wcdma codes

8/3/2019 Wcdma Codes

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CODES AND CHANNELS

There are 3 types of codes used in WCDMA: orthogonal codes, pseudorandom noisecodes and synchronization codes.

The channelization codes are orthogonal codes, and the scrambling code is a pseudo-

noise sequence. Orthogonality can only be achieved if the codes are timesynchronized. As UEs are not time synchronized, their uplink transmissions areasynchronous and not orthogonal.

In the uplink, different users have different long code offsets, and the network canrecognize different users from their offsets.

Each Node B has only one unique primary scrambling code, and this is used to

separate various base stations.

All dedicated physical channels have an existing uplink channel for feedbackinformation.

A SHO is a procedure once it is performed, the result is a macrodiversity situation.Macrodiversity is achieved through SHO. Macrodiversity also provides protectionagainst shadowing. Without macrodiversity (and multipath diversity) a UE can easily

get shadowed if a large obstacle gets between the UE and the base station. In SHOthe UE has at least one other path that can maintain the service if one radio link

suffers from shadowing. Macrodiversity components will be combined in the physicallayer, and not in the protocol stack.

Site-selection diversity transmission (SSDT) is a special case of SHO

(see Figures 3.17 and 3.18). The principle of SSDT is that the best cell of theactive set is dynamically chosen as the only transmitting site, and the other

cells involved turn down their DPDCHs. The DPCCH is transmitted as it

normally would be via all cells. Because only one base station is transmittingin downlink data channels, the interference level is lower than with normal

SHO.

The working principle of the SSDT is that the UE selects one of thecells from its active set to be the primary cell, and the other cells are nonprimary.

Only the primary cell transmits in the downlink data channels.

Each cell in the active set is assigned a temporary identification. The UEperiodically informs the UTRAN about the current primary cell ID. This

information is delivered via the uplink feedback information (FBI) field.In order for the UE to continuously perform measurements and to

maintain synchronization, the nonprimary cells must continue to transmitpilot information on the DPCCH in case the situation in the air interface

changes.



Transmitter-antenna diversity can be used to generate multipath diversity in

places where it would not otherwise exist. Multipath diversity is a usefulphenomenon, especially if it can be controlled. It can protect the UE against

fading and shadowing. TX diversity is designed for downlink usage. Transmitterdiversity needs two antennas, which would be an expensive solution

for the UEs.In fact, the whole system of transmit-antenna diversity is optional

for the UTRAN, but the support for all TX diversity modes is mandatory

for the UE.If the UTRAN employs any form of TX diversity, it informs the UE about it.

As the capacity in a typical UTRAN cell is uplink limited

There are three separate channel concepts in the UTRAN: logical, transport,

and physical channels.

Logical channels define what type of data is transferred. These channels define thedata-transfer services offered by the MAC layer; that is, the concept of logical

channels is used in the interface above the MAC. Note that logical channels are notactually a layer 1 concept, but logically they exist in the interface between the MAC

and the RLC protocol layers; that is, clearly within layer 2.Logical channels can be divided into control channels and traffic channels. A

control channel can be either common or dedicated. A common channel is a point-to-multipoint channel; that is, common to all users in a cell, and a dedicated channel is

a point-to-point channel; that is, used by only one user. Control channels transferControl plane (C-plane) information and traffic channels User plane (U-plane)

information.



The defined logical control channels are:• Broadcast control channel (BCCH): Downlink common channel; Broadcasts

system and cell-specific information.• Paging control channel (PCCH): Downlink channel; Transfers paging

information and some other notifications.• Dedicated control channel (DCCH): Bidirectional point-to-point channel;

Transfers dedicated control information.• Common control channel (CCCH): Bidirectional point-to-multipoint channel;Transfers control information. This logical channel is always mapped onto

RACH/FACH transport channels.

The used logical traffic channels are:• Dedicated traffic channel (DTCH): Bidirectional point-to-point channel;

Transfers user info.• Common traffic channel (CTCH): Downlink point-to-multipoint channel;

Transfers dedicated user information for a group of users.

Transport channels define how and with which type of characteristics the data istransferred by the physical layer. These channels are used in the interface between

the MAC and the PHY layers. The transport channel is a new concept if WCDMA is

compared to the GSM system. The transport channel is a concept applied to theinterface between the physical layer and the MAC layer. These channels are used bythe MAC layer to access physical layer services. All transport channels are

unidirectional. Some types of transport channels can exist in both the uplinkand downlink directions, but these entities are still separate resources.A dedicated channel (DCH) is mapped onto two physical channels. The DedicatedPhysical Data Channel (DPDCH) carries higher layer information, including user data,whilethe Dedicated Physical Control Channel (DPCCH) carries the necessary physical layercontrol information. These two dedicated physical channels are needed to supportefficientlythe variable bit rate in the physical layer. The bit rate of the DPCCH is constant, whilethe bitrate of DPDCH can change from frame to frame.

