WO1995022876A1

WO1995022876A1 - Quality check for a mobile cellular radio system

Info

Publication number: WO1995022876A1
Application number: PCT/SE1994/000128
Authority: WO
Inventors: Harald Kallin; Roland Bodin
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1994-02-16
Filing date: 1994-02-16
Publication date: 1995-08-24
Also published as: AU6387494A

Abstract

In a mobile cellular radio system, 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 at the mobile station and the base station. In addition, the disturbance levels on the available traffic channels are also measured at the mobile station and at the base station and then are compared with the signal strength of the mobile station requesting a channel. Based on the comparison, the mobile station is assigned to a traffic channel which meets or exceeds a predetermined set of conditions.

Description

QUALITY CHECK FOR A MOBILE CELLULAR RADIO SYSTEM

FIELD OF THE INVENTION

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

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., 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 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 information 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 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 distur¬ bance 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 deter¬ mines 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 tem¬ porarily unsealed thereby becoming an available traffic channel. In one embodiment of the present invention, a first signal strength of a signal between a base station and a requesting mobile station is measured at the base station. It is then determined whether there are any available traffic channels in the base station. If a traffic channel is not available, the request is terminated. However, if at least one traffic channel is available, up to a predetermined number of the available traffic channels are selected to form a first set of available traffic channels. The requesting mobile station then measures the disturbance levels on each of the selected traffic channels. The signal strength of the signal between the base station and the mobile station is then measured at the mobile station. The disturbance levels on the selective channels is then measured at the base station. Finally, the signal strength and the disturbance levels measured at the base station are compared and the signal strength and the disturbance levels measured at the mobile station are compared, and the reques ing mobile station is assigned to a channel if the channel meets a predetermined set of conditions based upon the comparisons. In another 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 stationminus 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 another 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 reques¬ ting 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 requesting 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 traffic 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 simplified block diagram of a base station with four channels;

Figure 3 illustrates a mobile station operating in a cellular system;

Figure 4 illustrates a flow chart of a subroutine utilized by the cellular system in one embodiment of the present invention;

Figure 5 illustrates a flow chart of a subroutine used by computer in one embodiment of the present invention.

Figure 6 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 c lular 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 C1-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 suf- ficient.

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 il¬ lustrated 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 B1-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 connec¬ tion 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 con¬ sideration, 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 cr.iestion would have to take over the transmission. If a hanc ff 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.

Figure 2 illustrates a base station 20 with four radio channels. Normally, a base station according to the present invention would be implemented with more than four radio channels. However, for the purposes of simplicity, the invention can be explained using the simplified represen¬ tations illustrated in Figure 2. In this example, the channel operating on frequency f₀ receives signals from a mobile station 22. As a result, the base station 10 can measure the signal strength of the signal received from the mobile station 22. The signal level is labeled C^^ for the carrier mobile-to-base station on frequency f₀.

The other three channels in the base station 20 are available, i.e. , not presently in use, and operate on frequen¬ cies f_1# f₂, and f₃. As long as these channel are not in use, the base station can measure the interference levels on each of the channels. The interference can be caused by a variety of factors including: other mobile stations (not illustrated) which are operating on co-channels in other cells; inter- modulation products, spurious transmissions from other radio equipment; and other man-made noises, such as car ignition, factories, electric trains, etc... The interference levels measured at the base station 20 are called I_πi._f1, I_rnbf2r and I^^. Figure 3 illustrates a mobile station 22 which is receiving a variety of signals. In this example, the mobile station 22 is communicating with a base station 20 on a channel with a frequency f₀. After receiving a signal on the channel, the mobile station can measure the signal strength of the received signal. The measured signal strength is labelled ^C _mbfo for the carrier from base station-to-mobile on frequency f₀. Furthermore, the mobile station 22 can measure the signal strength of a number of carriers/frequencies. As a result, the mobile station 22 can measure the signal strengths of the channels operating on frequencies f₁, f₂, and f₃.

In the present embodiment of the present invention, the cellular system uses the above-described principles during a handoff procedure to ensure that a handoff is only made to a channel with acceptable quality.

Referring now to Figure 4, 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 signal strength of a signal from a mobile station that is requesting access to a base station is measured at the base station in step 201. The signal repre¬ sents a request for a handoff. the measured signal strength is labelled C_mbfo for the carrier from mobile-to-base station on frequency f₀. 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 tra fic channel. For example, a mobile station may contact the base station using a control channel which has a power level 3 corresponding to -6dBw in an .AMPS system. The traffic channels in the cell may however allow a power level of 2 which correspond to -2dBw. As a result, the signal strength of the requesting mobile station should be increased by 4dB. In another example, the mobile station may have a very high signal strength. After a 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 traffic channels in the system are then scanned in order to determine whether there are any traffic channel available to handle the requesting mobile station in the step 203. 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 prede¬ termined level. If there are not available traffic channels in the system, the system checks to see if there are any sealed channels in step 205. If there are not sealed chan¬ nels, access is denied to the requesting mobile station in step 207, and the request is terminated. However, if there are channels in the system that have been sealed, a sealed traffic channel is selected and the traffic channel is unsealed in step 209, thereby making the traffic channel available to handle calls. The sealed channel can be selected 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 system determines that there is at least one traffic channel for handling incoming calls in step 203, up to a predetermined number of available traffic channels are selected in step 211 and the mobile station is ordered to measure the signal strength of each of the selected channels in step 213. The measured signal strengths for each of the selected channels is labelled as 1^., where i is the number of selected traffic channels. The mobile station also measures the signal strength of the channel on which the mobile station is communicating with the base station. This measured signal strength is labelled as C^^. The mobile station, in step 217, then sends the measured signal strength values back to the base station.

