basics in wcdma

8/13/2019 Basics in WCDMA

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Basics in WCDMA

ANAND KUMAR RV



WCDMA

• Wide Band Code Division Multiple Access is a third generation mobile communicationsystem. It’s a wireless system where the telecommunication, datacom & media industryconverge and is based on a Layered Architecture.

• 3G WCDMA systems have 5MHz bandwidth (one direction). 5MHz is neither wide nornarrow, it is just the bandwidth. New 3G WCDMA systems have wider bandwidth thanexisting 2G CDMA systems (CDMA - 1.25MHz), that's why the "Wide". There arecommercial CDMA systems with 20MHz bandwidth

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Layered Architecture

ApplicationsLayer

ControlLayer

MSC-S HLR

GGSN

GMSC/Transit

SG

ConnectivityLayer

GSM/ EDGE

Access

WCDMA

Access

M-MGW M-MGW

Internet

Intranet

PSTNISDN

SGSN

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WCDMA BAND

3G point of view three CDMA variants in use.

DS-WCDMA-FDD: Direct Sequence- Wideband Code Division Multiple Access -Frequency Division Duplex

• Uplink: 1920 -1980 MHz

• Downlink: 2110 -2170 MHz

• Duplex Distance: 190 MHz ?

DS-WCDMA-TDD: Direct Sequence- Wideband Code Division Multiple Access – TimeDivision Duplex

• Lower Band: 1900 -1920 MHz

• Upper Band: 2010 -2025 MHz

MC-CDMA: Multi Carrier - Code Division Multiple Access

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UTRAN

• The main task of UTRAN is to create and maintain Radio Access Bearers (RAB) forcommunication between UEs and Core Network.

• With RAB the Core Network elements are given an illusion about a fixedcommunication path to the UEs thus releasing them from the need to takecare of radio communication aspects.

• UTRAN is located between two open interfaces being Uu and Iu.

From the bearer point of view the main task of UTRAN is to provide bearer

service over these interfaces.

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Radio Access Bearer (RAB)

RAB

The Radio Access Bearer (RAB) is the entity responsible for transporting radioframes of an application over the access network in UMTS.

• Controlled by the core network (CN)

• CN determines traffic class and QoS

Real-Time Applications

•

Streaming Class: Preserve time relation between entities (packets) in adata stream

• Conversational Class: Preserve time relation between entities within acertain delay

Non-Real Time Applications

• Background Class: Destination is not expecting data. Preserve Payload

• Interactive Class: Request / Response Pattern with preserved payload

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PARAMETERS OF WCDMA

• Channel bandwidth 5 MHz

• Duplex mode FDD and TDD

• Chip rate 3.84 Mcps

•

Frame length 10 ms(38400 Chips)• Spreading Modulation Balanced QPSK (downlink) Dual-channel QPSK(uplink)

• Data modulation QPSK (downlink) BPSK (uplink)

• Channel coding Convolutional and turbo codes

• Coherent detection User dedicated time multiplexed pilot (downlink and

uplink),common pilot in the downlink

• Time Slots: 15 slots per 10 msec frame

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Cont. PARAMETERS OF WCDMA

• Channel multiplexing in uplink Control and pilot channel time multiplexed

I&Q multiplexing for data and control channel

• Multi-rate Variable spreading and multi-code

• Spreading factors 4 – 256 (uplink), 4 – 512 (Downlink)

• Power control Open and fast closed loop (1.6 kHz)

• Spreading (downlink) OVSF sequences for channel separation,

Gold sequences 2^18-1 for cell and user separation (truncated

cycle 10 ms)

• Spreading (uplink) OVSF sequences, Gold sequence 2^41 for user separation

(different time shifts in I&Q channel, truncated cycle 10 ms)

• Handover Soft handover/Intra- frequency & Hard Handover/Inter-frequency

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PROTOCOL ARCHITECTURE

The air interface is layered into three protocol layers:

• The physical layer (layer 1, L1)

• The data link layer (layer 2, L2)

• Network layer (layer 3, L3)

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Radio Interface Protocol Model

PHY

MAC

RLC

Transport Channels

Logical Channels

User Plane

Radio BearersSignaling Radio

Bearers

PDCPBMC

RRC

USER PLANECONTROL PLANE

Control

L1(Radio Physical Layer)

