KR20070095333A - Cell search using rake searcher to perform scrambling code determination - Google Patents

Abstract

A spread spectrum receiver configured to perform cell searching can include a cell search system (120) and a searcher (125). The cell search system (120) is configured to perform primary and secondary synchronization, thereby identifying a scrambling code group. The searcher (125) is configured to determine a scrambling code from the scrambling code group identified by the cell search system.

Description

CELL SEARCH USING RAKE SEARCHER TO PERFORM SCRAMBLING CODE DETERMINATION}

The present invention relates generally to receiver architecture for use with spread spectrum and code division multiple access (CDMA) wireless networks.

Universal Mobile Telecommunications System (UMTS) is a standard for 3G wireless networks defined by the International Telecommunication Union (ITU). UMTS defines a communication scheme consisting of slots, in which 15 slots form a frame. Each slot specifies, among other data, synchronization information used to synchronize the inter-node communication of the UMTS compliant network.

An important procedure performed by a receiver in a UMTS network, for example a CDMA mobile receiver, is cell search operation. Cell search is typically performed by a cell search system that is included as part of the receiver. The cell search system is activated after the receiver is energized to determine synchronization information belonging to the cell in which the receiver is located. The cell search operation is a three step process. That is, the cell search system performs slot synchronization (first synchronization), frame synchronization and scrambling code group determination (second synchronization), and scrambling code determination.

When performing cell search, the cell search system accesses a synchronization channel (SCH) and a common pilot channel (CPICH) of the received radio signal. The SCH is a composite channel formed from the first SCH and the second SCH. Within each slot, the first SCH designates a first synchronization code (PSC). However, the first SCH contains only data for the first 256 chips of each 2560 chip slot. As is known, "chip" or "chip speed" means the proportion of spreading codes in a CDMA communication system.

The cell search system of the receiver achieves slot synchronization with the cell using the first SCH. This is typically done using a single matched filter or other similar device. The filter is matched to a PSC common to all cells. The slot timing of the cell can be obtained by detecting the peak of the matched filter output.

The cell search system of the receiver performs frame synchronization using the second SCH. The second SCH designates a second synchronization code (SSC) in each slot. Unlike the PSC, the SSC can be one of sixteen different codes. Each slot contains one SSC. The SSC used is different for each slot forming a period of one frame or a sequence with 15 slots. There are 64 possible SSC sequences, each sequence corresponding to one of the 64 possible scrambling code groups. By observing the entire frame of data, the receiver can determine which of the 64 SSC sequences is being transmitted. Since the SSC sequence is repeated at the same period as one frame, the sequence can be used to achieve frame synchronization of the receiver because it can identify frame boundaries. The transmitted SSC sequence also indicates which scrambling code group is used for the current cell.

Each scrambling code group contains eight possible scrambling codes. To determine the actual scrambling code, the cell search system of the receiver correlates each of the eight possible scrambling codes of the identified scrambling code group with the received CPICH signal until the correct scrambling code is determined. After the actual scrambling code is identified, a first common control channel (CCPCH) can be detected so that system and cell specific broadcast channel (BCH) information can be read.

Conventional cell search systems typically include a matched filter correlator for correlating received samples to CPICH sequences. We also find peaks of eight correlations for a given group of scrambling codes, including peak detection hardware. It also includes other logic that can determine which peak is the strongest of the eight to identify a particular scrambling code from the eight scrambling codes of the scrambling code group.

Conventional cell search designs are costly in terms of both the total hardware needed and the increasing power usage. Typically, a cell search system is implemented as an application specific semiconductor (ASIC). Because the system is large, the die size needs to be large for manufacturing. Larger ASIC designs include more gates, which consume more power. This can be crucial in terms of battery life in the mobile receiver. This problem is worse because scrambling code determination is rarely performed, typically only when the receiver is energized or unlocked.

As such, it is desirable to reduce the total amount of hardware needed in the receiver for the cell search process and thus to reduce the power consumption of the cell search system in the receiver.

In accordance with the principles of the present invention, it is understood that the total amount of hardware required for the cell search system of the receiver can be further reduced. In particular, the cell search system performs the first and second synchronization, while the identification of the scrambling code is transferred from the cell search system to the searcher of the receiver. Thus, hardware associated with scrambling code identification can be removed from the cell search system.

