IMPROVEMENTS IN, OR RELATING TO, MEASURING TRAFFIC INTENSITY IN A DIGITAL MOBILE RADIO TELEPHONY

Abstract: A method of measuring traffic intensity in a GSM digital mobile radio telephony system uses a spectrum analyser to measure a radio spectrum for radio emissions from the system. The spectrum analyser employs a rapid frequency sweep so that each channel of said GSM digital mobile radio telephony system is measured during a time period which is less than a GSM time gap. The measurements are performed on the uplink radio transmissions from mobile transceivers. The method can be used to measure traffic intensity in all GSM systems carrying traffic at a given location. The method includes the following steps: setting the frequency analyser to make frequency sweeps over selected GSM channels in a GSM frequency band; setting the frequency analyser's sweep time so that for each GSM channel a signal intensity is measured at a plurality of points during a time interval which is less than one GSM TDMA time gap; and if the signal intensity measured at said plurality of points exceeds a user set threshold value, recording a measured signal as traffic.

Description: IMPROVEMENTS IN, OR RELATING TO, MEASURING TRAFFIC INTENSITY IN A DIGITAL MOBILE RADIO TELEPHONY The present invention relates to a method of, and apparatus for, measuring the traffic intensity in mobile radio telephony systems, particularly GSM mobile radio telephony systems, at given locations. The increasing use of mobile radio telephony means that there is a continuing growth of traffic in such networks. Eventual, in a given part of any network, the traffic will grow to a point at which the networks capacity to handle new traffic is at risk. This means that, from the point of view of a subscriber, the quality of service will degrade unless the syste s's traffic capacity is improved. When planning and installing new base stations to increase the traffic capacity of a mobile radio telephony system, it is of the utmost importance that the new capacity, i.e. the new base stations, should be installed where they can take over a heavy traffic load from the existing network. It should be noted that the cost of a base station, including project planning, local lease, and installation, is very high and that a badly positioned base station, which does not take the expected traffic, will not generate sufficient revenues to cover the costs associated with installation. Badly siting a base station may lead to a need for further investments in sites and base stations becoming necessary, in order to boost traffic capacity in the area concerned. When new cells, for a cellular mobile radio telephony system, such as a GSM system, are planned, it is now possible to use planning tools and make measurements to rapidly decide what coverage a new base station will have, but, at the present time, it is only possible to make assumptions about where traffic is generated. If the planner has a knowledge of where traffic is generated, he/she has a much better opportunity to make a good choice for a base station site and antenna placement. With the aid of the present invention, which measures GSM traffic in an uplink, a traffic profile for a specific area can be measured and this provides network planners with a basis for making their decision. The present invention enables traffic carried by all operators in a given area to be logged simultaneously. This gives a better statistical basis for the results and, at the same time, yields information about areas where a network operator has no coverage but his competitor's networks do. There are a number of problems which must be overcome in order to estimate traffic intensity from measurements made on the uplink in GSM, these are listed below. 1. The TDMA structure itself provides a significant difficulty to measuring D traffic intensity. The problem stems from the fact that a call occupies only an eighth of a 200 kHz GSM frequency channel. This means that the traffic intensity cannot simply be estimated by measuring the signal intensity on one frequency channel. 5 2. The GSM modulation frequency spectrum is located in a branch channel suppressed by only 18 dB. This means that a strong signal in a branch channel may be wrongly estimated as traffic in the channel concerned. Since the spectrum sweep is not synchronised to the traffic of all mobiles, because the base stations in most GSM networks are not synchronised, 0 this means that one call can be logged twice in the same sweep. 3. DTX (Discontinuous Transmission) must also be catered for in making measurements of traffic intensity. The use of this function means that the transmitter, in a mobile radio telephony system, is active only when the 5 user speaks into the mobile telephone. This, in turn, means that the signal intensity for one call will only be logged for around half the time a call is active when DTX is activated. 