UMTS Protocols and architecture

EPL657

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UMTS Network Architecture

in general the UMTS architecture consists of three major parts:

Wi l i t f th t i th di i t f Wireless interface that is the radio interface from the mobile terminals to the radio access network.R di A N t k (RAN) th t id th Radio Access Network (RAN) that provides the radio access (i.e., CDMA or WCDMA) and Radio Resource Management functionalities to mobile stations for a cellular system stations for a cellular system. Core network (CN) that is similar to any other backbone network and provides the connection to transit networks, Internet or PSTN/ISDN to transit networks, Internet or PSTN/ISDN networks. Core network may be either packet-based (GPRS) or circuit-switched (GSM).

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UMTS Network Architecture

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UMTS Network ArchitectureUTRAN is composed of several Radio Network SubsystemsUTRAN is composed of several Radio Network Subsystems

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BBSS Iur

HLR

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UE

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WWiirreelleessssIInntteerrffaaccee RRaaddiioo AAcccceessss NNeettwwoorrkk ((RRAANN))

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Every Radio Network Subsystem is composed of a Radio Network Controller (RNC) and one or more “Node Bs”.

UMTS Network ArchitectureRadio Access Network (RAN) is responsible for managing the radio resources and mobility of the users (UE). These functions are known as Radio Resource Management (RRM) and Mobility Management (MM). A core network (CN) in UMTS has the similar characteristics and QoS

i t th b kb t k M i f ti RAN requirements as any other backbone network. More information on RAN and CN can be found in the textbook.In UMTS between each of the wireless network architecture parts there are open interfaces defined:

Uu Interface is the interface that connects the mobile terminal or as it is called in Uu Interface is the interface that connects the mobile terminal or as it is called in UMTS the user equipment (UE) it connects the base station or Node B (as is called in UMTS). Currently this interface supports rates up to 2Mbps. But it is expected that this interface in the downlink will support higher data rates in range from 8-10Mbit/s. These rates are enabled by means of HSDPA (High Speed Downlink Packet Access) protocol .Iu Interface is the interface between the RAN and CN The packet switched data Iu Interface is the interface between the RAN and CN. The packet switched data is transmitted through Iu-PS interface and circuit switched data is transferred over Iu-CS interface.Iur Interface is the logical interface between the RNC in RAN. This interface enables handling of RRM (Radio Resource Management) and eliminates the burden from CN. Logically it is seen as a point to point link between RNC, but its physical g y p p p yrealisation maybe other than point-to-point.Iub Interface is the interface between the RNC and Base Stations.Wireless network supports GSM mode connections in which case the MS connect to the CN through Um interface to BSS and BSS connects through A (Gb interface in GPRS) interface to CN.

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Wireless InterfaceThe wireless interface is the same as any other cellular last hop interface that needs to be optimised for flexibility and spectrum efficiency.

But as this is an air interface the flexibility and spectrum efficiency in transmission of IP packets over the air is added to the characteristics of a standard last hop interfaces. An IP-based application expects the same network service from the i eless net o k as f om Inte net T picall the adio access the wireless network as from Internet. Typically the radio access network requires certain input parameters from the application and IP layer such that it can provide the radio access bearer for.

Based on these characteristics the QoS requirements associated with the wireless link include the associated with the wireless link include the

support for fine granularity QoS parameters, differentiation within a user flow and within a user packet, bi-directional communication, multi-casting, receiver control and local resource reservation

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Base StationThe Base Station is the gateway to the wired network for all the mobile stations in its coverage area, i.e., in a cell. It performs the functionality related to the

reception/transmission of radio signals at the wireless interface and the reception/transmission of the radio frames into the wired link. A radio frame is a short data segment coded/decoded and transmitted/received by the base station. These radio frames must be delivered on a unicast basis from RNC to base station and vice versa in a timely fashion with limited delay. Otherwise, the base station or RNC will discard them.

Base Station is actually responsible for the time synchronisation between the wireless link and the wired link h bl f ll ili i f di h ithat enables full utilisation of radio resources. Due to the time

constraints on the delivery of radio frames the majority of the traffic can be considered to be real-time traffic. This means that the traffic is very sensitive to delays and delay variation (jitt )(jitter).

