University of Sheffield Knowledge Transfer Account. KTA Development Hot House

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1 University of Sheffield Knowledge Transfer Account Development Hot House Project Name - ZigBee Wireless Quality Trials KTA Code DHH-010 Internal Department - Electronic and Electrical Engineering Internal Contacts - Prof Richard Langley ([email protected]) Dr Jon Rigelsford ([email protected]) External Company (1) - Ember ( External Contact (1) - David Egan, Product Marketing Director ([email protected]) External Company (2) - BRE ( KTA DHH Resource - Project Start Date - 16 th June 2010 Project Status - Finished Ms Kimm Sutter ([email protected]) Dr David Powell ([email protected]) Report - Reporting test results for 3 test sites Purpose - Final Report Date - 23rd May 2011 KTA Development Hot House Project: 009-DHH-010

2 Contents: Page No. 1. Introduction and background What is ZigBee? Why this project? Aims and Objectives 2 2. Methodology 2.1. Locations 3 Test site 1 3 Test Site 2 3 Test Site Testing kit Data measurements Received Signal Strength Indication (RSSI) Link Quality Indication (LQI ) Testing parameters 6 3. Results Test site Test site 1 Floor plan Test site 1 Trials Test site 1 Trial 1 Signal reference Test site 1 Trial 2 Window barrier Test site 1 Trial 3 Building reflection Test site Test site 2 Floor plan Test site 2 Trials Test site 2 Trial 1 Wall Barriers Test site 2 Trial 2 Window Barriers Test site Test site 3 Floor plan Test site 3 Trials Test site 3 Trial 1 Receiver in the kitchen Test site 3 Trial 2 Receiver in the bedroom Conclusions and recommendations References 26 KTA Development Hot House Project: 009-DHH-010

3 1. Introduction & Background This project is to assess the performance abilities of consumer band, wireless, digital network equipment (called ZigBee devices) under conditions that simulate the devices being placed in UK homes of different build styles and ages. 1.1 What is ZigBee? ZigBee is a specification for a suite of high-level, communication protocols using small, lowpower digital radios based on the IEEE [1] standard for Wireless Home Area Networks (WHANs), such as wireless light switches with lamps, electrical meters with inhome displays and consumer electronics equipment via short-range radio. The technology defined by the ZigBee specification is intended to be simpler and less expensive than other Wireless Personal Area Networks (WPANs), such as Bluetooth and WIFI. ZigBee is targeted at radio-frequency (RF) applications that require a low data rate, long battery life, and secure networking [2]. 1.2 Why This Project? The UK Government is committed to having a smart meter in every UK home by approximately The objective being to make domestic energy consumers aware of their usage and behaviours, to reduce overall energy usage and to put together a national smart electrical grid to reduce the country-wide energy consumption and to reduce the carbon footprint of the UK [3, 4]. To allow remote, two-way communication between room-based monitors, the smart meter and the energy suppliers, a suitable wireless solution is required. An obvious choice is the standard for Wireless Personal Area Networks and the low-power, low-cost ZigBee Alliance devices that are designed to operate to this standard [5]. The BRE Group (formerly The Building Research Establishment Group, now a registered charity, with a mission to support built environment research for the public benefit) and other observers have voiced concern that there is little data to describe how the insulating properties of the vast selection of building materials in use around the UK will affect the wireless signal propagation to be used by proposed SMART meters. The University of Sheffield was approached by Ember Corp, a supplier of ZigBee devices, and asked to explore and characterize how ZigBee devices, operating at their specific frequency and data rate, perform in a number of building styles and materials so that outputs can be compared and contrasted. 1

4 1.3 Aims & Objectives a) Design a test environment that will simulate a smart meter communication relay (meter device and sensor device) using ZigBee development boards, as provided by Ember Corporation [6]. b) Identify a number of buildings types in the local area that will conform to the guidelines suggested by BRE [7]. c) Perform site trials by taking the test environment kit to the buildings and take measurements at a number of locations to characterize the building. d) Draft a report that details the testing, that can form the basis of a White Paper by the ZigBee Alliance. e) Attempt a simple characterization of the transfer functions of the buildings materials based upon the results of the trials. 2