4. Mobiles in the measurement area, coupled to different base stations, may 30 have different power outputs. This leads to judgements of the size of the take up area, i.e. the area in which a mobile transceiver must be located in order for its traffic to be registered, being somewhat poorer than would be the case if all mobiles transmitted at the same power. According to a first aspect of the present invention, there is provided a method of measuring traffic intensity in a digital mobile radio telephony system, characterised by using a spectrum analyser to measure a radio spectrum for radio emissions at one, or more, locations using a rapid frequency sweep within a frequency band assigned to said digital mobile radio telephony system. According to a second aspect of the present invention, there is provided a method of measuring traffic intensity in a GSM digital mobile radio telephony system, characterised by using a spectrum analyser to measure a radio spectrum for radio emissions from said system, and by said spectrum analyser employing a rapid frequency sweep so that each channel of said GSM digital mobile radio telephony system is measured during a time period which is less than a GSM time gap. Said method may be adapted to measure traffic intensity in all GSM systems carrying traffic at a given location at which measurements are performed. Uplink radio transmissions from mobile transceivers may be measured using said spectrum analyser. Said method may include the following steps: setting said frequency analyser to make frequency sweeps over selected GSM channels in a GSM frequency band; setting said frequency analyser's sweep time so that for each GSM channel a signal intensity is measured at a plurality of points during a time interval which is less than one GSM TDMA time gap; and if the signal intensity measured at said plurality of points exceeds a user set threshold value, recording a measured signal as traffic. Said plurality of points may be seven points. Said user set threshold value may be varied to vary an area of take up for radio emissions from mobile transceivers. Sweeps of said spectrum analyser may be synchronised to said GSM TDMA time gaps and traffic may be measured on each channel in a total of eight sweeps by said spectrum analyser. Said spectrum analyser sweeps may not be synchronised to said GSM TDMA gaps and traffic may be measured on each channel by making a large number of sweeps with said frequency analyser. A detected signal intensity in two adjacent channels may be recorded as a single call and a calibration factor, K1, may be employed to correct measurements of traffic intensity to allow for cases in which two adjacent channels carry separate calls. A calibration factor, K2, may be used to correct for under reading of traffic intensity caused by discontinuous transmission. K2 may be set equal to 2. Said calibration factors, K1 and K2, may be calibrated for a network under investigation by performing measurements in a network cell with limited propagation and placing an antenna connected to said spectrum analyser near an antenna of a base station serving said cell. Said method may be applied to measure traffic intensity at a large number of points in a selected area thereby facilitating production of a traffic profile for said area. According to a third aspect of the present invention, there is provided apparatus for measuring traffic intensity in a GSM digital mobile radio telephony system, characterised in that said apparatus includes a spectrum analyser, connected via a radio receiver to a receiving antenna, for measuring the radio spectrum of emissions from said system and sweep means for causing said spectrum analyser to rapidly sweep through frequencies in a GSM radio band so that each channel of said GSM digital mobile radio telephony system is measured during a time period which is less than a GSM time gap. Said apparatus may be adapted to measure traffic intensity in all GSM systems carrying traffic at a given location at which measurements are performed. Said apparatus may be adapted to measure uplink radio transmissions from mobile transceivers. Said sweep means may be adapted to make frequency sweeps over selected GSM channels in a GSM frequency band; said frequency analyser's sweep time may be adapted to be set so that each GSM channel is measured at a plurality of points during a time interval which is less than one GSM TDMA time gap; and detector means may be provided to determine whether, or not, the signal intensity measured at said plurality of points exceeds a user set threshold value. Means may be provided to vary said user set threshold value so that an area of take up for radio emissions, from mobile transceivers, can be varied. Synchronising means may be provided to synchronise sweeps of said spectrum analyser to said GSM TDMA time gaps and said spectrum analyser and sweep means may be adapted to measure traffic on each channel in a total of eight sweeps of said spectrum analyser. A detected signal intensity in two adjacent channels may be recorded as a single call and first compensation means may be provided to apply a calibration factor K1 to correct measurements of traffic intensity to allow for cases in which two adjacent channels carry separate calls. Second compensation means may be provided to apply a calibration factor, K2, to correct for under