A METHOD RELATING TO A RADIO COMMUNICATIONS SYSTEM

Abstract: The present invention relates to a cellular radio communications system in which each radio connection is allocated a dedicated radio channel. The quality of the radio connection over the radio channel is checked regularly and compared with a threshold value (RLT_UL, RLT_DL). If the connection quality is found to lie beneath the threshold value, the radio connection is terminated and the radio channel can then be reused for a new connection. According to the present invention, the threshold value is changed in accordance with the traffic load in the cell concerned. When the traffic load in the cell is high, i.e. only a few channels are available, the radio connection is terminated when the quality reaches a given level of impairment. The radio channel can then be used for a new radio connection. On the other hand, when the traffic load is low, the quality of the connection must be still poorer than the quality in a high load situation in order for the channel to be released.

Description: A METHOD RELATING TO A RADIO COMMUNICATIONS SYSTEM FIELD OF INVENTION The present invention relates to a method of handling radio channels that have been allocated to a radio base station for communication with radio terminals . DESCRIPTION OF THE BACKGROUND ART WO 96/00482 describes handling of radio channels in a mobile communications system where a number of radio connections use a small number of radio channels. It is possible that a connection will obtain access to the radio channel only when there is a need to transmit information over said channel. WO 96/00482 addresses the problem of the small delay that may arise before a channel is found available for the connection when information shall be transmitted over said connection. This may mean that the first word in a sequence speech message will not reach the receiver. The problem is solved according to WO 96/00482, by allowing a connection to retain access to the channel for a short period of time after the latest information transmission. It is said that this is an advantage, because one information transfer is often followed by another. The length of time over which the radio channel is retained for the latest connection is determined by a number of parameters. One of these parameters determines the magnitude of the load on remaining radio channels, in other words the urgency of the need to use the radio channel concerned for some other connection. Distinct from WO 96/00482, the present invention relates to a radio communications system where a connection is allocated a dedicated radio channel, i.e. the connection obtains exclusive access to the radio channel for the geographical area in which the mobile is situated. SUMMARY OF THE INVENTION The present invention addresses a problem where a radio based station bars new radio connections, because no free radio channels are available. The block is manifested, inter al ia , by the in-ability of a mobile station that moves into the area covered by the radio base station during an ongoing connection to handover to the radio base station. This means that the quality of the radio connection will be impaired and there is also the risk that the connection will be terminated. The mobile station may also interfere with other connections . It is unreasonable to solve the aforesaid problem by extending the capacity of the base station so that there will always be found available a free channel for new connections, in view of economy and spectrum efficiency. Accordingly, one object of the present invention is to release occupied radio channels for use with new radio connections . When the quality of a radio connection becomes so poor that the information transmitted is lost, a radio channel will nevertheless be dedicated to this connection over a period of time in case the connection quality can be improved. According to the present invention, the radio channel is released from the connection after a short time period when only a few free channels are available in the radio base station. On the other hand, if there is a good supply of free channels, the radio channel is not released until a longer period of time has lapsed. According to one embodiment, the radio quality is measured over one time period and compared with a given quality level. The radio channel is released, if the radio quality falls beneath said given level over this time period. The level is normally set to a value which is so low that the large part of the information sent over the connection will be lost. The present invention has the advantage of flexibly adapting the risk .of premature release of an ongoing connection against the need of free radio channels in the radio base station. By means of the present invention, it is possible to adapt the level at which the channel shall be released to the need of using the channel for a new connection at that moment in time. Thus, the channels can be utilised more effectively, i.e. can accommodate more traffic, when the load on the radio base station is high. When the traffic load is low, on the other hand, there is less risk of losing a call connection as a result of premature release of the channel. The invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a cellular radio communications system, or specifically the mobile communications system GSM. Fig. 2 is a diagrammatic illustration of the value of a given counter relative to a given level . Fig. 3 is a flowchart illustrating a method of changing a level that determines whether or not a radio channel shall be released from a given connection. