The results of these experiments are discussed in Chapter 6. Experiments were designed to show the variation in the received signal strength on a single channel, multiple channels and to investigate the effect of walls. Initially the effect of the environment was investigated by using a laptop (client) and a single access point (sensor). The effect of the environment and different directions on RSS was also demonstrated by deploying multiple sensors. The location experiments were conducted using a minimum of 4 sensors.
5.4.1 Single channel - RSS variation against distance
An experiment was designed to observe the effect of indoors multipath on received signal strength. The TravelMate 240 Laptop was placed on a stool (0.6m high) or a trolley (0.7m high) in one of the laboratories of the test area (figure 5-1), with a wireless adaptor card (3COM Office Connect). The AP (WRT54G) was fixed at a common point on another stool 0.8m in height. The Laptop was placed at 1m distance from AP (access point). RSS reading was recorded on a single frequency - channel 1, before it was moved to 2m distance from the sensor for next RSS reading on the same channel. Same process was repeated to record RSS values up to 10m. Readings were plotted using Matlab and are shown in figure 6-1. The set up is shown in figure 5-9
Figure 5-9 RSS of transmitted signal measured by sensor and passed back to the client for recording
Signal strength Reading (RSS) extracted using web interface utility running on WRT54G wirelessly by laptop.
5.4.2 Multi-channel RSS measurements
• Four different channels-RSS variation vs distance: The above experiment (figure 5-9) was extended to see the multipath propagation effect by taking readings on four different channels. It was intended to observe the variation level of signals spread over different channels, which were randomly chosen as 1, 4, 6 and 9. The experiment was conducted in the same room HL 15A. The environment of this room was discussed in section 5.2. The method of performing the experiment was the same as explained in section 5.4.1 repeated for 4 different channels (including channel 6) at each distance. The results are discussed in section 6.3.1.
Figure 5-10 Signal strength readings recorded for 4 different channels
• Effect of frequency selective readings on RSS measurements: The same experiment layout of figure 5-10 was used to extract RSS values at distances of 2m, 4m and 7m. At each distance, RSS values were recorded on all channels (1- 13). Channels were changed on the AP using the web interface utility as explained in section 5.3.1. The results are discussed in Chapter 6 (6.3.2)
• Effect of averaging across the channels: The experiment layout in figure 5-10 was used to collect RSS values across the channels at multiple distances. The 3COM Office connect wireless card was associated to WRT54G. At fixed distances the signal strength of the associated client was extracted using the web-based utility on WRT54G. The collected RSS readings were averaged and plotted against distance. The effect on variation in RSS values is shown and plotted in figure 6-5
Channel 1 Channel 4 Channel 6 Channel 9
5.4.3 Effect of walls on propagation
In order to develop the geolocation algorithm, it was considered an important factor to study the amount of absorption by the walls in the test area (figure 5-1). Walls pose one of the major concerns in design of indoor location algorithms. Correct identification of walls in an indoor environment can drastically improve the chances of reporting accurate location estimation.
An experiment was designed so that RSS readings collected beyond a single wall were recorded. For this a common wall between two rooms in the test area was chosen between an AP and the client as shown in figure 5-11.
Figure 5-11 Test bed area showing equipment layout for measuring wall loss
The client was placed in the old café and the sensors were placed in 15A and the EW lab. The sensor in 15A reports RSS value for the client at randomly selected distances
distances. At each distance RSS readings were collected for all channels and averaged. The results are discussed in section 6.4
5.4.4 Experiments with multiple Sensors
All experiments so far were conducted with a single sensor. This section presents the experiments extended to multiple sensors.
• Variation in RSS from different locations: An experiment was designed to compare the effect in change of reported signal strength values from three different positions around the sensor. An experiment was set up in HL 15A (figure 5-1). Three WRT54Gs (sensors) were placed at fixed points inside the lab as shown in figure 5-12. The client was associated to each sensor (AP) one by one after setting up a small private network so that the client could communicate to each of the access points placed in the room. While the client was associated to each AP (sensors), in it from the signal strength of the associated client was collected using web interface utility of WRT54G as explained in section 5.3. Signal strengths (RSS) were collected across the channel 1 to 13 for every distance and averaged.
Only one AP is ‘ON’ (powered up) at a time to collect readings. This is necessary since by default clients will always associate to the AP with the highest received signal strength. For every change of channel, the client disassociates and will connect to the next available AP with strongest signal. Results from this experiment are explained and discussed in section 6.5.
Figure 5-12 Equipment set-up to measure the effect of multipath on signal strength as received from three different directions
• Effect on RSS values collected in an alternate indoor environment: To continue towards development of a model for any indoor environment, an experiment was designed to investigate RF propagation in different environments. For this experiment the electrical laboratory was selected (electrical lab) which had a different environment as mentioned in section 5.2. The sensors were placed at fixed positions within the room (Electrical lab) as shown in the layout of figure 5-12. A small private wireless network was established and each sensor was associated in turn to obtain signal strength readings spread over randomly selected distances. Section 6.5.2 presents the results.
• RSS values with single/double walls between multiple sensors/client: The client in a practical indoor environment faced the challenge of multiple walls separating it and different sensors. The sensors were placed in different directions/ locations as shown in figure 5-13.
Figure 5-13 Layout of multiple walls between client and multiple sensors
Each sensor collected RSS value on all 13 channels which were then averaged. The results showing the absorption affect on RSS readings with single and double walls are presented in figure 6-11.