HK1110736B - Improved cell search for handover conditions - Google Patents

Description

Improved cell search for handover state

Technical Field

The present invention relates to handover conditions in carrier based systems, and more particularly to primary cell search procedures.

Background

The basic unit of time in a UMTS radio signal is a 10 millisecond (ms) radio frame that is divided into 15 slots of 2560 chips each. UMTS radio signals from a cell (or base station) to a UMTS receiver are "downlink signals" and radio signals in the opposite direction are referred to as "uplink signals".

The physical layer of Universal Mobile Telecommunications System (UMTS) Wideband Code Division Multiple Access (WCDMA) uses Direct Sequence Spread Spectrum (DSSS) modulation with a chip rate of 3.84 Mcps. The Frequency Division Duplex (FDD) mode carries uplink and downlink channels on separate frequency bands of 5 MHz each. This mode is typically used for larger outdoor cells because it can support a larger number of users than Time Division Duplex (TDD) mode. In TDD mode, transmissions share the same uplink and downlink channels during different time slots. TDD mode does not support as many users as FDD mode, and therefore TDD mode is more suitable for smaller cells. TDD mode is also better suited to carry asymmetric traffic than FDD mode.

An important procedure performed by a receiver within a UMTS network, such as a CDMA mobile receiver, is a cell search operation. Cell search is typically performed by a cell search system included as part of the receiver. After powering on the receiver, the cell search system is activated to determine synchronization information belonging to the cell in which the receiver is located. The cell search operation is a three-phase process. That is, the cell search system performs slot synchronization (primary synchronization), frame synchronization and scrambling code group determination (secondary synchronization), and scrambling code determination.

After power-up, the Mobile Terminal (MT) must perform several operations before voice/data communication can begin. First, the receiver needs to perform Automatic Gain Control (AGC) in order to scale (scale) the received signal power and avoid clipping (clipping) at the analog-to-digital converter. This procedure may be performed on the Synchronization Channel (SCH) first and then once the cell's scrambling code is acquired, the descrambled common pilot channel (CPICH) may be used.

Next, the receiver needs to acquire timing synchronization. Timing synchronization can be obtained from the SCH channel. The MT searches for the strongest SCH signal it can find and that signal determines with which cell the MT will initiate communication. Since the SCH channel is periodic, the receiver can correlate (correlate against) the primary SCH to derive a timing error. Based on this channel, the receiver can obtain chip, symbol, and slot synchronization.

The primary SCH carries the same signal for all cells in the system. The secondary SCH is different for each cell and carries a pattern (pattern) of Secondary Synchronization Codes (SSCs) that repeat every frame. Once the MT receives the sequence, it will have frame synchronization.

In performing cell search, the cell search system accesses a Synchronization Channel (SCH) and a common pilot channel (CPICH) of a received wireless signal. The SCH is a composite channel formed of a primary SCH and a secondary SCH. In each slot, the primary SCH specifies a Primary Synchronization Code (PSC). However, the primary SCH only contains data during the first 256 chips of each 2560 chip slot. As is known, "chip" or "chip rate" refers to the rate of a spreading code in a CDMA communication system.

In addition, the pattern identifies which scrambling code group the scrambling code of the current cell belongs to. There are 64 scrambling code groups and each group includes 8 scrambling codes. Once the MT determines the scrambling code group of the current cell, the search for the scrambling code of the current cell is narrowed to the eight codes in that group.

A typical acquisition procedure for a carrier-based receiver is as follows:

1. primary cell search

2. Secondary cell search

3. Scrambling code determination

4. Multipath searching

5. Finger (finger) assignment

6. Code tracking and Automatic Frequency Control (AFC) loop locking

7. Maximum Ratio Combining (MRC) of finger output

8. Acquisition of receiver lock and data can be sent to upper layers is lengthy and complex and may take on the order of seconds to complete. This waiting period is annoying to the user of the cellular phone/mobile station/mobile device when he/she turns on his/her phone, and a method of shortening the acquisition process is clearly desirable.

EP1179962 to cho et al, entitled "Mobile station handover method for asynchronous receiving communication system, less switching between ues and non-ues modes", describes a method of switching between Uplink Synchronous Transmission Scheme (USTS) and non-USTS modes based on the power strength ratio between the current cell and the adjacent cell. The method is used for handover of a mobile station in a wireless telecommunication system and for increasing a data transmission rate based on a compressed mode. The method described by Cho et al does not address cell search and the pilot power strength ratio described and used by Cho et al varies from cell to cell. The power strength ratio may be determined based on snapshots (snapshots) (transients) of the correlation data.