5 2. Methodology 2.1 Locations Tests were carried out in a variety of locations around west Sheffield, South Yorkshire during the winter months of Tests covered 3 building types covering a mix of multiple and standard, domestic residences. Further testing would be desirable in other types of buildings. To date, the building types tested are: Test site 1 New build, completed in External stone cladding over inner-leaf thermolite block, with plaster finish. Flat roof consisting rock asphalt or pre-cast slab. Windows are all double glazed with metal frame. Internal heating system is a drylined wet heating system (radiator with gauge in each room). The ground floor is level with the surrounding land. The building has a 40-year life span and 4 inhabited floors. Current use is as a Halls of Residence but style could simulate a modern block of flats. Test Site 1: Front door, facing northwest. Test site 2 - Built late 1980's. Brick clad, cavity walls, plaster finish. Cavity walls constructed with Duplex plasterboard board (this is plasterboard with foil backing to resist water vapour and provide added insulation. It was used in UK construction until mid-1990's). Windows are PVC-framed, double glazing. Roof constructed of slate and felt. Wet heating system centrally controlled. Four inhabited floors. Current use is as a Halls of Residence but style could simulate modern block of flats 3

6 Test Site 2: Front door, facing east-northeast Test site 3 Late-Victorian terraced house (built approx. 1890). Brick construction with brick and stud internal walls (possibly some rooms still of lathe/plaster internal walls and insulated with rubble). Slate and lining roof. Original windows replaced with PVC double glazing. Independent wet heating system with radiators in each room. Two inhabited floors plus stone-walled, uninhabited basement. Test site 3 Front door, facing east- northeast 4

7 The house lies on a steeply inclined street with neighbour buildings connected to both side walls of the house. There is a back yard that extends approx. 10m behind the house, and a paved street immediately outside the front door Comments on physical and geographical features in close location of tests sites will be added to the results section, where appropriate. To complete the list of housing options suggestion by BRE, further tests will be required on: 1930 s housing (red brick, semi-detached or detached) Modern family home (post 2000) 2.2 Testing Kit 2 * Ember Corp Zigbee modules, fitted with an Antenova dielectric SMT antenna POE (Power Over Ethernet) system to power system USB powerbrick and/or battery power (dependent on location) InSight software adapters (for programming and data capture) Notepad ++ software Assembled node for transmitting and receiving wireless signal 2.3 Data measurement Received Signal Strength Indication (RSSI) The RSSI is a measurement of the power, or radio energy, present in a received radio signal, regardless of its source. Values are unit-less with kit manufacturers expressing outputs in different ways. The Ember kit expressed the readings in power levels between 0 and -96 decibels (dbm), with -96d Bm being the lowest possible result before 5

8 communication was lost between nodes. The measurement is based on the highest energy level detected over a specific period of time, of the data packet being received. For these trials the energy is in a wireless environment, but it is worth noting that the energy received may be from any transmitter on that frequency, and not necessarily our partner node Link Quality Indication (LQI) The reported LQI value is a unit-less number ranging from that represents the link quality of the connectivity between neighbouring nodes as they communicate. The measurement is based on the reliability of a data packet being received when being sent from one node to another. The maximum value represents the best possible link quality, and conversely lower values, i.e. < 200, represent high errors rates. For example, an LQI value of 200 represents approximately 80% reliability of receiving a transmitted data packet intact. This can be summarized as shown in Table 1 below. It is possible to achieve 99%+ packet success rate with more than 4 chip errors per byte, but once chip errors occur, you are very close to the point of losing connectivity. Chip errors per byte LQI Table 1: Link Quality Indication (LQI) vs Chip Errors per byte. 2.4 Testing parameters All reported results are for the application-level test software (including TCP-style retransmission upon failure), not the raw packet range testing. The objective of the propagation trials was to measure any change in signal strength as the distance and barriers between 2 boards (a transmitter and a receiver) increased, noting influence of physical features such as: Number walls/ceiling barriers between the boards Whether windows were open or closed Reflection off neighbouring buildings Within the report, the Zigbee modules are referred to as: Transmitter board = Reference node (fixed location during trials) Receiver board = Sensor node (mobile during trials) 6

