Patent Application
20090219893
This invention relates to CDMA systems, and more specifically to handover area detection in CDMA systems.
In Code Division Multiple Access (CDMA) systems, a soft handover (SHO) area is characterized by similarly strong pilot power signals (CPICH Ec/lo in Wideband CDMA (WCDMA)). Pilot powers are measured by the mobile in idle as well as in connected mode. In connected mode, it is very important that the mobile is always connected to the strongest cell(s). Otherwise, it would cause significant interference in uplink and waste network capacity. In idle mode, it is important to camp in the strongest cell to allow a quick call initiation and not cause interference at call initiation.
A new situation arises if the mobile has to detect a SHO area in another band than the currently serving. When new downlink (DL) carriers are allocated for frequency division duplex (FDD)-WCDMA it is possible to be connected in a DL2 carrier (e.g., extension band carrier) and cause uplink (UL) interference without being able to detect the interference situation in the DL2 carrier. The current third generation partnership program (3GPP) procedures don't forsee the SHO area detection in another band to avoid UL interference. Connected mode inter-frequency measurements in compressed mode are event triggered and for handover purposes.
When multiple DL carriers are assigned to one UL carrier, a mobile device (e.g., User Equipment (UE), Mobile Station (MS), cellular phone, etc.) can interfere in the UL to a close base station that it cannot hear in the DL. Interference in this soft handover (SHO) area may also happen at call setup. To avoid the interference at call initiation, the SHO area in the other band should be detected already in idle mode and Cell_Paging Channel (PCH), UMTS Terrestrial Radio Access Network Registration Area(URA)_PCH state. Efficient measurements in idle mode, CELL_PCH and URA_PCH states can also avoid unnecessary inter-band measurements using compressed mode and inter-band handover right after the call setup. Continuous idle mode measurements of other bands however are draining on the mobile battery power in those states. Therefore, a search criteria to detect SHO areas (i.e., overlapping areas) in other bands and cell reselection criterion to avoid UL interference situations are needed.
A method and system for soft handover area detection for uplink interference avoidance that includes a network device and mobile device in a communications network. A trigger criteria threshold is determined for the mobile device. The mobile device is using a downlink carrier. If a trigger criteria has risen above or fallen below the trigger criteria threshold, inter-frequency measurements of co-sited cells are performed and compared to determine if a soft handover area exists. Co-sited cells are searched for downlink carriers and a reselection is initiated from the downlink carrier to a co-sited cell downlink carrier if the co-sited cell downlink carrier is useable by the mobile device. A reselection is initiated from the downlink carrier to a non co-sited cell downlink carrier if no co-sited cell downlink carrier useable by the mobile device is found. The system provides for reselection while uplink carrier interference is avoided.
The present invention is further described in the detailed description which follows in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present invention in which like reference numerals represent similar parts throughout the several views of the drawings and wherein:
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention. The description taken with the drawings make it apparent to those skilled in the art how the present invention may be embodied in practice.
Further, arrangements may be shown in block diagram form in order to avoid obscuring the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements is highly dependent upon the platform within which the present invention is to be implemented, i.e., specifics should be well within purview of one skilled in the art. Where specific details (e.g., circuits, flowcharts) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that the invention can be practiced without these specific details. Finally, it should be apparent that any combination of hard-wired circuitry and software instructions can be used to implement embodiments of the present invention, i.e., the present invention is not limited to any specific combination of hardware circuitry and software instructions.
Although example embodiments of the present invention may be described using an example system block diagram in an example host unit environment, practice of the invention is not limited thereto, i.e., the invention may be able to be practiced with other types of systems, and in other types of environments.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
The present invention provides a method and apparatus for soft handover detection using inter-band measurements. Uplink interference occurs when not all downlink neighbors are co-sited in the second downlink carrier band of frequencies. According to embodiments of the present invention, soft handover area detection may occur while the mobile device is in any mode or state, including, for example, CELL_DCH state, idle mode, CELL_FACH state, CELL_PCH state, URA_PCH state, etc., thus preventing uplink carrier interference.
According to embodiments of the present invention, search criteria may be a cell specific trigger for the actual SHO area measurement in other bands in idle mode. This differs from existing search criteria as, according to the present invention, the subsequent measurements may be inter-band measurements for SHO area detection, i.e., measurements of the one and only core DL band (DL1) associated with the current DL2 band where the mobile camps in. Moreover, measurements for SHO area detection differ from nowadays 3GPP standardized inter-frequency measurements as, according to the present invention, the associated cells in the core band may be co-sited and thus synchronized. Also, SHO area detection may be continuous as long as the criteria is fulfilled, but possibly with lower repetition rate.