Transport channels are divided into common channels and dedicated channels. They

are all unidirectional.

Common transport channels include:

• Broadcast channel (BCH): A downlink channel for broadcast of system and

cell-specific info.• Paging channel (PCH): A downlink channel used for transmission of paging and

notification Messages. Transmission associated with transmission of paging indicator

in PICH physical channel.• Random access channel (RACH): A contention-based uplink channel; Used

for initial access or non-real-time dedicated control or traffic data; A limited-size datafield.

• Forward access channel (FACH): A common downlink channel; May carrysmall amounts of user data. This is used, for example, after a random accessmessage has been received by the base station. It is also possible to transmit packetdata on the FACH.

• Downlink shared channel (DSCH): A downlink channel shared by several

UEs;



Used for dedicated control or traffic data; always associated with a DCH (does notexist alone). it can be shared by several users.

• Common packet channel (CPCH): uplink; shared channel; is an extensionto the RACH channel that is intended to carry packet-based user data in theuplink direction.

The only dedicated transport channel type is:

• Dedicated channel (DCH): For one UE only; Either uplink or downlink.

Physical channels define the exact physical characteristics of the radio channels.These are the channels used below the PHY layer; that is, in the radio interface.

The Synchronisation Channel (SCH), the Common Pilot Channel (CPICH) and theAcquisition Indication Channel (AICH) are not directly visible to higher layers and aremandatory from the system function point of view, to be transmitted from every basestation. The CPCH Status Indication Channel (CSICH) and the CollisionDetection/Channel Assignment Indication Channel (CD/CA-ICH) are needed if CPCH(transport channel) is used.

The physical channels in the FDD mode are described below.• Synchronization channel (SCH): Used for cell search; Downlink; Two sub

channels, the primary and secondary SCH. Transmitted only during the first 256chips (i.e., one-tenth) of each timeslot.

• Common pilot channel (CPICH): Downlink; The common pilot channel is an

unmodulated code channel, which is scrambled with the cell-specific primaryscrambling code. UTRA has two types of common pilot channel, primary andsecondary. The difference is that the Primary CPICH is always under the primaryscrambling code with a fixed channelisation code allocation and there is only onesuch channel for a cell or sector. An important area for the primary common pilotchannel is the measurements for the handover and cell selection/reselection. The useof CPICH reception level at the terminal for handover measurements has theconsequence that, by adjusting the CPICH power level, the cell load can be balancedbetween different cells. Reducing the CPICH power causes part of the terminals to



hand over to other cells, while increasing it invites more terminals to hand over tothe cell, as well as to make their initial access to the network in that cell. SecondaryCPICH may have any channelisation code of length 256 and may be under asecondary scrambling code as well. The typical area of Secondary CPICH usage wouldbe operations with narrow antenna beams intended for service provision at specific

‘hot spots’ or places with high traffic density. With fixed channelization code ,

Primary CPICH (P-CPICH) is the phase reference for SCH, primary CCPCH, AICH, andPICH, and the default reference for other downlink physical channels; Secondary

CPICH (S-SPICH), with any channelization code of length 256 may be the referencefor the downlink DPCH, and for the associated PDSCH. The presence of S-CPICH in a

cell is optional.• Primary common control physical channel (P-CCPCH): Fixed rate of 30 Kbps;

Carries BCH; Not transmitted during the first 256 chips of each timeslot. no power

control information;• Secondary common control physical channel (S-CCPCH): Variable rate;

Carries FACH and PCH; FACH and PCH can be mapped to the same or separatechannels;

Carries FACH and PCH and at least one S-CCPCH is required in each cell. no powercontrol information.

• Physical downlink shared channel (PDSCH): Carries DSCH (downlinkshared channel); Always associated with a downlink DPCH (dedicated physical

channel for packet data, it is additionally used), which carriers its controlinformation.

• Paging indicator channel (PICH): Carries page indicators to indicate thepresence of a page message on the PCH.

• Acquisition indicator channel (AICH): Carries acquisition indicators (=signatures for the random access procedure). Once the base station has detected thepreamble with the random access attempt, then the same signature sequence thathas been used on the preamble will be echoed back on AICH.For the uplink CPCH (transport channel) access procedure, a set of CPCH specificdownlink physical control channels has been specified. These channels carry notransport channels, but only information needed in the CPCH access procedure.

• CPCH Access preamble acquisition indicator channel (AP-AICH): Carries APacquisition indicators of the associated CPCH. identical to the AICH used with RACH

• CPCH status indicator channel (CSICH): Carries CPCH status information.

