WRAN OPERATIONS

DAY 2

WCDMA TECHNOLOGY OVERVIEW

AKASH SHIVHARE

Functional Types of RNC Controlling RNC (CRNC)

The CRNC controls a Node B. Its functions include code allocation, admission control, scheduling of system information, cell configuration, and radio link management.

Serving RNC (SRNC)

The SRNC terminates the Iu interface (data and signaling) for a specific mobile. The signaling between the UE and the UTRAN is Radio Resource Control (RRC), which is terminated at the SRNC. SRNC functions also include Radio Resource Management (RRM), outer loop power control, and handovers. The UE can only be connected to one SRNC at one time.

Drift RNC (DRNC)

It is possible that the UE is using a cell belonging to an RNC other that the SRNC, for example in a soft handover situation. This RNC is called a Drift RNC (DRNC). The Drift RNC means an RNC involving radio link addition/deletion/reconfiguring procedure but not having the bearer – radio link mapping control

Contd..

Scrambling & Spreading Codes

Scrambling codes = multiplication of frequency with PN codes, does not affect bandwidth, Primary 256 chip kasami code, Secondary- segment of long gold code

The primary scrambling codes are divided into 64 code groups, each consisting of 8 codes so a total number of codes will be 512.

Each primary scrambling code has a set of 16 secondary scrambling codes.

Channelization code/ OVSF ( Orthogonal variable spreading factor ) codes/ Spreading codes: Direct sequence spread spectrum codes ( DSSS, QAM ) are spreader orthogonally with a pulse of +1,-1. Downlink = 4 to 512 SF, Uplink= 4 to 256 SF, increases signal bandwidth.

Contd..

The codes are fully orthogonal, i.e., they do not interfere with each other, only if the codes are time synchronized.

In downlink OVSF codes are used to distinguish users with in cell/sector & scrambling codes are used to reduce inter-base-station interference which may be caused due to the use of same OVSF codes in two adjacent cell/sectors.

In uplink OVSF codes are used to separate the physical channels/services of one user ( that is why we can use data and speech at a time) and scrambling codes are used to distinguish between users.

OVSF Code

OVSF-Orthogonal variable spreading factor.

SF=Chip rate/Symbol rate Chip rate is constant in WCDMA=3.84 Mcps

A physical channel may use a certain code in the tree if no other physical channel uses a code from an underlying branch.

Therefore shorter the Spreading code greater is the bit rate over air interface.

Contd…

Channels, Channels and Channels

WCDMA system uses a 3- layer channel structure to carry control information and user data between layer 2 and layer 1. These three types of channels are:

Logical Channel- provide data transfer service from the MAC layer.

Transport Channel- responsible for mapping L2 information to L1 ( Error detection & correction).

Physical Channel- Specified by the carrier frequency, codes ( Channelization & Scrambling codes)

Mapping of Channels

Logical ChannelsLogical channels are of two types

1. Control Channels: -BCCH- Broadcast control channel: A downlink channel for broadcasting system control information, such as the spreading code values of a cell and neighboring cells, the allowed transmitted power, and other system parameters.

-PCCH- Paging control channel: A downlink channel used for transferring paging information, It is utilized when the network wants to communicate with UE but does not know its exact location.

-CCCH- Common control channel: Bi -directional channel for transmitting control information between the network and UE’s.

-DCCH- Dedicated control channel: A point to point bi-directional channel that transmits dedicated control information between a single UE and the network.

Contd..

2. Traffic channels

-DTCH- Dedicated traffic channel: A point to point channel that dedicated to one UE service for the user information transfer.

-CTCH- Common traffic channel- A point to multipoint downlink channel to transmit dedicated user information for all for a group of specified UE’s

Tx Channels

1. Dedicated transport channel DCH – Dedicated Channel: This is the only type of dedicated transport channel.

The DCH is transmitted over the entire cell and carries all the information intended for the given user from the higher layers, which includes data and control information ( UR measurement, handover, command, etc.)

2. Common Transport channels:

BCH- Broadcast channel: A downlink channel that carries system and cell specific information, and is transmitted over the entire cell.

FACH- Forward access channel: A downlink channel carries control information to UE’s

PCH- Paging channel: A downlink transport channel that is always transmitted over the entire cell. The PCH carries the information relevant to the paging procedure.

RACH- Random access channel: An uplink channel that carries control information form the terminal, such as RRC connection setup request.

CPCH- Common packet channel: An uplink packet based transport channel . CPCH is associated with a dedicated channel on the downlink that provides power control.

