03 TM51103EN03GLA2 Air Interface

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Contents

1 UMTS Air interface technology 3

1.1 Duplex methods 4

1.2 UMTS Frequency 7

1.3 Access method 8

2 UMTS Air interface description 11

2.1 Principle 12

2.2 Data processing 13

2.3 Codes 21

2.4 Logical, transport and physical Channels 28

2.5

Air interface protocol stack 35

3 High Speed Downlink Packet Access HSDPA 51

3.1 HSDPA performance 52

3.2 HSDPA implementation : 53

3.3 HSDPA channels : 55

3.4 MAC Layer Split 57

3.5 Adaptive Modulation and Coding (AMC) Scheme : 58

3.6 Error Correction (HARQ) 59

3.7 Fast packet scheduling 61

3.8 Impact on the Iub Interface 633.9 Handset Capabilities 64

4 Exercises 65

5 Solution 67

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1 UMTS Air interface technology


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1.1 Duplex methods

The duplex define the way how two communicating entities will communicate witheach others.

We define here three ways:

Simplex: This is one way communication method used for broadcasting (TV,Radio)

Half-duplex: This is a two ways communication method; the two communicatingentities cannot transmit and receive simultaneously.

Full-duplex: This method is the same as half duplex except that the two entitiescan communicate simultaneously.

We define two means to achieve full or half duplex method:

FDD: Frequency Division Duplex :

The frequency band is split into two sub-band one for the uplink and the other for thedownlink. Then the receiver and the transmitter use two carriers at the same time.

Advantages: Using this method we can avoid collision between uplink anddownlink.

Drawbacks: Frequency resources are wasted


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UplinkUplink DownlinkDownlink

Fig 1 Frequency division duplex


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TDD: Time Division Duplex :

The two communicating entities use the same frequency band, but it doesnt

communicate simultaneously. It uses two different time period, one period for theuplink and the other one for the downlink.

Advantages: The frequency resources are not wasted.

Drawbacks: Collision may occur during communication.

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Time

Frequency

UplinkUplink

DownlinkDownlink

Fig 2 Time division duplex


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1.2 UMTS Frequency

The IMT-2000 has allocated the band from 806 MHz - 960 MHz, 1710 MHz - 2025MHz, 2110 MHz 2200 MHz and finally 2500Mhz 2690 MHz for a worldwidemobile communication implementation.

The frequency band which is used for UMTS use is summarized in the followinggraph:

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Fig 3 IMT-2000 frequency allocation for different mobile system

1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paireduplink and downlink, channel spacing is 5 MHz. An Operator needs 3 - 4 channels(2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network.1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired,channel spacing is 5 MHz. Tx and Rx are not separated in frequency.1980-2010 and 2170-2200 MHz Satellite uplink and downlink.


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1.3 Access method

The goal of a network operator is to achieve a higher capacity with fewer resources.In order to do this different access methods are used following is defining thesemethods:

1.3.1 FDMA: Frequency division mult iple access

The used frequency band is divided into different carriers as shown below. The samenumber of carrier is used for both uplink and downlink. Each carrier is indexed withUARFCN (UTRA absolute radio frequency carrier number).

UAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FCUAR

FC

UAR

FC

Frequency

UplinkDownlink

Fig 4 Frequency division multiple access technique

The advantage of this technique is that the bandwidth is used more efficiently. Itmeans that one operator can reuse its set of frequency according to a certain patterncalled cluster.


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1.3.2 TDMA: Time divis ion multiple access

This is a time domain multiplexing technique. The principle is simple one carrier isdivided into different timer period called timeslot.

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Time

F

R

E

Q

U

E

N

C

Y

Timeslot

Fig 5 TDMA Time division multiple access

In order to increase the capacity of the network the two previously discussedtechniques are used together.


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1.3.3 CDMA: Code divis ion multiple access

This is a technique which is using code division in the air interface. Lets assume thatthere is a crowd of people speaking together, so if everyone will speak loudly nobodycan listen to his talker. The principle introduced by CDMA is as simple as that: eachone will speak with low level and with his own language so everybody can have acoherent discussion without disturbing his neighbor. So in CDMA system thesubscribers share the same frequency and the same time but they got differentcodes.

Fig 6 CDMA Concept expressed in terms of power, frequency and time

The capacity of the cell is not anymore function of number of timeslots in the airinterface but its expressed in function of power allowed within one cell, or to be morespecific this capacity is expressed with allowed signal to interference ratio within onecell.Then when more subscribers acess the cell then they will add more interference levelto the cell till the interference level reach a planed level or threshold.


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2 UMTS Air interface description


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2.1 Principle

The air interface is the interface located between the UE and the base station and inthe standard it is referred as Uu interface.The transmission in the air interface is based on CDMA technology and its calledW-CDMA (Wideband CDMA) because its using 3 times the bandwidth which is usedby the CDMA and then for the WCDMA we allocate 3.84 MHz effective band.Adding the guard band the total bandwidth will reach 5 MHz.

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Fig 7 UMTS bandwidth

Different variants bandwidths are specified by the standard 5 MHz, 10 MHz and 20MHz, the mostly used by operators is 5 Mhz.


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2.2 Data processing

Before sending the data over the Uu interface data need to be processed in order tocomply with the air interface requirement in term of bandwidth and QoS. Thisprocessing in the following steps:

Fig 8 Data process


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2.2.1 Channel coding

Channel coding refers to a technique used to protect data against losses in the airinterface. The technique used here is adding redundancy to the signal giving it morechance to be transmitted correctly over the Uu interface.

For the channel coding in UTRA two options are supported for FDD and three optionsare supported for TDD:

Convolutional coding.

Turbo coding.

No coding (only TDD).

Channel coding selection is indicated by higher layers. In order to randomizetransmission errors, bit interleaving is performed further.

2.2.2 Rate Matching

After channel coding data needs to be put into radio frames and sometime theamount of data is less or exceed the size of these radio frames. So in order make acorrect framing bits are added or by puncturing in a controlled way and this processis called rate matching. The following graph shows which are allowed data rate to be

matched:


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Fig 9 UMTS Rate matching


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2.2.3 Radio framing, and Spreading

After rate matching the data from previous block comes with a tight bandwidth and ahigher output power. So in order to reduce the power of the signal we multiply it by acode, channelization code, so that the signal will be spread all over the totalbandwidth reducing then the power under the noise level.

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Fig 10 Spreading

By doing that the receiver transmits the signal with a lower level allowing then less

interference in the air interface.The length of the code that the signal will be multiplied with is expressed as follow:

The chip is the smallest logical unit in a code it means a chip is a bit in the code. Thecode frequency is higher than the signal frequency so that we obtain spreading of thesignal over the bandwidth. The chip rate used is 3.84 million chips per second(Mcps/s) and it is fixed.The characteristics of the spreading codes will be discussed later.


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2.2.4 Modulation

There are several considerations that were taken into account when making thechoice for the overall format for the UMTS WCDMA modulation formats. Some of theconsiderations were:

It is necessary to ensure that the data is carried efficiently over the availablespectrum, and therefore maximum use is made of the available spectrum, andhence the capacity of the system is maximized.

