Scaling Laws for Infrastructure Single and Multihop Wireless Networks in Wideband Regimes

With millimeter wave bands emerging as a strong candidate for 5G cellular networks, next-generation systems may be in a unique position where spectrum is plentiful. To assess the potential value of this spectrum, this paper derives scaling laws on the per mobile downlink feasible rate with large bandwidth and number of nodes, for both Infrastructure Single Hop (ISH) and Infrastructure Multi-Hop (IMH) architectures. It is shown that, for both cases, there exist \emph{critical bandwidth scalings} above which increasing the bandwidth no longer increases the feasible rate per node. These critical thresholds coincide exactly with the bandwidths where, for each architecture, the network transitions from being degrees-of-freedom-limited to power-limited. For ISH, this critical bandwidth threshold is lower than IMH when the number of users per base station grows with network size. This result suggests that multi-hop transmissions may be necessary to fully exploit large bandwidth degrees of freedom in deployments with growing number of users per cell.Comment: 5 pages, 3 figure

Unified Capacity Limit of Non-coherent Wideband Fading Channels

In non-coherent wideband fading channels where energy rather than spectrum is the limiting resource, peaky and non-peaky signaling schemes have long been considered species apart, as the first approaches asymptotically the capacity of a wideband AWGN channel with the same average SNR, whereas the second reaches a peak rate at some finite critical bandwidth and then falls to zero as bandwidth grows to infinity. In this paper it is shown that this distinction is in fact an artifact of the limited attention paid in the past to the product between the bandwidth and the fraction of time it is in use. This fundamental quantity, called bandwidth occupancy, measures average bandwidth usage over time. For all signaling schemes with the same bandwidth occupancy, achievable rates approach to the wideband AWGN capacity within the same gap as the bandwidth occupancy approaches its critical value, and decrease to zero as the occupancy goes to infinity. This unified analysis produces quantitative closed-form expressions for the ideal bandwidth occupancy, recovers the existing capacity results for (non-)peaky signaling schemes, and unveils a trade-off between the accuracy of approximating capacity with a generalized Taylor polynomial and the accuracy with which the optimal bandwidth occupancy can be bounded.Comment: Accepted for publication in IEEE Transactions on Wireless Communications. Copyright may be transferred without notic

Dynamic Time-domain Duplexing for Self-backhauled Millimeter Wave Cellular Networks

Millimeter wave (mmW) bands between 30 and 300 GHz have attracted considerable attention for next-generation cellular networks due to vast quantities of available spectrum and the possibility of very high-dimensional antenna ar-rays. However, a key issue in these systems is range: mmW signals are extremely vulnerable to shadowing and poor high-frequency propagation. Multi-hop relaying is therefore a natural technology for such systems to improve cell range and cell edge rates without the addition of wired access points. This paper studies the problem of scheduling for a simple infrastructure cellular relay system where communication between wired base stations and User Equipment follow a hierarchical tree structure through fixed relay nodes. Such a systems builds naturally on existing cellular mmW backhaul by adding mmW in the access links. A key feature of the proposed system is that TDD duplexing selections can be made on a link-by-link basis due to directional isolation from other links. We devise an efficient, greedy algorithm for centralized scheduling that maximizes network utility by jointly optimizing the duplexing schedule and resources allocation for dense, relay-enhanced OFDMA/TDD mmW networks. The proposed algorithm can dynamically adapt to loading, channel conditions and traffic demands. Significant throughput gains and improved resource utilization offered by our algorithm over the static, globally-synchronized TDD patterns are demonstrated through simulations based on empirically-derived channel models at 28 GHz.Comment: IEEE Workshop on Next Generation Backhaul/Fronthaul Networks - BackNets 201

Bandwidth occupancy of non-coherent wideband fading channels

Peaky and non-peaky signaling schemes have long been considered species apart in non-coherent wideband fading channels, as the first approaches asymptotically the linear-in-power capacity of a wideband AWGN channel with the same SNR, whereas the second reaches a nearly power-limited peak rate at some finite critical bandwidth and then falls to zero as bandwidth grows to infinity. In this paper it is shown that this distinction is in fact an artifact of the limited attention paid in the past to the product between the bandwidth and the fraction of time it is in use. This fundamental quantity, that is termed bandwidth occupancy, measures average bandwidth usage over time. The two types of signaling in the literature are harmonized to show that, for any type of signals, there is a fundamental limit-a critical bandwidth occupancy. All signaling schemes with the same bandwidth occupancy approach the capacity of wideband AWGN channels with the same asymptotic behavior as the bandwidth occupancy grows to its critical value. For a bandwidth occupancy above the critical, rate decreases to zero as the bandwidth occupancy goes to infinity

