HK1095471B - Methods for performing cell reselection by using power scan and parallel decoding of a list of radio frequency channels, and corresponding apparatuses - Google Patents

Abstract

Upon power on, a terminal performs cell selection, finds the most suitable cell to receive communication service, and camps on this cell (the serving cell). The terminal thereafter performs "C2-based" cell reselection (332) if a better cell is found, "non-C2 based" cell reselection if the current serving cell cannot be camped on (342), "power scan" cell reselection if the C2-based or non-C2 based cell reselection fails (352), and cell selection if the power scan cell reselection fails. For the power scan cell reselection, the terminal initially performs a power scan and obtains received signal strength measurements for a list of RF channels. This list includes fewer than all RF channels evaluated by the cell selection. The terminal then acquires and decodes the N strongest RF channels; preferably in parallel, to find a suitable cell. The terminal selects a suitable cell, if found, with the highest C2 value as the new serving cell from which to receive service.

Description

Method for performing cell reselection by using power scan and parallel decoding of a list of radio frequency channels and corresponding device

Technical Field

The present invention relates generally to communication techniques, and more specifically to techniques for performing cell reselection in a wireless communication system.

Background

In a global system for mobile communications (GSM) system, a terminal that has just been powered on or has just lost coverage searches for a suitable cell for the terminal to receive communication services. A "cell" can refer to a base station and/or the coverage area of the base station in the system, depending on the context in which the term is used. A "suitable" cell refers to a cell on which the terminal may receive service. GSM specifies a set of criteria that a cell must meet in order to be identified as a suitable cell. If a suitable cell is found, the terminal registers (if necessary) in the cell. The terminal will camp on the cell if the terminal is in an idle mode and not actually communicating with the cell. During camping on the cell, the terminal performs GSM specified tasks to enable the terminal to: (1) receiving system information from the cell, (2) receiving a paging message from the cell (i.e., alerting the terminal of an incoming call), and (3) initiating a call setup for an outgoing call or other action. The cell on which the terminal camps is called the "serving" cell.

During camping on the cell, the terminal periodically checks whether there is a better cell (i.e., another cell with a higher received signal level) that the terminal can camp on and receive service. If such a cell exists, the terminal selects the cell as the new serving cell through a process commonly referred to as "cell reselection". In some cases, the terminal may also need to perform cell reselection immediately in order to select the serving cell as another cell. For example, if the current serving cell is barred, the terminal needs to perform cell reselection immediately if the terminal cannot receive a signal from the current serving cell because the channel condition has deteriorated. In either case, the terminal performs cell reselection in idle mode so that it can monitor incoming page messages for the system and initiate a call if the channel conditions change (e.g., if the terminal moves to a new location).

For a cell reselection where the initial reselection fails or there is no information about neighboring cells, the terminal needs to obtain received signal strength measurements and needs to collect relevant system information for the new cell, both of which typically take a long time to implement. During the time that the terminal performs these tasks, the terminal cannot accept service from the system and also loses all paging messages sent to it, both of which are undesirable.

There is therefore a need in the art for techniques to collect information about neighboring cells during cell reselection to reduce both down time and the likelihood of missed paging information.

Disclosure of Invention

Techniques for performing cell reselection through power scanning and/or parallel decoding are provided. These techniques may reduce downtime and provide improved performance. Once powered on, a terminal performs cell reselection to find the most suitable cell for the terminal to camp on and receive communication services. And if the terminal is in an idle mode, the terminal selects the most suitable cell as a serving cell of the terminal and camps on the cell. Thereafter, the terminal may perform cell reselection to select another suitable cell to receive service. Cell reselection may need to be performed for any reason, e.g., if a better cell is found, if the terminal can no longer camp on the current serving cell, etc. If a better cell is found, the terminal may perform "C2" based cell reselection and may perform "non-C2" based cell reselection for any other reason. In an embodiment, the terminal performs a "power scan" cell reselection if the C2-based or non-C2-based cell reselection fails. In other embodiments, the power scanning cell reselection may be triggered by several other events or conditions.

In one embodiment of power scan cell reselection, the terminal first performs a power scan on a first list of RF channels to obtain received signal strength measurements for those RF channels. The first list may include different RF channels according to an event triggering the power scanning cell reselection. In all cases, the first list includes fewer than all RF channels evaluated by the cell reselection. Based on the results of the power scan, the terminal obtains a second list of at least one RF channel. For example, the second list may include the N strongest RF channels in the first list. Wherein N is more than or equal to 1. The terminal processes (e.g., acquires and decodes) at least one RF channel within the second list to find a suitable cell. If N >1, then the RF channels in the second list may be processed in parallel as described below to speed up the power scanning cell reselection. The terminal will select the most suitable cell (if found) as the new serving cell it is serving. And if the power scanning cell reselection fails, the terminal performs cell selection.

Drawings

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth above when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout. Wherein:

FIG. 1 shows a wireless communication system;

FIG. 2 shows a channel configuration for a control channel in GSM;

FIG. 3 shows the complete operation of a terminal in a GSM system;

fig. 4 shows a cell selection process;

fig. 5 shows a C2 based cell reselection procedure;

fig. 6 shows a non-C2 based cell reselection procedure;

FIG. 7 shows a power scan cell reselection process with serial decoding;

FIG. 8 shows a power scanning cell reselection process with parallel decoding;

FIG. 9 shows a parallel decoding process;

FIG. 10 shows parallel decoding of an exemplary list of 4 RF channels; and

fig. 11 shows a block diagram of a terminal.

