NEC Corporation has developed and demonstrated a radio-over-fibre (RoF) system utilising a 1-bit fibre transmission method, enabling the affordable construction of stable millimetre-wave communication networks for Beyond 5G/6G.
This method allows high-frequency analogue signals to be transmitted using an economical electrical-to-optical converter for general-purpose digital communications, facilitating the creation of compact distributed antenna units at a lower cost. Consequently, a stable millimetre-wave communication environment can be achieved cost-effectively in high-rise buildings, underground malls, factories, railways, indoor facilities, and other challenging environments.
NEC will present its findings at the IEEE MTT-S International Microwave Symposium (IMS2024) in Washington, D.C., starting on 16 June.
Development background
High-speed wireless communications using millimetre-wave technology are expected to be crucial for Beyond 5G/6G. Since around 80% of mobile communication traffic occurs indoors, millimetre-wave technology is being considered for indoor solutions. However, due to significant propagation loss and high linearity in the millimetre-wave frequency band, maintaining line of sight between base stations and terminals is essential for ensuring adequate quality of service (QoS).
While densely installing distributed antenna units (DAs) for direct data transmission and reception with terminals is effective in addressing these issues, the size, power consumption, and cost of installing the necessary number of DAs pose significant challenges. To tackle these problems, NEC has developed a radio-over-fibre (RoF) system and a new transmission method.
Issues with conventional RoF systems and solutions
Conventional RoF systems can be categorised as digital RoF and analogue RoF. Digital RoF involves transmitting digital signals generated in the radio unit (RU) to the DA over fibre. This requires the DA to be equipped with a digital signal processor (DSP) and a digital-to-analogue converter (DAC), resulting in high power consumption and costs. In contrast, analogue RoF transmits high-frequency analogue signals from the RU to the DA over fibre. Although this method simplifies the configuration as the DA does not need a DSP or DAC, it requires a dedicated converter with high linearity for the electrical-to-optical conversion, increasing device costs.
To address these issues, NEC developed a 1-bit fibre transmission method and a 1-bit RoF system. The 1-bit fibre transmission method converts high-frequency analogue signals into 1-bit pulse signals for transmission over fibre, with the desired analogue signal being reproduced through a filter. This approach allows the use of an economical general-purpose electrical-to-optical converter, as in digital RoF, while eliminating the need for DSPs and DACs in the DA, as in analogue RoF. Thus, the 1-bit RoF system combines the advantages of both digital and analogue RoF systems.
Developing a 1-bit modulator with a high signal-to-noise and distortion ratio (SNDR) for converting signals into 1-bit pulse signals has been a challenge. NEC addressed this by developing a vector decomposition method with superior SNDR for downlink traffic and a digital reproduction method for uplink traffic that cancels signal distortion generated by 1-bit fibre transmission. This has enabled NEC to suppress SNDR degradation during 1-bit fibre transmission in both downlink and uplink directions.
Additionally, to ensure that the newly developed 1-bit RoF system complies with mobile communication standards, NEC created a new radio-over-fibre prototype for the 40 GHz band, consisting of an RU and a compact DA. Verification tests confirmed the system’s compliance with standards. The new system allows for the installation of compact, low-cost DAs in high density and is expected to improve the millimetre-wave communication environment by ensuring line of sight between DAs and terminals.
Future prospects
NEC’s 1-bit fibre transmission method enables the creation of compact, low-power, and cost-effective millimetre-wave distributed antennas. The radio-over-fibre system using this method facilitates the affordable construction of stable millimetre-wave communication networks in various challenging environments. This development will promote the adoption of high-speed, large-capacity communications using millimetre waves for Beyond 5G/6G. NEC will continue to advance high-speed and high-capacity technologies for Beyond 5G/6G.
NEC will present these findings at the session on Photonic-Enabled Systems and Solutions at IMS2024, held from 16 June in Washington, DC. The presentation is titled “1-bit Digital Radio-over-Fiber System with Hybrid Architecture for 40-GHz Band” and will take place on 18 June 2024.
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