9 Test sites 1 and 2 were not inhabited during our trials so rooms were free of people, electronic gadgets and domestic appliances. During tests, the team also vacated the rooms during link-up, to avoid the influence of water-rich human bodies on the readings. Test site 3 was a fully furnished, inhabited property but the team left the rooms between testing. As each test site had its own physical make up, not all trials have produced 3 sets of results to compare. Each site should be viewed independently, not necessarily as a comparison with the other 2 sites. 3. Results 3.1. Test Site Test Site 1 Floor plan The diagram below shows the floor plan for Test Site 1, including locations of all trials. 7

10 Test Site 1: Floor Plan, showing trial locations Test Site 1 Trials Three trials were carried out on Test Site 1. The trials were designed to extract data with respect to the following factors: Trial 1 Signal Reference - The propagation of the signal along an uninterrupted corridor, carried out in internal, Corridor 1, west side of building; Trial 2 Barriers - The effect of internal walls, floors and windows (both open and closed in the rooms containing the test equipment) on signal, carried out in Corridor 1, west side of building; Trial 3 Building reflection Exploring the effect of external masses on the signal propagation. Carried out in Corridor 2, northeast side of building, this trial featured windows open and mirrored the locations of the tests carried out in Trial Trial 1 Signal Reference This trial was to measure the propagation of the signal from the reference board node (located at point L10, to be coincident with the straight line distances in the other trials) along a corridor, with no barriers between the nodes. The sensor node was moved along 5 points (L11, L12, L15, L18, L19), gradually moving further away from the reference node. At each position a minimum of 6 application layer measurements were taken and averaged to provide a single result for each location. The average RSSI reading is given on the floor plan, plotted on a graph and tabulated below. The signal strength (RSSI) generally drops away as the sensor is moved further from the reference node. Point Dist. (mm) RSSI (min) RSSI (avg) RSSI (max) LQI L L L L L Test Site 1: Trial 1 Signal Reference - Data Table The values of the trial are shown in the data table above for the minimum, maximum, and averaged results of the RSSI readings. 8

11 The LQI column shows the link quality reading at each point. Test Site 1: Trial 1 - Reference RSSI (avg) Distance (mm) Test Site 1: Trial 1 - Reference The data plot above shows the values for the average measured RSSI in each one of the locations in the corridor. There are no internal walls between the boards in this test, so it can be considered the reference result for the other data to measure the effects of the walls (assuming a very simple model) Trial 2 Barriers (in Corridor 1, west side of the building) 9

12 The second trial explored the propagation of the signal with respect to the internals walls and floors between the rooms as the mobile node was moved to various locations in adjoining rooms (See locations L01 - L09 on Test site floor plan), gradually being moved horizontally and vertically further away from fixed location L00, as annotated above on Trial 2 detailed floor plan. Testing was repeated at locations L01-L09 on the next two floors up (called First and Second floor, using UK naming protocol). The layout of the rooms on all levels is identical to the ground floor so, for example First Floor location L01 is directly above Ground Floor location L01, etc. Using Ground Floor data from Trial 1 and Trial 2 we can observe the effects that the internal walls have on the propagation of the signal. Tests were also repeated with all windows in rooms housing the nodes, open instead of closed. For all test cases, at each position a minimum of 6 application layer measurements were taken and averaged to provide a single result for each location. The data tables below show results including: the minimum, maximum and average gain (RSSI), the link quality numbers at each location (LQI) and the distance between nodes at point of communication. Point Distance(mm) RSSI (min) RSSI (avg) RSSI (max) LQI (GROUND FLOOR) L L L L L L L L L (FIRST FLOOR) L L L L (SECOND FLOOR) L L (THIRD FLOOR) L L Test Site 1: Trial 2 Barriers (Windows Closed) Data Table 10

