E-Band 76-GHz Coherent RoF Backhaul Link Using an Integrated Photonic Mixer
An E-band 76-GHz coherent radio-over-fiber (CRoF) system has been developed employing an integrated coherent photonic mixer (CPX) in the radio access unit (RAU) and a Schottky envelope detector in the wireless receiver. The CPX basically consists of a 2×2 multi-mode interferometer (MMI) coupled to a balanced photodiode (PD). It enables direct conversion of the optical baseband signal to the wireless RF signal with a carrier frequency at 76 GHz using a local oscillator located in the RAU. The developed CPX module features a V-type connector. The 3-dB cut-off frequency and 1-dB saturation output power of the CPX module are estimated as∼70 GHz and−2.43 dBm, respectively. It is shown that due to the integration of the MMI and the balanced PD, the developed CPX outperforms a commercial 110-GHz PD in terms of conversion efficiency up to 92 GHz. Experimentally, long-distance wireless transmission is shown using the constructed CRoF system and highly directive antennas with a gain of 43 dBi each. A wireless transmission of 1 Gb/s non-return-to-zero data signal at 76 GHz carrier frequency is demonstrated up to 230 m. The receiver sensitivity of the constructed wireless receiver for a pre-FEC bit error rate of 10−3 has been measured to be −34.8 dBm. This has enabled wireless data transmission over 92 m and 230 m at transmit power levels as low as −11.17 dBm and −3.36 dBm, respectively. It is shown that the experimental transmit power levels agree well with the expected figures calculated using an analytic Friis model describing the wireless channel. It is also estimated that the system could support maximum wireless distances up to 2000 m.