WITH THE great boom of mobile-phone (MP) services,

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005 2235 Microwave System for the Detection and Localization of Mobile Phones in Large Buildings Premysl Hudec, Milan Polivka, Member, IEEE, and Pavel Pechac, Senior Member, IEEE Abstract A novel monitoring system for the detection and localization of mobile (cellular) phones in large buildings based on a matrix of microwave sensors has been designed and practically employed. The system enables detection of all active mobile services and ensures very high immunity against all other radio signals. The designed antennas, system structure, and signal processing provide high probability of correct localization of the transmitting mobile phone. Thus far, microwave detection systems with several thousands of sensors have been installed and successfully operated in several prisons in the Czech Republic. The system has been found to be an efficient tool for improving security in highly guarded areas. Index Terms Localization, microwave sensor (MS), mobile phone (MP), signal detection. I. INTRODUCTION WITH THE great boom of mobile-phone (MP) services, problems have arisen from the fact that, in certain areas and buildings, the usage of MPs is undesired or even illegal. Therefore, it is necessary to find tools that are able to detect and localize active MPs, i.e., to determine the presence of an active MP in the vicinity of a special microwave sensor (MS). An active MP generally acts as a radio transmitter that transmits the RF power with a definite frequency, definite modulation, and definite time frames. These features can be used for MP detection, identification, and localization. Specific requirements for design of an MP detection and localization system (DLS) were presented by the local Administration of Prisons, Czech Republic. Their activity was accelerated by escape attempts organized with the help of many illegally used MPs. II. TECHNICAL REQUIREMENTS, BASIC CONCEPTS The DLS must detect all mobile services active in the given region (see Table I). For the system, it is very important to detect only MPs and not to react to any other radio signals. The final objective of the DLS Manuscript received October 3, 2004; revised March 8, 2005. This work was supported in part by the Ministry of Education, Youth, and Sports of the Czech Republic under the Research in the Area of the Prospective Information and Navigation Technologies Research Program MSM 6840770014 and the Research Methods and Systems for Measurement of Physical Quantities and Measured Data Processing Research Program MSM 6840770015, and by the Grant Agency of the Czech Republic under Multiband Planar Antennas with Compact-Shaped Radiators Grant 102/04/P131. The authors are with the Faculty of Electrical Engineering, Department of Electromagnetic Field, Czech Technical University Prague, 166 27 Prague 6, Czech Republic (e-mail: hudecp@feld.cvut.cz; polivka@feld.cvut.cz; pechac@feld.cvut.cz). Digital Object Identifier 10.1109/TMTT.2005.848750 TABLE I MOBILE SERVICES DETECTED BY THE DESIGNED MS. *) SERVICE BEING PREPARED; SENSOR BEING TESTED operation should be to detect illegally transmitting MPs and to assist in finding and deactivating them. This can be aided by as precise as possible localization of the illegal MPs. The required DLS must be able to detect very short MP transmissions, e.g., when switching on the MP or sending a short message service (SMS). The DLS should be able to detect active MPs even under intentionally worsened conditions MP shaded by a person s head, horizontal antenna polarization, lower output power, etc., MSs must not be installed inside cells. Basic information on the designed detection and localization sensor and system were reported in [1]. For the design and construction of an MS of this kind, two basic technical solutions can be used. The first is based on the spectrum analyzer concept [2]. This type of the MS has a wide dynamic range and it is able to exactly determine the type of the detected mobile service (according to a precisely measured received frequency). It also rejects influences of all other radio services very efficiently. However, for the detection of MPs, it also has several substantial disadvantages. Scanning monitored frequency bands is a successive process and, when using small frequency steps, it cannot, in some cases, be fast enough. When scanning a definite frequency band, the sensor can miss a short MP transmission (e.g., a short SMS) in another frequency band. Beside that, the GSM and DCS services employ a frequency-hopping technique. The radio link frequency changes with the definite frequency jumps. In this case, the spectrum analyzer can lose captured MP power and has to start to search for it again. The second technical solution applicable for MP detection is based on an employment of wide-band RF detectors [3]. The wide-band RF detector is a transducer that converts input RF power into an output dc voltage (if the RF power is time constant) or low-frequency voltage (if the RF power is time dependent). It is able to detect immediately any incident RF power at any RF in its active frequency band. For MP monitoring and detection, it also exhibits some disadvantages. The RF wide-band detector is unable to measure the received frequency and, therefore, to determine the type of the received mobile service. The detector is unable to differentiate 0018-9480/$20.00 2005 IEEE