13 Point Distance(mm) RSSI (min) RSSI (avg) RSSI (max) LQI (SAME FLOOR) L L L L L (FIRST FLOOR) L L L L (SECOND FLOOR) L L (THIRD FLOOR) L L Test Site 1: Trial 2 Barriers (Windows Open) Data Table The data plot below shows the results for the Barriers trial for both windows closed and windows open, for a direct comparison. The result is difficult to interpret. At the locations closer to the L0 reference node the open windows facilitated a stronger signal, but the locations furthest from the reference node (L8 and L9) the open windows produced a weaker signal than when the windows were closed. Test Site 1: Trial 2 - Corridor 1 RSSI (avg) Distance (mm) Ground Floor - Windows Closed Ground Floor - Windows Open Second Floor - Windows Open First Floor - Windows Closed First Floor - Windows Open Test Site 1: Trial 2 Barriers Data Plot 11

14 The land outside of the windows is a flat, concrete car park, sparsely populated on the day of testing. More details of the surrounding land can be found in chapter Trial 3, below Trial 3 Building reflection (in Corridor 2, northeast side of the building) This trial is to explore if reflection of signals off external features can affect propagation results when windows are open during trials. In this case there is a neighbouring building to the northeast of Test site 1 as well as a dustbin storage site in between the two buildings. Please see image below: Surrounding areas to Test Site 1: Test site 1 Neighbouring building Dustbin storage Front door as shown in photo in Ch2 Test site 1 is located on flat, open ground, with car parks located to the south and southwest. Highlighted on the areal image is a communal dustbin store and the neighbouring building is located 10 metres to the northeast of the test site. Carried out in Corridor 2, northeast side of building, this trial featured windows open and mirrored the locations of the tests carried out in Trial 2 in Corridor 1, e.g. Ref point L00 in Corridor 1 is mirrored on to Ref point L20 in Corridor 2, etc. 12

15 N.B. The floor plan above shows the open window results from Trial 2, taken in Corridor 1. The numbers in parenthesis are the results from the mirror image locations in Corridor 2 but the schematic for Corridor has not been replicate. Locations for Trial 3 are annotated on the main floor plan as locations L20 L29. Trial 3 Building reflection was performed in Corridor 2, on the northeast side of the test site, to mirror the test points closest to the external wall of Trial 2, to the west. Trial 3 used a fixed reference node at L20 (mirroring the location of L00) and the sensor node was moved from location L21 to L29 as data was gathered. Windows were open for all tests and readings were taken up to the 2 nd floor. The logged numbers for Trial 3 Building Reflection are shown in the data table below along with the link quality figure. 13

16 RSSI (min) RSSI (avg) RSSI (max) Point Distance(mm) LQI (GROUND FLOOR) L L L L L (FIRST FLOOR) L L L L (SECOND FLOOR) L L (THIRD FLOOR) L L Test Site 1: Trial 3 Building reflection (Windows Open) Data Table The data plot below shows the results of Trial 3 carried out in Corridor 2 and compared against measurements taken in Corridor 1. This test was done to see if the presence of the second building outside Corridor 2 would have an effect on the results in comparison to the open land outside Corridor 1. Test Site 1: Trial 3 - Corridor (1) and Corridor (2) RSSI (avg) Distance (mm) Ground Floor - Windows Open (1) First Floor - Windows Open (1) Ground Floor - Windows Open (2) First Floor - Windows Open (2) Test Site 1: Trial 3 West side (Corridor 1) against Northeast side (Corridor 2) - Data Plot 14

17 The results are inconclusive. Results for the first floor are improved in all locations, however the results for ground floor locations show a drop in gain for the closer nodes (L2 and L5) but a +15dB result at L Test Site Test Site 2 Floor plan The diagram below shows the floor plan for Test Site 2, including test locations for all trials. Test Site 2: Floor Plan, showing test locations Test Site 2 - Trials Two trials were carried out on Test Site 2. The trials were designed to extract data with respect to the following factors: Trial 1 Wall Barriers - The propagation of the signal with respect to the internals walls and floors between rooms; Trial 2 Window Barriers - The effects on signal of having the windows open during the trials Test site 2 Trial 1 Wall Barriers Test Site 2 Trial 1 Wall Barriers was designed to test the propagation of the signal from the mobile sensor node (moved through locations L1 - L13) with respect to the reference node fixed at location L0. Testing covered ground floor, first floor and second floors of the building and all windows and doors were all closed throughout the trial. 15

