02- OWA200003 WCDMA Radio Interface Physical Layer

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Transcript of 02- OWA200003 WCDMA Radio Interface Physical Layer

Course code Course nameISSUE 1.0
The physical layer offers data transport services to higher layers.
The access to these services is through the use of transport channels via the MAC sub-layer.
The physical layer is expected to perform the following functions in order to provide the data transport service, for example Modulation and spreading/demodulation and despreading, Inner - loop power control etc.
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TS 25.201 Physical layer-general description
TS 25.211 Physical channels and mapping of
transport channels onto physical channels (FDD)
TS 25.212 Multiplexing and channel coding (FDD)
TS 25.213 Spreading and modulation (FDD)
TS 25.214 Physical layer procedures (FDD)
TS 25.308 UTRA High Speed Downlink Packet Access (HSDPA); Overall description; Stage 2
TR 25.877 High Speed Downlink Packet Acces (HSDPA) - Iub/Iur Protocol Aspects
TR 25.858 Physical layer aspects of UTRA High Speed Downlink Packet Access
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Upon completion of this course, you will be able to:
Outline radio interface protocol Architecture
Describe key technology of UMTS physical layer
Describe UMTS physical layer procedures
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Chapter 2 Physical Layer Key Technology
Chapter 3 Physical Layer Procedures
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UTRAN:UMTS Terrestrial Radio Access Network.
The UTRAN consists of a set of Radio Network Subsystems connected to the Core Network through the Iu interface.
A RNS consists of a Radio Network Controller and one or more NodeBs. A NodeB is connected to the RNC through the Iub interface.
Inside the UTRAN, the RNCs of the RNS can be interconnected together through the Iur. Iu(s) and Iur are logical interfaces. Iur can be conveyed over direct physical connection between RNCs or virtual networks using any suitable transport network.
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The radio interface (Uu) is layered into three protocol layers:
the physical layer (L1)
the network layer (L3).
The layer 1 supports all functions required for the transmission of bit streams on the physical medium. It is also in charge of measurements function consisting in indicating to higher 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 2 protocol is responsible for providing functions such as mapping, ciphering, retransmission and segmentation. It is made of four sublayers: MAC (Medium Access Control), RLC (Radio Link Control), PDCP (Packet Data Convergence Protocol) and BMC (Broadcast/Multicast Control).
The layer 3 is split into 2 parts: the access stratum and the non access stratum. The access stratum part is made of “RRC (Radio Resource Control)” entity and “duplication avoidance” entity. The non access stratum part is made of CC, MM parts.
Not shown on the figure are connections between RRC and all the other protocol layers (RLC, MAC, PDCP, BMC and L1), which provide local inter-layer control services.
The protocol layers are located in the UE and the peer entities are in the node B or the RNC.
Many functions are managed by the RRC layer. Here is the list of the most important:
Establishment, re-establishment, maintenance and release of an RRC connection between the UE and UTRAN: it includes an optional cell re-selection, an admission control, and a layer 2 signaling link establishment. When a RNC is in charge of a specific connection towards a UE, it acts as the Serving RNC.
Establishment, reconfiguration and release of Radio Bearers: a number of Radio Bearers can be established for a UE at the same time. These bearers are configured depending on the requested QoS. The RNC is also in charge of ensuring that the requested QoS can be met.
Assignment, reconfiguration and release of radio resources for the RRC connection: it handles the assignment of radio resources (e.g. codes, shared channels). RRC communicates with the UE to indicate new resources allocation when handovers are managed.
Paging/Notification: it broadcasts paging information from network to UEs.
Broadcasting of information provided by the non-access stratum (Core Network) or access Stratum. This corresponds to “system information” regularly repeated.
UE measurement reporting and control of the reporting: RRC indicates what to measure, when and how to report.
Outer loop power control: controls setting of the target values.
Control of ciphering: provides procedures for setting of ciphering.
The RRC layer is defined in the 25.331 specification from 3GPP.
The RLC’s main function is the transfer of data from either the user or the control plane over the Radio interface. Two different transfer modes are used: transparent and non-transparent. In non-transparent mode, 2 sub-modes are used: acknowledged or unacknowledged.
