OKI evaluated the performance of the system using an acquired experimental transmitter. The testing confirmed that communication is possible over 10 km when transmission output of 250 mW, the specified maximum. The testing also confirmed direct communications in urban locations among many obstacles such as buildings at a transmission output of 70 mW within an area of approximately 150 meters in radius.

Making smart communities and smart houses a reality requires a communications infrastructure to transmit data, such as electricity control data. The communications infrastructure will require home networks that connect home appliances and electrical power equipment within the home or office, and field area networks that connect devices such as smart meters between outside buildings.

Wireless multi-hop communications are expected to make such infrastructures possible. Electrical power equipment such as smart taps and smart meters are often installed in locations with poor signal access, underscoring the significance of the 920 MHz band with its high signal propagation compared with 2.4 GHz which broke communications.

—Takeshi Kamijoh, General Manager of Research and Development Center at OKI

Features of the 920 MHz band wireless multi-hop communication system include:

• Large scale. The technology achieves scalability to accommodate more wireless devices by reducing controlled traffic for route searching and by establishing retransmission procedures. By use of this technology, it will be possible to establish large-scale multi-hop networks consisting of thousands of wireless devices per base station with smart meters, OKI says. Fewer base stations are required, reducing overall system setup costs. OKI has already evaluated experimental systems with up to 100 devices using the 950 MHz band, both indoors and outdoors. The results indicate these systems are expected to accommodate networks of several thousand devices.

• High reliability. The system provides improved connectivity between wireless devices by using specially-developed retransmission and dynamic route controls to handle fluctuations in adjacent wireless links caused by obstructions such as vehicles. The system is also resistant to base station faults and line disturbances between base stations and the hub, thanks to functions that allow individual wireless devices to switch independently to different base stations. It also allows easy base station maintenance without shutting down.

• Energy savings. To this point, energy savings for end-point wireless devices have been achieved via ZigBee by continuously operating routers with relay functions. It is now possible to switch routers to sleep mode using energy-saving functions complying with IEEE 802.15.4e. This eliminates the need to install power supplies for routers and increases the overall energy savings achieved. Adding functions to automatically adjust sleep mode timing in conjunction with data flows will minimize transmission delays while maintaining energy savings. The Ministry of Internal Affairs and Communications Network Integrated Control System Standardization Promotion project has been seeking to standardize IEEE 802.15.4e. In this project, tests were performed in March 2011 of various energy-saving functions using an IEEE 802.15.4e-compliant test bed.

• Simple installation. Automated route searching functions allow easy network construction based on automated and independent connections to temporary base stations and wireless devices. This configuration also makes it possible to provide low-cost temporary networks after disasters. Wireless multi-hop communications are subject to network instability due to the lack of relay routes in configurations with a low density of installed wireless devices—for example, on the initial rollout of smart meters or smart house applications. However, stable network operations can be achieved from the start by adding a function that automatically adjusts transmission output to suit the distance separating devices. The relaxation of transmission output limits following the shift to the 920 MHz band allowed higher output, enabling wireless multi-hop communications even in locations with considerable distance separating devices.

Moving forward, OKI says it intends to play a major role in promoting widespread use of smart communities, smart houses and smart meters based on 920 MHz wireless multi-hop technology. In addition to incorporating this technology into commercial products, OKI will draft plans for the ZigBee standards that support 920 MHz for smart houses, and will also continue working to strengthen interconnectivity for home appliances, electrical power equipment, and home gateways.

The achievements to date will be presented at the Institute of Electronics, Information and Communication Engineers Society Conference to be held at Hokkaido University on 14 September.

Encompassing frequencies between 915.9 MHz and 929.7 MHz in Japan, the 920 MHz frequency band frequency band is widely used outside Japan for applications such as smart meters, due to high signal propagation compared to the 2.4 GHz band generally used for sensor networks.

While the 950 MHz band has been in use in Japan since the band was institutionalized in 2008, the shift to the 920 MHz band is expected to stimulate the market through international partnerships, since the same frequency band is used in the United States and Asia. The 920 MHz band also permits higher transmission output (20 mW or 250 mW) than the previous 10 mW limit.

Wireless multi-hop communications such as sensor networks generally comply with IEEE 802.15.4 international standards for wireless communications and ZigBee for network communications. Implemented as a standard in 2009 following deliberations headed mainly by OKI, IEEE 802.15.4 includes a physical layer extension known as IEEE 802.15.4d that supports the Japanese 950 MHz band.