Disclosure of Invention

The present invention enhances the cell search process of a carrier-based receiver. Although the invention will be described below for a 3g wcdma receiver, it should be understood that the invention can be applied to any carrier-based system.

The control logic block controls the primary cell search when the cellular receiver is in a handover scenario (e.g., in communication range of two or more cell towers). The largest peak is identified and a search is made for other peaks. If only one peak is above the threshold, an 8-step acquisition process for the receiver is performed by synchronizing the receiver to that peak. However, if multiple peaks are identified, the control logic re-runs the primary cell search X times repeatedly and keeps track of the change in amplitude of each peak. The control logic then identifies the peak (representing the cell tower to which the cell phone/mobile station/mobile device is moving) that increases in magnitude the most. The 8-step acquisition process of the receiver is performed by synchronizing the receiver to the peak whose amplitude increases the most.

A method and apparatus for controlling primary cell search operations are described, comprising: generating a profile (profile) of the correlation peak (peak); determining whether the magnitude of the correlation peak changes; if the magnitude of the correlation peak remains relatively constant, the signal acquisition process is synchronized to the correlation peak having the largest magnitude among the identified correlation peaks, and if the magnitude of the correlation peak changes, the signal acquisition process is synchronized to the peak whose magnitude is increasing. The method and apparatus further includes identifying an index and magnitude of a maximum correlation peak of the correlation peaks, determining if other correlation peaks are present in the profile, synchronizing a signal acquisition process to the originally identified maximum correlation peak if no other correlation peaks are present, storing the index and magnitude of all other correlation peaks located, and comparing changes in correlation peak magnitude for all previously identified peaks.

Drawings

Fig. 1 shows a portion of an illustrative wireless communication system in accordance with the principles of the invention.

Fig. 2 shows a single significant (significant) peak (peak a) and a second peak (peak B) with a magnitude below the N% threshold.

Fig. 3 shows three peaks, where peak a is a significant peak, peak B is a second significant peak with an amplitude above the N% threshold, and peak C is a third peak with an amplitude below the N% threshold.

Fig. 4 is a block diagram of the acquisition process of the present invention for a carrier based receiver.

Fig. 5A is a flow chart of the control logic portion of the block diagram of fig. 4.

Fig. 5B is a continuation of fig. 5A.

These and other aspects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings. Finally, like numbers on the figures represent like elements.

Detailed Description

In the following description, familiarity with UMTS-based wireless communication systems is assumed and will not be described in detail herein. For example, knowledge about spread spectrum transmission and reception, cells (base stations), User Equipment (UE), downlink channels, uplink channels, and RAKE receivers is assumed and not described herein. In addition, the present invention may be implemented using conventional programming techniques, which, as such, will not be described herein.

The invention solves the specific problems that: how to reduce acquisition time during a common situation (i.e., a handoff scenario) where a phone is located in the communication range of two or more cell towers.

The current approach of the prior art is to perform the 8-step acquisition process described above. When this occurs during a handoff scenario, the phone may start the acquisition process (e.g., cell search) using the signal from the closest base station (since it appears as the strongest peak at the receiver). However, if the phone is moving away from the base station, by the time the 8-step acquisition process is complete, the peak may be so low or non-existent that the lock fails and the entire 8-step acquisition process must be restarted using signals from a different cell tower.

It takes a long time to perform the acquisition process once and longer time to perform it twice. In terms of user experience, a method of "just (right) going through the acquisition process for the first time" is desired.

An illustrative portion of a UMTS wireless communications system 10 in accordance with the principles of the invention is shown in fig. 1. A cell (or base station) 15 broadcasts a downlink Synchronization Channel (SCH) signal 16 that includes the PSCH and SSCH subchannels described above. As previously described, the SCH signal 16 is used by UMTS User Equipment (UE) for synchronization purposes as a prerequisite to voice/data communications. For example, during a "cell search" operation, the UE processes the SCH signal. In this example, the UE 20 (e.g., a cellular phone) initiates a cell search when, for example, the UE 20 is turned on or powered up. The purposes of the cell search operation include: (a) synchronizing to cell transmissions at the slot and frame level (1 ev) of a UMTS radio frame, and (b) determining the scrambling code group of the cell (e.g., cell 15). As described below and in accordance with the principles of the invention, the UE 20 adaptively controls the duration of the process of determining the SSCH portion of the SCH for frame synchronization. It should be noted that although the following examples illustrate the inventive concept in the context of this initial cell search (i.e., when the UE 20 is turned on), the inventive concept is not so limited and may be applied to other cell search instances, such as when the UE is in "idle mode".

