GB2375687A

GB2375687A - Power control in cellular communications systems

Info

Publication number: GB2375687A
Application number: GB0111773A
Authority: GB
Inventors: Nicola Greco; Stephen Billington; Jafar Faroughi-Esfahani
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2001-05-14
Filing date: 2001-05-14
Publication date: 2002-11-20
Also published as: WO2002093780A1; GB0111773D0

Abstract

Power control of a mobile station (1) incorporating a downloaded software defined protocol stack (3) is achieved by means of a secure core module (4) which monitors emitted power levels and which only hands over power control to the communication system when an authorisation message has been received from its serving base station (2). In this way, spurious emissions from rogue terminals whose protocol stack has a malfunction, can be controlled.

Description

POWER CONTROL IN CELLULAR COMMUNICATIONS SYSTEMS This invention relates to power control in cellular communications systems and is particularly applicable to the field of software definable radio.

In a cellular communication system each of the user terminals (typically mobile stations) communicates with typically a fixed base station. Communication from the user terminal to the base station is known as the uplink and communication from the base station to the user terminal is known as the downlink. The total coverage area of the system is divided into a number of separate cells, each predominately covered by a single, serving base station.

A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the mobile stations and the base stations. This spectrum must be shared between all mobile stations simultaneously using the system. Multiple access techniques permit the simultaneous transmissions from several mobile stations to a single base station. The GSM system (Global System for Mobile Communications) uses time division multiple access (TDMA), in which a communications channel consists of a time-slot in a periodic train of time intervals over the same frequency. Each mobile station is allocated one specific time slot for communication with a base station in a repeating time frame. Another type of multiple access technique, and one proposed for the third generation universal mobile telecommunications system (UMTS) is known as code division multiple access (CDMA) which employs spread-spectrum signaling. Individual users in the CDMA communications system use the same RF carrier frequency, but are separated by use of individual spreading codes. Hence, multiple communications channels are allocated using a plurality of spreading codes within the portion of radio spectrum, each code being uniquely assigned to a mobile station.

Until recently, one problem for the user was that to be able to go any where in the world and use a cellular communications system, the user needed to carry different types of user terminal, since one terminal alone would not operate on all of the systems. However, multi-mode and re-configurable user terminals are now becoming commercially available, allowing the user to select anyone of several available communications systems, eg GSM or UMTS.

A re-configurable mobile station adapts itself for operation on any given type of cellular communications system, by down-loading the appropriate software over the air and subsequently, installing and running it.

The quality of the radio communication between a mobile station and a base station is determined by the signal to noise level of the communication signals, where the noise includes both thermal noise and interference noise. Other base stations and mobile stations generate interference, which increases the noise level and thus reduces the quality. In order to attain an acceptable quality level, the interference must be kept sufficiently low. A major technique for interference in a cellular system is the use of power control whereby the transmitted power of each mobile station and base station is maintained at the minimum level required for the signal to be received at an acceptable quality. Uplink power control can be implemented by the base station measuring the received signal quality and transmitting power-up information to the mobile station when the signal quality is below an acceptable level, and power-down information when the signal quality is above this level.

Similarly, downlink power control can be implemented by the mobile station transmitting power-up or power-down information depending on the signal quality of the signal received at the mobile station.

Currently, emission power in mobile stations is strictly regulated and a mobile station can be shipped only when a well-defined set of tests to ensure that the emission is compliant with existing regulations, has been satisfactorily completed. However, there is currently no means for protecting a communications system from software malfunctions after communications devices have been introduced into the market.

With the advent of software definable radios (eg re-configurable mobile stations), the combination of hardware and software will be subject to changes after equipment has been shipped and as a result, the complexity of the testing procedures will increase and so the function of equipment cannot be easily guaranteed. The increased complexity of testing mechanisms can result in a much more difficult type approval procedure and therefore undermine the commercial success of new technologies based on software definable radio concepts. In software definable radio, the functionality of a mobile station will change every time it re-configures itself to a new communication system (or radio access technology) with the aid of software downloaded. A malfunction resulting from the software download could create operational problems within the communication system that the mobile

station is attempting to register with. Multi-mode or re-configurable terminals must preferably have the capability of over-the-air software download in order to fulfill the user requirements of the next generation of mobile communications. Software download re-configuration will change the functionality of mobile stations and enable them to connect to different networks by taking into account the network availability, quality of service and user profile. Even though high level tests will be applied to such multi-mode terminals during their re-configuration, there is still a possibility of a malfunction allowing the mobile station to become a rogu terminal.

