fundamentos de redes móviles

8/12/2019 Fundamentos de Redes Móviles

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GSM Radio Interface

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GSM Radio Interface




WCDMA Air Interface Physical Layer N-75

The layer 1 supports all functions required for the transmission of bit streams on the

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physical medium. It is also in charge of measurements function consisting in indicating tohigher layers, for example, Frame Error Rate (FER), Signal to Interference Ratio (SIR),interference power, transmit power, … It is basically composed of a “layer 1

management” entity, a “transport channel” entity, and a “physical channel” entity.The layer 1 (physical layer) is used to transmit information under the form of electricalsignals corresponding to bits, between the network and the mobile user. Thisinformation can be voice, circuit or packet data, and network signaling.

The UMTS layer 1 offers data transport services to higher layers. The access to theseservices is through the use of transport channels via the MAC sub-layer.

These services are provided by radio links which are established by signalingprocedures. These links are managed by the layer 1 management entity . One radiolink is made of one or several transport channels, and one physical channel.

The UMTS layer 1 is divided into two sub-layers: the transport and the physical sub-layers. All the processing (channel coding, interleaving, etc.) is done by the transportsub-layer in order to provide different services and their associated QoS. The physicalsub-layer is responsible for the modulation, which corresponds to the association of bits(coming from the transport sub-layer) to electrical signals that can be carried over the airinterface. The spreading operation is also done by the physical sub-layer.

These two parts of layer 1 are controlled by the layer 1 management (L1M) entity. It ismade of several units located in each equipment, which exchange information throughthe use of control channels.

The la er 2 rotocol is res onsible for rovidin functions such as ma in ci herin




In terms of protocol layer, the WCDMA radio interface has three types of channels:


physical channel, transport channel and logical channel.

Logical channel: Carrying user services directly. According to the types of the carriedservices, it is divided into two types: control channel and service channel.

Transport channel: It is the interface between radio interface layer 2 and layer 1, and it isthe service provided for MAC layer by the physical layer. According to whether theinformation transported is dedicated information for a user or common information for allusers, it is divided into dedicated channel and common channel.

Physical channel: It is the ultimate embodiment of all kinds of information when they aretransmitted on radio interface. Each channel which uses dedicated carrier frequency,code (spreading code and scramble) and carrier phase (I or Q) can be regarded as aphysical channel.




As in GSM, UMTS uses the concept of logical channels.


A logical channel is characterized by the type of information that is transferred.As in GSM, logical channels can be divided into two groups: control channels for controlplane information and traffic channel for user plane information.

The traffic channels are:Dedicated Traffic Channel (DTCH): a point-to-point bi-directional channel,that transmits dedicated user information between a UE and the network. Thatinformation can be speech, circuit switched data or packet switched data. Thepayload bits on this channel come from a higher layer application (the AMRcodec for example). Control bits can be added by the RLC (protocol information)in case of a non transparent transfer. The MAC sub-layer will also add a headerto the RLC PDU.

Common Traffic Channel (CTCH): a point-to-multipoint downlink channel fortransfer of dedicated user information for all or a group of specified UEs. Thischannel is used to broadcast BMC messages. These messages can either becell broadcast data from higher layers or schedule messages for support ofDiscontinuous Reception (DRX) of cell broadcast data at the UE. Cell broadcastmessages are services offered by the operator, like indication of weather, traffic,location or rate information.

The control channels are:

Broadcast Control Channel (BCCH): a downlink channel that broadcasts allsystem information types (except type 14 that is only used in TDD). For example,system information type 3 gives the cell identity. UEs decode system informationon the BCH except when in Cell_DCH mode. In that case, they can decode




In order to carry logical channels, several transport channels are defined. They are:


Broadcast Channel (BCH): a downlink channel used for broadcast of systeminformation into the entire cell.Paging Channel (PCH): a downlink channel used for broadcast of controlinformation into the entire cell, such as paging.Random Access Channel (RACH): a contention based uplink channel used forinitial access or for transmission of relatively small amounts of data (non real-timededicated control or traffic data).Forward Access Channel (FACH): a common downlink channel used fordedicated signaling (answer to a RACH typically), or for transmission of relativelysmall amounts of data.Dedicated Channel (DCH): a channel dedicated to one UE used in uplink ordownlink .




