Constant Envelope Precoding for Large Antenna Arrays

5G, the new generation of mobile communications, is expected to provide huge improvements in spectral efficiency and energy efficiency. Specifically, it has been proven that the adoption of large antenna arrays is an efficient means to improve the system performance in both of these efficiency measures. For these reasons, the deployment of base stations with large amount of antennas has attracted a substantial amount of research interest over the recent years. However, when pure digital beamforming is pursued in large array system context, a large number of transmitter and receiver chains must also be implemented, increasing the complexity and costs of the deployment. In general, power consumption of the cellular network is recognized as a major concern. Radio transmitters tend to be really power hungry, especially because of the potential energy inefficiency of their power amplifiers. Due to the characteristics of the current and future waveforms utilized in wireless communications, power amplifiers need to work in a relatively linear regime in order not to distort the signal, making the energy efficiency of such highly linear amplifiers to be rather low. If power amplifiers were capable of working in the nonlinear regime without degrading system performance, their energy efficiency could be notably increased, resulting in considerable savings in energy, costs and system complexity. In this Thesis, the development and evaluation of a constant envelope spatial precoder is being addressed. The precoder is capable of generating a symbol-rate constant envelope signal, which despite pulse-shape filtering yields substantial robustness against the nonlinearities of power amplifiers. This facilitates pushing power amplifiers into heavily nonlinear regime, with the consequent increase in their energy efficiency. At the same time, the precoder is able to perform spatial beamforming processing in order to mitigate the multi-user interference due to spatial multiplexing. It is assumed that the number of antennas in the base station is much larger than the number of simultaneously scheduled users, implying that large-scale MU-MIMO scenarios are considered, which allows us to exploit the additional degrees of freedom to perform waveform shaping. For the sake of evaluating the proposed precoder performance, different metrics such as PAPR, BER, multi-user interference and beamforming gain are compared to those of currently used precoding techniques. The obtained results indicate that the studied constant-envelope precoder can facilitate running the PA units of the large-array system in heavily nonlinear region, without inducing substantial nonlinear distortion, while also simultaneously providing good spatial multiplexing and beamforming characteristics. These, in turn, then facilitate larger received SINRs for the scheduled users, and therefore larger system throughputs and a more efficient utilization of the power amplifiers

New OFDM schemes based on orthogonal transforms for mobile communications systems :

PhD ThesisIn this thesis, two new orthogonal frequency division multiplexing (OFDM) systems are presented. The first scheme proposes a new OFDM system transceiver based on the C-transform, which is termed C-OFDM. Over multipath channels, the C-OFDM achieves 10 dB signal-to-noise ratio (SNR) gain at 10−4 bit-error-rate (BER), in comparison to the OFDM that based on the is discrete cosine transform (DCT-OFDM) and the conventional OFDM schemes. It also reduces the peak-to-average power ratio (PAPR) of the OFDM signal by about 1 dB and in some cases up to 3 dB. In the second scheme, a new fast, orthogonal X-transform is produced. The proposed X-transform is then used in a new OFDM named X-OFDM to greatly reduce the complexity, the PAPR and the BER. The proposed scheme achieves around 15 dB SNR gain in comparison to the conventional OFDM at 10−4 BER and reduces the average PAPR (over 105 OFDM symbol) by about 6 dB for N =1024 subcarriers. Furthermore, in this study, the X-transform is utilized to produce a new Alamouti space-time OFDM (ST-OFDM). The proposed ST-X-OFDM scheme reduces the transmitter complexity and achieves important SNR gain over the conventional ST-OFDM systems. The BER performance of the proposed schemes in the presence of solid-state power amplifiers (SSPAs) is also investigated analytically and by simulation. It shows that the X-OFDM is resilient to the SSPAs nonlinear distortion whereas the C-OFDM may lead to BER impairment in the presence of the SSPA. Furthermore, a coding technique to mitigate the sensitivity of the COFDM scheme to the SSPA is also proposed in this study. In this research, mathematical models for the proposed C-OFDM, XOFDM and ST-X-OFDM, which tightly match the simulation results over a diverse range of transmission scenarios and mapping schemes, are also derived. In addition, the BER performance of the proposed COFDM and X-OFDM schemes in the presence of the carrier frequency offset (CFO), with and without frequency synchronization algorithm, are also investigated. The proposed C-OFDM and X-OFDM schemes are more sensitive to the CFO than the conventional schemes. However, when frequency synchronization algorithm is used, both the proposed schemes retain their significant BER improvement in comparison to the conventional schemes.Ministry of Higher Education and Scientific Research (MOHSR), Iraq and to the Iraqi cultural attach- London for supporting me financially during my study in England