reading of traffic intensity caused by discontinuous transmission. K2 may be set equal to 2. Calibration means may be provided to facilitate calibration of said apparatus for a network under investigation, by performing measurements in a network cell with limited propagation, said antenna being locatable near an antenna of a base station serving said cell. Said apparatus may be adapted to measure traffic intensity at a large number of points in a selected area thereby facilitating production of a traffic profile for said area. According to a fourth aspect of the present invention, there is provided a method of selecting a site for a GSM base station from a plurality of possible base station sites characterised by basing said selection, at least in part, on data derived from the method, as set out in any preceding paragraph, or data derived by use of the apparatus as set out in any preceding paragraph. According to a fifth aspect of the present invention, there is provided a method of determining antenna placement for a GSM base station, characterised in that the method set out in any preceding paragraph, or the apparatus set out in any preceding paragraph, is used to measure traffic intensity for at least some possible antenna placements. Embodiments of the invention will now be described, by way of example, with reference to measurement of traffic intensity in a GSM mobile radio telephone system. The measurement technique of the present invention is based on using a spectrum analyser to measure the radio spectrum in the GSM band. The spectrum sweep is made sufficiently fast so that each channel is measured during a time period of a duration which is no longer than a GSM time gap, i.e. 0.577 ms. As mentioned in the introduction to this patent specification, the method of measurement of the present invention must overcome four basic problems. The manner in which these problems are overcome in the present invention is discussed below. The first problem to be overcome relates to the intrinsic structure of the GSM TDMA frame structure. Because a call occupies only an eighth of a 200 kHz GSM frequency channel, the traffic intensity cannot simply be estimated by measuring the signal intensity on one frequency channel. However, by using the following method steps, this problem is adequately overcome. The TDMA radio transmission in the uplink on a frequency channel is measured using the following methodology: 1. The spectrum analyser is set so that it makes frequency sweeps over the required channels in the frequency band. 2. The sweep time is set so that each 200 kHz channel is measured in, for example, 7 points, during a time interval which is one TDMA time gap long, i.e. 0.577 ms. This implies that one sweep contains information as to whether there is radio signal traffic over one time gap for each frequency channel in the selected frequency range. A 200 kHz GSM channel is thus measured in 0.577 ms, i.e. in a time interval which is one time gap long, after which the following GSM channel is measured in the next 0.577 ms, etc.. 3. If the signal intensity in the measured values exceeds a threshold value, as selected by the user, the measured signal is recorded as traffic and logged in a file. By varying the threshold value, the area of the take up area can be varied. Provided that the sweeps can be synchronised to time gaps in the TDMA frame structure, the traffic on each frequency channel can be measured with eight sweeps. An absence of synchronisation can be compensated for by making a large number of sweeps, since an arbitrary time gap is encountered during each sweep. That is to say, if a sufficiently large number of sweeps are made, measurements will be performed over each time gap for about one eighth of the sweeps. As previously noted, the GSM modulation frequency spectrum is in a branch channel suppressed by only 18 dB. This means that a strong signal in a branch channel may be wrongly estimated as traffic in the channel concerned. Since the spectrum sweep is not synchronised to the traffic of all mobiles, because the base stations in most GSM networks are not synchronised, there is a risk that one call can be logged twice in the same sweep. To avoid one signal during one sweep being recorded as a call twice, in two branch channels, the signal intensity in two adjacent channels is recorded as one call. This means that a call may not always be recorded. This problem increases, somewhat, with traffic loading on surrounding base stations. However, it can be compensated by using a calibration factor K1. GSM mobile telephony systems employ DTX (Discontinuous Transmission). The use of this function means that a mobile transmitter is active only when the user speaks into the mobile telephone. This means that logging of the signal intensity for one call is performed for only around half the time when DTX is activated. The error introduced by DTX into the measurements, i.e. the channel not being used the whole time, can be compensated by introducing a calibration factor K2, which scales up the measured value to give the correct traffic intensity. Experience suggests that a caller speaks for