In Time Division Synchronous Code Division Multiple Access (TD-SCDMA) systems,the Global Positioning System (GPS) signal is often blocked or experiences interference. As a result,the GPS satellite cannot be found and synchronization cannot occur. Long out-of-sync periods can lead to timing differences between base stations,and if these differences are too large,they can affect the ability of Mobile Stations (MS) to search neighboring cells. This can also affect cell switchover,and cause Downlink Pilot Time Slot (DwPTS) to interfere with Uplink Pilot Time Slot (UpPTS). All these manifest as handover failures,call dropouts during handover,and a declining rate of call completion. As a consequence,user experience within the network is diminished. Researchers have found that when GPS synchronization is lost for more than 4 chips,network quality deteriorates noticably. When the loss is more than 10 chips,the MS may fail to locate a neighboring cell. When the loss is below 16 chips,the interference of DwPTS with UpPTS is not obvious in the first and second circles of the GPS out-of-sync cells. Studies show that,to protect network performance,the TD-SCDMA system allows for up to 4 chips of timing difference in the case of GPS synchronization loss.
展开▼