Detailed Description

The term "exemplary" is used herein to mean "serving as an example, instance, or illustration". Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

Fig. 1 shows a wireless communication system 100 having a number of base stations 110 that provide communication services for a number of terminals 120. A base station is a fixed station and may also be referred to as a Base Transceiver Station (BTS), a node B, an access point, or some other terminology. Terminals 120 are typically dispersed throughout the system. A terminal may be fixed or mobile and may also be referred to as a Mobile Station (MS), a Mobile Equipment (ME), a User Equipment (UE), a wireless communication device, or some other terminology. A Mobile Switching Center (MSC)130 provides coordination and control for the base stations 110, and further controls the routing of data to and from terminals served by these base stations. An MSC may also be referred to as a Radio Network Controller (RNC) or several other terms.

System 100 may be a time division multiple access system (TDMA) that may implement one or more TDMA standards, such as GSM. System 100 may also be a code division multiple access system (CDMA) that may implement one or more CDMA standards, such as wideband CDMA (W-CDMA), IS-2000, IS-856, IS95, and so on. These standards are well known in the art.

The techniques described herein for performing cell reselection through power scanning and/or parallel decoding may be used for various wireless communication systems. For clarity, the techniques are described herein as being specifically set forth for use in a GSM system.

Fig. 2 shows a channel configuration for a control channel within GSM. The timeline for data transmission is divided into multiple frames. For the control channel, each multiframe has a duration of 235.365 milliseconds and may be divided into 51 TDMA frames, labeled TDMA frames 0 through 50. Although not shown in fig. 2, each TDMA frame is further divided into 8 time slots, labeled as time slots 0 through 7. Time slot 0 is for the control channel and time slots 1 through 7 are for the traffic channel. The data transmission in each time slot may be referred to as a "burst". In GSM, the cells are all synchronized and the timing of each cell cannot be consistent with the timing of the other cells. Thus, the multiframe of each cell can start at any arbitrary point in time.

Control channels for GSM include a Frequency Correction Channel (FCCH), a Synchronization Channel (SCH), a Broadcast Control Channel (BCCH), and a Common Control Channel (CCCH). The FCCH may allow a terminal to set its frequency and coarse timing and send within TDMA frames 0, 10, 20, 30, and 40 of each multiframe. The SCH may carry (1) a reduced TDMA frame number (RNF) used by a terminal to synchronize its timing with the frame number, and (2) a base transceiver station identification code (BSIC) used to identify the transmitting base station. The SCH is transmitted in TDMA frames 1, 11, 21, 31 and 41 of each multiframe. The BCCH may carry system information and is sent within TDMA frames 2, 3, 4, and 5 of each multiframe. The CCCH carries control information and is also used to construct a Paging Channel (PCH). The PCH carries a paging message, e.g. to alert an idle mode terminal of an incoming call. The CCCH includes 9 radio blocks within each multi-frame, and some or all CCCH radio blocks may be used for the PCH. A CCCH radio block for the PCH may be referred to as a "paging block. Each idle mode terminal may be assigned to a particular paging group based on the terminal's International Mobile Subscriber Identity (IMSI) and the number of available call blocks within a CCCH. Each paging group includes a paging block transmitted in the mth CCCH radio block of each n multiframe, where 8 ≧ m ≧ 0 and 9 ≧ n ≧ 2.

Fig. 2 shows one of a variety of channel configurations for the control channel. But other possible combinations of control channels for the 51-frame multiframe exist. In addition, other time slots than time slot 0 may carry control channels. However, the specific channel combination shown in fig. 2 is found only within slot 0. The channel configuration for the control channels in GSM is described in detail in a publicly available 3GPP TS 05.01 document.

A terminal may be designed to operate in one or more frequency bands. Each frequency band covers a specific frequency range and may be divided into a number of 200KHz RF channels. Each RF channel is identified by a specific ARFCN (absolute radio frequency channel number). For example, the GSM900 band includes ARFCNs 1-124, the GSM 1800 band includes ARFCNs 512-885, and the GSM 1900 band includes ARFCNs 512-810.

Each cell transmits data and sends signals over a set of RF channels assigned to the cell by a network operator. To reduce inter-cell interference, different sets of RF channels may be assigned to cells located close to each other so that transmissions from the cells do not interfere with each other. Each cell may broadcast system information on one or more RF channel cells assigned to the cell. An RF channel used for broadcasting system information may be referred to as a BCCH carrier. If a terminal does not know which RF channels are BCCH carriers, the terminal needs to acquire and evaluate all RF channels to determine if the RF channels are a BCCH carrier for a cell.

Each cell broadcasts a BCCH Allocation (BA) list that includes up to 32 ARFCNs for BCCH carriers of up to 32 cells, one ARFCN/BCCH carrier being used by each cell. The BA lists broadcast by cells located close to each other may include many identical ARFCNs, but these lists are typically different from each other. A terminal obtains the BA list from its serving cell and performs measurements on the cells included in the list, as specified by GSM and described below.

In GSM, each cell broadcasts all system information in segments on the BCCH using different types of system information messages. Each system information message carries certain system information and is broadcast at a specified time. A system information type 3 message ("SI 3") may carry information needed for a terminal to perform cell reselection and to receive paging messages from a cell. A system information type 4 message ("SI 4") may carry information needed for a terminal to perform cell reselection but does not include information needed to receive paging messages. All system information may be broadcast within system information messages of the type 1 to 20 (which are not consecutively numbered). A terminal is not allowed to camp on the cell and transmit data to a cell on the uplink until the terminal collects all system information from the cell.

Fig. 3 shows a flow diagram of an overall process 300 for operation of a terminal in a GSM system. Upon power up, the terminal performs cell selection and finds a suitable cell for it to receive communication services (step 310). For GSM, a cell is considered to be a suitable cell if it meets the following criteria:

the cell is located in a selected or equivalent Public Land Mobile Network (PLMN);

the cell is not barred by the network operator;

the cell is not within a forbidden Location Area (LA).

The radio path loss between the terminal and the cell is less than a specified threshold, and

the cell is not a SoLSA-specific cell to which the terminal cannot subscribe.