Iterative detection for pretransformed OFDM by subcarrier reconstruction

In this paper, an iterative detection method for an uncoded pretransformed (PT) orthogonal frequency division multiplexing (OFDM) system where the channel is not known at the transmitter is proposed. The iterative detection starts with linear detection. The noiseless received signal at the weakest subcarrier (corresponding to the smallest channel amplitude) is estimated based on all the detected data symbols using a hard or soft decision. Then, the actual received signal at the weakest subcarrier is replaced by the estimated one. This process is referred to as reconstruction here. After reconstruction, linear detection is carried out again to generate the next set of symbol estimates. The whole process proceeds iteratively to reconstruct the received signal at the next-weakest subcarrier. The transform coefficients and the iterative process are designed to maximize the minimum signal-to-noise power ratio. Under the assumption that the previous detection is error free, it is shown analytically that the iterative method achieves a diversity advantage of i+1 in the ith iteration, thus providing an explanation of its superior performance. Due to the flexibility of the transform design, the analysis conducted is applicable for other common systems as well. Simulations in realistic channels are carried out, and the bit-error rate performance of the iterative detection is superior as compared to that of the conventional detectors for the PT-OFDM or OFDM system

New Hybrid Schemes for PAPR Reduction in OFDM Systems

الـ 3GPP قدمت مشروع LTE لتلبية الطلبات المتزايدة لخدمات الاتصالات ذات السرعة العالية والجودة العالية. يستخدم نظام الـ LTE تقنية مضاعفة تقسيم التردد المتعامد (OFDM) في شكل (OFDMA) في الوصلة الهابطة (Downlink) وشكل الـ (SCFDMA) في الوصلة الصاعدة (Uplink) مجتمعة مع تقنية الـ MIMO لتقديم معدل بيانات عالي، قدرة عالية، وحصانة ضد القنوات متعددة المسارات. ومع ذلك لا يزال ارتفاع نسبة القدرة العظمة إلى المتوسط (PAPR) لإشارة الـ LTE المرسلة هي المشكلة الرئيسية التي تعمل على تدهور كفاءة النظام بشك عام وإمكانية استهلاك الطاقة. لذلك كرست الكثير من البحوث للحد من تدهور الأداء بسبب مشكلة الـ PAPR في أنظمة LTE-OFDM. تعتبر طرق ضغط الإشارة (Companding Methods) جزءاً من الطرق المعروفة والتي تعتبر سهلة ومنخفضة التعقيد، وبلا قيود على شكل التضمين وحجم عدد الحوامل (Subcarrier Size)، ولها خصائص طيفية جيدة، ومع ذلك فإن هذه الطرق تقلل الـ PAPR بمقدار ضئيل. وقد اقترح هذا البحث سبعة طرق هجينة جديدة على أساس مزيج من Zaddoff Chu Matrix Transform (ZCT) مع ست أساليب مختلفة من طرق ضغط الإشارة وهي Rooting Companding (RCT)، New Error Function Companding (NERF)، Absolute Exponential Companding (AEXP)، Logarithmic Rooting Companding (LogR)، Cosine Companding (COS)، وTangent Rooting Companding (TanhR). بالإضافة إلى ذلك تم تطوير الطريقة الهجينة السابعة وتجمع الـ Zaddoff Chu Matrix Transform (ZCT) مع طريقة جديدة مقترحة تسمى Advanced AEXP (AAEXP). أظهرت النتائج أن هذه الطرق المتطورة تجمع بين خصائص طريقة الـ ZCT مع خصائص طرق ضغط الإشارة، وتحقق أداء أمثل وانخفاضاً أفضل من حيث PAPR وBER. كما حققت طريقة الـ ZCT+AAEXP أفضل النتائج مقارنة بالطرق الأخرى.The 3rd Generation Partnership Project (3GPP) introduced LTE to meet increasingly demands for communication services with high speed and quality. LTE uses OFDM in the form of OFDMA in the downlink and SCFDMA in the uplink combined with MIMO offering high data rate, high capacity and immunity against multipath channels. However, still the high PAPR of the LTE transmitted signal is the major problem affecting overall system performance degradation and power efficiency. A plenty of research has been devoted to reduce the performance degradation due to the PAPR problem inherent to LTE OFDM systems. A portion of the current techniques such companding methods have low-complexity, no constraint on modulation format and subcarrier size, good distortion and spectral properties; however, they have limited PAPR reduction capabilities. This paper proposes seven new hybrid schemes including Zaddoff Chu Matrix Transform (ZCT) precoding and six modern companding methods; Rooting Companding (RCT), New Error Function Companding (NERF), Absolute Exponential Companding (AEXP), Logarithmic Rooting Companding (LogR), Cosine Companding (COS) and Tangent Rooting Companding (TanhR) companding. Furthermore, the seventh proposed hybrid scheme has been added incorporating ZCT precoding with new proposed companding called Advanced AEXP (AAEXP) companding. The developed methods are combining properties of both ZCT & Compandings, and achieving superior PAPR performance and optimal BER. Simulations results illustrate that the new seven proposed hybrid schemes can achieve better PAPR reduction, and BER performance and the best achievement has been achieved by ZCT+AAEXP scheme

