WO2002021756A2

WO2002021756A2 - System and method of determining signal quality for different frequencies in a mobile telecommunications network

Info

Publication number: WO2002021756A2
Application number: PCT/CA2001/001231
Authority: WO
Inventors: Angelo Cuffaro; Hongxi Li
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2000-09-06
Filing date: 2001-09-05
Publication date: 2002-03-14
Anticipated expiration: 2003-03-06
Also published as: AU2001287439A1; WO2002021756A3

Abstract

A method and system for estimating signal quality in a mobile telecommunication network involves generating an estimated signal quality offset(32) between first and second frequency bands in a code division multiple access network. A base station transmits the estimated signal quality offset to a plurality of mobile stations in the CDMA network. At least one of the mobile stations operating in the first frequency band measures a current signal quality (34) on the first frequency band and uses the measured value along with the received estimated signal quality offset to calculate an expected signal quality for the second frequency band (36).

Description

SYSTEM AND METHOD OF DETERMINING SIGNAL QUALITY FOR

DIFFERENT FREQUENCIES IN A MOBILE

TELECOMMUNICATIONS NETWORK

BACKGROUND OF THE INVENTION

Technical Field of the Invention

The present invention relates generally to the estimation of signal quality in a cellular telecommunication network, and in particular to the facilitation of efficient bandwidth allocation among mobile stations.

Description of Related Art

According to wide band code division multiple access (WCDMA) standards, multiple users are able to communicate via the same radio interface by using unique spreading codes to encode each of their respective signals rather than by using numerous different time slots and frequencies as in time division multiple access (TDM A) systems. As a result, a large number of users can simultaneously communicate via the same frequency. Because each WCDMA frequency requires a large amount of bandwidth (e.g., currently 5 MHz, and up to 20 MHz in the future), however, most WCDMA providers are only authorized to provide services on one or two frequency bands. One consequence of this limitation is that adjacent cells typically operate on the same frequency or frequencies.

Because each WCDMA frequency band is shared by a large number of users, as the number of users operating on the same WCDMA frequency increases, the amount of interference on each frequency band also increases. The amount of noise or interference from other users, therefore, can provide a good estimate of the capacity of the WCDMA system. In particular, as the interference approaches some maximum threshold, it can be determined that the system is approaching its maximum capacity.

Some WCDMA operators (i.e., those who have been allocated a large amount of bandwidth) have sufficient capacity to provide WCDMA services on two different frequencies in each cell. In other words, a particular cell in the WCDMA network might have a first and a second frequency while an adjacent cell would have the same first and second frequencies, wherein the first and second frequencies in each cell provide two distinct layers of frequencies for use in communications. Such a configuration provides operators with additional capacity and can be used to balance or reduce the amount of interference in the network. To perform such load balancing, however, mobile stations much perform a hard handover between the first and second frequencies within the same cell. Before performing a hard handover, the mobile station and or the system should determine whether the quality of service on the target frequency will be sufficient to provide adequate service. One way of making such a determination is to allow WCDMA mobile stations to measure the quality of service on the target frequency by operating in a slotted mode, wherein the mobile station briefly tunes to the target frequency and measures the quality of service on that frequency. To minimize the costs of mobile stations, mobile stations are generally designed to handle only one frequency at a time. In other words, mobile stations have only one receiver. Accordingly, when a mobile station is involved in a communications session, the mobile station is using one frequency and therefore cannot receive on another frequency. The slotted mode of operation allows data that is transmitted and received by the mobile station to be compressed, thereby opening up a time window in which the mobile station can switch to the target frequency and make quality measurements.

In addition to being complex, the slotted mode scheme has other disadvantages as well. First, compressing the data reduces the processing gain. To compensate for this reduction, the transmit power of the mobile station must be increased, which increases the drain on the battery. Furthermore, data compression causes a deterioration in signal quality, which is not acceptable in some situations, particularly in the case of high data rate services.

