BY DAVID EGAN, SILICON LABORATORIES
The U.K. government requires energy suppliers to take all reasonable steps to deploy smart meters in customer residences in Great Britain by 2019, with the mass rollout to start in 2014. ZigBee Smart Energy is a solution to be used in smart metering home-area networks (HANs), which will include electric meters, gas meters and in-home displays. In addition to the maturity and popularity of the ZigBee Smart Energy application profile, ZigBee's essential strength is its self-healing mesh networking capability, which ensures robust communications.
In its second Smart Metering Equipment Technical Specification (SMETS2) published for consultation Aug. 13, 2012, the U.K.'s Department of Energy and Climate Change (DECC) proposed a combination of ZigBee Smart Energy Profile and Device Language Message Specification (DLMS) commands and data between devices in a smart metering HAN, permitting solutions based on 2.4 gigahertz radio and 868 megahertz radio, acknowledging that 2.4 gigahertz (IEEE 802.15.4) solutions are available as part of the ZigBee standard and supported by many manufacturers of consumer equipment and metering equipment. According to SMETS2, DECC's HAN radio frequency propagation trial suggests that 2.4 gigahertz radio likely will achieve some 70 percent coverage of homes in Great Britain without using additional equipment, effectively using ZigBee Smart Energy in a point-to-point or star-type network and not using the mesh networking capabilities of ZigBee. The reason is primarily that gas meters in particular cannot rely upon router and repeater equipment that is powered and controlled by consumers, therefore prone to power failures or tampering.
Many of these early 2.4 gigahertz-based HAN installations will not contain enough mains-powered devices to create a mesh with alternate paths. In many cases, an in-home display or gas meter will need to be able to communicate directly with an electric meter or communications hub. Additional routers would ensure robust communications, but they would add to support and hardware costs and be prone to power loss and tampering. This challenge highlights a need to examine the point-to-point propagation properties of ZigBee's operating in unlicensed radio bands in the 2.4 gigahertz spectrum. Some developers who have had poor experiences with propagation of other 2.4 gigahertz solutions might fear that ZigBee will be challenged similarly in home applications, resulting in communication failures and negatively impacting the consumer experience.
Widespread use of ZigBee Smart Energy in the U.S. smart metering program is little comfort because U.S. regulations allow devices to transmit at much higher power levels (100 mW/+20 dBm) than in the U.K. and Europe (10 mW/+10 dBm). It also could be argued that U.K. housing stock uses different building materials. Yet, numerous ZigBee products already are in use in Europe, mainly in home automation, smart lighting and automatic meter reading markets, and major European manufacturers have been satisfied with the propagation performance of ZigBee. Published test data, however, has not always been generally available to inform the market.
With the increased interest in smart metering in the U.K. tempered by the lack of available published material, Silicon Labs has experience with organizations on ZigBee propagation tests. Ember ZigBee technology, now a Silicon Labs offering resulting from its acquisition of Ember Corp., has been used extensively for these tests, including the Silicon Labs' Ember ZigBee EM357 system-on-chip (SoC), as well as technical information and advice on radio frequency and ZigBee matters.
Silicon Labs' Ember ZigBee SoC is particularly suitable for this type of testing for two reasons:
It can support transmission power of +8 dBm and receive sensitivity of -102 dBm, a dynamic link budget of 110 dBm and transmitting close to the EU legal limit of +10 dBm without the expense and power consumption of an external power amplifier (PA) or low-noise amplifier (LNA).
- Silicon Labs' Ember ZigBee EM357 SoC is a popular solution used in many ZigBee Smart Energy HAN devices in the U.S. and U.K., so it is directly relevant to the market.
- The following two examples show how Silicon Labs' Ember ZigBee technology has supported this type of testing at the University of Sheffield and EDF Energy.
University of Sheffield Tests
The University of Sheffield's Communications Group within the Department of Electronic and Electrical Engineering has expertise in radio propagation modeling, active and passive frequency selective surfaces (FSS), antenna systems and wireless systems design. This group has conducted ZigBee propagation tests using Silicon Labs' EM357 SoC. It published a report that details controlled, scientific tests carried out in reference building types including a terraced house with basement and apartment blocks.
The tests incorporate typical positions of meters and in-home displays. They include a fixed device, usually positioned where a meter would be located, such as a basement or at the front door. A second device, which is located at different positions around the building, sends messages to the fixed device. The signals were measured recording both RSSI (Received Signal Strength Indicator, in dBm) and LQI (Link Quality Indicator, a 0-255 value).
Figure 1 from University of Sheffield shows results from one of the apartment blocks tested.
The results show the reference device (fixed-node) in one corner of the building on the ground floor and RSSI readings from positions on ground floor, first floor and second floor.
The RSSI values shed light on how the signal weakens as it goes through internal walls, as well as free space. It also indicates the importance of good choice of radio and good printed circuit board design. For example, a test using radios capable of transmitting at +5 dBm with -95 dBm receive sensitivity would represent a 10 dB difference in dynamic link budget and would have negatively impacted these results.
The LQI values also are interesting. Values of 255 indicate a good-quality link; lesser values indicate chip errors in the received packets. An LQI of 0 still can result in a successfully received packet but indicates it would take little to change in the environment to result in a lost signal.
EDF Energy Site Survey
Silicon Labs' Ember ZigBee technology was used by EDF Energy in testing ZigBee propagation as part of a site survey for a smart metering installation in an apartment block. Silicon Labs' Ember ZigBee EM357 SoC was used in this site survey, which subsequently was published by EDF Energy into the SMDG HAN Workgroup working within the Smart Metering Implementation Programme in the U.K.
Figure 2 shows a similar pattern to the University of Sheffield tests. In this case, the tests are being carried out from a fixed node inside a meter cabinet in the center of one apartment block floor connected to different points in each of six apartments on that same floor.
With growing interest in using ZigBee for smart metering HANs in the U.K. and elsewhere around the world, testing propagation properties of these devices at 2.4 gigahertz is important to advise technology selection, architectural design and deployment plans. Independently conducted tests are becoming available to advise this process, and these tests can encourage greater understanding of ZigBee propagation in the smart metering community.
These test results, as well as the testing commissioned by DECC, suggest that ZigBee at 2.4 gigahertz can be a satisfactory solution for smart metering HANs in most U.K. homes, even when the considerable power of ZigBee's mesh networking cannot be used, especially if best-in-class ZigBee radios and robust radio frequency implementations are used.
David Egan is a senior product marketing manager for Silicon Labs' Wireless Embedded Systems business, focusing on the company's Ember ZigBee solutions. He has more than 20 years of experience in product development, support and operations. He also has played a key role representing the ZigBee Alliance in Europe and since 2006 has worked to introduce and support ZigBee within the U.K. government's smart metering program. Egan has bachelors' degrees in computer applications from Dublin City University and technology management from ITT Dublin, as well as a master's degree in strategic information technology management from the University of Derby.