System and method for detecting use of mobile telecommunications devices

Abstract

In some environments, use of mobile telecommunications devices is prohibited or restricted. The invention provides for detecting use of a mobile telecommunications device (12) in a controlled environment (14) by broadcasting an acoustic identification signal (32) from a loudspeaker or other acoustic broadcasting device (30) in the controlled environment, so that the identification signal may be captured by the microphone of the mobile device during any call made or received by the mobile device and be included in the payload of the call, and the identification signal may then be detected in the call payload by spectral energy analysis. The acoustic identification signal is preferably above the audible frequency band for human hearing, but within the acoustic range of a typical mobile device microphone and also within the audio frequency band processed and analysed by a telecommunications network (18): the identification signal may be between 15kHz and 25kHz, and preferably between 17kHz and 24kHz. Some of the rooms (22) or outdoor areas (24) within the controlled environment may not have a broadcasting device, if there is no restriction on mobile device use within them.

Description

This invention relates to systems and methods for detecting use of mobile, or cellular, telecommunications devices within a controlled environment. BACKGROUND TO THE INVENTION In some controlled environments, the use of mobile, or cellular, telecommunications devices, in particular mobile telephones, may be restricted or prohibited. Known techniques for implementing such restrictions or prohibitions are outlined below. Radio Frequency (RF) jamming can be employed, but this is relatively indiscriminate, both in terms of personal impact and geographical coverage. The outcome is usually short-term disruption, rather than lasting device management, recovery, or personal accountability. The impact is almost impossible to define responsibly, lacking the ability to quantify the unintended consequences or success. Also, even in the most controlled environment, there can be communication / IOT devices within the emergency infrastructure, and these will also be affected. An alternative approach is to provide a private telecommunications network covering the controlled environment. The private network is unattached to public telecommunications networks and is intended to provide service to mobile devices operating in the controlled environment. As such local mobile devices connect with the private network rather than a public one, and the private network is able to both prohibit or restrict service as required, and recover device hardware identities for further action. However, there is still a trade-off between comprehensive coverage of the intended environment and overreach into public spaces and provider networks. Active management of this perimeter risk is necessary for corporate accountability since device identities can be disclosed. Both of the options outlined above have practical drawbacks due to the primary radio effect on which they are based. The RF jamming option has fallen out of favour for reasons of cost, benefit, and risk. The private network option addresses a more strategic requirement and attempts to manage the external impact. However, in the case of larger environments, such installations are disproportionately expensive, both in terms of the physical logistics and later risk management. A third option is to request or require that the operators of public mobile telecommunications networks provide relevant data from their network. However, although in general public network operators can share evidential data under legal constraint, it is up to the requesting party to identify the mobile devices. As such, it is the requesting party who is the pro-active enquirer and not the public network operator, whose first responsibility is to its customers. It would be desirable to provide an improved system and method for detecting use of mobile, or cellular, telecommunications devices within a controlled environment, and preferably also to identify the mobile device. SUMMARY OF THE INVENTION From a first aspect the invention provides a method of detecting use of a mobile telecommunications device in a controlled environment, the method comprising: broadcasting an identification signal in said controlled environment; and detecting said identification signal in a call made by, or received by, said mobile telecommunications device across a telecommunications network. The identification signal may be captured by the microphone of the mobile device during the call and included in the payload of the call. The identification signal may be detected in the call payload by any convenient means, e.g. spectral energy analysis. Typically, said mobile communication device includes a microphone for detecting sounds within a microphone frequency band during said call, and said broadcasting involves broadcasting said identification signal in a frequency band within said microphone frequency band. Preferably, said broadcasting involves broadcasting said identification signal in a frequency band outside of, preferably above, the telephony voice frequency band. Preferably, said broadcasting involves broadcasting said identification signal in an inaudible, or nonaudio, frequency band, optionally in the ultrasonic frequency band. Typically, at least one component of said telecommunications network is configured to perform frequency analysis or energy analysis, optionally spectral energy analysis, of said call in an analysis frequency band, and wherein said broadcasting involves broadcasting said identification signal within said analysis frequency band. Preferably, said broadcasting involves broadcasting said identification signal in a frequency band between 15 kHz and 25 kHz, optionally between 17 kHz and 24 kHz. In preferred embodiments the method includes incorporating an identifier into said identification signal. Incorporating said identifier into said identification signal may involve varying the frequency and / or amplitude of said identification signal to represent the identifier. Varying the frequency of said identification signal may involve varying the frequency between a first frequency and a second frequency, or otherwise transmitting said identification signal at said first frequency or said second frequency, wherein, preferably, said first frequency represents one binary value and the second frequency represents the other binary value. Typically, said identifier comprises a string of symbols, and wherein incorporating said identifier into said identification signal involves transmitting said symbols in sequence. Optionally, the method includes incorporating a synchronisation identifier into said identification signal in association with said identifier. Incorporating said synchronisation identifier into said identification signal may involve transmitting said synchronisation identifier before said symbols to indicate the beginning of said identifier. Incorporating said synchronisation identifier into said identification signal may involve transmitting said identification at a third frequency. Optionally, incorporating said identifier into said identification signal is performed by frequency keying, or frequency-shift keying, or on-off keying. Optionally, said broadcasting involves broadcasting said identification signal at at least one location