Mobile networks or wireless systems of the 5th generation, also referred to as 5G, aim to achieve a higher capacity than 4G. In a 5G radio context, the number of antennas is expected to increase dramatically. To reduce energy and allow sharing of resources, a centralized basestation architecture is most advantageous. Furthermore, different 5G techniques under study, such as Massive multiple input multiple output (MIMO), require synchronization between different antennas, which further increase the need for centralized processing and RF generation. However, the distribution of the RF signals from the centralized basestation toward the antennas is not trivial. Because with increasing distance and frequency, copper loss will degrade performance.
RF signals may be transported over fiber by digital encapsulation of I/Q samples of the RF signal using for example the Common Public Radio Interface (CPRI). The disadvantage of this solution is that significant processing is required at the antenna side. Furthermore, synchronization of different antennas is difficult, which complicates Massive MIMO scenarios.
Another option for transporting RF signals over fiber is direct modulation of the optical carrier with an analog signal. This can be done at the required RF frequency or at a lower IF frequency. If modulated at the RF frequency, only optical to electrical conversion is required at the remote antenna and the optical channel can be incorporated in the overall channel the system encounters, hence similar equalization techniques already used in wireless systems can be reused to remove fiber imperfections. If a lower IF frequency is used, an upconversion stage is still required at the remote antenna. The latter will again increase the difficulty of antenna synchronization.
However, to modulate the wireless signals on an optical carrier, modulators with a high dynamic range are required. They should add only little noise and should be sufficiently linear to ensure the wireless signal is not distorted. Typically, Mach Zehnder modulators or direct modulation of DFB lasers are employed. Both are expensive photonic devices.
There is therefore a need for systems and methods to transport RF signals from a centralized basestation over optical fiber towards the antennas.