An enhanced multicarrier modulation system for mobile communications

PhD ThesisThe recent revolution in mobile communications and the increased demand on more efficient transmission systems influence the research to enhance and invent new modulation techniques. Orthogonal frequency division multiplexing with offset quadrature amplitude modulation (OFDM/OQAM) is one of the multicarrier modulations techniques that overcomes some of the weaknesses of the conventional OFDM in term of bandwidth and power efficiencies. This thesis presents a novel multicarrier modulation scheme with improved performance in mobile communications context. Initially, the theoretical principles behind OFDM and OFDM/OQAM are discussed and the advantages of OFDM/OQAM over OFDM are highlighted. The time-frequency localization of pulse shapes is examined over different types of pulses. The effect of the localization and the pulse choice on OFDM/OQAM performance is demonstrated. The first contribution is introducing a new variant of multicarrier modulation system based on the integration of the Walsh-Hadamard transform with the OFDM/OQAM modulator. The full analytical transmission model of the system is derived over flat fading and frequency selective channels. Next, because of the critical requirement of low implementation complexity in mobile systems, a new fast algorithm transform is developed to reduce the implementation complexity of the system. The introduced fast algorithm has demonstrated a remarkable 60 percent decrease in the hardware requirement compared to the cascaded configuration. Although, the problem of high peak to average power ratio (PAPR) is one of the main drawbacks that associated with most multicarrier modulation techniques, the new system achieved lower values compared to the conventional systems. Subsequently, three new algorithms to reduce PAPR named Walsh overlapped selective mapping (WOSLM) for a high PAPR reduction, simplified selective mapping (SSLM) for a very low implementation complexity and Walsh partial transmit sequence (WPTS), are developed. Finally, in order to assess the reliability of the presented system in this thesis at imperfect environments, the performance of the system is investigated in the presence of high power amplifier, channel estimation errors, and carrier frequency offset (CFO). Two channel estimations algorithms named enhanced pair of pilots (EPOP) and averaged enhanced pair of pilots (AEPOP), and one CFO estimator technique called frequency domain (FD) CFO estimator, are suggested to provide reliable performance.Ministry of Higher Education and Scientific Research (MOHSR) of Ira

