1. Field of the Invention
This invention relates to a method and apparatus for implementing a cell search in a mobile wireless communication system. More specifically, the invention relates to a reduced circuit and power cell search method and circuit in a W-CDMA (Wide-band CDMA) system.
2. Description of the Related Arts
Wireless communication systems, adopting a CDMA (Code Division Multiple Access) cellular schema as a technique for multiple mobile terminals access, due to terminal movement and propagation condition changes, require continuous searching for new cells on the UE (User Equipment) side for initial synchronization establishment (scrambling code/frame timing identification) with the transmitting cell.
One of the standards, supporting a CDMA system has been offered by a consortium named “3rd Generation Partnership Project” (3GPP) and stated in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213 and 3G TS 25.214 (the W-CDMA standard). Those documents are incorporated herein by reference. According to this standard, a following procedure for cell search is applied in a WCDMA system, consisting of three steps: STEP 1 for slot timing identification, STEP 2 for frame timing and code group identification, and STEP 3 for scrambling code identification.
FIG. 1 is a block diagram which illustrates a typical example of the structure of a prior art cell search circuit 7, accommodated in a conventional mobile terminal. The cell search circuit 7 applicable for any of the cell search stages (STEPs), includes a correlating unit 702 or matched filter 704 (which is implementation-dependent) for despreading of the base-band received signal on each STEP of the cell search procedure.
In spread spectrum communication, despreading indicates spread demodulation in a receiver side using the same spread code as that of a transmission side. A matched filter generally comprises a plurality of registers and multipliers for multiplying the output of each stage register by a coefficient and an adder for adding the outputs of the plurality of multipliers and outputting the sum. A correlating unit 702 with a bank of correlators performs the actual despreading by correlation between the codes generated by the code generator 703 and the base-band received signal.
Despreading in STEP 2 (frame timing and code group identification) and STEP 3 (scrambling code identification) is typically performed by correlating unit 702, though the matched filter 704 could also be used. Code generator 703 generates a required code for correlation with base-band received signal—SSCH code for frame timing and code group identification on STEP 2, a primary scrambling code for identification on STEP 3 and PSCH code for STEP 1 despreading when a correlating unit is used for slot timing detection. All codes are defined by 3GPP standard [3G TS 25.211-25.215].
A power calculation unit 705, to which the output of correlating unit 702 or matched filter 704 is an input, obtains a power of correlated signal based on I (in-phase) and Q (quadrature) components of incoming signal.
An accumulator 706, to which the output of power calculation unit 705 and a memory unit 707 are the inputs, performs the accumulation for current power results and the previous results stored in memory for the predetermined period of time. The accumulator 706 performs averaging in time to increase reliability of detection.
A memory unit 707 to which the output of accumulator 706 is the input, is used to save the intermediate power results. A detector unit 708 searches for a maximum value among the accumulated results in memory 707 for a peak candidate. A decision unit 709, compares a detected maximum against the calculated average value of the accumulated profile stored in memory 707 with regard to a predetermined threshold coefficient.
A control unit 701, which receives a system counter signal, controls the operation timing of each of the circuit components.
With the specified cell search method, to improve the detection the input to cell search unit (the received base-band signal) is usually over-sampled. Increased sampling rate of the incoming signal translates to finer time resolution and hence generally leads to better output in terms of accuracy. However, in some conditions it may cause losses in performance. To monitor cells on the other FDD frequencies (inter-frequency search) and on other radio access technologies supported by UE, a compressed mode [3G TS 25.212, 25.215] is adopted, when the search is not continuous and is performed in time slots or gaps specified by transmission gap pattern. Typical cell search procedure may require more than a single gap processing for each STEP in order to achieve reliable performance.
In conditions of high frequency offset low density of gaps results in significant timing drift between the processing intervals, causing drift of the peak location within single STEP processing as well as between the STEPs. It results in uncertainty and errors in exact detection as the true peak becomes spread over a few positions, a so called “blurring effect”, which becomes even more dramatic in an over-sampled data stream. At the same time, as different STEPs can not be usually processed in the same gap, due to timing drift the real peak position at the start of the STEP is shifted from the reference timing, provided by the previous processing, which leads to additional errors in detection. The effect may be even more severe if the drift is big enough, so that the next STEP simply loses the peak.
Therefore it is desirable to provide a cell search method and apparatus with an increase of cell search performance in compressed mode with low density of gaps in high frequency offset conditions.
Moreover, over-sampling of incoming base-band signals results in larger hardware size in both functional circuit scale and memory size. A consequent shortcoming with the conventional cell search circuit of larger hardware size, is the increase of electric current consumed, which is critical to a mobile station's performance. In this regard, it is desirable to optimize power consumption of the UE in order to maximize the portability and operation time of the device.
In light of the foregoing, it is desirable to provide an improved method and apparatus for acquiring UE synchronization with a transmitting base station under critical conditions. It is also desirable to provide an improved cell search method and circuit through which the scale of the circuitry and power consumption are reduced.