Date: 08/09/2011

920MHz frequency band wireless multi-hop communication sys for smart grid app

OKI has developed wireless multi-hop communication system for the new 920 MHz frequency for use in homes to connect smart meters.

"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," says Takeshi Kamijoh, General Manager of Research and Development Center at OKI. "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."

OKI said the testing has confirmed that communication is possible over 10km when transmission output of 250mW, the specified maximum. The testing also confirmed direct communications in urban locations among many obstacles such as buildings at a transmission output of 70mW within an area of approximately 150m in radius.

OKI also said it will draft plans for the ZigBee standards that support 920 MHz for smart houses to familiarize ZigBee standards which is recognized internationally, and will also continue working to strengthen interconnectivity for home appliances, electrical power equipment, and home gateways.

Key features of the 920 MHz band wireless multi-hop communication system developed by OKI:

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. 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 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.

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.

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.