The modulation format should be chosen to avoid the audio interference caused tomany nearby electronics equipment resulting from the pulsed transmission formatused on many 2G systems such as GSM

As the uplink and downlink have different requirements, the exact format for themodulation format used on either direction is slightly different.

UMTS modulation schemes for both uplink and downlink, although somewhatdifferent are both based around QPSK formats. This provides many advantagesover other schemes that could be used in terms of spectral efficiency and otherrequirements.

Fig 11 UMTS Modulation

The OQPSK is the Offset QPSK the difference with QPSK is that there is no jump

is permitted over the intermediate states.


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2.2.5 Multipath propagation

Due to the environment of serving area the microwave can be reflected by different

obstacles before it reach the BTS or the MS and this is the multipath propagation. Soat the receiver side there will be a combination of different signals. In order to dealwith such a propagation context the RAKE receiver is used.

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Fig 12 RAKE receiver block diagram


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In a W-CDMA receiver the following steps take place (excluding the error correctioncoding):

1. Descrambling:Received signals are multiplied by the scrambling code anddelayed versions of the scrambling code. The delays are determined by a pathsearcher prior to descrambling. Each delay corresponds to a separate multipaththat will eventually be combined by the Rake receiver.

2. Despreading:The descrambled data of each path are dispread by simplymultiplying the descrambled data by the spreading code.

3. Integration and dump: The dispread data is then integrated over one symbolperiod, giving one complex sample output per quadrature phase-shift keying(QPSK) symbol. This process is carried out for all the paths that will be combinedby the RAKE receiver.

4. The same symbols obtained via different paths are then combined together usingthe corresponding channel information using a combining scheme like maximumratio combing (MRC).

5. The combined outputs are then sent to a simple decision device to decide on thetransmitted bits.

6. The objective of the channel estimation block is to estimate the channel phaseand amplitude [denoted in Figure 1 as g(t, i)] for each of the identified paths.Once this information is known, it can be used for combining each path of thereceived signal.


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2.3 Codes

Previously we talked about spreading codes, spreading is done using achannelization codes and scrambling codes:

Fig 13 Spreading using channelization and scrambling codes


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2.3.1 Channelization code

Channelization codes are used

UL: to separate physical data and control data from same terminal

DL: to separate connection to different terminals in a same cell.

For a good separation these code are orthogonal and then we use OVSF codes(orthogonal variable spreading factor codes) these codes are also called Walshcodes. It uses a different spreading factor according to bandwidth requirementincreasing then the data rate of the signal.

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Fig 14 Code tree

One important limitation of OVSF-WCDMA is that the system must maintain theorthogonality among the assigned codes. The maintenance of the orthogonalityamong the assigned OVSF codes causes the code blocking problem due to their treestructure. When users are using a higher data rate then they will use a shorter codethis will lead to a blocking to the remaining tree branch and then limiting the access tothe other users.


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2.3.2 Scrambling codes

Scrambling codes has a different use from the channelization code, they are used to

distinguish in the UL between different users and downlink between different Node B.One scrambling code then is allocated by cell or by user. The scrambling codes havea lower orthogonality than the channelization codes.These codes are organized into 512 code sets. We define then 512 primaryscrambling codes and in a lower hierarchical level we define from 1 to 15 secondaryscrambling codes achieving then a total number of 8096 codes. The scrambling codeis identified by first identifying its code set to significantly reduce the degree of codeuncertainly.

Fig 15 Scrambling codes set


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2.3.3 Code management

Code management is devoted to managing the downlink OVSF (Orthogonal Variable

Spreading Factor) code tree used to allocate physical channel orthogonality amongdifferent users. Clearly, the advantage of the OVSF codes used in the UTRANdownlink is perfect orthogonality. However, the drawback is the limited number ofavailable codes. Therefore, it is important to be able to allocate/reallocate thechannelization codes in the downlink with an efficient method, in order to preventcode blocking. Code blocking indicates the situation where a new call could beaccepted on the basis of interference analysis and also on the basis of the sparecapacity of the code tree but, due to an inefficient code assignment, this sparecapacity is not available for the new call that must, therefore, be blocked. Thissituation is depicted in Figure 4.24, where two transmissions with SF 4 and twotransmissions with SF 8 are assumed to have been assigned the correspondingcode sequences Cch,4,2, Cch,4,3, Cch,8,1 and Cch,8,3, respectively, which preventthe use of the codes marked with a cross in Figure 4.24. It is worth noting that, withsuch OVSF code tree occupancy, the arrival of a new call requesting for SF 4would experience code blocking, since no code at that layer is available. On thecontrary, if the code allocation shown in below figure was used, it would allow thesupport of the two SF 4 users, the two SF 8 users and still would provide roomto support a new SF 4 request with code Cch,4,1.


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Fig 16 Example of code blocking


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Fig 17 :Example of code allocation preventing code blocking

In general terms, a code allocation strategy would aim at minimizing code treefragmentation, preserving the maximum number of high rate codes and eliminatingcode blocking. Nevertheless, since the purpose of the code allocation/reallocationstrategies is to prevent code blocking, this may require code handover, that is, a callusing a given code is forced to use a different code belonging to the same layer.


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2.3.4 Multiuser detection in WCDMA systems

Before sending user data in the air interface it must be multiplied by a scrambling

code C1. While sending over the air interface different signals of different users arecombined. In order to extract the user data from the other signals we must multiply itby the same code again.

1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 User data

1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1

X

Code

0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0

1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1

Scrambled Signal

Code

X

Fig 18 Scrambling process

The characteristic of this scrambling code is that they are not orthogonal but theyhave very good orthogonality propriety and they are pseudo random.

Fig. 1


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2.4 Logical, transport and physical Channels

UTRA FDD radio interface has logical channels, which are mapped to transportchannels, which are again mapped to physical channels. Logical to Transportchannel conversion happens in Medium Access Control (MAC) layer, which is alower sub-layer in Data Link Layer (Layer 2).

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Radio resources controlRadio resources control

Medium Access ControlMedium Access Control

Physical LayerPhysical Layer

Logical channel

Transport channel

Physical channel

Control

and

measur

ement

Fig 19 Protocol stack

Different channels transport channels can be mapped into one physical channel anddifferent logical channel can be mapped to a transport channel. This channel

organization allows signaling information to be transfer to the concerned andappropriate protocol level in a network element.


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Fig 20 Channels

Fig 21 Channels 2


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2.4.1 Physical channels

A physical channel is basically defined by a frequency and a spreading code. The

physical channel uses a cosine or sine waveform as a signal carrier. We candistinguish between two kinds of physical channels:

o Dedicated physical channel.

o Common physical channel.