The search criteria trigger may be any of many parameters related to the mobile device and the UL and DL carriers. Preferably, the parameter is a cell quality indicator of the currently serving cell, e.g., CPICH Ec/lo. However, the criteria may also be a signal strength indicator, a mobile speed indicator, a positioning indicator, or any combination of the above. Moreover, information on cells, which are co-sited on the DL1 and on the DL2 band may be utilized as cell reselection criterion from a DL2 band to a DL1 band.
There may be a threshold set or determined for trigger parameters/search criteria for the mobile device. The thresholds may be pre-programmed in the mobile device, set by the mobile device, or set by a network node based on the situation. If the search trigger is fulfilled, (e.g., in the case of the trigger being measured signal quality, the measured quality CPICH Ec/lo of the serving cell being less than a threshold), the mobile device may start searching for core band cells on the frequency DL1. If the network has informed that the cells of DL2 are co-sited with cells on DL1, this information may be utilized in cell reselection criterion and furthermore in cell reselection evaluation in order to initiate inter-band cell reselection at the right time. If the mobile device has not detected all of the corresponding cells or some of the corresponding cells on DL2 that are detectable on the core band DL1, the mobile device may trigger cell reselection to the best cell on the core band DL1.
The network may inform the mobile device whether the mobile device has to find all the same cells from the DL2 band as from the DL1 band or only a subset. The network may exclude some cells from the comparison in order to avoid unnecessary inter-band cell reselections in cases that the mobile device is close to the edge area of the coverage area of the band DL2 but still there is more than one cell. The network also may inform a timer over which time the evaluation and comparisons of cells on the frequency DL2 and DL1 should occur prior to cell reselection to DL1 band is made. For example, if the mobile device has not been able to find and measure all the indicated co-sited cells from DL2, which are detectable on DL1, cell reselection to DL1 may be performed.
Network devices 12-22 may be any type of network node or device that supports wireless devices connected to a telecommunications network, for example, a Radio Network Controller (RNC), a Base Station Controller (BSC), etc. Network device 12 and mobile device 36 transfer data and control information between each other via uplink 35 and downlink 37 channels. A base station or cell (not shown) may supply frequencies from a particular band of frequencies that allow a mobile device 36 to select from and use for a downlink carrier 35 and uplink carrier 37. The uplink carrier frequency and downlink carrier frequency may be from the same band of frequencies, or from different bands of frequencies.
As a mobile device moves from one location to another, the base station or cell closest to the mobile device will likely then supply the uplink and downlink carriers for the particular mobile device. Generally, if the same band of frequencies is available at the neighboring base station, the network device may direct a soft handover to occur between the downlink and uplink carriers supplied from the original base station to downlink and uplink carriers supplied from the neighboring base station.
According to the present invention, a currently used network device 12 and/or neighboring network device 14, possibly along with mobile device 36, may detect soft handover areas before a handover is to occur such that a handover may occur without causing uplink channel interference. As noted previously, uplink interference may be caused when a mobile device moves to a location that does not supply the same bands of frequencies currently being used by the mobile device for its downlink carrier.
Each mobile device 30-48 and/or network device 12-22 may perform various measurements in a periodic or continuous basis to detect soft handover areas for uplink interference avoidance. For example, measurements such as signal strength, signal quality, etc. may be made and compared with similar measurements of carriers from neighboring or co-sited bands to determine if a soft handover area exists and whether a handover should occur to avoid uplink interference. A network device and/or mobile device may determine the types of measurements made and when they are made. Moreover, a network device and/or mobile device may perform the measurements, where in the latter case, a network node may instruct the mobile device to perform the measurements or the mobile device perform the measurements without instruction from the network device. Further, the mobile device may perform the measurements and report the results to the network device whereby the network device decides whether a soft handover area exists and whether a soft handover should occur to avoid uplink interference.
Signal quality of a carrier (downlink or uplink) may include interference from other cells and is related to the signal quality at a specific mobile device. In contrast, signal strength may include the sum of all the signals and indicates the total strength in a specific frequency. With signal strength measurements, there is no differentiating between a particular mobile device's signal and other signals. Co-sited downlink carriers are downlink carriers from the same antenna or same base station or cell as the downlink carrier currently being used by a mobile device.
