Smart Energy Metering: The Coexistence of ZigBee and WiFi
3e-Houses is an EU-sponsored smart energy metering research project being carried out by a consortium of organisations (including IP Performance - see news item here) from Spain, Germany, The UK and Bulgaria. The aim of the project is to integrate established ICT technologies in social housing in order to give residents the ability to manage their energy consumption. Real time monitoring of energy usage is central to 3e-Houses project and is heavily dependent on wireless networking. The smart metering systems used in the 3e-Houses project incorporate devices that adopt both 802.11 (WiFi) and 802.15.4 (ZigBee) wireless standards. In this article we discuss cross technology interference (CTI) between these two technologies.
ZigBee wireless technology is based upon the IEEE 802.15.4 standard for wireless personal area networks (WPANs). ZigBee was designed to be low-cost as well as low-power. While low-power limits the communications range, it extends battery life. ZigBee is used extensively in wireless sensor network (WSN) applications where providing mains power can be problematic and devices need to rely on battery power.
ZigBee is ideal for energy smart metering systems. A typical system would consist of a number of device sensors and controllers distributed throughout a household (possibly in a mesh configuration) communicating back to a data aggregation device. Battery powered devices are obviously more flexible (than main powered devices) because they are not restricted to locations that are within a cable’s length of a mains socket.
ZigBee’s low-power communication, however, does present a potential problem. 802.15.4 can operate in one of three possible frequency bands which are shown in Table 1. As we can see, when 802.15.4 operates in the 2.4 GHz band, it has to coexist with IEEE 802.11 networks (WiFi). WiFi is a wireless LAN standard that is widely used in both commercial and domestic environments.
WiFi and ZigBee are different technologies and they do not co-operate with regard to spectrum sharing. There is, therefore, the potential for interference between ZigBee networks and neighbouring WiFi networks. This is known as cross technology interference (CTI).
CTI is a concern of the UK consortium working on the 3e-Houses project. ZigBee smart metering systems will be installed in each household and will relay data back to a central database. State-of-the-art Toshiba tablet computers feedback energy information to participating residents. The tablet computers rely (solely) on WiFi for communication. In the 3e-Houses project we will, therefore, be installing WiFi networks alongside ZigBee.
There are number research papers which investigate the coexistence of WiFi and ZigBee. The extent of the CTI problem between WiFi and ZigBee reported by the research literature varies. Sikora et. al.  report 802.15.4 packet error rates as high as 95%. On the other hand Thonet et.al.  claim “ZigBee operates satisfactorily, even in the most adverse interference conditions”. Thonet, however, does advice increasing the transmit power of 802.15.4 in order maintain satisfactory operation. The consequence of this is, it will diminish battery life.
Other research studies focus on methods to mitigate the effects of WiFi interference on ZigBee. [1, 8, 4]. We believe, this in itself is significant and suggests concerns over CTI should not be taken lightly.
WiFi operates in 2.4 GHz and 5 GHz unlicensed bands. As we are concerned with ZigBee co-existence, we confine our discussion to the 2.4 GHz band. In Europe, there are 13 802.11 channels (in the US there are only 11). The channels are 22 MHz wide with a 5 MHz separation between the centre frequencies of adjacent channels. This means that, in the entire spectrum band, there can only be three non-overlapping channels . The bandwidth of 802.15.4 channels are 2 MHz and are separated by 5 MHz. Thus, there are 16 non-overlapping ZigBee channels.
Research has shown that, in 802.11 networks, there can be interference even between devices on channels deemed to be non-overlapping . Non-overlapping channel interference is a result of low path-loss, i.e., when the devices are close together. Adjacent channel interference can be reduced/eliminated by increasing the distance between devices.
CTI between WiFi and ZigBee is unavoidable. As WiFi transmission power is 30 times greater than ZigBee, it is unlikely that ZigBee will cause undue interference to WiFi. Clearly it is ZigBee that will suffer from interference from WiFi. From our examination of the current research, we should not be unduly worried about WiFi/ZigBee CTI. Provided WiFi signals are sufficiently weak, a signal-to-interference ratio of 5-6 dB yields a high probability of a ZigBee frame being successfully received .
However, it would be advisable to implement some precautionary measures and plan both network installations accordingly. We should not underestimate the effects of adjacent channel interference as “non-overlapping” channels may not necessarily be non-overlapping if devices are close together. We need, therefore to ensure there is sufficient separation of channel frequencies and physical distances between devices. Thonet et al  outline a few rules of thumb:
- WiFi and ZigBee channels should be separated by 30 MHz. Clearly we only have control over the actual networks we are installing. It is likely that there will be other WiFi network in the vicinity. Hopefully the pass-loss will be sufficiently high that CTI is avoided. Nevertheless, some band planning may be necessary.
- WiFi and ZigBee devices should be at least 2 meters apart (e.g. we should avoid placing the ZigBee data collector on top of the WiFI access-point).
- Ideally, ZigBee devices should be less than 9 meters apart. Sometimes this may not be possible. ZigBee, however, is a mesh technology, intermediate repeaters can be used to reduce hop distances.
It will also be necessary to examine the trade-off between transmission power and battery longevity.
The rollout of the 3e-Houses project in the UK is about to begin, while a pilot scheme is currently under way in Spain. This will give us an opportunity to observe WiFi/ZigBee CTI first hand and study the effects on real-time energy monitoring.
 Minimizing 802.11 Interference on Zigbee Medical Sensors. 4th International ICST Conference on Body Area Networks, May 2009.
 3e Houses. 3e houses: Saving energy and the environment across Europe, 2011. http://www.3ehouses.eu/.
 Alan Holt and Chi-Yu Huang. 802.11 Wireless Networks, Security and Analysis. Springer, London, UK, 2010.
 Chieh-Jan Mike Liang, Nissanka Bodhi Priyantha, Jie Liu, and Andreas Terzis. Surviving wi-fi interference in low power Zigbee networks. In Proceedings of the 8th ACM Conference on Embedded Networked Sensor Systems, SenSys ’10, pages 309–322, New York, NY, USA, 2010. ACM.
 Axel Sikora and Voicu F. Groza. Compatibility of IEEE 802.15.4 (Zigbee) with IEEE 802.11 (WLAN), Bluetooth, and Microwave Ovens in 2.4 GHz, ISM-band.
 Gilles Thonet, Patrick Allard-Jacquin, and Pierre Colle. ZigBee WiFi Coexistence, 4 2008. http://www.zigbee.org/.
 Eduard Garcia Villegas, Elena Lopez-Aguilera, Rafael Vidal, and Josep Paradells. Effect of adjacent-channel interference in IEEE 802.11 wlans. 2007 2nd International Conference on Cognitive Radio Oriented Wireless Networks and Communications, pages 118–125, 2007.
 Wei Yuan, XiangyuWang, and Jean Paul M. G. Linnartz. A coexistence model of IEEE 802.15.4 and IEEE 802.11.4
by Alan | 16 September 2011
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