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.
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).
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
• 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
PRIMARY SYNCHRONIZATIONCHANNELUses 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 synchronizationUsed to determine the beginning of a slot
SECONDARY SYNCHRONIZATION CHANNELIt also consists of 256 chip length known as Secondary Synchronization Code (SSC)SSC also consists of first 256 chips of every slotThere are 15 different SSC’s & they can form 64 unique secondary SCH sequences (code groups).It is used in numbering of slots.
FRAME SYNCHRONIZATIONIt is used to determine: 1. slot number 2. the radio frame boundary 3. code group of the cellFrame 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.
SCRAMBLING CODEIDENTIFICATIONIn 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)
Difference between Primary Scramblingcode & Primary synchronization codes
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.
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).
(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.
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.
POWER WEIGHTING INUPLINKIn 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.
PROCEDUREOne 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.
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.
Simplified diagram depicting power weight assignments in uplink
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.
SIMPLIFIED DIAGRAM OF POWERWEIGHTING IN DOWNLINK
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