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Radio Access Network (RAN)

boundaries of RAN are the radio transmission and reception access points (terminated by Node Bs) at one end and the interfaces to the gateways (e g MSC and SGSN/GGSN) which in interfaces to the gateways (e.g., MSC and SGSN/GGSN), which in turn provide connections to the fixed public network at the other end.RAN is responsible for managing the radio resources and mobility of the users These functions are known as Radio Resource of the users. These functions are known as Radio Resource Management (RRM) and Mobility Management (MM).

MM refers to functions needed to keep the UE connected to RAN considering the mobility of the user within the RAN and also the type of traffic this user is using. RRM refers to a collection of algorithms for management of radio resources. These algorithms are related to admission control and packet scheduling, different handover types, power control for the radio connections, etc.

RAN consists of a huge number of nodes Bs widely spread over a RAN consists of a huge number of nodes Bs widely spread over a large geographic network usually connected with the RNCs via expensive leased transmission lines.

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Radio Access Network (RAN)

controls a number of Node Bs including the radio channels and the connections to mobile stations mobile stations.

responsible for the allocation of transport resources within the radio access network. And in case of a WCDMA RAN, the RNC provides soft handover, , p ,combining and splitting between streams from different base stations belonging to the same mobile station.

For maximal utilisation of radio spectrum fast and For maximal utilisation of radio spectrum, fast and frequent handover operations between radio channels and radio base stations are required. Furthermore, in current RANs, the RNC is capable of supporting the initiation and management of the resource reservations for both gdirections, i.e., bi-directional, both to and from the base station, simultaneously.

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Radio Access Network (RAN)As packet delay in the RAN is most important QoS aspect, both, the RNC and Node Bs have the responsibility to transport the received packets in time to the next node:

packet transportation between BS and RNC requires strict packet transmission and receiving times. In UMTS, time alignment techniques are used in the RNC and the BS to support the precise timing between the two elements. Needs time synchronisation between Uu interface and the RANNeeds time synchronisation between Uu interface and the RAN.Besides that, another time alignment mechanism is active between the RAN (RNC) and the CN. At this part of the network, it is needed to synchronise the packet transmission times at the Iu-cs interface. The main goal of the time alignment mechanisms is to assure that the end-to end delay and delay variation are not violated in order to provide to-end delay and delay variation are not violated in order to provide the mobile user the proper QoS.

From traffic point of view, RAN contains a large number of flows. Presently, this traffic is mainly composed of voice traffic and smaller contribution of data traffic Basically traffic and smaller contribution of data traffic. Basically, the RAN traffic is heterogeneous and is composed of small volume traffic like, compressed voice packets, and signalling traffic. Because of this, the Iub and Iur connections are in general 2 Mbps

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connections are in general 2 Mbps.

core network (CN)A core network (CN) in UMTS has the similar A core network (CN) in UMTS has the similar characteristics as any other backbone network and it is responsible for the switching and control of the connections. Part of the mobility management y gis handled in the Core Network as well. Depending on the transport technology used CN may be either

circuit – switched Packet - switched.

In packet switched domain the SGSN/GGSN nodes provide amongst other things support for packet provide, amongst other things, support for packet switched services towards mobile stations, including mobility management, access control and control of packet data protocol contexts. p pIn addition, the GGSN provides interworking with external packet-switched networks such as the public Internet.

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core network (CN)In 3GPP R99 the circuit-switched domain is quite similar In 3GPP R99 the circuit-switched domain is quite similar to the current cellular networks. In 3GPP R4 there are substantial changes introduced to the circuit-switched domain mainly related to the MSC/VLR node that evolves in MSC server and Media gateway nodes and IP is used in in MSC server and Media gateway nodes and IP is used in many instances in the network. The focus in 3GPP R5 of the core network is shifted to the packet-switched domain. In 3GPP R5 the traffic is always packet-switched and basically it is expected that there will be no need for and basically it is expected that there will be no need for circuit-switched networks. A core network (CN) in UMTS has similar characteristics and QoS requirements as any other backbone network. Q q yThe amount of traffic in the Core Network is always large that requires resource management on aggregates basis. When compared to the RAN the impact of the handover procedures in the CN performance is insignificant. p p gConsequently CN has less stringent QoS requirements than RAN.

