1. Field of the Invention
The present invention generally relates to a method and apparatus for detecting synchronization in a mobile communication system, and in particular, to a method and apparatus for detecting synchronization during demodulation of a Primary Common Control Physical Channel (PCCPCH) in an asynchronous mobile communication system.
2. Description of the Related Art
With the rapid development of communication technology, mobile communication systems have reached the phase of providing not only general voice communication service but also a high-speed data service capable of supporting a multimedia service. A packet data system providing the high-speed data service is roughly classified into a synchronous system adopted in the United States and an asynchronous system adopted in Europe and Japan, and the synchronous and asynchronous systems undergo different standardizations according to their standard groups.
The synchronous packet data system managed by 3rd Generation Partnership Project 2 (3GPP2) is evolving into Code Division Multiple Access (CDMA) 2000 1x currently in service, Evolution Data Only (1x EV-DO) capable of high-speed packet transmission, and Evolution of Data and Voice (EV-DV) capable of supporting both voice and packet services, and the asynchronous packet data system managed by 3GPP includes Universal Mobile Telecommunication Systems (UMTS), which is also called Wideband-CDMA (W-CDMA).
A description will now be made of a frame synchronization detection technology for a Broadcasting Channel (BCH) among the synchronization detection technologies applied to UMTS.
In UMTS, a User Equipment (UE) of a subscriber acquires 10-ms frame synchronization through an initial cell search process. The UE matches timing synchronization of a forward link through a forward broadcasting channel, i.e. BCH, and acquires information related to a Random Access Channel (RACH). The BCH is carried on a PCCPCH.
The UE decodes a 10-ms PCCPCH frame for 20 ms (Transmission Time Interval (TTI) of a BCH), and delivers the decoded PCCPCH frame to an upper layer in a TTI size. Therefore, because the BCH has 20-ms frame synchronization, the UE should correctly detect synchronization of a PCCPCH frame having a 20-ms TTI boundary in order to correctly decode the BCH.
FIG. 1 shows a method for detecting synchronization of a BCH in a conventional asynchronous mobile communication system. The conventional method shown in FIG. 1 performs BCH decoding, and then matches a TTI boundary of a BCH using a Cyclic Redundancy Check (CRC) result on the decoded BCH.
According to the conventional technology, in order to acquire frame synchronization of a PCCPCH having a 20-ms TTI boundary, a UE receives and decodes a 20-ms BCH in step 101.
In step 103, the UE performs a CRC check on the decoded data. If the CRC result is “Good,” the UE determines that it has correctly detected synchronization of a BCH, and if the CRC result is “Bad,” the UE determines that it has failed to detect synchronization of a BCH. That is, if the CRC result is “Good,” the UE determines that a TTI boundary between a UMTS Terrestrial Radio Access Network (UTRAN) and the UE is matched. If the CRC result is “Bad,” the UE determines that the TTI boundary between the UTRAN and the UE is mismatched.
Therefore, if the CRC result is “Good” in step 103, the UE proceeds to step 105 where it delivers a transport block obtained by decoding a BCH to an upper layer with the acquired frame synchronization. However, if the CRC result is “Bad” in step 103, the UE proceeds to step 107 where it discards the next 10-ms PCCPCH frame and then prepares for decoding of a PCCPCH frame. In step 109, the UE receives and decodes a 20-ms BCH, and then proceeds to step 105 where it delivers a transport block obtained by decoding the BCH to an upper layer with the acquired frame synchronization.
The conventional synchronization detection method matches a TTI boundary by shifting a mismatched TTI boundary of a UE one frame. If a TTI boundary between a UTRAN and a UE is mismatched by one frame as shown by reference D1 of FIG. 8, the CRC result on a decoded BCH will be “Bad.” It is possible to match the TTI boundary between the UTRAN and the UE with a method of shifting a TTI boundary of the UE 10 ms (one frame).
However, in a poor radio environment, a CRC result on a decoded BCH that is “Bad” even though the TTI boundary is matched occurs frequently. Therefore, when the CRC result is “Bad,” it is hard to determine whether the “Bad” CRC result is due to the mismatch of the TTI boundary, or due to the poor radio environment. If the “Bad” CRC result has occurred in the condition where the TTI boundary is matched and the radio environment is not good, the UE using the method described in FIG. 1 shifts the TTI boundary, determining that the TTI boundary is not matched. A “Bad” CRC result will necessarily occur in the next TTI, and as a result, the UE should shift again the TTI boundary.
Therefore, for the “Bad” CRC result caused by the poor radio environment, the UE should observe a change in CRC results on a BCH decoded for more TTIs, and shift the TTI boundary according to the observation result. In this way, the conventional BCH synchronization detection method requires a long time in order to detect a reliable TTI boundary. In particular, when the UE performs BCH decoding to obtain information on neighbor cells after awaking from a sleep state, the use of the conventional method requires a longer time, causing a reduction in performance of the UE.