QUALITY BASED HANDOVER PROCEDURE BETWEEN CO-LOCATED CELLS

• US 20100290435A1
• Filed: 03/13/2008
• Published: 11/18/2010
• Est. Priority Date: 03/13/2008
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Kazmi, Muhammad
• Granted Patent: US 8,411,555 B2
• Time in Patent Office: Days
• US Class Current: 370/332
• CPC Class Codes: H04W 36/144 over a different radio air interface technology
• CPC Class Codes: H04W 36/302 due to low signal strength

Abstract

The present invention relates to cellular system and especially to handover between cells on different frequency carriers in an OFDM system or between cells in 2 systems adopting different radio access technologies (RAT), at least one of them being OFDM technology. A problem is the intra-cell interference impact on the quality a connected mode terminal measures in the own cell and in neighbour cells. This is a problem for cells based on OFDM technology, because intra-cell interference has no impact as compared to the inter-cell interference on the quality provided by the cell to a connected mode terminal, whereas in UTRA the intra-cell and inter-cell interference have the same impact on the quality provided. The solution to the problem is based on the insight that for cells that are located on the same site, the eNode B has possess information on the power transmitted on the respective frequency carrier, and can adjust the quality as reported from the terminal on co-located OFDM cells. The adjusted quality measure enables an improved evaluation of the quality of co-located cells, for a potential handover. The invention also relates to embodiments on a 2-step handover, wherein handover to inter-frequency, or an inter-RAT cell on another site is made as a handover to co-located inter-frequency/inter-RAT cell and a handover to a intra-frequency cell located at the other site.

Claims

1. Method for evaluating a second cell for a potential handover of a terminal that is served by a first cell, wherein the first and the second cells are co-located, and wherein the second cell is operating on a second frequency carrier in a OFDM system and the first cell operating on a first frequency carrier other than the second frequency carrier in the OFDM system or the first cell is operating in another radio access technology system than the OFDM system, the method is comprising the steps of:

receiving measures from the terminal on the quality of the first and the second cell, wherein the quality measure of the first and second cells comprises an intra-cell interference component and the quality measure of the first cell includes an intra-cell interference component if the first cell is included in the OFDM system;

detecting the downlink carrier power of the second cell;

characterised by the further step of;

adjusting the quality measure received from the second cell, by compensating for the intra-cell interference component in the quality measures and compensating for the intra-cell interference component in the quality measure of the first cell if the first eel! is included in the OFDM system;

evaluating the best cell for serving the terminal by comparing the adjusted quality measure of the second cell with the quality measure of the first cell, as adjusted if the first cell is included in the OFDM system.

3. A method according to claim 1 wherein handover to the second cell is decided provided the adjusted quality measure of the second cell is better than that of the first cell, and the signal strength of the second cell is at least at a predefined level.

4. A method according to claim 1 wherein the terminal was served by a cell A from which it was handed over to the first cell, and wherein the first cell and the A cell are operating on the same frequency carrier in the OFDM system or are operating in the same radio access technology system other than the OFDM, and wherein the A cell is located on another site than the first and second cells, comprising the steps of:

evaluating signal strength measures on the A cell, and signal strength measures on the first cell and that are received from the terminal;

selecting the first cell for supporting the terminal if the signal strength from the first cell is better than the signal strength of the A cell.

5. A method according to claim 4, wherein the evaluating step also comprises evaluating quality measures on the second cell and that are received from the terminal.

6. A method according to claim 4, wherein the first cell is selected on the further condition of the quality measure of the first cell is not worse than a offset value below the quality of the A cell.

7. A method according to claim 4, wherein the first cell is selected on the further condition of the quality measure of the first cell is at least at a predefined level.

8. A method according to claim 5 wherein the A cell when serving the terminal produces a list of recommended cells, and information on those of the recommended cells that are co-located with first cell and their respective quality measure as reported by the UE when served by the A cell, is forwarded to the first cell, and if quality is not improved in the first cell and if the second cell quality is better than that of the first cell the terminal is handed over to the second cell.