There is a need, therefore, for a system and method of efficiently determining a quality of communications on different frequencies in a cellular communications system. Preferably, such a system and method would be relatively simple, would not result in any significant drain on power resources, and would not cause a deterioration of the quality of ongoing communications. SUMMARY OF THE INVENTION

The present invention comprises a method and system for estimating signal quality between different frequencies in a cellular telecommunication network. In one embodiment of the invention, a telecommunication system includes a plurality of mobile stations that communicate using a code division multiple access (CDMA) protocol. At least one of the mobile stations communicates on a first CDMA frequency band. The system also includes a plurality of cellular base stations for communicating with the plurality of mobile stations using the CDMA protocol. Each base station transmits a quality offset indication representing an estimated quality offset between the first CDMA frequency band and a second CDMA frequency band in each sector. The at least one of the mobile stations measures a signal quality for the first CDMA frequency band and calculates an expected signal quality for the second CDMA frequency band using the measured signal quality and the quality offset indication transmitted by the base station. In another embodiment of the invention, there is provided a method for estimating signal quality in a mobile telecommunication network. The method comprises the steps of generating an estimated offset between a first frequency band and a second frequency band in a code division multiple access (CDMA) network and measuring a signal quality for the first frequency band. Finally, an estimated signal quality for the second frequency band is calculated using the estimated quality offset and the measured signal quality for the first frequency band.

In yet another embodiment of the invention, there is provided an additional method for estimating signal quality in a mobile telecommunication network. In this method, a first estimated quality offset between a first frequency band and a second frequency band in a first code division multiple access (CDMA) cell is generated, and a second estimated quality offset between the first frequency band and the second frequency band in a second code division multiple access (CDMA) cell is also generated. Then, a mobile station that is in a soft handover mode involving the first cell and the second cell measures a signal quality for the first frequency band for signals received from each of the first and second cells. Finally, an estimated signal quality for the second frequency band is calculated using the measured signal qualities and the estimated quality offsets. The calculation includes weighting the measured signal qualities and the estimated quality offsets based on a percentage of signal received from the first cell and a percentage of signal received from the second cell.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:

FIGURE 1 illustrates a sectorized cell in a cellular telecommunications system in which the present invention can be implemented;

FIGURE 2 illustrates a flow diagram of a preferred method for determining quality offset estimates between different frequencies in a cellular communications system;

FIGURE 3 depicts an illustrative chart showing interference levels on each of two frequency bands over time in connection with a handover initiation scheme that balances interference on each of two cellular communication frequencies; and

FIGURE 4 depicts an illustrative chart showing interference levels on each of two frequency bands over time in connection with a scheme for triggering handovers when interference on a particular frequency band reaches a preselected threshold.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to the drawings wherein like reference characters denote like or similar parts throughout the various figures. Referring now to FIGURE 1, there is illustrated a sectorized cell 2 in a WCDMA cellular telecommunications system in which the present invention can be implemented. The sectorized cell 2 includes three sectors 10, each of which can also be referred to as an individual cell.

Communications within each of the sectors 10 is supported by a single base station, or site, 12. The base station 12 includes three directional antennas (not shown), one for each of the sectors 10. The base station 12 supports communications in each sector 10 on two different channels or frequencies. Thus, the base station 12 might communicate with a first mobile station 14(1) over an interface 16 using a first frequency, while communicating with a second mobile station 14(2) via the air interface 16 using a second frequency.

In certain situations, interference on one of the frequencies might cause a sufficient deterioration of the signal quality that it becomes desirable to move some mobile stations 14 to a different frequency. This can be accomplished by performing a hard handoff of particular mobile stations 14 from a current channel, which corresponds to a first frequency band to a target channel, which corresponds to a second frequency band. Before performing such a hard handoff, however, it is desirable to determine whether each mobile station 14 to be handed off will obtain an acceptable quality level on the target channel.