Relative signal quality may also be a measurement performed. In this method, signal quality may be measured and compared with the signal quality of downlink carriers from another base station. Differences between the two may be then used to determine if a soft handover area exists. Moreover, a mobile device currently using a current downlink carrier from a current cell moving closer to a neighboring cell may look for a downlink carrier from the neighboring cell from the same frequency band as the current downlink carrier. If a downlink carrier is missing in this band, then the network device and mobile device know that a soft handover area exists where uplink interference may occur if the handover doesn't occur earlier.
Soft handover area detection may occur while a mobile device is in any mode or state, for example, the mobile device may be in an idle mode, or a connected mode where it is waiting for data or actively transmitting data. Depending on the mode or state of the mobile device, may determine what types of measurements (e.g., inter-frequency measurements) may be made.
One reason for handover may be because the mobile device has reached the end of coverage of a frequency carrier in an extension (e.g., 2.5 GHz) band. The end of extension band coverage may invoke inter-band, inter-frequency or inter-system handover. The trigger criteria may always be the same. As inter-band handovers can possibly be done faster, separate trigger thresholds might be implemented. Some example coverage triggers for example implementations according to the present invention may include but are not limited to: handover due to uplink dedicated channel (DCH) quality, handover due to UE Tx power, handover due to downlink dedicated physical channel (DPCH) power, handover due to common pilot channel (CPICH) received signal chip power (RSCP), and handover due to CPICH chip energy/total noise (Ec/No).
Coverage may be another reason for handover. A coverage handover may occur if: (1) the extension band cell has a smaller coverage area (=lower CPICH power or different coverage triggers) than a core band, (2) currently used core band coverage ends (then also extension band), or (3) the UE enters a dead zone.
Intra-frequency measurements may be another reason for soft handover. A soft handover procedure in an extension band may work in principle the same way as in core bands with branch addition, replacement and deletion procedures. SHO procedures may be based on CPICH Ec/IO measurements. Despite stronger attenuation in the extension band, Ec/IO as a ratio may be about the same for both bands. Therefore, in principle the same SHO parameter settings may be used in the extension band. However, if stronger attenuation in an extension band is not compensated for by additional power allocation, the reliability of SHO measurements (Ec/lo) may suffer. Moreover, an extension band cell might have neighbors on extension band frequencies and on core band frequencies at the same time. Then, the UE may have to measure both intra-frequency and inter-band neighbors.
UL interference in the core bands due to delayed SHO at the extension band coverage edge may occur. An extension band cell may have both extension band neighbors and core band neighbors at the same time. While for the extension band neighbor the normal SHO procedure may be sufficient, for the core band neighbor an early enough inter-band handover may have to be performed. Otherwise, serious UL interference could occur in the core band neighbor cell. SHO areas might be located relatively close to the base station and thus not necessarily relate to high UE Tx (transmit) power (or base transceiver station (BTS) Tx power). Coverage handover triggers may not be sufficient.
In this example embodiment, a mobile device (UE) 50 is using a downlink carrier from an extension band of frequencies 52 from base station 53 closest to the mobile device 50. As a mobile device 50 moves from the left side of base station 53 and approaches cell coverage overlap areas, the mobile device 50 uses UL and DL carriers from neighboring cells (i.e., middle cell 53 and rightmost cell 55). Generally, if the mobile device 50 is using an UL and DL carrier in an extension band (e.g., a band of frequencies starting at approximately 2.5 GHz) cell, once the mobile device 50 moves towards the coverage of a neighboring extension band cell, a soft handover will occur between the DL and UL carriers of the neighbor cells. However, in a situation where there is no neighboring extension band cell as shown here, a soft handover cannot occur since the mobile device 50 must now obtain a DL and UL carrier from a core band (e.g., a band of frequencies starting at approximately 2 GHz) cell. This may cause interference in the UL carrier (not shown) of the neighboring cell. However, according to the present invention, a network device may monitor this situation and cause selection of a different DL carrier in an existing band early to allow a soft handover from the extension band 52 (e.g., 2.5 GHz) in middle cell 53 to the core band of frequencies 58 (e.g., 2.0 GHz) in the neighboring cell 55, therefore, avoiding potential interference in the UL carrier of the neighboring cell 55.
In order to prevent a directed setup into an interfering area, the UE (mobile device) 50 may need to report in a Random Access Channel (RACH) message the measured neighbors in the core band 56. The message attachment may be standardized but may need to be activated. A network node (e.g., Radio Network Controller (RNC)) then may need to check that all measured cells have a co-sited neighbor in the extension band.