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates parts of a cellular radio communications system, more specifically a GSM system. The Figure shows a number of radio terminals or mobile stations MS1-MS3 dispersed over a geographical area, and a number of base transceiver stations BTS1, BTS2. Each base transceiver station BTS1-BTS2 is connected to a base station controller BSC. A number of base transceiver stations BTS1-BTS2 are connected to the base station controller. The base station controller BSC is connected to a mobile switching centre MSC, not shown. Telecommunications connections can be setup between a mobile station MS1-MS3 and the mobile communications system via a radio connection between the mobile station MS1-MS3 and a base transceiver station BTS1, BTS2. The base transceiver station BTS1, BTS2 that serves, e.g., a given mobile station MSI will depend on where the mobile station MSI is situated geographically. In the illustrated case, the mobile station MSI is situated within the area covered by the first base transceiver station BTS1, that is to say it is situated within a first cell Cl served by the first base transceiver station BTS1. The base transceiver station BTSl governs an array of radio channels that can be used for communication with mobile stations MS1-MS3 that are situated within the own cell Cl . For the connection established with the mobile station MSI, there is allocated a dedicated radio channel, i.e. a radio channel Jtehat is used solely for the connection with the mobile station MSI. The dedicated radio channel is comprised of a physical uplink channel and a physical downlink channel. In the case of the illustrated example related to the GSM system, a physical channel is comprised of a specific timeslot on a specific carrier frequency. In the GSM system, a physical channel can also be divided into different logic channels. Unless otherwise mentioned, a channel or radio channel shall be understood in the following as meaning a physical duplex radio channel. There are times when the number of radio channels available in a base transceiver station BTSl, BTS2 is insufficient to serve all mobile stations that request such service within the cell Cl, C2 of the base transceiver station BTSl, BTS2. A connection is usually terminated by sending a release signal in one of the terminal points of the connection, in conjunction with a person wishing to terminate a call. The radio connection is terminated subsequent to the transmission of release signals and subsequent to obtaining a response to the transmitted release signal. The radio channel used for this particular connection is then free to be used for a new connection. However, it sometimes occurs that connections are broken and the radio channel released from the connection without prior transmission of a release signal. The present invention applies in such cases. The quality of the radio connection may be so poor that information transmitted over the channel does not reach the receiver end. Connection quality is measured over a time period and compared with a given quality level. If the quality is below this level, the connection is terminated and the radio channel can be reused for another connection. The parameter that sets this level in the GSM system is designated Radio Link Time-Out. The quality is measured in each respective uplink and downlink. There is a respective level in the uplink and the downlink against which the quality is compared; the comparison level in uplink is designated Radio Link Time-Out Uplink. If the quality is found to be below the comparison level, the connection is terminated and the radio channel released for reuse. Radio connection reports are regularly sent over said connection in both uplink and downlink. Each report has a well-defined format. The quality of the radio connection is measured in accordance with an earlier known method, by stepping a counter up one step or increment from a starting value in the event the absence of a measurement report from the receiver. Fig. 2 is a diagrammatic illustration that shows how the uplink counter is reset over a time period on the basis of a starting value in relation to the level RLT_UL . The X-axis of the diagram shows the number of measurement reports received, which also corresponds to time that has lapsed. The Y-axis shows the counter value. If three reports have been lost in succession and the counter thus counts-up three increments from the starting value, the counter will be stepped down two increments should a measurement report be received correctly. In Fig. 2, the correct measurement report received is followed by 15 sequentially excluded measurement reports. The counter has been stepped-up by 16 increments from the starting value, after the fifteenth measurement report in the succession of excluded reports. In the illustrated example, an increase of 16 increments represents the level RLT_UL at which the uplink radio connection is so poor as to terminate the radio connection. The