A SoLSA (localized service area supported) dedicated cell is a cell that only allows a terminal with a Localized Service Area (LSA) subscription to camp on. The cell suitability standard is specified by GSM in chapter 3.2 of a publicly available 3GPP TS03.22 document. The terminal selects the most suitable cell (i.e., the suitable cell having the strongest received signal strength) as the serving cell and, if necessary, performs registration within that cell (also in step 310). Cell selection is described in further detail below.

If the terminal is in idle mode, it will camp on the serving cell and perform idle mode tasks (step 320). These tasks include:

measuring at least the received signal level of the serving cell at each paging block;

decoding the BCCH of the serving cell at least every 30 seconds to obtain all system information;

measuring the received signal level of the non-serving cell (i.e., the "neighbor cell") within the BA list;

decoding the SCH of the 6 strongest non-serving cells at least every 30 seconds to obtain the BSIC to confirm that the same cell is being monitored; and

decoding the BCCH of the 6 strongest non-serving cells at least every 5 minutes to obtain system information (SI3 or SI4) that affects cell reselection.

Idle mode tasks within GSM are described in chapter 6.6.1 of a publicly available 3GPP TS 05.08 document. The terminal typically measures received signal strength values for the serving and neighboring cells during or immediately after its paging block. The measurements and system information are used both to determine whether there is a better cell for the terminal to camp on and receive service from, and to select another serving cell if the terminal cannot maintain camping on the current serving cell.

A determination is made periodically as to whether cell reselection is required (step 322). The terminal will perform cell reselection to select a new serving cell if any of the following events occur:

the path loss of the current serving cell has become too high;

the presence of a downlink signaling failure;

the current serving cell has been barred;

a better cell exists in the same registration area or an even better cell exists in another registration area with the same PLMN or an equal PLMN;

the terminal is not able to transmit data to the network; and

the network fails a validation check.

The path loss for a cell is determined based on a path loss criterion parameter C1 as a function of received signal strength measurements and other parameters for the cell. If the C1 value is less than 0 for at least 5 seconds, the path loss is too high.

A preferred cell is determined based on a path loss criteria parameter C2 and other parameters as a function of C1. A cell is determined to be better than the current serving cell if the C2 value of the cell is higher than the C2 value of the current serving cell for at least 5 seconds.

Downlink signaling failure is a common event in the field and is determined based on a downlink signaling failure counter (DSC). When the terminal camps on a cell for the first time, the DSC is initialized to a starting value. Thereafter, the DCS increments one count (but limited to the starting value) whenever a page message from the cell is correctly decoded, and decrements 4 counts whenever a page message is incorrectly decoded. When the DCS is near or below zero, a downlink signaling fault is declared.

A cell may be barred so that the terminal cannot camp on the cell. Whether a given cell is barred or not is indicated by system information broadcast by the cell. Since the conditions of the barred cells are dynamically changing, the terminal will periodically check this information of the serving cell and take corresponding action.

If the terminal makes a maximum specified number of random access attempts but fails to access the network (i.e., is not answered), it may be assumed that the terminal is unable to communicate with the network.

The events that may trigger cell reselection are described in chapter 4.5 of 3GPP TS 03.22. If no cell reselection is required (as determined in step 322), the terminal returns to step 320 and continues camping on the current serving cell. If cell reselection is required, a determination is made whether to trigger the cell reselection due to the discovery of a better cell (step 324).

If a preferred cell is found (i.e., the answer to step 324 is yes), the terminal will perform a "C2 based cell reselection" as set forth below (step 332). Since the preferred cell is determined according to the C2 values of the preferred cell and the current serving cell, cell reselection to a preferred cell is referred to as C2-based cell reselection. If it is determined that the terminal should still remain within the current serving cell (step 334), the terminal returns to step 320 and continues camping on this cell. Otherwise, if a preferred cell is found to be suitable (as determined in step 336), the terminal selects the preferred cell as the new serving cell (step 360) and then camps on this cell (step 320). If a suitable cell is not found (as determined in step 336), the terminal also performs "power scan" cell reselection (step 352) as set forth below.

If cell reselection is triggered by an event other than the discovery of a preferred cell (i.e., the answer to step 324 is no), the terminal also performs "non-C2" based cell reselection (step 342) as set forth below. GSM requires immediate implementation-C2-based cell reselection since the terminal cannot be served from the current serving cell and needs to reselect another cell to be served. If the non-C2 based cell reselection finds a suitable cell (as determined in step 344), the terminal selects this suitable cell as the new serving cell (step 360) and then camps on this cell (step 320). If a suitable cell is not found (as determined in step 344), the terminal performs power scan cell reselection (step 352).

In one embodiment, if neither the C2-based cell reselection in step 332 nor the non-C2-based cell reselection in step 342 finds a suitable cell, the terminal performs a power scanning cell reselection (step 352). If the power scanning cell reselection finds a suitable cell (as determined in step 354), the terminal selects this suitable cell as the new serving cell (step 360) and then camps on this cell (step 320). If a suitable cell cannot be found (as determined in step 354), the terminal returns to step 310 and performs cell selection.

Cell selection, C2-based cell reselection, non-C2-based cell reselection, and power scanning cell reselection are described in further detail below.

Fig. 4 shows a flow chart of a cell selection process 310a, which may be used in step 310 of fig. 3. For "normal" cell selection, in which the terminal does not know in advance which RF channel is the BCCH carrier, the terminal performs a "power scan" to obtain received signal strength measurements for all relevant RF channels (block 410). The number of ARFCNs scanned depends on the specific frequency bands supported by the terminal. For the power scan, the terminal acquires strength measurements for at least 5 received signals distributed over 3 to 5 seconds for each ARFCN (step 412). These received signal strength measurements are also referred to as monitor values, power measurements, and received signal level measurements. The terminal then calculates an average of the measurements acquired for each ARFCN (step 414). The average value of a given ARFCN is referred to as "RLA _ C" in GSM. The terminal then classifies the RLA _ C values of all ARFCNs. In the embodiment shown in fig. 4, the terminal provides a list of the L strongest ARFCNs sorted in descending order of their RLA _ C values, referred to as the "AA" list (step 416).

