WCDMA Air Interface
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This presentation discusses about the WCDMA air Interface used in 3G i.e. UMTS. This Radio Interface has great capability on which Third Generation of Mobile Communication is built, with backward compatibility.
Transcript of WCDMA Air Interface
- Lowest layer in this interface is physical layer, (PHY). The physical layer has interface to both MAC and RRC sub-layer.
- Layer 2 Consists of Medium Access Control (MAC) Radio Link Control (RLC) The Broadcast Multicast Control (BMC) Packet Data Convergence Protocol (PDCP)
- Layer 3 consists of: RRC Mobility Management (MM) GPRS Mobility Management (GMM) Call control (CC) Supplementary services (SS) Short Message Service (SMS) Session Management (SM) GPRS Short Message Service support(GSMS)
- FEC encoding/decoding of transport channels Radio measurements and indications to higher layers Macro diversity distribution/combining and SHO execution Error detection on transport channels Multiplexing of transport channels and de multiplexing of coded composite transport channels (CCTrCHs)
- Rate matching Mapping of CCTrCHs on physical channels Modulation, spreading/demodulation, and de-spreading of physical channels Frequency and time synchronization Closed-loop power control Power weighting and combining of physical channels RF processing
- FEC scheme aims to reduce transmission error UTRAN employs two FEC schemes (i) Convolution codes (ii) turbo codes
- Radio measurements is to be carried by physical layer and result is to be reported to higher layers. These measurements can be specific to either UE or Node B
- Possible measurement types of UE : Received Signal Code Power (RSCP) Received signal strength indicator (RSSI) Block error rate (BLER) UE transmitted power CFN-SFN observed time difference UE Rx-Tx time difference Observed time difference to GSM cell
- Possible measurement types for UTRAN Received total wide band power Signal to interference ratio (SIR) SIR error Transmitted carrier power Transmitted code power Bit error rate (BER) Round trip time SFN-SFN observed time difference
- Macro diversity (SHO) is a situation in which a receiver receives the same signal from different sources. This macro diversity should be combined, Without combining interference level would increase and capacity decreased by considerable amount. This combining is done at physical layer using RAKE receiver
- RAKE receiver
- The purpose of error detection is to find out whether a received block of data was recovered correctly. CRC is used for this. There are five CRC polynomial lengths in use (0, 8, 12, 16, and 24 bits)
- Each UE can have several transport channels in use simultaneously. Every 10 ms, one radio frame from each transport channel is multiplexed into a (CCTrCH), serially. For uplink, FDD mode has 1 CCTrCH TDD mode has multipleCCTrCHs For downlink, both FDD andTDD modes have multipleCCTrCHs
- The number of bits on a transport channel can vary with every transmission time interval. However, the physical channel radio frames must be completely filled. To match bit rate after transport channel with total physical channel bit rate either repeating or puncturing bits techniques is used.
- If there is more than one physical channel in use, then the bits in the CCTrCH must be divided among them.This is done by segmenting the input bits evenly for each physical channel. Since rate matching is already done in an earlier phase, so the bits should fit nicely into physical channels.
- In the scrambling procedure, the I- and Q- phases are further (after channelization) multiplied by a scrambling code. These scrambling codes have good autocorrelation properties. The modulation scheme in the UTRAN is quadrature phase shift keying(QPSK). Modulation rate = 3.84 Mcps
- This procedure takes place when the power is turned on in the UE, starts with downlink SCH synchronization. The UE knows the SCH primary synchronization code, which is common to all cells. P-SCH and S-SCH both are sent over first 256 chips of each slot (of 2560 chips). There are 15 such slots in each radio frame.
- Structure of synchronization channel
- In UTRAN two forms of power control : open loop & closed loop. Closed loop power control is further divided into : inner loop & outer loop power control. Outer loop power control sets the Signal to Interference Ratio (SIRtarget) and Inner loop power control adjusts the peer entity transmit power so that measured SIR fulfills the SIRtarget requirement.
- In Uplink
- In the uplink, one UE can transmit simultaneously one DPCCH and up to six DPDCHs. The control channel (DPCCH) will be sent in the Q-plane, and the data channels (DPDCH) in both planes. The channelization codes are orthogonal codes, and the scrambling code is a pseudo- random sequence.
- In downlink
- All channels have their own power weight factor G. All physical channels (except the SCH) are processed in the same way as a DPDCH. All channels (except the SCH) are scrambled with the same scrambling code.
- Four UE Power Classes
- Frequency bands
- RF specification
- There are three separate channel concepts in the UTRAN: logical, transport and physical channels.
- Logical channels define what type of data is transferred. It can be divided into control channel and traffic channel. Control channels transfer Control plane (C- plane) information and traffic channels User plane (U-plane) information. A control channel can either be common or dedicated.
- A common channel is a point-to-multipoint channel; i.e. common to all users in a cell. A dedicated channel is a point-to-point channel; i.e. used by only one user. The defined logical control channels are: Broadcast control channel (BCCH) Paging control channel (PCCH) Dedicated control channel (DCCH) Common control channel (CCCH) Shared channel control channel (SHCCH) The used logical traffic channels are: Dedicated traffic channel (DTCH) Common traffic channel (CTCH)
- The transport channels define how and with which type of characteristics the data is transferred by the physical layer. Transport channels are divided into common channels and dedicated channels. All these channels are unidirectional.
- Common transport channels include: Broadcast channel (BCH) Paging channel (PCH) Random access channel (RACH) Common packet channel (CPCH) Forward access channel (FACH) Downlink shared channel (DSCH) High-speed downlink shared channel (HS-DSCH) Uplink shared channel (USCH) The only dedicated transport channel type is: Dedicated channel (DCH)
- Physical channels define the exact physical characteristics of the radio channels. In frequency-division duplex (FDD) mode, both the uplink and downlink bands have their own frequency channels. In time-division duplex (TDD) mode, there is only one frequency channel, which is then dynamically time-divided for both uplink and downlink slots.
- Downlink Synchronization channel (SCH) Common pilot channel (CPICH) Primary common control physical channel (P-CCPCH) Secondary common control physical channel (S-CCPCH) Physical downlink shared channel (PDSCH) Paging indicator channel (PICH) Acquisition indicator channel (AICH) Downlink and Uplink Dedicated physical data channel (DPDCH) Dedicated physical control channel (DPCCH) Uplink Physical random access channel (PRACH) Physical common packet channel (PCPCH) Uplink dedicated control channel for HS-DSCH (HS-DPCCH)
- Downlink Primary common control physical channel (P-CCPCH) Secondary common control physical channel (S-CCPCH) Synchronization channel (SCH) Paging indicator channel (PICH) Physical downlink shared channel (PDSCH) Physical Node B synchronization channel (PNBSCH) High speed physical downlink shared channel (HS-PDSCH) Shared control channel for HS-DSCH (HS-SCCH) Downlink and Uplink Dedicated physical channel (DPCH) Uplink Physical random access channel (PRACH) Physical uplink shared channel (PUSCH) Shared info