2236 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005 Fig. 2. Block diagram of the UMTS channel. Fig. 1. Block diagram of the realized MS. the transmission of an MP from any other radio service with a comparable RF power. Beside that, their noise floor can be too high to detect specific mobile services. III. MS CIRCUITS The designed MS is a special multiband RF receiver. Its structure combines the advantages of the spectrum analyzer concept (distinction of detected services, immunity against other radio services) with the advantages of wide-band RF detectors (parallel reception of all frequencies, immediate response) (see Fig. 1). The sensor consists of three RF channels (two active, the third one the Universal Mobile Telecommunication System (UMTS) channel, is being tested) that cover all required frequency bands. The post-detection signal analysis is applied in order to differentiate individual mobile services and to suppress all other nearby radio signals. RF Channel 1 detects only the Nordic Mobile Telephony (NMT) service, its antenna and filter operate in the 450 460-MHz frequency band, all other frequencies from 0 to 3 GHz are suppressed. The service identification is enhanced by a time-domain signal analysis focused on the dc component of the RF receiver output signal. RF Channel 2 monitors the global system for mobile communications (GSM), digital cellular system (DCS), and digital European cordless telecommunications (DECT) services, its dual-band antenna and filter operate in 880 925- and 1710 1900-MHz frequency bands. These mobile services are detected and identified by means of a complex time-domain analysis. RF Channel 3 intended to monitor the UMTS 1920 1980-MHz uplink. It is based on a very sensitive RF receiver and frequency-domain signal analysis. Output power transmitted by a UMTS MP will be typically very low (from 20 to 40 dbm). Therefore, the UMTS receiver must be implemented as a spectrum analyzer (see Fig. 2). The input low-noise amplifier (LNA) together with the IF filter typically ensure 104-dBm noise floor. This channel is controlled by its own microcomputer. With the help of the included D/A converter and voltage-controlled oscillator (VCO), the receiver scans the required 1920 1980-MHz frequency band and searches for nearby active MPs. Received signals are converted to IF, amplified, and logarithmically detected. DC voltage Fig. 3. Sensor board with 455- and 900/1800-MHz quarter-wavelength antennas (above the main board) and RF receiver and microcomputer boards (below the board). corresponding to measured RF power is processed by an A/D converter, and further analyzed. The UMTS channel is designed as a separate screened box that will be connected to the main board of all existing MSs after activating the UMTS service. Each MS consists of the antenna board, RF-receiver board, and microcomputer board (see Fig. 3). The included microcomputer performs A/D conversion, channel switching, basic signal analysis, and range/threshold setting and ensures connection of the sensor to the RS-485 bus. Each sensor board is mounted in an opened metallic housing and installed on corridor walls outside monitored cells. IV. MS ANTENNAS The MS antennas were designed with respect to specific properties of wave propagation in the given indoor environment (especially multiple reflections and standing waves). Three separate antennas cover all four frequency bands (i.e., NMT, GSM, DCS, and UMTS). They should have as low as possible cross-polarization ratio, suitable (directive) radiating patterns, as high as possible protection against mechanical attacks, and low sensitivity to detuning caused by the presence of a wall as a dielectric superstrate layer in an aperture of the housing. Designed MS antennas use a narrow patch-like structure with a relatively high (15 mm) air substrate [3] [5]. This increases impedance bandwidth, and long vertical shorting pins substantially increase the antennas ability to receive cross-polarized signals (see Fig. 4). The NMT antenna operates in the 455-MHz band, and the dual-band GSM/DCS antenna in 900/1800-MHz bands. Their performance is similar to that of F-type antennas. Antenna feeders are formed by coaxial probes; shorting walls were reduced to two shorting pins. The dual-band GSM/DCS antennas employs a single common feeding point. The final UMTS antenna type has not been chosen yet.