in said controlled environment, optionally at more than one different location in said controlled environment. Optionally, the method includes incorporating a respective different identifier into the respective identification signal broadcast at each location. The, or each, location may comprise a room, or a building, or an outdoor area. In preferred embodiments, detecting said identification signal involves detecting said identification signal in a payload of said call. Typically, said payload includes any voice content and / or any other audio content of said call. Advantageously, detecting said identification signal is performed during said call. In typical embodiments, detecting said identification signal involves performing frequency analysis and / or energy analysis, preferably spectral energy analysis, of said call, preferably on the payload of the call. Typically, detecting said identification signal involves detecting said identifier in said call, preferably in the payload of the call. Detecting said identification signal may involve detecting said synchronisation identifier in said call, preferably in the payload of the call. In some embodiments, the method includes performing said detecting said identification signal at a codec or audio coder included in said telecommunications network. Advantageously, the method includes performing said detecting said identification signal at said codec or audio coder prior to, or during, determining a sampling rate for said call, in particular a sampling rate for the payload of the call. Conveniently, detecting said identification signal involves monitoring the energy of the call, preferably the payload of the call, in respect of at least one frequency, or at least one frequency band, or energy bin, that corresponds to said identification signal. Typically said call, in particular the payload of the call, comprises call signals, typically comprising said identification signal and voice signals and / or other audio signals, and wherein detecting said identification signal involves monitoring the energy of the call signals in respect of at least one frequency, or at least one frequency band, or energy bin, that corresponds to said identification signal. Detecting said identification signal may involve monitoring the energy of the call, preferably the payload of the call, at said first frequency and said second frequency. Detecting said identification signal may involve monitoring the energy of the call, preferably the payload of the call, at said third frequency. Monitoring the energy may involve determining whether or not the energy level exceeds a threshold value. From a second aspect the invention provides a system for detecting use of a mobile telecommunications device in a controlled environment, the system comprising means for broadcasting an identification signal in said controlled environment; and means for detecting said identification signal in a call made by, or received by, said mobile telecommunications device across a telecommunications network. The system typically includes at least one broadcasting device configured to broadcast said identification signal in said controlled environment. Said at least one broadcasting device may be configured to broadcast said identification into at least one location in said controlled environment, wherein, optionally, said at least one location comprises a room, or a building or an outdoor area. Conveniently, said detecting means comprises a codec or audio coder included in said telecommunications network. Embodiments of the invention mitigate at least some of the problems outlined above in connection with conventional approaches, in particular avoiding inappropriate intrusion and prohibitive expense by removing the need for an over-powering primary effect, e.g. jamming signal or private network. Advantageously, preferred embodiments involve the use of inexpensive acoustic broadcasting devices which have no influence in public spaces or on the general use of mobile networks. The broadcasting devices allow calls made by mobile devices that are within a controlled location to be tagged or marked with an identifying signal, and so to be detected. Advantageously, embodiments of the invention allow the mobile devices to be identified and / or located, at least to within the locality of the relevant broadcasting device. Preferred embodiments provide telecommunication networks, advantageously public telecommunications networks and in particular public telephone networks, with means for identifying calls that are made by a mobile telephone, or other mobile device capable of making calls, within a controlled environment. In particular, such calls can be detected by detecting the identification signal 5 in the call. When such a call is detected, information relating to the call and / or the device that made the call can be obtained and communicated to a monitoring system. Advantageously, the network operator is not required to make any subjective analysis of the call or handle any case information. Advantageously, each broadcasting device broadcasts an identification signal with a unique ID, which allows the location of the mobile device to be determined, i.e. as being within range of the respective broadcasting device. In some embodiments, a plurality of broadcasting devices are distributed around the controlled environment, optionally being concealed or incorporated into a host device. For example, each broadcasting device may be a stand-alone device, in which case it is preferably concealed, or may be incorporated into another device, such as a loudspeaker, radio, television, or other device capable of rendering audio or acoustic signals. In preferred embodiments, the identification signal is an ultrasonic signal. More generally, it is preferred that the identification signal is transmitted at non-audible (at least to humans) frequencies. Preferably, the identification signal includes a broadcast identifier, or unique ID, that is unique to the system and / or is unique to the broadcasting device. When any mobile device within the broadcast range of the broadcasting device establishes a call, the identification signal is detected by the microphone of the mobile device and so is included in the microphone output signal of the call. At network level, call traffic from relevant cell sites, i.e. cell(s) associated with the controlled environment, is monitored for the identification signal. The monitoring may conveniently be performed by an audio codec, in particular a speech or voice codec, included in the network infrastructure. The monitoring may be performed in a manner similar to that of conventional network compression codecs, except that, advantageously, in order to implement embodiments of the invention, access the voice content of the call is not required. If the identification signal is detected in a call, the unique ID of the signal may be obtained and communication to an authorised party, e.g. the operator of a monitoring system, optionally along with the any identifier(s) associated with the device making the call. Advantageously, no other call content is processed or communicated to the monitoring system. The authorised party can follow-up, checking the relevance of the time and location of the call, and / or determining if the call was made by an authorised or unauthorised mobile device. Further action can be taken as required, e.g. depending on whether or not the call is deemed to have been made by an