• Add to Favorites
  • Subscribe
  • Friend us on Facebook
  • Follow Caijing on Twitter

Subway's Use of Public 2.4 GHz Band Risky

11-20 14:59 Caijing
The forced shutdowns in Shenzhen reflect the clash between subway communication systems and wireless networks, two industries on the fast track of development.

By staff reporters Ling Xin, Lu Wei, and Gao Shengke

Shenzhen Metro Group Co., Ltd. (SZMC) attributes frequent forced shutdowns of its Line 2 and Line 5 in early November to passengers' portable 3G devices interfering with train line signals. After the shutdowns, the group wrote to communications authorities, asking them to block 3G signals in subway carriages. The request was denied.

Both lines employ communications based train control (CBTC) systems that transmit wireless data on the common 2.4GHz band, a free band open to the public and used by subway communications systems in most Chinese cities.

As the market grows, more 3G wireless routers are reaching 100 milliwatts, the upper limit of international standards on transmitting power. Moreover, the transmitting power of some routers manufactured by underground factories can be up to 300 milliwatts. Experiments by SZMC indicate that the subway communications system registers immediate interference by external signals when seven or eight wireless routers are carried into each carriage.

However, several professors at the School of Electronic and Information Engineering, Beijing Jiaotong University, said that interference on subway communications systems by signals transmitted from passengers' wireless devices has been studied for many years in China, and despite the absence of national standards or industry norms, anti-interference technologies are readily available if subway companies take the time to learn about them.

The leap-frog development of subway projects in recent years has also led to insufficient system testing. It normally takes three to four years to complete a subway communications system abroad, compared to one or two years in China. An article published in Railway Signaling & Communication in 2011 writes that the CBTC system in Shenzhen's Subway Line 2 became fully functional within 19 months, setting a record for the shortest construction period in subway history. The project is a "miracle in the history of communications system project implementation," according to the article.

The forced shutdowns in Shenzhen reflect the clash between subway communication systems and wireless networks, two industries on the fast track of development. The 2.4GHz range is the standard for all subway lines throughout China, thus interference by external signals may become a nationwide problem.

To solve the problem, the subway industry should abandon the free 2.4GHz range and use an exclusive, paid range instead, say industry experts.

However, the wireless spectrum is a scarce strategic resource under strict control. Currently only a few sectors including the military, aviation, public security, meteorological, broadcasting, and telecommunications have exclusive ranges.

Xie Feibo, director of the Radio Administration Bureau, Ministry of Industry and Information Technology, said that there is no unitary administrative department to lead the application for a frequency range nationwide, nor are there uniform technological specifications and standards for the country's subway industry. Each city runs its subway system in its own way. It is unclear which technical department will be responsible for regulating and maintaining the subway industry's exclusive range should their application be approved.

Full article in Chinese: http://magazine.caijing.com.cn/2012-11-18/112291331.html

Editors’ Picks »