• CPCH Collision-detection/channel-assignment indicator channel (CD/CA-ICH): Carries CD (collision detection) indicators only if the CA (channel assignment)

is not active, or both CD indicators and CA indicators at the same time if the CA isactive. Note that the three previous channels are control channels for uplink PCPCH.

If CPCH functionality is not supported by the network, then above channelsdo not exist.

Downlink and Uplink: A dedicated channel (DCH transport channel) is mappedonto two physical channels

• Dedicated physical data channel (DPDCH): Carries DCH (dedicated channel);

Carries data generated at layer 2 and above. The Dedicated Physical Data Channel(DPDCH) carries higher layer information (layer 2 and above), including user data.

• Dedicated physical control channel (DPCCH): Carries control information

generated at layer 1. carries the necessary physical layer control information. These two dedicated physical channels are needed to support efficiently the variablebit rate in the physical layer. The bit rate of the DPCCH is constant, while the bit rate

of DPDCH can change from frame to frame. Note that in the uplink these two

channels are I/Q code multiplexed,but in the downlink they are time multiplexed.Sometimes the DPDCH



and DPCCH together is an entity known as a dedicated physical channel (DPCH).

Uplink• Physical random access channel (PRACH): Carries RACH; Uses slotted ALOHA

technique with fast acquisition indicators.• Physical common packet channel (PCPCH): Carries CPCH (common packet

channel); Uses DSMA-CD technique with fast acquisition indication.

Shared Channels: To save capacity, the network does not assign a dedicated

channel for every user. If the expected/measured data traffic is of low or medium

volume, or it is a bursty type, then shared channels could be used instead of dedicated channels. On a shared channel the resource is open to be used by

everybody, and each user can request a temporary allocation for a short timeusing a special resource-reservation procedure. The temporary reservation is

typically quite short, and once it has expired, the allocation procedure must beperformed again in case new data. has to be sent. Thus a shared channel can be

used by only one active user at a time, but that user may change frequently. Theshared channels support fast power control, but they do not support soft handovers.

In the uplink direction a shared channels, the common packet channel (CPCH) in the

FDD mode. uplink shared channels must be combined with other channel types fordata transfer, which is a RACH/FACH pair. The RACH/FACH is used for the sharedchannel allocation and the FACH also for relaying the downlink acknowledgements.

Acknowledgements can also be sent via a downlink shared channel(DSCH), if such is allocated. Note that the RACH and FACH are not considered shared

channels but common channels. The difference between these concepts is thatcommon channels cannot be allocated to one user; they are common to everybody.

The RACH is a contention-based uplink channel where the permission tosend each burst must be acquired separately. The FACH is a common downlink

channel.

Channel Mapping: There are many physical channels, especially in the

downlink FDD diagram, that do not map into any transport channel at all.This is because they are some type of indication channel, which indicates

something to the receiving physical channel in a unidirectional scheme. Theinformation is of no interest to the higher layers, and thus it is not

necessary to map these channels to any kind of transport channels.



Transport Format: The physical layer operates in 10-ms time slices (radio frames) in

the connected mode (however, HSDPA employs 2-ms frames). These frames will befilled with data sent from the MAC layer to the physical layer for processing and

transmitting (and similarly something will be received from the physicallayer). This data is sent using transport blocks. The MAC layer generates a

new transport block every 10 ms (or a multiple of that), fills it with the necessaryinformation, and sends it to the physical layer. The CRC is added to

the transport block by the physical layer. It is possible to send several transportblocks via the same transport channel within one ‘frame time’ in parallel.

A set of simultaneous transport blocks is called the transport block set.

The transmission time interval (TTI) is defined as the inter-arrival timeof transport block sets. This is always a multiple of an L1 radio frame duration,the exact value being either 10, 20, 40, or 80 ms (again, in HSDPA this

is 2 ms). Each transport channel can have its own TTI. Note that a TTIvalue does not tell you anything about the amount of data to be sent, but just

how often the MAC layer sends data to the physical layer. The size of theindividual data chunk is determined by the transport block size and the

transport block set size parameters. TTI indicates how often the transport channeldata rate can be modified.



With 10 ms TTI, the rate can be modified every 10 ms; with an 80-ms settingthe modification can be done only every 80 ms. Note that it is also possible

to have a zero size transport block in a TTI. Each transport format within a transportformat set has a unique identifier

called the transport format identifier (TFI). It is used in the interlayercommunication between the MAC layer and the physical layer to indicate

the transport format. Several transport channels can exist simultaneously, each of them havingdifferent transport characteristics. These transport channels are multiplexed

together in layer 1, and the composite is called the coded composite

transport channel (CCTrCH).

HSDPA

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