NEC develops compact millimetre-wave distributed antenna for Beyond 5G/6G

By utilising this method, high-frequency analog signals can be transmitted using an inexpensive electrical-to-optical converter for general-purpose digital communications, thereby enabling the realisation of a compact distributed antenna unit at low cost. As a result, a stable millimetre-wave communication environment can be inexpensively achieved in high-rise buildings, underground malls, factories, railways, indoor facilities, and other obstacle-laden environments. NEC will be presenting its results at the IEEE MTT-S International Microwave Symposium (IMS2024) starting on June 16 (local time) in Washington, D.C.

Development background

High-speed wireless communications leveraging millimetre-wave technology are expected to be a key technology for Beyond 5G/6G. In particular, since approximately 80% of mobile communication traffic occurs indoors, millimetre-wave is being considered as an indoor solution. However, since there is significant propagation loss and high linearity in the millimetre-wave frequency band, it is imperative to ensure line of sight between base stations and terminals to achieve sufficient quality of service (QoS). While dense installation of distributed antenna units (DA) 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 system (RoF) and a related transmission method.

Issues surrounding conventional RoF systems and approaches to resolving them

As shown in Figure 2, conventional RoF systems are categorised as digital RoF systems (digital RoF) and analog RoF systems (analog RoF). With digital RoF, digital signals generated in the radio unit (RU) are transmitted to the DA over fibre. Since DA used in digital RoF must be equipped with a device for processing digital signals (i.e., 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 DA 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 (1-bit RoF). 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, an affordable 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.

Initiatives to realise 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  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. 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 millimetre-wave communication environment by ensuring the line of sight between DAs and terminals.

Future prospects

NEC developed a 1-bit fibre transmission method enabling a compact millimetre-wave distributed antenna, low power consumption, and cost reduction. The radio-over-fibre system applying this method enables inexpensively building stable millimetre-wave 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. Going forward, NEC will continue to develop high-speed and high-capacity technologies for Beyond 5G/6G.