METHOD FOR DETECTING AND COMPENSATING FOR RADIO LINK TIMING ERRORS

Abstract: A method for detecting and compensating for radio link timing errors in a mobile communication system is disclosed, whereby a mobile station (16) determines that one or more radio link transmissions (14) from the network are outside or nearly outside of that mobile station's (16) receiving time window (18), and reports such information to the network for appropriate action.

Description: METHOD FOR DETECTING AND COMPENSATING FOR RADIO LINK TIMING ERRORS CROSS-REFERENCES TO RELATED APPLICATIONS This Application for Patent claims the benefit of priority from, and hereby incorporates by reference the entire disclosure of, co-pending U.S. Provisional Application for Patent Serial No. 60/174,064, filed December 30, 1999. BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates in general to the mobile communications field and, in particular, to a method that can be used in a mobile communication system to detect and compensate for radio link timing errors. Description of Related Art The operations of certain mobile communication systems such as, for example, Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) mobile communication systems depend on the use of a timing reference for synchronization purposes. Locally, in a Base Station (BS), this timing reference is generated by an internal oscillator. However, this internal oscillator can also be locked to and synchronized with an external long-term timing reference. Certain design problems are associated with the use of BS timing references for network time synchronization. For example, an oscillator used by a BS to generate an internal timing reference can drift in frequency, which reduces the accuracy of the timing reference being generated. Also, a BS's timing (as perceived by a Mobile Station (MS)) can shift or slide in time because the MS is moving toward or away from the BS (mobility), or the radio environment is changing and essentially moving over time (time dispersion). This problem is sometimes referred to as "MS timing shift". In any event, it can be seen that the propagation delay on the radio link can vary over time. In a mobile communication network, each BS can define one or more cells. Each cell connects the BS (and the radio network) to a particular MS via a unique radio link (radio air interface connection). Normally, there are no particular timing problems between different/multiple radio links belonging to one BS and directed to one MS. However, when two or more cells belong to different BSs, the timing between such cells may create a significant design problem. In this regard, each MS has a "receiving time window" or interval within which all radio link transmissions are to be received. The timing references defined for mobile systems operating in accordance with the "IS-95" and "CDMA2000" protocols are based on the use of highly accurate Global Positioning System (GPS) timing signals, which provide a relatively stable timing reference in all network nodes. Consequently, for example, in IS-95 and CDMA2000 systems, each BS has a relatively accurate timing reference with respect to inter-BS timing that can be used to maintain the radio link transmissions in the MSs' receiving time windows. However, the radio link transmissions can still move within the receiving time windows due to the effects of MS mobility and/or time dispersion. In any event, a significant design problem exists for those (e.g., CDMA) mobile systems that use the less accurate inter-BS time synchronization methods, because some (one or more) of the radio links in such systems can move out of the MSs' receiving time windows. In existing and developing mobile communication systems (e.g., certain Wideband CDMA or WCDMA systems), when an MS is connected by radio links to numerous cells of different BSs, the synchronization between these cells can drift in phase. This problem arises because these systems do not use (or plan to use) an absolute time reference (e.g., such as the GPS) for synchronizing the different BSs involved. Actually, the problem of improper "phase drift" in this context can be assumed to occur because no absolute timing reference is being used for inter-BS synchronization, or some form of inadequate (e.g., sub-optimal or not always available) network time synchronization method is being used. A solution for handover evaluations in existing cellular systems with macro- diversity is based on the power level of the signal received by the MS. However, with this approach, it is normally not possible for a network to determine whether or not a radio link transmission is outside an MS's receiving time window. Consequently, it is clear that a significant need exists for a method that will enable a mobile communication system which is operating without an accurate network timing reference, to determine when one or more radio link transmissions have moved outside or nearly outside of an MS 's receiving time window. As described in detail below, the present invention successfully resolves this problem and other related problems. SUMMARY OF THE INVENTION In accordance with a preferred embodiment of the present invention, a method for detecting and means for compensating for radio link timing errors is provided, whereby an MS determines that one or more radio link transmissions are outside or nearly outside of that MS's receiving time window, and reports such information to the network for appropriate action. An important technical advantage of the present invention is that a method for mobile communication network time synchronization is provided that does not require the use of an absolute timing reference such as the GPS. Another important technical advantage of the present invention is that a method is provided for minimizing the effects of phase drift between cells in a mobile communication system. Still another important technical advantage of the present invention is that a method is provided for improving the accuracy of inter-BS timing in a mobile communication system and thereby decreasing the uncertainty (for an MS) of other BS timing, which improves cell searching processes. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: FIGURES 1A, IB and 1C are related diagrams that illustrate how radio link timing errors can be detected and compensated for, in accordance with a preferred embodiment of the present invention; FIGURES 2A and 2B are related diagrams that illustrate a second aspect of the preferred embodiment; FIGURES 3 A and 3B are related diagrams that illustrate a third aspect of the preferred embodiment; and FIGURES 4A and 4B are related diagrams that illustrate a fourth aspect of the preferred embodiment. DETAILED DESCRIPTION OF THE DRAWINGS The preferred embodiment of the present invention and its advantages are best understood by referring to FIGURES 1 A-4B of the drawings, like numerals being used for like and corresponding parts of the various drawings. Essentially, in accordance with a preferred embodiment of the present invention, a method for detecting and compensating for radio link timing errors is provided, whereby an MS determines that one or more radio link transmissions are outside or nearly outside of that MS's receiving time window, and reports such information to the network on the uplink for appropriate action. The threshold levels that are used to determine whether or not a radio link transmission is outside the MS's receiving time window, are sent to the MS from the network. Specifically, FIGURES 1 A, 1 B and 1 C are related diagrams that illustrate how radio link timing errors can be detected and compensated for, in accordance with a preferred embodiment of the present invention. Referring to FIGURE 1 A, a section 10 of an exemplary mobile communication system is shown. For example, such a system can be a CDMA system that functions in accordance with an existing or future CDMA protocol (e.g., IS-95, CDMA2000, or the so-called 3 rd Generation system). However, the present invention is not limited to a system that functions in accordance with a particular protocol and can include, for example, any telecommunication system with radio link timing problems that need to or should be resolved. System 10 includes a Base Station (BS) 12, which is connected via an appropriate communications link to a network. With the configuration shown, the BS 12 defines a single cell and associated radio link 14. As such, an MS 16 can be connected for communication with the network via the radio link 14 and BS 12. FIGURE IB is a timing diagram that illustrates how an MS (e.g., 16) can determine if a radio link (e.g., 14) transmission is moving out (or has nearly moved out) of the MS's receiving time window (e.g., 18), in accordance with the preferred embodiment of the present invention. In general, one downlink (forward) code is used for each radio link. These codes should be orthogonal with respect to a BS. Consequently, for this exemplary embodiment, a new radio link setup should be aligned to the closest 256-chip boundary. Therefore, in a 3 rd Generation CDMA system, for example, since no tight/accurate inter-BS synchronization exists, a radio link can be positioned relatively close to a border of an MS's receiving time window. As shown, for this exemplary embodiment, one radio link (14) is involved. If the received radio link 14 is located relatively close to the center of the MS's timing window 18, and there is no sudden change in the MS's distance from the BS or in the radio environment, the MS can follow the downlink signal relatively slowly and accurately with the timing of the uplink transmission. However, on the other hand, if the received radio link is located far enough away from the center of the window 18, then this situation can create a more complicated problem (which, as described below, the present invention resolves). For this exemplary scenario, now referring to FIGURE 1 C, it can be assumed that the MS 16 has just completed a handover from one or more other cells (not shown) to the cell associated with the BS 12 and radio link 14. Also, each of the other radio links (connected prior to the handover) has been released from its respective connection with the MS 16. The remaining radio link 14 being received by the MS 16 is offset from the center of the receiving time window 18 by, for this example, approximately 128 chips. Since the width of the receiving time window from boundary to boundary is, for this example, slightly larger than 256 chips, the received radio link 14 is located near a boundary of the receiving time window 18. Consequently, a slight movement of the radio link (e.g., due to the effects of time- dispersion or radio environment changes) can cause the radio link to slide out of the receiving time window. As such, in accordance with the preferred embodiment of the present invention, the MS 16 transmits a message to the network (on the uplink) which reports that the radio link 14 has moved beyond the borders that the radio network had previously signaled to the MS. These borders could represent a slightly smaller window size than that of the MS's receiving time window 18. The network can then take appropriate action in response to such a report (as discussed in detail below). Alternatively, before the radio link 14 actually moves outside the receiving time window, the MS 16 can transmit a message to the network which reports that the radio link 14 is near a boundary of the window 18. Again, as discussed in more detail below, the network can take appropriate action in response to such a report. For example, the MS can be directed to transmit a report message to the network if a radio link transmission is offset from the center of the timing window by an amount which is greater than selected predetermined values (i.e., each value related to a "side" of the window). For example, a timing window can contain two threshold values. One such threshold value can be related to a "less than" (e.g., located to the left of the first border) report from the MS. The other threshold value can relate to a "greater than" report from the MS (e.g., located to the right of the second border). As such, in response, an appropriate action by the network is to adjust the radio link timing in order to re-position the radio link to an acceptable location within the window. FIGURES 2A and 2B are related diagrams that illustrate a second aspect of the preferred embodiment. In this case, referring to FIGURE 2 A, the MS 16 is connected to the network by two radio links 14 and 114 (via BS 12 and 112, respectively). However, as shown in FIGURE 2B, the two radio links 14 and 114 are