18 Test Site 2: Trial 1 Wall Barriers - Results on floor plan The data table below shows the trial results including the minimum, maximum and average gain (RSSI) and the link quality numbers at each location (LQI) as well as the distance between nodes. Point Distance (mm) RSSI (min) RSSI (avg) RSSI (max) LQI (GROUND FLOOR) L L L L L L L L L L L L

19 Point Distance (mm) RSSI (min) RSSI (avg) RSSI (max) LQI (FIRST FLOOR) L L L L L L L L L (SECOND FLOOR) L L L L L Test Site 2: Trial 1 Wall Barriers Data Table It is important to note that the link quality fell significantly from the maximum value (255) for the communication between the L0 device and the mobile device at point L6 for both the ground floor and first floor measurements. This is probably due to the geometry of the building, whereby the direct line of sight between the two test locations is obscured by the outside wall. Communication was not possible with point L7 on the first floor, but it is included for completeness. Any points not mentioned were unable to communicate. Test Site 2: Trial 1 - Windows Closed RSSI (avg) Distance (mm) Ground Floor First Floor Second Floor Test Site 2: Trial 1 Wall Barriers Data Plot 17

20 We have found it difficult to read anything conclusive from the graph of the signal strength plotted against distance between modes. Further work could be done to generate radar graphs depicting signal strength over distance Test site 2 Trial 2 Window Barriers Test Site 2: Trial 2 Windows Barriers Results on floor plan Trial 2 was designed to test the effects of opening the windows in the rooms containing the sensor nodes as they were moved around points L1 through L13 with respect to the reference node fixed at location L0. The data tables below show the results including the minimum, maximum and average gain (RSSI) and the link quality numbers at each location (LQI). Note that the link quality has fallen significantly from the maximum value (255) for the communication between L0 and L9 on the ground floor, L0 and L4 on the first floor, and L0 and L3/L4 on the second floor. No reason could be identified for lack of communication with point L4 on the ground floor this was trialled three times without success. Points L5 (first floor) and L2 (second floor) were also unable to communicate. 18

21 Point Distance (mm) RSSI (min) RSSI (avg) RSSI (max) LQI (GROUND FLOOR) L L L L L L L L L L L (FIRST FLOOR) L L L L L L L (SECOND FLOOR) L L L L L Surrounding areas to Test Site 2: Test Site 2: Trial 2 Window Barrier Data Table Trial Site 2 East facing corridor running north to south. Low hillock close to test locations L8-L13 Test Site 2 lies on flat ground at the front (due west), which has been converted into a tarmac car park. The closest building lies 12 meters, directly to the west. The testing corridor for Trials 1 and 2 runs approximately north to south, on the east side of the block. To the east of the block is a low hillock, in the green space to the rear of the building. This may produce reflections which could impact the results taken on the ground floor. 19

22 Test Site 2: Trial 2 - Windows Open RSSI (avg) Distance (mm) Ground Floor First Floor Second Floor Test Site 2: Trial 2 Window Barrier Data Plot 3.3. Test Site Test Site 3 Floor plan Test Site 3: Floor Plan showing Trial 1 node locations 20

23 Test Site 3 Trials Test Site 3: Floor Plan showing Trial 2 node locations Two trials were planned for Test Site 3. The trials were designed to extract data simulating communication between a hypothetical, hand-held wireless monitoring device (as proposed by the government for energy monitoring) at various devices located around a typical house and an electric metering point, repeating those preformed at the other test sites. As with previous trials, results would capture the propagation of the signal with respect to the internals walls between the rooms. Trial 1 Receiver in the Kitchen, located on Ground Floor (typical location) Trial 2 Receiver in Bedroom, located on First Floor (furthest location from the existing gas and electricity meters at L3) Ideally, the fixed reference node would have been located in the basement next to the existing gas and electricity meters at L3 but due to power availability and temperature, this was not possible. Due to reciprocity this will not affect the validity of the data Test Site 3 Trial 1 Receiver in the kitchen The first trial at Test Site 3 was designed to test the propagation of the signal from the mobile sensor node (that was moved to points L1 through L7) with respect to the reference node fixed at location L0 in the kitchen. Location L1 was used to simulate a device that would take a doorstep meter reading. 21