In current implementation of CDMA systems, the mobile station works in either open loop or closed loop mode. When in open loop mode, the mobile station performs power measurements and decides the power level. When in closed loop mode, the mobile station sends measurements to the system via its serving base station and the system responds with a message to adjust the power emission appropriately. The procedure is part of a standardised protocol specified in the third generation partnership project. If the protocol stack necessary for power control is to be implemented in the software and downloaded over-the-air, any malfunction of the open or closed loop power control modes cannot be corrected by current methods.

Hence, there is a need for a simplified testing process and mechanism whereby a rogue mobile station which causes high levels of interference within the communication system through persistent emission of high levels of power, can be identified.

According to the present invention there is provided a method of controlling power of emissions from a user terminal operating in a cellular communications system, wherein the user terminal includes a software-defined protocol stack, the method including the steps of; at the user terminal, measuring a level of emissions from the user terminal, comparing the measured level with a reference level, if the measured level exceeds the reference level, adjusting the emission to a level not exceeding the reference level, setting a timer so that emissions continue for a pre-set period of time, storing a value of an operational characteristic of the user terminal,

transmitting, via the software defined protocol stack, the value to a remote base station, at the remote base station, receiving the transmitted value, generating an authorisation token comprising the required transmitted value, transmitting the authorisation token to the user terminal, and at the user terminal, receiving the authorisation token comprising the transmitted value, comparing the transmitted value with the stored value, if the transmitted value and the stored value are different, rejecting the authorisation token and retaining control of emissions of the user terminal, and if the transmitted value and the stored value are identical, accepting the authorisation token, de-activating the timer and relinquishing power control of the user terminal to the remote base station.

In a second aspect, the present invention consists of a user equipment capable of interfacing with a software defined protocol stack, and including apparatus for controlling power emissions from the user equipment, the apparatus including; means for measuring a level of emissions from the user equipment, means for comparing the measured level with a reference level, means for adjusting the emissions to a level not exceeding the reference level if the measured level exceeds the reference level, a timer, coupled to the means for adjusting, for allowing emissions to continue for a pre-set period of time, a store for storing a value of an operational characteristic of the user terminal, means for transmitting, via the software defined protocol stack, the value to a remote base station, means for receiving from the remote base station, an authorisation token comprising the transmitted value, means for comparing the transmitted value with the stored value, means for rejecting the authorisation token and retaining control of emissions of the user terminal if the transmitted value and the stored value are different, and means for accepting the authorisation token, de-activating the timer and relinquishing power control of the user terminal to the remote base station if the transmitted value and the stored value are identical.

The user equipment may be a card or module which can be plugged into a personal computer (the combination comprising a user terminal), which in turn, is capable of installing the software defined protocol stack which has been downloaded over the air.

Alternatively, the user equipment may be a user terminal such as a mobile station (eg. mobile'phone) which incorporates the software defined protocol stack.

In a third aspect, the present invention consists of base station apparatus for assisting in controlling power emissions from a user terminal operating in a cellular communication system, the base station apparatus including, means for receiving from the user terminal a value of an operational characteristic of the user terminal, means for generating an authorisation token comprsing the received value in response to receiving the value, and means for transmitting the authorisation token to the user terminal.

The authorisation token may further comprise additional information relating to frequency bands which the user terminal is allowed to use and/or restrictions regarding spurious emissions or other operational instructions. In this way, the communication system can relax or tighten its control on the operation of a user terminal depending on the geographical location of the terminal and the time of day of its operation.

The authorisation token may be encrypted with a symmetric or asymmetric key or a combination of the two.

The operational characteristic may be for example, a measurement of power level emitted by the user terminapower received from the base station, the software configuration or synchronisation information.

The user terminal may be configured to shut down emissions completely if an authorisation token is not received or not received correctly within a pre-set time period from switch-on.

By virtue of the invention, a downloaded protocol stack is prevented from controlling emitted power levels until authorisation from the communications system (via a serving base station) has been received at the user terminal (mobile station) which incorporates the protocol stack. This is achieved by providing the mobile station with non-reconfigurable means for checking that the software protocol which has been used to send a value of a terminal characteristic to a serving base station, is working correctly.

In a preferred embodiment, a secure core module (which is not re-configurable and cannot be changed by any software download) in the mobile station analyses the spectrum emissions of the mobile station in order to control power emission, thus providing a mechanism to automatically reduce the power emission to a minimum level determined by the regulatory bodies when a mobile station does not have authorisation to emit high levels of power under the control of the communication system. The authorisation token allows a secure management of the secure core module. Preferably the mobile station contains an analogue to digital converter that converts the signal emitted by the mobile station via its antenna to a digital signal and sends it to a spectrum analyser incorporated in the secure core module. The spectrum analyser then performs detailed measurements of the mobile station's emissions in allocated and adjacent frequency bands. It utilises a spectrum mask to decide whether the emission level is compliant to standards. The spectrum mask may be modified using the authorisation token. The timer is utilised by the secure core module to set the amount of time for which the mobile station is allowed to transmit at maximum power. The authorisation token is used by the communication system to access the secure core module digital and timer. The authorisation token contains all the security information required to securely manage the secure core module and also carries additional information on allowed emission masks and commands to set the timer.

