KR20060065670A - Frequency synchronization during cell search in a universal mobile telephone system receiver - Google Patents

Abstract

A Universal Mobile Telephone System (UMTS) receiver performs slot synchronization using a received primary synchronization channel (PSCH) (305). Subsequent to completion of slot synchronization, the UMTS receiver performs frame synchronization using a received secondary synchronization channel (SSCH) (320) in such a way that the UMTS receiver uses the received primary synchronization channel (PSCH) to adjust for the presence of frequency offset (325, 330, 335).

Description

FREQUENCY SYNCHRONIZATION DURING CELL SEARCH IN A UNIVERSAL MOBILE TELEPHONE SYSTEM RECEIVER}

TECHNICAL FIELD The present invention generally relates to wireless receiving devices, and more particularly to user equipment (UE) in spread spectrum based wireless systems such as Universal Mobile Telephone System (UMTS).

The basic time unit in a UMTS radio signal is a 10 ms radio frame, which is divided into 15 slots of 2560 chips each. The UMTS radio signal from the cell (or base station) to the UMTS receiver is a "downlink signal" while the radio signal in the reverse direction is called an "uplink signal". When the UMTS receiver is turned on for the first time, the UMTS receiver performs a "cell search" for searching for a cell to communicate with. In particular, as described below, the UMTS receiver initially searches for a downlink synchronization channel (SCH) transmitted from the cell, synchronizes to the channel at the slot and frame level, and performs a special scrambling code group of the cell. Is determined. Only after successful cell search can voice / data communication be started.

In terms of cell search, the SCH is a sparse downlink channel active only for the first 256 chips of each slot. The SCH consists of two subchannels, namely, a primary SCH (PSCH) and a secondary SCH (SSCH). The PSCH 256 chip sequence or PSCH code is the same in every slot of the SCH for every cell. In contrast, an SSCH 256 chip sequence or SSCH code may be different in each of the 15 slots of a radio frame and is used to identify one of the 64 possible scrambling code groups. That is, each radio frame of the SCH repeats a scrambling code group sequence associated with each transmission cell. Each SSCH code is obtained from the alphabet of 16 possible SSCH codes.

As part of cell search, a UMTS receiver first uses a PSCH to obtain slot synchronization. In this regard, the UMTS receiver correlates the received sample of the received PSCH against a known PSCH 256 chip sequence (which is the same for all slots) and based on the location of the correlation peak. The slot reference time is determined. When the slot reference time is determined, the UMTS receiver can slot synchronize and determine when each slot starts in the received radio frame.

After slot synchronization, the UMTS receiver stops processing the PSCH and starts processing the SSCH. In particular, the UMTS receiver correlates a particular sequence of 15 SSCH codes in a received radio frame against a known sequence to obtain frame synchronization and determine a group of scrambling codes of the cell. Next, due to the identification of the scrambling code group, the UMTS receiver will descramble all other downlink channels (e.g., Common Pilot Channel (CPICH)) of the cell to begin voice / data communication. Can be.

Unfortunately, the cell search process described above has some disadvantages. One of them is time. Since SSCH processing identifies a sequence of 15 special SSCH codes, typically SSCH code processing occurs for a number of received radio frames, for example 10-20. Therefore, it may take 100 to 200 ms to complete the cell search. Another disadvantage is that the UMTS receiver does not acquire frequency synchronization until the CPICH is descrambled, which frequency synchronization occurs after successful completion of the cell search described above. As such, the frequency offset between the cell and the UMTS receiver may degrade the performance of SSCH processing during cell search (eg, the correlation peak may be very far from background noise and may not be noticeable). Such frequency offset occurs due to the low accuracy of the reference oscillator in the UMTS receiver, for example, used for down conversion. In addition, some frequency offset effects may be exacerbated by the Doppler effect when the UMTS receiver is mobile. As a result, the frequency offset may further lengthen the time required for the UMTS receiver to perform the SSCH processing portion of cell search, especially if such a frequency offset can cause SSCH processing to restart.

Therefore, according to the principles of the present invention, the wireless receiver performs slot synchronization using the received first synchronization channel, and when slot synchronization is completed, performs frame synchronization using the received second synchronization channel, One synchronization channel is used by the wireless receiver to adjust the frequency offset. So the frequency offset effect on the process of frame synchronization is eliminated or reduced.