In an exemplary embodiment, the receiver is a CDMA receiver. By way of example, the cell search system is deactivated after performing the second synchronization to determine the associated scrambling code group. Next, the searcher is activated after the cell search system performs the second synchronization. The searcher correlates each scrambling code of the associated scrambling code group with the received common pilot channel (CPICH) signal and selects the scrambling code with the largest correlation peak. The searcher may also correlate different offsets of each scrambling code of the scrambling code group.

According to a feature of the invention, the receiver may comprise a scrambling code generator for dynamically generating at least one scrambling code of the associated scrambling code group. The searcher may select a particular scrambling code using the dynamically generated scrambling code (s). In another embodiment, the method may include a memory that stores at least one pre-generated scrambling code of the related scrambling code group. In that case, the searcher reads one or more scrambling codes of the associated scrambling code group to select a particular scrambling code.

Another aspect of the invention may include a cell search method for use by a spread spectrum receiver such as a CDMA receiver. The method may include performing first and second synchronization using a cell search system, thereby identifying a scrambling code group, and using a searcher to identify a scrambling code from the scrambling code group.

The method may include deactivating the cell search system after the execution step and activating the searcher before the selection step. The scrambling code can be selected by correlating each scrambling code of the scrambling code group with the received CPICH signal and selecting the scrambling code having the largest correlation peak. In performing the correlation, different offsets of each scrambling code of the scrambling code group may be correlated.

In exemplary embodiments, the method may include storing the scrambling code of at least one of the scrambling code groups in memory, reading the scrambling code of one or more of the scrambling code groups from memory, and performing a correlation step if necessary. In yet another embodiment, the method may include dynamically generating at least one scrambling code for use in the correlation step.

Preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings.

1 is a schematic diagram illustrating one embodiment of a receiver in accordance with the inventive apparatus disclosed herein.

2 is a flowchart illustrating a method of determining a scrambling code in accordance with another embodiment of the present invention.

In addition to the concept of the present invention, the components shown in the drawings are well known and will not be described in detail. In addition, the UMTS-based wireless communication system is assumed to be well known and will not be described in detail herein. For example, spread spectrum transmission and reception, cells (base stations), user equipment (UE), downlink channels, uplink channels, searchers, and RAKE receivers, in addition to the concepts of the present invention, are well known and are not described herein. In addition, the inventive concept may be implemented using conventional programming techniques not described herein. Finally, like numerals in the drawings indicate like elements.

The present invention provides a solution in the receiver for determining the scrambling code. According to the apparatus of the present invention disclosed herein, the cell search is implemented as a two-step process in which the cell search system performs the first synchronization and the second synchronization, not the three-step process, and the scrambling code determination is performed in another existing method in the receiver. This is done using hardware such as an explorer.

In particular, after cell search hardware obtains the second SCH, the cell search hardware or system may be deactivated to save power. Next, the searcher can be activated. Typically, searcher hardware is used to identify multipath components. Since the searcher is already configured to perform such processing, it is understood that existing searcher hardware can also be used for scrambling code determination in accordance with the principles of the present invention. Thus, the hardware required to implement the cell search system can be reduced.

1 is a schematic diagram illustrating one embodiment of a receiver 100 in accordance with the apparatus of the present invention disclosed herein. In one aspect of the invention, the receiver 100 is a UMTS receiver, such as a CDMA receiver. As shown in FIG. 1, receiver 100 includes an analog to digital converter 105 for converting a received analog signal into a digital representation. The digital signal accordingly is provided to the matched filter 110.

The matched signal is provided to the tapped delay line 115, and the tapped delay line provides a signal to the cell search system 120, the searcher 125 as well as one or more fingers 130A-130N. Tapped delay line 115 includes a plurality of taps from which various samples of the multipath signal may be provided. Each tap may provide samples for different delayed versions of the received multipath signal.

The signal provided to the cell search system 120 includes timing information. In particular, the signal includes a composite synchronization channel (SCH) and a joint pilot channel (CPICH) and allows the cell search system 120 to determine timing information from the provided signal. The cell search system 120 performs a two-step cell search operation. Specifically, slot synchronization is performed using the first SCH, and frame synchronization and scrambling code group determination are performed using the second SCH. As noted, the cell search system 120 does not need to execute the third step of scrambling code determination. Thus, such hardware can be excluded from the cell search system 120, thereby reducing size and power consumption.