Some embodiments of the invention will now be described by way of example only with reference to the drawings of which: Figure 1 is a schematic block diagram of part of a cellular communications system including a user terminal in accordance with the invention, and Figure 2 is a flow diagram illustrating power control of a user terminal in accordance with the invention.

In Figure 1 a mobile station 1 is served by a base station 2 which forms part of a cellular communications system. The mobile station 1 has downloaded the appropriate software in order to enable it to operate within the system supported by the base station 2, and has installed the software in a module labelled software defined protocol stack 3.

A secure core module 4 has outputs which control a timer 5 and a waveform generation and power control module 6. The software defined protocol stack module 3 is also linked to the waveform generation and power control module 6 and to the secure core module 4. The output of the waveform generation and power control module 6 is fed to a transceiver 7 which is also connected to an antenna 8 and analogue to digital converter 9.

In operation, the mobile station 1 begins to transmit a signal generated by the waveform generation and power control module 6 via the transceiver 7 and antenna 8, in accordance with known procedures, for the purposes of registering with the communication system supported by the base station 2 and for setting up and receiving calls.

An open loop power control mechanism is enabled as follows. The signal from the transceiver 7 is digitised by the analogue to digital converter 9 and fed to the secure core module 4. The secure core module 4 is adapted to analyse the spectral content of the transmitted signal with reference to a spectrum mask incorporated in the secure core module 4. In particular, the secure core module 4 is able to detect spurious emissions which are outside the acceptable limits of performance of the mobile station and which, therefore, could give rise to unacceptable levels of interference within the communications system.

If the measured transmitted levels lie within the pre-determined limits defined by the mask, then the digital signal processor takes no further action and the mobile station continues to transmit its signals.

If, on the other hand, the measured transmitted levels exceed the limits, the secure core module 4 sends a command to the waveform generation and power control module 6 instructing it to reduce its output power to the appropriate levels. The secure core module 4 continues to monitor the transceiver's 7 output and makes

further adjustments whenever necessary so that emissions from the transceiver 7 and antenna 8 are kept within the limits.

Power control is handed over to the communications system (for closed loop mode operation) on completion of authentication procedure as follows. Initially and during a period of time where emissions are under the control of the secure core module 4 (as described above), the mobile station 1 instigates transmission of an authorisation token from the base station 2. The purpose of the authorisation token is to confirm that the downloaded protocol stack 3 is functioning correctly and therefore closed loop power control can be commenced. In order to detect malfunction of the protocol stack 3, the authorisation token includes some information which can be used as a check-sum on its receipt by the mobile station 1.

The secure core module 4 compares the received check-sum with a stored checksum and only if a match occurs will power control be handed over to the base station and its associated system controllers (not shown). A mismatch on the other hand, indicates that there is likely to be a malfunction of the protocol stack 3.

Whilst the mobile station's power emissions are under the control of the secure core module 4 and the mobile station 1 is awaiting receipt of the authorisation token, the timer 5 is set by the secure core module 4 in order to control the amount of time for which the waveform generation and power control module 6 outputs signals at a level defined by the secure core module 4. For example, the secure core module 4 and timer 5 can arrange for the mobile station 1 to transmit at its maximum power level in bursts, whose duration decreases with each subsequent burst. The time between consecutive bursts is also set to increase. When power control is handed over to the base station, the timer 5 is deactivated.

Some examples of the authentication procedure will now be described with reference to Figure 2. At step 10, the secure core module 4 measures the transmitted power level of the mobile station 1 and stores the measured value. At step 11, the protocol stack 3 receives the measured value from the secure core module 4. The next stage in the process requires the protocol stack 3 to report to the base station 2, via the transceiver 7 and antenna 8, the measured value. Now, if the protocol stack 3 is not functioning correctly, then the value transmitted will be an incorrect, modified value. The procedure in this case, takes the left hand fork of Figure 2 where at step 12, a modified value is transmitted to the base station 2.

This modified value is passed on to the base station's controller (not shown) which

instructs the base station 2 to send an authorisation token including the modified value (as received) to the mobile station 1 (step 13).