HUDEC et al.: MICROWAVE SYSTEM FOR THE DETECTION AND LOCALIZATION OF MPs IN LARGE BUILDINGS 2237 Fig. 4. Measured antenna radiation patterns of dual-band 900/1800-MHz patch antenna in housing: (a) and (b) without and (c) and (d) with 100-mm brick cover. All antennas were designed, analyzed, and measured, taking into account influences of nearby walls. Radiators are buried in the housing; the chosen distance between them and any wall contributes to a lower sensitivity of antenna parameters with respect to the wall s properties. The measured reflection coefficient of the GSM/DCS antenna without and with a 100-mm brick dielectric layer in its aperture was presented in [1]. Fig. 4(a) (d) shows measured radiation diagrams of the GSM/DCS antenna, again, without and with the same 100-mm-thick dielectric wall. It can be seen that, in both cases, the antenna has suitable radiating patterns, especially in the most important 180 240 range ( -plane). This angle range covers the best part of each monitored cell. In the majority of angle ranges, cross-polar components are close enough to co-polar components, especially in case when dielectric brick cover is used [see Fig. 4(c)]. The MP detection is to a high degree independent of the MP position in the monitored cell. V. PRACTICAL DLS IMPLEMENTATION A typical structure of the DLS installed in a large prison building can be seen in Fig. 5. Each monitored room (cell) is covered by one MS (see Fig. 6). Approximately 20 sensors are connected to a segment unit (SU), which ensures remote powering of all sensors, basic data processing, and galvanic separation of the segment RS-485 bus and the main RS-485 bus, which connects all SUs to the master computer.

2238 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 6, JUNE 2005 Fig. 5. Scheme diagram of the DLS. System consists of MS matrix, SUs, and master computer. Fig. 7. Outline of testing scenario floorplan with periodical structure of cells with a typical probabilities of correct MP localization (in percentage). in the power balance. There are several phenomena influencing the propagation loss, which are discussed below. A. Wall Attenuation of the I-Link Quite high values can be expected since heavy walls are usually used for cells. In [6], a floor loss factor of 6.9 db is used in the MultiWall propagation model at 900 MHz. Different values of wall attenuation can be found in the literature for different wall types and frequency bands [7]. Fig. 6. Part of installed MS matrix. VI. MP LOCALIZATION As has already been mentioned, localization of any active illegal MP helps in finding and deactivating it, which is the main task of the DLS. In the case of an MP transmission, at the same instant, the system reads values of the received RF powers at all activated sensors. The most probable position of the transmitting MP can be defined as a cell with the highest RF power detected. However, due to the behavior of radio waves propagating in such a complicated indoor structure, in some cases, this can be wrong. A simple radio-wave propagation study and extensive measurements were accomplished in order to assess the extent of this problem. A floor plan with a periodical structure of cells was used as the testing scenario. Fig. 7 shows one typical cell (dimensions 4.5 3.0 m) with two identical neighboring cells. All are equipped with antennas ( ) installed on a corridor wall above each entrance. A source of radiation the MP to be detected is marked as. depicts the RF power measured by the correct sensor. represents RF power measured by a sensor in a neighboring cell. If the RF power measured in the cell with an active MP is lower than the RF power measured in any other cell, then the process of MP localization is wrong. It means that the false localization in the testing case occurs when db). The same radiated power and receiver sensitivity can be considered for both the wanted (C link) and unwanted (I link) paths so that the ratio is controlled by the signal propagation loss B. Multipath Propagation Due to Reflections and Scattering on Walls, Furniture, etc. The phenomena often qualified as fast fading can be modeled by classical ray tracing or a ray-launching technique [7] causing deep fades in approximately a half-wavelength separation distance. In reality, the fades are not so deep thanks to a much more complicated environment than can be comprehended through the model. C. Mobile Antenna Directivity The sensor antenna can be treated as hemispherical in our case (worst case, influence of walls) so that the same gain is considered for both the C-link and I-link. The key role is played by the transmitting antenna of the MP. Based on the orientation of the MP antenna and the user s body, antenna type, and losses in the human body, the total gain may vary for different directions quite significantly [9]. D. Intentional Shadowing of the C-Link The intentional shadowing of the C-link an be done by the MP user to avoid detection. The used propagation model is based on a semiempirical COST231 MultiWall model [6] with additional loss factors to consider the above-mentioned phenomena influencing the total loss. The (in decibels) can be expressed as (1) where the free-space loss (FSL) is given for a distance between the phone and sensor antennas, is the wall attenuation, is a loss factor introduced due to multipath propagation, and