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User Plane

The User Plane of a wireless network with Packet-Domain-CN and IP-based RAN (as in later releases of

UMTS).

TCPTCP

Application Application

MAC

RLC

PDCP

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PDCP L2

IP

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IPIP

UDP

IP

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UDP/IP UDP/IP UDP/IP

UDP

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BS RNC SGSN GGSN Server

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RAN CN

User Planetwo IP levels in the protocol stack:

The end-to-end IP layer which is going from the mobile terminal (UE) up to the remote host and its usage is the same as the one of the IP layer in a usual TCP/IP modelas the one of the IP layer in a usual TCP/IP model.Edge-to-edge IP layer, which is actually the IP-based transport layer used for the transmission of the radio frames between the base stations (BS) and RNC and between the RNCs The same or a different edge-to-edge IP layer is used RNCs . The same or a different edge-to-edge IP layer is used also between the RNCs and SGSN/GGSN nodes in the core network.

In the IP-based RAN radio frames will be transmitted using IP as transport technology IP will have to provide the same transport technology. IP will have to provide the same functionality as ATM does. But this is a tough demand on IP as current IP is optimised for best effort services and these services are not good enough for time synchronisation of reception and transmission of radio frames in radio access networks. Due to this and when considering the mobility of the UEs, the edge-to-edge layer in IP-based RAN is the most important part of the wireless networks from the perspective of the resource management. Note that the mobile user is unaware of the t t t h l d i th i l t k d thi h ld

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transport technology used in the wireless network and this should remain as such also in the case of the IP-based RAN.

Air interface protocol architecture

layered into three protocol layers:

Network layer (layer 3, L3).

The data link layer (layer 2, L2);

The physical layer (layer 1, L1);

Figure 2 – Air Interface Protocol Architecture

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Figure 2 Air Interface Protocol Architecture

layersThe physical layer

interfaces the medium access control (MAC) sub layer of layer 2 and the radio resource control (RRC) layer of layer 3. The physical layer offers different transport channels to MAC. A transport channel is offers different transport channels to MAC. A transport channel is characterized by how the information is transferred over the radio interface.

Transport channels are channel coded and then mapped to the physical channels specified i h h i l l MAC ff diff l i l h l h di in the physical layer. MAC offers different logical channels to the radio link control (RLC) sub layer of layer 2.

A logical channel is characterized by the type of information transferred. Layer 2 is split into following sub layers: MAC RLC packet data convergence protocol into following sub layers: MAC, RLC, packet data convergence protocol (PDCP) and broadcast/multicast control (BMC). Layer 3 and RLC are divided into control and user planes. PDCP and BMC exist in the user plane only.

In the control plane, layer 3 p yis partitioned into sub layers where the lowest sub layer, denoted as RRC, interfaces with layer 2. The RLC sub layer provides ARQ functionality closely coupled with the radio transmission technique used.

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Logical Channels

The MAC layer provides data transfer services on logical channels. A set of logical channel types is defined for different kinds of data types is defined for different kinds of data transfer services as offered by MAC. Each logical channel type is defined by the type of information that is transferred Logical channel information that is transferred. Logical channel types are depicted in Figure. Logical channels are classified into two groups:

Control channels for the transfer of control plane information (Table 1)Traffic channels for the transfer of user plane information (Table 2 ).

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Logical Control ChannelsLogical Control ChannelsBroadcast control channel (BCCH) Downlink channel for broadcasting system control information.

Paging control channel (PCCH) Downlink channel that transfers paging information and is used when:• Network does not know the location cell of the mobile station;• The mobile station is in the cell connected state (utilizing sleep mode procedures)mode procedures).

Common control channel (CCCH) Bidirectional channel that transfers control information between network and mobile stations. This channel is used:• By the mobile stations having no RRC connection with the network;

B th bil t ti i t t h l h• By the mobile stations using common transport channels when accessing a new cell after cell reselection

Dedicated control channel (DCCH) Point-to-point bidirectional channel that transmits dedicated control information between a mobile station and the network This channel isinformation between a mobile station and the network. This channel is established through RRC connection setup procedure.