Nouvelles Techniques de Précodage et d Égalisation pour les Systèmes Multiporteuses

Dans cette thèse, de nouvelles techniques d'égalisation et de précodage pour des systèmes multiporteuses ont été proposées et analysées. D'abord, la performance d'erreur des systèmes multiporteuses à base de bancs de filtres (FBMC) précodées a été analysée. Il a été découvert que cette performance est très sensible à l'égalisation complète des sous-canaux. Quand il y a de l'interference inter-symbole residuel qui vient de l'égalisation imparfaite du sous-canaux, il y a une perte de diversité; cette diversité peut être recuperée avec l'adoption d'un nombre de sous-canaux assez grand pour que chaque sous-canaux subisse de l'évanouissement plat ou avec l'utilisation d'un égaliseur de sous-canaux avec une longueur assez suffisante pour compenser cette réponse en fréquence. Après, une approximation pour la distribution du rapport signal/bruit-plus-interfèrence (SINR) des systèmes SC-FDE qui utilisent égalisation MMSE linéaire a été proposée. Cette approximation utilise la distribution lognormal avec la plus petit distance de Kullback-Leibler vers la vraie distribution, et il s'est révélé qu'elle est precise dans la performance d'erreur; elle sert aussi comme une abstraction de cette système. Avec cette abstraction, une méthode précise pour obtenir la performance d'erreur analytique codée de ces systèmes a été proposée. Finalement, des précodeurs Tomlinson-Harashima (THP) et égaliseurs (linéaires et à retour de décision) largement linéaires pour des systèmes SC-FDE ont été proposés. Ces précodeurs et égaliseurs ont une performance d'erreur mieux quand comparés avec ses versions strictement linéaires si des signaux de constellations impropres sont transmises. Aussi, la performance d'erreur quand des égaliseurs à retour de décision sont utilisés est moins sensible au longueur du filtre de retour. Quand des précodeurs largement linéaires sont utilisés, cette performance devient moins sensible à des erreurs d'estimation du canaux.In this thesis, new precoding and equalization techniques for multicarrier systems were proposed and analyzed.First, the error performance of precoded filterbank multicarrier (FBMC) systems was analyzed. It was found out that this performance is highly sensitive to complete subchannel equalization. When there is residual intersymbol interference (ISI) stemming from imperfect subchannel equalization there is a loss of diversity; this loss can be prevented with the adoption of a number of subchannels large enough so that each subchannel suffers flat fading or with the utilization of a subchannel equalizer with sufficient length to compensate the subchannel frequency response.After that, an approximation for the signal to interference-plus-noise ratio (SINR) distribution of SC-FDE systems using linear MMSE equalization was proposed. This approximation uses the lognormal distribution with the smallest Kullback-Leibler distance to the true distribution, and was shown to be precise in the error performance sense; it serves as a system abstraction. With this abstraction, a precise method to obtain the analytical coded error performance of these systems was proposed.Finally, widely linear Tomlinson-Harashima precoders and equalizers (linear and decision-feedback) for SC-FDE systems were proposed. These precoders and equalizers have better error performance when compared to their strictly linear versions if signals coming from an improper constellation are transmitted. Their error performance when decision-feedback equalizers are used is less sensitive to the length of the feedback filter. When widely linear precoders are used, this error performance becomes less sensitive to channel estimation errors.PARIS-CNAM (751032301) / SudocSudocFranceF

Esquemas de pré-codificação e equalização para arquiteturas híbridas sub-conectadas na banda de ondas milimétricas