A dedicated physical channel is allocated only for one connection but commonchannels are used simultaneously or alternatively by different connections.The physical layers map under control of the MAC the transport channels to thephysical channel according totheir physical requirement.

o Dedicates Physical Data Channel DPDCH:Used in uplink direction to

transmit signaling and user data from higher layer.o Dedicated physical control channel DPCCH: This channel is used to

control the data transmission over the air interface. The informationincluded in this channel are power control commands, pilot bits

o Dedicated physical channel DPCH: The DPDCH and the DPCCH areimplemented on DPCH.

o Physical Random Access Channel PRACH: This physical channel isused during the initial access procedure or call setup. The informationcontained on this channel is RACH.

o Physical common packet control channel PCPCH:Packet data of theCPCH is sent via PCPCH through the use of CSMA/CD technique.

o Common Pilot Channel CPICH: CPICH is an important channel used forcell phase and time reference as well as channel estimation. This channelwill help the UE to identify the primary scrambling code by sending a bitpattern at a fixed data rate at 30 kb/s and with a known 256 spreadingfactor. The same channel code is always used by the CPICH.

o Common control physical channel CCPCH: This is a downlink channelwhich is used to carry broadcast information and synchronization to themobile station. We have two CCPCH:

P-CCPCH:Primary common control channel which used to broadcast BCH cell

info for different users within a cell serving area.

S-CCPCH:Secondary Common control physical channel which is used to carrythe FACH Forward Access channel and PCH paging channel.

Synchronization channel SCH:Physical channel is used to for cell search andframe synchronization. We can distinguish two SCH:

Primary SCH: The 10ms radio frames of the SCH are divided into 15 slots, eachof length 2560chips. The Primary SCH consists of a modulated code PSC(Primary synchronization code) of length 256chips, and is transmitted once everyslot.


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Fig 22 WCDMA Frame

Secondary SCH: consists of repeatedly transmitting a length 15 sequence ofmodulated codes of length 256 chips, the Secondary Synchronization Codes(SSC), transmitted in parallel with the Primary SCH. The SSC is denoted csi,k ,where i = 0, 1, , 63 is the number of the scrambling code group, and k = 0, 1, ,14 is the slot number. Each SSC is chosen from a set of 16 different codes oflength 256. This sequence on the Secondary SCH indicates which of the codegroups the cell's downlink scrambling code belongs to.

o Physical downlink shared channel: This channel is used to carry dataover DSCH and different connections can share this channel. A DPCH isalways allocated to the PDSCH.


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2.4.2 Transport channels

The output from, and input to, MAC is in the form of transport channels, which can beseen as service between MAC and Layer 1 (physical layer). Generally, transportchannels map onto specific physical channels and have specific characteristics interms of direction, data rate (including variation) and power control requirements. Theconfiguration of a transport channel is related dynamically to QoS requirements.

Random Access Channel RACH: This channel is mapped to PRACH and it isused to send a small amount of data for a connection setup or initial access inuplink direction. When it use the RACH the mobile send a first preamble and thenwait for an indication from the network that a first preamble was received and thenit send a second preamble.

Broadcast channel BCH: This downlink channel is used to transmit cell specificinformation to the mobile. This information is contained on the BCCH which is itselfmapped to BCH.

Forward Access channel FACH:This channel is used to transfer a small amountof user data or signaling over the air interface and also to grant access to themobile during initial access procedure after receiving second preamble of theRACH.

Dedicated channel DCH: This channel to carry user data traffic different logicalchannel can be mapped over this channel (DCCH or DTCH).

Data Shared Channel DSCH:In UMTS, the Downlink Shared Channel (DSCH) isused to transmit data packets from the Node B to the User Equipment (UE). EachDSCH is associated with a Dedicated Channel (DCH) which is used for powercontrol, channel estimation and transmission of associated control information for

the DSCH.

Common pilot channel CPICH: This channel is used in UMTS to enable channelestimation. The CPICH uses a pre defined bit sequence. It has a fixed rate of30Kbps with a SF (Spreading Factor) of 256. This allows the UE (User Equipment)to equalize the channel in order to achieve a phase reference with the SCH(Synchronization Channel) and also allows estimations in terms of power control.The same channel code is always employed on the Primary CPICH.


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2.4.3 Logical channels

Logical channel are used for different purpose depending on the information carried

within these channels. This information could be paging information or BCCH or othersignaling information.

Broadcast Control Channel BCCH: This logical channel carry specificinformation and parameter about the cell.

Paging Control Channel (PCCH): A downlink channel that transfers paginginformation.

Dedicated Control Channel (DCCH):A point-to-point bidirectional channel thattransmits dedicated control information between a UE and the RNC. This channelis established during the RRC connection establishment procedure.

Common Control Channel (CCCH):A bidirectional channel for transmittingcontrol information between the network and UEs. This logical channel is alwaysmapped onto RACH/FACH transport channels. A long UTRAN UE identity isrequired (U-RNTI, which includes SRNC address), so that the uplink messagescan be routed to the correct serving RNC even if the RNC receiving the message

is not the serving RNC of this UE.

The Traffic Channels are:

Dedicated Traffic Channel (DTCH):A Dedicated Traffic Channel (DTCH) is apoint-to point channel, dedicated to one UE, for the transfer of user information. ADTCH can exist in both uplink and downlink.

Common Traffic Channel (CTCH): A point-to-multipoint downlink channel fortransfer of dedicated user information for all, or a group of specified, UEs.


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Fig 23 Channels mapping


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2.5 Air interface protocol stack

The protocol stack in the air interface is 3 level layered as shown below:

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SignalingSignaling

RLCRLC

Logical ChannelsLogical Channels

MACMAC

Transport ChannelsTransport Channels

Physical layerPhysical layer

Control and

measurements

BMCBMC

PDCPPDCP

Control plane

User plane Radio Bearer

Control plane Radio Bearer

Physical ChannelsPhysical Channels

1

2

3

User plane

RRCRRC

Fig 24 Air interface protocol stack


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The physical layer offers services to the MAC layer via transport channels that werecharacterized by how and with what characteristics data is transferred. The MAClayer, in turn, offers services to the RLC layer by means of logical channels. The

logical channels are characterized by what type of data is transmitted. The RLC layeroffers services to higher layers via service access points (SAPs), which describe howthe RLC layer handles the data packets and if, for example, the automatic repeatrequest (ARQ) function is used. On the control plane, the RLC services are used bythe RRC layer for signaling transport. On the user plane, the RLC services are usedeither by the service-specific protocol layers PDCP or BMC or by other higher-layeru-plane functions (e.g. speech codec). The RLC services are called Signaling RadioBearers in the control plane and Radio Bearers in the user plane for services notusing the PDCP or BMC protocols. The RLC protocol can operate in three modes transparent, unacknowledged and acknowledged mode. The Packet DataConvergence Protocol (PDCP) exists only for the PS domain services. Its main

function is header compression. Services offered by PDCP are called Radio Bearers.The Broadcast Multicast Control protocol (BMC) is used to convey over the radiointerface messages originating from the Cell Broadcast Centre. In Release 99 of the3GPP specifications, the only specified broadcasting service is the SMS CellBroadcast service, which is derived from GSM. The service offered by BMC protocolis also called a Radio Bearer.The RRC layer offers services to higher layers (to the Non-Access Stratum) viaservice access points, which are used by the higher layer protocols in the UE sideand by the Iu RANAP protocol in the UTRAN side. All higher layer signaling (mobilitymanagement, session management, and so on) is encapsulated into RRC messagesfor transmission over the radio interface.