ODMA common control channel(OCCCH)

Bidirectional channel for transmitting control information between mobile stations.( )

ODMA dedicated control channel(ODCCH)

Point-to-point bidirectional channel that transmits dedicated control information between mobile stations. This channel is established through RRC connection setup procedure.

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Traffic Channels

Dedicated traffic channel (DTCH) Point-to-point channel, dedicated to one mobile station, for the transfer of user information. A DTCH can exist in both uplink and downlink. p

ODMA dedicated traffic channel Point to point channel dedicated to one mobileODMA dedicated traffic channel (ODTCH)

Point-to-point channel, dedicated to one mobile station, for the transfer of user information between mobile stations. An ODTCH exists in relay link. A point-to-multipoint unidirectional channel for transfer of dedicated user information for all or a group of specified mobile stations.

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Transport Channels

A transport channel is defined by how and with what characteristics data is transferred over the air interface. There exist two types of transport channels:

Dedicated channels;Common channels listed in Table 3Common channels, listed in Table 3.

There is one dedicated transport channel, the dedicated channel (DCH), which is a downlink or uplink transport channel. p pThe DCH is transmitted over the entire cell or over only a part of the cell using beam-forming antennas. The DCH is characterized by the possibility of fast rate change (every 10 ms) fast power control and rate change (every 10 ms), fast power control, and inherent addressing of mobile stations.

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Release ’99 WCDMA Downlink Packet Data Capabilities

Various methods for packet data transmission in WCDMA downlink already yexist in Release ’99. Three different channels in Release ’99/Release 4 WCDMA specifications that can be used for downlink packet data are

D di t d Ch l (DCH)Dedicated Channel (DCH);Downlink-shared Channel (DSCH);Forward Access Channel (FACH).Forward Access Channel (FACH).

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DCH channel

The DCH can be used basically for any type of service, and it has a fixed spreading factor (SF) in th d li k Th it th d the downlink. Thus, it reserves the code space capacity according to the peak data rate for the connection.

For example, with Adaptive Multirate (AMR) speech service and packet data, the DCH capacity reserved is equal to the sum of the highest rate used for the AMR speech and the highest rate allowed to be sent simultaneously with full rate AMR. This can be used even up to 2 Mbps, but reserving the code tree for a very high peak rate with low actual duty cycle is obviously not a very efficient use of code resources.

The DCH is power controlled and may be p yoperated in soft handover as well.

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DSCH channel

developed to operate always together with a DCH. This way, channel properties can be defined to best suit packet data needs while leaving the data with tight delay data needs, while leaving the data with tight delay budget, such as speech or video, to be carried by the DCH. The DSCH in contrast to DCH (or FACH) has a The DSCH, in contrast to DCH (or FACH), has a dynamically varying SF informed on a 10 ms frame-by-frame basis. The DSCH code resources can be shared between several The DSCH code resources can be shared between several users and the channel may employ either single code or multicode transmission.

b f t t ll d ith th i t d DCH may be fast power controlled with the associated DCH but does not support soft handover. The associated DCH can be in soft handover, for example speech is provided on DCH if present with packet data provided on DCH if present with packet data.

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FACH channel

The FACH, carried on the secondary common control physical channel (S-CCPCH) can be used for d li k k t d t ll downlink packet data as well. The FACH is operated normally on its own, and it is sent with a fixed SF and typically at rather high yp y gpower level to reach all users in the cell, owing to the lack of physical layer feedback in the uplink. There is no fast power control or soft handover There is no fast power control or soft handover for FACH. FACH cannot be used in cases in which simultaneous speech and packet data service is required.

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HSDPA Concept

key idea of the HSDPA concept is to increase packet data throughput using several techniques including link adaptation and fast physical layer (L1) retransmission adaptation and fast physical layer (L1) retransmission combining. The transport channel carrying the user data with HSDPA operation is denoted as the High speed Downlink Shared operation is denoted as the High-speed Downlink Shared Channel (HS-DSCH). The Node B estimates the channel quality of each active HSDPA user on the basis of for instance power control HSDPA user on the basis of, for instance, power control, ACK/NACK ratio, and HSDPA-specific user feedback. Scheduling and link adaptation are then conducted at a fast pace depending on the active scheduling algorithm fast pace depending on the active scheduling algorithm and the user prioritisation scheme.