DSCH -Downlink Shared channel: A downlink channel shared by several UE’s . The DSCH carries dedicated user data and / or control information and is associated with one or several downlink DCH’s

Physical Channel1 .Common physical channel

P-CCPCH, S-CCPCH, PRACH, PCPCH,SCH

2. Dedicated physical channel

DPDCH, DPCCH, PDSCH

3. Indication physical channel

AICH,PICH,CD/CA-ICH,CSICH

4. Common Pilot Channels

P-CCPICH,S-CCPICH

Common Physical channel

P- CCPCH- Primary common control physical channel: used to carry the BCH transport channel. It carries all system information, neighbor list & network information. D/L channel

S-CCPCH- Secondary common control physical channel: used to carry the FACH and PCH. It is used for call set up & handoff. D/L channel

SCH – Synchronization channel: used for cell search. Consists of system information, Scrambling code group. D/L channel

Contd..

PRACH-Physical random access channel:

carries the RACH information from the transport channel. U/L channel

PCPCH-Physical Common Packet Channel:

carries the transport channel CPCH information. CPCH, transmission is based on the Digital sense multiple access- collision Detection(DSMA-CD) approach. U/L channel

DPCh

DPDCH- Dedicated Physical Data Channel- carries the user data and higher layer signaling from the DCH transport channel, and its bit rate can be changed frame-to-frame ( 10 ms). The spreading factor for the DPDCH ranges from 4 to 256. (Bi directional)

DPCCH- Dedicated Physical Control Channel: carries control information such as the pilot bits, feedback information (FBI),Transmit power control (TPC).The spreading factor for the DPCCH is always 256.(Bidirectional)

PDSCH- Physical downlink shared channel: used to carry the DSCH transport channel. (D/L only)

Pilot Channel dl only

P-CPICH- Primary common pilot channel: Used to carry scrambling codes and also used in cell selection & reselections. It is broadcasted over the complete cell. It is scrambled with primary scrambling codes.

S-CPICH- Secondary common pilot channel: It also carries similar information that of P-CPICH, but the information is broadcasted only over a part of cell. It is scrambled either primary or a secondary scrambling code.

Indication channel ul only

PICH- Page indication channel: a fixed rate (SF=256) physical channel used to carry the paging indicators. The PICH is always associated with an S-CCPCH to which a PCH transport channel is mapped.

AICH- Acquisition indication channel: carries the acquisition indicators (AI), which are used to acknowledge the correctly received preambles from a PRACH

CSICH- CPCH Status indicator channel: provides the status of CPCH. The UE will only monitor this channel prior to packet transmission.

CD-CAICH-Collision detection and channel assignment channel :

used to carry CD indicator & CA indicator.

QOS in WCDMA

Power Control

In WCDMA , traffic is controlled by power control Mechanism, as all UE’s share same frequency.

Power Control is of two types:

Open Loop Power Control (Used in Access Mode)

Close Loop Power Control (Used in Dedicated Mode)

Close Loop Control is further of two types:

Inner Loop Power Control (Fast)

Outer Loop Power Control(Slow)

Contd..

Open loop

Open loop power control is the ability of the UE transmitter to set its output power to a specific value.

It is used for setting initial uplink and downlink transmission powers when a UE is accessing the network.

In this process, the UE estimates the transmission signal strength by measuring the received power level of the signal from the Node B in the downlink and adjust its transmission power level.

Closed Loop

In the closed loop power control technique, the quality measurements are done in Node B and the result are then sent back to mobile transmitter so that it can adjust its transmitted power.

Close Loop Power Control is of two types:Inner Loop Power ControlOuter Loop Power Control

Inner Loop

Inner Power control is the ability if the ability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target

Frequency of Inner loop power control is 1500Hz & the step size is 1,2&3 dB.

Outer Loop

Outer loop power control is used to maintain the quality of communication at the level of bearer service quality requirement, while using as low power as possible.

The uplink outer loop power control is responsible for setting a target SIR in the Node B for each individual uplink inner loop power control.

The downlink outer power control is the ability of the UE received to converge to required link quality (BER) set by the network (RNC) in downlink.

Handover

The downlink signals, received by the UE, are combined by the UE, are combined in the RAKE receiver, that allows for multipath reception and thereby gives protection against fading.

Handover are mainly of two types in WCDMA.

Soft/Softer Handover: When handover occurs between same carrier frequency, eg- WCDMA handover between same carrier frequency.

Hard Handover: When handover occurs between different carrier frequencies or different RAT eg: IRAT handover.

Soft/ Softer Handover

In soft handover, the UE connection consists of at least two radio links established with cells belonging to different RBS.

In softer handover the UE connection consists of at least two radio links established with cells belonging to the same RBS.

Hard handover

Inter-Frequency Handover: Inter- Frequency handover allows service continuation on dedicated channels when the UE is moving our of coverage on another WCDMA RAN frequency exists.

IRAT Handover: IRAT handover allows service continuation on dedicated channels for circuit-switched services when the UE is moving out from WCDMA RAN coverage to an area where only GSM network coverage exists.

UE need to be in compressed mode to initiate IF or IRAT handover

IRAT Handover procedure

Compressed Mode

Compressed mode control mechanism whereby certain idle periods are created in radio frames during which the UE can perform measurements on other frequencies. The UE can carry out measurements in the neighboring cell, such are GSM Cell and FDD cell on another frequency.

Thank You..