authorised or unauthorised device. Preferred embodiments of the system may comprise corresponding means, which may be implemented in hardware and / or software as convenient, for perform any one or more of the method steps recited in the method claims appended hereto. Further advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: Figure 1 illustrates a system for detecting use of mobile telecommunications devices in a controlled environment, the system embodying one aspect of the invention; Figure 2 is a block diagram of part of the system of Figure 1, illustrating the controlled environment and an acoustic broadcasting device and mobile telecommunications device located in the controlled environment; Figure 3 is a block diagram of another part of the system of Figure 1, illustrating part of a telecommunications network, including means for detecting an identification signal in calls; and Figures 4A to 4C are graphic illustrations of energy bins resulting from call analysis by an exemplary codec. DETAILED DESCRIPTION OF THE DRAWINGS Referring now in particular to Figure 1 of the drawings there is shown, generally indicated as 10, a system for detecting use of mobile telecommunication devices 12 in a controlled environment 14. The, or each, mobile telecommunications device 12 (hereinafter referred to as mobile device 12) typically comprises a telephone, in particular a mobile or cellular telephone (including smartphones), but may comprise any telephony and / or computing device that is capable of establishing calls, in particular telephone calls (including audio calls and / or video calls), with other mobile communications devices 16 using a telecommunications network 18. In the illustrated embodiment, it is assumed that the other mobile devices 16 are located outside of the controlled environment 14, but it will be understood that embodiments of the invention also apply to calls established between mobile devices 12 that are located in the controlled environment 14. If for example a call is established between a mobile device 12 and a mobile device 16, in preferred embodiments it will be apparent which mobile device 12, 16 is the device located in the controlled environment, as is described in more detail below. The controlled environment 14 may comprise any physical site(s) or location(s) in respect of which the use of mobile devices 12 is restricted or prohibited. The controlled environment 14 may for example comprise one or more room, and / or one or more building, and / or grounds or outdoor areas, any of which may be located at the same site (e.g. included in an estate or campus), or at different sites. In typical embodiments, the controlled environment 14 comprises one or more building, and optionally any associated grounds, that are associated with a given institution. In the illustrated embodiment, the controlled environment 14 comprises, by way of example, a plurality of buildings 20 each having one or more room 22, and outdoor area(s) 24 (e.g. a garden, yard or sports facility). The system 10 comprises at least one broadcasting device 30, in particular an acoustic broadcasting device, located in the controlled environment 14. The system 10 typically includes a plurality of broadcasting devices 30. Each broadcasting device 30 is associated with (and be located in or at least partly located in) a respective location, e.g. a single room 22 or group of two or more rooms, or outdoor area 24, within the environment 14, in particular a location in respect of which use of mobile devices 12 is restricted or prohibited. Optionally, more than one broadcasting device 30 may be associated with the same location 22, 24. Each broadcasting device 30 is configured to broadcast, or transmit, acoustic, or sound, signals, in particular into the respective location of the environment 14 with which it is associated. Preferably, each broadcasting device 30 is configured to broadcast acoustic signals with a broadcast range that corresponds to the respective location, e.g. room, of the environment 14 with which it is associated, preferably such that the broadcast signal does not travel beyond the respective location, e.g. room, or at least not in a detectable manner. Controlling the range of the broadcast signals can be achieved using any conventional means, e.g. power controlling means and / or signal shaping means. In the example of Figure 1, some of the rooms 22 have no broadcasting device 30 and it is assumed that in these rooms 22 there is no restriction or prohibition of mobile device 12 use. This exemplifies an advantageous aspect of preferred embodiments, namely that the system 10 can be configured to define the controlled environment 14 such that one or more room or other location that is not part of the controlled environment 14 can be located adjacent or amid the controlled environment 14 and not be affected by the system 10, i.e. the identification signal 32 is not present in the other room(s) / location(s), or at least not at a level that is detectable by the microphone of mobile devices. As is described in more detail below with reference to Figure 2, each broadcasting device 30 is configured to broadcast an acoustic identification signal 32, preferably comprising an identifier (ID). Optionally, the identifier is unique for each broadcasting device 30, i.e. each broadcasting device 30 transmits a respective unique ID. Alternatively, the, or each, broadcasting device 30 associated with a given location 22, 24 is configured to transmit the same unique ID, i.e. the unique ID is associated with the location 22, 24 rather than the device 30. Alternatively, the, or each, broadcasting device 30 associated with a given environment 14 may be configured to transmit the same unique ID, i.e. the unique ID is associated with the environment 14 rather than the location 22, 24 or device 30. In any event, the unique ID may service as a location identifier as is described in more detail hereinafter. Optionally, the identifier ID may include a system ID, which identifies the signal 32 as belonging to the system 10, and / or a site ID, which identifies the controlled environment 14. The system ID and / or the site ID are preferably provided in addition to the unique ID. The system ID and / or the site ID may be provided as or in a preamble portion of the signal 32. The acoustic identification signal 32 is transmitted or broadcast in a frequency band that lies outside of, and is preferably above, the telephony (human) voice frequency band, which is typically defined as being from approximately 300 Hz to 3400 Hz. Advantageously, the frequency band of the identification signal 32 is outside of, preferably above, the audible frequency band, i.e. the frequency 8 band associated with human hearing, which is normally considered to be approximately 20 Hz to 20 kHz, although in practice the upper limit of human hearing may be lower, e.g. approximately 15 kHz. The frequency band of the identification signal 32 is also within the frequency band within which the microphone(s) of the mobile devices 12 is capable of detecting acoustic signals. Microphone frequency bands can vary from device to device, but in typical mobile devices today the microphones are capable of detecting acoustic signals within the range 20 Hz to 25 kHz (approximately). Advantageously, the frequency band of the identification signal 32 is also within the frequency band that is analysed, or processed, by the telecommunications network 18 during a call, which