In an embodiment of the invention, the wireless receiver is part of a UMTS user equipment (UE), the first synchronization channel is a PSCH subchannel, and the second synchronization channel is an SSCH subchannel. The wireless receiver continues to process the PSCH during SSCH processing to adjust the frequency offset. In particular, frequency adjustment is performed by rotating the received samples of the PSCH with different frequency offsets and then correlating the PSCH codes. The frequency offset corresponding to the highest correlation peak is used as an estimate of the actual frequency offset between the cell and the wireless receiver.

According to another aspect of the present invention, the wireless receiver continuously processes the PSCH during SSCH processing to continuously approximate the frequency offset. For example, a coarse estimate of the frequency offset is first determined by adjusting the estimate of the frequency offset in a large frequency step (or coarse step), for example, in 2.5 Hz increments. After the rough estimate of the frequency offset has been determined, further adjustment of the rough estimate of the frequency offset can be made, for example using smaller steps (or precise steps) of 1.25 Hz increments and then 0.625 Hz increments. Determine the final estimate.

1 illustrates part of an exemplary wireless communication system in accordance with the principles of the invention;

2 and 3 illustrate an exemplary embodiment of a wireless receiver in accordance with the principles of the invention.

4, 5 and 6 illustrate exemplary flowcharts in accordance with the principles of the present invention.

Of the elements shown in the drawings are not related to the concept according to the invention is known and will not be described in detail. In addition, it is assumed that the UMTS-based wireless communication system is familiar and will not be described in detail herein. For example, spread spectrum transmission and reception, a cell (base station), a user equipment (UE), a downlink channel, an uplink channel and a RAKE receiver, which are not related to the concept according to the present invention, are known and described herein. I never do that. In addition, the concepts according to the invention may be implemented using conventional programming techniques, which will not be described herein. Finally, like numerals in the drawings indicate like elements.

An exemplary portion of a UMTS wireless communication system 10 in accordance with the principles of the present invention is shown in FIG. The cell (or base station) 15 broadcasts a downlink synchronization channel (SCH) signal 16 including the above-described PSCH and SSCH subchannels. As noted above, the SCH signal 16 is used as a pre-condition to voice / data communication by a UMTS user equipment (UE) for synchronization purposes. For example, the UE processes the SCH signal during a "cell search" operation. In this example, the UE 20, for example a mobile phone, initiates cell search, for example when the UE 20 is turned on or powered up. The purpose of the cell search operation includes (a) synchronization for cell transmission at the slot and frame level of the UMTS radio frame, and (b) determination of the scrambling code group of the cell (eg, cell 15). As described below, in accordance with the principles of the present invention, the UE 20 processes the SSCH subchannels to adjust the frequency offset using the PSCH subchannels while acquiring frame synchronization with the cells 15. The following example illustrates the concept according to the present invention in this initial cell search, i.e., when the UE 20 is turned on, but the concept according to the present invention is not limited thereto, and is another example of cell search. For example, it should be noted that the UE is also applicable when in "idle mode".

Referring now to FIG. 2, shown is an exemplary block diagram of a portion of a UE 20 in accordance with the principles of the present invention. The UE 20 includes a front end 105, an analog to digital (A / D) converter 110, a cell search element 115, a searcher element 120, a RAKE receiver 125, a host interface block. 130 and processor 135. Additional elements known in the art, which are not related to the concept according to the present invention, may be included in the block shown in FIG. 2, but it should also be noted that for simplicity of description they are not described herein. For example, the A / D converter 110 may include a digital filter, a buffer, and the like.

The front end 105 receives a radio frequency (RF) signal 101 transmitted from the cell 15 (see FIG. 1) through an antenna (not shown) and represents a base representing PSCH and SSCH subchannels. Provides a base band analog signal 106. The front end 105 provides a baseband analog signal 106 including a reference frequency source 103 for use in the processing of the RF signal 101. The baseband analog signal is sampled by the A / D converter 110, which provides a stream 111 of received samples. The received sample 111 is available to three components: cell search element 115, searcher element 120, and RAKE receiver 125. The cell search element 115 processes the PSCH and SSCH subchannels in accordance with the principles of the present invention described below. After successful cell search, searcher element 120 evaluates the received sample for multipath assignment to each of the fingers of RAKE receiver 125, and the RAKE receiver is, for example, a decoder (not shown). Providing a symbol for subsequent decoding by means of combining data from multiple paths may provide voice / data communication. Since only cell search element 115 relates to the concept according to the invention, search component 120 and RAKE receiver 125 are no longer described herein. The host interface block 130 combines the data between the three aforementioned components and the processor 135, in which the processor receives the results from the cell search component 115 via signaling 134. do. The processor 135 is illustratively a program stored controller processor, for example a microprocessor, and includes a memory (not shown) for storing programs and data.