RLC provides services to upper layers:
data transfer (transparent, acknowledged and unacknowledged modes),
QoS setting: the retransmission protocol (for AM only) shall be configurable by layer 3 to provide different QoS,
notification of unrecoverable errors: RLC notifies the upper layers of errors that cannot be resolved by RLC.
The RLC functions are:
mapping between higher layer PDUs and logical channels,
ciphering: prevents unauthorized acquisition of data; performed in RLC layer for non-transparent RLC mode,
segmentation/reassembly: this function performs segmentation/reassembly of variable-length higher layer PDUs into/from smaller RLC Payload Units. The RLC size is adjustable to the actual set of transport formats (decided when service is established). Concatenation and padding may also be used,
error correction: done by retransmission (acknowledged data transfer mode only),
flow control: allows the RLC receiver to control the rate at which the peer RLC transmitting entity may send information.
MAC services include:
Data transfer: service providing unacknowledged transfer of MAC SDUs between peer MAC entities.
Reallocation of radio resources and MAC parameters: reconfiguration of MAC functions such as change of identity of UE. Requested by the RRC layer.
Reporting of measurements: local measurements such as traffic volume and quality indication are reported to the RRC layer.
The functions accomplished by the MAC sublayer are listed above. Here’s a quick explanation for some of them:
Priority handling between the data flows of one UE: since UMTS is multimedia, a user may activate several services at the same time, having possibly different profiles (priority, QoS parameters...). Priority handling consists in setting the right transport format for a high bit rate service and for a low bit rate service.
Priority handling between UEs: use for efficient spectrum resources utilization for bursty transfers on common and shared channels.
Ciphering: to prevent unauthorized acquisition of data. Performed in the MAC layer for transparent RLC mode.
Access Service Class (ACS) selection for RACH transmission: the RACH resources are divided between different ACSs in order to provide different priorities on a random access procedure.
PDCP
UMTS supports several network layer protocols providing protocol transparency for the users of the service.
Using these protocols (and new ones) shall be possible without any changes to UTRAN protocols. In order to perform this requirement, the PDCP layer has been introduced. Then, functions related to transfer of packets from higher layers shall be carried out in a transparent way by the UTRAN network entities.
PDCP shall also be responsible for implementing different kinds of optimization methods. The currently known methods are standardized IETF (Internet Engineering Task Force) header compression algorithms.
Algorithm types and their parameters are negotiated by RRC and indicated to PDCP.
Header compression and decompression are specific for each network layer protocol type.
In order to know which compression method is used, an identifier (PID: Packet Identifier) is inserted. Compression algorithms exist for TCP/IP, RTP/UDP/IP, …
Another function of PDCP is to provide numbering of PDUs. This is done if lossless SRNS relocation is required.
To accomplish this function, each PDCP-SDUs (UL and DL) is buffered and numbered. Numbering is done after header compression. SDUs are kept until information of successful transmission of PDCP-PDU has been received from RLC. PDCP sequence number ranges from 0 to 65,535.
BMC (broadcast/multicast control protocol)
The main function of BMC protocol are:
Storage of cell broadcast message. the BMC in RNC stores the cell broadcast message received over the CBC-RNC interface for scheduled transmission.
Traffic volume monitoring and radio resource request for CBS. On the UTRAN side, the BMC calculates the required transmission rate for the cell broadcast service based on the messages received over the CBC-RNC interface, and requests appropriate .CTCH/FACH resources from from RRC
Scheduling of BMC message. The BMC receives scheduling information together with each cell broadcast message over the CBC-RNC interface. Based on this scheduling information, on the UTRAN side the BMC generates schedule message and schedules BMC message sequences accordingly. On the UE side ,the BMC evaluates the schedule messages and indicates scheduling parameters to RRC, which are used by RRC to configure the lower layers for CBS discontinuous reception.
Transmission of BMC message to UE. The function transmits the BMC messages according to the schedule
Delivery of cell broadcast messages to the upper layer. This UE function delivers the received non-corrupted cell broadcast messages to the upper layer
The layer 1 (physical layer) is used to transmit information under the form of electrical signals corresponding to bits, between the network and the mobile user. This information can be voice, circuit or packet data, and network signaling.
The UMTS layer 1 offers data transport services to higher layers. The access to these services is through the use of transport channels via the MAC sublayer.
