Multiuser MIMO-OFDM for Next-Generation Wireless Systems

This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station’s or radio port’s coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems

Super-orthogonal space-time turbo coded OFDM systems.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.The ever increasing demand for fast and efficient broadband wireless communication services requires future broadband communication systems to provide a high data rate, robust performance and low complexity within the limited available electromagnetic spectrum. One of the identified, most-promising techniques to support high performance and high data rate communication for future wireless broadband services is the deployment of multi-input multi-output (MIMO) antenna systems with orthogonal frequency division multiplexing (OFDM). The combination of MIMO and OFDM techniques guarantees a much more reliable and robust transmission over a hostile wireless channel through coding over the space, time and frequency domains. In this thesis, two full-rate space-time coded OFDM systems are proposed. The first one, designed for two transmit antennas, is called extended super-orthogonal space-time trellis coded OFDM (ESOSTTC-OFDM), and is based on constellation rotation. The second one, called super-quasi-orthogonal space-time trellis coded OFDM (SQOSTTCOFDM), combines a quasi-orthogonal space-time block code with a trellis code to provide a full-rate code for four transmit antennas. The designed space-time coded MIMO-OFDM systems achieve a high diversity order with high coding gain by exploiting the diversity advantage of frequency-selective fading channels. Concatenated codes have been shown to be an effective technique of achieving reliable communication close to the Shannon limit, provided that there is sufficient available diversity. In a bid to improve the performance of the super orthogonal space-time trellis code (SOSTTC) in frequency selective fading channels, five distinct concatenated codes are proposed for MIMO-OFDM over frequency-selective fading channels in the second part of this thesis. Four of the coding schemes are based on the concatenation of convolutional coding, interleaving, and space-time coding, along multiple-transmitter diversity systems, while the fifth coding scheme is based on the concatenation of two space-time codes and interleaving. The proposed concatenated Super-Orthogonal Space-Time Turbo-Coded OFDM System I. B. Oluwafemi 2012 vii coding schemes in MIMO-OFDM systems achieve high diversity gain by exploiting available diversity resources of frequency-selective fading channels and achieve a high coding gain through concatenations by employing the turbo principle. Using computer software simulations, the performance of the concatenated SOSTTC-OFDM schemes is compared with those of concatenated space-time trellis codes and those of conventional SOSTTC-OFDM schemes in frequency-selective fading channels. Simulation results show that the concatenated SOSTTC-OFDM system outperformed the concatenated space-time trellis codes and the conventional SOSTTC-OFDM system under the various channel scenarios in terms of both diversity order and coding gain

Orthogonal multicarrier modulation for high-rates mobile and wireless communications

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN037085 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Oblique Sounding and HF Communication Techniques for Very Long Haul Ionospheric Links