may for example be up to approximately 24 kHz, e.g. for analysis performed by an audio codec or coder. In preferred embodiments, the frequency band of the identification signal 32 is 15 kHz to 25 kHz. It will be understood however that higher or lower frequencies may be used depending on the frequency band parameters of the relevant system components, which are subject to change particularly as technology advances. In some embodiments, the frequency band of the identification signal 32 is within the ultrasound frequency band, i.e. the broadcasting devices 30 are configured to transmit ultrasound signals. In this context the word ultrasound is intended to embrace frequencies that are inaudible to humans rather than being within a specific frequency bandwidth. The human hearing range can vary depending on various factors including age. For example, signals at a frequency of approximately 17 kHz are audible to children but tend not to be audible to adults. In preferred embodiments, the identification signal 32 is broadcast at one or more frequency within a frequency band in which signals are normally inaudible to humans, are within the detection range of microphones commonly found on mobile devices and which are within the processing range of voice codecs that are typically used in telephone networks. As indicated above, the frequency band used for the identification signal 32 in preferred embodiments is typically 15 kHz to 25 kHz, a more preferred frequency band being 17.5 kHz to 24 kHz. Each broadcasting device 30 may be configured to broadcast the identification signal 32 at the same frequency or frequencies, or may be configured to broadcast the identification signal 32 at a different frequency or frequencies to avoid interference with each other. Microphones on modern mobile devices can respond to frequencies in the ultrasound range, even though the voice / music frequencies of interest are much lower. This provides two advantages: firstly, the existing microphone(s) on conventional mobile devices will detect the identification signal 32 even though it is not in the frequency range of normal interest (i.e. the identification signal is advantageously a non-audio signal rather than an audio signal; and secondly, while the identification signal 32 is detected together with other call content, i.e. voice and / or other audio signals, there is spectral separation. This facilitates recovery, or detection, of the identification signal 32, as is described in more detail below. In typical embodiments, the telecommunications network 18 comprises a telephone network, in particular a mobile or cellular telephone network. The network 18 may be of any conventional configuration, typically including a base system subsystem (BSS) and a network switching subsystem (NSS), and may include or be connected to a public standard telephone network (PSTN) 9 and / or computer networks as would be apparent to a skilled person. Advantageously, the network 18 is a public network. The mobile devices 12 are capable of making and receiving calls across the network 18 in conventional manner. The system 10 may include a monitoring system 40 connected to the network 18 in order to receive information relating to calls that are determined to have been made by or received by a mobile device 12 in the controlled environment 14, as is described in more detail hereinafter. The monitoring system 40 may comprise a computer system and / or telephony system as is convenient. Referring now in particular to Figure 2, an exemplary embodiment of the broadcasting device 30 is described. The broadcasting device 30 comprises a signal generator 50 configured to generate the identification signal 32. In preferred embodiments, the signal generator 50 is operable to generate the identification signal at more than one different frequency within the desired frequency band. Typically, the signal generator 50 comprises a signal source 52, e.g. an oscillator or clock signal source, and preferably a signal controller 54 for controlling the frequency of the generated signal. In embodiments where the signal source 52 generates a clock signal or digital signal, the signal generator 50 may include a signal converter 56 for converting the signal to an analogue form. Any suitable digital-to-analogue converter may be used for this purpose. More generally, the signal generator 50 may take any conventional form. For example the signal generator 50 may comprise variable oscillator or a variable, or programmable, clock signal generator. The broadcasting device 30 includes a transducer 60 for converting the electrical signal generated by the signal generator 50 into an acoustic signal and broadcasting the resulting acoustic identification signal 32 as described above. The transducer 60 may take any conventional form, for example a loudspeaker, ultrasonic speaker or any other electroacoustic transducer capable of producing sound in the desired frequency band. The broadcasting device 30 may be provided as a dedicated, stand-alone device. Alternatively, the broadcasting device 30 may be incorporated into one or more other devices or systems as is convenient, in particular devices or systems that include a loudspeaker or other electroacoustic transducer capable of producing sound in the desired frequency band. For example, the broadcasting device 30 may be incorporated into a television, radio, smart speaker, or public address system. The components of the broadcasting device 30 may be co-located or distributed as is convenient so long as the transducer 60 is located such that it can deliver the identification signal 32 to the desired location in the controlled environment 14. For example, if the transducer 60 is provided by a smart speaker in a given room 22, the signal generator 50 may be located remotely (within or outside of the controlled environment 14) and in communication with the smart speaker by any suitable communications network in order to provide the identification signal to the smart speaker. The system 10 may comprise different such types of broadcasting device 30 as is convenient. The broadcasting device 30, in particular when embodied as a stand-alone unit, may be referred to as a beacon. The identification signal 32 may be described as a beacon signal. Optionally, the broadcasting device 30 may be voice-activated such that it transmits the identification signal in response to detection of the human voice. This may be achieved using any conventional voice-activation means. When a call, in particular a voice call, is established between any one or more of the mobile devices 12 and another device 12,16 across the network 18, a microphone 13 (or more than one microphone depending on the configuration of the respective mobile device 12) of the, or each, mobile device 12 in the controlled environment 14 detects the identification signal 32 transmitted by the, or each, broadcasting device 30 associated with the location of the mobile device 12 in the controlled environment 14. Current mobile devices 12 commonly use the microphone in a dedicated mode when a voice call is commenced. This usually denies it to other applications until the call is ended. The identification signal 32 is detected by the microphone 13 together with any speech or other audio input, e.g. music, that the user (not shown) of the mobile device 12 may make or may otherwise be detectable during the call. Therefore, the output signal produced by the, or each, microphone 13 includes the identification