One of the main possible applications of SP technology, as mentioned above, is it as enabling technology for 5G wireless networks. In this section I’m going to present a case study on its application as a transceiver between base station and the antennas in a communication network, based on the technology introduced in the previous section.?To achieve the goals of 5G mentioned in section 1 (increased bandwidth, low latency, faster bit transmission), there is a need of rethinking and reorganizing the radio access networks (RANs).
Since the number of connected devices is expected to increase hugely, radio-over-fiber links (RoF, i.e. light is modulated by a radio signal and transmitted over an optical fiber link) appear to be the solution: given the large number of required links, therefore, a cheap, scalable SP transceiver is needed. ?In Ref. 6, a III-V-on-silicon distributed feedback laser is used as a transmitter (fig. 17).
The laser is built by an adhesive wafer bonding on a InP/AlGaInAs multi-quantum-well epitaxial stack to a integrated circuit based on silicon photonics. The silicon waveguides are 400 nm thick, 180 nm deep, and a DFG grating is defined in them, below the III-V gain material. Light is coupled from the gain section to the Si waveguide by a tapering in two steps, that minimizes reflections and guarantees no light losses due to coupling to higher order modes.
At the receiver side a high-bandwidth waveguide-couples Ge-on-Si photodetector (to translate the signal from optical to electrical) are used (fig. 18). More specifically, the RX is a 14 ?m Ge-on-Si waveguide-coupled avalanche photodetector on a SP integrated circuit, where the Germanium slab (400 nm thick and 1000 nm wide) is p-doped, built over a n-doped Silicon waveguide.
The receiver works in avalanche operation at low voltages, in order to improve the link gain without needing very high power.?These devices appear to be a scalable and a cost-effective solution for future mass usage. They have been tested for 1) a 64-QAM OFDM signal at 3.5 and 5 GHz and for 2) a 16 Gbps 16-QAM at 20 GHz data signal, showing both the feasibility of a high bit rate transmission with these devices, as well as good linear behavior and low noise level.?The bit-error rate (BER) for experiment n.2 has been found to be below 2.
5 · 10?8 and of 5 · 10?5 respectively with and without the presence of an equalizer. The received constellation is reported in fig. 19, along with an electrically transmitted back-to-back 4 GBd for comparison purpose.
The obtained results from the study show that the silicon photonics transceivers can be used to transmit a high complex-modulated data (as this is the case of a quadrature-to-amplitude modulation) of on a high frequency carrier. Is it then possible to operate at a frequency of about 28 GHz, that is the range of interest of fifth generation wireless networks.