WO2007113267A2

WO2007113267A2 - A power control method for soft handover in mobile communication systems

Info

Publication number: WO2007113267A2
Application number: PCT/EP2007/053128
Authority: WO
Inventors: Henrik Andersson
Original assignee: Andrew Corporation
Priority date: 2006-03-31
Filing date: 2007-03-30
Publication date: 2007-10-11
Also published as: WO2007113267A3

Abstract

A method for soft handover in a cellular mobile radio network comprises the steps of determining Signal-to Interference-Ratio (SIR) targets for a mobile and sending the SIR targets to base stations (A, B). In the method the CPICH power from the base stations (A, B) is measured when the mobile terminal is entering the soft handover region between the starting cell (A) and a neighbouring new cell ( B), and a link is added to the mobile, when the CPICH power is within a preset range, providing connection in uplink and downlink directions between the mobile terminal and the base stations A and B, respectively. In the method the base station (A) is commanded to increase the SIR target for the uplink connection of the mobile terminal for forcing the mobile terminal to increase the transmission power until the determined SIR target in the cell (B) has been reached, and when it is confirmed that both uplink and downlink connections are available through cell B, the soft handover is completed by commanding the cell (A) to delete the connection to the mobile terminal. The RNC is configured to control the soft handover for a mobile terminal and comprises means for performing the soft handover.

Description

A SOFT HANDOVER METHOD AND A RADIO CONTROLLER FOR MOBILE COMMUNICATION SYSTEMS

Field of invention The invention relates to a method of performing a soft handover between cells in a cellular mobile radio network. Furthermore, the invention relates to a radio network controller configured to control soft handover in a cellular mobile radio network.

Background

A mobile communication system is built up by radio base stations, each station covering one or several cells. When a mobile terminal moves from one cell to another it performs what is called a handover. Handover is the essential feature for dealing with the mobility of the mobile users. At soft handover the mobile terminal will connect to several base stations simultaneously. Compared with conventional hard handover, soft handover has better performance on both link and system level.

Soft handovers provides decreased interference levels and better cell edge performances than ordinary hard handovers. The system will obtain higher capacity and improved coverage with soft handovers due to e.g. lower transmission power, less interference, macro diversity gain, micro diversity gain and downlink load sharing between cells.

A weakness with current soft handover methods is that the handover process is not successful in cases where the uplink and downlink links are connected to different base stations. In order to complete a soft handover successfully, both uplink and downlink links must be up and running on the terminating new cell before taking down the connection towards the old cell(s). In currently known methods, there is no guarantee that both the uplink link and the downlink link towards the terminating cell is up before taking down the connection towards the old cell, resulting in a large amount of failed handovers and dropped calls in situations where the uplink and downlink directions are not correlated. Problems will arise when there is an asymmetry, e.g. at a soft handover between base stations having different power levels of the common pilot channel (CPICH). These power levels are transmitted as a broadcast from all Wideband Code Division Multiple Access (WCDMA) base stations and are used by the mobile terminal to decide the base station(s) to connect with.

Failed handovers will occur in a number of various situations, such as:

At handover between a small cell and a large cell, in which case the transmitted CPICH power levels between the cells are large, for example between pico-macro, micro-macro or pico-micro cells. - At handover between base stations from different vendors, which have different characteristics, e.g. requirements regarding sensitivity levels or C/I (Carrier to Interference ratio). At handover between two cells experiencing different interference levels in uplink or downlink. - At handover between WCDMA (Wideband Code Division Multiple

Access) cells with different power level settings. At handover between cells with different load situation. At handover between indoor located cells and outdoor located cells.

Disclosure of the invention

An object of the present invention is to eliminate or alleviate at least one of the drawbacks mentioned above, which is achieved by the method according to claim 1.

A further object of the invention is to provide a radio network controller (RNC) configured to control soft handover for a mobile terminal of a cellular mobile radio network.

According to one aspect of the invention, there is provided a method for soft handover in a cellular mobile radio network comprising one or several radio network controllers (RNC), base stations having coverage areas forming one or several cells, and soft handover regions between the cells. The RNC(s) is/are in communication with mobile terminals moving within the network and the base stations. The method comprises the steps of determining Signal-to Interference- Ratio (SIR) targets for the mobile terminal by the RNC; sending the SIR targets from the RNC to the base stations; providing the mobile terminal within a starting cell A, wherein the mobile terminal is connected to the base station A having a highest CPICH power; measuring the CPICH power from the base stations A, B when the mobile terminal is moving and is entering the soft handover region between the starting cell A and a neighbouring new cell B; adding a link to the mobile terminal from the base station B when the CPICH power from the base station to the mobile terminal is within a preset range, providing connection in uplink and downlink directions between the mobile terminal and the base stations A and B, respectively. Furthermore, the method is characterized by the steps of commanding the base station A to increase the SIR target for the uplink connection of the mobile terminal for forcing the mobile terminal to increase the transmission power until the determined SIR target in the cell B has been reached; confirming that both uplink and downlink connections are available through cell B by the RNC; and completing the soft handover by commanding the cell (A) to delete the connection to the mobile terminal.

