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Sync in 3 g

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  • 1. NEED OF SYNCHONIZATION• Synchronization in 3G is basically used for Cell search procedure.• In this procedure, the UE(User Equipment) searches for an appropriate cell and latches on to it.
  • 2. Cell Search Procedure is carried out in 3 steps:1. Slot synchronization2. Frame synchronization & code group identification3. Scrambling Code IdentificationFor these purposes, we use the Synchronization Channel (SCH).
  • 3. SYNCHRONIZATIONCHANNEL(SCH)• SCH is a downlink physical channel.• It is used in the cell search procedure.• SCH consists of 2 channels: 1. Primary Synchronization Channel(P-SCH) 2. Secondary Synchronization Channel(S-SCH)• The Primary and Secondary SCH are sent in parallel
  • 4. • Every cell transmits information in the form of frames• 1 frame has 15 slots• 1 slot has length = 2560 chips• Frame duration = 10 milliseconds• Slot Duration = 667 microseconds• Chip duration=260 nanoseconds• Chip rate=3.84 Mcps
  • 5. PRIMARY SYNCHRONIZATIONCHANNELUses a 256 chip spreading sequence known as Primary Synchronization Code (PSC).PSC consists of first 256 chips of every slot.It is identical for every cell.Used for slot synchronizationUsed to determine the beginning of a slot
  • 6. SECONDARY SYNCHRONIZATION CHANNELIt also consists of 256 chip length known as Secondary Synchronization Code (SSC)SSC also consists of first 256 chips of every slotThere are 15 different SSC’s & they can form 64 unique secondary SCH sequences (code groups).It is used in numbering of slots.
  • 7. FRAME SYNCHRONIZATIONIt is used to determine: 1. slot number 2. the radio frame boundary 3. code group of the cellFrame Synchronization is done by correlating the received signal with all possible SSC sequences, and identifying the maximum correlation value.Since the cyclic shifts of the sequences are unique the code group as well as the frame synchronization is determined.
  • 8. SCRAMBLING CODEIDENTIFICATIONIn this step, the UE determines the exact primary scrambling code used by the found cell.Each code group identifies 8 possible primary scrambling codes, & the correct one is found by correlating each one in turn over the CPICH of that cell.Then the correct primary scrambling code can be used to decode BCH information from the primary common control physical channel (P-CCPCH)
  • 9. Difference between Primary Scramblingcode & Primary synchronization codes
  • 10. INTRODUCTIONWCDMA defines two dedicated physical channels - (a)DPDCH(Dedicated Physical Data Channel):Carries dedicated data generated at layer 2 and above . (b)DPCCH(Dedicated Physical Control Channel):Carries layer 1 control information.• Each connection is allocated one DPCCH and zero , one or several DPDCHs.• It is used for combining physical channels.
  • 11. SPREADING TECHNIQUE(anoverview)• Spreading is applied to the mentioned dedicated channels. It basically consists of two operations:- (A)CHANNELIZATION OPERATION:------Transforms each data symbols into chips thus increasing the bandwidth of the signal.It makes use of orthogonal codes(OVSF codes-orthogonal variable spreading factor codes).
  • 12. (B)SCRAMBLING OPERATION:-------Used on top of spreading , scrambling code (PNsequence code) is added to the spread signal.It is used for cell separation in DL and userseparation in UL, this implies it does not change signalBW but only makes signal from different sourcesseparable to each other.
  • 13. RELATIONSHIP BETWEENSPREADING AND SCRAMBLING As the chip rate is already achieved in spreading by the channelization code, the symbol rate is not affected by the scrambling.
  • 14. POWER WEIGHTING INUPLINKIn UL, data modulation of both DPDCH and DPCCH is BPSK.The modulated DPCCH will be sent in Q plane, while first DPDCH will be sent in I plane and then subsequently added DPDCHs are mapped alternatively to I/Q plane.Now spreading modulation used in UL is dual channel QPSK which is used after data modulation. Spreading modulation comprises of spreading and scrambling operation as mentioned.
  • 15. PROCEDUREOne DPCCH and up to six parallel DPDCHs can be transmitted simultaneously.The data on the I and Q branches are multiplied by different channelization codes.After channelization, the real valued spread signal are weighted by gain factors which are different for data and control channels , however all data channels should have an equal factor.
  • 16. The weights designate the amount of power with which each of the data and control channels are to be transmitted.The gain factors βd and β are used to set different QOS c (Quality of Service) requirements for different channels (channels with higher QOS requirement can be sent with high power levels).Gain factors may vary frame to frame based on CTF(current transport format) combination.After transforming signal from real to complex, it is then scrambled by a complex valued scrambling code which can be long or short.
  • 17. Simplified diagram depicting power weight assignments in uplink
  • 18. Power weighting in uplink
  • 19. POWER WEIGHTING INDOWNLINK (PROCEDURE)QPSK is used for data modulation in DL.Each pair of two bits are serial to parallel converted and mapped to the I and Q branches respectively. Mapping is done in a way that odd and even numbered symbols are mapped to I and Q branch respectively.The data in the I and Q branches in downlink are spread to the chip rate by the same channelization code as used in uplink.The signal which is spread is subsequently scrambled by a cell specific scrambling code.
  • 20. SIMPLIFIED DIAGRAM OF POWERWEIGHTING IN DOWNLINK
  • 21. DIFFERENCES IN POWER WEIGHTMECHANISM IN UPLINK ANDDOWNLINK UPLINK DOWNLINK(1)Data modulation here is (1)Data modulation here isdone by BPSK done by QPSK(2)Data rates used in I and Q (2)Data rates used in I and Qplanes are different planes are same(3)Scrambling code is mobile (3)Scrambling code is cellstation specific in uplink specific in downlink
  • 22. •THANK YOU!!!!!!!