Context-based Cell Search in Millimeter Wave 5G Networks

I. IN T RO D U C T I O N In this starting phase of the process that will lead to the definition of the 5th generation (5G) of wireless access networks, the mobile industry sector is facing big challenges due to the incredibly fast increase in data traffic volumes and the need to expand the network and provide high peak rates everywhere with low infrastructure costs. The limited resources available in the traditional spectrum portions below 3GHz is pushing the research community to consider much higher frequency bands where large and continuous spectrum segments are available. In particular, millimeter waves (mmwaves) with frequencies above 30GHz are considered excellent candidates for the definition of some of the components of future 5G systems [1], [2]. Traditionally, mm-waves have been used for point-to-point radio links and they constitute an important and widely used technology for the backhauling of current mobile networks. For 5G, the main goal is that of enabling the use of mmwaves in the access part of the network so as to enlarge the available capacity and increase dramatically the data rate available for end users. This objective, however, presents a number of technical challenges because of the harsh propagation at so high frequencies and the transmitter technology that limits the power available at antenna elements. Propagation is characterized by severe limitations in the transmission range due to path loss that increases with the square of the frequency. Moreover, most of the objects appear as opaque and cannot be penetrated by signals; also reflection is in most cases very limited depending on the material and the arrival angle. The characterization of the propagation channel is actually a challenge by itself due to the number of different parameters that can influence signal and the very diverse physical scenarios to be considered [3]. On the other side, the short wave length allows the use of antenna arrays with a relatively large number of elements that can be accommodated in a small space both on the base station (BS) and the mobile terminal (UE). Research in transmission technology is currently focusing on the use of beamforming techniques to concentrate radiated power on very small angles so as to extend the transmission range and to track user terminals while they move in the coverage area. This is considered sufficient to enable the design of pico BSs for serving small cells with a coverage area up to a few hundreds of meters. Nevertheless, it is quite evident that even with the massive use of sophisticated directive transmissions the availability of mm-wave access in future mobile networks cannot be continuous either in space, due to limited size of the coverage areas, and in time, due to the obstacles (including human body) that can obstruct the propagation. For this reason, in the MiWEBA project [2], we are considering a system architecture based on heterogeneous technologies where continuous connectivity is provided everywhere using legacy or advanced technologies at micro waves, while additional capacity layers based on mmwaves small cells are added to provide high transmission rates and traffic offload in the areas with dense traffic concentration. The non continuous coverage of mm-wave small cells can be an issue not only for data traffic, but also for signaling that in wireless access networks is essential for a number of functions related to mobility management and service access. Therefore, the system architecture that is currently considered by MiWEBA project and other projects on 5G considers a functional split between the user (U) plane and control (C) plane that allows mobile terminal to exchange signaling messages with base stations with legacy technology (typically a macro cell) and get radio resources for the data connection on high capacity mm-wave small cells when possible [4]. Obviously, some of the low level control functions that are strictly related on the physical layer cannot be delegated to a separated signaling connection with a different base station. Among these functions, for mm-waves access the cell discovery with initial synchronization is particularly critical being considered where mmwave small cells can exploit an overlay coverage layer based on legacy technology. The additional mm-wave layer can also take advantage of a functional split between control and user plane, that allows to delegate most of the signaling functions to legacy base stations and to gather context information from users for resource optimization. Abstract—The exploitation of the mm-wave bands is one of the most promising solutions for 5G mobile radio networks. However, the use of mm-wave technologies in cellular networks is not straightforward due to mm-wave severe propagation conditions that limit access availability. In order to overcome this obstacle, hybrid network architectures are