According to another aspect of the invention, there is provided a radio network controller (RNC) configured to control soft handover for a mobile terminal of a cellular mobile radio network having base stations A, B with coverage areas forming one or several cells A, B and a soft handover region (SHO region) between the cell A and cell B, comprising means for determining Signal-to Interference- Ratio (SIR) targets for the mobile terminal, means for sending the SIR targets to the base stations A, B, means for commanding the base station B of a new neighbouring cell B to add a link to the mobile terminal when the mobile terminal is entering the SHO region and the CPICH power from the base station B is within a preset range, characterized by comprising means for commanding the base station A of the starting cell A to increase the SIR target for the uplink connection of the mobile terminal for forcing the mobile terminal to increase the transmission power until the determined SIR target in the cell B has been reached; means for confirming that both uplink and downlink connections are available through cell B ; and means for commanding the cell A to delete the connection to the mobile terminal for completing the soft handover.

Further objects, features and advantages of the present invention will appear from the detailed description, from the attached drawings as well as from the dependent claims.

Brief description of the drawings

In order to explain the invention, a number of embodiments of the invention will be described below with reference to the drawings, in which: Fig l is a schematic view of an arrangement for mobile communication systems having a soft handover region (SHO region),

Fig 2 shows the arrangement in Fig 1 , wherein a mobile terminal is communicating with a base station A,

Fig 3 shows the arrangement in Fig 1 , wherein the mobile terminal has entered the SHO region and is communicating with base stations A and B,

Fig 4 illustrates the arrangement in Fig 3, wherein the mobile terminal uses a Cell B for downlink transmission and a Cell A in uplink transmission,

Fig 5 shows the arrangement in Fig 1 when Cell A has deleted the connection with the mobile terminal and the mobile terminal has left the SHO region,

Fig 6 shows the arrangement in Fig 1 , wherein the mobile terminal is communicating with both stations when being outside the SHO region,

Fig 7 shows the arrangement in Fig 4, wherein the mobile terminal has increased the transmitted power to guarantee the communication towards Cell B, and

Fig 8 illustrates that both uplink and downlink is available through Cell B and hence the handover is completed.

Same references have been used to indicate the same parts in the figures to increase the readability of the description and for the sake of clarity. Description of embodiments of the invention

A standard Radio Access Network comprises typically a large number of base stations (BS) and one or several Radio Network Controllers (RNC). Each base station controls one or several cells of the network, and each RNC controls several base stations. The base station has the functions to set up, supervise and delete radio connections with the mobile terminals located within its cell(s). The RNC has more advanced functions, such as mobility management, admission control and load control. The mobility management is the function for controlling which cell(s) that are or will be connected to each mobile terminal and the handover function. The RNC continuously supervises the link performance by instructing the base station(s) and the mobile terminal(s) to send measurement reports periodically or at specific events. A base station always monitors the uplink performance for all mobile terminals within its cell(s) and sends an error message to the RNC when observing that an uplink connection has failed. This radio link failure error message is an example of an event triggered report. Similarly the mobile terminals monitor the downlink connection(s) from the base station(s) and sends measurement reports to the RNC. Examples of a typical event triggered measurement report related to SHO could be an event IA, which is triggered when the mobile terminal measures the CPICH power from a neighbouring cell to be within a specified relative power limit compared to the CPICH power from the existing cell. The reception of this event IA measurement report triggers the RNC to instruct the base station in the neighbouring cell to add a SHO connection to the mobile terminal. Similarly, an event IB measurement is triggered when the CPICH from an existing SHO connection is too weak and should be deleted. The "quality levels" or power levels that should trigger a measurement report, from either the mobile terminals or the base stations, are determined by the RNC.

The RNC can also control the quality of each individual radio link by sending SIR targets to the base stations, the SIR targets being individual values for each connection. By changing these targets values, the RNC can indirectly control the power levels transmitted from each base station and mobile terminal, independently for each radio link. According to the invention, actions can be taken based on the measurement reports received, and it is possible to change the target levels in an old cell in order to guarantee that both the uplink and downlink connections are available in a new cell and have a sufficiently good quality before deleting the connections of the old cell(s). The following description of one embodiment of the invention is based on

