WO1992011736A1

WO1992011736A1 - Quality check for a mobile cellular radio system

Info

Publication number: WO1992011736A1
Application number: PCT/SE1991/000870
Authority: WO
Inventors: Harald Kallin; Stig Roland Bodin
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1990-12-21
Filing date: 1991-12-17
Publication date: 1992-07-09
Also published as: DE4193285T; GB9217860D0; GB2257336A; JPH05505925A; CA2075984A1

Abstract

In a mobile cellular radio system, when setting up a new call or when handing over an existing call to a new traffic channel, it is necessary to avoid disturbances on the new traffic channel which can cause poor transmission quality. In order to avoid unnecessary disturbances on the new traffic channel, the signal strength of the mobile station requesting a new channel is measured. In addition, the disturbance levels on the available traffic channels are also measured and then compared with the signal strength of the mobile station requesting a channel. Based on the comparison, a traffic channel is assigned to the mobile station so that the disturbance level on the traffic channel is at least less than or equal to a minimum threshold value.

Description

QUALITY CHECK FOR A MOBILE CELLULAR RADIO SYSTEM

_f FIELD OP THE INVENTION

* The present invention relates generally to cellular radio systems having channels for transmitting information between base

5 ^■ stations and mobile stations. More precisely, the invention relates to a method for selecting and assigning traffic channels on the basis of signal strength and signal disturbance,

BACKGROUND OF THE INVENTION

In cellular mobile radio systems, it is fundamental that a mobile 10 station with an established connection on a radio channel shall be able to maintain the established connection when moving from one cell serviced by a base station to another cell serviced by another base station. It is also highly desirable that the mobile station with an established connection on a radio channel 15 shall be able to maintain the established connection when moving within the same cell and when the channel which is used is subject to increased interference. The process by which a mobile station can maintain an established connection when moving in a cellular radio system is generally called a hand off.

20 In general, radio communication is only possible when the desired information-carrying radio signals have sufficient signal strength and are sufficiently strong relative to the noise and interfering radio signals at the receiver. The minimum strength, of course, depends on particular features of the system, e.g.,

25 the kind of modulation and the type of receiver. In order to insure that the established connection may continue on an intended radio channel between a mobile station and an intended base station, the handoff process includes measurement of the parameters of the radio signals at the intended base station

30 and/or at the mobile station. The first cellular mobile systems placed in public use were analog systems typically used for speech and other types of analog information. The systems include multiple radio channels for transmitting analog in¬ formation between base and mobile stations by transmitting analog modulated radio signals. In general, first cellular mobile radio systems had relatively large cells, and the signal measurements in the handoff process in such systems were performed by the base station. One such system is the Nordic Mobile Telephone System, NMT 450. Another known cellular mobile radio system is the AMPS mobile radio system in the United States. An excellent general description of a mobile cellular radio system can be found in a publication entitled "CMS 88 Cellular Mobile Telephone System" published by Ericsson Telecom AB, 1988. The rapidly increasing usage of these radio systems often cause the cells to be utilized at maximum capacity.

In a cellular mobile radio system, it is important when setting up a call or when handing over a call to a new traffic channel to assign a traffic channel which will not have transmission problems. In existing systems, poor transmission quality is prevented by sealing off channels, i.e., not using traffic channels which have disturbances above a predetermined level. However, the simplicity of this method decreases the call handling capacity of the system since all channels which have disturbances greater than a predetermined level are sealed off regardless of the signal strength between the requesting mobile station and the base station. In addition, poor transmission quality can result if the signal of the requesting mobile station has a relatively low signal strength.

SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of the prior art by choosing a traffic channel on which the disturbance is proportionally less than the signal strength of the requesting mobile station. In the present invention, the signal strength of the requesting mobile station is measured when the mobile station requests a traffic channel between the base station and the mobile station. A computer then determines whether there are any available traffic channels in the cellular mobile system. If there are no available traffic channels in the cellular system, the request for a traffic channel by the mobile station is denied unless the system contains sealed traffic channels. If the system contains sealed traffic channels, a sealed traffic channel is temporarily unsealed thereby becoming an available traffic channel.

In one embodiment of the present invention, a traffic channel is selected and the disturbance level on the traffic channel is determined. A comparison is then made between the signal strength of the requesting mobile station and the disturbance level on the traffic channel to determine if the disturbance level on the traffic channel is below a minimum threshold value. If the disturbance level on the traffic channel is below the signal strength of the requesting mobile station minus the minimum threshold value, then the requesting mobile station is assigned to the selected traffic channel. However, if the disturbance level on the selected traffic channel is above the minimum threshold value, then another traffic channel is selected. This cycle continues until a traffic channel is found with a disturbance level below the signal strength of the requesting mobile station minus the minimum threshold value.