Thereafter, the terminal attempts to acquire the ARFCNs in the AA list one ARFCN at a time to find the most suitable cell for it to camp on. The terminal selects the strongest ARFCN in the AA list as the current ARFCN (step 420). The terminal then performs cell acquisition and attempts to acquire the current ARFCN (block 430). For cell acquisition, the terminal first acquires the FCCH of the current ARFCN to obtain a coarse frequency timing for this ARFCN (step 432). The terminal then decodes the SCH of the current ARFCN to obtain the BSIC of this ARFCN and the precise timing and information needed to acquire the BCCH (step 434). Thereafter, the terminal decodes the BCCH for the current ARFCN to obtain SI3 or SI4 (step 436). The information includes the PLMN of the current ARFCN and parameters for verifying the suitability of the cell for the current ARFCN (i.e., whether the terminal can camp on the cell).

From all the information obtained in block 430, a determination is made whether to acquire the cell of the current ARFCN and whether the cell is suitable (step 440). If the answer is "yes," the terminal selects the cell of the current ARFCN as the serving cell and decodes the BCCH of this cell to collect all system information (step 450). Since the ARFCNs are evaluated sequentially according to their RLA _ C values and sorted in descending order, the first suitable cell found is the most suitable cell. At this point, the cell selection process 310a ends. Otherwise, if the cell of the current ARFCN is not suitable (i.e., the answer to step 440 is no), then the current ARFCN is cleared from the AA list (step 442). A determination is then made as to whether the AA list is empty (step 444). If the answer is "no", the terminal returns to step 420 and selects another ARFCN in the AA list to attempt acquisition. Otherwise, an indication is provided that no suitable cell is found (step 446), and the cell selection process 310a is then ended.

Fig. 5 shows a flow chart of a C2-based cell reselection procedure 330a, which may be used for block 330 in fig. 3. First, the terminal obtains a list of cells that are deemed to be better than the current serving cell, referred to as a "BC" list (step 510). A cell is determined to be better than the current serving cell if the C2 value for the cell is greater than the C2 value for the current serving cell for at least 5 seconds. The preferred cell is located in a neighboring cell within the BA list that is monitored by terminals in idle mode. The BC list may include one or more preferred cells.

For the embodiment shown in fig. 5, the terminal continues camping on the current serving cell as a foreground task and attempts to decode the BCCH of the preferred cell as a background task. Typically, the terminal may perform both foreground and background tasks in a Time Division Multiplexed (TDM) fashion. Foreground tasks have higher priority and execute first, while background tasks have lower priority and execute second. The terminal only switches from the current serving cell to a preferred cell if the BCCH of the preferred cell can be correctly decoded. By "acknowledging" before switching, the terminal may continue to receive service from the current serving cell during a portion of the C2-based cell reselection. The acknowledgement prior to the transition may also reduce the likelihood of lost service due to the terminal transitioning to a preferred cell too early and failing to decode the cell.

The terminal attempts to acquire a preferred cell in the BC list one cell at a time. The terminal selects the best cell within the BC list as the current cell (step 512). The terminal decodes the BCCH of the current cell (as a background task) to obtain SI3 or SI4 to carry information for verifying the suitability of the current cell (step 514). If the BCCH for the current cell cannot be decoded (as determined in step 516), then this cell is cleared from the BC list (step 518). A determination is then made whether the BC list is empty (step 520). If the answer is "no", the terminal returns to step 512 and selects another preferred cell to attempt acquisition. Otherwise, if the BC list is empty, the terminal reserves the current serving cell (step 522) and continues camping on this cell (step 320 in fig. 3).

If the BCCH for the current cell is decoded correctly (i.e., the answer to step 516 is no), then the terminal transitions to the current cell and begins decoding the BCCH for this cell to obtain all of the system information needed to camp on and transmit to the cell (step 530). If all system information is successfully obtained (as determined in step 532), the terminal selects the current cell as the new serving cell (step 360 in fig. 3) and then camps on this cell (step 320 in fig. 3).

If all of the system information for the current cell is not available (as determined in step 532), the terminal performs power scanning cell reselection to a cell list, referred to as the "PS 1" list (step 352 of FIG. 3). In an embodiment, the PS1 list includes all cells in the BA list including previous/current serving cells. In another embodiment, the PS1 list includes the 6 strongest neighbor cells in the BA list. In yet another embodiment, the PS1 list includes all cells within the BA list plus additional cells that the terminal can camp on. These additional cells may be cells that the terminal has recently camped on, cells from a neighbor cell BA list, and so on. In summary, the PS1 list may include any cell that the terminal may camp on.

Fig. 5 shows an embodiment of a C2-based cell reselection process. Cell reselection based on C2 may be implemented in other ways. The event triggering the power scanning cell reselection may be different from the event shown in fig. 5.

A conventional terminal typically performs cell selection immediately upon failure of C2-based cell reselection and evaluates all ARFCNs to find a suitable cell. As such, cell selection may require an extended period of time, and the terminal is typically not served during this time. Since power scanning cell reselection may be performed in a shorter time than cell selection and since power scanning cell reselection may in many cases find a suitable cell to camp on, performing power scanning cell reselection instead of or prior to the cell selection described above in fig. 3 and 5 may reduce service loss and improve performance.