El sistema de comunicació ràdio d’alta freqüència (HF, en anglès) és usat arreu del món per agències governamentals i no governamentals sempre que calgui una alternativa a les comunicacions via satèl•lit: vaixells a alta mar, avions fora de cobertura de xarxes ràdio amb visió directa, operacions militars, zones on la infraestructura ha estat destruïda per algun tipus de desastre o bé zones llunyanes sense cap altre tipus de comunicació. La ràdio HF representa una alternativa, o un sistema de backup al satèl•lit per a comunicacions de llarg abast i en redueix els costos, evita la vulnerabilitat i els problemes de sobirania. En aquesta tesi s’ha estudiat l’enllaç HF entre la base antàrtica espanyola Juan Carlos I, situada a l’illa Livingston a l’arxipèlag de les Shetland del Sud, i Espanya. L’objectiu d’aquest treball és estudiar els problemes que afecten la propagació; és a dir, la relació senyal a soroll i interferència, la dispersió multicamí i la dispersió per efecte Doppler, i dissenyar la capa física d’un enllaç HF de baixa velocitat, poca potència i llarg abast. Pel que fa aquest últim punt es fan un parell de propostes: espectre eixamplat per seqüència directa (DSSS, en anglès) i multiplexació per divisió en freqüència ortogonal (OFDM, en anglès). El repte que es planteja és el de la definició de les característiques dels símbols que millor encaixen en aquest canal per tal d’obtenir un benefici de la diversitat temporal i freqüencial que ofereix el canal. Des de l’any 2003 diverses campanyes han permès estudiar aquest canal HF, però no va ser fins la campanya 2009/2010 que s’obtingué un foto de les característiques, diürnes i nocturnes, de la ionosfera. En els articles que es presenten en aquesta tesi hem estès el rang freqüencial d’estudi respecte a investigacions prèvies i hem mostrat diferències de comportament entre el dia i la nit. Hem usat els resultats de la caracterització del canal per a dissenyar i comparar la bondat dels símbols DSSS i OFDM. Ambdues possibilitats han resultat ser candidates a implementar l’enllaç HF entre l’Antàrtida i Espanya. Tot i així, ambdues tècniques representen visions diferents de la implementació del mòdem: mentre que DSSS obté bons resultats a baixa velocitat en entorns amb baixa relació senyal a soroll, OFDM aconsegueix tasses de velocitat més elevades en canals més benignes.Los sistemas de radio de alta frecuencia (HF, en inglés) son usados por agencias gubernamentales y no gubernamentales en todo el mundo siempre que se necesite una alternativa a las comunicaciones por satélite: barcos en alta mar, aviones fuera del rango de cobertura de las redes radio de visión directa, operaciones militares, zonas donde la infraestructura ha sido destruida por algún desastre. Ésta ofrece una alternativa, o representa un sistema de backup, a las comunicaciones vía satélite, evitando los costes, la vulnerabilidad y los problemas de soberanía de las comunicaciones por satélite. En esta tesis se ha estudiado el enlace HF entre la base antártica española Juan Carlos I en la isla Livingston, en las Shetland del sur y España. El objetivo de este trabajo es el estudio de las limitaciones de la propagación ionosférica (como la relación señal a ruido e interferencia, la dispersión multicamino y la dispersión por efecto Doppler) y el diseño de la capa física de un enlace HF de baja velocidad, baja potencia y largo alcance. Se han estudiado un par de propuestas para este enlace, como son el espectro ensanchado por secuencia directa (DSSS, en inglés) y la multiplexación por división en frecuencia ortogonal (OFDM, en inglés). El reto ha sido definir las características que mejor se adecuan a este enlace para poder aprovechar la diversidad temporal y frecuencial que ofrece el canal HF. Desde el año 2003 diversas campañas de sondeo han permitido estudiar el canal HF pero no es hasta la campaña 2009/2010 que se consigue una fotografía de la actividad ionosférica tanto nocturna como diurna. En los artículos que se presentan en esta tesis hemos extendido los estudios previos a todo el rango de frecuencias HF y hemos mostrado las diferencias entre el día y la noche. Hemos usado estos resultados de caracterización del canal para diseñar y comparar símbolos DSSS y símbolos OFDM. Ambas posibilidades han resultado ser posibles candidatas para implementar un enlace HF de baja velocidad entre la Antártida y España. Sin embargo ambas técnicas representan dos aproximaciones distintas a la implementación del módem. Mientras que DSSS consigue un buen funcionamiento a baja velocidad en escenarios con baja relación señal a ruido, OFDM consigue tasas de transmisión más altas en escenarios más benignos.High Frequency (HF) radio is used by governmental and non nongovernmental agencies worldwide whenever an alternative to satellites for sky wave communication is needed: ships at sea, aircraft out of range of line-of-sight radio networks, military operations, disaster areas with communication infrastructure destroyed or distant regions lacking other communications. It offers an alternative to satellites, or a backup, for long-haul communications, thus avoiding the costs, vulnerabilities and sovereignty concerns of satellite communications. In this thesis the HF link between the Antarctic Spanish Station Juan Carlos I in Livingston Island, South Shetlands and Spain is studied. The aim of this study is to address the impairments that affect HF propagation (i.e., signal-to-noise plus interference ratio, multipath and Doppler shift and spread) and to design the physical layer of a low rate, low power and long-haul HF link. Some proposals regarding this last issue are addressed, i.e., direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM). The challenge is to define the symbol characteristics that best fit the link to benefit from time and frequency diversity that offers the HF channel. Since 2003 several transmission campaigns have allowed to study the HF channel but it is not until the 2009/2010 campaign that we have achieved a whole picture of both diurnal and nocturnal ionospheric activity. In the papers presented in this thesis we have extended the previous research to the whole range of HF frequencies and we have shown the differences on performance between day and night. We have used the results from channel characterization to design and compare the performance of DSSS and OFDM symbols. Both techniques have turned out to be possible candidates to implement a low rate HF link between Antarctica and Spain. However, both techniques stand for different approaches of the modem: DSSS achieves good performance at low data rate in low SNR scenarios, whereas OFDM achieves higher data rate in benign channel

Optimizing LDPC codes for a mobile WiMAX system with a saturated transmission amplifier