In a second embodiment of the present invention, the disturbance level is measured for all available traffic channels. The traffic channels are then ordered according to their disturbance level. The signal strength of the requesting mobile station is then compared with the disturbance levels of the traffic channels to determine if at least one traffic channel has a disturbance level below a minimum threshold value. The minimum threshold value can fluctuate depending upon the traffic activity in the cellular mobile system. If at least one traffic channel has a disturbance level below the signal strength of the requesting mobile station minus the minimum threshold value, the requesting mobile station is assigned to the traffic channel which has a disturbance level closest to the signal strength of the reques- ting mobile station minus the minimum threshold value or any other predetermined order, e.g., the furthest away from the minimum threshold. However, if all the available traffic channels have disturbance levels greater than the signal strength of the requesting mobile station minus the minimum threshold value, then the computer measures the disturbance levels on all the available tra fic channels again and continues to do so until a traffic channel is found to be compatible with the signal strength of the requesting mobile station wherein the signal strength is continuously monitored by the system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a portion of a cellular mobile radio system having cells, a mobile switching center, base stations and mobile stations.

Figure 2 illustrates a flow chart of a subroutine used by computer in one embodiment of the present invention. Figure 3 illustrates a flow chart of a subroutine utilized by a computer for another embodiment of the present invention.

DETAILED DESCRIPTION

Figure 1 illustrates ten cells C1-C10 in a cellular mobile radio system. Normally, a cellular mobile radio system according to the present invention would be implemented with more than ten cells. However, for the purposes of simplicity, the present invention can be explained using the simplified representations illustrated in Figure 1.

For each cell C1-C10, there is a base station B1-B10, with the same reference number as the corresponding cell. Figure 1 illustrates the base stations as situated in the vicinity of the cell center and having omni-directional antennas. The cells Cl- C10 are, therefore, schematically represented as hexagons. The base stations of adjacent cells may, however, be located in the vicinity of cell borders and have directional antennas as is well known to those skilled in the art. Figure 1 also illustrates nine mobile stations M1-M9, movable within a cell and from one cell to another. In a typical cellular radio system there would normally be more than nine cellular mobile stations. In fact, there are typically many times the number of mobile stations as there are base stations. However, for the purpose of explaining the invention, the reduced number of mobile stations is sufficient.

Also illustrated in Figure 1 is a mobile switching center MSC. The mobile switching center MSC illustrated in Figure 1 is connected to all ten base stations B1-B10 by cables. The mobile switching center (MSC) is also connected to cables to a fixed public switching telephone network or similar fixed network. All cables from the mobile switching center MSC to the base station B1-B10 and cables to the fixed network are not illustrated.

In addition to the mobile switching center MSC illustrated, there may be another mobile switching center connected by cables to base stations other than those illustrated in Figure 1. Instead of cables, other means, for example, fixed radio links may be used for connecting base stations B1-B10 to the mobile switching center. The mobile switching center MSC, the base stations Bl- B10, and the mobile stations M1-M9 are all computer controlled.

The cellular mobile radio system illustrated in Figure 1 includes a plurality of radio channels for communication. Such systems can be designed for either analog information on digital information and the present invention is applicable to either analog or digital systems, but in the following description an analog system is assumed. Some of the radio channels are used for control channels, and each base station B1-B10 has at least one control channel. Normally a control channel is not used for the transfer of speech information. Control channels are typically used for monitoring and controlling mobile stations during the set up of the connection and during the registration of a mobile (i.e., when the mobile reports to the land system in which it is located) . Each cell is always equipped with a signal strength receiver which consists of a receiver and a control unit. The signal strength receiver is typically the same design as the receiver used for each traffic channel. The signal strength receiver in each cell performs cyclical measurements, sampling the radio frequencies received from the mobile stations. All the system frequencies may be sampled but only the traffic channel frequency allocated to mobile stations in the neighboring cells are of interest for handoff. The information about which channel should be taken under consideration, during the above mentioned sampling is originally received from the MSC. The measurement results are updated in the control unit as a mean value after cyclical sampling. In this way, each cell knows what the transmission parameters within a mobile station currently using the neighbor's traffic channel would be if the cell in question would have to take over the transmission. If a handoff has been requested by a cell, the MSC will ask the neighboring cell to send the measurement results of the signal strengths from the mobile station.

Referring now to Figure 2, a simplified flow chart illustrates the subroutine of a computer located at either the MSC or a base station, for one embodiment of the present invention. The computer supervises the measurement of the signal strength from a mobile station that is requesting access to the base station. The signal can represent a new call or a preexisting call needing a handoff. The signal strength of the mobile requesting access to the base station may need to be adjusted for expected changes in the received signal strength on the traffic channel. For instance, the power level used by the mobile station on a control channel may be different than the power level the mobile station is expected to use on a traffic channel. For example, a mobile station may contact the base station using a control channel which has a power level 3 corresponding to -6dB in an AMPS system. The traffic channels in the cell may however allow a power level of 2 which corresponds to -2dB . As a result, the signal strength of the requesting mobile station should be increased by 4dB. In another example, a mobile station may have a very high signal strength. After the call set-up or the handoff, the Mobile Station Power Regulation Control in the candidate traffic channel may reduce the output power of the mobile station, hence reducing the received signal strength.