signal 32 (in electrical form). The microphone output signal also includes electrical signals representing any speech or other sounds detected during the call. During the call, the content of microphone output signal is incorporated into the call 17 that is transmitted across the network 18 by the mobile device 12 in conventional manner. The payload of the call as transmitted across the network 18 includes the identification signal 32 (or more particularly an electrical or optical representation thereof) together with any voice signals. The payload of the call comprises the content of the call resulting from sound captured by the microphone(s) of the mobile device 12 during the call. The payload may comprise not only voice content and the identification signal, but also any ambient acoustic signals that may be present. The payload may be said to comprise signals representing the acoustic content of the call. As is described in more detail below, the identification signal 32 is recoverable from, or detectable in, the call payload because it is in a frequency band that is separate from the voice frequency band. In a simple embodiment, detection in the payload of any signal in the frequency band designated for the identification signal 32 may be deemed to be detection of the identification signal 32 and therefore detection of use of a mobile device 12 in the controlled environment 14. However, in preferred embodiments, the identification signal 32 comprises the identifier (ID) as described above. As such, detection of the identification signal 32 in the call payload involves detection of the identifier ID in the payload, in particular detection of the identifier (ID) in the frequency band designated for the identification signal 32. The identifier (ID) may be incorporated into the identification signal 32 by any suitable means, e.g. by varying the frequency of the identification signal 32 or by on-off keying using a single frequency. Typically, the identifier (ID) comprises a string, or sequence, of symbols, e.g. digits, bits or characters, that is encoded in, or otherwise incorporated into, the identification signal 32. Conveniently, this may be achieved by varying the frequency of the Identification signal 32 (e.g. frequency keying) to represent the identifier (ID), or using on-off keying to represent the identifier (ID). For example, the frequency of the identification signal 32 may be varied over time to represent the sequence of symbols. Conveniently, each symbol may be represented in binary form, i.e. comprising one or more bit. Optionally, each symbol may be encoded in the identification signal 32 by varying the frequency between first and second frequencies, wherein the first frequency represents one bit value (e.g. “1 ” or “0") and the second frequency represents the other bit value (e.g. “0” or “1”). The frequency may be varied sequentially in order represent the sequence of symbols that make up the identifier (ID). Alternatively, each symbol may be encoded in the identification signal 32 by switching the signal 32 on and off (or between a high level and a low level), wherein ON or HIGH represents one bit value (e.g. “1 ” or “0”) and OFF or LOW represents the other bit value (e.g. “0” or “1”). The signal 32 may be switched in this manner sequentially in order represent the sequence of symbols that make up the identifier (ID). A preamble ID may be transmitted before the identifier (ID) to facilitate detection of the identifier (ID). The preamble may comprise one or more transmittable symbols. Preferably, a synchronisation indicator is incorporated into the identifier signal 32 before transmission of the identifier (ID). The synchronisation indicator may take any convenient form, for example comprising transmission of a synchronising tone (e.g. a period during which the identification signal 32 is transmitted at a third frequency that is different from the first and second frequencies) and / or transmission of a of one or synchronisation symbols. Advantageously, the synchronisation identifier allows synchronisation of the detection means with the identifier (ID), i.e. detection of the synchronisation identifier indicates the subsequent beginning of the transmission of the identifier (ID). More generally, the identifier (ID) may be incorporated into, or encoded in, the identification signal 32 using any convenient technique, e.g. on-off keying, frequency keying, or frequency-shift keying, chirp spread spectrum (CSS), or frequency modulation, amplitude modulation or other modulation of a carrier signal. In preferred embodiments, the, or each, frequency used to transmit, or encode, the identifier (ID) is within the frequency band designated for the identifier signal 32, as described above. In the frequency-varying example above, the first and second frequencies may for example be 19200 Hz and 18400 Hz respectively. The third frequency of the synchronisation tone may be 17600 Hz. It will be understood that the invention is not limited to these frequencies. In this example, the frequency band associated with the identification signal 32 may be 17 kHz to 20 kHz. Referring to Figure 1, the network 18 may include one or more identification signal detection device 70 configured to detect the identification signal 32 in calls made across the network 18, preferably in real time, i.e. during the call. Each detection device 70 may be implemented in hardware and / or software as is convenient. In preferred embodiments, the detection device 70 is configured to detect the identification signal 32, preferably the identifier (ID), in the payload of the call 32. The detection device 70 may be configured to detect the identification signal 32 in any convenient manner, e.g. by performing frequency analysis and / or energy analysis of the signals of which the call, in particular the payload of the call, is comprised. In particular, the identification signal 32, and more particularly the identifier (ID), may be detected by analysing the frequency content or spectral energy content of the payload in the frequency band of the identification signal 32, e.g. at the, or each, frequency used to transmit the identification signal 32. In preferred embodiments, the identifier (ID) is detected by analysing the spectral content or energy content of the payload at the first and second frequencies that are used to transmit the identifier (ID). Each detection device 70 may be located at any suitable location in the network 18. Typically, each cell or base station of the network 18 that provides service to mobile devices 12 in the controlled environment 14 includes at least one detection device 70. The detection device 70 may be a dedicated device configured and deployed in the network 18 for the purpose of detecting the identification signal 32. Preferably, however, one or more existing components of the network 18 are adapted to serve as the detection device 70. Conveniently, any component(s) of the telecommunications network 18 that performs analysis, in particular spectral analysis and / or energy analysis, of the content of the call payload may be adapted to detect the identification signal 32. For example, the network 18 typically includes one or more audio coding device that performs audio coding of the payload (including the voice content and any other audio content such as music) of calls for transmission across the network 18. The processing performed by the audio coders may vary but typically includes any one or more