Now we will begin to discuss the physical channel. Physical channel is the most


important and complex channel, and a physical channel is defined by a specific carrierfrequency, code and relative phase. In CDMA system, the different code (scramblingcode or spreading code) can distinguish the channel. Most channels consist of radio

frames and time slots, and each radio frame consists of 15 time slots. There are twotypes of physical channel: UL and DL.




The different physical channels are:


Synchronization Channel (SCH): used for cell search procedure. There is theprimary and the secondary SCHs.Common Control Physical Channel (CCPCH): used to carry common controlinformation such as the scrambling code used in DL (there is a primary CCPCH

and additional secondary CCPCH).Common Pilot Channels (P-CPICH and S-CPICH): used for coherent detectionof common channels. They indicate the phase reference.Dedicated Physical Data Channel (DPDCH): used to carry dedicated datacoming from layer 2 and above (coming from DCH).Dedicated Physical Control Channel (DPCCH): used to carry dedicated controlinformation generated in layer 1 (such as pilot, TPC and TFCI bits).Page Indicator Channel (PICH): carries indication to inform the UE that paginginformation is available on the S-CCPCH.Acquisition Indicator Channel (AICH): it is used to inform a UE that thenetwork has received its access request.High Speed Physical Downlink Shared Channel (HS-PDSCH): it is used tocarry subscribers BE service data (mapping on HSDPA) coming from layer 2.High Speed Shared Control Channel (HS-SCCH): it is used to carry controlmessage to HS-PDSCH such as modulation scheme, UE ID etc.




The different physical channels are:


Dedicated Physical Data Channel (DPDCH): used to carry dedicated datacoming from layer 2 and above (coming from DCH).Dedicated Physical Control Channel (DPCCH): used to carry dedicated controlinformation generated in layer 1 (such as pilot, TPC and TFCI bits).Physical Random Access Channel (PRACH): used to carry random accessinformation when a UE wants to access the network.High Speed Dedicated Physical Control Channel (HS-DPCCH): it is used tocarry feedback message to HS-PDSCH such CQI,ACK/NACK.




When a UE is turned on, the first thing it does is to scan the UMTS spectrum and find a


UMTS cell. After that, it has to find the primary scrambling code used by that cell inorder to be able to decode the BCCH (for system information). This is done with the helpof the Synchronization Channel.

Each cell of a NodeB has its own SCH timing, so that there is no overlapping.

The SCH is a pure downlink physical channel broadcasted over the entire cell. It istransmitted unscrambled during the first 256 chips of each time slot, in time multiplexwith the P-CCPCH. It is the only channel that is not spread over the entire radio frame.The SCH provides the primary scrambling code group (one out of 64 groups), as well asthe radio frame and time slot synchronization.

The SCH consists of two sub-channels, the primary and secondary SCH. These sub-channels are sent in parallel using code division during the first 256 chips of each timeslot. P-SCH always transmits primary synchronization code. S-SCH transmits secondarysynchronization codes.

The primary synchronization code is repeated at the beginning of each time slot. Thesame code is used by all the cells and enables the mobiles to detect the existence of theUMTS cell and to synchronize itself on the time slot boundaries. This is normally donewith a single matched filter or any similar device. The slot timing of the cell is obtainedby detecting peaks in the matched filter output.

This is the first step of the cell search procedure. The second step is done using thesecondary synchronization channel.