24 Location L3 simulates a node at the electricity meter, in the basement, talking to the energy monitor device in the kitchen. Other locations simulate devices that may be in a standard domestic property. The first trial simulated devices in the basement, ground floor and first floor of the house. Note that the external windows and doors were all closed throughout the trial. Test Site 3: Trial 1 Receiver in Kitchen Results on floor plan The data tables below show the numbers measured directly including the minimum, maximum and average gain (RSSI) and the link quality numbers at each location (LQI). It is important to note that the quality figure has fallen significantly from the maximum value (255) for the communication between the device in the kitchen and the device outside the front door (L1). Point Distance(mm) RSSI (min) RSSI (avg) RSSI (max) LQI (GROUND FLOOR) L L L (BASEMENT) L (FIRST FLOOR) L L L Test Site 3: Trial 1 Receiver in Kitchen Data Table 22

25 The data plot below shows the results of the first trial. Test Site 3: Trial 1 - Ground Floor Receiver RSSI (avg) Distance (mm) Ground Floor Basement First Floor Test Site 3: Trial 1 Receiver in Kitchen Data Plot Test Site 3 Trial 2 Receiver in the bedroom The second trial was designed to test the propagation of the signal from the bedroom to the basement and the location outside the front door. Location L3 was used to simulate a meter in the basement. Location L1 was used to simulate a device that would take a doorstep meter reading from the setup. Note that the reference node was moved to Bedroom 2, and the new reference location is named L10. 23

26 Test Site 3: Trial 2 Receiver in the Bedroom Results on floor plan The data tables below show the results including the minimum, maximum and average gain (RSSI), the link quality numbers at each location (LQI) and the distance between nodes. There appears to be a good link quality measurement between all nodes in the building now. L10 communicates with maximum link quality between the node in the basement (L3). Point Distance(mm) RSSI (min) RSSI (avg) RSSI (max) LQI (FIRST FLOOR) L (GROUND FLOOR) L L L (BASEMENT) L Test Site 3: Trial 2 Receiver in Bedroom Data Table 24

27 Test Site 3: Trial 2 - First Floor Receiver RSSI (avg) Distance (mm) Ground Floor Basement First Floor Test Site 3: Trial 2 Receiver in the Bedroom Data Plot 25

28 4. Conclusions and recommendations This preliminary study aimed to explore and characterize how ZigBee devices developed by Ember Corp for smart metering applications perform in a variety of building styles. These devices allow remote, two-way communication between room-based monitors and the smart meter. Results have shown that the wireless signals will penetrate several adjacent walls within a building and through at least one floor, indicating that problems are unlikely to be encountered in most domestic properties. The results are presented as representative to those found in real buildings and incorporate the complex multipath of the measurement environment. No attempt has been made to optimise the signal strength due to changing the antenna orientation as the authors believe that this is highly improbable in real deployment as smart metering customers will put their displays where it suits them, not necessarily at the optimum location! Careful radio and infrastructure planning will be required for blocks of flats, where wireless repeaters or cabled solutions may be preferential if the point-to-point range is too great. For installations where Zigbee based devices are used, other nodes could be utilised to create a mesh network and hence act as repeaters. Future work will address 1930s and modern housing types (post 2000) and methods for simulating wireless propagation in the built environment. A simple characterization of the transfer functions of the buildings materials encountered has not been attempted. References [1] IEEE Working Group for Wireless Personal Area Networks, IEEE WPAN Task Group 4, [2] Wikipedia, the free encyclopedia, Wikipedia entry for ZigBee, [3] Guardian News, Smart energy meters in every UK home by 2020, [4] Department of Energy and Climate Change, Smart electricity and gas meters,, [5] Zigbee Alliance, [6] Ember Corporation, [7] BRE, Design parameters: 26