro de bit. Um duplo filtro de Wiener para melhorar a equaliza¸c˜ao dos sinais de baixa potˆencia, durante a ressurgˆencia, tambem foi proposto. Utilizando dados dos 4 hidrofones de um arranjo piramidal, as respostas impulsivas foram estimadas para observar a variabilidade das condi¸c˜oes de propaga¸c˜ao. Em uma condi¸c˜ao de perfil de temperatura isot´ermico, as respostas impulsivas apresentaram multicaminhamento curto, com chegadas mais fortes nos receptores. `A medida que a ressurgˆencia ocorria, foram observadas quedas abruptas de temperatura superiores a 10◦C, na posi¸c˜ao dos hidrofones, acarretando uma forte refra¸c˜ao da onda sonora para o fundo marinho. Em consequˆencia, sinais mais fracos foram observados nos hidrofones. Os resultados obtidos com dados do BioCom’19 mostram que, para uma rela¸c˜ao sinal/ru´ıdo variando entre −3.9 e 7.3 dB, o duplo filtro de Wiener forneceu um ganho do erro m´edio quadr´atico de at´e 2.8 dB, comparado com o filtro de Wiener simples. As t´ecnicas de tempo reverso passivo (TRP) tem sido amplamente empregadas nas comunica¸c˜oes submarinas. Por´em, as r´apidas altera¸c˜oes das condi¸c˜oes de propaga¸c˜ao em canais submarinos variantes no tempo, durante as transmiss˜oes da sequˆencia de treinamento e da mensagem, degradam o desempenho das t´ecnicas TRP de equaliza¸c˜ao. Assim, esse trabalho prop˜oe um m´etodo de TRP utilizando treinamento superposto, para sistemas com um ou m´ultiplos sensores. O m´etodo proposto utiliza uma sequˆencia de treinamento, com o efeito Doppler corrigido, para estimar o canal e realizar o TRP. O m´etodo compara 3 differentes estrat´egias para melhorar a performance do sistema de comunica¸c˜ao: a diversidade temporal devido aos sinais idˆenticos transmitidos continuamente, a diversidade espacial devido aos 2 arranjos de hidrofones, piramidal e linear, com 4 hidrofones cada um, al´em da combina¸c˜ao dos 2 arranjos (8 hidrofones). Neste m´etodo, a t´ecnica de TRP com treinamento superposto minimiza o multicaminhamento e realiza a corre¸c˜ao de erros atrav´es da soma coerente dos diferentes canais. A interferˆencia intersimb´olica residual ´e removida pelo filtro de Wiener. Resultados obtidos com dados do experimento BioCom’19 mostram que o m´etodo proposto pode fornecer um ganho do erro médio quadrático de até 1.62 dB para canais independentes, e 3.13 dB, para canais combinados, comparativamente ao m´etodo sem o TRP, usando apenas o filtro de Wiener. Neste contexto, o m´etodo de TRP utilizando treinamento superposto alcan¸cou taxas de erro de bit < 10−2 para uma rela¸c˜ao sinal ru´ıdo, na banda de transmiss˜ao, variando entre −5 a +6 dB. Focado em comunica¸c˜oes com baixa probabilidade de detec¸c˜ao para pequenos ve´ıculos submarinos, este trabalho tamb´em apresenta um estudo sobre a combina¸c˜ao dos canais de press˜ao e velocidade de part´ıcula dos sensores vetoriais. Os sensores vetoriais possuem pequenas dimens˜oes, adequadas `a utilizac˜ao em ve´ıculos autˆonomos, e permitem obter um ganho de diversidade para as comunica¸c˜oes BPD. Para testar o m´etodo de treinamento superposto com o tempo reverso passivo utilizando sensores vetoriais, um experimento foi realizado, em ´aguas rasas, na costa do Algarve/Portugal. Para reduzir a rela¸c˜ao sinal/ru´ıdo para uma faixa de 0 a −10 dB, foi adicionado ru´ıdo gravado no experimento. Os resultados experimentais indicam que a combina¸c˜ao dos canais podem fornecer um ganho da rela¸c˜ao sinal/ru´ıdo e do erro m´edio quadr´atico de at´e 9.4 e 3.1 dB, respectivamente, comparados com os resultados do sensor de press˜ao. Portanto, as principais contribui¸c˜oes dessa tese s˜ao (i) a proposta do m´etodo de treinamento superposto para comunica¸c˜oes com baixa probabilidade de detec¸c˜ao, (ii) a compreens ˜ao dos efeitos da ressurgˆencia costeira sobre as comunica¸c˜oes, e seus impactos sobre a coerˆencia temporal e espacial, (iii) o m´etodo de treinamento superposto em conjunto com o tempo reverso passivo para lidar com a varia¸c˜ao do canal entre o tempo de recep¸c˜ao do sinal de treinamento e da mensagem, e (iv) a combina¸c˜ao dos m´ultiplos canais dos sensores vetoriais para comunica¸c˜oes BPD. Dessa forma, uma melhor compreens˜ao do canal submarino e a utiliza¸c˜ao dos m´etodos propostos combinados com sensores do estado da arte, como os sensores vetoriais, se configura como um avan¸co neste campo do conhecimento, permitindo aumentar a robustez do sistema BPD, bem como reduzir a probabilidade de detec¸c˜ao, mantendo a ocultação das comunicações.I am also grateful for the sponsorship provided by the Brazilian Navy through the Postgraduate Study Abroad Program, Grant No. Port.227/MB/2019