The Common Pilot Channel (CPICH) is a pure physical control channel broadcasted


over the entire cell. It is not linked to any transport channel. It consists of a sequence ofknown bits that are transmitted in parallel with the primary and secondary CCPCH.The PCPICH is used by the mobile to determine which of the 8 possible primaryscrambling codes is used by the cell, and to provide the phase reference for common

channels.Finding the primary scrambling code is done during the cell search procedure through asymbol-by-symbol correlation with all the codes within the code group. After the primaryscrambling code has been identified, the UE can decode system information on the P-CCPCH.The P-CPICH is the phase reference for the SCH, P-CCPCH, AICH and PICH. It isbroadcasted over the entire cell. The channelization code used to spread the P-CPICHis always Cch,256,0 (all ones). Thus, the P-CPICH is a fixed rate channel. Also, it isalways scrambled with the primary scrambling code of the cell.




The Primary Common Control Physical Channel (P-CCPCH) is a fixed rate (SF=256)


downlink physical channel used to carry the BCH transport channel. It is broadcastedcontinuously over the entire cell like the P-CPICH.

The figure above shows the frame structure of the P-CCPCH. The frame structure is

special because it does not contain any layer 1 control bits. The P-CCPCH only has onefix predefined transport format combination, and the only bits transmitted are data bitsfrom the BCH transport channel. It is important to note that the P-CCPCH is nottransmitted during the first 256 chips of the slot. In fact, another physical channel (SCH)is transmitted during that period of time. Thus, the SCH and the P-CCPCH are timemultiplexed on every time slot.

Channelization code Cch,256,1 is always used to spread the P-CCPCH.




The Page Indicator Channel (PICH) is a fixed rate (30kbps, SF=256) physical channel


used by the NodeB to inform a UE (or a group of UEs) that a paging information will soonbe transmitted on the PCH. Thus, the mobile only decodes the S-CCPCH when it isinformed to do so by the PICH. This enables to do other processing and to save themobiles’ battery.

The PICH carries Paging Indicators (PI), which are user specific and calculated byhigher layers. It is always associated with the S-CCPCH to which the PCH is mapped.

The frame structure of the PICH is illustrated above. It is 10 ms long, and alwayscontains 300 bits (SF=256). 288 of these bits are used to carry paging indicators, whilethe remaining 12 are not formally part of the PICH and shall not be transmitted. Thatpart of the frame (last 12 bits) is reserved for possible future use.

In order not to waste radio resources, several PIs are multiplexed in time on the PICH.Depending on the configuration of the cell, 18, 36, 72 or 144 paging indicators can bemultiplexed on one PICH radio frame. Thus, the number of bits reserved for each PIdepends of the number of PIs per radio frame. For example, if there is 72 PIs in oneradio frame, there will be 4 (288/72) consecutive bits for each PI. These bits are allidentical. If the PI in a certain frame is “1”, it is an indication that the UE associated withthat PI should read the corresponding frame of the S-CCPCH.




The Secondary Common Control Physical Channel (S-CCPCH) is used to carry the


FACH and PCH transport channels. Unlike the P-CCPCH, it is not broadcastedcontinuously. It is only transmitted when there is a PCH or FACH information to transmit.At the mobile side, the mobile only decodes the S-CCPCH when it expects a usefulmessage on the PCH or FACH.

A UE will expect a message on the PCH after indication from the PICH (page indicatorchannel), and it will expect a message on the FACH after it has transmitted somethingon the RACH.

The FACH and the PCH can be mapped on the same or on separate S-CCPCHs. If theyare mapped on the same S-CCPCH, TFCI bits have to be sent to support multipletransport formats

The figure above shows the frame structure of the S-CCPCH. There are 18 different slotformats determining the exact number of data, pilot and TFCI bits. The data bitscorrespond to the PCH and/or FACH bits coming from the transport sub-layer. Pilot bitare typically used when beamforming techniques are used.

The SF ranges from 4 to 256. The channelization code is assigned by the RRC layer asis the scrambling code, and they are fixed during the communication. They are sent onthe BCCH so that every UE can decode the channel.