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HSDPA Concept

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HSDPA Concept

With HSDPA, two of the most fundamental features of WCDMA, variable SF and fast power control, are disabled and replaced by means of adaptive modulation disabled and replaced by means of adaptive modulation and coding (AMC), extensive multicode operation and a fast and spectrally efficient retransmission strategy. The use of more robust coding fast Hybrid Automatic The use of more robust coding, fast Hybrid Automatic Repeat Request (HARQ) and multicode operation removes the need for variable SF.To allow the system to benefit from the short term To allow the system to benefit from the short-term variations, the scheduling decisions are done in the Node B. The idea in HSDPA is to enable a scheduling such that if desired most of the cell capacity may be allocated that, if desired, most of the cell capacity may be allocated to one user for a very short time, when conditions are favourable. In the optimum scenario, the scheduling is able to track the fast fading of the usersable to track the fast fading of the users.

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HSDPA Concept

The physical layer packet combining basically means that the terminal stores the received data packets in soft memory and if decoding has failed the new transmission memory and if decoding has failed, the new transmission is combined with the old one before channel decoding. The retransmission can be either identical to the first transmission or contain different bits compared with the transmission or contain different bits compared with the channel encoder output that was received during the last transmission. With this incremental redundancy strategy, one can achieve a diversity gain as well as improved one can achieve a diversity gain as well as improved decoding efficiency.

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Mapping between logical and pp g gtransport channels.

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Common Transport ChannelsBroadcast channel (BCH) Downlink transport channel that is used toBroadcast channel (BCH) Downlink transport channel that is used to

broadcast system- and cell-specific information. The BCH is always transmitted over the entire cell with a low fixed bit rate.

Forward access channel (FACH) Downlink transport channel. The FACH istransmitted over the entire cell or over only a part of the cell using

beam-forming antennas. The FACH uses slow power control.

Paging channel (PCH) Downlink transport channel. The PCH is always transmitted over the entire cell The transmission of the PCH is associated withthe entire cell. The transmission of the PCH is associated with the transmission of a physical layer signal, the paging indicator , to support efficient sleep mode procedures

Random access channel (RACH) Uplink transport channel. The RACH is always received from the i ll Th RACH i h i d b li i d i dentire cell. The RACH is characterized by a limited size data

field, a collision risk and by the use of open loop power control.

Common packet channel (CPCH)

Uplink transport channel. The CPCH is acontention-based random access channel used for transmission of (C C ) contention based random access channel used for transmission of

bursty data traffic. CPCH is associated with a dedicated channel on the downlink, which provides power control for the uplink CPCH.

Downlink shared channel Downlink transport channel shared by several mobile stations The

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Downlink shared channel (DSCH)

Downlink transport channel shared by several mobile stations The DSCH is associated with a DCH.

Physical Channels

The transport channels are channel coded and matched to the data rate offered by physical channels. Thereafter the transport channels are mapped on the Thereafter, the transport channels are mapped on the physical channels. Physical channels consist of radio frames and time slots.

The length of a radio frame is 10 ms and one frame consists of 15 time slots time slots. A time slot is a unit, which consists of fields containing bits. The number of bits per time slot depends on the physical channel.Depending on the symbol rate of the physical channel, the configuration of radio frames or time slots varies.configuration of radio frames or time slots varies.

There are two uplink dedicated physical and two common physical channels:

The uplink dedicated physical data channel (uplink DPDCH) and the uplink dedicated physical control channel DPDCH) and the uplink dedicated physical control channel (uplink DPCCH);The physical random access channel (PRACH) and physical common packet channel (PCPCH).