In the last years, the demand for high data rates increased substantially and the mobile communications are currently a necessity for our society. Thus, the number of users to access interactive services and applications has increased. The next generation of wireless communications (5G) is expected to be released in 2020 and it is projected to provide extremely high data rates for the users. The millimeter wave communications band and the massive MIMO are two promising keys technologies to achieve the multi Gbps for the future generations of mobile communications, in particular the 5G. The conjugation of these two technologies, allows packing a large number of antennas in the same volume than in the current frequencies and increase the spectral efficiency. However, when we have a large number of antennas, it is not reasonable to have a fully digital architecture due to the hardware constrains. On the other hand, it is not feasible to have a system that works only in the analog domain by employing a full analog beamforming since the performance is poor. Therefore, it is required a design of hybrid analog/digital architectures to reduce the complexity and achieve a good performance. Fully connected and sub-connected schemes are two examples of hybrid architectures. In the fully connected one, all RF chain connect to all antenna elements while in the sub-connected architecture, each RF chain is connected to a group of antennas. Consequently, the sub-connected architecture is more attractive due to the low complexity when compared to the fully connected one. Also, it is expected that millimeter waves be wideband, however, most of the works developed in last years for hybrid architectures are mainly focused in narrowband channels. Therefore, in this dissertation it is designed a low complex analog precoder at the user terminals and a hybrid analog-digital multi-user linear equalizer for broadband sub-connected millimeter wave massive MIMO at the base station. The analog precoder at the transmitter considers a quantized version of the average angle of departure of each cluster for its computation. In order to remove the multi-user interference, it is considered a hybrid sub-connected approach that minimizes the bit error rate (BER). The performance results show that the proposed hybrid sub-connected scheme is close to the hybrid full-connected design. However, due to the large number of connections, the full-connected scheme is slightly better than the proposed sub-connected scheme but with higher complexity. Therefore, the proposed analog precoder and hybrid sub-connected equalizer are more feasible to practical applications due to the good trade-off between performance and complexity.Nos últimos anos, a necessidade por elevadas taxas de transmissão de dados tem vindo a aumentar substancialmente uma vez que as comunicações móveis assumem cada vez mais um papel fundamental na sociedade atual. Por isso, o número de utilizadores que acedem a serviços e aplicações interativas tem vindo a aumentar. A próxima geração de comunicações móveis (5G) é esperada que seja lançada em 2020 e é projetada para fornecer elevadas taxas de transmissão de dados aos seus utilizadores. A comunicação na banda das ondas milimétricas e o MIMO massivo são duas tecnologias promissoras para alcançar os multi Gb/s para as comunicações móveis futuras, em particular o 5G. Conjugando essas duas tecnologias, permite-nos colocar um maior número de antenas no mesmo volume comparativamente às frequências atuais, aumentando assim a eficiência espectral. No entanto, quanto se tem um grande número de antenas, não é viável ter uma arquitetura totalmente digital devido às restrições de hardware. Por outro lado, não é viável ter um sistema que trabalhe apenas no domínio analógico. Assim sendo, é necessária uma arquitetura híbrida analógica-digital de modo a remover a complexidade geral do sistema. É esperado que os sistemas de comunicação baseados em ondas milimétricas sejam de banda larga, no entanto, a maioria dos trabalhos feitos para arquiteturas híbridas são focados em canais de banda estreita. Dois exemplos de soluções híbridas são as arquiteturas completamente conectada e sub-conectada. Na primeira, todas as cadeias RF estão ligadas a todas as antenas enquanto na arquitetura sub-conectada cada cadeia RF é ligada apenas a um grupo de antenas. Consequentemente, a arquitetura sub-conectada é mais interessante do ponto de vista prático devido à sua menor complexidade quando comparada à arquitetura completamente conectada. Nesta dissertação é projetado um pré-codificador analógico de baixa complexidade no terminal móvel, combinado com um equalizador multiutilizador desenhado para uma arquitetura híbrida sub-conectada, implementado na estação base. O pré-codificador no transmissor assume um conhecimento parcial da informação do canal e, de modo a remover eficientemente a interferência multiutilizador, é proposta também uma arquitetura híbrida sub-conectada que minimiza a taxa média de erro. Os resultados de desempenho mostram que o esquema híbrido sub-conectado proposto está próximo da arquitetura híbrida completamente conectada. No entanto, devido ao grande número de conexões, a arquitetura híbrida completamente conectada é ligeiramente melhor que a arquitetura sub-conectada proposta à custa de uma maior complexidade. Assim sendo, o pré-codificador analógico e o equalizador sub-conectado híbrido proposto são mais viáveis para aplicações práticas devido ao compromisso entre o desempenho e a complexidade.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Esquemas de cooperação entre estações base para o LTE no sentido descendente