The control interfaces between the RRC and all the lower layer protocols are used bythe RRC layer to configure characteristics of the lower layer protocol entities,including parameters for the physical, transport and logical channels. The samecontrol interfaces are used by the RRC layer, for example to command the lowerlayers to perform certain types of measurement and by the lower layers to reportmeasurement results and errors to the RRC.


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2.5.1 Medium Access Control MAC

In the Medium Access Control (MAC) layer the logical channels are mapped to thetransport channels. The MAC layer is also responsible for selecting an appropriatetransport format for each transport channel depending on the instantaneous sourcerate of the logical channels. The transport format is selected with respect to thetransport format combination set which is defined by the admission control for eachconnection.

The functions of the MAC layer include:

Mapping between logical channels and transport channels.

Selection of appropriate Transport Format (from the Transport FormatCombination Set) for each Transport Channel, depending on the instantaneous

source rate.

Priority handling between data flows of one UE. This is achieved by selecting highbit rate and low bit rate transport formats for different data flows.

Priority handling between UEs by means of dynamic scheduling. A dynamicscheduling function may be applied for common and shared downlink transportchannels FACH and DSCH.

Identification of UEs on common transport channels. When a common transportchannel (RACH, FACH or CPCH) carries data from dedicated-type logicalchannels

(DCCH, DTCH), the identification of the UE (Cell Radio Network TemporaryIdentity

(C-RNTI) or UTRAN Radio Network Temporary Identity (U-RNTI)) is included inthe MAC header.

Multiplexing/demultiplexing of higher layer PDUs into/from transport blocksdelivered to/from the physical layer on common transport channels. MAC handlesservice multiplexing for common transport channels (RACH/FACH/CPCH). This isnecessary, since it cannot be done in the physical layer.

Multiplexing/ demultiplexing of higher layer PDUs into/from transport block setsdelivered to/from the physical layer on dedicated transport channels. MAC allowsservice multiplexing also for dedicated transport channels. While the physical layermultiplexing makes it possible to multiplex any type of service, including serviceswith different quality of service parameters, MAC multiplexing is possible only forservices with the same QoS parameters.


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Traffic volume monitoring. MAC receives RLC PDUs together with statusinformation on the amount of data in the RLC transmission buffer. MAC comparesthe amount of data corresponding to a transport channel with the thresholds set by

RRC. If the amount of data is too high or too low, MAC sends a measurementreport on traffic volume status to RRC. The RRC can also request MAC to sendthese measurements periodically. The RRC uses these reports for triggeringreconfiguration of Radio Bearers and/or Transport Channels.


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2.5.2 The Radio Link Control Protocol

The radio link control protocol provides segmentation and retransmission services forboth user and control data. Each RLC instance is configured by RRC to operate inone of three modes: transparent mode (Tr), unacknowledged mode (UM) oracknowledged mode (AM). The service the RLC layer provides in the control plane iscalled Signaling Radio Bearer (SRB). In the user plane, the service provided by theRLC layer is called a Radio Bearer (RB) only if the PDCP and BMC protocols are notused by that service; otherwise the RB service is provided by the PDCP or BMC.

Each mode provides a different set of services defining the use of that mode by thehigher layers. Transfer of user data is a service which is common to all three modes.

Transparent modeis defined for quick and dirty data transfer across the radio

interface, and is the only one of the three modes which does not involve the additionof any header information onto the data unit. Erroneous data units are discarded ormarked as erroneous.

Transparent mode isthe mode normally used by both the PNFE and BCFE entitieswithin RRC, for paging/notification and cell broadcast messaging.

In Unacknowledged mode, as in transparent mode, no retransmission protocol isused, and so data delivery is not guaranteed. Received erroneous data can be eithermarked or discarded, depending on configuration.

For both Transparent mode data transfer & unacknowledged mode data transfer,RLC provides a function for the segmentation of large data units into smaller ones

(and re-assembly at the receive end). The segment lengths are defined when thechannel is established. In unacknowledged mode, segment lengths are given by alength indicator which is within the header added to the data unit.

Unacknowledged mode additionally provides a service whereby small packet dataunits can be concatenated together (again indicated within a header field), aciphering service, and a sequence number check which allows the receiver to checkwhether or not data has been lost.

The functions of the RLC layer are:

Segmentation and reassembly. This function performs segmentation/reassembly

of variable-length higher layer PDUs into/from smaller RLC Payload Units (PUs).One RLC PDU carries one PU. The RLC PDU size is set according to the smallestpossible bit rate for the service using the RLC entity. Thus, for variable rateservices, several RLC PDUs need to be transmitted during one transmission timeinterval when any bit rate higher than the lowest one is used.

Concatenation. If the contents of an RLC SDU do not fill an integral number ofRLC PUs, the first segment of the next RLC SDU may be put into the RLC PU inconcatenation with the last segment of the previous RLC SDU.

Padding. When concatenation is not applicable and the remaining data to betransmitted does not fill an entire RLC PDU of given size, the remainder of thedata field is filled with padding bits.


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Transfer of user data. RLC supports acknowledged, unacknowledged andtransparent data transfer. Transfer of user data is controlled by QoS setting.

Error correction. This function provides error correction by retransmission in theacknowledged data transfer mode.

In-sequence delivery of higher layer PDUs. This function preserves the order ofhigher layer PDUs that were submitted for transfer by RLC using theacknowledged data transfer service. If this function is not used, out-of-sequencedelivery is provided.

Duplicate detection. This function detects duplicated received RLC PDUs andensures that the resultant higher layer PDU is delivered only once to the upperlayer.

Flow control. This function allows an RLC receiver to control the rate at which thepeer RLC transmitting entity may send information.

Sequence number check (Unacknowledged data transfer mode). This functionguarantees the integrity of reassembled PDUs and provides a means of detectingcorrupted RLC SDUs through checking the sequence number in RLC PDUs whenthey are reassembled into an RLC SDU. A corrupted RLC SDU is discarded.

Protocol error detection and recovery. This function detects and recovers fromerrors in the operation of the RLC protocol.

Ciphering is performed in the RLC layer for acknowledged and unacknowledged

RLC modes. The same ciphering algorithm is used as for MAC layer ciphering, theonly difference being the time-varying input parameter (COUNT-C) for thealgorithm, which for RLC is incremented together with the RLC PDU numbers. Forretransmission, the same ciphering COUNT-C is used as for the originaltransmission (resulting in the same ciphering mask); this would not be so ifciphering were on the MAC layer. An identical ciphering mask for retransmissionsis essential from Release 5 onwards when the HSDPA feature with physical layerretransmission combining is used. The ciphering details are described in 3GPPspecification TS 33.102 [4].

Suspend/resume function for data transfer. Suspension is needed during the

security mode control procedure so that the same ciphering keys are always usedby the peer entities. Suspensions and resumptions are local operationscommanded by RRC via the control interface.