around half the time, which means that a value of 2 is reasonable for K2. Mobiles in the measurement area may be coupled to different base stations and may, consequently, have different outputs. This means that judgements relating to the size of the take up area may be suspect. Simulations have, however, shown that this phenomenon has little effect on the possibility of hitting a high traffic point in the network. The simulations showed that the traffic intensity levels and resolution only changed to a minor degree. To make a good estimate of traffic in an area, the compensation factors must be calibrated for the network concerned. Calibration can be performed on a cell with limited propagation, where the measurement apparatus of the present invention has its antenna placed near the antenna of the base station concerned. By varying the threshold value, the take up area for the measurement can be selected. A high threshold value gives information on a small area, since only the very closest mobiles give a sufficiently high signal intensity, whereas for a low thresh ld value traffic in a larger area is recorded. The present invention has three principle applications, namely: - by measuring the traffic intensity at a large number points in one area, a traffic profile for that area can be created which makes it possible to see where traffic is generated. use of the present invention can facilitate decisions on the choice between a few conceivable sites, if the designer has a number of places available for a new base station site, the present invention can be used to provide information on which of the available BTS sites will take most traffic. if a site has been selected, antenna placement may be decisive in how the cell appears, the present invention can be used to examine the traffic intensity for different antenna placements. An operator can use the present invention to record the traffic intensity in his own and/or his competitors' TDMA networks (GSM). This has not previously been possible for a technique in which the signal intensity is logged with a measuring receiver, or spectrum analyser. During measurement in analogous systems, it is sufficient to judge the signal intensity on different channels to estimate the traffic intensity. Details of the measuring apparatus have been described herein in functional terms, it will be readily apparent to those skilled in the art how to construct and use measuring equipment according to the present invention from the description thereof provided herein.

Claims: CLAIMS 1 A method of measuring traffic intensity in a digital mobile radio telephony system, characteπsed by using a spectrum analyser to measure a radio spectrum for radio emissions at one, or more, locations using a rapid frequency sweep within a frequency band assigned to said digital mobile radio telephony system 2 A method of measuring traffic intensity in a GSM digital mobile radio telephony system, characterised by using a spectrum analyser to measure a radio spectrum for radio emissions from said system, and by said spectrum analyser employing a rapid frequency sweep so that each channel of said GSM digital mobile radio telephony system is measured during a time period which is less than 3 A method, as claimed in either claim 1, or 2, characterised by said method being adapted to measure traffic intensity in all GSM systems carrying traffic at a given location at which measurements are performed 4 A method, as claimed in either claim 2, or claim 3 when dependent on claim 2, characteπsed by measuπng uplink radio transmissions from mobile transceivers using said spectrum analyser 5 A method as claimed in either claim 2, or claim 3 when dependent on claim 2, or claim 4, characterised by said method including the following steps - setting said frequency analyser to make frequency sweeps over selected GSM channels in a GSM frequency band, setting said frequency analyser's sweep time so that for each GSM channel a signal intensity is measured at a plurality of points during a time interval which is less than one GSM TDMA time gap, and - if the signal intensity measured at said plurality of points exceeds a user set threshold value, recording a measured signal as traffic. 6. A method, as claimed in claim 5, characterised by said plurality of points being seven points. 7. A method, as claimed in claim 5, or 6, characterised by varying said user set threshold value to vary an area of take up for radio emissions from mobile transceivers. 8. A method, as claimed in any of claims 5 to 7, characterised by synchronising sweeps of said spectrum analyser to said GSM TDMA time gaps and by measuring traffic on each channel in a total of eight sweeps by said spectrum analyser. 9. A method, as claimed in any of claims 5 to 7, characterised by said spectrum analyser sweeps not being synchronised to said GSM TDMA gaps and by measuring traffic on each channel by making a large number of sweeps with said frequency analyser. 10. A method, as claimed in of claims 5 to 9, characterised by recording a detected signal intensity in two adjacent channels as a single call and employing a calibration factor, K1 , to correct measurements of traffic intensity to allow for cases in which two adjacent channels carry separate calls. 