Patent Application
20130329561
The present invention relates to methods and devices for estimating available cell load in a cellular radio system.
Because the uplink in a Wideband Code Division Multiple Access (WCDMA) cellular radio system is non-orthogonal the so called near-far problem must be handled, i.e. a terminal close to the base station could easily drown the signal from a terminal at the cell border. To ensure the coverage of terminals at the cell border a limit is introduced on the total received power at the base station. A relative measure of total interference is rise over thermal (RoT) or noise rise, i.e. total interference relative to thermal noise, see Harri Holma, Antti Toskala, “WCDMA for UMTS—Radio Access For Third Generation Mobile Communications”, third Edition.
The uplink total interference may come from both intra-cell interference and inter-cell interference. In order to be able to control the RoT a cell should estimate the level of inter-cell cell interference it is currently experiencing.
Load control is a functionality used for WCDMA uplink. Load control determines for each cell the maximum available load room that can be used by the scheduling function based on the uplink interference situation in that cell, see Harri Holma, Antti Toskala, “WCDMA for UMTS—Radio Access For Third Generation Mobile Communications”, third Edition.
There are many different options to implement load control. Below two different exemplary implementations are described.
In a first example all the load which originates from users who are not served by one cell is treated (and estimated) in a single term called Lother, where Lother is caused by the inter cell interference. The load available Lavail for the scheduler of one in the cell is then calculated as:
L
avail
=L
max
−L
other (1)
Where Lmax is the maximum allowed load in one cell, which is determined by the target (or maximum allowed) RoT in that cell. Lavail is the load available for said cell (own cell available load room). The total load in the cell can be said to be Lavail (if it is used) plus Lother, hence, this will be referred to as the two term approach.
In a second example the fact that transmissions stemming from User Equipments (UEs) who are not served but in soft handover (SHO) with said one cell are received and decoded by that cell is used. Since the transmission is received, the load of the transmission can be calculated. With this information the available load is calculated as:
L
avail
=L
max
−L
SHO
−L
other (2)
i.e. the available load is equal to the maximum load in that cell, minus the load stemming from UEs in SHO with that cell but have some other cell as serving cell (LSHO), minus the load from all the other UEs (Lother), where Lother is caused by the inter cell interference. This is referred to as the three term approach. Thus, in the three term approach the load caused by UEs in SHO are separated from the inter cell interference.
There is a desire for new methods and devices providing improved estimation of the available load for a cell in a cellular radio system, in particular a WCDMA system in order to improve the load scheduling in the cells of the cellular radio system.
It is an object of the present invention to provide an improved methods and devices to address the problems as outlined above.
This object and others are obtained by the methods and devices as set out in the appended claims.
As has been realized by the inventors one property of inter-cell interference is that it is typically highly variable. As a consequence the estimation process of Lother will typically require a high level of filtering. Such a filtering will make it difficult to adapt to abrupt changes in the inter-cell interference. As has been further realized by the inventors the filter will not be able to make a distinction between random changes (which should be filtered) and abrupt changes which would be beneficial to take into account when calculating the available load room and used when scheduling the load in the cell.
The problem (i.e. an abrupt change in inter-cell interference (or Lother)) has been discovered to arise when a user makes a serving cell change (referred to herein as case 1) or a cell is added into or removed from the active set of the user (referred to herein as case 2) and the load contribution of that UE should move between the different load terms. As has been realized this problem is especially critical if the user transmit with high data rate. This would potentially be a small problem if not for the slow estimation of Lother.
When a UE changes serving cell (case 1) the load from that UE which was prior incorporated in Lother will suddenly be under the control of said cell and the current Lother estimate will be an overestimation of the actual value. This will lead to that said cell would underestimate the available load room and, consequently, schedule bitrates that are lower than what is achievable under the given load constraints.
For case 1, when a UE leaves one cell (i.e. changes serving cell to another cell) there is also a problem. The true value of Lother of said cell is instantly increased but the estimated value will require some time to converge to this new value. This results in an overestimation of the available resources with overload as a possible outcome. The overload may cause the coverage of the cell to shrink with possible dropped calls and failed access attempts as consequence.
For case 2, when a cell is added into active set of one user, the cell will be able to measure the load contribution of said user (part of LSHO). However, since this load is still taken into account in the Lother term the issue of underestimation arises also in this case.
Similarly, when a cell is removed from the active set (case 2) this may result in overestimation of the load in said cell. It will take some time before the Lother estimation has been able to detect an actual change in the inter-cell interference and act accordingly.
In accordance with embodiments described herein methods and devices are provided that provide faster estimation and/or compensation for abrupt changes in inter-cell interference during serving cell change (case 1) or when a cell is added into or removed from the active set (case 2). This will in turn thus avoid (or at least reduce) either overestimation or underestimation of the available uplink resources in a cellular radio system.
In accordance with one embodiment a method of determining the available load in a cell of cellular radio system is provided where the determination of the available load is made in response to an estimated inter cell interference load. In accordance with the method an estimated inter cell interference is first determined. Then inter cell interference change events are monitored by determining when a User Equipment changes serving cell or when a User Equipment adds or removes a cell to the active set of the User Equipment. The determined inter cell interference is adjusted by an estimated load caused by the User Equipment causing an interference change event, and the available load is determined in response to the adjusted inter cell interference. The method can continuously update inter cell interference. Moreover, the update can be made fast and thereby allow an improved estimation of the available cell load.