L2(Radio Link Layer)

L3(Radio Network Layer)U-Plane Radio Bearers

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WCDMA Radio Channels

Physical Channels

•

Transmission media.• Two types of physical channels defined in L1 - FDD and TDD.• FDD is characterized by frequency, code, I/Q phase• Follow a layered structure of “radio frames” and “time slots”

Transport Channels

• describes the way information is transferred over the radio interface

Logical Channels

• the type of information transferred characterizes a logical channel

UE BS RNC

Logical Channels

Transport Channels

Physical Channels

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WCDMA Logical Channels

Classified as

• Control channels for the transfer of control plane

• Traffic channels for the transfer of user plane

Control channel (CCH)

•

Broadcast control channel (BCCH)• Paging control channel (PCCH)

• Dedicated control channel (DCCH)

• Common control channel (CCCH)

• Shared Channel Control Channel (SHCCH)

Traffic channel (TCH)• Dedicated traffic channel (DTCH)

• Common traffic channel (CTCH)

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WCDMA TRANSPORT CHANNELS

Common Channels

• BCH Broadcast Channel

• FACH Forward Access Channel

• PCH Paging Channel

• RACH Random Access Channel

• CPCH Common Packet Channel

Dedicated Channels

• DCH Dedicated Channel

• DSCH Downlink Shared Channel

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WCDMA PHYSICAL CHANNELS

Common Control Channels

• P-CCPCH Primary Common Control Physical Channels (DL)

• S-CCPCH Secondary Common Control Physical Channels (DL)

• P-SCH Primary Synchronisation Channel (DL)

• S-SCH Secondary Synchronisation Channel (DL)

• CPICH Common Pilot Channel (DL)

• AICH Acquisition Indicator Channel (DL)

• PICH Paging Indicator Channel (DL)

• PDSCH Physical Downlink Shared Channel (DL)

• PRACH Physical Random Access Channel (UL)

• PCPCH Physical Common Packet Channel (UL)

• AP-AICH Access Preamble Acquisition Indicator Channel (DL)

Dedicated Channels

• DPDCH Dedicated Physical Data Channel (DL & UL)

• DPCCH Dedicated Physical Control Channel (DL & UL)

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CHANNELS WCDMA-FDD

Downlink Uplink

LogicalChannels

SHCCH

BCCH

PCCH CCCH DCCH DTCH CCCH DTCH DCCH

TransportChannelsSCH BCH PCH FACH DCH DCHRACH CPCH

PhysicalChannels

SCH1/2 PCCPCH SCCPCH DPCH (DPDCH+DPCCH)

PRACH

DPDCH DPCCH

PCPCH

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PHYSICAL CHANNEL

• PCCPCH (downlink) - This channel continuously broadcasts system identification and access control information.

• SCCPCH (downlink) - This channel carries the Forward Access Channel (FACH) providing control information,and the Paging Channel (PACH) with messages for UEs that are registered on the network

• PRACH (uplink) - This channel enables the UE to transmit random access bursts in an attempt to access a

network.• DPDCH (up and downlink) - This channel is used to transfer user data.

• DPCCH (up and downlink) - This channel carries control information to and from the UE. In both directions thechannel carries pilot bits and the Transport Format Combination Identifier (TFCI). The downlink channel alsoincludes the Transmit Power Control and Feedback Information (FBI) bits.

• PDSCH (downlink) - This channel shares control information to UEs within the coverage area of the node B.

• PCPCH - This channel is specifically intended to carry packet data. In operation the UE monitors the system tocheck if it is busy, and if not it then transmits a brief access burst. This is retransmitted if no acknowledgement is

gained with a slight increase in power each time. Once the node B acknowledges the request, the data istransmitted on the channel.

• SCH - The synchronisation channel is used in allowing UEs to synchronise with the network.

• CPICH This channel is transmitted by every node B so that the UEs are able estimate the timing for signaldemodulation. Additionally they can be used as a beacon for the UE to determine the best cell with which tocommunicate.

• AICH - The AICH is used to inform a UE about the Data Channel (DCH) it can use to communicate with the nodeB. This channel assignment occurs as a result of a successful random access service request from the UE.