significantly separated in time (e.g., the BSs 12 and 112 are connected to different BSs). In this case, the MS 16 can select one of the radio links to follow in the receiving time window (e.g., the strongest radio link). Nevertheless, a number of different timing problems can occur. For example, because of time-dispersion effects or the effects of mobility along with changes in the line-of-sight between the transmitter and receiver (i.e., the radio link can alternate between being in and out of the line-of-sight), one of the radio links can slide away from the other, or a radio link can just disappear from the window and then pop up (be detected) again outside. For this example, in accordance with the present invention, the MS can transmit a message to the network which reports a radio link that has not been detected for a predetermined length of time. Notably, one design choice (but not to be considered a limitation on the present invention) is that two windows can be used for detecting the radio link. A larger window can be used whereby an MS (e.g., User Equipment or UE) can detect the radio link within that window. A smaller window can also be used whereby an MS can report to the network that the radio link is located close to the border of the larger window. However, for this example, if the radio link pops up outside the larger window, the MS likely would not detect the radio link and, therefore, not be able to report this information to the network. FIGURES 3 A and 3B are related diagrams that illustrate a third aspect of the preferred embodiment. In this case, referring to FIGURE 3 A, the MS 16 is connected to the network by three radio links 14, 114 and 214 (via BS 12, 112 and 212, respectively). Referring to FIGURE 3B, the MS can determine what action to take depending on the particular circumstances involved. Preferably, an MS reports all detected radio links to the network. Also, if necessary, the network can request relevant information from an MS about the measured cell(s) and then decide what action to take. Notably, although the exemplary terminology "MS" is being used herein to denote a mobile station, mobile terminal, mobile radiotelephone, etc., the equivalent terminology "User Equipment or UE" can also be used. As an alternate approach, in accordance with an embodiment of the present invention, if the weakest radio link (e.g., 214) moves out of the MS's receiving time window, the MS (16) can transmit a message to the network which reports that radio link (214) is outside the window. On the other hand, if the strongest radio link (e.g., 14) moves out of the window, then the MS 16 can be ordered by the network to maintain the connection with (and follow in the window) the strongest radio link and terminate the connections with the two weaker links. As such, the network can interpret the MS's radio link (or related) report and instruct the MS to ignore or terminate a radio link that threatens the transmit power control timing to the uplink. Similar to the second aspect of the embodiment, a radio link can slide out of the receiving time window. FIGURES 4A and 4B are related diagrams that illustrate a fourth aspect of the preferred embodiment. In this case, referring to FIGURE 4A, the MS 16 is connected to the network by four radio links 14, 114, 214 and 314. However, in this case, the four radio links are associated with four BSs and two different timing domains (i.e., radio links 14 and 114 associated with BS 12 and BS 112, and radio links 214 and 314 associated with BS 212 and 312). Referring to FIGURE 4B, if two of the radio links (e.g., 214 and 314) move out of the MS ' s timing window, then the MS 16 can transmit a message to the network which the network can interpret, for example, as a report that the two radio links can be dropped from the active set of radio links. Notably, according to the 3GPP, the terminology "active set" can be defined as the set of radio links simultaneously involved in a specific communication service between user equipment and a UTRAN (Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network) access point. A "UTRAN access point" can be defined as a conceptual point within the UTRAN that performs radio transmission and reception. A UTRAN access point is associated with one specific cell, such that one UTRAN access point exists for each cell. A UTRAN access point is the UTRAN-side endpoint of a Radio Link. In response to a report from an MS that a radio link is outside or nearly outside the MS's receiving time window, the radio network can respond in a number of different ways. For example, the network can delete that radio link from the active set and wait until the MS transmits a message which reports that the MS has detected a new neighbor cell and associated radio link to be placed in the active set. The network then connects that radio link to the MS, which receives the transmission in the timing window. Alternatively, the network can directly connect the radio link which was reported to be outside the timing window to the MS, and direct the MS to include that radio link in the active set. As such, the network does not have to wait for a new neighbor cell report from an MS, and the network can directly setup a radio link after it has been deleted. As another alternative, the radio network can connect another copy of the radio link which was reported outside or nearly outside the MS's receiving time window, in the same cell and then remove the first radio link from the active set. As still another alternative, the radio network can update the active set by sending a "replace" message to the MS, and then removing the first radio link and adding a second radio link as a replacement. In this case, the timing of the new radio link may be different than that of the first radio link. Although a preferred embodiment of the method and apparatus of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment 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 method for detecting and compensating for radio link timing errors in a mobile communication network, comprising the steps of: detecting a radio link transmission timing from said mobile communication network; determining whether or not said detected radio link transmission timing is outside a receiving time window; and if said detected radio link transmission timing is outside said receiving time window, sending a message to said network, said message indicating that said detected radio link transmission timing is outside said receiving time window. 