Referring now to FIG. 3, an exemplary block diagram of cell search element 115 is shown. The cell search element 115 includes a PSCH element 205, an SSCH element 210, and a rotor 215. Reference should also be made to FIG. 4, which illustrates an exemplary flowchart in accordance with the principles of the present invention for processing downlink PSCH and SSCH subchannels in the cell search element 115 of FIG. 3. The processor 135 of the UE 20 initiates cell search by processing the downlink PSCH subchannel in step 305, at step 305, attempting to obtain slot synchronization. In particular, the processor 135 activates the PSCH element 205 via signaling 206 to process the received sample 111. In addition, the processor 135 controls the rotor 215 via signaling 216, at which point it provides a zero rotation of the received sample 111, ie the received sample 111. Passes through the rotor 215 as if there is no rotor 125 without rotating. In step 305, the received sample 111 is processed by the PSCH element 205 known in the art. For example, since the downlink PSCH subchannel is a known PSCH 256 chip sequence or PSCH code that occurs periodically (i.e., repeats every slot of the downlink SCH signal), the PSCH element 205 is a received sample 111. ) Is correlated to the PSCH code and provides an associated peak correlation value. In this regard, the PSCH element 205 includes a matched filter and a buffer that stores the output signal of the matched filter (both not shown). The PSCH element 205 provides the peak value to the processor 135 via signaling 206. This peak value is averaged over a plurality of slots, e.g., 4-20 slots, of the received radio frame, reducing the probability of "false lock." If the peak value is less than the predetermined threshold, processor 135 controls PSCH element 205 to continue processing any received signal to continue searching for the cell. However, if the peak value is greater than the preset threshold, the UE 20 completes slot synchronization, the processor 135 continues the cell search process for frame synchronization, and determines a special scrambling code group for the relevant cell. do. An alternative method considers slot synchronization to be complete if the peak correlation value exceeds the next highest correlation value by a predetermined addition or multiplication factor.

In particular, in accordance with step 310 and the principles of the present invention, processor 135 enables both SSCH element 210 and PSCH element 205. The SSCH element processes the received sample 111 as is known in the art. The PSCH element is used to determine an estimate of the frequency offset, and the processor 135 uses this estimate to adjust the reference frequency 103 via the signaling 136 of FIG. 2 to compensate for the frequency offset during SSCH processing. So the effect of frequency offset on the process of frame synchronization is eliminated or reduced.

Referring now to FIG. 5, step 310 of FIG. 4 is shown in detail. As shown, step 310 includes step 320 associated with SSCH processing and steps 325, 330, and 335 associated with estimating frequency offset. Step 320 corresponds to SSCH processing known in the art and is performed by each of the SSCH elements 210 and processor 135 of FIGS. 2 and 3 as shown. The SSCH element 210 connects with the processor 135 via signaling 211. As mentioned above, the SSCH 256 chip sequence or SSCH code is different in each of the 15 slots of a radio frame for a particular cell. As such, each radio frame repeats a particular 15 SSCH code associated with a particular cell. When active by the processor 135, the SSCH element 210 uses a particular sequence of 15 SSCH codes in a received radio frame to obtain frame synchronization and uses a group of scrambling code groups (here, cell 15). Correlated to a known sequence for use in determining the scrambling code group associated with < RTI ID = 0.0 > As mentioned above, SSCH processing may require processing of numerous, for example, 10-20 received radio frames. During this processing, the PSCH element 205 is used by the processor 135 to estimate the frequency offset between the cell 15 and the UE 20.

In particular, at step 325, processor 135 adjusts rotor 215 to provide received sample 111 to PSCH element 205 in varying rotations. The use and placement of rotor 215 as shown in FIG. 3 prevents various rotations from affecting SSCH processing. For example, when searching for a frequency offset, instead of directly adjusting the reference frequency 103 of FIG. 2, the received sample 111 is multiplied by a complex number that rotates at the desired frequency before applying it to the PSCH element 205. . As such, and as can be seen from FIG. 3, this multiplication or rotation affects only the sample processed by the PSCH element 205, and does not affect the sample processed by the SSCH element 210. However, the use and placement of the rotor 215 is merely exemplary, and the concept according to the present invention is not limited thereto. For example, all received samples may rotate despite the effect on SSCH processing.