11. A method, as claimed in any of claims 5 to 10, characterised by using a calibration factor, K2, to correct for under reading of traffic intensity caused by discontinuous transmission. 12. A method, as claimed in claim 10, characterised by setting K2 equal to 2. 13. A method as claimed in any of claims 5 to 11 , characterised by calibrating said calibration factors, K1 and K2, for a network under investigation by performing measurements in a network cell with limited propagation and placing an antenna connected to said spectrum analyser near an antenna of a base station serving said cell. 14. Apparatus for measuring traffic intensity in a GSM digital mobile radio telephony system, characterised in that said apparatus includes a spectrum analyser, connected via a radio receiver to a receiving antenna, for measuring the radio spectrum of emissions from said system and sweep means for causing said spectrum analyser to rapidly sweep through frequencies in a GSM radio band so that each channel of said GSM digital mobile radio telephony system is measured during a time period which is less than a GSM time gap. 15. Apparatus, as claimed in claim 14, characterised in that said apparatus is adapted to measure traffic intensity in all GSM systems carrying traffic at a given location at which measurements are performed. 16. Apparatus, as claimed in either claim 14, or claim 15, characterised in that said apparatus is adapted to measure uplink radio transmissions from mobile transceivers. 17. Apparatus as claimed in any of claims 14 to 16, characterised in that said sweep means is adapted to make frequency sweeps over selected GSM channels in a GSM frequency band; in that said frequency analyser's sweep time is adapted to be set so that each GSM channel is measured at a plurality of points during a time interval which is less than one GSM TDMA time gap; and in that detector means are provided to determine whether, or not, the signal intensity measured at said plurality of points exceeds a user set threshold value. 18. Apparatus, as claimed in claim 17, characterised in that means are provided to vary said user set threshold value so that an area of take up for radio emissions, from mobile transceivers, can be varied. 19. Apparatus, as claimed in either claim 17 or 18, characterised in that synchronising means are provided to synchronise sweeps of said spectrum analyser to said GSM TDMA time gaps and in that said spectrum analyser and sweep means are adapted to measure traffic on each channel in a total of eight sweeps of said spectrum analyser. 20. Apparatus, as claimed in any of claims 17 to 19, characterised in that a detected signal intensity in two adjacent channels is recorded as a single call and in that first compensation means are provided to apply a calibration factor K1 to correct measurements of traffic intensity to allow for cases in which two adjacent channels carry separate calls. 21. Apparatus, as claimed in any of claims 17 to 20, characterised in that second compensation means are provided to apply a calibration factor, K2, to correct for under reading of traffic intensity caused by discontinuous transmission. 22. Apparatus, as claimed in claim 21 , characterised in that K2 is set equal to 2. 23. Apparatus, as claimed in any of claims 20 to 22, characterised in that calibration means are provided to facilitate calibration of said apparatus for a network under investigation, by performing measurements in a network cell with limited propagation, said antenna being locatable near an antenna of a base station serving said cell. 24. A method, as claimed in any of claims 2 to 13, characterised by said method being applied to measure traffic intensity at a large number of points in a selected area thereby facilitating production of a traffic profile for said area. 25. Apparatus, as claimed in an of claims 14 to 23, characterised in that said apparatus is adapted to measure traffic intensity at a large number of points in a selected area thereby facilitating production of a traffic profile for said area. 26. A method of selecting a site for a GSM base station from a plurality of possible base station sites characterised by basing said selection, at least in part, on data derived from the method claimed in any of claims 2 to 13, or data derived by use of the apparatus as claimed in any of claims 14 to 23. 27. A method of determining antenna placement for a GSM base station, characterised in that the method claimed in any of claims 2 to 13, or the apparatus as claimed in any of claims 14 to 23, is used to measure traffic intensity for at least some possible antenna placements.

Inventors: OLSSON BO, WIDELL SVANTE, WINROTH MATS OLOF, ZETTERBLAD JOHAN, OLSSON, BO, WIDELL, SVANTE, WINROTH, MATS, OLOF, ZETTERBLAD, JOHAN
Applicants: OLSSON BO, TELIA AB, WIDELL SVANTE, WINROTH MATS OLOF, ZETTERBLAD JOHAN, OLSSON, BO, TELIA AB (PUBL), WIDELL, SVANTE, WINROTH, MATS, OLOF, ZETTERBLAD, JOHAN
Application year: 1998
Main classification: H04Q7
IPC codes: H04B17, H04W24