2. The method of Claim 1, wherein said determining step further comprises determining whether or not said detected radio link transmission timing is nearly outside said receiving time window; and if said detected radio link transmission timing is nearly outside said receiving time window, sending said message to said network, said message indicating that said detected radio link transmission timing is nearly outside said receiving time window. 3. The method of Claim 1 or 2, further comprising the step of: responsive to said message, said network adjusting a timing of said radio link transmission. 4. The method of Claim 1 or 2, further comprising the step of: responsive to said message, said network removing said radio link from an active set. 5. The method of Claim 4, further comprising the step of: responsive to a second message, connecting said radio link to a mobile station. 6. The method of Claim 1 , further comprising the steps of: responsive to said message, said network connecting said radio link to a mobile station; and directing said mobile station to include said radio link in an active set. 7. The method of Claim 1 or 2, further comprising the steps of: responsive to said message, said network connecting a copy of said radio link in a same cell; and removing said radio link from an active set. 8. The method of Claim 1 or 2, further comprising the steps of: responsive to said message, said network sending a replace message to a mobile station to update an active set; removing said radio link from said active set; and adding a second radio link to said active set. 9. The method of Claim 1, wherein said network comprises a CDMA network. 10. The method of Claim 1 , wherein the determining step further comprises the steps of: a mobile station receiving at least one radio link transmission timing threshold from said mobile communication network; and comparing said detected radio link transmission timing to said at least one radio link transmission timing threshold. 11. A method for detecting and compensating for radio link timing errors in a mobile communication network, comprising the steps of: detecting a plurality of radio link transmission timings from said mobile communication network; determining whether or not at least one of said plurality of detected radio link transmission timings is inside a receiving time window; and if said at least one of said plurality of detected radio link transmission timings is inside said receiving time window, determining whether or not a second one of said plurality of detected radio link transmission timings is separated from substantially a center of said receiving time window by a predetermined value, and if so, sending a message to said network, said message indicating that said second one of said plurality of detected radio link transmission timings is separated from substantially said center of said receiving time window by said predetermined value. 12. The method of Claim 11 , further comprising the step of: responsive to said message, said network adjusting a timing of said second one of said plurality of detected radio link transmissions timings. 13. The method of Claim 11, wherein said network comprises a CDMA network. 14. The method of Claim 11, wherein the determining step further comprises the steps of: a mobile station receiving at least one radio link transmission timing threshold from said mobile communication network; and comparing said at least one of said plurality of detected radio link transmission timings to said at least one radio link transmission timing threshold. 15. The method of Claim 11 , wherein said predetermined value comprises a threshold value received by a mobile station from said mobile communication network. 16. A system for detecting and compensating for radio link timing errors in a mobile communication network, comprising: means for detecting a radio link transmission timing from said mobile communication network; means for determining whether or not said detected radio link transmission timing is outside a receiving time window; and means for sending a message to said network if said detected radio link transmission timing is outside said receiving time window, said message indicating that said detected radio link transmission timing is outside said receiving time window. 17. The system of Claim 16, wherein said means for determining further comprises means for determining whether or not said detected radio link transmission timing is nearly outside said receiving time window; and means for sending said message to said network if said detected radio link transmission timing is nearly outside said receiving time window, said message indicating that said detected radio link transmission timing is nearly outside said receiving time window. 18. The system of Claim 16 or 17, further comprising: means for adjusting a timing of said radio link transmission timing responsive to said message. 19. The system of Claim 16 or 17, further comprising: means for removing said radio link from an active set responsive to said message. 20. The system of Claim 16, further comprising: means for connecting said radio link to a mobile station responsive to a second message. 21. The system of Claim 16, further comprising: means for connecting said radio link to a mobile station and directing said mobile station to include said radio link in an active set, responsive to said message,. 22. The system of Claim 16 or 17, further comprising: means for connecting a copy of said radio link in a same cell and removing said radio link from an active set, responsive to said message. 23. The system of Claim 16 or 17, further comprising: means for sending a replace message to a mobile station to update an active set, removing said radio link from said active set, and adding a second radio link to said active set, responsive to said message. 24. The system of Claim 16, wherein said network comprises a CDMA network.

Inventors: ANDERSSON KENTH, LUNDH PETER, MUELLER WALTER, NYBERG BO, ANDERSSON, KENTH, LUNDH, PETER, MUELLER, WALTER, NYBERG, BO
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H04Q7
IPC codes: H04B7, H04J13, H04Q7