These services are provided by radio links which are established by signaling procedures. These links are managed by the layer 1 management entity. One radio link is made of one or several transport channels, and one physical channel.
The UMTS layer 1 is divided into two sublayers: the transport and the physical sublayers. All the processing (channel coding, interleaving, etc.) is done by the transport sublayer in order to provide different services and their associated QoS. The physical sublayer is responsible for the modulation, which corresponds to the association of bits (coming from the transport sublayer) to electrical signals that can be carried over the air interface. The spreading operation is also done by the physical sublayer. These sublayers are well described in chapters 6 and 7.
These two parts of layer 1 are controlled by the layer 1 management (L1M) entity. It is made of several units located in each equipment, which exchange information through the use of control channels.
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Spreading consists of 2 steps
Channelization operation, which transforms data symbols into chips. Thus increasing the bandwidth of the signal, The number of chips per data symbol is called the Spreading FactorSF.The operation is done by multiplying with OVSF code.
Scrambling operation is applied to the spreading signal .
Data bit
Chips after spreading
Spreading is applied to the physical channels. It consists of two operations. The first is the channelization operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The number of chips per data symbol is called the Spreading Factor (SF). The second operation is the scrambling operation, where a scrambling code is applied to the spread signal.
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OVSF code is used as channelization code
The channelization codes are uniquely described as Cch,SF,k, where SF is the
spreading factor of the code and k is the code number, 0 k SF-1.
The channelization codes are Orthogonal Variable Spreading Factor (OVSF) codes. They are used to preserve orthogonality between different physical channels. They also increase the clock rate to 3.84 Mcps. The OVSF codes are defined using a code tree.
In the code tree, the channelization codes are individually described by Cch,SF,k, where SF is the Spreading Factor of the code and k the code number, 0 k SF-1.
A channelization sequence modulates one user’s bit. Because the chip rate is constant, the different lengths of codes enable to have different user data rates. Low SFs are reserved for high rate services while high SFs are for low rate services.
The length of an OVSF code is an even number of chips and the number of codes (for one SF) is equal to the number of chips and to the SF value.
The generated codes within the same layer constitute a set of orthogonal codes. Furthermore, any two codes of different layers are orthogonal except when one of the two codes is a mother code of the other. For example C4,3 is not orthogonal with C1,0 and C2,1, but is orthogonal with C2,0.
Each Sector of each Base Station transmits W-CDMA Downlink Traffic Channels with up to 512 code channels.
Code tree repacking may be used to optimize the number of available codes in downlink.
Exercise: Find code Cch,8,3 and code Cch,16,15
OVSF shortage
Scrambling enables neighboring cells to use the same channelization codes. This allows the system to use a maximum of 512 OVSF codes in each cell. Notice that the use of an OVSF code forbids the use of the other codes in its branch. This reduces considerably the number of available codes especially for high rate services. This may lead to an OVSF shortage. In such a case, secondary scrambling codes may be allocated to the cells and enable the reuse of the same OVSF in the same cell.
SF = 1
SF = 2
SF = 4
Cch,1,0 = (1)
Cch,2,0 = (1,1)
Cch,2,1 = (1,-1)
Cch,4,0 =(1,1,1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,4,2 = (1,-1,1,-1)
Cch,4,3 = (1,-1,-1,1)
Scrambling code period: 10ms ,or 38400 chips.
The code used for scrambling of the uplink DPCCH/DPDCH may be of either long or short type, There are 224 long and 224 short uplink scrambling codes. Uplink scrambling codes are assigned by higher layers.
For downlink physical channels, a total of 218-1 = 262,143 scrambling codes can be generated. scrambling codes k = 0, 1, …, 8191 are used.
Uplink scrambling code
All the physical channels in the uplink are scrambled. In uplink, the scrambling code can be described as either long or short, depending on the way it was constructed. The scrambling code is always applied to one 10 ms frame. Different scrambling codes will be allocated to different mobiles.
In UMTS, Gold codes were chosen for their very low peak cross-correlation.
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A primary scrambling code and 15 secondary scrambling codes are
included in a set.
Set 0
Set 1
……
8192 scrambling codes
Downlink link scrambling code
The scrambling codes used in downlink are constructed like the long uplink scrambling codes. They are created with two 18-cell shift registers.