In mobile communication, the user’s information is transmitted through a wireless communication link that is subjected to a range of deteriorating effects. The quality of the transmission can be presented by the rate of transfer and the reliability of the received stream. The capacity of the communication link can be reached through the use of channel coding. Channel coding is the method of adding redundant information to the user’s information to mitigate the deteriorating effects of the communication link. Mobile WiMAX is a technology that makes use of orthogonal frequency division multiplexing (OFDM) modulation to transmit information over a wireless communication channel. The OFDM physical layer has a high peak average to power ratio (PAPR) characteristic that saturates the transmitter’s amplifier quite easily when proper backoff is not made in the transmission power. In this dissertation an optimized graph code was used as an alternative solution to improve the system’s performance in the presence of a saturated transmission’s amplifier. The graph code was derived from a degree distribution given by the density evolution algorithm and provided no extra network overhead to implement. The performance analysis resulted in a factor of 10 improvement in the error floor and a coding gain of 1.5 dB. This was all accomplished with impairments provided by the mobile WiMAX standard in the construction of the graph code.Dissertation (MEng)--University of Pretoria, 2009.Electrical, Electronic and Computer Engineeringunrestricte

Performance of IEEE 802.11a wireless LAN standard over frequency-selective, slowly fading Nakagami channels in a pulsed jamming environment

Wireless local area networks (WLAN) are increasingly important in meeting the needs of the next generation broadband wireless communication systems for both commercial and military applications. In 1999, the Institute of the Electrical and Electronics Engineers (IEEE) 802.11a working group approved a standard for a 5 GHz band WLAN that supports a variable bit rate from 6 to 54 Mbps, and orthogonal frequency-division multiplexing (OFDM) was chosen because of its well-known ability to avoid multipath effects while achieving high data rates by combining a high order sub-carrier modulation with a high rate convolutional code. This thesis investigates the performance of the OFDM based IEEE.802.11a WLAN standard in frequency-selective, slowly fading Nakagami channels in a pulsed-noise jamming environment. Contrary to expectations, the signal-to-interference ratio (SIR) required to achieve a specific does not monotonically decrease when the bit rate decreases. Furthermore, the results show that the performance is improved significantly by adding convolutional coding with Viterbi decoding, and thus highlights the importance of forward error correction (FEC) coding to the performance of wireless communications systems.http://archive.org/details/performanceofiee109453638Lieutenant Junior Grade, Turkish NavyApproved for public release; distribution is unlimited

Resource allocation software algorithms for AMC-OFDM systems

PhD ThesisIn recent years, adaptive modulation and coding (AMC) technologies, resource allocation strategies and user scheduling for single-cell downlink orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) systems have been widely researched in order to ensure that capacity and throughput are maximised. In terms of AMC technologies, the correlation between the channel coefficients corresponding to the transmitted sub-carriers has not been considered yet. In the literature of resource allocation and user scheduling, either channel coding is not considered or only a fixed code rate is specified. Consequently, with a fixed number of data sub-carriers for each user, all these criteria restrict the flexibility of exploiting the available channel capacity, which reflects negatively on system throughput. At the same time, the presented scheduling algorithms so far managed the data of each user regardless the fair services of all users. The philosophy of this thesis is to maximise the average system throughput by proposing novel AMC, resource allocation and user scheduling strategies for OFDM and OFDMA systems based on developed software engineering life cycle models. These models have been designed to guarantee the scalability, extendibility and portability of the proposed strategies. This thesis presents an AMC strategy that divides the transmitted frame into sub-channels with an equal number of sub-carriers and selects different modulation and coding schemes (MCSs) amongst them rather than considering the same MCS for the entire frame. This strategy has been combined with a pilot adjustment scheme that reduces the pilots used for channel estimation in each sub-channel depending on the measured coherence bandwidth, signal to noise ratio (SNR), and SNR fluctuation values. The reduced pilots are replaced with additional data sub-carriers in order to improve the throughput. Additionally, a novel resource allocation strategy has been introduced in order to maximise the system throughput by distributing the users, transmission power and information bit streams over the employed sub-channels. The introduced method utilises the proposed AMC strategy in combination with pilot adjustment scheme to tackle the problem of channel capacity exploiting efficiently. It presents the throughput as a new cost function in terms of spectral efficiency and bit-error rate (BER), in which both convolutional coding rates and modulation order can be varied. The investigated throughput maximisation problem has been solved by producing two approaches. Firstly, optimised approach that solves the adopted problem optimally using the well known Lagrange multipliers method. This approach requires a huge search processes to achieve the optimal allocation of the resources, which yields a high computational complexity. To overcome the complexity issue, the second approach decouples the considered maximisation problem into two sub-problems based on the decomposition method on the cost of performance particularly for low SNR values. The proposed resource allocation strategy has been developed to work with multi-input-multi-output (MIMO) based AMC-OFDMA systems. In this project, two MIMO transmission criteria are considered, i.e. traditional and eigen-mode. In contrast, a user scheduling algorithm combined with the proposed resource allocation and AMC strategies is presented. The user scheduling algorithm aims to maximize the average system throughput by arranging the users in distinct queues according to their priorities and selecting the best user of each queue individually in order to guarantee a fair user service amongst different priority levels. When the involved users are scheduled, the scheduled users have been passed to the resource allocation to implement the distribution of the available resources. The proposed strategies have been tested over different international telecommunication union (ITU) channel profiles. The obtained simulation results show the superior performance of the introduced approaches in comparison with the related conventional methods. Furthermore, the gradually improvement in the throughput performance of the AMC-OFDM/ODMA system throughout the combination of the proposed strategies is clearly explained.Ministry of Higher Education and Scientific Research/IRAQ