quality in downlink is measured separately in the same way as in 1 » the uplink, although the downlink level is set independently of the uplink level RLT_UL . Accordingly, separate counters are included in the uplink and downlink respectively, for registering the quality of the radio connectio . According to earlier known technology, the level RLT_UL is set to a fixed value, by an operator of the mobile communications system. According to the present invention, the level RLT_UL that represents how poor a radio connection shall be in order to release the channel is changed in accordance with the number of free channels available in the base transceiver station BTSl. For instance, the level RLT_UL can be set in increments or steps between an upper limit value RLT_UL_max and a lower limit value RLT_UL_min. If only a few free channels are available, it is thereby possible to release a channel from a given radio connection subsequent to the counter having stepped up solely 10 increments, which in the illustrated case corresponds to the lower limit value RLT_UL_min for the level RLT_UL . On the other hand, if there is an abundance of free radio channels a given radio channel may continue to be dedicated to a given connection until the counter value has been increased by 22 increments, which in the illustrated case corresponds to the upper limit value RLT_UL_max for the level RLT_UL. Fig. 3 is a flowchart illustrating how the uplink level RLT UL and the downlink level RLT_DL are changed in respect of time, depending on the number of free channels in the base transceiver station BTSl. It is assumed in a first step SI that the level RLT_UL, RLT_DL has a start value, and consequently there is a time delay before the value is tested for any change that may be necessary"/ * In a following step S2, a check is made to establish whether or not the number of free channels in the base transceiver station 1 exceeds a given limit. Step S3 is carried out if the answer is No, i.e. only a few free channels are available . In step S3, a check is made to establish whether or not the uplink level RLT_UL is set to the lower limit value RLT_UL_min. Step S5 is carried out if the answer is Yes, the level RLT_UL is set to the lower limit value. A step S4 is carried out before step S5 if, on the other hand, the answer is No. In step S4, the uplink level RLT__UL is decreased preferably by one increment. The counter therewith needs to be increased by one increment fewer, prior to releasing the channel from the connection. In the following step S5, a check is made to establish whether or not the downlink level RLT_DL is set to the lower limit value RLT_DL_min . A new step SI and its following steps are carried out if the answer is Yes, the level RLT_DL is set to the lower limit value RLT_DL_min. On the other hand, if the answer is No, a step S6 is carried out before step SI and its following steps . In step S6, the downlink level RLT_DL is decreased by one increment . On the other hand, should the check carried out in accordance with step S2 above reveal that the number of free channels exceeds a given limit and the answer to the question is therefore Yes there is a sufficient number of free channels in the base transceiver station 1, a following step S7 is carried oiife . In step S7, a check is made to establish whether or not the uplink level RLT_UL is set to a highest permitted limit value RLT_UL_max. A step S9 is carried out if the answer is Yes, i.e. that the level RLT_UL is set to the upper limit value RLT_UL_max. On the other hand, if the answer is No then a step S8 is carried out before step S9. In the step S8, the uplink level RLT_UL is increased by one increment . In step S9, a check is made to establish whether or not the downlink level RLT_DL is set to the upper limit value RLT_DL_max . Step SI and its following steps are repeated if the answer is Yes, i.e. that the level RLT_DL is set to the upper limit value RLT_DL_max. On the other hand, if the answer is No there is carried out a step S10 in which the uplink level RLT_DL is increased through one increment, whereafter step SI is repeated. A measurement report is sent every 480 ms in uplink and in downlink over a logic channel designated Slow Associated Control Channel, referred to by the acronym SACCH. The measurement report has a strict structure and its information is coded so that said information can be established upon receipt, even though some data may have been distorted in the radio transmission. The information cannot be recovered, however, if an excessively large part of the data has been distorted. The measurement report is then considered to be lost and the counter counts-up one increment. Normally, uplink and downlink are inflicted simultaneously with poor radio quality, for instance when the mobile station accompanies its user into a garage and becomes situated in radio shadow. In such a case when the uplink counter has reached the uplink level RLT_UL, for instance after 16 measurement reports have been lost in succession, there is sent in downlink a channel release message. Transmission in downlink is then terminated. A channel release acknowledgement shall arrive in uplink from the mobile station MSI by way of response. Because of the poor quality of the connection, no channel release message will reach