In accordance with the present invention, the WCDMA cellular system stores an estimated quality offset between the two frequencies. The system transmits the estimated quality offset value from the base station 12 to each mobile station 14 via the air interface 16. Then, when a mobile station 14 is to be handed off to another frequency using a hard handoff procedure, the mobile station uses the received quality offset estimate along with a measured signal quality on the current frequency to calculate an expected quality level on the target frequency. The expected quality level can then be used to determine whether the handoff would be beneficial. If so, the mobile station 14 is handed over to the new frequency. In accordance with a preferred embodiment of the invention, the mobile station 14 then measures the signal quality on the new frequency, calculates the actual quality offset, and reports the calculated value to the base station. Alternatively, the mobile station 14 can simply report the actual signal quality on each of the two channels and allow the base station 12 to calculate the actual offset. An adoptive algorithm in the base station 12 or elsewhere in the system then adjusts the quality offset estimate using the reported data. Although the invention is described in connection with a WCDMA system, it can also be implemented in other types of code division multiple access (CDMA) systems as will be recognized by those of ordinary skill in the art. Typically, when cell planning is performed, the two or more WCDMA frequencies in each sector or cell 10 are designed to handle the same coverage area. Accordingly, the estimated quality offset of the present invention is based on the assumption that the two frequencies have the same geographic coverage area. If the cells 10 do not have the same coverage area, an additional quality offset (e.g., 3 d B) that defines a quality difference in terms of path loss between the two frequencies should be used.

By using an iterative approach wherein actual quality offset measurements are made in connection with every hard handoff, the quality offset estimate of the present invention can be continuously updated. In other words, as the actual quality offset changes over time, data received by the system as a result of ongoing hard handoffs can be used to adjust the quality offset estimate accordingly. Furthermore, the more handoffs that occur per unit of time, the more accurate the quality offset estimate becomes. Thus, when there is a significant amount of traffic in the particular sector or cell 10, hard handoffs will occur relatively frequently. These frequent handoffs will result in frequent adjustments to the quality offset estimate, thereby improving the accuracy of the estimate.

In one embodiment of the invention, one estimated quality offset value is provided for each pair of frequencies in each cell 10. In an alternative embodiment, different quality offset estimates can be provided for different areas of the cell 10. For example, the system might provide a first quality offset estimate for mobile stations located close to the base station 12 (e.g., within a first zone 18), a second quality offset estimate for mobile stations 14 located in the middle of the cell 10 (e.g., within a second zone 20), and a third quality offset estimate for mobile station 14 located near the outer edge of the cell 10 (e.g., within a third zone 22). In yet another alternative embodiment, additional quality offset estimates can be provided between different sectors or cells 10. For instance, assuming a mobile station 14 is located near a border between two cells 10, a quality offset estimate could be provided between a first frequency in a first cell 10(1) and a second frequency in an adjacent cell 10(2).

Also in accordance with the invention, the values represented by the quality offset estimates and calculations can relate to a number of different types of quality measurements. For example, signal quality can be measured in terms of path loss or of a signal strength to interference (C/I) ratio, as measured by the mobile station 14. Similarly, signal quality 10 can be represented simply by the received signal strength. As additional alternatives, signal quality can be represented by the measured interference on uplink signals or by the bit error rate for voice communications or the unit error rate for packet data communications. Referring now to FIGURE 2, there is illustrated a flow diagram of a preferred method for determining quality offset estimates between different frequencies in a cellular communications system. First, an initial quality offset estimate value is set at step 32. The initial value can be selected, for example, to represent a rough estimate of the actual quality offset between the two frequencies (i.e., based on the amount of interference in each frequency band or on the number of users that are initially using each of the frequencies). As an alternative, the initial quality offset value can be set to represent some preselected threshold above which mobile stations 14 will begin to be off-loaded to a different frequency band. Once the quality offset estimate is initialized, each mobile station 14 periodically measures the signal quality on its respective current channel at step 34.

At step 36, the mobile station estimates the signal quality on other available channels, such as channels that operate on other frequencies within the same cell 10 or in other adjacent cells 10, using the measured signal quality. In particular, the mobile station calculates the estimated signal quality according to: Q_E = Qc -t- Qoffset, wherein Q_E is the estimate of signal quality on the potential target WCDMA frequency band, Q_c is the signal quality on the current WCDMA frequency band (as measured by the mobile station 14), and Q_offset is the quality offset estimate between the current and potential target frequency bands (as received by the mobile station 14 from the system). In some instances, the estimated signal quality will be calculated for one potential target frequency band in the same cell 10 as the current frequency band. In other instances, multiple signal quality estimates will be calculated (e.g., when there are multiple potential target frequency bands).

