Man arrested over operation of rogue cellular base station in Tokyo

A Chinese man has been arrested on suspicion of illegally operating a mobile phone base station without a license in busy entertainment districts in central Tokyo, the Metropolitan Police Department said.

Yu Hai, a 31-year-old Chinese national living in Tokyo’s Kita Ward, is alleged to have installed radio equipment in a vehicle and operated an unlicensed base station in Tokyo’s Ginza district on Saturday, the police said.

He has denied the allegations against him, reportedly telling investigators, “I’ve never used any radio equipment before. I don’t know anything about it.”

The police also referred him to prosecutors on suspicion of violating the radio law by interfering with important radio communications, saying he disrupted NTT Docomo’s mobile phone services.

This is believed to be the first case in Japan in which an individual has been arrested for interfering with mobile phone communications, according to the police.

Investigators said the suspect allegedly used specialized equipment installed in a vehicle to transmit radio signals that interfere or block normal mobile phone communications.

He then caused nearby smartphones to connect to an unlicensed base station that transmitted signals using the outdated second-generation (2G) mobile communication standard.

Taking advantage of weaker authentication mechanisms in 2G technology compared with current 4G and 5G standards, Yu is believed to have sent fraudulent text messages in Chinese through the Short Message Service (SMS).

The messages reportedly told recipients that payments using their credit cards had been temporarily suspended due to the risk of unauthorized use overseas. Recipients were then directed to websites where they were asked to enter their credit card details and other personal information.

The suspect is believed to have repeatedly carried out the operation in Tokyo’s Ginza, Shinjuku and Shibuya districts. The signals he allegedly transmitted reportedly caused as many as 10,000 smartphones a day to lose mobile service within a radius of at least 500 meters.

Since January last year, police have received inquiries from 18 people, including Chinese tourists, who lodged reports over suspected fraud. Their combined losses are estimated at about ¥4 million ($25,300). The police are investigating whether the operation involved an organized group.

Investigators examine a vehicle allegedly used to operate the unlicensed mobile phone base station in Tokyo's Chuo Ward on Saturday. | JIJI

In December 2024, NTT Docomo received reports from customers about communication disruptions. An investigation conducted the following month led the company to identify the suspect.

The Ministry of Internal Affairs and Communications, which was notified of the findings, issued him a verbal warning. However, he allegedly continued the illegal activities.

The discovery of rogue base stations attracted attention on social media last year. The ministry issued a public warning in May last year, saying that the interference of mobile phone services had occurred due to what it suspected to have been the operation of unlicensed radio equipment.

Nobuo Suzuki, a professor at Kindai University well-versed in mobile networks, said dedicated software for building base stations can be downloaded by anyone from the internet.

Advances in radio equipment have also reduced the cost of setting up such stations compared with the past, Suzuki said, adding that this might have made them a tool for criminal activities.

Although operating an unlicensed base station requires specialized technical knowledge and remains difficult, advances in artificial intelligence may be making the process easier, he said.

In Thailand and Hong Kong, the media has reported the discovery of similar cases between last year and this year involving fraudulent SMS messages being sent through rogue base stations.

Japan has already discontinued services using the 2G mobile communication standard exploited in the case. Smartphone users are therefore advised to exercise caution if their devices suddenly display a 2G connection or lose mobile services.

Mobile carriers and other organizations are advising users who receive suspicious text messages not to open any links and to delete them immediately.

Some smartphone models allow users to disable connections to 2G networks.