Algoritmos en transmisión y recepción para OFDM en entornos multi-usuario

Desde su incursión en la vida de los ciudadanos de forma cotidiana, las comunicaciones móviles han ido adquiriendo cada vez más protagonismo. Lo que hace unos años era un elemento de lujo o causa de envidias, hoy es algo tan habitual como coger el autobús. Este auge en las comunicaciones móviles ha llevado consigo un aumento del número de usuarios de estos servicios, así como una mayor exigencia por parte de éstos en cuanto a velocidad de transmisión, seguridad, servicios, calidad ... Por este motivo, se han ido introduciendo nuevas redes, así como mejoras en las ya existentes. Actualmente estamos asistiendo al inicio de la implantación de las redes denominadas de tercera generación (3G). No obstante, estas redes 3G fueron de nidas a mediados de los años 90. En la actualidad nos encontramos en la de nición de lo que será la cuarta generación móvil (4G) a la que probablemente preceda una intermedia, tal vez Super 3G. Muchas son las tecnologías candidatas para esta Super 3G o para la 4G, pero es OFDM (Multiplexación por División en Frecuencias Ortogonales - Orthogonal Frequency Division Multiplexing) una de las más prometedoras para terminar siendo la tecnología elegida, debido principalmente a la robustez que ofrece frente al multi-trayecto, la flexibilidad que proporciona por el hecho de dividir el ancho de banda en sub-portadoras que pueden ser moduladas independientemente para adaptarse a las condiciones específicas de los canales móviles y al aprovechamiento de la diversidad multi-usuario cuando se combina en OFDMA (Acceso múltiple por división en frecuencias ortogonales - Orthogonal Frequency Division Multiple Access). Sin embargo, para que estos sistemas OFDMA sean una realidad en redes futuras como la Super 3G o la 4G, es preciso reducir la complejidad que ellos conllevan, en aspectos como la sincronización multi-usuario o la implementación de la modulación adaptativa. Esta Tesis está enfocada a reducir la complejidad de implementación para que la tecnología OFDMA pueda ser un hecho en las futuras redes inalámbricas. La sincronización en sistemas OFDMA, principalmente en escenarios ad-hoc plantea un reto importante a la hora de implementarla en este tipo de redes. Dado que existe una literatura extensa dedicada a resolver el problema de la sincronización en entornos OFDM mono-usuario, en esta Tesis se propone un procedimiento para reducir el problema de la sincronización multiusuario al caso mono-usuario, lo que permite la re-utilización de todas las técnicas propuestas en la literatura. Además de las buenas prestaciones incluso en situaciones de saturación de la red, permite un ahorro de la energía necesaria para la realización de las tareas de sincronización. Asimismo, a la vez que se resuelve la sincronización multi-usuario, también en esta Tesis se aporta un algoritmo para realizar el seguimiento de la desviación de frecuencia en sistemas tanto OFDM como OFDMA, de una forma sencilla y eficiente. Este algoritmo utiliza las sub-portadoras piloto de los símbolos, y ofrece unas prestaciones razonables, incluso cuando el número de estas sub-portadoras piloto es reducido. La modulación adaptativa u otras técnicas avanzadas de comunicaciones como MIMO (Multiple Input Multiple Output) o el conformado de haz (beamforming), que se hacen más atractivas cuando se combinan con OFDM u OFDMA, requieren de cierta realimentación hacia el transmisor por parte del receptor, además de un conocimiento del canal bastante preciso. En esta Tesis se aportan algoritmos para la compresión de esta información (que puede ser muy elevada dependiendo de la granularidad de la adaptación y la variabilidad del canal), lo que permite un mejor aprovechamiento de los recursos radio, y simpli ca la implementación de este tipo de sistemas. Se consiguen reducciones superiores a cuatro veces la tasa original, lo que implica grandes ventajas en todos los sentidos. Además, se realiza un estudio teórico sobre cómo afectan los errores en la estimación de canal en la capacidad del sistema, obteniéndose dos cotas numéricas para ésta. Gracias a estas cotas se puede concluir que las técnicas actuales de estimación de canal son su cientemente buenas como para que las pérdidas producidas por los errores en esta estimación no sean muy significativas; No obstante, estas pérdidas aumentan a medida que se incrementa la relación señal a ruido, y, por tanto, es conveniente seguir mejorando la estimación de canal si se pretende que estos sistemas trabajen a altas relaciones señal a ruido. Por último, destacar que las simulaciones