Based on the results of the signal quality estimate, it is determined whether a hard handoff is necessary at step 38. The determination of whether to perform a hard handoff can be made in a variety of different ways. In general, the purpose of the hard handoff is to prevent interference in one frequency band from becoming too excessive and/or to balance the load between frequency bands. Accordingly, as the quality offset between two frequency bands grows, it might be desirable to switch some mobile stations 14 to the other frequency band. Generally, a mobile station 14 is in communication with one or more sectors

10, which form that mobile station's active set of sectors 10. Each sector 10 in the system has a defined set of neighbor cells/sectors, a list of which is broadcast by the system to the mobile station 14. The mobile station 14 measures the signal quality of signals that are received from the neighbor cells/sectors (i.e., the measurement set). If the mobile station 14 is in soft handover with two serving sectors 10(1) and 10(3), the measurement set consists of the collection of the neighbor cells/sectors 10 for the serving sectors 10(1) and 10(3).

By conducting signal quality measurements on the cells/sectors 10 in the measurement set, a hard handover decision can be made in one of two situations. In both situations the signal quality of a potential target sector 10 can be calculated by:

Q₂ = Qι + o₁₂, where Q_! is the measured quality (in terms of uplink interference) for a section S_x, in the measurement set having a first frequency, O₁₂ is the offset value between the sector S_! and a potential target sector S₂, and Q₂ is the calculated signal quality for the potential target sector S₂, which is in the same sector 10 as the sector S_l5 but is on a different, second frequency. In a first situation, the sector S, is the active set. If

Ql ^"*^" O₁₂ ≥ Ql_Threshold5 wherein Qι_τ_hreshoi_d represents a selected threshold quality, then a hard handover to the target sector S₂ is performed. In this case, the hard handover involves a switch to the second frequency in the same sector 10.

In the second situation, the sector S_[ is not in the active set. If Qi + Oι₂ " Qb > ^Hard Handover Threshold wherein Q_b denotes the quality value of the best sector in the active set, then a hard handover to the target sector S₂ is performed. In this case, the hard handover involves a switch both to the second frequency and to a sector 10 that is not currently part of the active set. Thus, the second situation facilitates hard handover when the quality (i.e., Q_j + Q₁₂) obtainable in a different sector and on a different frequency would be better than the current quality (i.e., Q_b) by some predetermined amount (i.e., the Hard Handover Threshold).

In one embodiment, the initial quality offset value can be set (at step 32) such that, if the interference in one frequency band exceeds some preselected threshold, mobile stations 14 in that frequency band will begin to handoff to a different frequency band. For instance, when the sector S _{ in the above example is in the active set, the Q_{{ ThreshoId} value can be set to be X percent of the maximum uplink tolerable interference. Accordingly, the system will start to move some mobile stations 14 to the second frequency when the interference on the first frequency reaches the limit established by the Q_{{ τhresho}ι_d value. In another alternative, the system can take into account the handover interval t, together with the Qj _Threshol value to determine when mobile stations 14 should be moved to the second frequency.

In another alternative, the system can be designed such that interference on the two or more frequency bands will remain relatively balanced. For example, the quality offset could be initially set to zero and could subsequently be adjusted depending upon the amount of traffic or interference in the two frequency bands. By initiating hard handoffs to counteract changes in the quality offset, however, traffic and interference could be maintained at a relatively even distribution between the frequency bands .

Referring now to FIGURE 3, there is depicted an illustrative chart showing interference levels 60 and 62 over time on each of two frequency bands f_x and f₂ in connection with the latter of the above alternatives. Because mobile stations 14 are redistributed evenly between the frequency bands, in accordance with changes in interference levels, the interference curves 60 and 62 for each frequency band are essentially identical. Referring now to FIGURE 4, there is depicted an illustrative chart showing interference levels 64 and 66 on each of two frequency bands f_t and f₂ over time in connection with the former of the above handover initiation alternatives. In this case, most or all of the mobile station 14 traffic in the cell 10 is placed in the first frequency band f, until the interference level in that frequency band f, reaches a preselected threshold level (as indicated at 68). Mobile stations 14 are subsequently directed to or handed over to the second frequency band f₂ to prevent too much interference in the first frequency band fj.