If on the other hand, the protocol stack 3 is functioning correctly, then the procedure takes a right hand fork of Figure 2 and the correct value is sent from the protocol stack 3 via the transceiver 7 and antenna 8 to the base station 2 (step 14) and thereon to the system controller. The system controller subsequently instructs the base station 2 to send the mobile station 1 (step 15) an authorisation token which incorporates the (correct) measured value.

At step 16 the authorisation token comprising the value for power level measurement (which may or may not have been modified by the protocol stack 3) is received at the mobile station 1. At step 17 the secure core module 4 compares the received value with the previously stored value of the power measurement. If the values are not identical then this implies that there is a malfunction of the protocol stack 3. In this case, at step 18, the secure core module 4 rejects the authorisation token and retains charge of the power control function of the mobile station 1. It may for example, reduce power emissions and allow the timer 5 to continue intermittent transmissions while a second attempt at authentication is tried.

If, however, the received and stored values are identical, then this implies that the protocol stack 3 has been installed correctly and is working satisfactorily. In this case, the secure core module 4 accepts the authorisation token, the timer 5 is deactivated and the secure core module 4 relinquishs the power control function (step 19) which can now be done in closed loop fashion under the control commands from the base station 2.

Claims

CLAIMS 1. A method of controlling power of emissions from a user terminal operating in a cellular communications system, wherein the user terminal includes a software defined protocol stack, the method including the steps of; at the user terminal, measuring a level of emissions from the user terminal, comparing the measured level with a reference level, if the measured level exceeds the reference level, adjusting the emissions to a level not exceeding the reference level, setting a timer so that emissions continue for a pre-set period of time, storing a value of an operational characteristic of the user terminal, transmitting, via the software defined protocol stack, the value to a remote base station, at the remote base station, receiving the transmitted value, generating an authorisation token comprising the received transmitted value, transmitting the authorisation token and transmitted value to the user terminal, and at the user terminal, receiving the authorisation token comprising the transmitted value, comparing the transmitted value with the stored value, if the transmitted value and the stored value are different, rejecting the authorisation token and retaining control of emissions of the user terminal, and if the transmitted value and the stored value are identical, accepting the authorisation token, de-activating the timer and relinquishing power control of the user terminal to the remote base station.
2. A method as claimed in Claim 1 and including the further step of curtailing emissions from the user terminal if an authorisation token is not received within a pre-determined period of time.
3. A method as claimed in Claim 1 and including the further step of curtailing emissions from the user terminal if an authorisation token is not received correctly within a pre-determined period of time.

<Desc/Clms Page number 11>
4. A method as claimed in any preceding Claim in which the operational characteristic is a measurement of emitted power level.
5. A method as claimed in any preceding Claim and including the further step of at the base station, encrypting the authorisation message prior to its transmission to the user terminal.
6. A method as claimed in any preceding Claim in which the authorisation token further includes operational instructions for the user terminal.
7. A method as claimed in any preceding Claim in which the operational characteristic is a measurement of received power level.
8. A user equipment capable of interfacing with a software defined protocol stack and including apparatus for controlling power emissions from the user equipment, the apparatus including: means for measuring a level of emissions from the user terminal, means for comparing the measured level with a reference level, means for adjusting the emissions to a level not exceeding the reference level if the measured level exceeds the reference level, a timer coupled to the means for adjusting for allowing emissions to continue for a pre-set period of time, a store for storing a value of an operational characteristic of the user terminal, means for transmitting, via the software defined protocol stack, the value to a remote base station, means for receiving from the remote base station, an authorisation token comprising the transmitted value, means for comparing the transmitted value with the stored value, means for rejecting the authorisation token and retaining control of emissions of the user terminal if the transmitted value and the stored value are different, and means for accepting the authorisation token, de-activating the timer and relinquishing the power control of the user terminal to the remote base station if the transmitted value and the stored value are identical.

<Desc/Clms Page number 12>
9. A user equipment as claimed in Claim 8 and further including means for curtailing emissions therefrom if an authorisation message is not received or not received correctly within a pre-determined period of time.
10. Base station apparatus for assisting in controlling power emissions from a user terminal operating in a cellular communication system, the base station apparatus including ; means for receiving from the user terminal, a value of an operational characteristic of the user terminal, means for generating an authorisation token comprising the received value in response to receipt of the value, and means for transmitting the authorisation token to the user terminal.
11. A method of controlling power of emissions from a user terminal operating in a cellular communication system, wherein the user terminal includes a software defined protocol stack, the method being substantially as hereinbefore described with reference to the drawings.
12. A user equipment substantially as hereinbefore described with reference to the drawings.
13. Base station apparatus substantially as hereinbefore described with reference to the drawings.