Cooperative wireless communications : present and future

Wireless data traffic grows exponentially while, at the same time, communications in known point-to-point channels seem to have almost achieved the Shannon Limit. In order to satisfy demand the fundamental point-to-point limitation needs to be circumvented by considering communications in wireless networks jointly in all nodes. Cooperative wireless communications achieve greater communication performance through networked collaboration not unlike the networking of computers has empowered computation. This topic covers a broad range of theoretical and practical aspects, as well as the evolution of wireless networks in an extended span of time from the state of the art and current standards, to the backwards-compatible modification of current technologies in the near future, while they are still in the middle of their life cycle, and the preparation of theoretical ground works for the writing of new standards in the distant future. The first part of the thesis focuses on the present. In the theoretic aspect, a survey is performed on the state of the art in research literature about the analysis of cooperative diversity gains at multiple levels of the protocol stack. In the practical aspect, the implementation of a detailed system level simulator puts to test the rules for relaying in the most recent cellular standard, which is an early instance of cooperation. The second part of the thesis focuses on the near future. The scarcity of spectrum in the microwave bands employed by current technologies is identified as a major problem that can be tackled with cooperation. In the theoretic aspect, an analytical study is performed to compute the social spectrum gains in cognitive radio cooperative spectrum leassing mechanisms. In the practical aspect, the results illustrate that the limited gains of cooperation in current standards can be improved in some emergent application niches such as machine-to-machine communications, much more suitable to be used with cooperative spectrum leasing than personal data services. The last part of the thesis focuses on the distant future. The spectrum crunch will be palliated through the introduction of massive degrees of freedom made possible by moving communication up to millimeter-wave bands that allow more bandwidth, antennas and node density. In the theoretical aspect, a scaling law analysis proves that there is a limit to the increment of capacity through massive resources, and that said limit must be pushed further back with cooperative multi-hop communications in new standards. In the practical aspect, a summary covers the future research topics in information theory, physical and medium access control layers that must pave the way for the design of the hardware in future wireless networks.Esta tesis estudia el papel de las comunicaciones cooperativas en las redes sin hilos, para lo que hemos identificado tres pasos distintos para la incorporación de la cooperación en redes inalámbricas en general, y en las redes celulares de telefonía y datos en particular. La tesis se divide en tres partes que corresponden a cada una de estas etapas. La primera parte de la tesis se centra en el presente, en el que el modelo teórico de canales de cooperación está relativamente desarrollado, pero su implementación en dispositivos reales es prácticamente inexistente, con la excepción de algunos gestos menores para acercarse como los relays estáticos en la versión más reciente del estándar de redes celulares. La segunda parte de la tesis se centra en el futuro cercano (aproximadamente desde el presente hasta la década de 2020), que corresponde a la duración del ciclo de vida de la actual generación de tecnologías. En este período la compatibilidad entre los dispositivos de la misma familia será una prioridad, y las posibles mejoras llegarán en forma de incremento en la eficiencia de uso de las mismas frecuencias (microondas) y arquitecturas (dúplex estático y topologías en árbol). La última parte se refiere en un futuro más avanzado (a partir de 2020), cuando expire la actual generación de tecnologías y nuevos estándares de comunicaciones tomen su lugar. Durante este período, se abre una ventana de oportunidad para introducir cambios drásticos en la filosofía de diseño, y será posible ampliar el espectro de las redes hacia nuevas bandas (ondas milimétricas) o nuevas topologías (dúplex dinámico y topología totalmente conectada).Esta tese estuda o rol das comunicacións cooperativas nas redes sen fíos, para o cal se identificaron tres etapas distintas para a incorporación da cooperación nas redes sen fíos en xeral, e nas redes celulares de telefonía e datos en particular. A tese está dividida en tres partes que se corresponden con cada unha destas etapas. A primeira parte da tese céntrase no presente, en que o modelado teórico de canles cooperativas está relativamente desenvolvido, pero a súa implementación en dispositivos reais é prácticamente inexistente coa excepción dalgúns leves xestos de aproximación como os relays estáticos na última versión do estándar de redes celulares. A segunda parte da tese céntrase no futuro próximo (aproximadamente dende o presente ata a década de 2020), que se corresponde coa extensión do ciclo de vida das tecnoloxías da actual xeración. Dado que neste periodo a compatibilidade entre dispositivos da mesma familia será unha prioridade, as posibles melloras han de chegar na forma de refinamentos na eficiencia de uso das mesmas frecuencias (microondas) e arquitecturas (dúplex estático e topoloxías en árbore). A última parte correspondese co futuro máis avanzado (a partir de 2020), cando se extinga a actual xeración de comunicacións e novos estándares tomen o lugar dos actuais. Durante este periodo abrirase unha ventá de oportunidade para a introdución de cambios drásticos na filosofía de deseño, e será posible expandir o espectro das redes cara novas bandas (ondas milimétricas) ou novas topoloxías (dúplex dinámico e topoloxía completamente conectada)

Optimal link scheduling in millimeter wave multi-hop networks with space division multiple access

In this paper, we introduce a model for Multiple-Input Multiple-Output (MIMO) Space Division Multiple Access (SDMA) into the analysis of a multi-hop millimeter wave network under the classic Network Utility Maximization (NUM) framework with Maximum Back Pressure scheduling (MBP). We show that the proof of convergence of MBP remains valid when we allow the scheduler to select multiple links to the same receiver in the same frame. Conventional MBP with a single link per receiver is traditionally implemented using the Maximum Weighted Matching (MWM) algorithm over the network graph. Under our modification, the problem becomes a Maximum Weighted Partition of the graph. Message Passing (MP) algorithms are efficient and have been successfully applied to graph partitioning problems in the past, so we use one to approximate the optimal MBP scheduling. Through simulation over a randomized mmWave picocell, we compare the MWM reference without SDMA, the efficient MP approximation, and the exact optimal MBP scheduler with SDMA (obtained by brute force). Simulations show that by leveraging SDMA in multi-hop mmWave network scheduling, a 50% capacity increase is obtained on average