of: compression, bit rate determination, sampling rate determination, bandwidth allocation, channel allocation and so on. Such coders are normally provided as part of a codec, in particular an audio codec. Examples of such codecs include the Adaptive Multi-Rate (AMR) codec Adaptive Multi-Rate Wideband (AMR-WB) codec and the Enhanced Voice Services (EVS) codec. Such audio coders or codecs are normally configured to perform spectral analysis and / or spectral energy analysis of the call payload (in particular the voice or other audio (e.g. music) content) in order to determine how to process the payload, e.g. to determine the sampling rate, and can be adapted to detect the identification signal 32 and so to serve as the detection device 70. Preferably, the signal energy analysis performed by the detection device 70 is configured to detect the identification signal 32. Existing codecs may perform spectral analysis within a relatively narrow frequency band, e.g. the voice frequency band, that excludes the frequency band used for the identification signal 32, and it is simpler therefore to detect the identification signal 32 by performing energy analysis. Conveniently, therefore, detection of the identification signal 32 may be performed by one or codec, in particular voice codec(s) or audio codec(s), included in the network 18. The, or each, codec may be provided at various locations in the network 18 depending on the network configuration, e.g. at a base station. It will be understood that the invention is not limited to detection of the identification signal 32 atcodec(s). An exemplary detection device 70 is illustrated in Figure 3, which shows a block diagram of a codec 72 included in the network 18, the codec 72 being adapted to detect the identification signal 32 in the call 17 received from any one of the mobile devices 12. The detection device 70 may be implemented in computer software and / or hardware as is convenient. In this example, the codec 72 is an Enhanced Voice Services (EVS) codec modified to detect the identification signal 32 but it will 13 be understood that the invention is not limited to use with EVS codecs. Only those components of the codec 72 that are helpful for understanding the present invention are shown. The codec 72 may include pre-processing means 74 configured to detect the type of content contained in the payload of the call 17, e.g. voice and / or music, or silence, and / or to perform other pre-processing task(s) such as filtering. The outcome of the pre-processing may determine how the payload is analysed downstream of the pre-processing means 74. The pre-processing may be performed in any conventional manner and is not essential to performance of the invention. The codec 72 typically includes spectral analysis means 76 configured to perform spectral, or frequency, analysis of the payload of the call 17 in order to determine the spectral or frequency content of the payload. The spectral analysis may be performed using any conventional spectral analysis technique(s). Typically, the spectral analysis is performed during initial sample analysis (e.g. when determining sampling rate) prior to audio coding. In conventional codecs the frequency band over which spectral analysis is performed may be limited to the voice frequency band, or may be wider to cover other audio signals such as music. In the example of the EVS codec, the spectral analysis is performed up to 12.8 kHz. In typical embodiments, such bands are not wide enough to detect the identification signal 32. Optionally, the spectral analysis means 76 may be configured or adapted to perform frequency analysis over a bandwidth that does include the frequency band of the identification signal 32, e.g. up to 20 kHz in the example provided above. As such, the identification signal 32 may be detected by the spectral analysis. In particular the identifier (ID) may be detected and recovered from the payload. The spectral analysis means 76 may be implemented in hardware and / or software as is convenient. In preferred embodiments, the codec 72 includes energy analysis means 78 configured to perform spectral energy analysis of the payload of the call 17 in order to determine the spectral energy content of the payload. The spectral energy analysis may be performed using any conventional spectral analysis technique(s), e.g. by analysing signal power across frequency sub-bands. In some conventional codecs the frequency band over which spectral energy analysis is performed may be wide enough to cover the frequency band associated with the identification signal 32. In the example of the EVS codec, the spectral energy analysis is performed up to 24 kHz. In any event, the spectral energy analysis means 78 may be configured or adapted to perform energy analysis over a bandwidth that does include the frequency band of the identification signal 32, e.g. up to 20 kHz in the example provided above. As such, the identification signal 32 may be detected by the energy analysis. In particular the identifier (ID) may be detected and recovered from the payload. The energy analysis means 78 may be implemented in hardware and / or software as is convenient. In preferred embodiments, the energy analysis means 78 includes energy level determining means 80 for determining the energy level of the payload signals in the frequency band associated with the identification signal 32. In particular, the energy level determining means 80 is configured to determine the energy level of the payload at the or each frequency associated with the identification signal 32. In preferred embodiments the energy level determining means 80 is configured to determine the energy level at each frequency used to encode the identifier (ID). Detecting energy at a level above a threshold value in respect of a given frequency is an indication that the payload contains a signal at the given frequency, while detecting energy below the threshold value is an indication that the payload does not contain a signal at the given frequency. In this way, the detection device 70 can detect the presence of signals at the first and second frequencies and so detect the symbols of the identifier (ID). Similarly, the detection device 70 can detect the presence of signals at the third frequency and so detect the synchronisation identifier. Conventional codecs, such as the EVS codec, are not normally concerned with the energy content of the payload in the frequency band of the identification signal 32, instead focusing on the energy content of payload signals in voice frequency band, or higher to cover music and other audio signals. Such codec can readily be modified to include the energy level determining means 80. The energy level determining means 80 may be implemented in hardware and / or software as is convenient. As is illustrated in Figures 4A to 4C, the energy analysis means 78 typically determines the respective energy level in each of a plurality of frequency sub-bands or bins (which may also be referred to as energy bins or frequency bins), wherein each bin corresponds to a respective interval or sub-band of frequencies contained within the payload signal. For example, the EVS codec provides 60 bins. Figures 4A to 4C show the energy levels of bins 43 to 48, which in the EVS codec correspond to the frequencies used to transmit the identification signal 32. It is noted that the depicted bin numbers are exemplary only and are not limiting to the invention. In this example, bins 43 