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2.5.3 The Packet Data Convergence Protocol PDCP

The Packet Data Convergence Protocol (PDCP) [6] exists only in the user plane and

only for services from the PS domain. The PDCP contains compression methods,which are needed to get better spectral efficiency for services requiring IP packets tobe transmitted over the radio. For 3GPP Release 99 standards, a headercompression method is defined, for which several header compression algorithmscan be used. As an example of why header compression is valuable, the size of thecombined RTP/UDP/IP headers is at least 40 bytes for IPv4 and at least 60 bytes forIPv6, while the payload, for example for IP voice service, can be about 20 bytes orless.The main PDCP functions are:

Compression of redundant protocol control information (e.g. TCP/IP and

TP/UDP/IP headers) at the transmitting entity, and decompression at the receivingentity. The header compression method is specific to the particular network layer,transport layer or upper layer protocol combinations, for example TCP/IP andRTP/UDP/IP. The only compression method that is mentioned in the PDCPRelease 99 specification is RFC2507.

Transfer of user data. This means that the PDCP receives a PDCP SDU from thenon access stratum and forwards it to the appropriate RLC entity and vice versa.

Support for lossless SRNS relocation. In practice this means that those PDCPentities which are configured to support lossless SRNS relocation have PDUsequence numbers, which together with unconfirmed PDCP packets are forwardedto the new SRNC during relocation. Only applicable when PDCP is usingacknowledged mode RLC with in sequence delivery.


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2.5.4 The Broadcast /Multicast Control Protocol BMC

The Broadcast/Multicast Control (BMC) protocol exists also only in the user plane.This protocol is designed to adapt broadcast and multicast services, originating fromthe Broadcast domain, on the radio interface. In Release 99 of the standard, the onlyservice using this protocol is the SMS Cell Broadcast service. This service is directlytaken from GSM. It uses UM RLC using the CTCH logical channel which is mappedinto the FACH transport channel. Each SMS CB message is targeted to ageographical area, and RNC maps this area into cells.

The main functions of the BMC protocol are:

Storage of Cell Broadcast messages. The BMC in RNC stores the Cell Broadcastmessages received over the CBCRNC interface for scheduled transmission.

Traffic volume monitoring and radio resource request for CBS. On the UTRANside, the BMC calculates the required transmission rate for the Cell BroadcastService based on the messages received over the CBCRNC interface, andrequests appropriate CTCH/ FACH resources from RRC.

Scheduling of BMC messages. The BMC receives scheduling information togetherwith each Cell Broadcast message over the CBCRNC interface. Based on thisscheduling information, on the UTRAN side the BMC generates schedulemessages and schedules BMC message sequences accordingly. On the UE side,the BMC evaluates the schedule messages and indicates scheduling parametersto RRC, which are used by RRC to configure the lower layers for CBSdiscontinuous reception.

Transmission of BMC messages to UE. This function transmits the BMCmessages (Scheduling and Cell Broadcast messages) according to the schedule.

Delivery of Cell Broadcast messages to the upper layer. This UE function deliversthe received non-corrupted Cell Broadcast messages to the upper layer.

When sending SMS CB messages to a cell for the first time, appropriate capacityhas to be allocated in the cell. The CTCH has to be configured and the transportchannel used has to be indicated to all UEs via (RRC) system informationbroadcast on the BCH. The capacity allocated for SMS CB is cell-specific and mayvary over time to allow efficient use of the radio resources.


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2.5.5 The Radio Resource Control Protocol RRC

The major part of the control signaling between UE and UTRAN is Radio Resource

Control messages. RRC messages carry all parameters required to set up, modifyand release Layer 2 and Layer 1 protocol entities. RRC messages carry in theirpayload also all higher layer signaling (MM, CM, SM, etc.). The mobility of userequipment in the connected mode is controlled by RRC signaling (measurements,handovers, cell updates, etc.).


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2.5.6 Radio Access Bearer

A bearer is a data stream that spans some part of the system and has a specific

quality of service (QoS). Figure below shows the most important bearers in UMTS.


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Fig 25 Bearers used in UMTS. (Adapted from 3GPP TS 23.107.)

When the mobile and the network agree to set up a data stream, the system firstimplements it using a UMTS bearer. This carries information such as voice or packetdata between the mobile termination and the far end of the core network (MSC,GMSC or GGSN). If the MT and TE are implemented as two different devices, thenanother bearer transports information between them. However, this bearer liesoutside the scope of UMTS, so we will not consider it further. The same applies to thebearer that lies beyond the far end of the core network.The UMTS bearer is associated with a number of QoS parameters. These describe

the service that the user expects to receive, using parameters such as the requireddata rate, error rate and delay.The system cannot supply this quality of service right away, because the UMTSbearer spans different interfaces that use different transport protocols. It thereforebreaks the UMTS bearer down into bearers that have a smaller scope. A CN bearerhandles the path over the core network, while a radio access bearer (RAB) handlesthe path between the mobile and its first point of contact there. In turn, the radioaccess bearer is broken down into an Iu bearer between the core network and theSRNC, and a radio bearer between the SRNC and the mobile. Each bearer is thenimplemented using the transport protocols that are appropriate for the correspondinginterface, which provide the user with the quality of service expected. On the air

interface, for example, the radio bearer is implemented using the RLC, MAC andphysical layer protocols.


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Five special radio bearers carry signaling messages between the mobile and itsserving RNC. They are known as signaling radio bearers (SRBs), and they are:

RB0 for all CCCH messages (RLC unacknowledged mode and RLC transparent

mode)

RB1 for DCCH signaling using RLC unacknowledged mode

RB2 for DCCH signaling using RLC acknowledged mode (except those carryingNAS signaling)

RB3 for DCCH signaling using RLC unacknowledged mode and carrying NASsignaling. (Optionally RB4 also)

RB5 RB31 for DCCH signaling using RLC transparent mode.

Each of them is implemented in a particular way that is appropriate for a particular

type of message. RB 0 is used to set up signaling communications between themobile and the network; the other signaling radio bearers handle all subsequentcommunications.RBs 1 and 2 carry RRC messages between the mobile and its serving RNC, themain difference between them being in the configuration of the RLC protocol. RBs 3and 4 are used to forward non-access stratum messages that begin or end in thecore network. RB 4 is optional, but if it is implemented, then RB 3 is used for highpriority messages, and RB 4 is used for low priority ones.


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RRCRRC

RLCRLC

MACMAC

Physical layerPhysical layer

Control

and

measure

ments

BMCBMCPDCPPDCP

Control plane signaling radio bearer User plane Radio bearer

RB0 RB1 RB2 RB3 RB4 RB5 RB31

1

2

3

Mapped to TRM SAP/UM SAP/AM SAP

TRM SAP UM SAP AM SAP

Physical channels

Transport channels

Logical channels

Fig 26 Bearers


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2.5.7 Access and non Access Stratum :

During the specification of the UMTS by 3GPP the stratified structure of UMTS

network was introduced. This structure which is conformal to ISO-OSI model allowsdistinguishing between independent services in the UMTS network.Then the UMTS network is divided into two levels, the Access Stratum and NonAccess stratum, these two levels correspond to a repartition of logical functions withinthe network.