• PICH - This channel provides the information to the UE to be able to operate its sleep mode to conserve itsbattery when listening on the Paging Channel (PCH). As the UE needs to know when to monitor the PCH, data isprovided on the PICH to assign a UE a paging repetition ratio to enable it to determine how often it needs to'wake up' and listen to the PCH.

• CSICH - This channel, which only appears in the downlink carries the status of the CPCH and may also be usedto carry some intermittent, or "bursty" data. It works in a similar fashion to PICH.

• CD/CA-ICH - This channel, present in the downlink is used to indicate whether the channel assignment is activeor inactive to the UE.

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LOGICAL CHANNEL

• BCCH (downlink) - This channel broadcasts information to UEs relevant to the cell, such asradio channels of neighbouring cells, etc.

• PCCH (downlink) - This channel is associated with the PICH and is used for pagingmessages and notification information.

• DCCH (up and downlinks) - This channel is used to carry dedicated control information inboth directions.

• CCCH (up and downlinks) - This bi-directional channel is used to transfer controlinformation.

• SHCCH (bi-directional) - This channel is bi-directional and only found in the TDD form ofWCDMA / UMTS, where it is used to transport shared channel control information.

• DTCH (up and downlinks) - This is a bidirectional channel used to carry user data or traffic.

• CTCH (downlink) - A unidirectional channel used to transfer dedicated user information toa group of UEs.

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TRANSPORT CHANNELS

• DCH (up and downlink) - This is used to transfer data to a particular UE. Each UE has itsown DCH in each direction.

• BCH (downlink) - This channel broadcasts information to the UEs in the cell to enablethem to identify the network and the cell.

• FACH (down link) - This is channel carries data or information to the UEs that are registeredon the system. There may be more than one FACH per cell as they may carry packetdata.

• PCH (downlink) - This channel carries messages that alert the UE to incoming calls, SMSmessages, data sessions or required maintenance such as re-registration.

• RACH (uplink) - This channel carries requests for service from UEs trying to access thesystem

• CPCH (uplink) - This channel provides additional capability beyond that of the RACH andfor fast power control.

• DSCH (downlink) - This channel can be shared by several users and is used for data that is"bursty" in nature such as that obtained from web browsing etc.

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UPLINK CHANNEL MAPPING

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DOWNLINK CHANNEL MAPPING

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Power Control in WCDMA

• The purpose of power control (PC) is to ensure that each user receives and transmits just enoughenergy to have service but to prevent

1-Removes Near-Far effects.2-Mitigates Fading.

3-Compensates changes in propagation conditions.

4-Decrease Intra-cell (UL) and Inter-cell interference (UL & DL).

UE1UE2

UE3

UE1

UE2

UE3

UE1 UE2 UE3

Without PC received

power levels would be

unequal

With ideal PC received

power levels are equal

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POWER CONTROL

In WCDMA, power control is employed in both uplink and downlink.

Downlink power control is basically for minimising the interference to other cells and compensating for other cells

interference as well as achieving acceptable SIR(Signal to Interference).

To manage the power control properly in WCDMA, the system uses different two defined power control:Open Loop Power ControlClosed Loop Power Control (CLPC)/Inner Loop Power Control

Open Loop Power Control (OLPC)It is the ability of the UE transmitter to sets its output power to a specific value. It is used for settinginitial uplink and downlink transmission powers when a UE is accessing the network. The open loop power controltolerance is ± 9 dB (normal conditions) or ± 12 dB (extreme conditions)

The UE adjusts its transmission power based on estimate of the received signal level from the BS Common PilotChannel (CPICH) when the UE is in idle mode and prior to Physical Random Access Channel (PRACH) transmission.

Closed-Loop Power Control (CLPC):It is utilised for adjusting the transmission power when the radio connection has already been established. Its maintarget is to compensate the effect of rapid changes in the radio signal strength and hence it should be fast

enough to respond to those changes.

Used In the uplink is the ability of the UE transmitter to adjust its output power in accordance with one or moreTransmit 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. Inner loop power control frequency is 1500Hz.



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Power Control in WCDMA

Open loop Power control (OLPC) mechanism is used in the initial random access through Physical RandomAccess Channel (PRACH).

The UE increase it's initial power with defined level and timing (received from the broadcasted information stepsuntil it gains access to the cell .