Aspects of the present invention are implemented in the control logic block shown in fig. 4. The control logic implements the logic described by the flow diagrams in fig. 5A and 5B. First, an initial primary cell search step is run and the correlation peak profile is checked to locate the index and magnitude of the largest peak. After the maximum peak (the "original" peak) is identified, the remainder of the correlation peak profile is scanned to identify other peaks that are more than M samples away from the original peak and have an amplitude greater than or equal to N% of the original peak. M and N are design parameters determined by simulation or adaptive parameters, e.g., M4 and N80%. A dead zone (dead zone) of M samples is used because the peaks actually have a sinusoidal function shape and samples very close to the main/original peak are attributed to the main peak itself and do not represent separate peaks. An N% amplitude threshold is used in order to isolate only strong peaks, which represent signal energy from those base stations that are close enough for the receiver to lock onto the base station.

Fig. 2 shows the case of a single significant peak (peak a) and a second peak (peak B) whose amplitude is below the N% threshold. When this occurs, the remaining 8-step acquisition process is performed by synchronizing the receiver to peak a.

Another aspect of the present invention is when the situation shown in fig. 3 occurs. In fig. 3, the peak a is an original peak. However, there is a second peak (peak B) that is also above the N% threshold. Peak C also appears on fig. 3, but the magnitude of peak C does not exceed the N% threshold. In this case, the control logic block of the present invention is expressed as follows. It records the index and magnitude of peak a and peak B and it reruns the primary cell search X times (X is a design parameter, e.g., X ═ 10). The change in amplitude (if any) is recorded for each iteration of the primary cell search. If the amplitudes of peak a and peak B are relatively constant, the remainder of the 8-step acquisition process is performed by synchronizing the receiver to peak a. This would correspond to a scenario where the user's phone/mobile station is not moving significantly.

However, if the amplitude of peak a and peak B is changing, it indicates that the user's phone/mobile is moving. The arrangement determines which cell tower the mobile station is moving towards-whether the cell tower corresponding to peak a or the cell tower corresponding to peak B. For example, if the amplitude of peak B is increasing and the amplitude of peak a is decreasing, it indicates that the phone is moving towards cell tower B and away from cell tower a.

Therefore, even if peak B is lower in magnitude than peak a, an 8-step acquisition process should be performed by synchronizing the receiver to peak B. The algorithm examines all correlation peaks (i.e., correlation peaks above the threshold) and runs an 8-step acquisition process by synchronizing to the peak whose amplitude increases the most.

If there are additional peaks that exceed the N% threshold, the receiver will be synchronized to the most increased peak, rather than the peak with the largest magnitude. For example, if peak B and peak C both exceed the N% threshold and peak C has a smaller threshold than peak B, but the magnitude of peak C increases more, the receiver will be synchronized to peak C.

Fig. 4 is a block diagram of the acquisition process of the present invention for a carrier based receiver. The control logic of the present invention is conducted after the primary cell search is run/performed and before the secondary cell search is run/performed. The control logic is executed/run (repeated) multiple times. The number of repetitions of the control logic process is either a design parameter determined based on simulation or an adaptive parameter.

Fig. 5A and 5B together are a flow chart of the control logic portion of the block diagram of fig. 4. A primary cell search is run/performed at 501 and a profile of correlation peaks is generated. The index and magnitude of the largest peak is identified at 503. A test is made at 505 to determine if there are other peaks that are more than M samples away from the maximum peak identified in the primary cell search, where the magnitude of any other peak is determined to be greater than or equal to N% of the maximum peak, where M and N are design parameters or adaptive parameters set based on simulation. If the test result is negative, the remainder of the acquisition process (previous secondary cell search) is performed by synchronizing to the originally identified maximum peak at 535. If the test result is positive, the indices and magnitudes of all correlation peaks that meet the above criteria are stored at 510. Another test is performed at 515 to determine if the number of iterations is equal to X, where X is a design parameter set based on simulation or X is an adaptive parameter. If X iterations have not been performed, control returns to 505. If X iterations have been performed, a change in correlation peak amplitude is performed at 520 for all peaks previously identified. A further test is performed at 525 to determine if the magnitude of the correlation peak remains relatively constant, which means that the mobile station/device is not moving. If the amplitude remains relatively constant, the remainder of the acquisition process (previous secondary cell search) is performed by synchronizing to the identified maximum peak at 540. If the test at 525 is negative, then the entire acquisition process (previous secondary cell search) is performed at 530 by synchronizing to the peak whose magnitude is increasing by the maximum amount, which means that the mobile station/device is moving toward the corresponding base station.

