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.

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  • In details.
  • All channels have their own power weight factor G. Thus it is possible for theUTRAN to set different weights on different channels according to theirQuality of Service (QoS) requirements.
  • The total bandwidth for 3G is divided between the FDD and the TDDmodes. The paired band is allocated for the FDD mode and the smallerunpaired band for the TDD mode.
  • WCDMA Air Interface

    1. 1.  Lowest layer in this interface is physical layer, (PHY).  The physical layer has interface to both MAC and RRC sub-layer.
    2. 2. Layer 2 Consists of  Medium Access Control (MAC)  Radio Link Control (RLC)  The Broadcast Multicast Control (BMC)  Packet Data Convergence Protocol (PDCP)
    3. 3. 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)
    4. 4.  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)
    5. 5.  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
    6. 6.  FEC scheme aims to reduce transmission error  UTRAN employs two FEC schemes (i) Convolution codes (ii) turbo codes
    7. 7.  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
    8. 8. 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
    9. 9. 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
    10. 10.  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
    11. 11. RAKE receiver
    12. 12.  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)
    13. 13.  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
    14. 14.  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.
    15. 15.  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.
    16. 16.
    17. 17.  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
    18. 18.  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.
    19. 19. Structure of synchronization channel
    20. 20.  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.
    21. 21. In Uplink
    22. 22.  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.
    23. 23. In downlink
    24. 24.  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.
    25. 25.  Four UE Power Classes
    26. 26.  Frequency bands
    27. 27.  RF specification
    28. 28.  There are three separate channel concepts in the UTRAN: logical, transport and physical channels.
    29. 29.  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.
    30. 30.  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)
    31. 31.  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.
    32. 32.  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)
    33. 33.  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.
    34. 34.  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)
    35. 35.  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 information channel for HS-DSCH (HS-SICH)
    36. 36.  The spreading process actually consists of two phases, spreading and scrambling, and both of them use different types of codes with different characteristics.
    37. 37.  Spreading increases the bandwidth of the signal.  Spreading codes used are orthogonal  Scrambling shuffles the bit using a code called scrambling code.  Scrambling is done after the spreading in the transmitter  Scrambling codes are pseudo-random sequence of bits.
    38. 38.  A signal can be subjected to time diversity, multipath diversity, and antenna diversity.
    39. 39.  Time diversity means that the signal is spread in the time domain.  Time diversity spreads the faulty bits over a longer period of time, and thus makes it easier to reconstruct the original data.
    40. 40.  Multipath diversity is a phenomenon that happens when a signal arrives at the receiver via different paths.
    41. 41.  In aWCDMA system the receiver is typically able to track and receive several multipath components and combine them into a composite signal.  The receiver is of the RAKE variety. RAKE receiver
    42. 42.  In a CDMA system the same signal can be transmitted over the air interface, on the same frequency, from several base stations separated by considerable distances.This scheme is called the soft handover (SHO).  In a SHO all the participating base stations use the same frequency, and the result is a macro diversity situation.
    43. 43.  A typical SHO situation
    44. 44.  Radio interface protocols can be divided in two ways: horizontal layers and vertical planes.
    45. 45.  The UTRAN MAC can even contain different functionalities depending on whether it supports FDD,TDD, or both modes.  The MAC is not a symmetric protocol; the entities in the UE and in the UTRAN are different.
    46. 46. The services MAC provides to the upper layers include the following:  Data transfer;  Reallocation of radio resources and MAC parameters;  Reporting of measurements to RRC.
    47. 47.  Mapping between logical channels and transport channels;  Priority handling between data flows of one UE;  Priority handling between UEs by means of dynamic scheduling;  Identification of UEs on common transport channels;  Traffic-volume monitoring;
    48. 48.  Transport-channel type switching;  Ciphering for transparent RLC;  Selection of the appropriate transport format for each transport channel depending on the instantaneous source rate;
    49. 49.  RLC layer is in charge of the actual data packet transmission over the air interface.  The RLC task maintains a retransmission buffer, performs ciphering, and routes the incoming data packets to the right destination task (RRC, BMC, PDCP, or voice codec).