9. A method according to claim 1, wherein when the second cell serves the terminal, the terminal measures and report quality and signal strength of the second cell and at least one third cell, wherein the second and third cells are located on separate sites, and the second and third cell operate on the same frequency carrier, wherein handover to one of the third cells is decided if the signal strength of said one third cell is better that that of the second cell.

10. A method according to claim 1, wherein RSRQ is the quality measure and RSRP is the signal strength respectively measured by the terminal in a cell applying OFDM technology.

11. A method according to claim 1, wherein CPICH Ec/Io is the quality measure and CPICH RSCP is the signal strength measured by the terminal in a cell applying WCDMA technology.

2. A method according to claim 2 wherein the quality measures are the received reference signal power of respectively the first and second cell relative to the total interference comprising of intra-cell interference, inter-cell interference and noise from all sources and in the step of adjusting the quality, the intra-cell interference is excluded while comparing the quality of the said first cell with that of the second said cell wherein the said comparison in logarithmic scale is expressed by the operation:

Quality_—

1st_cell−

Quality_—

2nd_cell+P_—

1st_cell−

P_—

2^nd_cell;

where P_—

1^st_cell and P_—

2^nd_cell are the power transmitted on the frequency carrier of the 1^st and 2^nd cells respectively and Quality_—

1st_cell and Quality_—

2nd_cell are the quality measure on respective cell as received from the terminal.

12. A mobile terminal arranged for communication with a cellular radio network via a serving cell, and arranged for performing quality measurements on the serving ceil and on OFDM technology cells neighbouring the serving cell, and further arranged for receiving information on which neighbouring cell/s that is/are co-located with a serving radio base station, characterized by further comprising:

means for receiving, from the serving radio base station, a compensation factor for the co-located neighbouring cell/s and for the serving cell if the serving cell is an OFDM technology cell;

means for adjusting the quality measure of the co-located neighbouring cell/s with their respective compensation factor, and for adjusting the quality of the serving cell if the serving cell is of OFDM technology and,

means for evaluating the cells by comparing the adjusted quality measure/s of the one or more co-located neighbouring cell/s with the quality measure of the serving cell, as adjusted if adjustment has been made on the serving cell, for the purpose of potentially reporting an event to the serving radio base station,

13. The mobile terminal of claim 12, wherein the receiving means is adapted for receiving the compensator factor as included in a threshold value for the respective co-located neighbour cell/s, and the difference in the quality measures of the serving cell and neighbour cell is compared to the threshold value, and dependent on the comparison the event report is triggered.

14. The mobile terminal of claim 12, wherein, for each of the co-located neighbouring cells, the adjusting means is arranged to adjust in logarithmic scale by the operation:

Quality_of_serving_cell−

Quality_of_neighbour_cell+compensation factor.

wherein the compensation factor comprises power transmitted on the frequency carrier in the serving cell minus the power transmitted on the frequency carrier in the serving cell.

15. The mobile terminal of claim 12 wherein the compensator factor is received for the respective neighbouring cells that belong to an OFDM system.

16. A radio base station that is,

arranged for providing at least two cells on a respective OFDM frequency carrier,

arranged for supporting communication with one or more mobile terminal/s in each cell,

arranged for adapting the power transmitted on each frequency carrier in accordance to a varying traffic load in the respective cell, and further,

arranged for receiving from a first mobile terminal quality measures on a first of said two or more cells that is serving the mobile terminal, and a quality measure on the second of said two or more cells, wherein the quality measures comprise an intra-cell interference component,

characterized by being,

arranged for adjusting the quality measure of respectively the first and the second cell by compensating for the intra-cell component by use of the information on the power transmitted on the frequency carrier of the respective cells;

arranged for evaluating the best fitted of the first and second cells for serving the first mobile terminal, by comparing the adjusted quality measure of the first cell with the adjusted quality measure of the second cell.

17. The radio base station of claim 16, wherein the radio base station is further arranged to provide a compensation factor to the one or more mobile terminal/s, for the mobile terminal/s to adjust the quality measure of the first and second cell, and where the said compensation factor in logarithmic scale is expressed by:

P 1st cell−

P 2^nd cell.