NEC develops mmWave distributed antenna for Beyond 5G/6G

NEC Corporation has successfully developed and demonstrated a radio-over-fibre system with a 1-bit fibre transmission method, making it possible to affordably build stable millimetre-wave (mmWave) communication networks for Beyond 5G/6G. The company presented its results at the IEEE MTT-S International Microwave Symposium (IMS2024), held in June.

Development background

High-speed wireless communications leveraging mmWave technology are expected to be a key technology for Beyond 5G/6G. In particular, since approximately 80% of mobile communication traffic occurs indoors, mmWave is being considered as an indoor solution.

However, since there is significant propagation loss and high linearity in the mmWave frequency band, it is imperative to ensure line of sight between base stations and terminals to achieve sufficient quality of service. While dense installation of distributed antenna (DA) units for direct transmission and reception of data with terminals and avoiding obstacles is known to be effective in resolving these issues, the size, power consumption and cost of installing the required number of DA have proven to be major issues. To overcome these issues, NEC developed a radio-over-fibre (RoF) system and a related transmission method.

Issues surrounding conventional RoF systems

Conventional RoF systems are categorised as either digital RoF or analog RoF. With digital RoF, digital signals generated in the radio unit (RU) are transmitted to the DA over fibre. Since DAs used in digital RoF must be equipped with a digital signal processor (DSP) and a digital-to-analog converter (DAC), both power consumption and costs are high. Meanwhile, analog RoF uses a method with which high-frequency analog signals are generated in the RU and transmitted to the DA over fibre. With analog RoF, configuration is simplified since DAs are not required to be equipped with DSP or DAC; however, a dedicated converter with high linearity for analog signals is required for the electrical-to-optical converter. This in turn drives up device costs.

NEC therefore set out to develop a 1-bit fibre transmission method and a 1-bit RoF system. The 1-bit fibre transmission method converts high-frequency analog signals into 1-bit pulse signals that are transmitted over fibre, and the desired analog signal can be reproduced through a filter. For 1-bit RoF using this method, a general-purpose electrical-to-optic converter for digital communications can be utilised, as is the case with digital RoF. At the same time, like analog RoF, there is no need for the DA to be equipped with DSP or DAC. As a result, 1-bit RoF is a system that combines the advantages of digital RoF and analog RoF.

Realising the 1-bit RoF system

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 in the practical application of 1-bit RoF. NEC, therefore, developed a vector decomposition method (where a radio signal is decomposed into two signals with a constant amplitude) that has a superior SNDR as a 1-bit fibre transmission method for downlink traffic. Also, for uplink traffic, NEC developed a digital reproduction method that cancels signal distortion generated by 1-bit fibre transmission and regenerates the original signal. As a result, NEC succeeded in suppressing the degradation of the SNDR during 1-bit fibre transmission in both downlink and uplink directions.

In addition, to confirm that the newly developed 1-bit RoF system conforms to mobile communication standards, NEC developed a new radio-over-fibre prototype for the 40 GHz band consisting of an RU and a compact DA. A verification test confirmed the system’s compliance with the standards.

The newly developed system makes it possible to install compact, low-cost DAs in high density. In addition, it is expected to improve the mmWave communication environment by ensuring the line of sight between DAs and terminals.

Future prospects

NEC’s 1-bit fibre transmission method enables a compact mmWave distributed antenna, low power consumption and cost reduction. The RoF system applying this method enables inexpensive, building-stable mmWave communication networks in high-rise buildings, underground malls, factories, railways, indoor facilities and other obstacle-laden environments. It will therefore promote the uptake of high-speed and large-capacity communications using millimetre waves for Beyond 5G/6G.

Image: The millimetre-wave mobile network utilising a radio-over-fibre system.