Thesis defense K. Chandra: 5G

27 February 2017 | 15:00
location: Aula, TU Delft
by Webredactie

Towards Realizing 5G: Efficient Medium Access and Beamwidth Adaptation in 60 GHz. Promotor: I.G.M.M. Niemegeers (EWI).

The unprecedented growth of mobile devices and high data rate applications have resulted in an enormous surge in the wireless data traffic. The existing wireless communication systems operating in the sub-6GHz frequency band have  already reached their capacity limits. This has led to research on the next generation of wireless communications, also known as 5G. Due to the availability of large unused bandwidth in the millimeter wave (mmWave) frequency band (30—300GHz), new air interfaces in this band have emerged as promising candidates for multi-Gbps wireless access in 5G communications. Although the first  ever demonstration of wireless signal reception was in the year 1889 by Sir J. C. Bose consisting of 60 GHz signals, mmWave radios have mainly remained confined to the military. This was mainly due to the exorbitant cost of equipment and the not so conducive propagation properties. The recent developments in silicon-based complementary metal–oxide–semiconductor (CMOS) processes allow inexpensive implementation of mmWave systems for consumer applications. The possibility of low-cost implementation coupled with the demand for multi-Gbps wireless access, has accelerated the investigations into mmWave based wireless local area networks (WLANs) and mobile communications. The high free-space path loss in the mmWave band mandates the use of directional antennas to provide the required signal power at the receiver. This brings many challenges at the medium access control (MAC) and network layers. mmWave communication standards such as IEEE 802.11ad and IEEE 802.15.3c – targeting short-range high data rate communications in the 60GHz frequency band – have specified the use of directional antennas. The aim of this dissertation is to investigate the MAC and network layer challenges of 60GHz directional communications in the context of multi-Gbps connectivity for 5G networks. We propose a modelling framework for the performance evaluation of the IEEE 802.11ad MAC protocol that considers the presence of both contention and noncontention channel access modes. Further, we consider the parameters unique to mmWave communications such as transmit and receive beamwidths and the spatio-angular distribution of communicating devices and propose a beamwidth allocation scheme that minimizes the packet collision probability in contention based channel access and maximizes the channel utilization. While using directional antennas, a common assumption is that a narrow-beamwidth link provides more capacity compared to a wide beamwidth link. This is because a decrease in beamwidth results in an increase in the antenna gain. However, narrow beamwidth links are highly susceptible to beam alignment errors. Further, selection of the best transmit and receive directions requires the transmission of training packets resulting in beam setup overhead. The beam alignment and setup overheads depend on the transmit and receive beamwidths, resulting in trade-offs between antenna gains and the corresponding beam alignment and link setup overheads. We investigate the impact of these overheads on the capacity of directional mmWave links to determine the optimum beamwidths. We also propose an efficient beam searching mechanism, employing an  pproach called decrease-and-conquer, resulting in a significant reduction in the link setup time. Further, to solve the problem of frequent link misalignment caused by the movement of users, we propose to use motion sensors (accelerometer, gyroscope and magnetometer) that are present in consumer devices, such as smartphones, to detect and circumvent beam misalignment. We show that motion sensor data can be used to predict the next location and orientation of the user. This information is used to reconfigure the directional antennas in advance and hence avoid frequent link disruptions. Multiple access points (APs) are required to facilitate seamless multi-Gbps connectivity in indoor environments since mmWave signals are subject to very high attenuation across walls. This triggers frequent handovers in case of mobile users. Moreover, beam blockage can happen because of humans obstructing the beam, and because of the orientation of the devices with respect to APs, prompting recurrent beam searching. These issues require methods for efficient network management to ensure seamless multi-Gbps connectivity in the mmWave bands. Therefore, we propose a hybrid network architecture consisting of both 2.4/5GHz and 60GHz links that exploits the excellent coverage provided by the 2.4/5GHz signals for control and the enormous capacity potential of the mmWave band for data transmissions. This results in a faster device discovery, leading to a speedynetwork association and reduced latency in the medium access. Further, we also investigate the radio-over-fiber (RoF) based network architecture which promises excellent central management of 60GHz APs. The RoF based network architecture is particularly attractive for mmWave communication systems as multiple APs would be operating in a small area where dynamic capacity allocation and seamless handover can be provided by the RoF-based central coordinator. We investigate the performance of the IEEE 802.11ad MAC protocol for a 60GHz RoF network architecture and discuss the crucial constraints on MAC parameters due to the extra delay introduced by the fiber. The proposed solutions in this dissertation, which were investigated for the 60 GHz band, concerning the MAC and network layers, we argue, will provide efficient multi-Gbps wireless access in the mmWave bands in general. 

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