When the cellular receiver is in a handover scenario (e.g., within communication range of two or more cell towers), the control logic block controls the primary cell search in order to improve performance and reduce acquisition time. The largest peak is identified and a search is made for other peaks that are further away than the M samples from the largest peak and have an amplitude greater than or equal to N% of the amplitude of the largest peak. If only one peak is above the threshold, an 8-step acquisition process for the receiver is performed by synchronizing the receiver to that peak. However, if multiple peaks are above the threshold, the control logic repeatedly re-runs the primary cell search X times and keeps track of the change in amplitude of each peak. The control logic then identifies the peak whose amplitude increases the most (representing the cell tower to which the handset is moving). The 8-step acquisition process of the receiver is performed by synchronizing the receiver to the peak whose amplitude increases the most.

It should be understood that: the invention may be implemented in various forms of hardware, software, firmware, special purpose processors, or a combination thereof, for example, in a mobile terminal, access point, or a cellular network. Preferably, the present invention is implemented as a combination of hardware and software. Further, the software is preferably implemented as an application program tangibly embodied on a program storage device. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. The machine is preferably implemented on a computer platform having hardware such as one or more Central Processing Units (CPU), a Random Access Memory (RAM), one or more input/output (I/O) interfaces. The computer platform also includes an operating system and microinstruction code. The various processes and functions described herein may either be part of the microinstruction code or part of the application program (or a combination thereof), which is executed via the operating system. In addition, various other peripheral devices may be connected to the computer platform such as an additional data storage device and a printing device.

It should also be understood that: because some of the constituent system components and method steps depicted in the accompanying figures are preferably implemented in software, the actual connections between the system components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention.

Claims (8)

1. A method for controlling cell search operations, the method comprising:

generating a profile of correlation peaks;

identifying an index and an amplitude of a maximum correlation peak of the correlation peaks;

determining whether there are other correlation peaks in the profile;

synchronizing the signal acquisition process to the originally identified maximum correlation peak if no other correlation peaks exist;

storing the index and magnitude of all other correlation peaks if there are other correlation peaks;

repeating the steps for a preset number of times;

comparing the change in magnitude of the correlation peak for all previously identified peaks;

synchronizing a signal acquisition process to a correlation peak having a largest magnitude among the identified correlation peaks if the magnitude of the correlation peak remains relatively constant;

synchronizing the signal acquisition process to a correlation peak of increasing amplitude if the amplitude of the correlation peak is changing.

2. The method of claim 1, further comprising storing the index and the magnitude of a maximum correlation peak of the correlation peaks that is more than a predetermined number of samples away from the maximum correlation peak.

3. The method of claim 1, further comprising storing the index and the magnitude of a maximum correlation peak of the correlation peaks that is greater than or equal to a predetermined percentage of the maximum correlation peak.

4. An apparatus for controlling cell search operations, comprising:

means for generating a profile of correlation peaks;

means for identifying an index and magnitude of a maximum correlation peak of the correlation peaks;

means for determining whether there are other correlation peaks in the profile;

means for synchronizing the signal acquisition process to the originally identified maximum correlation peak if no other correlation peaks exist;

means for storing the index and magnitude of all other correlation peaks, if any;

a component repeating the functions of the above components a predetermined number of times;

means for comparing the change in correlation peak amplitude for all previously identified peaks;

means for synchronizing a signal acquisition process to a correlation peak having a largest magnitude among the identified correlation peaks if the magnitude of the correlation peak remains relatively constant;

means for synchronizing the signal acquisition process to a correlation peak of increasing amplitude if the amplitude of the correlation peak is changing.

5. The apparatus of claim 4, further comprising means for storing the index and the magnitude of a maximum correlation peak of the correlation peaks that is more than a predetermined number of samples away from the maximum correlation peak.

6. The apparatus of claim 4, further comprising means for storing the index and the magnitude of a maximum correlation peak of the correlation peaks that is greater than or equal to a predetermined percentage of the maximum correlation peak.

7. The apparatus of claim 4, wherein the apparatus is a receiver.

8. The apparatus of claim 7, wherein the receiver is a mobile terminal.