The system then measures the disturbance level on each of the selected traffic channels at the base station in step 219. The disturbance levels are labelled as I^. where i is the number of selected traffic channels. The measured signal strength C_|lfcfo is then compared with the measured disturbance levels l _fi to determine if

^Cmbfo ~ ^mbfi ^{> T}1 where T₁ is a predetermined threshold value. The measured signal strength C^^ is then compared with the measured signal strengths 1^. to determine if

^Cbmfo ^"*

^{> T}2 where T₂ is a predetermined threshold value. The threshold values T, and T₂ can be set equal to each other or they can be set at different levels. In addition, the threshold values can be varied depending on the amount of traffic in the system. For example, the first and second predetermined threshold levels can be set at 20 decibels when a cell or a relevant area near a cell is operating under relatively low traffic conditions, 16 decibels when the system is operating under relatively heavy traffic conditions, and 18 decibels when the system is operating under relatively normal traffic conditions.

If a traffic channel meets both of the above-described conditions, in step 221, the traffic channel is assigned to the requesting mobile station. If a plurality of selected channels fulfill the above-described conditions, the selec¬ tion of one of these channels can be carried out in several different ways. For instance, the first channel to fulfil the requirements can be chosen. In the alternative, the channel that exceeds the threshold values the least may be selected or the channel that exceeds the threshold value the most can be selected.

If it is determined in step 221 that none of the selected traffic channels fulfills the requirements, the system determines whether there are any other traffic channels available is step 225. If there are other traffic channels available, the above-described process starts over again. However, if it is determined that there are no other traffic channels available in step 225, the system denies access to the requesting mobile station and the request is terminated.

Referring now to Figure 5, 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 represents a request for 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 -6dBW 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 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 requestingmobile 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 predeter¬ mined 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 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 mol^{* '}le 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 reques¬ ting mobile station is terminated.

Referring now to Figure 6, 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 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 -6dBW 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 distur- bance 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 thres¬ hold 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 requestingmobile stationwith 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 themeasured 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

CLAIMS :

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 a first signal strength of a signal between a base station and a requesting mobile station at said base station; determining whether there are any traffic channels available in said base station; selecting a first set of up to a predetermined number of available traffic channels; measuring first disturbance levels on each of the selected channels at the requesting mobile; measuring a second signal strength of said signal at said mobile station; measuring second disturbance levels on said selected channels at said base station; comparing said first measured signal strength with said second disturbance levels and said second measured signal strength with said first disturbance levels; assigning said requesting mobile station to a channel which meets a predetermined set of conditions based on said comparison.

2. A method for selecting and assigning traffic channels according to claim 1, wherein said predetermined set of conditions are met when said first signal strength minus the second measured disturbance level on said channel is greater than a first predetermined threshold level and when said second signal strength minus the second measured distur¬ bance level on said channel is greater than a second predeter¬ mined threshold level.

3. A method for selecting and assigning traffic channels according to claim 2, wherein said first and second predetermined levels are set equal to each other.

4. A method for selecting and assigning traffic channels according to claim 1, wherein the first traffic channel to meet said predetermined set of conditions is assigned to said requesting mobile station.

5. A method for selecting and assigning traffic channels according to claim 1, wherein the channel which exceeds the predetermined set of conditions the least is assigned to said requesting mobile station.

6. A method for selecting and assigning traffic channels according to claim 1, wherein the channel which exceeds the predetermined set of conditions by the largest amount is assigned to said requesting mobile station.

7. A method for selecting and assigning traffic channels according to claim 1, wherein said available traffic channels are selected at random.

8. A method for selecting and assigning traffic channels according to claim 1, wherein said available traffic channels are selected according to which channels have been idling the longest.

9. A method for selecting and assigning traffic channels according to claim 1, wherein said first signal strength is adjusted for expected changes in the signal strength of the requesting mobile station.

10. A method for selecting and assigning traffic channels according to claim 2, wherein said first and second predetermined threshold levels decrease as traffic intensity in a cell or a relevant area near a cell increases.

11. A method for selecting and assigning traffic channels according to claim 2, wherein said first and second predetermined threshold levels are set at substantially 20 decibels when a cell or a relevant area near a cell is operating under relatively low traffic conditions.

12. A method for selecting and assigning traffic channels according to claim 2, wherein said first and second predetermined threshold levels are 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 according to claim 2, wherein said first and second predetermined threshold levels are set at substantially 18 decibels when a cell or a relevant area near a cell is operating under normal traffic conditions.

14. A method for selecting and assigning traffic channels according to claim 1, further comprising a step sealing off all traffic channels which have a predetermined disturbance level.

15. A method for selecting and assigning traffic channels according to claim 14, further comprising a step of unsealing a traffic channel when there are no available traffic channels in the cellular system.

16. A method for selecting and assigning traffic channels according to claim 1, further comprising a step of selecting a second set of different available traffic channels when said first set of available traffic channels does not contain an acceptable traffic channel.