the mobile station MSI and consequently no response is sent in uplink. Even if a response were to be sent, it would not reach the base transceiver station 1 in uplink. The radio channel cannot be released from the connection in this state, since it is not precluded that the mobile station MSI will attempt to transmit in uplink. The radio channel continues to dedicate the connection until the counter of the mobile station MSI has also reached the downlink level RLT_DL, wherewith any attempt to transmit further in uplink on the part of the mobile station MSI can be excluded. The time before the channel can be released from the time at which the mobile entered radio shadow will be: (R linkup + R linkdown) * 480 ms . Assuming that the level downlink RLT_DL and the level uplink RLT_UL is set to 16, there will be a delay of approximately 16 s before the channel can be released and allocated to a new connection. In the GSM system, it is the base station controller that controls the radio connection. It is thus the base station controller BSC that handles the control of radio channels and allocates radio channels to different users. The uplink measurement reports are received by the base transceiver station 1 and forwarded to the base station controller BSC. The counter which counts the number of lost measurement reports ιέ* implemented in the base station controller. The base station controller BSC also checks the counter value relative to the level RLT__UL in uplink and releases the radio channel, depending on the result of this comparison. The mobile station MSI includes a counter for establishing the quality of the downlink connection, and the counter value is compared with the level RLT__DL in the mobile station MSI. If the downlink quality falls below the level RLT_DL, the mobile station MSI initiates the release of the connection. The base station controller BSC also includes a counter corresponding to the counter in the mobile station MSI which is compared with the level RLT_DL in downlink with regard to the possible release of the channel. This counter is used when a channel release message is sent in downlink, for instance as in the above radio shadow example. The counter is stepped-up from a starting value after the channel release message has been sent in downlink. Upper and lower limit values RLT_DL_max, RLT_UL_max, RLT_DL_min, RLT_UL_min, are set to a value by the operator of the mobile communications system in the same way as the level RLT_DL, RLT_UL while earlier set to a given value. The downlink level RLT_DL is calculated in the base station controller on the basis of these limit values in accordance with the method illustrated in Fig. 3, and is sent downlink to the mobile station MSI. It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the following Claims.

Claims: CLAIMS 1. A method of handling a first radio channel of a number of radio channels that have been allocated a base transceiver station (BTSl) for enabling a corresponding number of connections to be established in duplex with radio terminals (MSl-MS2)γ . , wherein the first radio channel has been dedicated k for a first connection, and wherein the first radio channel is released from the first connection when the radio quality of said first connection falls beneath a given level (RLT_DL, RLT_UL) , characterised by setting the level (RLT_DL, RLT_UL) in accordance with the number of free radio channels available . 2. A method according to Claim 1 wherein said first radio connection regularly sends reports and said radio quality is measured in accordance with the proportion of reports received incorrectly. 3. A method according to Claim 2 wherein the quality of the connection in downlink is measured on the basis of the proportion of reports received incorrectly in downlink and the quality of the connection in uplink is measured on the basis of the reports received incorrectly in uplink, wherein the quality of the connection in downlink is compared with a first level (RLT_DL) and the quality of the connection in uplink is compared with a second level (RLT_UL) , wherein the comparison determines whether or not the first radio channel shall be released, and wherein said first and said second level (RLT_DL, RLT__UL) are set to vary in time between two limit values (RLT_DL_max, RLT_UL_max, RLT__DL_min, RLT_UL_min) on the basis of the number of free radio channels available. 4. A method according to Claim 3 wherein said first and said second levels (RLT_DL, RLT_UL) are changed on the basis of a starting value in accordance with the following steps: a) waiting (SI) over a time period; b) checking (S2) : is the number of free channels greater than a given number; c) if the swer to the question in step b) is No: decreasing (S4, S6) the first and the second level by one increment provided (S3, S5) that said levels do not become smaller than the lower limit value (RLT_DL_min, RLTJJLjnin) , thereafter returning to step a); d) if the answer to the question in step b) is Yes: increase (S8, S10) the first and the second levels through one increment provided (S7, S9) that said levels do not exceed the upper limit value (RLT_DL_max, RLT_UL__max) and then return to step a) .

Inventors: MILEN ANDERS, MILEN, ANDERS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H04Q7
IPC codes: H04W72, H04W76