Referring to FIG. 5, it is assumed that, based on the accuracy of the local receiver oscillator of the UE, the frequency offset between the UE and the cell may be as large as ± 10 kHz, which is a known a priori. As such, step 325 is executed to iteratively step through the rotation values, i.e., frequency offsets of 0, ± .25, ± .5, ± .75, ± 1.00, ..., ± 10.0 Hz. For each rotation value, the PSCH element 205 correlates the rotated received sample against a known PSCH code and provides the associated correlation peak value to the processor 135 via signaling 206. Processor 135 tracks the magnitude of correlation peaks resulting from various rotational settings. Without rotation, any substantial frequency offset between the cell 15 and the UE 20 may have a lower correlation peak for the PSCH code than the correlation peak resulting from the zero frequency offset between the cell 15 and the UE 20. Will be. So when the received sample 111 rotates, the rotation value associated with the largest correlation peak is an estimate of the actual frequency offset between the cell 15 and the UE 20. In step 330, processor 135 examines all correlation peaks and determines the largest correlation peak according to the associated rotation value representing an estimate of the frequency offset. In step 335, processor 135 appropriately adjusts the local reference, eg, reference frequency 103 of FIG. 2, by the associated rotation value. Although FIG. 5 shows that the frequency offset is compensated for in a single pass environment passing through steps 325, 330 and 335, the present invention is not so limited, and for example steps 325, 330 and 335 are SSCH processing. You can repeat this several times. When the SSCH processing is completed in step 320, the scrambling code group of the cell 15 is identified, which causes the UE 20 to use all other downlink channels of the cell (eg, for frequency synchronization, and identify the From the group of scrambling code may descramble a CPICH (Common Pilot Channel) that determines the actual scrambling code for the cell, voice / data communication can be started.

In addition, the above-described processing may be performed as shown in FIG. 6 similar to the flowchart of FIG. 5. As can be seen from FIG. 6, there is one or more processing levels represented by coarse estimation step 405 and fine estimation step 410. Steps 405 and 410 each include processing similar to that shown in steps 325 and 330 of FIG. 5 to provide an estimate of frequency offset. Similarly, step 405 or 410, or both 405 and 410, may repeat several times during SSCH processing. 6, consider the following example. Again, based on the accuracy of the local receiver oscillator, assume that it is a known priori that the frequency offset between the UE and the cell can be as large as ± 10 Hz. As such, step 405 is executed to first determine a rough estimate of the frequency offset. For example, processor 135 performs PSCH processing using large frequency steps, e.g. 2.5 Hz steps which are frequency offsets of 0, ± 2.5, ± 5, ± 7.5 Hz for rotor 215. . Next, step 410 further refines the result of the rough estimation of the frequency offset by using smaller steps. For example, after step 405, a rough estimate of the frequency offset associated with the largest peak is assumed to be 5 Hz. Processor 135 then proceeds, at step 410, with a .25 Hz frequency that is a frequency offset of 5, 5 ± .25, 5 ± .5 and 5 ± .75 Hz for small frequency steps, e.g. rotor 215. PSCH processing is performed using the step of to determine the estimate of the frequency offset as described above. If an estimate of the frequency offset is determined, the processor 135 appropriately adjusts the local reference with the estimated frequency offset, eg, the reference frequency 103 of FIG. 2, at step 335. In fact, PSCH processing is used to continuously approximate the frequency offset during SSCH processing.

As noted above, in accordance with the principles of the present invention, the PSCH subchannel is used during the processing of the SSCH subchannel, which allows the wireless receiver to obtain at least coarse frequency synchronization before the SSCH processing is completed. . As such, this approach may improve the performance of SSCH processing in the presence of a frequency offset. Although described in the context of an initial cell search process, the concept according to the invention is applicable to any part of wireless operation in which a downlink channel, such as an SSCH subchannel, is processed in the presence of a frequency offset.