As said before, FACH can be used to carry user data. The difference with the dedicatedchannel is that it cannot use fast power control, nor soft handover. The advantage is thatit is a fast access channel.




The Physical Random Access Channel (PRACH) is used by the UE to access the


network and to carry small data packets. It carries the RACH transport channel. ThePRACH is an open loop power control channel, with contention resolution mechanisms(ALOHA approach) to enable a random access from several users.

The PRACH is composed of two different parts: the preamble part and the message partthat carries the RACH message. The preamble is an identifier which consists of 256repetitions of a 16 chip long signature (total of 4096 chips). There are 16 possiblesignatures, basically, the UE randomly selects one of the 16 possible preambles andtransmits it at increasing power until it gets a response from the network (on the AICH).That preamble is scrambled before being sent. That is a sign that the power level is highenough and that the UE is authorized to transmit, which it will do after acknowledgmentfrom the network. If the UE doesn’t get a response from the network, it has to select anew signature to transmit.

The message part is 10 or 20 ms long (split into 15 or 30 time slots) and is made of theRACH data and the layer 1 control information.




The data and control bits of the message part are processed in parallel. The SF of the


data part can be 32, 64, 128 or 256 while the SF of the control part is always 256. Thecontrol part consists of 8 pilot bits for channel estimation and 2 TFCI bits to indicate thetransport format of the RACH (transport channel), for a total of 10 bits per slot.

The OVSF codes to use (one for RACH data and one for control) depend on thesignature that was used for the preamble (for signatures s=0 to s=15: OVSF control =Cch,256,m , where m=16s + 15; OVSF data = C ch,SF,m , where m=SF*s/16.




The PRACH transmission is based on the access frame structure. The access frame is


.To avoid too many collisions and to limit interference, a UE must wait at least 3 or 4access slots between two consecutive preambles.The PRACH resources (access slots and preamble signatures) can be divided betweendifferent Access Service Classes (ASC) in order to provide different priorities of RACHusage. The ASC number ranges from 0 (highest priority) to 7 (lowest priority).




The Acquisition Indicator Channel (AICH) is a common downlink channel used to control


the uplink random accesses. It carries the Acquisition Indicators (AI), eachcorresponding to a signature on the PRACH (uplink). When the NodeB receives therandom access from a mobile, it sends back the signature of the mobile to grant itsaccess. If the NodeB receives multiple signatures, it can sent all these signatures backby adding the together. At reception, the UE can apply its signature to check if theNodeB sent an acknowledgement (taking advantage of the orthogonality of thesignatures).

The AICH consists of a burst of data transmitted regularly every access slot frame. Oneaccess slot frame is formed of 15 access slots, and lasts 2 radio frames (20 ms). Eachaccess slot consists of two parts, an acquisition indicator part of 32 real-valued symbolsand a long part during which nothing is transmitted to avoid overlapping due topropagation delays.

The SF used is always 256 and the OVSF code used by the cell is indicated in systeminformation type 5.




There are two kinds of uplink dedicated physical channels, the Dedicated Physical


Data Channel (DPDCH) and the Dedicated Physical Control Channel (DPCCH). TheDPDCH is used to carry the DCH transport channel. The DPCCH is used to carry thephysical sub-layer control bits.

Each DPCCH time slot consists of Pilot, TFCI FBI TPCPilot is used to help demodulation

TFCI: transport format combination indicator

FBI:used for the FBTD. (feedback TX diversity)

TPC: used to transport power control command.




On the figure above, we can see the DPDCH and DPCCH time slot constitution. The


parameter k determines the number of symbols per slot. It is related to the spreadingfactor (SF) of the DPDCH by this simple equation: SF=256/2 k. The DPDCH SF rangesfrom 4 to 256. The SF for the uplink DPCCH is always 256, which gives us 10 bits per

slot. The exact number of pilot, TFCI, TPC and FBI bits is configured by higher layers.This configuration is chosen from 12 possible slot formats. It is important to note thatsymbols are transmitted during all slots for the DPDCH




The uplink DPDCH and DPCCH are I/Q code multiplexed. But the downlink DPDCH and


DPCCH is time multiplexed. This is main difference.