218-1 = 262,143 different scrambling codes can be formed using this method. However, not all of them are used. The downlink scrambling codes are divided into 512 sets, of one primary scrambling code and 15 secondary scrambling codes each.
The primary scrambling codes are scrambling codes n=16*i where i=0…511. The 15 secondary scrambling codes associated to one primary scrambling code are n=16*i + k, where k=1…15. For now 8192 scrambling codes have been defined.
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Group 0
……
512 primary scrambling codes
Each group consists of 8 primary scrambling codes
There is a total of 512 primary codes. They are further divided into 64 primary scrambling code groups of 8 primary scrambling codes each. Each cell is allocated one and only one primary scrambling code. The group of the primary scrambling code is found by the mobiles of the cell using the SCH, while the specific primary scrambling code used is given by the CPICH. The primary CCPCH and the primary CPICH channels are always scrambled with the primary scrambling code of the cell, while other channels can be scrambled by either the primary or the secondary scrambling code.
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Chapter 2 Physical Layer Key Technology
Chapter 3 Physical Layer Procedures
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Section 1 Physical Channel Structure and Functions
Section 2 Channel Mapping
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WCDMA radio interface has three kinds of channels
In terms of protocol layer, the WCDMA radio interface has three channels: Physical channel, transport channel and logical channel.
Logical channel: Carrying user services directly. According to the types of the carried services, it is divided into two types: Control channel and service channel.
Transport channel: It is the interface of radio interface layer 2 and physical layer, and is the service provided for MAC layer by the physical layer. According to whether the information transported is dedicated information for a user or common information for all users, it is divided into dedicated channel and common channel.
Physical channel: It is the ultimate embodiment of all kinds of information when they are transmitted on radio interfaces. Each kind of channel which uses dedicated carrier frequency, code (spreading code and scramble) and carrier phase (I or Q) can be regarded as a dedicated channel.
In UMTS, there are 3 types of channels:
Logical channels: each logical channel type is defined by  <what type of information > is transferred.
Transport channels: each transport channel is described by <how > and with <what characteristics > data is transmitted over the radio interface.
Physical channels: provide the real transmission resource, being in charge of the association between bits and physical symbols (electrical signals). It corresponds, in UMTS, to a frequency , a specific set of codes and phase.
As a conclusion:
Logical Channel = specification of the information global content
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As in GSM, UMTS uses the concept of logical channels.
A logical channel is characterized by the type of information that is transferred.
For example, some channels are used to transfer dedicated information, some for transfer of general control information, etc..
As in GSM, logical channels can be divided into two groups: control channels for control plane 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. That information can be speech, circuit switched data or packet switched data. The payload bits on this channel come from a higher layer application (the AMR codec for example). Control bits can be added by the RLC (protocol information) in case of a non transparent transfer. The MAC sublayer will also add a header to the RLC PDU.
Common Traffic CHannel (CTCH): a point-to-multipoint downlink channel for transfer of dedicated user information for all or a group of specified UEs. This channel is used to broadcast BMC messages. These messages can either be cell broadcast data from higher layers or schedule messages for support of Discontinuous Reception (DRX) of cell broadcast data at the UE. Cell broadcast messages are services offered by the operator, like indication of weather, traffic, location or rate information.
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Dedicated traffic channel (DTCH)
Common traffic channel (CTCH)
Broadcast control channel (BCCH)
Paging control channel (PCCH)
Dedicate control channel (DCCH)
Common control channel (CCCH)
The control channels are:
Broadcast Control CHannel (BCCH): a downlink channel that broadcasts all system information types (except type 14 that is only used in TDD). For example, system information type 3 gives the cell identity. UEs decode system information on the BCH except when in Cell_DCH mode. In that case, they can decode system information type 10 on the FACH and other important signaling is sent on a DCCH.
Paging Control CHannel (PCCH): a downlink channel that transfers paging information. It is used to reach a UE (or several UEs) in idle mode or in connected mode (Cell_PCH or URA_PCH state). The paging type 1 message is sent on the PCCH. When a UE receives a page on the PCCH in connected mode, it shall enter Cell_FACH state and make a cell update procedure.
Dedicated Control CHannel (DCCH): a point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. This channel is used for dedicated signaling after a RRC connection…