The computer then causes the traffic channels in the system to be scanned in order to determine whether there are any traffic channels available to handle the requesting mobile station in step 103. An available traffic channel is any unsealed channel which is not in use in a cellular system which seals channels with disturbance levels over a predetermined level or any channel which is not already in use in a cellular system which does not seal channels with disturbance levels over a predetermined level. If there are no available traffic channels in the system, the computer checks to see if there are any sealed channels in step 105. If there are no sealed channels, the computer then denies access to the requesting mobile station in step 107 and the request is terminated. However, if the computer determines that there are channels in the system which have been sealed, the computer selects a sealed traffic channel and unseals the traffic channel thereby making it available to handle incoming calls and handoffs in step 111. The computer selects a sealed traffic channel by selecting the traffic channel which has been idling the longest, the traffic channel which has the lowest disturbance level or at random.

After the computer determines that at least one traffic channel is available for handling incoming calls in step 103, the computer selects one of the available traffic channels in step 113. The computer can select an available traffic channel either at random or by selecting the channel that has been idling the longest. The computer then supervises the measurement of the strength of any disturbance on the selected traffic channel in step 115. The disturbance level is preferably measured by sampling the traffic channel for 300 ms and calculating an average disturbance level. The strength of the disturbance on the selected traffic channel is then compared with the signal strength of the requesting mobile station in step 117. If in step 119 the strength of the disturbance level is less than the signal strength of the mobile station requesting access by at least a predetermined minimum threshold value, then the computer assigns the requesting mobile station to the traffic channel, thereby completing the request in step 123. If the computer determines in step 119 that the selected traffic channel has a disturbance level greater than the signal strength of the requesting mobile station minus the minimum threshold value, the computer returns to step 103 wherein the computer checks to see if there are any available traffic channels. This cycle continues until a traffic channel is found which is compatible with the requesting mobile station or the requesting mobile station is terminated.

Referring now to Figure 3, a simplified flow chart illustrates the subroutine of a computer, located at the MSC or a base station, for another embodiment of the present invention. The computer first supervises the measurement of the signal strength of a mobile station requesting access to a traffic channel in step 151. The signal strength of the mobile reques¬ ting access to the base station may need to be adjusted for expected changes in the received signal strength on the traffic channel. For instance, the power level used by the mobile station on a control channel may be different than the power level the mobile station is expected to use on a traffic channel. For example, a mobile station may contact the base station using a control channel which has a power level 3 corresponding to - 6dB in an AMPS system. The traffic channels in the cell may however allow a power level of 2 which corresponds to -2dBW. As a result, the signal strength of the requesting mobile station should be increased by 4dB. In another example, a mobile station may have a very high signal strength. After the call set-up or the handoff, the Mobile Station Power Regulation Control in the candidate traffic channel may reduce the output power of the mobile station, hence reducing the received signal strength. The computer then determines whether there are any available traffic channels in the system in step 153. An available traffic channel is any unsealed channel which is not in use in a cellular system which seals channels with disturbance levels over a predetermined level or any channel which is not already in use in a cellular system which does not seal channels with disturbance levels over a predetermined level. If the computer determines that there are no available traffic channels in the system at the time of the request, the computer determines whether there are any sealed channels in the system in step 155. If there are no sealed channels in the system, the computer denies access to the mobile station requesting access to a traffic channel in step 157. However, if the computer determines that there are sealed channels in step 155, the computer selects a sealed traffic channel and unseals the traffic channel in step 161 thereby making the channel available to handle incoming calls and handoffs. The computer selects a sealed traffic channel by selecting the traffic channel which has been idling the longest, the traffic channel which has the lowest disturbance level or at random.

After the computer determines that at least one traffic channel is available in step 153, the computer measures the strength of the disturbance levels on all of the available traffic channels in the system in step 163. The disturbance levels on each of the traffic channels are then arranged according to their disturbance levels from lowest to highest in step 165.