Returning now to FIGURE 2, if it is determined at step 38 that a hard handoff is not necessary, the process 30 returns to step 34 where the mobile station 14 periodically measures the signal quality on the current channel. If, on the other hand, it is determined that a hard handoff is necessary, then the mobile station 14 selects a channel and a sector 10 at step 40. In addition, the mobile station 14 can, in some cases, select more than one sector. A selection of more than one sector or cell 10 can occur, for instance, when the mobile station 14 is at or near a border between sectors or cells 10. This is because WCDMA systems use a soft handoff procedure, wherein mobile stations 14 that are moving between sectors or cells 10 communicate with more than one base station 12 or sector antenna at the same time. In other words, a mobile station 14(3) in soft handover might be receiving sixty percent of its signal via an air interface 16(1) connection in a third cell 10(3) and forty percent of its signal via an air interface 16(2) connection in the first cell 10(1), wherein both connections utilize the same frequency band.

Accordingly, when a hard handoff involves a mobile station 14 that is in soft handoff, the mobile station will typically switch from a soft handoff procedure (involving the same base stations 12 or cells 10) in one frequency band to a soft handoff procedure in another frequency band. In such a situation, the calculation of the estimated signal quality involves a use of quality offset estimates in both of the sectors or cells 10 that are involved in the soft handover. In particular, the quality estimate for the target sectors or cells 10 can be calculated using a weighted algorithm, wherein the weighting corresponds to the signal distribution percentages between the cells 10:

QE ^{= C}lQci + ^C ₂Qc₂ ^{" "} clQoffsetl ^{+ C} ₂Qofset2-

The value c_x and c₂ in this equation represent the signal distribution percentages between the two cells (i.e., the mobile station 14 receives c_λ percent of its signal from a first cell 10 and c₂ percent of its signal from a second cell 10), while Q_cl and Q_c2 represent the signal quality of the current WCDMA frequency band in each cell 10.

Finally, Q_offsetl is the offset between the current and target frequency bands in the first cell 10 and Q_offset2 is the offset between the current and target frequency bands in the second cell 10. As a result, the estimated signal quality for a soft handoff connection in the target frequency band can be determined and used in steps 38 and 40 to decide whether a hard handoff should be performed and to select the target frequency and the sectors or cells 10.

Once the channel and the sector or sectors 10 are selected, a hard handoff is performed at step 42. After the handoff, the mobile station 14 measures the actual signal quality on the new channel at step 44 and reports the measured signal quality to the system at step 46. Finally, the system calculates a new estimated quality offset at step 48. The new estimated quality offset is calculated using an adaptive algorithm that is designed to cause the estimate to approximate the actual offset. The calculation can be a function of a number of parameters, including the measured signal quality reported by the mobile station 14, the previous quality offset estimate (i.e., the one used by the mobile station 14 to calculate the target cell signal quality estimate), the estimated signal quality calculated by the mobile station 14 before the hard handoff, and the signal quality on the original channel as measured by the mobile station 14, as well as additional parameters such as the handover threshold, the time interval between or frequency of handoffs between the two channels (which provides information as to the reliability of the previous estimate), and the mobile station's approximate location within the cell. Once the new estimated quality offset is calculated, the process 30 returns to step 34 where each mobile station 14 periodically measures the quality on its respective current channel.

Although various preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A telecommunication system comprising: a plurality of mobile stations for communicating using a code division multiple access (CDMA) protocol, at least one of the plurality of mobile stations communicating on a first CDMA frequency band; at least one cellular base station for communicating with the plurality of mobile stations using the CDMA protocol, said at least one base station transmitting a quality offset indication representing an estimated quality offset between the first CDMA frequency band and a second CDMA frequency band; and wherein the at least one of the plurality of mobile stations measures a signal quality for the first CDMA frequency band and calculates an expected signal quality for the second CDMA frequency band using the measured signal quality and the quality offset indication transmitted by the at least one base station.