As described above, GSM requires cell reselection to be performed immediately for any reason other than the discovery of a preferred cell. If valid RLA _ C values are not available for neighboring cells within the BA list, the terminal needs to wait until these values are available and then perform cell reselection if still needed. For example, a valid rlac value may not be available for a cell if the required number of measurements for the cell has not been completed. To reduce cell reselection delay caused by waiting for a valid rlac value, GSM allows the terminal to speed up idle mode measurement procedures. However, this can complicate idle mode design and the cell reselection procedure can provide little improvement.

Fig. 6 shows a flow chart for non-C2 based cell reselection, which may be used for block 340 in fig. 3. First, the terminal obtains a list of cells to attempt acquisition, referred to as the "NC" list (step 610). In an embodiment, the NC list includes neighboring cells for which the terminal currently has a valid RLA _ C value. Improvements in cell reselection performance may be obtained by immediately starting non-C2 based cell reselection for those cells whose valid RLA _ C values are currently available, and then performing power scanning cell reselection for cells within the BA list.

For the embodiment shown in fig. 6, the terminal attempts to acquire cells in the NC list one cell at a time. The terminal selects the best cell in the NC list as the current cell (step 612). The terminal decodes the BCCH of the current cell to obtain the full system information (step 614). Since the terminal cannot get service from the current serving cell, the terminal collects all system information (instead of SI3 or SI4) in order to find a suitable cell as soon as possible. If all system information is successfully obtained and the current cell is deemed suitable (as determined in step 616), the terminal selects the current cell as the new serving cell (step 360 of fig. 3) and then camps on this cell (step 320 of fig. 3).

Otherwise, if all system information for the current cell is not available or if the cell is deemed unsuitable (as determined in step 616), the cell is purged from the NC list (step 618). A determination is then made as to whether the NC list is empty (step 620). If the answer is "no", the terminal returns to step 612 and selects another cell in the NC list to attempt acquisition. Otherwise, if the NC list is empty, the terminal performs power scan cell reselection to a cell list, referred to as the "PS 2" list (step 352 of FIG. 3). In an embodiment, the PS2 list includes all cells in the BA list except the previous/current serving cell that cannot be camped on. In another embodiment, the PS2 list includes the 6 strongest neighbor cells in the BA list. In yet another embodiment, the PS2 list includes all cells within the BA list plus additional cells that the terminal can camp on. In summary, the PS2 list includes any cell that the terminal may camp on.

Fig. 7 shows a flow diagram of a power scan cell reselection process 350a with serial decoding. Process 350a may be used for block 350 of fig. 3. First, the terminal obtains a list of cells, referred to as a "PS" list, to attempt to perform power scanning cell reselection. In an embodiment, the PS list may be (1) a PS1 list from the C2-based cell reselection, which may include all cells in a BA list including previous/current serving cells; or (2) a PS2 list from non-C2 based cell reselection that includes all cells within the BA list except the previous/current serving cell. For this embodiment, the PS list may include up to 32 ARFCNs for up to 32 cells, which is substantially less than the number of ARFCN sets used for a full power scan for a normal cell selection.

For process 350a, the terminal first performs a power sweep to obtain received signal strength measurements for all ARFCNs in the PS list (block 710). For power scan, the terminal obtains at least 5 measurements of each ARFCN in the PS list over 3 to 5 seconds (step 712), calculates RLA _ C values for each ARFCN from the measurements (step 714), and classifies the RLA _ C values for all ARFCNs in the PS list (step 716). In one embodiment shown in fig. 7, the terminal provides a list of the N strongest ARFCNs after classification, referred to as the "PSN" list. The value of N may be selected based on different considerations, such as the expected amount of time available for power scanning cell reselection. For example, N may be chosen equal to 6, which corresponds to the number of neighboring cells for which GSM requires the terminal to periodically acquire system information. In another embodiment, the PSN list includes all ARFCNs within the PS list. In summary, the PSN list comprises one, several or all ARFCNs within the PS list.

The power scan in block 710 may be performed relatively quickly (e.g., within about 5 seconds for an exemplary terminal design). For power scanning, the terminal remains awake to perform as many measurements as necessary. Conversely, in idle mode, the terminal may sleep between its paging blocks and wake up just prior to a paging block to receive paging messages and perform measurements. The terminal may perform more measurements in idle mode, either by waking more frequently or by remaining awake for a longer period of time; both can complicate the design of the idle mode procedure. The cell reselection power scan (block 710) may be advantageously implemented by program code and/or a processing unit to perform the cell selection power scan (block 410 in fig. 4), although a different ARFCN list may need to be used.

After the power scan, the terminal attempts to acquire the ARFCNs in the PSN list one ARFCN at a time to find the most suitable cell for its camping. The terminal selects the strongest ARFCN within the PSN list as the current ARFCN (step 720). The terminal then attempts to acquire the current ARFCN (step 730). For cell acquisition, the terminal first acquires the FCCH of the current ARFCN to obtain frequency and coarse timing (step 732), then decodes the SCH to obtain BSIC, fine timing, and information needed to acquire the BCCH (step 734), and finally decodes the BCCH to obtain all system information (step 736).

If the current ARFCN is acquired and deemed suitable (as determined in step 740), the terminal selects the cell for the current ARFCN as the serving cell (step 360 in fig. 3) and then camps on this cell (step 320 in fig. 3). The first suitable cell found is also the most suitable cell, since the ARFCNs are evaluated in descending order according to their RLA _ C values. If the current ARFCN is not appropriate (i.e., the answer to step 740 is "no"), then the current ARFCN is purged from the PSN list (step 742). If the PSN list is not empty (as determined in step 744), the terminal returns to step 720 to select another ARFCN to attempt acquisition. Otherwise, if all ARFCNs in the PSN list have been attempted to be acquired without finding a suitable cell, the terminal performs cell selection (step 310 in fig. 3).