and 44 correspond to frequencies in the range 17200 Hz to 18000 Hz, and their energy levels may therefore be used to detect the third frequency and therefore the synchronisation identifier, as is illustrated in Figure 4A in which the energy level of bins 43 and 44 are above a threshold level T thereby indicating the presence of the synchronisation identifier in the payload. Bins 45 and 46 correspond to frequencies in the range 18000 Hz to 18800 Hz, and their energy levels may therefore be used to detect the second frequency, which in this example is assumed to represent binary level “0”, as is illustrated in Figure 4B in which the energy level of bins 45 and 46 are above a threshold level T thereby indicating the presence of the second frequency and therefore binary “0” in the payload. Bins 47 and 48 correspond to frequencies in the range 18800 Hz to 19600 Hz, and their energy levels may therefore be used to detect the first frequency, which in this example is assumed to represent binary level “1”, as is illustrated in Figure 4C in which the energy level of bins 47 and 48 are above a threshold level T thereby indicating the presence of the first frequency and therefore binary “1 ” in the payload. In alternative embodiments in which only one frequency is used for the identification signal 32, only one frequency bin corresponding to the relevant frequency needs to be monitored in order to determine the identifier (ID). Optionally, a preamble may be transmitted before the identifier (ID) using the same frequency. The preamble may serve as synchronisation means to facilitate detection of the identifier. It will be understood that in alternative embodiments different bin(s) may correspond to the frequency or frequencies of interest. More generally, the, or each frequency associated with the identification signal 32 may correspond to one or more bin. Advantageously, in embodiments where more than one frequency is used for the identification signal 32, the frequencies associated with the identification signal 32 are spaced apart by an amount corresponding to at least one bin, e.g. two bins in the present example. Advantageously, the bin(s) that correspond to the identification signal 32 are otherwise quiet, i.e. do not normally register signal energy, at least not above the threshold level T, as a result of the normal audio content of the payload. By monitoring the energy levels of the relevant bins over time, the detection device 70 is able to detect the identifier (ID) in the payload. Conveniently, the EVS codec provides an indication of energy levels in all 60 bins and so no modification of the EVS codec is required for this purpose. Advantageously, the identifier recovery method does not induce latency of network processes or traffic copying to escape it. Accordingly, identifier recovery may involve a simple a readout of existing EVS metrics. The call 17 is typically transmitted and processed by the network 18 in frames. It is preferred to transmit each symbol of the identifier (ID) for a period corresponding to multiple frames of the codec, or other detection device 70, intended to detect the identifier (ID). This reduces the possibility that symbols are detected erroneously, and allows error checking to be performed at individual bit level and at full identifier level. By way of example, an EVS audio frame is 0.02s long and the bits of the identifier (ID) may be transmitted for, say, 8 frames. The synchronisation ID and / or preamble can be included using the same frequency but at a different symbol period. This facilitates detection of the system ID, site ID, or other preamble or synchronization ID as distinct from the unique ID of the device 30. On-off keying may be used to transmit the unique ID and a preamble at a single frequency. It will be apparent from the foregoing that the detection device 70 is able to detect the presence of the identification signal 32 in the signals that comprise the payload of the call 17, and in preferred embodiments also to detect or determine the identifier (ID). A indicated at 82 in Figure 3, the detection device 70 is configured to determine whether or not the identification signal 32, and more particularly the identifier (ID) is present in the payload of the call 17. If the identification signal 32 is present, then this is an indication that the call 17 is being made to or from a mobile device 12 in the controlled environment 14, i.e. within range of one or more of the broadcasting devices 30. Detection of the identification signal 32 may be reported to the monitoring system 40. The operator of the monitoring system 40 may decide what further action needs to be taken. In the example of Figure 3, the reporting is performed by control module 84, conveniently comprising an API. The control module 84 is optionally bi-directional, supporting command-and-control opportunities such as network blacklisting. In preferred embodiments, the identifier (ID) is unique to one or more of the broadcasting devices 30 and so provides an indication of the location of the mobile device 12 in the controlled environment 14. The monitoring system 40 may also be provided with the time of the call. Optionally, the incoming call may be examined at the detection device 70, e.g. for a few microseconds, to determine if the identification signal 32 is present in the payload. If it is, then the identifier (ID) is obtained as described above. Detection of the identifier (ID) involves no interference with the voice content of the call, either by the identification signal 32 or the network 18. The operator of the network 18 can provide one or more identifier (e.g. International Mobile Subscriber Identity (IMSI) and / or International Mobile Equipment Identity (IMEI)) of the mobile device 12 involved in the call. The call content is not interfered with and continues without interruption at the informed sample rate. In particular, detection of the identification signal does not require any action in the frequency band associated with the voice content of the call. It is noted that the codec 72, in particular the pre-processing means 74 and analysis means 76, 78 do not examine or decode the call content per se, e.g. the voice message contained in the payload; instead they determine the type and / or other characteristic(s) of the payload in order to determine how best to apply the audio coding. In particular, the pre-processing means 74 and analysis means 76, 78 enable the codec 72 to control the sampling rate dynamically during the call (block 86 in Figure 3). Should the codec 72 decide that a particular frame of the call is a voice frame, it may reduce the sample rate below what is necessary for detection of the identification signal 32. In preferred embodiments, therefore, the identification signal 32 is detection prior to, or during, such decisions being made, or at least before the sampling rate is applied to the contents of the call. Preferred embodiments of the invention provide a solution to mobile device management in controlled areas, that is relatively low cost and simple to deploy, and which does not interfere with the operation of public telephone networks. Unlike alternative approaches, the local effect does not compete with commercial mobile services in the vicinity and does not interfere in their spectrum. The preferred system is an augmentation of normal service, operating at content level as opposed to connectivity level. The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention.