Basically, there are two types of downlink DPCH. They are distinguished by the use ornon use of the TFCI field. TFCI bits are not used for fixed rate services or when the TFC

doesn’t change.




We have known that the uplink DPDCH and DPCCH are I/Q code multiplexed. But the


downlink DPDCH and DPCCH is time multiplexed. This is main difference. Theparameter k in the figure above determines the total number of bits per time slot. It isrelated to the SF, which ranges from 4 to 512. The chips of one slot is also 2560.

Downlink physical channels are used to carry user specific information like speech, dataor signaling, as well as layer 1 control bits. Like it was mentioned before, the payloadfrom the DPDCH and the control bits from the DPCCH are time multiplexed on everytime slot. The figure above shows how these two channels are multiplexed. There isonly one DPCCH in downlink for one user.




HS-PDSCH is a downlink physical channel that carries user data and layer 2 overhead


bits mapped from the transport channel: HS-DSCH.

The user data and layer 2 overhead bits from HS-DSCH is mapped onto one or severalHS-PDSCH and transferred in 2ms subframe using one or several channelization code

with fixed SF=16.




HS-SCCH uses a SF=128 and has q time structure based on a sub-frame of length 2 ms,


i.e. the same length as the HS-DSCH TTI. The timing of HS-SCCH starts two slot priorto the start of the HS-PDSCH subframe.

The following information is carried on the HS-SCCH (7 items)

Modulation scheme(1bit) QPSK or 16QAM

Channelization code set (7bits)

Transport block size ( 6bits)

HARQ process number (3bits)

Redundancy version (3bits)

New Data Indicator (1bit)

UE identity (16 bits)

In each 2 ms interval corresponding to one HS-DSCH TTI , one HS-SCCH carries

physical-layer signalling to a single UE. As there should be a possibility for HS-DSCHtransmission to multiple users in parallel (code multiplex), multiplex HS-SCCH may beneeded in a cell. The specification allows for up to four HS-SCCHs as seen from a UEpoint of view .i.e. UE must be able to decode four HS-SCCH.




The uplink HS-DPCCH consists of:


Acknowledgements for HARQ

Channel Quality Indicator (CQI)

As the HS-DPCCH uses SF=256, there are a total of 30 channel bits per 2 ms sub frame(3 time slot). The HS-DPCCH information is divided in such a way that the HARQacknowledgement is transmitted in the first slot of the subframe while the channel qualityindication is transmitted in the rest slot.




This page indicates how the mapping can be done between logical, transport and


physical channels. Not all physical channels are represented because not all physicalchannels correspond to a transport channel.

The mapping between logical channels and transport channels is done by the MAC sub-

layer.Different connections can be made between logical and transport channels:

BCCH is connected to BCH and may also be connected to FACH;

DTCH can be connected to either RACH and FACH, to RACH and DSCH, toDCH and DSCH, to a DCH or a CPCH;

CTCH is connected to FACH;

DCCH can be connected to either RACH and FACH, to RACH and DSCH, toDCH and DSCH, to a DCH or a CPCH;

PCCH is connected to PCH;CCCH is connected to RACH and FACH.

These connections depend on the type of information on the logical channels.



Signaling Analysis of Typical UTRAN Procedures N-104

By RRC signaling, RNC controls UE. So the RRC is the most important signaling in


UTRAN. The access success rate , handover success rate are all calculated based onthe RRC signaling statistic. RRC signaling is very useful to analyze the fault.

IUB interface signaling is called NBAP signaling, which can be used for locating NodeB’s

faults.By analyze RANAP signaling between RNC and CN, we can locate a fault in CN or inUTRAN, especially for RAB assignment ,call drop, inter-RAT relocation and pagingprocedure, RANAP signaling is very useful.