In accordance with one embodiment the determined available load is used to schedule transmission in the uplink whereby an uplink transmission can be improved.
In accordance with one embodiment when a UE is changing serving cell (case 1), the target cell (i.e. the radio base station controlling the cell that will become the new serving cell) will reduce Lother according to the estimated load from said user in said target serving cell, and the serving cell (i.e. the radio base station controlling the serving cell) will increase Lother according to the estimated load from said user in said original serving cell. Note that the estimated load of said user is typically available in both cells (i.e. the radio base stations controlling the two cells) due to soft handover.
In case 2, a non-serving cell (i.e. the radio base station controlling a non-serving cell) which is removed from the active set of the UE will increase Lother according to the estimated load from said UE in said non-serving cell, and a non-serving cell which is added to the active set of said UE will decrease Lother according to the estimated load from said UE in said non-serving cell.
The invention also extends to and a radio base stations Node B arranged to perform the methods in accordance with the above. The radio base station Node B can be provided with a controller/controller circuitry for performing the above methods. The controller(s) can be implemented using suitable hardware and or software. The hardware can comprise one or many processors that can be arranged to execute software stored in a readable storage media. The processor(s) can be implemented by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared or distributed. Moreover, a processor or may include, without limitation, digital signal processor (DSP) hardware, ASIC hardware, read only memory (ROM), random access memory (RAM), and/or other storage media.
The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawing, in which:
The scenario when a user equipment makes a serving cell change (case 1) is depicted in
When a UE leaves one cell (i.e. changes serving cell to another cell) there is also a problem. The true value of Lother of said cell is instantly increased but the estimated value will require some time to converge to this new value. This results in an overestimation of the available resources with overload as a possible outcome. The overload may cause the coverage of the cell to shrink with possible dropped calls and failed access attempts as consequence.
The scenario when a cell is added into or removed from the active set of a user equipment (case 2) is also depicted in
Similarly, when a cell is removed from the active set case 2 this may result in overestimation of the load in said cell. It will take some time before the Lother estimation has been able to detect an actual change in the inter-cell interference and act accordingly.
In
In
The cell which used to be the serving cell will always have a measurement of the load of the UE available and can add this quantity to its estimated Lother value.
When a UE performs handover to said NodeB the “true” Lother value will drop in accordance with the load of that UE, the proposed solution allows for a faster adaptation.
For case 2 (the three-term approach) the proposed solution works in a similar fashion as depicted in
In
In accordance with one embodiment when the two term approach is adopted (case 1), suppose a UE is changing serving cell from cell A to cell B, and the load that the UE generates in cell A and cell B is loadA and loadB, respectively. The NodeBs that control cell A and cell B respectively can then be configured to update Lother for these two cells to compensate the abrupt inter-cell interference change as:
LotherAnew=LotherAold+loadAest (3)
LotherBnew=LotherBold−loadBest (4)
LoadAest and loadBest are load estimated by NodeBs that control cell A and cell B respectively.
The Lother updates can be triggered by Node-B Application Protocol (NBAP) signaling requesting the serving cell change, which is sent from RNC to the NodeBs.
LoadAest is typically always available in the NodeB that controls cell A when serving cell change is triggered. For LoadBest, this may not be the case if cell B is not in the active set of said UE (which is typically not the case). In this case NodeB that controls cell B needs to get LoadBest first after the serving cell change. This information can be obtained fast. For example the LoadBest can be based on Dedicated Physical Control Channel (DPCCH) Carrier-to-Interference Ratio (CIR) target and enhanced transport format combinations (E-TFC) read on enhanced DPCCH (E-DPCCH). Thus the update for LotherB needs to be delayed for a (predefined) very short time.
A special case is that cell A and cell B are controlled by the same NodeB (e.g. in a main remote radio system). In this case, even if cell B is not in the active set of said UE, loadAest and LoadBest are both available in said NodeB, and the NodeB can update Lother for both cell A and cell B once serving cell change is triggered. According to a second embodiment of the invention where the three term approach is adopted (case 2), suppose a cell is added into active set of one user. The NodeB that controls said cell update Lother for said cell to compensate the abrupt inter-cell interference change according to Eq. (4) above.
The Lother update can be performed a (predefined) very short time after the cell is added into the active set (the completion of active set addition can be known from NBAP signaling for active set addition). This is to make sure that the estimated load from the UE can be made available at the NodeB controls the cell before the update.
In case that (some of) the cells that are currently in the active set of the UE and the cell that is being added into the active set are controlled by the same NodeB (e.g. softer handover or in a main remote radio system). The Lother update can be performed when the active set addition is triggered by the corresponding NBAP signaling.
In case that a cell is removed from active set of one user, the NodeB that controls said cell update Lother for said cell to compensate the abrupt inter-cell interference change according to Eq. (3) above.
In this case the estimated load from the UE is always available at the NodeB that controls the cell, and the Lother update can be performed when the active set deletion is triggered by the corresponding NBAP signalling.
Using the methods and devices as described herein will provide efficient compensation for abrupt inter-cell interference change during serving cell change and/or soft handover. As a result harmful under- and over-estimations of the available own cell load can be avoided or at least reduced. Also a more stable uplink behaviour (less RoT oscillation) and better use of available uplink resources can be achieved.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2011/050217 | 2/25/2011 | WO | 00 | 8/23/2013 |
