Patent Publication Number: US-2005143089-A1

Title: Location aided cell search

Description:
FIELD OF INVENTION  
      The invention generally relates to wireless communication systems. In particular, the invention relates to cell search in such systems.  
     BACKGROUND  
      In cell search, a wireless transmit/receive unit (WTRU) identifies and begins synchronization with a cell of a wireless network. This procedure uses extensive resources of the WTRU.  
      To illustrate, in the proposed third generation partnership project (3GPP) universal mobile telecommunications system (UMTS) for wideband code division multiple access (W-CDMA), a three step process is used for cell search. In the first step, the WTRU searches for primary synchronization code (PSC) locations in a radio frame. Each cell within the system transmits a PSC. In the second step, the WTRU uses the PSC locations to detect secondary synchronization codes (SSCs). The SSCs indicate certain cell specific information. In the third step, the WTRU either identifies the scrambling code of the common pilot channel (CPICH), for frequency division duplex (FDD) mode, or the midamble of the broadcast channel (BCH), for time division duplex (TDD) mode of one or multiple cells. After completing cell search, the WTRU synchronizes with one of the detected cells.  
      As a WTRU moves, it may move between cells. To facilitate the handover between cells, the WTRU performs cell search to synchronize with new cells. Due to the complex nature of cell search, this procedure is undesirable. This procedure consumes a considerable amount of memory, power, and processing time, and is susceptible to false detection.  
      Accordingly, it is desirable to have alternate approaches to facilitate handover.  
     SUMMARY  
      Positions of a wireless transmit/receive unit (WTRU) are determined over time. The determined positions are used to determine a movement direction of the WTRU. Based on a current position of the determined positions and the movement direction of the WTRU, at least one cell that the WTRU is approaching is identified. Information is sent to the WTRU for the identified at least one cell. The information for the identified at least one cell is used to reduce the complexity of cell search. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S)  
       FIG. 1  is a simplified diagram of an embodiment for a location aided cell search system.  
       FIG. 2  is an illustration of a movement vector with respect to cells.  
       FIG. 3  is an illustration of known transportation routes in conjunction with cells.  
       FIG. 4  is a flow diagram of location aided cell search. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      Location aided cell search can be applied to many wireless communication systems. Hereafter, a wireless transmit/receive unit (WTRU) includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes but is not limited to a base station, Node-B, site controller, access point or other interfacing device in a wireless environment.  
       FIG. 1  is a simplified illustration of a location aided cell search system. A WTRU  10  communicates with a base station  20  and the wireless network  22  of the base station  20 , via an air interface  34 . The WTRU  10  has a transceiver  28  and an antenna/antenna array  32  for receiving and transmitting signals using the air interface  34 . A positioning device  26  is used to determine the geographic location of the WTRU  10 . The positioning device  26  may use the global positioning satellite (GPS) system or a cellular based positioning system to determine the location. Although the positioning device  26  is shown at the WTRU  10 , the positioning device in alternate embodiments may be located at the base station  20 /wireless network  22 . In many cellular based systems, the base station  20 /wireless network  22  utilize time difference of arrival (TDOA) and time of arrival (TOA) calculations and the correlation of this information can be performed at the base station  20 /wireless network  22 . The positioning information is sent to the base station  20 , such as by a signal or message. A cell search device  30  is used to perform cell search. A controller  24  is used to control the cell search device  30 .  
      The base station  20 /wireless network  22  receives the positioning information from the WTRU  10 , such as by a transceiver  38  and antenna/antenna array  36 . The positioning information is received by a cell reselection device  40 . The cell reselection device  40  uses the position information to determine a movement vector (indicating speed and direction) for the WTRU  10 . In a 3GPP UMTS system, typically the cell reselection device  40  is located at a radio network controller (RNC), although it may be located at the Node-B, core network or other places. The movement vector is determined from the change in the WTRU position over time. The movement vector used may be the most current vector from the latest two position estimates, an averaging over multiple estimates may be used or a weighted average may be used. Preferably, the movement vector, position information and cell operating areas are constructed using a horizontal plane model (two dimensional), although a three dimensional model can be used with some increased complexity.  