By analysis of RNSAP signaling between two RNC, we can trace inter-RNC handoverprocedure.

Most of the procedures are realized by cooperation of several interfaces.




The logical structure of system information is like a tree.


By the index in MIB and SB, UE can read the detail contents in SIB




MIB: master information block

PLMN ID


PLMN is broadcasted in MIB, that is MCC and MNC of the network, which is used for UEPLMN selection

Besides that, MIB includes scheduling information of SB and SIB, that is the index, bywhich UE can acquire the location of SB and SIB, so as to read the detail information inSIB.

Scheduling informationforSB and SIB




IU Interface paging signaling

Source of the paging

UE IMSI


Paging signaling on IU interface include:

Type of the domain (CS or PS)

UE ID IMSI is mandatory, but TMSI/PTMSI is optional

LAC/RAC

Paging cause

LAC

Paging cause



Course Name P-112

By setting up RRC connection with UTRAN, UE enters connected mode. This procedure


is triggered by UE sending “RRC CONNECTION REQUEST” on RACH.

Based on the cause value in “RRC CONNECTION REQUEST” signaling, RNC decidesthe channel type for RRC connection. The channel for RRC connection could be DCH

(DCH3.4k ,DCH6.8k ,DCH13.6k ,DCH27.2k), CCH (RACH/FACH), and even HSPAchannel.

For RRC Setup on DCH or HSPA, RNC must configure NodeB to prepare RL firstly, afterthat, by “RRC CONNECTION SETUP” signaling, RNC configures UE

UE sends “RRC CONNECTION SETUP COMPLETE” as a response, in this signaling,UE reports the capability of itself.



Course Name P-114

NAS Procedure


NAS means “non access stratum”.

After RRC connection setup, UE transmits “Initial UE message” to CN to triggerNAS signaling procedure.

Via NAS signaling, UE can complet location registration (IMSI ATTACH), GPRSATTACH, PDP activation, authentication, security mode procedures, and so on.

“RAB Assignment” is after the NAS procedures.

Most of the NAS signalings are transparent messages in UTRAN, so they arecalled “directed transfer” messages.




This signalings are called “direct transfer” message, because UTRAN does not process


these messages, just repeats to CN in uplink or to UE in downlink.




RAB: The service that the access stratum provides to the non-access stratum for


transfer of user data between User Equipment and CN.

RB: The service provided by the layer 2 for transfer of user data between UserEquipment and Serving RNC.

RL: A "radio link" is a logical association between single User Equipment and asingle UTRAN access point. Its physical realization comprises one or more radiobearer transmissions.



Course Name P-118

RAB Setup Procedure


By “RAB Assignment” procedure, the service bearer is setup. CS “RAB Assignment”procedure is after the “call proceeding” DT message, while PS “RAB Assignment”procedure is after the “PDP Context Activation” DT message.

In this example, after UE’s “call setup” DT message, CN sends “RAB ASSIGNMENTREQUEST” to RNC ,informs the QoS of the service and priority of the UE to RNC.

RNC will make a decision based on the required QoS and start the radio bearer setupprocedure.

By NBAP , RNC requests NodeB reconfigure RL (Question: Why it is Reconfigure, notsetup), and send the relative configuration to UE by “RB Setup” signaling.

UE sends a ”RB SETUP COMPLETE” signaling to RNC as a response.RNC then sends a



GSM Radio Interface




WCDMA UTRAN Interface and Signaling Procedure N-120

UE to UE CS Call Process (1)







Ass: Assignment CM: Call Management Cmd: CommandCmp: Complete DL: Downlink DT: Direct TransferRel: Release Req: Request RL: Radio LinkRsp: Response Prep: Prepare Recfg: ReconfigurationUL: Uplink



GSM Radio Interface


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