2. The system of claim 1, wherein the CDMA protocol comprises a wideband CDMA (WCDMA) protocol.

3. The system of claim 1 , wherein the at least one mobile station uses the expected signal quality to determine whether to handoff to the second CDMA frequency band.

4. The system of claim 3, wherein the determination of whether to handoff includes comparing the expected signal quality to a quality threshold.

5. The system of claim 3 , wherein the at least one mobile station performs a handoff to the second CDMA frequency band based on said determination.

6. The system of claim 5, wherein the at least one mobile station further measures a signal quality for the second CDMA frequency band after performing the handoff and reports the measured signal quality for the second frequency band to the at least one base station.

7. The system of claim 6, wherein the measured signal quality reported by the at least one mobile station is used to calculate an updated quality offset indication for subsequent transmission by the at least one base station.

8. The system of claim 5 , wherein the at least one mobile station further measures a signal quality for the second CDMA frequency band after performing the handoff, calculates an actual quality offset between the first CDMA frequency band and the second CDMA frequency band using the measured signal quality for each of the first and second CDMA frequency bands, and reports the actual quality offset to the at least one base station.

9. The system of claim 8, wherein the reported actual quality offset is used to calculate an updated quality offset indication for subsequent transmission by the at least one base station.

10. The system of claim 1, wherein the quality offset indication is associated with a predetermined portion of a territory served by the at least one base station.

11. The system of claim 1 , wherein the at least one mobile station is in a soft handoff mode involving the at least two CDMA base stations, said at least one mobile station calculating an expected signal quality for the second CDMA frequency band using a quality offset indication transmitted by each of the two CDMA base stations.

12. A method for estimating signal quality in a mobile telecommunication network, comprising the steps of: generating an estimated offset between a first frequency band and a second frequency band in a code division multiple access (CDMA) network; measuring a signal quality for the first frequency band; and calculating an estimated signal quality for the second frequency band using the estimated quality offset and the measured signal quality for the first frequency band.

13. The method of claim 12, wherein the step of generating an estimated quality offset includes setting an initial estimated quality offset based on an amount of interference in each of the first frequency band and the second frequency band.

14. The method of claim 12, wherein the step of generating an estimated quality offset includes setting an initial estimated quality offset based on a preselected threshold for initiating hard handoffs between the first frequency band and the second frequency band.

15. The method of claim 12, further comprising the step of performing a hard handoff from the first frequency band to the second frequency band in response to the estimated signal quality being above a predetermined threshold.

16. The method of claim 15, further comprising the steps of: measuring an actual signal quality for the second frequency band after performing the hard handoff; and calculating an actual quality offset using the measured signal quality for the first frequency band and the measured signal quality for the second frequency band.

17. The method of claim 16, further comprising the step of generating an updated estimated quality offset using the current estimated quality offset and the calculated actual quality offset.

18. The method of claim 12, wherein the step of generating an estimated quality offset comprises updating a previous estimated quality offset based at least in part on measurements conducted by a mobile station in the first frequency band before performing a hard handoff and in the second frequency band after performing the hard handoff.

19. The method of claim 12, wherein the estimated quality offset is associated with a predetermined territory.

20. A method for estimating signal quality in a mobile telecommunication network, comprising the steps of: generating a first estimated quality offset between a first frequency band and a second frequency band in a first code division multiple access (CDMA) cell; generating a second estimated quality offset between the first frequency band and the second frequency band in a second code division multiple access (CDMA) cell; measuring, at a mobile station in a soft handover mode involving the first cell and the second cell, a signal quality for the first frequency band for signals received from each of the first and second cells; and calculating an estimated signal quality for the second frequency band using the measured signal qualities and the estimated quality offsets, wherein said calculation includes weighting the measured signal qualities and the estimated quality offsets based on a percentage of signal received from the first cell and a percentage of signal received from the second cell.