llevadas a cabo para examinar las prestaciones de los algoritmos desarrollados han proporcionado datos prácticos que ayudarán al diseño e implementación de futuras redes móviles 4G.Nowadays mobile communications are common in our society. In less than two decades they have changed the way people understand personal communications. This interest is still growing and makes users (every day larger in number) ask for more and more services, speed, security and quality, at least comparable to the wired technologies. For this reason, new networks have been introduced in the scene, as well as improvements in the already existing ones. At the moment we are witnessing the beginning of the deployment of the so-called third generation networks (3G). However, these 3G networks were defined in the middle of years 90. At the present time we are in the definition of what will be the fourth mobile generation, which is called the 4G and probably an intermediate stage as Super 3G could be defined first. Many different technologies are in consideration, but OFDM (Orthogonal Frequency Multiplexing Division) is one of most promising candidates to end up becoming the technology to be used, due mainly to the robustness that it others to the multi-path channel, the flexibility that it provides by the fact that it divides the bandwidth into sub-carriers which can be modulated independently to adapt the signal to the specific channel conditions and to the advantage of the multi-user diversity when it is used to multiplex several users in the form of OFDMA (Orthogonal Frequency Division Multiple Access). Nevertheless, in order for these OFDMA systems to be a reality in future networks as Super 3G or 4G, the reduction in complexity has to be considered, in aspects such as the multi-user synchronization or the implementation of the Adaptive Modulation. This doctoral Thesis is focused on reducing the implementation complexity in both multi-user synchronization and Adaptive Modulation so that OFDMA technology can be a fact in the near future radio networks. The synchronization in OFDMA systems, specially in ad-hoc scenarios, raises an important challenge at the time of implementing this technology. Since an extensive literature is devoted to solve the problem of the synchronization in single-user OFDM systems, in this Thesis we have designed and proposed a procedure to reduce the multi-user synchronization problem to the single-user case, and therefore it allows the reuse of all the already proposed techniques in the literature. The procedure exhibits a good performance even in network saturation situations, and in addition it does not incur in a high power consumption when performing the synchronization tasks. Besides, solving the multi-user synchronization problem, the Thesis also proposes an algorithm for frequency offset tracking in an efficient and simple way, for both OFDM and OFDMA systems. This algorithm uses the pilot sub-carriers scattered in each OFDM symbol, and o ers reasonable performance, even when the number of these pilot sub-carriers is small. Adaptive Modulation and other advanced techniques as MIMO (Multiple Input Multiple Output) or beamforming become more attractive when they are combined with OFDM or OFDMA. However they require certain feedback information from receiver to the transmitter, and in addition, a quite precise knowledge about channel conditions. In this Thesis several algorithms for compressing the feedback information have been developed. This feedback information can be very highly demanding (depending on the granularity of the adaptation and the variability of the channel), and therefore this compression allows a better use of the resources, and simplifies the implementation of this kind of systems. Reductions of more than four times the original flow are obtained, which implies great advantages. In addition, a theoretical study is made on how errors in channel estimation affect the channel capacity, and several upper-bounds (one of them very tight) for this effect are obtained. One of the conclusions is that the actual state of the art in channel estimation techniques is good enough to be used without too much loss; However, the loss increases as the signal to noise ratio increases, and therefore, it is suggested to continue improving channel estimation algorithms for improving performance in high signal to noise ratio scenarios. Finally, thanks to simulations and designs carried out in order to examine the performance of the developed algorithms, some practical data are provided that will help in the future design of new mobile networks as 4G