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Fig 27 Access and non Access Stratum

The Access Stratum defines all the network function that are related to the Accessnetwork as an example the RRC and HO. As the UTRAN is defined as the Accessnetwork for UMTS then it is totally included in the Access Stratum. And then theAccess stratum include a part of the CN which is the Iu and the and a part of the UEfunctions (RRM)The Access stratum support, by service provision, the Non access Stratum. As anexample when a connection is established the Access stratum is responsible, after

request from the non Access stratum, to establish a signaling link and radio bearer inthe UTRAN according to a required QoS. This QoS is negotiated at the non Accessstratum level between the network and the mobile station.


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3 High Speed Downlink Packet Access

HSDPAHigh speed downlink packet access (HSDPA) is an extension of the capabilities ofUMTS included in3GPP release 5, with the target of providing higher bit rates and capacity. It is alsocalled 3.5G, with transmission rates up to 14.4 Mbps and 20 Mbps (for MIMOsystems) over a 5MHz bandwidth.HSDPA can significantly enhance downlink speeds, with average realisticthroughputs of 400700 kbps and bursts at over 1 Mbps, even in the initial stage.This dramatically improves the user experience of different applications such as web

browsing, streaming or Intranet access. Also, in combination with HSUPA, it can bethe driver for advance services like VoIP. HSDPA shares the spectrum and codesfrom WCDMA and, most of the time, only requires a software upgrade of existingUMTS R99 base stations.HSDPA offers a lower cost per bit and is mainly intended for non-real-time (NRT)traffic, but potentially allows new application areas with higher data rates and lowerdelay variances. The maximum number of UEs on HSDPA does in theory depend onthe number of available channellization codes for the associated DPCHs.The most critical parameter affecting HSDPA performance is the transmission power.Since the total power of the base station is shared with R99 DCH, a trade-offbetween HSDPA and R99 users needs to be considered. This trade-off is affected by

the strategy chosen using HSDPA, depending on whether it is introduced as a highbit rate service for top users or as a way to improve efficiency and capacity ofbackground NRT traffic.


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3.1 HSDPA performance

In terms of performance, in a 5MHzchannel HSDPA can provide maximum peakrates of up to 14.4 Mbps with 15 spreading codes and with no channel coding.However, this would mean that one unique subscriber will have to use all availablecodes on the high speed downlink share channel (HSDSCH). This is not a realisticapproach, especially for initial implementations, since typically the capacity is alsoshared with regular UMTS DCH channels. Codes allocated for HSDPA are fixed, andnot usable for DCH, so in practice implementations with 15 codes will require at leastseveral 5MHz carriers to be available in the system. It is expected that HSDPArealizations will follow a progressive approach, starting with five codes, and evolvingto 10 or 15 as higher capacity or resources to support it become available.Additionally, it is important to consider that not all UE classes will support 10 or 15

codes, so in order to get the full benefit from maximum throughputs, terminalavailability needs to be considered. A realistic data rate will be about 600800 kbps,taking real network conditions into account, while an estimated network round triptime (RTT) would be 80100 ms.As a summary, the overall performance of an HSDPA network will depend on:

The number of spreading codes (support of 5, 10 or 15 multicodes);

The modulation mode (QSPK (quadrature phase shift keying), 16-QAM(quadrature amplitude modulation)), where 16-QAM is optional for the network andalso for the UE;

The error correction level; Capabilities of end user devices.

On the other hand, the impact of HSDPA on WCDMA R99 will be driven by sharingresources and the allocation of fixed codes and constant transmission power toHSDPA. HSDPA will cause a drop in downlink, since fixed codes are fully allocated.Additionally, HSDPA may lead to quality problems and lower data rates for WCDMAR99 connections if the network RF planning is not designed to tolerate the extrainterference caused by lack of power control in the HSDPA transmission. In somecases, it may be a need to limit the amount of power for HSDPA in order to protectWCDMA R99. In such cases, the gain in HSDPA performance coming from

increasing its transmission power should be closely checked with the degradation inWCDMA R99 performance. The type of modulation (QPSK or 16-QAM) can have animportant impact in this case.


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3.2 HSDPA implementation :

Table below present basic features introduced in UMTS architecture and protocols inorder to support HSDPA. In addition, other capabilities like the multiple-input multiple-output (MIMO) receiver would be supported to provide further signal gain and higherthroughputs.HSDPA introduces a new radio bearer in the UMTS system, the high speed downlinkshare channel (HSDSCH). This channel allows several users to be time-multiplexedso that during silent periods the resources are available to other users. The HSDSCHuses 2ms transmission time intervals (TTIs) and a fixed spreading factor of 16, whichallows a maximum of 15 parallel codes for user traffic andsignaling.

Feature HSDPA

MAC layer split Functionality moved to Node B toimprove efficiency of packet schedulerand retransmissions

Downlink frame size 2ms TTI (3 slots)

Channel feedback Channel quality reported at 2ms rate(500 Hz) for CQI (channel qualityindication), ACK (acknowledged)/ACK,TPC (transmission power control)

Adaptive modulation and coding (AMC) QPSK and 16-QAM mandatory scheme

HARQ Fast layer 1 retransmission (improvesRTT); hase or incremental redundancy(IR)

Packet scheduling Fast scheduling done in Node B with2ms time basis; types: round-robin,proportional fair, fair throughput, etc.

Shared channel transmission Dynamically shared in the time and codedomains


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In addition to accelerating service access for users and improving data transfers, thisreduced TTI allows the system to adapt itself faster to changing conditions. Theuplink data transmission of the HSDPA user initially relies on release 99 DCH with

different available rates (i.e. 64, 128 or 384 kbps).A new MAC-hs entity in added on the BTS to handle all these new features neededfor HSDPA traffic, as shown in Figure B.1. Layers above MAC-hs (for the high speeddownlink shared channels), such as

MAC-d (for the dedicated transport channels) and RLC are similar to those in therelease 99 networks.

The adaptive modulation and coding (AMC) technique is used in order tocompensate for variations in radio transmission conditions, while the transmissionpower remains constant. HSDPA-enabled user equipment sends channel qualityreports to the base station at 2ms intervals, which are used to adapt the modulationor resources accordingly. At layer 1, the hybrid automatic repeat request (HARQ)with a Stop and Wait (SAW) Protocol is used as a retransmission mechanism. Unlikethe UMTS R99, the HARQ is processed directly in Node B, which allows a fasterresponse, instead of being handled by the RNC.

The fast scheduling feature is also implemented in Node B, compared to UMTS R99,where the scheduler is located in the RNC. The scheduler determines to whichterminal the transmission in HSDSCH will be directed and, depending on the AMC, atwhat data rate.


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3.3 HSDPA channels :

There are five different physical channels that are used by HSDPA services. HSDPAdata are carried on HSPDSCH, which is a shared channel for all HSDPA users in thecell. There are two physical control channels, one dedicated channel in uplink(HSDPCCH) and one shared channel in downlink (HSSCCH).In addition to these there are associated DPCHs for uplink and downlink.

