3G/UMTS

Frame Structure and Physical Channel - Downlink

• Overall Timing relationship among physical channels

• SCH (Synchronization Channel)

• CPICH(Common Pilot Channel)

• P-CCPCH(Primary Common Control Physical Channel)

• S-CCPCH (Secondary Common Control Physical Channel)

• AICH(Acquisition Indicator Channel)
• DPCH(Dedicated downlink physical channels)
• HS-SCCH(HS Shared Control Channel)
• HS-PDSCH (High Speed Physical Downlink Shared Channel)

Overall Timing relationship among physical channels

< 25.211 - Figure 29: Radio frame timing and access slot timing of downlink physical channels >

SCH (Synchronization Channel)

• The SCH consists of two sub channels,the Primary and Secondary SCH.
• The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots,each of length 2560 chips

< 25.211 - Figure 18: Structure of Synchronisation Channel (SCH) >

< Primary SCH >

• Primacy SCH has the length of 256 chips
• The PSC is the same for every cell in the system

< Secondary SCH >

• Made up of consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips
• Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH.

CPICH(Common Pilot Channel)

• Data Rate : a fixed rate (30 kbps =  20 bits/slot * 15 slots/frame * 100 frames/sec, SF=256)
• Contents :  a pre-defined bit sequence.
• Two Types of CPICH : Primary CPICH(P-CPICH) and Secondary CPICH(S-CPICH)

< 25.211 - Figure 13: Frame structure for Common Pilot Channel >

<  P-CPICH(Primary CPICH) >

• The same channelization code is always used for the P-CPICH
• The P-CPICH is scrambled by the primary scrambling code(PSC).// UE detect PSC from P-CPICH
• There is one and only one P-CPICH per cell
• The P-CPICH is broadcast over the entire cell.

<  S-CPICH(Secondary CPICH) >

• An arbitrary channelization code of SF=256 is used for the S-CPICH
• An S-CPICH is scrambled by either the primary or a secondary scrambling code
• There may be zero, one, or several S-CPICH per cell
• An S-CPICH may be transmitted over the entire cell or only over a part of the cell
• An S-CPICH that is intended to be used as phase reference for the second, third or fourth transmit antenna by UEs configured in MIMO mode or in MIMO mode with four transmit antennas shall be transmitted over the entire cell using the primary scrambling code and the antenna 1 pattern.

P-CCPCH(Primary Common Control Physical Channel)

• Data Rate : a fixed rate (30 kbps =  20 bits/slot * 15 slots/frame * 100 frames/sec, SF=256)
• Contents :  BCH(MIB / SIB) Transport data.
• Not transmitted during the first 256 chips of each slot. Instead, Primary SCH and Secondary SCH are transmitted during this period
• no TPC commands, no TFCI and no pilot bits are transmitted within this channel

< 25.211 - Figure 15: Frame structure for Primary Common Control Physical Channel >

S-CCPCH (Secondary Common Control Physical Channel)

• Data Rate : a variable rate ( 20*(2^k) bits/slot * 15 slots/frame * 100 frames/sec, SF=256/(2^k)), where k=0..6
• Contents :  FACH / PCH data.

< 25.211 - Figure 17: Frame structure for Secondary Common Control Physical Channel >

< 25.211 - Table 18: Secondary CCPCH fields >

< 25.211 - Table 19: Pilot Symbol Pattern >

AICH(Acquisition Indicator Channel)

< 25.211 - Figure 21: Structure of Acquisition Indicator Channel (AICH) >

DPCH(Dedicated downlink physical channels)

< 25.211 - Figure 9: Frame structure for downlink DPCH >

HS-SCCH(HS Shared Control Channel)

HS-SCCH is to carry the control information for HS-PDSCH and it has fixed rate of 60 kbps and spreading factor of 128.

<25.211-Figure 26A: Subframe structure for the HS-SCCH >

HS-SCCH Type 1 which is normally used for HSDPA in non-MIMO configuration has structure as shown below (See 25.212 4.6 Coding for HS-SCCH type 1 for details)

HS-PDSCH (High Speed Physical Downlink Shared Channel)

HS-PDSCH is the physical downlink channel that carries HSDPA user data. One HS-PDSCH corresponds to one channelization code of SF(Spreading Factor) 16. Depending on UE capability, multiple HS-PDSCH (i.e, multiple channelization code) can be transmitted simultaneously.

The frame structure of HS-PDSCH is shown below. The modulation scheme for this channel can be QPSK, 16 QAM, 64 QAM. Depending the modulation scheme, M in the following diagram varies. That is, M becomes 2 when the modulcation scheme is QPSK, it becomes 4 for 16 QAM and it become 6 for 64QAM.

< 25.211-Figure 26B: Subframe structure for the HS-PDSCH >

The slot format for HS-DPSCH (i.e, Data field) are as shown below.

< 25.211-Table 26: HS-DSCH fields >