The natural source of mobile telephony data is from the data centres of mobile network operators. However, there is a number of difficulties when acquiring information in this manner, most notably the legal and privacy issues that prevent operators delivering such information to outside researchers. In addition, even with best efforts, there is no guarantee that data from theses sources is always available, complete or accurate. Network operators continually optimise their network throughout the day, using temporary towers. This adds a level of uncertainty to these fixed point measurements as network topologies become more dynamic. A more fundamental issue arises regarding spatial accuracy as the spatial resolution of the usage statistics is dependent on both the operator’s network topology and base station hardware. As a result, approaches have emerged which aim to address these issues by placing either embedded software applications on the mobile devices to log data [53], or by constructing custom sensing platforms which monitor mobile devices in their vicinity [21, 22].
Using an embedded sensing application, Ahas and Mark [165] tracked the mobile phones of 300 users through a social positioning application. They combined spatio-temporal data from phones with demographic and attitudinal data from surveys to view a map of social spaces in
Estonia. MITs Reality Mining project [54, 166] also illustrated that it was possible to extract common mobility patterns from the activities of mobile phone users. The subjects were issued with mobile phones pre-installed with several pieces of software that recorded and sent research data on call logs, Bluetooth devices in proximity, cell tower IDs, application usage, and phone status. Other research efforts have examined social connection [167, 168], mobility [30, 169] and subscriber behaviour [170]. Another application to make use of embedded sensing applications is Google Mapsr real time traffic estimator. The traffic information used in this feature comes from a combination of third party sources and data provided by Android users which have chosen to share information by opting into the “My Location” feature on Google Mapsr.
However, there are issues with embedded sensing applications. Aside from potential ethical concerns, embedded sensing applications require the cooperation of the device user to install intrusive software applications onto their mobile devices that enable the logging of information. This can have a limiting effect on the number of devices which can be sensed due to lack of cooperation from users. A compatibility issue may also arise as software applications on mobile devices are often platform dependent. As a result, researchers has started to develop custom-built sensing platforms for the passive collection of mobile telephony data.
Normal mobile network operational functions, such as device paging, are initiated when mobile devices are connected to a network. All mobile device activities occur in designated frequency bands and may be passively sensed using mobile receiver technology. Examples of custom-built sensing platforms include [21, 22] and [171]. Path Intelligence [171] have devised sophisticated sensing devices to return mobile device mobility patterns for the purpose of foot fall analysis in shopping centres. Their applications gather information from scanning the mobile phone frequency ranges and localising devices based on the characteristics of the radio signals observed. Typically, information which may be collected through custom sensing platforms includes in-range device positional estimates, in-range device count estimates and mobile spectral energy.
Doyle et al.[21, 22], highlighted the capabilities of cumulative received signal strength indications (RSSI) for the measurement of overall mobile device transmissions within the proximity of custom built sensing devices. The research, carried out on the north campus of the National University of Ireland Maynooth (NUIM), evaluated these sensing devices by
undertaking two experiments. The first experiment investigated whether such sensors could detect spectral emissions from an SMS and phone call under controlled conditions. Results from this initial experiment are depicted in Figure 2.5 and Figure 2.6 respectively. The second experiment investigated each sensing device’s capability to record mobile spectrum RSSI in an uncontrolled environment. Here, normal mobile device activity was observed from mostly the student population of NUIM over two consecutive time periods. Results from this experiment are given in Figure 2.7 and Figure 2.8.
The results of the first experiment indicate that the sensing nodes were capable of detecting normal mobile phone activity as the spectral energy associated with a text message and phone call were clearly visible. The second experiment performed on a non-controlled environment highlight events occurring close to each hour mark. These events relate to times where classes finished and started, and are as expected. These preliminary findings suggest that monitoring cumulative receiver signal strength measurements of mobile phone signals can be a valuable tool in gathering information for mobile phone usage independent of mobile phone operators on localised scales. However, these results are preliminary and the parameters of the temporal processing technique used, detailed in [21, 22], require further tuning.
In the experimental setup, only an aggregated measure of spectral energy was recorded. Using more sophisticated sensors, the spectral energy of each unique device may be monitored. With this information, it is possible to obtain a more meaningful result for user occupancy. Such data could also be used to complement traditional techniques for mapping mobile device activity. For instance, one could use the network operator data, if available, to model the dynamics of a city or town, while localised RSSI data mapping could be employed to observe the dynamics of specific buildings or localised areas.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 2.5 3 Time, min RSSI, volts (a) Sensor A 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 2.5 3 Time, min RSSI, volts (b) Sensor B
Figure2.5: Observed mobile transmissions through recorded RSSI of mobile spectral energy
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 500 1000 1500 2000 2500 Time, min
Weighted Cumulative RSSI
(a) Sensor A 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 1000 2000 3000 4000 5000 6000 7000 8000 Time, min
Weighted Cumulative RSSI
(b) Sensor B
Figure2.6: Weighted RSSI, highlighting time periods of high activity.
12:42:230 12:52:23 13:02:23 13:12:23 13:22:23 13:32:23 13:42:23 13:52:23 14:02:23 14:12:23 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Time RSSI, volts (a) Sensor A 12:42:230 12:52:23 13:02:23 13:12:23 13:22:23 13:32:23 13:42:23 13:52:23 14:02:23 14:12:23 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Time RSSI, volts (b) Sensor B
Figure2.7: Normal mobile device activity from the student population of NUI Maynooth over consecutive time periods (1.5 hours).
12:42:23 12:52:23 13:02:23 13:12:23 13:22:23 13:32:23 13:42:23 13:52:23 14:02:23 14:12:230 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Time
Weighted Cumulative RSSI
(a) Sensor A 12:42:23 12:52:23 13:02:23 13:12:23 13:22:23 13:32:23 13:42:23 13:52:23 14:02:23 14:12:230 1000 2000 3000 4000 5000 6000 7000 8000 9000 Time
Weighted Cumulative RSSI
(b) Sensor B