But OLPC will be not effective especially in fast fading conditions so we add a fine tuning mechanismClosed loop power control

The outer closed loop at rate 100 times per second set the target SIR that can be compared with measured SIRin order to BS and UE know what to do with measured SIR at rate 1500 times per second in the inner fast closedloop power control



WCDMA CODES

• Spreading means increasing the signal bandwidth

• spreading includes two operations:

Channelisation (increases signal bandwidth) - using orthogonal codes Scrambling “(does not affect the signal bandwidth) – using pseudo noise codes

• Channelisation codes are orthogonal codes, based on Orthogonal Variable Spreading Factor(OVSF) technique

• In the downlink, it can separate different users

within one cell/sector

• In the uplink, it can only separate the physical

channels/services of one user because the

mobiles are not synchronised in time.

• In order to separate different users in the uplink, scrambling codes are used.

• The scrambling code can be a long code (a Gold code with 10 ms period) or a short code(S(2) code).

• In the downlink scrambling codes are used to reduce the inter-base station interference.

Typically, each Node B has only one scrambling code for UEs to separate base stations.• In the uplink scrambling codes are used to separate the terminals.

• SF = chip rate / symbol rateHigh data rates = low SF codeLow data rates = high SF code



Contd. WCDMA CODES



WCDMA Handovers

• Softer handover

o Handover between two sectors of the same Node B

o Special case of a soft handover

o No need for duplicate frames

• Hard handover

o The source is released first and then new one is

added

o Short interruption in data flow

• Soft handovero Handover between different Nodeb’s

o Several Nodeb’s transmit the same signal to the UE which combines the transmissions

• Advantages: lower Tx power needed for each Node B and UE

• Disadvantage: resources (code, power) need to be reserved for the UE in eachNodeB

o Excess soft handovers limit the capacity

o lower interference, battery saving for UEo No interruption in data transmission

o Needs RNC duplicating frame transmissions to two Nodeb’s

• In WCDMA system, the majority of handovers are intra-frequency soft handovers

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WCDMA NETWORK

Core Network

UTRAN

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RNC (Radio Network Controller)

• The RNC is switching and controlling element of the UTRAN. RNC is locatedbetween the Iub and Iu interface. It also has the third interface called Iur forinter-RNS connections.

• Referring to the Bearers, the RNC is a switching point between the Iu Bearerand Radio Bearer(s).

RNC tasks

Radio Resource Management• Radio Resource Control (RRC)

• Admission Control & Packet Scheduling

• Code Allocation

• Power Control

• Handovers and Micro Diversity



NodeB

• NodeB is the UMTS equivalent of base station transceiver .

• The term Node B refers to the base station equipment which communicates with thesubscriber’s handset via the radio link.

• It provides radio resources for a UMTS network, and uses UMTS channel allocation tocommunicate with the handset.

• It provides all the RF processing, enabling transmission and reception information to andfrom the mobile terminal.

• This information is encoded using the W-CDMA scheme.

• Node B uses WCDMA/TD-SCDMA as the air interface technology.

• The BS is located between the Uu and Iub interfaces. Its main tasks are to establish thephysical implementation of the Uu interface and, towards the network, theimplementation of the Iub interface by utilising the protocol stacks specified for theseinterfaces.

•

Realization of the Uu interface means that the BS implements WCDMA radio accessphysical channels and transfers information from transport channels to the physicalchannels based on the arrangement determined by the RNC.

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http://en.wikipedia.org/wiki/WCDMA

http://en.wikipedia.org/wiki/TD-SCDMA




http://en.wikipedia.org/wiki/WCDMA



UMTS UE ELEMENTS

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PROTOCOLS USED OVER IuB INTERFACE

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PROTOCOLS USED OVER IuR INTERFACE

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PROTOCOLS USED OVER Iu CS INTERFACE

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PROTOCOLS USED OVER Iu PS INTERFACE

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Usefull Links for UMTS

• http://www.umtsworld.com/umts/faq.htm#f31

• http://en.wikipedia.org/wiki/3g

• http://www.nmscommunications.com/file/3G_Tutorial.pdf

http://www.umtsworld.com/umts/faq.htm

http://en.wikipedia.org/wiki/3g

http://www.nmscommunications.com/file/3G_Tutorial.pdf

http://www.nmscommunications.com/file/3G_Tutorial.pdf

http://en.wikipedia.org/wiki/3g

http://www.umtsworld.com/umts/faq.htm

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