HUDEC et al.: MICROWAVE SYSTEM FOR THE DETECTION AND LOCALIZATION OF MPs IN LARGE BUILDINGS 2239 it helped to reduce illegal usage of MPs in monitored prisons to a minimum. It is applicable in any other similar large buildings. ACKNOWLEDGMENT This study was conducted in part at the Department of Electromagnetic Field, Czech Technical University, Prague, Czech Republic. Fig. 8. Floor plan of a cell showing three levels of false localization risk for intentional shadowing loss factor dla equals to: (a) 0, (b) 2, and (c) 4 db. represents a loss factor caused by the mobile antenna directivity and intentional shadowing. Reference values for (1) were chosen to perform the analysis. A reference frequency of 900 MHz (GSM900) was selected from the MS s wide frequency range (450 1980 MHz) as a typical value. A loss factor of 10 db was taken as a reference for. As mentioned above, in general, it is quite complicated to estimate the influence of the multipath propagation. In [8], the average enhancement or attenuation of 3 db at 900 MHz was reported. If the worst case is considered ( attenuated, enhanced), a reference value of 6 db can be assigned to. The factor, including gain of the MP antenna and losses in its user s body, was set to 4 db as a reference. It can then be seen that the sum is equal to zero in our testing case. The last factor, which has not yet been involved in,is the loss caused by intentional shadowing. The schematic results of the study within a single cell floor plan are demonstrated in Fig. 8; the intentional shadowing loss factors equal to 0, 2, and 4 db are used as a parameter. The white areas represent locations where db low risk of false localization; in light gray areas, is in (0 and 5 db) intervals moderate risk of false localization, and db in dark gray areas with high risk of false localization. Theoretical results of the above-stated study were compared with extensive practical measurements of a correct localization probability (see Fig. 7). At each shown point of the given cell, 32 calls (in the GSM band) were shortly activated. All of them with a different MP position, half of the measurements with the MP shaded by a head. DLS reactions were recorded. The presented numbers show a percentage of correct localization of the activated MP. Fig. 7 shows a good agreement between the study results and DLS behavior. False localization exists and cannot be completely avoided. The highest probability of incorrect localization is in the far corners; only very rarely is the error greater than one cell to the right- or left-hand side. The localization error also appears one floor up or down; this corresponds well with the MS antenna radiation pattern (see Fig. 4). From a practical point-of-view, with the DLS optimally set, the probability of correct MP localization is approximately 90%. VII. CONCLUSION The described microwave system has been developed according to the requirements of the Administration of Prisons. It has a novel circuit structure and employs complex software signal processing. During more than two years long operation, REFERENCES [1] P. Hudec and M. Polívka, Microwave system for the detection and localization of mobile phones in large high-guarded buildings, presented at the 34th Eur. Microwave Conf., Amsterdam, The Netherlands, 2004. [2] A. E. Bailey, Microwave Measurement. London, U.K.: Peregrinus, 1985, ch. 5 13. [3] P. Hudec and M. Polívka, Microwave sensor for the detection of mobile phones, in Proc. Radioelektronika, Brno, Czech Republic, May 2003, pp. 217 220. [4] M. Polívka, P. Hudec, and M. Mazánek, Dual-band quarter wavelength planar antenna for signal detection in GSM 900/DCS 1800 bands, in Proc. Comite, Pardubice, Czech Republic, Sep. 2003, pp. 69 71. [5] M. Polívka and P. Hudec, Study of trial band quarter wavelength planar antenna for signal detection in NMT 450/GSM 900/DCS 1800 bands, in Proc. Radioelektronika, Bratislava, Slovakia, May 2002, pp. 294 297. [6] J. Lähteenmäki, Indoor propagation models, COST, Brussels, Belgium, COST231 Final Rep., 1996. [7] J. D. Parsons, The Mobile Radio Propagation Channel, 2nd ed. New York: Wiley, 2000. [8] P. Pechac and M. Klepal, Empirical models for indoor propagation in CTU Prague buildings, Radioengineering, vol. 9, no. 1, pp. 3 36, Apr. 2000. [9] K. Siwiak, Radiowave Propagation and Antennas for Personal Communications. London, U.K.: Artech House, 1998. Premysl Hudec received the M.S. and Ph.D. degrees in radio electronics from the Czech Technical University Prague, Prague, Czech Republic, in 1982 and 1995, respectively. In 1982, he joined the Department of Electromagnetic Field, Czech Technical University Prague. His research interests are focused on microwave measurement and microwave systems. Milan Polivka (M 04) received the M.S. and and Ph.D. degrees in radio electronics from the Czech Technical University Prague, Prague, Czech Republic, in 1996 and 2003, respectively. In 1996, he joined the Department of Electromagnetic Field, Technical University of Prague, as an Assistant. His research interests are in the field of antenna and radiating systems. Pavel Pechac (SM 03) received the M.S. and Ph.D. degrees in radio electronics from the Czech Technical University Prague, Prague, Czech Republic, in 1993 and 1999, respectively. He is currently an Associate Professor with the Department of Electromagnetic Field, Czech Technical University Prague. His research interests are in the field of radio-wave propagation and wireless systems.