Both the cell selection process shown in fig. 4 and the power scanning cell reselection process shown in fig. 7 may decode the BCCH of a cell in series, one at a time, to determine if the cell is suitable. For example, the BCCH for a cell can be decoded to obtain SI3/SI4, and if decoding is successful, can be further decoded to obtain all system information. If the initial SI3/SI4 or all system information decoding fails, the next cell is processed. Each of these two methods only decodes the BCCH of the other cell if it is determined that the current cell is not suitable. Serial decoding of BCCHs for multiple cells can substantially extend the process of cell selection and cell reselection.

Parallel decoding of BCCHs for multiple cells may reduce the process of cell reselection. As shown in fig. 2, parallel decoding is possible since cells broadcast their system information in bursts.

Fig. 8 shows a flow diagram of a power scan cell reselection process 350b with parallel decoding. The process 350b may also be used for block 350 of fig. 3. First, the terminal performs a power scan on the ARFCNs in the PS list and obtains a list of PSNs with the N strongest ARFCNs (step 810). Step 810 may be implemented at block 710 of fig. 7. The terminal performs parallel decoding on the N ARFCNs within the PSN list, as described below (block 820). If a suitable cell is found by the parallel decoding (as determined in step 824), the terminal selects this suitable cell as the serving cell (step 360 in fig. 3) and then camps on this cell (step 320 in fig. 3). Otherwise, if no suitable cell is found in all ARFCNs within the PSN list, the terminal performs cell selection (step 310 of fig. 3).

Parallel decoding may be implemented in different ways. In one embodiment, the terminal processes the FCCH and SCH of each ARFCN to be decoded in parallel sequentially, one at a time, starting with the strongest ARFCN. The terminal schedules decoding of the BCCH for each ARFCN that has successfully acquired its FCCH and SCH. As described above, the cells within the network are asynchronous and each cell broadcasts system information based on a particular schedule. Thereby, the BCCH decoding time for each ARFCN is scheduled within the time that the BCCH for that ARFCN is broadcast. Whenever the terminal does not decode the BCCH of another ARFCN that has been previously scheduled, it can process the FCCH and SCH of an ARFCN and schedule BCCH decoding of the ARFCN.

Fig. 9 shows a flow diagram of a parallel decoding process 820a, which may be used for block 820 of fig. 8. First, the terminal obtains a list of ARFCNs sorted according to their RLA _ C values for parallel decoding (step 910). This sorted list may be a list of PSNs provided by the power scan.

The terminal selects the strongest ARFCN in the PSN list and identifies it as CHx (step 912). The terminal acquires the FCCH for CHx to obtain frequency and coarse timing and then decodes the SCH for CHx to obtain the information needed to acquire the BCCH (step 914). If the SCH for CHx is successfully decoded (as determined in step 916), the terminal schedules decoding of the BCCH for CHx at the earliest time that the BCCH is broadcast on CHx. After deciding on BCCH decoding of CHx in step 918 or if it is determined in step 916 that SCH decoding of CHx is not successful, the terminal clears CHx from the PSN list.

A determination is then made as to whether there is sufficient time to process the FCCH and SCH of another ARFCN within the PSN list before the next scheduled BCCH (step 922). As shown in fig. 2, the FCCH and SCH are broadcast more frequently than the BCCH. Thus, the FCCH and SCH for multiple ARFCNs can be processed between BCCH transmissions. If the answer to step 922 is no, the terminal proceeds to step 930. Otherwise, if there is sufficient time to process the FCCH and SCH of another ARFCN, a determination needs to be made as to whether the PSN list has been emptied (step 924). If the answer to step 924 is no, the terminal returns to step 912 and selects the strongest ARFCN in the PSN list for processing. Otherwise, if the PSN list is cleared, the terminal proceeds to step 938.

In step 930, the terminal acquires and decodes the next scheduled BCCH for an ARFCN identified as CHy, and obtains SI3 and SI4 for CHy. A determination is then made whether the BCCH for CHy is successfully decoded (step 932). If the answer is no, the terminal proceeds to step 922. Otherwise, it is further determined whether there is a better ARFCN (i.e., better than CHy) with a pending scheduled BCCH (step 934). Although the ARFCNs within the PSN list are scheduled and processed sequentially starting from the best ARFCN, the BCCH decoding for a better ARFCN may also be scheduled at a later time due to the asynchronous timing of the cells and the different BCCH broadcast schedules used by the cells. If CHy is better than all ARFCNs with pending scheduled BCCHs, the terminal selects the cell for CHy as the serving cell (step 360 in fig. 3) and then camps on this cell (step 320 in fig. 3). Otherwise, if there is a better ARFCN with a pending scheduled BCCH, and if CHy is the best ARFCN that has been successfully decoded so far, the terminal retains the result for CHy (step 936). Then, the terminal proceeds to step 922.

In step 938, it is determined whether all scheduled BCCHs have been decoded. If the answer is "no", the terminal returns to step 930 to decode the next scheduled BCCH. Otherwise, if all scheduled BCCHs have been decoded (i.e., "yes" to the answer to step 938), a determination is made as to whether there is a previously saved ARFCN (step 940). If the answer is "yes", the terminal selects the cell of this saved ARFCN as the serving cell (step 360 in fig. 3) and then camps on this cell (step 320 in fig. 3). Otherwise, the terminal performs cell selection (step 310 of fig. 3).

In fig. 9, steps 910 through 924 process the ARFCNs within the PSN list and schedule decoding of the BCCH for these ARFCNs. Steps 930 through 940 decode the scheduled BCCH and provide the best ARFCN that has been successfully decoded.