In step 167, the computer monitors the amount of traffic activity occurring in the system at the time the request for a traffic channel is made. The computer in step 169 assigns a minimum threshold value for the difference between the signal strength of the requesting mobile station and the disturbance level on the selected traffic channel depending upon the amount of traffic activity in the system. The level of traffic activity is determined as a function of the total number of channels in the cell versus the number of channels already in use. For example, if traffic activity in the system is normal when a mobile station requests access to a traffic channel, the computer will set the minimum threshold value at substantially 18 decibels. However, when the traffic activity in the system is relatively low, the minimum threshold value can be set at substantially 20 decibels while when the traffic activity in the system is relatively high the minimum threshold value can be set at substantially 16 decibels. In step 171, the computer then compares the signal strength of the requesting mobile station with the disturbance levels of all the traffic channels. In step 171, the computer then assigns the mobile station to a traffic channel which has a disturbance level equal to or less than the signal strength of the mobile station minus the minimum threshold value. However, if all the measured traffic channels have disturbance levels greater than the signal strength of the mobile station minus the minimum threshold value, the program returns to step 153 to determine whether there are any new traffic channels available. This cycle continues until a traffic channel is found which is compatible with the requesting mobile station or the request is terminated.

While the invention has been described in its preferred embodiments, it is to be understood that the words that have been used are words of description rather than of limitation and that changes within the purview of the amended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

Claims

WE CLAIM:

1. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations, comprising the steps of: determining the signal strength of a signal between the base station and the requesting mobile station; measuring the disturbance level on at least one available traffic channel; comparing the signal strength of the requesting mobile station with the disturbance level on the available traffic channel; and assigning the requesting mobile station to a traffic channel which has a disturbance level below the signal strength of the requesting mobile station minus a minimum threshold value.

2. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein the disturbance level is measured on all available traffic channels.

3. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 2, wherein the requesting mobile station is assigned a traffic channel which has a disturbance level closest to but below the signal strength of the requesting mobile station minus the minimum threshold value.

4. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 2, wherein the requesting mobile station is assigned a traffic channel which has a disturbance level the farthest below the signal strength of the requesting mobile station minus the minimum threshold value.

5. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein the disturbance levels on the available traffic channels are measured one at a time until a traffic channel is found which has a disturbance level equal to or less than the signal strength of the requesting mobile station minus the minimum threshold value.

6. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 5, wherein the available traffic channels are selected at random.

7. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 5, wherein the available traffic channels are selected according to which channels have been idling the longest.

8. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein the signal strength of the requesting mobile station is adjusted for expected changes in the signal strength of the requesting mobile station.

9. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said minimum threshold value decreases as traffic intensity in a cell or a relevant area near a cell increases.

10. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said minimum threshold value is set at substantially 20 decibels when a cell or a relevant area near a cell is operating under relatively low traffic conditions.

11. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said minimum threshold value is set at substantially 18 decibels when a cell or a relevant area near a cell is operating under normal traffic conditions.

12. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said minimum threshold value is set at substantially 16 decibels when a cell or a relevant area near a cell is operating under relatively heavy traffic conditions.

13. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said signal strength of the requesting mobile station is measured at the base station.

14. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said signal strength of the requesting mobile station is measured at the mobile station.

15. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said disturbance level of a traffic channel is measured at a base station.

16. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said disturbance level of a traffic channel is measured at a mobile station.

17. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, wherein said disturbance level of a traffic channel is measured at a mobile station and at a base station.

18. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 17, wherein the signal strength of the requesting mobile station is compared with said disturbance levels of a traffic channel measured at the mobile station and the base station.

19. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 1, further comprising a step of sealing off all traffic channels which have a predetermined disturbance level.

20. A method for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations according to claim 19, further comprising a step of unsealing a traffic channel when there are no available traffic channels in the cellular system.

21. An apparatus for selecting and assigning traffic channels in a cellular mobile radio system having at least one base station and a plurality of mobile stations, comprised of: a first measuring means for determining the signal strength of a signal between the base station and a requesting mobile station; a secondmeasuring means for determining the disturban¬ ce level on at least one available traffic channel; comparing means for comparing the signal strength to the requesting mobile station with the disturbance level on the available traffic channel; and assigning means for assigning the requesting mobile station to a traffic channel which has a disturbance level below the signal strength of the requesting mobile station minus a minimum threshold value.

22. An apparatus according to claim 21, wherein said minimum threshold value decreases as traffic intensity in a cell or a relevant area near a cell increases.

23. An apparatus according to claim 21, wherein said minimum threshold value is set at substantially 20 decibels when a cell or a relevant area near a cell is operating under relatively low traffic conditions.

24. An apparatus according to claim 21, wherein said minimum threshold value is set at substantially 18 decibels when a cell or a relevant area near a cell is operating under normal traffic conditions.

25. An apparatus according to claim 21, wherein said minimum threshold value is set at substantially 16 decibels when a cell or a relevant area near a cell is operating under relatively heavy traffic conditions.

26. An apparatus according to claim 21, which further includes means for sealing a traffic channel subject to disturbances, and means for unsealing a traffic channel when there is an increased demand for traffic channels.