Once the second SCH is obtained, the cell search system 120 may be deactivated or powered down. For example, cell search system 120 may be placed in a standby mode. Next, the navigator 125, which may be in standby or low power mode, may be activated or energized. Once energized, searcher 125 executes a third step of scrambling code determination. Cell search system 120 and searcher 125 may be implemented as one or more integrated circuits (ICs), ASICs, processors, controllers, individual components, or any combination thereof.

A processor (not shown) may include to facilitate communication between various components of the receiver 100 via the processor interface 150. Thus, the searcher 125 using the scrambling code group determined by the cell search system 120 may use it to identify the specific scrambling code to use. When the searcher 125 operates, the searcher searches or correlates a common pilot channel (CPICH) signal for each of the eight scrambling codes in the scrambling code group determined by the cell search system 120.

Since cell search system 120 has already achieved slot and frame synchronization, searcher 125 only needs to correlate with a small number of scrambling code offsets (eg, from offset 0 to offset M) at the start of the searcher space. In the UMTS standard, there are eight scrambling codes in a given scrambling code group. The highest correlation value of each of the eight searches is identified and the value can be stored. It should be appreciated that no similar hardware is required for the explorer 125 as similar functionality is executed during normal explorer operation. Next, the scrambling code used by the wireless transmitter is identified as the code corresponding to the highest correlation peak. In exemplary embodiments, processor 150 accessing memory (not shown) of searcher 125 executes the code corresponding to the highest correlation peak.

The scrambling code generation executed by the scrambling code generator 135 may be executed using any of a variety of different techniques. In one embodiment, eight scrambling codes may be generated on-the-fly. In other words, the scrambling code generator 135 may generate each scrambling code for use dynamically in correlation, if necessary. As known in the art, for example, a scrambling code generator uses a hardware implemented linear feedback shift register (LFSR) to generate the scrambling code, using one LFSR per scrambling code. The LFSR generates the scrambling code dynamically or "on the fly", and a new scrambling code chip value is generated for each chip. (The scrambling code covers a UMTS frame (38,400 chips) and includes a 38,400 chip value.)

In another embodiment, one or more or all eight scrambling codes may be generated before beginning the third step. For example, the processor may store the pre-generated scrambling code in a memory. Thus, the scrambling code generator 135 can be implemented as a memory or memory block, such as a memory having attached logic for storing 38,400 chip values of the scrambling code. (It should be noted that each scrambling code chip value may further include in-phase (I) and quadrature (Q) components.) In particular, if all eight scrambling codes are stored in the scrambling code memory, the receiver may For example, it is possible to communicate later with a plurality of base stations during a so-called "soft handoff", where each base station communicates using a different scrambling code.

Once the scrambling code is determined, the searcher 125 returns to the previous task of correlating the multipath signal with respect to the scrambling code to obtain a profile identifying various path locations within the multipath signal. Next, each of the fingers 130A to 130N may be assigned to different paths of the multipath signal determined by the searcher 125.

As is known in the art, fingers 130A-130N also process various paths using spreading codes provided by spreading code generator 140. The maximum ratio combiner (MRC) 145 phase assigns individual multipath signals from each finger 130A-130N using the CPICH signal. Maximum ratio combiner 145 supplies a structurally coupled signal provided to processor interface 150.

It should be appreciated that the delay hardware is unnecessary because each tap of the tapped delay line 115 provides a different version of the delayed multipath signal. That is, each different version of the multipath signal provided from a particular tap of the tabbed delay line 115 is time allocated with a different version provided from a different tap. As such, delay hardware need not be included between the fingers 130A-130N and the output of the MRC 145.

2 is a flowchart illustrating a method 200 used in a receiver to determine a scrambling code in accordance with the principles of the present invention. The method begins by way of example at step 205 where the cell search system 120 of FIG. 1 performs first and second SCH synchronization. Thus, the scrambling code group is identified from step 205. Since the cell search system is no longer needed after step 205, the cell search system can be powered down to further reduce power consumption.