Fig. 10 shows a timeline for decoding a typical list of 4 ARFCNs in parallel. These ARFCNs are identified as CH1, CH2, CH3, and CH 4; of the 4 ARFCNs, CH1 is the best ARFCN and CH4 is the worst ARFCN. At time T₁The terminal processes and decodes the FCCH and SCH of CH1 (identified as F1/S1). The terminal then decodes (identified as B1) the BCCH of CH1 scheduled at time T₆This time is the earliest time of receiving B1 of CH 1. At time T₂The terminal processes and successfully decodes the FCCH and SCH of CH2 (identified as F2/S2) and decodes the BCCH of C2 (identified as B2) scheduled at time T₄. Even though the processing of CH2 is later than CH1, since B2 arrives before B1 and has sufficient time to decode before B1 arrivesB2, so decoding of B2 is scheduled before decoding of B1.

At time T₃The terminal processing did not successfully decode the FCCH and SCH of CH3 (identified as F3/S3). Therefore, the terminal does not schedule decoding of the BCCH of CH 3. After processing CH3, the terminal determines that there is insufficient time to process CH4 before B2 of the scheduled CH 2. Then, the terminal waits and at time T₄Decoding B2. In this example, even if B2 is successfully decoded, the terminal cannot immediately select the cell of CH2 as the new serving cell because a better ARFCN (CH1) exists for a pending scheduled BCCH.

In an embodiment, once the BCCH for an ARFCN has been successfully decoded, the terminal stops scheduling of other ARFCNs. For this embodiment, the terminal does not process and schedule CH4, but simply waits for B1 of CH 1. In another embodiment, the terminal continues to process and schedule other ARFCNs even though the BCCH for an ARFCN has been successfully decoded. For this embodiment, the terminal will be at time T₅The FCCH and SCH of CH4 are processed and decoded (identified as 4/S4), and if the F4/S4 decoding was successful, the decoding of the BCCH of CH4 (identified as B4) is scheduled at time T₇。

In either case, at time T₆The terminal successfully decodes B1 of CH 1. Since there are no other ARFCNs better than CH1 for the pending scheduled BCCH, the terminal ends the cell reselection, selects the cell for CH1 as the new serving cell and camps on this cell.

As described above, parallel decoding may be used for power scanning cell reselection. Parallel decoding may also be used for cell selection, C2-based cell reselection, and non-C2-based cell reselection.

For the embodiment shown in fig. 3, power scan cell reselection is performed if either C2-based cell reselection or non-C2-based cell reselection fails. Power scan cell reselection may also be performed at other times and this is within the scope of the invention. Power scanning cell reselection may also be implemented, for example, if cell reselection is required and one or more other criteria are met. The criteria may be: an insufficient number of valid RLA _ C values are available; the effective RLA _ C value is below a threshold; no system information is available for any neighboring cell, and so on. Thereby, the power scanning cell reselection may be incorporated in other ways than the one shown in fig. 3 throughout the operation of the terminal and this is within the scope of the present invention.

GSM requires the terminal to perform cell reselection if a suitable cell is not found within 10 seconds after initiating cell reselection. To meet this requirement, a timer may be initialized to an appropriate value once cell reselection is initiated. When the timer expires, the terminal may discontinue cell reselection and begin performing cell selection. For simplicity, the process of the timer early terminating cell reselection is not shown in fig. 5 to 9.

Fig. 11 shows a block diagram of a terminal 120x capable of implementing the power scanning cell reselection techniques described herein. Terminal 120x is one of the terminals described in fig. 1. Base station 110x is for the current serving cell and base station 110y is for another cell (i.e., a neighboring cell). Base stations 110x and 110y are two of the base stations in fig. 1 and may belong to the same or different location areas.

On the downlink, the terminal receives downlink signals transmitted by different base stations (such as base station 110x and/or base station 110y) in the system. The received signal on an antenna 1112 is provided to a receiver unit (RCVR)1114 and conditioned and digitized to obtain data samples. A demodulator (Demod)1116 then demodulates the data samples in accordance with the GSM standard to obtain demodulated data. A decoder 1118 further decodes the demodulated data in accordance with the GSM standard to obtain decoded data, which can include system information and/or other signals (e.g., paging messages) transmitted by base station 110x and/or base station 110 y. The system information and/or signals may be provided to a controller 1120 and/or a memory unit 1122.

On the uplink, terminal 120x may transmit data and messages to base station 110x and/or base station 110 y. The data/message may be used to register at a new cell within a new location area, answer a page, and so on. An encoder 1142 may receive, format, and encode the data/message. The encoded data/message is then modulated by a modulator 1144 and further conditioned by a transmitter unit (TMTR)1146 to obtain an uplink signal for transmission to base station 110x and/or base station 110 y. Each base station receives and processes the uplink signal to recover the data/messages sent by the terminal, and may forward the messages to mobile switching center 130 for further processing.

Controller 1120 directs the operation of various processing units within terminal 120 x. For example, controller 120x can initiate, direct, and/or perform idle mode tasks, cell selection, cell reselection, and the like. Memory unit 1122 is used to store program codes and data used by controller 1120.

Controller 1120 may perform the cell selection and cell reselection processes described above in fig. 3-9. For cell selection and cell reselection, controller 1120 may direct receiver unit 1114 to make received signal strength measurements for the associated ARFCN. The measurements may be performed as part of idle mode tasks or power scanning for a cell selection and cell reselection. For a power sweep, controller 1120 may form a list of ARFCNs for which measurements are to be obtained, direct receiver unit 1114 to make measurements on the ARFCNs, receive measurements from receiver unit 1114, calculate RLA _ C values from the measurements, sort the results, and provide a list of the top L or N ARFCNs.

The controller 1120 may also determine whether cell reselection is required due to any of the events specified by GSM and may determine whether cell selection is due to a cell reselection failure. For cell selection and cell reselection, control unit 1120 may direct demodulator 1116 and decoder 1118 to collect system information for the evaluated ARFCNs, receive system information for these ARFCNs from decoder 1118, and use the information for cell selection and cell reselection.