Claims

1. A method of detecting use of a mobile telecommunications device in a controlled environment, the method comprising: broadcasting an identification signal in said controlled environment; and detecting said identification signal in a call made by, or received by, said mobile telecommunications device across a telecommunications network,2. The method of claim 1, wherein said mobile communication device includes a microphone for detecting sounds within a microphone frequency band during said call, and wherein said broadcasting involves broadcasting said identification signal in a frequency band within said microphone frequency band.

3. The method of claim 1 or 2, wherein said broadcasting involves broadcasting said identification signal in a frequency band outside of, preferably above, the telephony voice frequency band.

4. The method of any preceding claim, wherein said broadcasting involves broadcasting said identification signal in an inaudible, or non-audio, frequency band, optionally in the ultrasonic frequency band.

5. The method of any preceding claim, wherein at least one component of said telecommunications network is configured to perform frequency analysis or energy analysis, optionally spectral energy analysis, of said call in an analysis frequency band, and wherein said broadcasting involves broadcasting said identification signal within said analysis frequency band.

6. The method of any preceding claim, wherein said broadcasting involves broadcasting said identification signal in a frequency band between 15 kHz and 25 kHz, optionally between 17 kHz and 24 kHz.

7. The method of any preceding claim, further including incorporating an identifier into said identification signal, and wherein, optionally, incorporating said identifier into said identification signal involves varying the frequency and / or amplitude of said identification signal to represent the identifier, and wherein varying the frequency of said identification signal optionally involves varying the frequency between a first frequency and a second frequency, or otherwise transmitting said identification signal at said first frequency or said second frequency, wherein, preferably, said first frequency represents one binary value and the second frequency represents the other binary value.

8. The method of claim 7, wherein said identifier comprises a string of symbols, and wherein incorporating said identifier into said identification signal involves transmitting said symbols in sequence.

9. The method of claim 7 or 8, further including incorporating a synchronisation identifier into said identification signal in association with said identifier.

10. The method of claim 9, when dependent on claim 8, wherein incorporating said synchronisation identifier into said identification signal involves transmitting said synchronisation identifier before said symbols to indicate the beginning of said identifier.

11. The method of claim 9 or 10, wherein incorporating said synchronisation identifier into said identification signal involves transmitting said identification at a third frequency.

12. The method of any one of claims 7 to 11, wherein incorporating said identifier into said identification signal is performed by frequency keying, or frequency-shift keying, or on-off keying.

13. The method of any preceding claim, wherein said broadcasting involves broadcasting said identification signal at at least one location in said controlled environment, optionally at more than one different location in said controlled environment, wherein the, or each, location optionally comprises a room, or a building, or an outdoor area.

14. The method of claim 13 when dependent on any one of claims 7 to 12, including incorporating a respective different identifier into the respective identification signal broadcast at each location.

15. The method of any preceding claim, wherein detecting said identification signal involves detecting said identification signal in a payload of said call, wherein, typically, said payload includes any voice content and / or any other audio content of said call.

16. The method of any preceding claim, wherein detecting said identification signal is performed during said call.

17. The method of any preceding claim, wherein detecting said identification signal involves performing frequency analysis and / or energy analysis, preferably spectral energy analysis, of said call, preferably on the payload of the call.

18. The method of any preceding claim when dependent on any one of claims 7 to 12, wherein detecting said identification signal involves detecting said identifier in said call, preferably in the payload of the call.

19. The method of any preceding claim when dependent on any one of claims 9 to 11, wherein detecting said identification signal involves detecting said synchronisation identifier in said call, preferably in the payload of the call.

20. The method of any preceding claim, including performing said detecting said identification signal at a codec or audio coder included in said telecommunications network.

21. The method of claim 2, including performing said detecting said identification signal at said codec or audio coder prior to, or during, determining a sampling rate for said call, preferably for a payload of said call.

22. The method of any preceding claim, wherein detecting said identification signal involves monitoring the energy of the call, preferably the payload of the call, in respect of at least one frequency, or at least one frequency band, or energy bin, that corresponds to said identification signal, wherein monitoring the energy may involve determining whether or not the energy level exceeds a threshold value, and wherein, typically, said call, in particular the payload of the call, comprises call signals, typically comprising said identification signal and voice signals and / or other audio signals, and wherein detecting said identification signal involves monitoring the energy of the call signals in respect of at least one frequency, or at least one frequency band, or energy bin, that corresponds to said identification signal.

23. A system for detecting use of a mobile telecommunications device in a controlled environment, the system comprising means for broadcasting an identification signal in said controlled environment; and means for detecting said identification signal in a call made by, or received by, said mobile telecommunications device across a telecommunications network.

24. The system of claim 23, including at least one broadcasting device configured to broadcast said identification signal in said controlled environment, wherein, optionally, said at least one broadcasting device is configured to broadcast said identification into at least one location in said controlled environment, wherein, optionally, said at least one location comprises a room, or a building or an outdoor area.

25. The system of any claim 23 or 24, wherein said detecting means comprises a codec or audio coder included in said telecommunications network.20

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