Since the foregoing is merely illustrative of the principles of the present invention, those skilled in the art, although not explicitly described herein, embody the principles of the present invention and provide numerous alternative devices that exist within the spirit and scope of the present invention. It will be appreciated that arrangements can be devised. For example, while described in the context of separate functional elements, such functional elements may be implemented on one or more integrated circuits (ICs) and / or one or more stored program controlled processors (eg, microprocessors or digital signal processors (DSPs)). It may be. Similarly, although described in a UMTS-based system environment, the concept according to the invention is applicable to any communication system that processes a signal in the presence of a frequency offset. Therefore, it will be understood that numerous modifications may be made to the exemplary embodiments, and that other arrangements may be devised without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (14)

As a method for use in a wireless receiver, Receiving a wireless signal, Processing a first synchronization channel of the received wireless signal to obtain slot synchronization (305), and Processing a second synchronization channel of the received wireless signal to obtain frame synchronization, wherein the first synchronization channel is used to adjust a frequency offset (310) How to include. The method of claim 1, Wherein the first synchronization channel is a primary synchronization subchannel (PSCH) and the second synchronization channel is a secondary synchronization subchannel (SSCH) of a universal mobile telephone system (UMTS). The method of claim 1, Processing the second synchronization channel may include: Processing the first synchronization channel to estimate a frequency offset in the received wireless signal, and Adjusting the clock of the wireless receiver to compensate for the estimated frequency offset How to include. The method of claim 3, Estimating a frequency offset by processing the first synchronization channel, Rotating a signal associated with the first synchronization channel through a plurality of frequency offsets, Determining a corresponding plurality of correlation peaks for each of the rotated signals at each of the plurality of frequency offsets, Selecting at least one of the plurality of correlation peaks such that the magnitude of the selected correlation peak is at least as large as the magnitude of the remaining plurality of correlation peaks, and Using at least one of the corresponding plurality of frequency offsets associated with the selected correlation peak as the estimated frequency offset How to include. The method of claim 1, Processing the second synchronization channel may include: Processing the first synchronization channel to provide a rough estimate of a frequency offset in the received wireless signal, Processing the first synchronization channel to further refine the rough estimate of the frequency offset to provide a final estimate of the frequency offset, and Adjusting the clock of the wireless receiver to compensate for a final estimate of the frequency offset How to include. As a method for use in a universal mobile telephone system (UMTS) based wireless receiver, Obtaining slot synchronization from the primary synchronization signal of the received wireless signal, and After acquiring slot synchronization, acquiring frame synchronization from the secondary synchronization signal of the received wireless signal while adjusting a frequency offset using the primary synchronization signal How to include. The method of claim 6, Using the primary synchronization signal, Processing the primary synchronization signal to estimate a frequency offset in the received wireless signal, and Adjusting the clock of the wireless receiver to compensate for the estimated frequency offset How to include. The method of claim 7, wherein Estimating a frequency offset by processing the primary synchronization signal, Rotating a signal associated with the primary synchronization signal through a plurality of frequency offsets, Determining a corresponding plurality of correlation peaks for each of the rotated signals at each of the plurality of frequency offsets, Selecting at least one of the plurality of correlation peaks such that the magnitude of the selected correlation peak is at least as large as the magnitude of the remaining plurality of correlation peaks, and Using at least one of the corresponding plurality of frequency offsets associated with the selected correlation peak as the estimated frequency offset How to include. The method of claim 6, Using the primary synchronization signal, Processing the primary synchronization signal to provide a rough estimate of a frequency offset in the received wireless signal, Processing the primary synchronization signal to further refine the rough estimate of the frequency offset to provide a final estimate of the frequency offset, and Adjusting the clock of the wireless receiver to compensate for a final estimate of the frequency offset How to include. As a wireless device, A front end 105 for receiving a wireless signal and providing a stream of received samples, Operating on the received sample to obtain slot synchronization with respect to the primary synchronization signal of the received wireless signal, and when slot synchronization is obtained, first process for subsequent processing of the primary synchronization signal to estimate a frequency offset. Synchronization element 205, A secondary synchronization element 210 for operating on the received sample to obtain frame synchronization for a secondary synchronization signal of the received wireless signal, and A processor 135 for adjusting a frequency offset in the wireless equipment during operation of the secondary synchronization element in response to subsequent processing of the primary synchronization signal by the primary synchronization element. Wireless equipment comprising a. The method of claim 10, Subsequent to slot synchronization, the primary synchronization element continues to process the primary synchronization signal of the received radio signal concurrently with the processing of the received radio signal by the secondary synchronization element. The method of claim 10, The primary synchronization element determines an estimate of a frequency offset in the received wireless signal, and the processor adjusts a clock of the wireless equipment to compensate for the estimated frequency offset. The method of claim 10, While the secondary synchronization element acquires frame synchronization, a rotor 215 for rotating the received sample and applying the rotated received sample to the primary synchronization element that processes the primary synchronization signal Wireless equipment, including more. The method of claim 13, And the processor selects a rotation value of the rotor for use as the estimated frequency offset.

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