For clarity, techniques for implementing cell reselection with power scanning and/or parallel decoding are described herein with particular reference to GSM. These techniques may also be used for other wireless communication systems, such as CDMA systems.

The techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, various components for implementing cell reselection through power scanning and/or parallel decoding may be implemented within one or more devices: application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), field programmable logic arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, other electronic devices designed to perform the functions described herein, or a combination thereof.

For a software implementation, the techniques described herein may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory unit 1122 in fig. 11) and executed by a processor. The memory unit may be implemented within the processor or external to the processor, and if external, it is communicatively coupled to the processor via various means as is well known in the art.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (17)

1. A wireless device in a wireless communication system, comprising:

a receiver unit for performing a power scan on a first list of radio frequency channels and providing received signal strength measurements for the radio frequency channels in the first list, wherein the first list comprises radio frequency channels on which to find a suitable cell from which to receive communication services, and wherein the first list comprises less than all radio frequency channels evaluated for cell selection; and

a controller for: obtaining a second list of at least one radio frequency channel selected from the plurality of radio frequency channels in the first list; initiating processing of the at least one radio frequency channel within the second list to discover the suitable cell; and if the suitable cell is found, selecting the suitable cell as a new serving cell from which to receive service;

wherein when the second list includes N radio frequency channels in the first list, the N radio frequency channels have strongest received signal strength measurements, and if N >1, the N radio frequency channels in the second list are processed in parallel to find the suitable cell.

2. The wireless device of claim 1, wherein the controller is configured to identify a preferred cell that is better than a current serving cell and to initiate cell reselection to the preferred cell, and

wherein the power scan and the processing of the at least one radio frequency channel within the second list are performed only if the cell reselection to the preferred cell fails.

3. The wireless device of claim 1, wherein the controller is to: determining that the service can no longer be received from a current serving cell; obtaining a list of candidate cells from which service is available; and initiating cell reselection to the candidate cell, and

wherein the power scan and the processing of the at least one radio frequency channel within the second list are performed only if the cell reselection to the candidate cell fails.

4. The wireless device of claim 1, wherein the controller is to: directing the receiver unit to acquire the radio frequency channel for each of the N radio frequency channels; scheduling decoding of a broadcast channel of the radio frequency channels; and directing decoding of the broadcast channel of the radio frequency channel at a timing to determine whether the radio frequency channel is available for the suitable cell.

5. The wireless device of claim 4, wherein the controller is further configured to terminate processing of the N radio frequency channels within the second list immediately upon discovery of the suitable cell.

6. The apparatus of claim 1, wherein the wireless communication system is a global system for mobile communications.

7. A method of performing cell reselection in a wireless communication system, comprising:

obtaining a first list of radio frequency channels on which to find a suitable cell from which to receive communication services, wherein the first list includes fewer than all radio frequency channels evaluated for cell reselection;

performing a power scan to obtain received signal strength measurements for the radio frequency channels in the first list;

obtaining a second list of at least one radio frequency channel selected from said radio frequency channels in said first list;

processing the at least one radio frequency channel within the second list to discover the suitable cell; and

if the suitable cell is found, selecting the suitable cell as a new serving cell from which to receive service;

wherein when the second list includes N radio frequency channels in the first list, the N radio frequency channels have strongest received signal strength measurements, and if N >1, the N radio frequency channels in the second list are processed in parallel to find the suitable cell.

8. The method of claim 7, wherein the performing the power sweep comprises: for each of the radio frequency channels within the first list:

obtaining a sufficient number of received signal strength measurements of the radio frequency channels, an

An average of the received signal strength measurements of the radio frequency channels is calculated.

9. The method of claim 7, further comprising:

identifying a preferred cell that is better than a current serving cell;

performing cell reselection to the preferred cell; and

performing the obtaining a first list, performing a power scan, obtaining a second list, processing, and selecting if the cell reselection to the preferred cell fails.

10. The method of claim 7, further comprising:

determining that service cannot be received from a current serving cell;

obtaining a list of candidate cells from which service may be received;

performing cell reselection to the candidate cell; and

performing the obtaining a first list, performing a power scan, obtaining a second list, processing, and selecting if the cell reselection to the candidate cell fails.

11. The method of claim 7, wherein the second list includes all of the radio frequency channels within the first list.

12. The method of claim 7, wherein the processing comprises: for each of the N radio frequency channels:

the radio frequency channel is acquired and the radio frequency channel is acquired,

scheduling decoding of a broadcast channel of the radio frequency channels, and

decoding the broadcast channel of the radio frequency channel at a timing to determine whether the radio frequency channel is from the suitable cell.

13. The method of claim 12, wherein the processing further comprises:

the acquisition, scheduling, and decoding are terminated once the suitable cell is found.

14. The method of claim 12, wherein the processing further comprises:

the acquisition, scheduling and decoding is terminated once a best cell with the strongest received signal strength measurement is found among the suitable cells for all of the N radio frequency channels.

15. The method of claim 7, further comprising:

if a suitable cell is not found, cell selection is performed for all of the radio frequency channels evaluated for cell selection.

16. The method of claim 7, wherein the wireless communication system is a global system for mobile communications.

17. An apparatus in a wireless communication system, comprising:

means for obtaining a first list of radio frequency channels on which to find a suitable cell from which to receive communication services, wherein the first list comprises fewer than all radio frequency channels evaluated for cell selection;

means for performing a power scan to obtain received signal strength measurements for the radio frequency channels in the first list;

means for obtaining a second list of at least one of said radio frequency channels selected from said first list;

means for processing said at least one radio frequency channel within said second list to find said suitable cell, an

Means for selecting the suitable cell as a new serving cell from which to receive service if the suitable cell is found;

wherein when the second list includes N radio frequency channels in the first list, the N radio frequency channels have strongest received signal strength measurements, and if N >1, the N radio frequency channels in the second list are processed in parallel to find the suitable cell.