      Using a stored cell location database  42  and the position and vector of the WTRU  10 , the cell reselection device  40  determines which cell or cells that the WTRU  10  is moving towards. Using the position and vector of the WTRU  10 , a trajectory of the WTRU  10  is determined. Using that trajectory, cells along or close to that trajectory are determined.  
       FIG. 2  is an illustration of such a scenario. A WTRU  10 , located in cell  1   50   1 , is traveling east towards an eastern cell (cell  2   50   2 ). In this scenario, the cell reselection device  40  may send the WTRU  10  cell information for cell  2   50   2 , cell  3   50   3  and cell  4   50   4 .  
      The cell reselection device  40  may also use known transportation routes, such as highways, railroads, etc., to estimate the most likely cell that the WTRU  10  is moving towards. The positions of the WTRU  10  are compared to a known location of a transportation routes to determine whether the WTRU  10  is traveling along that route. One approach is to measure a distance of a locust of position points of a WTRU  10  from the route and see whether the distances are below a threshold.  
      The cell reselection device  40  may also use statistical information of past WRTU behavior to estimate the most likely cell that the WTRU  10  is moving towards. The cell location database  42  may include a compilation of destination cells for various WRTU positions and direction of travel. Accessing cell relation database  42 , the cell reselection device  40  determines the probability of the WRTU  10  entering each neighbor cell, and can signal the most likely next cell information to the WTRU  10 .  
      To illustrate as shown in  FIG. 3 , a WTRU  10  may be moving east towards an eastern cell (cell  2   50   2 ), but the road that the WTRU  10  is on will shortly take a sudden turn towards the south and towards a southern cell (cell  4   50   4 ). The cell reselection device  40  uses this transportation route information to determine the cell or cells that the WTRU  10  is traveling towards.  
      The cell reselection device  40  sends cell information to the WTRU  10  for the cells that the WTRU  10  is traveling towards. Information for these cells is relayed to the WTRU  10  through the air interface  34 . In the TDD mode of W-CDMA, the cell reselection device  40  may send the frequency and the “cell parameter.” In the FDD mode of W-CDMA, the cell reselection device  40  may send the cell frequency and primary scrambling code of the CPICH.  
      The cell information is sent to the WTRU  10  using the base station and WTRU transceivers  28 ,  38 , via the air interface  34 . The controller  24  receives the cell information and simplifies the traditional cell search procedure using this information. To illustrate, the midamble shifts for two TDD cells may be sent to a WTRU  10 . The WTRU  10  uses the midamble shifts to select one of the two TDD cells to use and decode that cell&#39;s BCH. Essentially, this information skips the first and second steps of cell search, which is highly desirable. The first step of cell search has large memory requirements and the second step is susceptible to an erroneously detected or not detected SSC. Accordingly, the handover between the cells is performed more efficiently.  
      Alternately, the WTRU  10  may be sent cell identifiers for the cells that it is moving towards. Using the cell identifiers, the WTRU  10  can eliminate other cells from the traditional cell search algorithms. As a result, the accuracy of cell search can be improved by discarding information from the eliminated cells in the cell search procedure, both simplifying the process and reducing the chances of a false detection of an eliminated cell.  
       FIG. 4  is a flow diagram of location aided cell search. Positions of a WTRU are determined over time, step  80 . The determined positions are used to determine a movement direction of the WTRU, step  82 . Based on a current position of the determined positions and the movement direction of the WTRU, the Cell Reselection Device  40  identifies cells that the WTRU is approaching, step  84 . Information is sent to the WTRU for the identified cells, step  86 . The information for the identified at least one cell is used to reduce the complexity of cell search, step  88 .