Adjacent cell interference (ACI) detection before the directed setup is automatically given if Forward Access Channel (FACH) decoding in the core band was successful. Load reason handover may be needed in addition to Directed Radio Resource Control (RRC) connection setup for congestion due to mobility. The load reason handover in current implementations is initiated by UL and DL specific triggers. By setting the trigger thresholds the operator can steer the load balancing:
Therefore,
In contrast, an inter-band handover core band-to-extension band may not occur in cells underlying a extension band coverage edge cell if the UE is in a SHO area. Specifically, a load/service reason inter-band handover during SHO in core bands may not be allowed. Also, inter-band handover core band-to-extension band due to an unsuccessful soft handover (branch addition) procedure may be disabled, but inter-frequency allowed.
Compressed mode may also be used for avoidance of adjacent channel protection (ACP)-caused UL interference. ACP caused UL interference may occur at certain UE Tx power levels where the UE location is close to an adjacent band base station. This is mostly a macro-micro base station scenario. The interfered base station may be protected in DL if it is operating in the adjacent extension band carrier otherwise not.
Adjacent channel interference (ACI) probability may directly relate to the mobile device's transmission power. Below certain powers the mobile cannot interfere to the micro base station and interference detection may not be required. A reasonable value for the power threshold that determines when to start interference detection may need to take into account the statistical probability of minimum coupling (MCL) loss situations, adjacent channel leakage ratio (ACLR), micro BTS noise level and desensitization. If the power is around the average UE Tx power (=−10 . . . 10 dBm) or higher, the number of mobile devices continuously checking for ACI interference may be reduced significantly.
An interfered base station may not be able to protect itself from ACI interference. The interfering mobile device must voluntarily stop transmission on its current band. Only by also operating in an extension band is the interfered base station self-protected.
Regarding compressed mode operation in an extension band (Cell_DCH), when the UE is operating in the extension band and needs to measure the core DL bands, CM usage in the core band can be applied normally and balancing of UL load may be triggering separately inter-frequency measurements. As described previously, there may be several reasons for inter-band CM measurements when the UE is in the extension band.
Since the DL load of the other band may be known, a network device (e.g., RNC) may initiate instead of an inter-band handover directly, an inter-frequency or inter-system handover in case of high load. Then, separate inter-frequency/inter-system measurements may be performed. In order to minimize the effects on network performance, CM may need to be used very efficiently and one consistent CM usage strategy may need to cover all inter-band measurements. The most excessive CM usage may come from “ACI detection” and “SHO area detection”. Both of these may be continuous in case they are needed. Both may be largely avoided either by intelligent carrier allocation in the extension band or by network planning.
Most of the carriers may be protected by carrier allocation. Only if an existing operator is not interested in extension band (e.g., 2.5 GHz) deployment, the UL adjacent carriers may need the ACI detection to protect another carrier from UL interference. Also, if operators want to have different numbers of extension band carriers, at some point, the UL carrier pattern may not be repeatable anymore in the extension band. Further, since a first operator may not use its additional carriers in the same geographical area and starting at the very same time as a second operator, ACI detection may be needed wherever protection from the extension band adjacent carrier is not provided.
UL carriers in the time division duplex (TDD) band may be automatically protected because here the UL carrier may exist only if also extension band is deployed. However, the adjacencies between TDD band and UL band may need special attention as again a first UL carrier can be interfered by a second if it is not (yet) operating in the extension band.
Regarding SHO area detection, network planning can reduce the need of CM by limiting the number of extension coverage edge cells and indicating edge cells via RNP parameters. If sectorized cells in the core band are fully repeated in the upper band, i.e., no softer handover area in the UL that is not a softer handover area in the extension band, the detection of SHO areas may be made dependent on the UE transmission power or CPICH Ec/lo. However here, it is more difficult to determine a threshold since there is no general limitation how close base stations can be to each other. If almost complete extension band coverage is needed it might be wise not to save on single sites and rather make the coverage as complete as possible. Moreover, if sparse capacity extension is needed, one can consider having less coverage area in the extension band cell by lowering the CPICH pilot power or applying different coverage handover thresholds. This lowers the average UE transmission power in the sparse cell and thus the probability of ACI or unwanted entering in UL SHO area.
Non-regarding network planning, there are still some cells where all reasons for CM are given. Here, the CM usage must be made efficient.