Performance of the CMS Tracker Optical Links and Future Upgrade Using Bandwidth Efficient Digital Modulation

The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) particle accelerator will begin operation in 2007. The innermost CMS subdetector, the Tracker, comprises ~10 million detector channels read out by ~40 000 analog optical links. The optoelectronic components have been designed to meet the stringent requirements of a high energy physics (HEP) experiment in terms of radiation hardness, low mass and low power. Extensive testing has been performed on the components and on complete optical links in test systems. Their functionality and performance in terms of gain, noise, linearity, bandwidth and radiation hardness is detailed. Particular emphasis is placed on the gain, which directly affects the dynamic range of the detector data. It has been possible to accurately predict the variation in gain that will be observed throughout the system. A simulation based on production test data showed that the average gain would be ~38% higher than the design target at the Tracker operating temperature of -10°C. Corrective action was taken to reduce the gains and recover the lost dynamic range by lowering the optical receiver's load resistor value from 100Ω to 62Ω. All links will have gains between 0.64 and 0.96V/V. The future iteration of CMS will be operated in an upgraded LHC requiring faster data readout. In order to preserve the large investments made for the current readout system, an upgrade path that involves reusing the existing optoelectronic components is considered. The applicability of Quadrature Amplitude Modulation (QAM) in a HEP readout system is examined. The method for calculating the data rate is presented, along with laboratory tests where QAM signals were transmitted over a Tracker optical link. The results show that 3-4Gbit/s would be possible if such a design can be implemented (over 10 times the equivalent data rate of the current analog links, 320Mbits/s).(Abridged version) The CMS experiment at the LHC will begin operation in 2007. The CMS Tracker sub-detector, comprises ~10 million detector channels read out by ~40 000 analog optical links. The optoelectronic components have been designed to meet the stringent requirements of a HEP experiment in terms of radiation hardness, low mass and low power. Extensive testing has been performed on the components and on complete optical links in test systems. Their functionality and performance in terms of gain, noise, linearity, bandwidth and radiation hardness is detailed. Particular emphasis is placed on the gain, which directly affects the dynamic range of the detector data. It has been possible to accurately predict the variation in gain that will be observed throughout the system. A simulation based on production test data showed that the average gain would be ~38% higher than the design target at the Tracker operating temperature of -10{\deg}C. Corrective action was taken to reduce the gains and recover the lost dynamic range by lowering the optical receiver's load resistor value from 100{\Omega} to 62{\Omega}. All links will have gains between 0.64 and 0.96V/V. The future iteration of CMS will be operated in an upgraded LHC requiring faster data readout. In order to preserve the large investments made for the current readout system, an upgrade path that involves reusing the existing optoelectronic components is considered. The applicability of Quadrature Amplitude Modulation (QAM) in a HEP readout system is examined. The method for calculating the data rate is presented, along with laboratory tests where QAM signals were transmitted over a Tracker optical link. The results show that 3-4Gbit/s would be possible if such a design can be implemented (over 10 times the equivalent data rate of the current analog links, 320Mbits/s)