The explosive growth in wireless traffic and in the number of connected devices as smart phones or computers, are causing a dramatic increase in the levels of interference, which significantly degrades the capacity gains promised by the point-to-point multi input, multi output (MIMO) based techniques. Therefore, it is becoming increasingly clear that major new improvements in spectral efficiency of wireless networks will have to entail addressing intercell interference. So, there is a need for a new cellular architecture that can take these factors under consideration. It is in this context that LTE-Advanced arises. One of the most promising LTE-Advanced technology is Coordinated Multipoint (CoMP), which allows base stations to cooperate among them, in order to mitigate or eliminate the intercell interference and, by doing so, increase the system’s capacity. This thesis intends to study this concept, implementing some schemes that fall under the CoMP concept. In this thesis we consider a distributed precoded multicell approach, where the precoders are computed locally at each BS to mitigate the intercell interference. Two precoder are considered: distributed zero forcing (DZF) and distributed virtual signal-to-interference noise ratio (DVSINR) recently proposed. Then the system is further optimized by computing a power allocation algorithm over the subcarriers that minimizes the average bit error rate (BER). The considered algorithms are also evaluated under imperfect channel state information. A quantized version of the CSI associated to the different links between the BS and the UT is feedback from the UT to the BS. This information is then employed by the different BSs to perform the precoding design. A new DVSINR precoder explicitly designed under imperfect CSI is proposed. The proposed schemes were implemented considering the LTE specifications, and the results show that the considered precoders are efficiently to remove the interference even under imperfect CSI.O crescimento exponencial no tráfego de comunicações sem-fios e no número de dispositivos utilizados (smart phones, computadores portáteis, etc.) está a causar um aumento significativo nos níveis de interferência, que prejudicam significativamente os ganhos de capacidade assegurados pelas tecnologias baseadas em ligações ponto-a-ponto MIMO. Deste modo, torna-se cada vez mais necessário que os grandes aperfeiçoamentos na eficiência espectral de sistemas de comunicações sem-fios tenham em consideração a interferência entre células. De forma a tomar em consideração estes aspectos, uma nova arquitectura celular terá de ser desenvolvida. É assim, neste contexto, que surge o LTE-Advanced. Uma das tecnologias mais promissoras do LTE-Advanced é a Coordenação Multi-Ponto (CoMP), que permite que as estações base cooperem de modo a mitigar a interferência entre células e, deste modo, aumentar a capacidade do sistema. Esta dissertação pretende estudar este conceito, implementando para isso algumas técnicas que se enquadram no conceito do CoMP. Nesta dissertação iremos considerar a implementação de um sistema de pré-codificação em múltiplas células, em que os pré-codificadores são calculados em cada BS, de modo a mitigar a interferência entre células. São considerados dois pré-codificadores: Distributed Zero Forcing (DZF) e Distributed Virtual Signal-to-Interferance Noise Ratio (DVSINR), recentemente proposto. De seguida o sistema é optimizado com a introdução de algoritmos de alocação de potência entre as sub-portadoras com o objectivo de minimizar a taxa média de erros (BER). Os algoritmos considerados são também avaliados em situações em que a informação do estado do canal é imperfeita. Uma versão quantizada da CSI associada a cada uma das diferentes ligações entre as BS e os UT é assim enviada do UT para a BS. Esta informação é então utilizada para calcular os diferentes pré-codificadores em cada BS. Uma nova versão do pré-codificador DVSINR é proposta de modo a lidar com CSI imperfeito. Os esquemas propostos foram implementados considerandos especificações do LTE, e os resultados obtidos demonstram que os pré-codificadores removem de uma forma eficiente a interferência, mesmo em situações em que a CSI é imperfeita