In accordance with the principles of the present invention, the searcher in step 210 operates using one of the scrambling codes from the identified scrambling code group. The searcher works through a short window of offsets from zero to some value M. Since slot and frame synchronization has already been made (at step 205), the searcher only needs to correlate for a small window of scrambling code offset. For example, in the case of correctly synchronized cell search (ie, the correct group of scrambling codes has been identified), the correlation only needs to involve a plurality of scrambling code offsets (M = 4). Otherwise, the window can be set to be a plurality of chips that sufficiently ensure that peaks are found. By way of example, if the searcher is deactivated until needed, the searcher may be activated before step 210.

In step 215, the largest correlation value found in the window of offsets from 0 to M in step 210 is stored. By way of example, the correlation is compared in terms of magnitude so that the largest correlation value with respect to magnitude is stored. In step 220, a determination is made whether all eight scrambling codes of the identified scrambling code group have been searched. If all of the scrambling codes have been processed, the method 200 proceeds to step 225. If not, the method 200 returns to step 210 where the searcher operates with the next scrambling code of the identified scrambling code group. Steps 210, 215 and 220 are repeated until all scrambling codes of the identified scrambling code group have been processed.

In step 225, the eight correlation peaks stored from eight scrambling code processing of the identified scrambling code group are evaluated. The largest correlation peak is selected and the scrambling code corresponding to the largest peak of the eight correlation peaks is identified as the scrambling code used by the wireless transmitter.

Once the scrambling code is determined, the various paths of the multipath signal can be processed as described above. In particular, the route can be extracted, derotate and combined.

The foregoing merely illustrates the principles of the invention, and therefore those skilled in the art will embody the principles of the invention, although not explicitly described herein, and devise various alternative devices that fall within the spirit and scope of the invention. Can be. For example, although illustrated in the context of separate functional components, these functional components may be placed on one or more integrated circuits (ICs) and / or on one or more stored program controlled processors (eg, microprocessors or digital signal processors (DSPs)). You can also implement Similarly, although illustrated in a UMTS-based system environment, the inventive concept is applicable to other communication systems. Therefore, it will be understood that various modifications may be made to the exemplary embodiments, and that other devices may be devised without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (19)

As a receiver, A cell search system for performing a first and second synchronization on the received radio signal to identify a related group of scrambling codes; and A searcher for use in identifying a scrambling code of said related scrambling code group Receiver comprising a. The method of claim 1, And the cell search system is deactivated after performing the second synchronization. The method of claim 1, The searcher is activated after the cell search system executes the second synchronization. The method of claim 1, And a scrambling code generator for dynamically generating at least one scrambling code of said associated scrambling code group for use by said searcher identifying a scrambling code. The method of claim 1, And a memory storing at least one pre-generated scrambling code of said associated scrambling code group. The method of claim 5, And the searcher reads the at least one scrambling code of the associated scrambling code group from the memory to identify a scrambling code. The method of claim 1, The searcher correlates each scrambling code of the group of related scrambling codes with a received common pilot channel (CPICH) signal and identifies a scrambling code having the largest correlation peak. The method of claim 7, wherein And the searcher correlates different offsets of each scrambling code of the associated scrambling code group. The method of claim 1, A spread spectrum receiver is a code division multiple access (CDMA) receiver. As a method for use in a receiver, Performing first and second synchronization using the cell search system to identify a group of related scrambling codes of the received wireless signal, and Identifying a scrambling code from said group of related scrambling codes using a searcher How to include. The method of claim 10, Deactivating the cell search system after the executing step. The method of claim 10, Activating the searcher prior to the identifying step. The method of claim 10, The identifying step, Correlating each scrambling code of the associated scrambling code group with a received pilot pilot channel (CPICH) signal, and Identifying the scrambling code with the largest correlation peak How to include. The method of claim 13, And said correlating step further comprises correlating different offsets of each scrambling code of said associated scrambling code group. The method of claim 13, Storing in the memory at least one scrambling code of said associated scrambling code group, and Reading the at least one scrambling code of the associated scrambling code group from the memory and executing the correlation step if necessary How to include more. The method of claim 13, Dynamically generating at least one scrambling code for use in the correlation step. The method of claim 10, And the receiver is a code division multiple access (CDMA) receiver. As a receiver, Processor, and Include explorer, The processor controls the searcher to determine (a) a scrambling code used to process the received wireless signal to scramble the received wireless signal, and (b) determine a multipath of the received wireless signal. . The method of claim 18, And a cell search system for processing the received radio signal to perform first and second synchronization on the received radio signal.

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