Most all reasons for CM require measurement of the associated DL core band, either own cell or neighbors. ACI detection can also be obtained by measuring the received signal strength indicator (RSSI) of the adjacent carriers in the core. If both SHO area detection and ACI detection is needed, it may be more efficient to rely for both on Ec/lo measurements provided that latter measurement can be done quickly enough. This may be enabled for two reasons: (1) CM in extension band operation can use the fact that extension band DL and core band DL are chip synchronized (assuming they are in the same base station cabinet, i.e., co-sited), and (2) both DL bands have the same or at least very similar propagation path differing merely in stronger attenuation for the extension band.
Two options for chip energy/system noise (Ec/lo) measurements may include: (1) measure core band Ec/lo (fast due to chip synchronization)—more accurate, may require a measurement gap of 4-5 timeslots, and (2) measure core band RSSI and use CPICH Ec correlation between bands=>Ec/lo—may require a measurement gap of 1-2 timeslots.
The second option may be preferred due to the short gaps. Basically, not even level measurements (Ec/lo) are required if the relative difference between both DLs RSSI is considered. Uncertainties on the network side (antenna pattern/gain, cable loss, loading, power amplifier (PA) rating, propagation loss/diffraction) as well as on the UE side (measurement accuracy) may disturb the comparison and may need to be taken into account if possible.
If a high difference in RSSIs (or low Ec/lo in the core band) is detected, the reason may be verified by:
Additionally, CM usage can be minimized by triggering it with some kind of UE speed estimate. If a UE is not moving CM can be ceased, when it moves again CM continues.
Regarding measurements for cell re-selection when the extension band is used, the UE in idle mode camps in the extension band as long as Ec/lo signal is good enough. In connected mode, PS services move to Cell_FACH, UTRAN registration area routing area paging channel (URA PCH), or Cell_PCH state after a certain time of inactivity (NRT). Then, idle mode parameters may control the cell re-selection. Cell re-selection may then happen for a coverage reason, i.e., when the extension coverage ends.
Interference detection may need to be provided also in states controlled by idle mode parameters to prevent UL interference due to RACH transmission. Here, for ACI and SHO area detection different mechanisms may be applied.
SHO area detection in idle mode (and Cell_PCH, URA_PCH) may be enabled by a two-step measurement and applied to the coverage edge cells: (1) a cell specific absolute Ec/lo-threshold triggers step, and (2) measure core band whether there is a cell without inter-band neighbor in extension band. To make the comparison, the UE may need to know the co-sited core neighbors. This may need to be added in extension band broadcast channel system information (BCCH SI). In Cell_FACH state, SHO areas may be detected by using the IF measurements occasions and checking if found neighbors in the core band have a co-sited neighbor in the extension band. Again additional BCCH information may be needed.
ACI may not be detected in idle mode but immediately before RACH transmission by measuring directly the two neighboring (adjacent) carriers in the core band. The delay in RACH transmission may be negligible due to the fast RSSI measurements. In Cell_FACH state, ACI detection may be provided by continuously measuring the adjacent core carriers (stealing slots for RSSI measurements).
In the case of the SHO area, the UE may initiate an inter-band handover to the core band. In case ACI is detected, the UE may initiate an inter-frequency handover (UL changes) similar to a conventional coverage reason cell re-selection.
In these example embodiments, the existing uplink frequency band is shown to include frequencies starting at approximately 1920 MHz, the existing downlink band to include frequencies starting at approximately 2110 MHz, and the new uplink and downlink bands to include frequencies starting at approximately 2500 MHz. However, the present invention is not limited by these frequency values but may be applied to any bands of possible frequencies. The frequencies being shown in
The embodiments shown in
An absolute or relative signal quality level can be applied for the process shown in
The present invention is advantageous in that it allows for the avoidance of severe interference scenarios. Moreover, soft handover detection according to the present invention allows for new frequencies from new bands to be used for uplink and downlink carriers.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to a preferred embodiment, it is understood that the words that have been used herein are words of description and illustration, rather than words of limitation. Changes may be made within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular methods, materials, and embodiments, the present invention is not intended to be limited to the particulars disclosed herein, rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. Especially, the present invention extends to all other CDMA systems other than WCDMA systems.
This application is a continuation of U.S. patent application Ser. No. 10/410,198 to Korpela, et al. filed Apr. 10, 2003 entitled “Method and Apparatus for Soft Handover Area Detection Using Inter-Band Measurements,” which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/375,809 filed Apr. 29, 2002, the contents of which are expressly incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60375809 | Apr 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10410198 | Apr 2003 | US |
| Child | 12421382 | US |
