ECE 5221 Personal Communication Systems

1. ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 24 – Basics of 3G – UMTS (3) Spring 2011

2. Intermediatecommunication nodesrequire layers 1 through 3 Internal operation within each layer is independent of the internal operation in any other layer OSI Communication model • WCDMA interfaces described using OSI model • OSI = Open System Interconnect • Developed by ISO as a general model for computer communication • Used as a framework for development and presentation of most contemporary communication standards Note: WCDMA covers Layers 1-3 of OSI Model Each layer communicates only with two adjacent layers and its peer on the other side Each layer receives services from the layer below and provides services to the layer above

3. UMTS Protocol stack • UMTS offers new Access stratum protocol stack • Non-Access Stratum is largely inherited from GSM • First three layers of the protocol stack are part of UTRAN Note: SMS exists on both circuit switched and packet switched side

4. UMTS CS protocols – control plane • Control plane – carries signaling • RNC terminates the Access Stratum (AS) • RRC, RLC and MAC terminate at RNC • PHY terminates at Node B except for outer loop power control • RAN (access stratum) acts as transport for NAS Note: UTRAN protocols are layered in an architecture that follows OSI model

5. UMTS CS protocols – user plane • User plane – caries user data • Application – end to end protocol • Access stratum the same for both control plane and user plane

6. UMTS PS protocols – control plane • Control plane for packet data • Very similar to control plane for PS • Identical access stratum

7. UMTS PS protocols – user plane • Additional protocol PDCP • PDCP – compression of IP headers • PDCP may or may not be used

8. Layout of the Access Stratum • Two planes • User plane - user data • Control plane – signaling • User data enters access through radio bearers (RABs) • Signaling is handled by RRC • Upper layer signaling – encapsulated through RRC messages (direct transfer) • RRC has a capability of reconfiguring all lower layers

9. Part 6 Elements of PHY Layer processing

10. UMTS-FDD PHY frame structure • UMTS-FDD PHY frame structure is based on 10ms frames • Frames are broken in 15 time slots • The number of bits/slot is variable • Chip rate is always the same (3.84 Mchips/sec)

11. UMTS-FDD DL processing Note: Number of channels depends on number of active users. P-SCH and S-SCH are always transmitted • There are 6 steps in DL PHY processing • I/Q separation • Variable spreading • Scrambling • Gain adjustment • Sync addition • Modulation

12. W-CDMA DL Modulation UMTS-FDD uses simple QPSK modulation scheme Complex code sequence is split into real and imaginary part and modulated using carriers in quadrature

13. W-CDMA Modulation Impulse response of the shaping filter Frequency response of the shaping filter Note: only 30dBc on the sidebands – may cause interference to GSM in non 1-1 overlay scenarios Analytical expression of the shaping filter impulse response UMTS-FDD uses root-raised cosine for the shaping filter The roll-off is a = 0.22

14. W-CDMA DL variable spreading UMTS-FDD available DL data rates • UMTS-FDD provides • high data rates through • variable spreading • code aggregation User data rates assume 1/2 convolutional encoding Different data channels have different rates The chip rate is always the same W-CDMA supports variable spreading on the DL Variable spreading is accomplished through use of orthogonal codes of different length

15. W-CDMA scrambling codes • OVSF codes provide orthogonality between signals coming from the same BTS – form of channelization • Scrambling codes allow mobile to distinguish signals coming from different base stations • Scrambling codes do not change signal bandwidth • Decoding a signal from a user is in 2 steps • Descrambling the signal from the Node B • De-spreading the signal from individual user

16. W-CDMA scrambling codes • UMTS-FDD uses 8192 complex scrambling codes • The codes are selected as parts of a 218 -1 long gold sequence (good correlation prperties) • Each of the codes are associated with left and right alternative scrambling code • Scrambling codes are 38400 chips long (10ms) • Scrambling code repeats every frame • Organized in 512 groups of 16 codes • The first code in each group is declared as the primary scrambling code (PrSC) • PrSC are used for cell identification Scrambling code tree

17. A cell is allocated one primary synchronization code The primary code is the same for all cells in the system Secondary code points to a group of primary scrambling codes W-CDMA synchronization codes Note: PSC allows mobile to synchronize to the time slots. SSC allows mobile to synchronize with the beginning of frame. Synchronization codes are used for system detection They are 256 chips long complex codes One primary and 64 secondary codes Secondary codes consist of 15 code words Secondary codes remain unique under cyclic shifts smaller than 15

18. W-CDMA primary scrambling codes • There are 512 primary scrambling codes • They are divided in 64 groups of 8 codes • Each cell is assigned one primary code • Primary scrambling code is used to provide orthogonality between different BS • Primary scrambling code is broadcast on the Common Pilot Channel (CPICH) Note: after decoding SSC, the mobile needs to consider only 8 out of 512 PrSC

19. W-CDMA code assignment example Task: use previous two slides to verify code assignments for the above cells Note: in practice network operator assigns only PrSC. SSC is assigned automatically on the basis of PrSC assignment Primary sync code is the same for all cells Secondary sync code number is the same as the group of the primary pSC

20. W-CDMA UL processing - dedicated channels Note: transmission from a single mobile can aggregate multiple codes to achieve higher data rate DPDCH - Dedicated Physical Data Channel DPCCH - Dedicated Physical Control Channel • There are 5 steps in the UL DCHs processing • Spreading • Gain adjustment • Complex addition • Scrambling • Modulation

21. W-CDMA UL variable spreading User data rates assume 1/2 convolutional encoding • Variable data rates are allowed on U DPDCH • Variable data rate achieved through • variable spreading 4 to 256 • code aggregation - up to 6 parallel codes • if code aggregation is used, spreading for all DPDCH is 4 • UL DPCCH is a constant rate channel ~ 15kb/sec (assigned code C256,0)