HSPDSCH (high speed physical downlink shared channel). This transfers actualHSDPA data of the transport HSDSCH and can use 1 to 15 code channels, with aspreading factor (SF) of 16. All these associated physical channels should beadjacent, QPSK or 16-QAM modulation is supported over 2 ms TTI slots. Nopower control is supported. In addition, the HSDSCH does not support softhandover due to the complexity of synchronizing the transmission and scheduling

from different cells. Instead, cell reselection through a normal DCH would beimplemented; i.e. the HSDPA user is given a DCH in the SHO area, and is thenmoved to the new cell, where it would get an HSDPA channel again after theprocedure is completed.

HSSCCH (high speed shared control channel). This includes information to tell theUE how to decode the next HSPDSCH frame. It uses QPSK modulation and afixed SF of 128. It shares the downlink power with the HSPDSCH, but may supportpower control in order to maximize the available power for the data channel. Morethan one HSSCCHs are required when code multiplexing is used, but a maximumof four is supported by the UE. Soft handover is not supported.

HSDPCCH (high speed dedicated physical control channel). This channel carriesthe ACK/NACK (not acknowledged) (repetition encoded) and channel qualityindicator transmitted from the UE in the uplink direction, which is needed for L1procedures. The primary modulation is BPSK (binary phase-shift keying) with anSF of 256 (15 kbps). The transmission power used is typically the same as thatused for the uplink DPCH plus additional offset to provide higher protection. TheHSDPCCH may be received by two different sectors in the same Node B, but ingeneral soft handover is not supported.

Associated DPCH (dedicated physical channel). Two DPCHs are needed for each

HSDPA UE, one in the downlink and another in the uplink. While the downlinkDPCH is only used for signaling purposes, the uplink DPCH is the complementarydata channel for the HSPDSCH, and may be allocated a data rate of 64, 128 or384 kbps. The primary modulation is QPSK and the SF can be from 4 to 512. Softhandover is supported for both DPCHs.


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3.4 MAC Layer Split

As a result of new functionalities to be carried by Node B in HSDPA, the MAC layer issplit into two entities. While MAC-d remains in the RNC in the same way as for R99,MAC-hs is located in Node B to allow rapid retransmission of NRT data.

Fig 28 UE and RNC

MAC-d is responsible for mapping between logical channels and transport channels,selection of and appropriate transport format and handling priorities. It also has toidentify UEs in the common transport channels and multiplex/demultiplex upper layer

PDUs and to measure the traffic volume. Ciphering for the transparent mode RLC isalso managed by MAC-d. MAC-hs is responsible for packet scheduling, linkadaptation and layer 1 error correction and retransmission (HARQ).

Due to this split in the functionality of the MAC layer, the user data buffers, whichused to be in the RNC, are moved to Node B. This makes the introduction necessaryof a flow control mechanism in the Iub interface, in order to avoid the buffer overflowand throughput degradation due to buffers becoming empty. MAC-d schedules thenumber of RLC PDUs according to the credits granted by MAC-hs at each interval of10 ms, and the aggregated rate of the HSDPA connections is controlled by the ratecontrol implemented in MAC-hs. The MAC-d PDUs are framed into FP-HSDSCHframes, while a maximum number of 16 MAC-d flows per BTS are supported.


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3.5 Adaptive Modulation and Coding (AMC) Scheme :

Link adaptation (LA) is the key feature to the success of HSDPA, since there is nopower control in HSPDSCH, and it is used to adapt HSPDSCH to different radioconditions. If LA does not work properly, cell capacity is lost and other techniquessuch as fast scheduling will not work.Link adaptation is done by changing the modulation and number of codes. The UEsignals information to the network about the highest data rate it can accept under thecurrent channel conditions while still maintaining a controlled block error rate (i.e.under 10 %). This CQI (channel quality indication) is signaled through theHSDPCCH. The network uses this in order to reconfigure the HSDSCH format forsubsequent transmission to that UE. For example, if the CQI shows that the quality isdegrading, the scheduler can choose a less aggressive coding/modulation format

that will cope better with the poor conditions.Typically the link adaptation is divided into two phases, known as the inner loop andouter loop algorithms:

Inner loop algorithm. This takes the decision for the modulation and codingscheme to be used in the next TTI. This selection will be done only for newtransmissions (i.e. not for retransmissions), and will be based on the received CQI,the available HSDSCH transmit power, the number of HSPDSCH codes, the RLCPDU size, input from the outer loop HSDSCH algorithm and the UE category. It isimportant that input parameters (CQI reports and DPCH power measurements) tothe inner loop algorithm are subject to a minimum delay, because otherwise the LAwould not be able to track fading in the radio channel properly.

Outer loop algorithm. The primary goal is to compensate any bias introduced bythe inner loop algorithm. This bias might be introduced due to offsets in relative UEperformance, due to improved receiver architecture, etc. Typically, the outer loopmay be based on the BLER target obtained from RLC ACK/NACK information, butalso the CQI may be used directly.


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3.6 Error Correction (HARQ)

Layer 1 retransmissions expand the system recovery capability from air transmissionerrors, and are subject to significantly shorter delays than RLC retransmissions, dueto the closeness of UE and Node B. This result in lower delay jitter, which can bevery beneficial for data services based on TCP or streaming applications.

The use of HARQ adds increased robustness to the system and a spectral efficiencygain. Two retransmission strategies are supported: incremental Redundancy (IR) andchase combining. The basic idea of the chase combining scheme is to transmit anidentical version of an erroneously detected data packet before the decodercombined the received copies weighted by the SNR prior to decoding. With the IRscheme, additional redundant information is incrementally transmitted if the decodingfails on the first attempt, by means of different puncturing schemes used in the

coding of the retransmitted data packets. In the case of HSDPA, IR with a one-thirdpunctured turbo code would typically be used for the retransmissions, although it hasthe drawback of requiring higher memory buffers in the UE than chase combining.

Fig 29 HARQ


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During the scheduling phase, the MAC-hs layer will give priority to retransmissionsover new RLC packets, which will be transmitted with the same code as the originaltransmission. HARQ can be used in the stop-and-wait mode or in the selective repeat

mode. Stop-and-wait is simpler, but waiting for the receivers acknowledgementreduces efficiency; thus multiple stop-and-wait HARQ processes are often done in

parallel in practice. When one HARQ process is waiting for an acknowledgement,another process can use the channel to send more data.There are a few aspects to consider for the HARQ mechanism:

If Node B receives an ACK from a UE, everything is fine.

If Node B receives a NACK from a UE, it means that the packet was received, butcould not be detected properly. In this situation, Node B should retransmit usingincremental redundancy.

If Node B never receives any ACK/NACK, it should retransmit using another self-decodable rate matching scheme.


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3.7 Fast packet scheduling

The objective of the packet scheduler is to optimize the cell capacity while delivering

the minimum required service experience for all active users excluding an allowedoutage target. Outage is defined from blocking, dropping and QoS requirementsrelated to a given application.