WCDMA FDD (Wideband Code Division Multiple Access Frequency Division Duplex) inter-cell intra- frequency handover from one Cell A (the old cell or starting cell) to another Cell B (the terminating cell). Cell A is smaller than Cell B, as illustrated in Fig 1. The coverage areas of each cell are determined by power transmitted from the base stations, especially the power level allocated to the common pilot channel, the CPICH. Power from the CPICH: s are transmitted as a broadcast from all WCDMA base stations, and each mobile terminal within the area will measure the power levels received from all surrounding base stations to determine and connect to the one(s) having the highest power transmission level. The power level selected for the CPICH is individual for each cell, but is typically 5-15% of the maximum total base station power. There is a trade-off between large cell range and capacity for a cell so the CPICH power should be set carefully. For a small pico cell base station the CPICH power will be in the order of 10-50 mW, which is very weak compared to a large macro cell base station that typically uses 1-4 W power on CPICH. The power level received on the CPICH determines the cell with which the mobile terminal is to communicate.

A small region defined by partly overlapping neighbouring cells is denoted the soft handover region (SHO region), as illustrated in Figl . This region is characterized in that the CPICH power received from several base stations will differ in power level only by a few decibels. In this region it is desirable for the mobile terminal to simultaneously connect to several cells, which is called a soft handover in WCDMA context. The cell sizes and the regions between the cells can be tuned by changing the power allocation to the CPICH.

Independent power control loops are used to control the transmission (TX) power levels in uplink (mobile terminal TX power) and in downlink (base station TX power). In a WCDMA system all users occupy the same frequency band and will interfere with each others. The power control loops are therefore essential in order to limit the interference levels. The aim is that every mobile terminal or base station shall use a power level that is as low as possible. A Radio Network Controller (RNC) is responsible for determining correct Signal-to-Interference- Ratio target (SIR-target) level for each individual link and is communicating the level values to involved base stations.

Based on the uplink SIR-target, which is received from the RNC for a specific connection, the base station will order the mobile terminal to increase or decrease the transmission power. The mobile terminal measures the signal quality on the downlink signal received, and will then send commands to the base station to increase or decrease the transmission power. The transmission power needed is based on a number of factors and can vary a lot depending on e.g. distance between terminal and base station, interference levels in uplink and downlink, load in the cells, and/or walls or other obstacles obstructing the radio signal. During soft handover two or more radio links are available in parallel. In this case the mobile terminal will only increase the transmission power if all the involved base stations simultaneously command increased power. For this reason the mobile terminal never transmits more power than needed to satisfy the SIR target in one of the base stations involved during the soft handover. The other base station(s) will measure a lower SIR value than the SIR target.

A typical WCDMA soft handover from Cell A to Cell B will proceed as follows. The mobile station is positioned within Cell A. It is only communicating with a base station A, as illustrated in Fig 2. Both the mobile terminal and the base station are using relatively low transmission power. The mobile terminal moves and enters the SHO region, as shown in Fig 3. Although the distances to the base stations differ a lot, the power levels received by the mobile terminal in downlink will be in the same range. Parameter settings of the RNC control the soft handover region, but typically all cells will be considered as soft handover alternatives if the CPICH power received is within a difference of 2-5 decibels. Based on measurement reports from the mobile terminal, the RNC will order Cell B to add a link to the mobile terminal. The mobile terminal is now communicating simultaneously with both cells. In downlink the mobile terminal is listening to both base stations and can select the strongest signal at each time instant, or can combine the signals from both stations if the signals are equal in power. In uplink both base stations is trying to listen to signals transmitted from the mobile terminal. However, the mobile terminal will regulate the transmitted power on the "best uplink", which in this example is towards Cell A because of a smaller distance between the cell and the base station, as indicated by the short continuous arrows in Fig 3. Cell B will therefore receive a very weak or a not detectable signal as illustrated by the dotted arrow in Fig 3. This is not a problem from a system point of view, the performance of the system will be good as long as at least one downlink and one uplink leg is available.

When the mobile terminal is within the SHO region, the RNC is able to communicate with the mobile terminal through one or several base stations, independently chosen for the uplink or downlink directions. An extreme case may occur when one cell is used for uplink and another cell is used for downlink transmission, as shown in Fig 4. In this case the downlink power from Cell A is too low (illustrated by the downward dotted arrow) to contribute to the performance because of power limitation in the base station, thus the mobile terminal will only select Cell B in downlink. However, the uplink direction will still be through Cell A (illustrated by the upward continuous arrow in Fig 4) because of the relatively close distance to the base station.

