cell search procedure in UMTS network

WCDMA and SWCDMA air interfaces are based on code division multiple access and as in all spread spectrum systems the initial acquisition procedure is a challenging issue. This procedure is identified as cell search in WCDMA and beam search in SW-CDMA.  Although the terrestrial air interface WCDMA and the satellite one SW-CDMA are very similar, the suggested cell/beam search procedures are different. The introduction of new attractive services in future 3G mobile systems will have a strong impact in the market penetration. In this multicast and broadcast services (MBMS) are definitely promisinh candidates.

A terrestrial network is highly inefficient to supply multicast broadcast services: the delivery of MBMS through  a terrestrial link requires information stream replication in all cells where atleast a UE registered to MBMS is located. Hence, many different resources such as frequencies , codes , network connections are basically reused to transmit the same information. A GEO satellite is infact able to cover wide area with a reduced number of resources.

In this two levels of spreading are used, channelization achieved with orthogonal variable spreading factor (OVSF) codes and scrambling obtained using long gold codes. The cell search procedure aims at identifing the downlink cell/beam specific primary scrambling code. Once the primary scrambling code is available the UE is able to acquire the broadcast and cell/beam specific information carried by the broadcast channel.

CELL/BEAM SEARCH PROCEDURE:
in the downlink of wcdma and swcdma air interfaces physical channels are arranged into 10ms radio frames which consists of 15 slots of .67 ms each corresponding to 2560 chips . without any knowledge about  slot and frame boundaries the cell search would require more time. The total number of employed scrambing code is 8192 each one 38400 chips long. They belong to gold family and are divided into 512 sets. The slot synchronisation pprovided by the first step is used to identify the frame synchronisation and the primary scrambling code in only one step. As type primary CCPCH is scrambled  with the primary synchronization code and is spread with OVSF code, once the FSW is detected, the frame synchronization and scrambling code are both identified.