Fig 30 Fast Packet Scheduling

The actual packet scheduling algorithm is not specified in 3GPP and there is a largedegree of freedom available to manufacturers. However, with the definition of variousQoS parameters such as discard timers and guaranteed bit rates, it is expected thatthe packet scheduler does its best to fulfill the requirements given for any user. Thisis especially significant in multivendor environments (e.g. RNC and node B fromdifferent vendors) where the QoS responsibility is distributed. The packet schedulerneeds to be flexible and adjustable by the parameters defined in 3GPP/release 5,including some indirect parameters such as complying with the power targetsspecified from the RNC.

Different approaches have been proposed for the packet scheduler, the simplestapproach is round-robin scheduling, or best effort, which performs a blind allocationof resources without using quality information. It has low complexity and allows a fair

distribution of power and code resources among users.


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On the other hand, algorithms like proportional fair scheduling, use information ofuser quality and fast fading behavior to select the most appropriate transmission turnfor each user. This scheduler has a higher complexity, but can provide 2060 %

gains in throughput compared with round-robin scheduling. The gain depends on thenumber of HSDPA users in the cell, the radio conditions and transmitted power.

UE capabilities also have an effect on scheduling. The UEs ability to receive datadepends on the UE category it supports. Category 1 and 2 UEs can receive data inevery third TTI. Categories 3, 4 and 11 UEs can receive data in every second TTI.The remaining UE categories can receive in every TTI.


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3.8 Impact on the Iub Interface

With higher throughputs in HSDPA, there is a high probability of congestion betweenNode B and the

RNC, which requires careful planning of the Iub interface. A careful flow controlmechanisms needs

to be introduced in order to avoid a strong reduction of data rates due to ATMdiscards, which would generate RLC retransmissions and TCP window reduction.

The understanding in 3GPP is that Node B is in control of the flow, so it will sendcapacity allocation messages to the RNC. Node B knows the status of the buffers inthe RNC from the capacity request, and uses this message to modify the capacity atany time, irrespective of the reported user buffer status.

Two messages are defined for Iub flow control, as shown in Figure B.5.

The HS-DSCH capacity request procedure allows the RNC to request HSDSCHcapacity by indicating the user buffer size in the RNC for a given priority level.

The HS-DSCH capacity allocation is used by Node B to allocate resources for agiven flow. It includes a number of parameters: the number of MAC-d PDUs that theRNC is allowed to transmit for the MAC-d flow (HSDSCH credits), the associatedpriority level indicated, the maximum MAC-d PDU length, the time interval duringwhich the HSDSCH credits granted may be transmitted (HSDSCH interval) and thenumber of subsequent intervals that the HSDSCH credits granted may be transmitted(HSDSCH repetition period).

There are different possible approaches for the flow control algorithm, but they needto be a trade-off between performance and implementation complexity. For instance,a simple flow control implementation may consist in sending periodic capacityallocations that either follow a round-robin approach among the UEs or is based onthe MAC-hs or has RLC buffer status. However, this kind of implementation can haveconstraints regarding the guaranteed bit rate and number of simultaneous users. On

the other hand, a more advanced flow control may take into account the buffer statusin Node B, the guaranteed bit rate, scheduling priority, the buffer status in the RNC,the air interface bit rate and the discard timer for sending the capacity allocationmessages.


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3.9 Handset Capabilities

HSDPA handsets are becoming more complex, due to the addition of several newfeatures to existing 3G devices in order to achieve maximum capability, such assupport of the 16-QAM modulation method, a roadmap for advanced receivers(equalizer, diversity), HARQ in layer 1, a faster turbo decoder, the need for increasedand faster buffer memory, etc.The UE capabilities, presented in Table B.4, are sent from the serving RNC (SRNC)to Node B when the HSDSCH MAC-d flow is established. They include amongothers:

The maximum number of bits a UE can receive within one TTI;

The maximum number of HSDSCH codes the UE can receive simultaneously;

The minimum inter-TTI arrival;

The total buffer size minus the RLC AM buffer size;

Five main parameters used to define the physical layer UE capability level (3GPPTS 25.306):

The maximum number of HSDSCH multicodes that the UE can simultaneouslyreceive; at least five multicodes must be supported in order to facilitate efficientmulticode operation;

The minimum inter-TTI interval, which defines the distance from the beginning of aTTI to the beginning of the next TTI that can be assigned to the same UE; e.g. ifthe allowed interval is 2 ms, this means that the UE can receive HSDSCH packetsevery 2 ms;

The maximum number of HSDSCH transport channel bits that can be receivedwithin a single TTI

The maximum number of soft channel bits over all the HARQ processes;

If the UE supports 16-QAM (e.g. code efficiency limitation);

Parameters are also available for specification of the L2 buffer capability

(RLC+MAC). A UE with a low number of soft channel bits will not be able tosupport IR for the highest peak data rates and its performance will thus be slightlylower than for a UE supporting a larger number of soft channel bits.


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4 Exercises

1. List the different duplex methods and their advantages and drawbacks?

FDD

2. Why we use stratification in UMTS?

3. Why cell are changing of boundaries ( breathing ) ?

4. What are the consequences of the cell breathing?

5. List different kinds of HO?

6. How we can make initial Access to the system?

7. Explain the HO mechanism

8. What is the difference between OQPSK and QPSK?

9. When the 16-QAM modulation is used ?

10. What is AMC and why we use it ?


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5 Solution

1. List the different duplex methods and their advantages and drawbacks?

FDD

Advantages: Using this method we can avoid collision between uplink anddownlink.

Drawbacks: Frequency resources are wasted

2. Why we use stratification in UMTS?

UMTS network is divided into two levels, the Access Stratum and NonAccess stratum, these two levels correspond to a repartition of logicalfunctions within the network.

3. Why cell are changing of boundaries ( breathing ) ?

As there is incoming and outgoing HO then the interference within one cellwill change and then the coverage of one cell will change

4. What are the consequences of the cell breathing?

As the cell is changing boundaries due to cell breathing we have toreconsider the HO margin during planning for the mobile station that isnear these boundaries

5. List different kinds of HO?

Softer HO

Soft HO

Hard HO

Intersystem HO

Inter-frequency HO


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6. How we can make initial Access to the system?

We send a RACH the mobile send a first preamble in that RACH and thenwait for an indication AICH from the network that a first preamble wasreceived and then it send a second preamble.

7. Explain the HO mechanism

Measurement report

Decision according to the threshold setting and algorithms for HO

Execution and allocation of resources

8. What is the difference between OQPSK and QPSK?

The difference is that there is no transition between intermediate statessymbols when we have to transmit two opposite symbols

9. When the 16-QAM modulation is used ?

HSDPA uses 16-QAM for transmitting 4 bit/ symbol and then achieving ahigher data rates

10. What is AMC and why we use it ?

AMC is the adaptative Modulation codec scheme we use it with HSDPA theprincipal is that the modulation is changed according to the air interface

radio conditions allowing then the signal more robustness

03 TM51103EN03GLA2 Air Interface

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