Estudo de tecnologias híbridas óptica-rádio para controle de fase em sistemas MIMO

Abstract

Currently, there is a great demand for higher transmission rates in mobile communication systems, mainly driven by 5G, which must be able to meet the growing increase in network-connected devices. To meet the new requirements, one of the most viable alternatives is to extend the use of the radio spectrum to higher frequencies (up to 100 GHz) while reducing the size of the RF cells, due to the more limited range of frequencies in this spectrum. The use of a large number of RF cells, each cell having limited range, makes it necessary to employ multiple-input multiple-output (MIMO) systems to ensure an adequate level of signal coverage. However, these alternatives impose technological and economical limitations, making radio systems more complex and expensive, since the increase in the transmission rate per channel brings the need for more advanced digital signal processing (DSP) techniques. In this context, the use of hybrid systems, in which an optical network integrates the radio access network architecture, emerges as an alternative to ensure high aggregate rate values with low complexity and low latency, allowing to exploit the high transmission capacity of fiber along with its low attenuation. The transport of RF signals over optical fiber, called radio over fiber (RoF), in addition to enabling the transport of large amounts of data, allows to transfer all the signal processing to a remote central unit, connected by an optical network to the radio base stations, and can keep the structure of the latter as simple as possible, thus reducing cost and latency. Hybrid optical-radio technologies represent an effective solution for providing broadband radio access for the new generation of mobile networks, as they can be applied especially for high-speed communications and for implementing multi-cell networks while keeping data processing centralized. In MIMO systems, one of the most critical requirements is the phase control of the transmitted signals by an array or vector (array) of antennas, which ensures the control of the directivity of the transmitted beam (beamforming) and the possibility to change it according to the position of one or more mobile devices (beamsteering). The phase control is usually done in the electrical domain in the remote antennas, however, this approach is not very convenient when the number of antennas is very high, because it increases the processing time and consequently the latency of the system. In this work, several configurations of hybrid optical-radio networks are studied, with the objective of performing the phase control entirely in the optical domain, thus avoiding the processing that would be necessary to implement this phase control in the RF cells themselves and allowing to centralize this function in a remote unit. Phase control in the optical domain is achieved through the chromatic dispersion characteristics of the fiber, which causes optical channels at different wavelengths to propagate at different speeds, with the time delay relationships as a function of spectral distance being deterministic. The results show the feasibility of the proposed phase control techniques, and their applicability to RF antenna array systems. Furthermore, the proposed architecture is capable of operating in the 5G frequency bands using digitally modulated signals. The proposed configuration is simple and cost-effective, which makes it suitable for application in high-density MIMO systems.