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Physical ChannelsThe uplink DPDCH is used to carry dedicated data generated at layer 2 and above (i.e., the dedicated transport channel (DCH)). There may be zero, one, or several uplink DPDCHs on each layer 1 connection on each layer 1 connection. The uplink DPCCH is used to carry control information generated at layer 1. Control information consists of known pilot bits to support channel estimation for coherent detection transmit power control (TPC) commands feedback detection, transmit power-control (TPC) commands, feedback information (FBI), and an optional transport-format combination indicator (TFCI). For each layer 1 connection there is only one uplink DPCCH. Th i d li k d di t d h i l h l h d There is one downlink dedicated physical channel, one shared and five common control channels:

Downlink dedicated physical channel (DPCH);Physical downlink shared channel (DSCH);Primary and secondary common pilot channels (CPICH);Primary and secondary common control physical channels (CCPCH);Synchronization channel (SCH).

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Physical ChannelsOn the DPCH, the dedicated transport channel is transmitted time multiplexed with control information generated at layer 1 (known pilot bits, power-control

d d l fcommands, and an optional transport-format combination indicator). DPCH can contain several simultaneous services when TFCI is transmitted or a fixed rate service when TFCI is TFCI is transmitted or a fixed rate service when TFCI is not transmitted. The network determines if a TFCI should be transmitted. When the total bit rate to be transmitted exceeds the maximum bit rate for a d li k h i l h l lti d t i i i downlink physical channel, multicode transmission is employed (i.e., several parallel downlink DPCHs are transmitted using the same spreading factor). In this case, the layer 1 control information is put on only the , y p yfirst downlink DPCH.

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Physical ChannelsThe physical downlink shared channel is used to carry the downlink shared channel.

shared by users based on code multiplexing. y p gAs the DSCH is always associated with a DCH, the PDSCH is always associated with a downlink DPCH. For PDSCH the spreading factors may vary from 256 to 4. If the spreading factor and other physical layer parameters can the spreading factor and other physical layer parameters can vary on a frame-by-frame basis, the TFCI shall be used to inform the mobile stations of the instantaneous parameters of PDSCH.

Common pilot channel (CPICH) is a fixed-rate Common pilot channel (CPICH) is a fixed rate (30 Kbps, SF=256) downlink physical channel that carries a predefined bit/symbol sequence. Two types of common pilot channels,

primary CPICHsecondary CPICH.

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Physical Channelsprimary CCPCH

fixed-rate (30 Kbps, SF=256) downlink physical channels used to carry the BCH. Common control yphysical channels are not inner-loop power controlled. The primary CCPCH is not transmitted during the first 256 chips of each slot. Instead, primary and secondary SCHs are transmitted during this period. g p

secondary CCPCH used to carry the FACH and PCH.

main difference between primary and secondary CCPCH is primary CCPCH has a fixed predefined ratesecondary CCPCH can support variable rate. Furthermore, a primary CCPCH is continuously transmitted over the entire cell while a secondary CCPCH is only transmitted when there is data available and may be transmitted in a narrow lobe in the same data available and may be transmitted in a narrow lobe in the same way as a dedicated physical channel (only valid for a secondary CCPCH carrying the FACH).

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Primary and Secondary CPICHPrimary CPICH Uses the same channelization code always;

• Scrambled by the primary scrambling code;• One per cell;p ;• Broadcast over the entire cell;• The primary CPICH is the phase reference for the SCH,

primary CCPCH, AICH, PICH. It is also the default phase reference for all other downlink physicalphase reference for all other downlink physical channels.

Secondary CPICH

• Zero, one, or several per cell;• May be transmitted over only a part of the cell;• A secondary CPICH may be the reference for the

secondary CCPCH and the downlink DPCH If this is thesecondary CCPCH and the downlink DPCH. If this is the case, the mobile station is informed about this by higher-layer signaling.

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Synchronisation Channel (SCH)consists of two sub channels, the primary and secondary SCH.

The primary SCH consists of a modulated code of The primary SCH consists of a modulated code of length 256 chips, the primary synchronization code (PSC), transmitted once every slot. The PSC is the same for every cell in the system. The secondary SCH consists of repeatedly The secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the secondary synchronization codes (SSC), transmitted in parallel with the primary SCH Each SSC is chosen from a set of 16 different SCH. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the secondary SCH indicates to which of the code groups the cell’s downlink scrambling code belongs.

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