    50. 50.  Transparent DataTransfer Service Segmentation and reassembly; Transfer of user data; SDU discard.  Unacknowledged DataTransfer Service Segmentation and reassembly; Concatenation; Padding; Transfer of user data; Ciphering; Sequence number check; SDU discard.
    51. 51.  Acknowledged DataTransfer Service Concatenation; Padding; Transfer of user data; Error correction; In-sequence delivery of higher-layer PDUs; Duplicate detection; Flow control; Protocol error detection and recovery; Ciphering; SDU discard.
    52. 52.  Segmentation and reassembly of higher- layer PDUs into/from smaller RLC payload units;  Concatenation (RLC SDUs may be concatenated so that they will fill  the RLC PUs);  Padding;  Transfer of user data;  Error correction;
    53. 53.  In-sequence delivery of higher-layer PDUs;  Duplicate detection;  Flow control;  Sequence number check (in unacknowledged data transfer mode);  Protocol error-detection and recovery;  Ciphering (in UM and AM modes);  Suspend/resume function.
    54. 54.  The RRC controls the configuration of the lower layers in the protocol stack, and it has control interfaces to each of the lower layers (PDCP, BMC, RLC, MAC, and layer 1).
    55. 55.  General control  Notification  Dedicated control
    56. 56.  Initial cell selection and cell reselection  Broadcast of information reception of paging messages;  Establishment, maintenance, and release of RRC connection;  Establishment, reconfiguration, and release of radio bearers;
    57. 57.  Handovers (HOs);  Measurement control;  Outer-loop power control;  Security mode control;  Control of requested QoS;  Contention resolution in theTDD mode;  Timing advance in theTDD mode;
    58. 58.  Mobility Management (MM) Main function of the MM is location management. MM procedures can be divided into three groups:  MM common procedures;  MM specific procedures;  MM connection-management procedures.
    59. 59.  GPRS Mobility Management (GMM) The GPRS mobility management (GMM) sub layer provides services to the session management (SM) entity and to the SMS support entity for message transfer. GMM procedures can be of two types: ▪ GMM common procedures; ▪ GMM-specific procedures.
    60. 60.  CallControl (CC) A CC entity supports the following elementary procedures: ▪ Call-establishment procedures; ▪ Call-clearing procedures; ▪ Call-information-phase procedures; ▪ Miscellaneous procedures.
    61. 61.  Supplementary Services (SS) Supplementary services (SS) are value-added services that may or may not be provided by the network operator.  Short Message Service (SMS) The purpose of the SMS is to provide a means to transfer short text messages between a UE and a short message service center (SMSC).These messages are sent using the control signaling resources, and their maximum length can be only 160 characters.
    62. 62.  Session Management (SM) The main function of the SM protocol is to support packet data protocol (PDP) context handling of the user terminal. there is no “connection” concept in a (IP) packet-switched system.  GPRS Short Message Service Support (GSMS) The GPRS Short Message Service (GSMS) protocol task handles the SMS service while the UE is attached to the GPRS system.This protocol is an extension of the circuit- switched SMS protocol.
    63. 63.  MAC and RLC are in both the planes.  PDCP and BMC exist only in user plane.  The U-plane is responsible for the transfer of user data, such as voice or application data, whereas the C-plane handles the control signaling and the overall resource management
    64. 64.  The functions the PDCP perform following:  Header compression and decompression of IP data streams;  Transfer of user data;  Maintenance of PDCP sequence numbering;  It uses the service provided by a lower layer called Radio Link Control (RLC).
    65. 65.  Broadcast/multicast control is a layer 2 sub- layer that exists only in the U-plane.  This layer handles only downlink broadcast/multicast transmission.  The functions of BMC are: ▪ Storage of cell broadcast messages; ▪ Traffic-volume-monitoring and radio resource requests forCBS; ▪ Scheduling of BMC messages; ▪ Transmission of BMC messages to UEs; ▪ Delivery of cell broadcast messages to the upper layer
    66. 66. Queries