Fig 5 shows the mobile terminal moving further into Cell B, leaving the SHO region. The handover shall now be completed and the connection to Cell A shall be taken down. The SHO regions should be kept limited in size, since too large areas will reduce system capacity. The RNC will according to currently used handover methods either command Cell A to delete the connection to the mobile, which will result in high call drop probability because the mobile terminal have to use sufficient power ( Fig 5), or detect that there is no uplink available in Cell B and leave the Cell A connection up (Fig 6), which will result in an increasing SHO region, capacity loss, increasing interference, and higher probability for lost connections. To avoid the situation described in above paragraph, the soft handover method according to the invention is adding some features in order to assure that both uplink and downlink is working toward the terminating Cell B before taking down the link to Cell A. Before the mobile terminal leaves the SHO region, the RNC shall command the base station in Cell A to increase the SIR target for the uplink connection. In this example this will force the mobile terminal to increase the transmitted power until the SIR target in Cell B is satisfied and communication towards Cell B is guaranteed, see Fig 7. When the RNC detects that both uplink and downlink connections are available through the targeting Cell B the handover is completed by commanding Cell A to delete the connection to the mobile, Fig 8. This will give much more determined and successful handovers, and the SHO regions can be kept limited. The presented embodiment is only one specific example. There are many other examples of handover problems where the power level from the mobile terminal is too low because of for example interference situation in the cells, load in the cells, etc. Note that the SIR target increase in Cell A will not decrease the performance in a case where the uplink towards Cell B already is the best link. Since the mobile terminal always regulates on the best link, the SIR target increase on the worst link will not influence or in any way impair the performance. The increase of the mobile terminal power that has to be made will range from O to 20- 40 decibels, or even more in extreme cases, but the exact value does not need to be known because of the best link limitation mentioned.

By using the soft handover method described, the network side can control the transmitted power from the mobile terminal to adapt according to target cell characteristics. The adaptation is done by changing the power control target level in a cell before completing the handover procedure. The radio network will achieve increased performance in terms of handover success rate, lower call drop rate and higher system capacity because of well defined handover zones and decreased interference levels. In an alternative embodiment the increased SIR target can be replaced by e.g.

.power- target, BLER- target or any other power control metric.

Thus, the invention can be used to solve a number of other problematic handover situations, such as e.g.: - Handover between cells with equipment from different vendors.

Handovers in congested cells.

Handovers between cells with different interference levels.

Handover between cells with different CPICH power configuration.

Handover between macro and micro cells. - Handover between macro and pico cells.

Handover between micro and pico cells.

Handover between indoor and outdoor located cells.

Handover between cells with different load.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Furthermore, a plurality of means, elements or method steps may be implemented. Additionally, individual features may be included in different embodiments, these may possibly be combined in other ways, and the inclusion in different embodiments does not imply that a combination of features is not feasible. In addition, singular references do not exclude a plurality. The terms "a", "an" does not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

1. A method for soft handover in a cellular mobile radio network comprising one or several radio network controllers (RNC), base stations (A, B) having coverage areas forming one or several cells (A, B), and soft handover regions between the cells (A, B), wherein the RNC(s) is/are in communication with mobile terminals moving within the network and the base stations (A, B), comprising the steps of:

- determining Signal-to Interference-Ratio (SIR) targets for the mobile terminal by the RNC,

- sending the SIR targets from the RNC to the base stations (A, B),

- providing the mobile terminal within a starting cell (A), wherein the mobile terminal is connected to the base station (A) having a highest CPICH power,

- measuring the CPICH power from the base stations (A, B) when the mobile terminal is moving and is entering the soft handover region between the starting cell (A) and a neighbouring new cell ( B),

- adding a link to the mobile terminal from the base station (B), when the CPICH power from the base station to the mobile terminal is within a preset range, providing connection in uplink and downlink directions between the mobile terminal and the base stations A and B, respectively, characterized by the steps of:

- commanding the base station (A) to increase the SIR target for the uplink connection of the mobile terminal for forcing the mobile terminal to increase the transmission power until the determined SIR target in the cell (B) has been reached, - confirming that both uplink and downlink connections are available through cell B by the RNC, and

- completing the soft handover by commanding the cell (A) to delete the connection to the mobile terminal.

2. A radio network controller (RNC) configured to control soft handover for a mobile terminal of a cellular mobile radio network having base stations (A, B) with coverage areas forming one or several cells (A, B) and a soft handover region (SHO region) between the cell (A) and cell (B), comprising means for determining Signal- to Interference-Ratio (SIR) targets for the mobile terminal, means for sending the SIR targets to the base stations (A, B), means for commanding the base station (B) of a new neighbouring cell (B) to add a link to the mobile terminal when the mobile terminal is entering the SHO region and the CPICH power from the base station (B) is within a preset range, characterized by comprising:

- means for commanding the base station (A) of the starting cell (A) to increase the SIR target for the uplink connection of the mobile terminal for forcing the mobile terminal to increase the transmission power until the determined SIR target in the cell (B) has been reached;

- means for confirming that both uplink and downlink connections are available through cell (B) ; and

- means for commanding the cell (A) to delete the connection to the mobile terminal for completing the soft handover.