wcdma principle 20110930 b v1.0
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WCDMA RAN Fundamental
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The first generation is the analog cellular mobile communication network in
the time period from the middle of 1970s to the middle of 1980s. The mostimportant breakthrough in this period is the concept of cellular networks put
forward by the Bell Labs in the 1970s, as compared to the former mobile
communication systems. The cellular network system is based on cells to
implement frequency reuse and thus greatly enhances the system capacity.
The typical examples of the first generation mobile communication systems
are the AMPS system and the later enhanced TACS of USA, the NMT and the
others. The AMPS (Advanced Mobile Phone System) uses the 800 MHz band
of the analog cellular transmission system and it is widely applied in North
America, South America and some Circum-Pacific countries. The TACS (Total
Access Communication System) uses the 900 MHz band. It is widely applied in
Britain, Japan and some Asian countries.
The main feature of the first generation mobile communication systems is that
they use the frequency reuse technology, adopt analog modulation for voice
signals and provide an analog subscriber channel every other 30 kHz/25 kHz.
However, their defects are also obvious:
Low utilization of the frequency spectrum
Limited types of services No high-speed data services
Poor confidentiality and high vulnerability to interception and number
embezzlement
High equipment cost
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Put forward in 1985 by the ITU (International Telecommunication Union), the
3G mobile communication system was called the FPLMTS (Future Public LandMobile Telecommunication System) and was later renamed as IMT-2000
(International Mobile Telecommunication-2000). The major systems include
WCDMA, cdma2000 and UWC-136. On November 5, 1999, the 18th
conference of ITU-R TG8/1 passed the Recommended Specification of Radio
Interfaces of IMT-2000 and the TD-SCDMA technologies put forward by China
were incorporated into the IMT-2000 CDMA TDD part of the technical
specification. This showed that the work of the TG8/1 in formulating the
technical specifications of radio interfaces in 3G mobile communication
systems had basically come into an end and the development and applicationof the 3G mobile communication systems would enter a new and essential
phase.
The 3GPP is an organization that develops specifications for a 3G system
based on the UTRA radio interface and on the enhanced GSM core network.
The 3GPP2 initiative is the other major 3G standardization organization. It
promotes the CDMA2000 system, which is also based on a form of WCDMA
technology. In the world of IMT-2000, this proposal is known as IMT-MC. The
major difference between the 3GPP and the 3GPP2 approaches into the airinterface specification development is that 3GPP has specified a completely
new air interface without any constraints from the past, whereas 3GPP2 has
specified a system that is backward compatible with IS-95 systems.
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ITU has allocated 230 MHz frequency for the 3G mobile communication
system IMT-2000: 1885 ~ 2025MHz in the uplink and 2110~ 2200 MHz in thedownlink. Of them, the frequency range of 1980 MHz ~ 2010 MHz (uplink)
and that of 2170 MHz ~ 2200 MHz (downlink) are used for mobile satellite
services. As the uplink and the downlink bands are asymmetrical, the use of
dual-frequency FDD mode or the single-frequency TDD mode may be
considered. This plan was passed in WRC92 and new additional bands were
approved on the basis of the WRC-92 in the WRC2000 conference in the year
2000: 806 MHz ~ 960 MHz, 1710 MHz ~ 1885 MHz and 2500 MHz ~ 2690
MHz.
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The WCDMA system uses the following frequency spectrum (bands other than
those specified by 3GPP may also be used): Uplink 1920 MHz ~ 1980 MHz anddownlink 2110 MHz ~ 2170 MHz. Each carrier frequency has the 5M band
and the duplex spacing is 190 MHz. In America, the used frequency spectrum
is 1850 MHz ~ 1910 MHz in the uplink and 1930 MHz ~ 1990 MHz in the
downlink and the duplex spacing is 80 MHz.
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Compatible with abundant services and applications of 2G, 3G system has an
open integrated service platform to provide a wide prospect for various 3Gservices.
Features of 3G Services
3G services are inherited from 2G services. In a new architecture, new service
capabilities are generated, and more service types are available. Service
characteristics vary greatly, so each service features differently. Generally,
there are several features as follows:
Compatible backward with all the services provided by GSM.
The real-time services (conversational) such as voice service
generally have the QoS requirement.
The concept of multimedia service (streaming, interactive,
background) is introduced.
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Formulated by the European standardization organization 3GPP, the core
network evolves on the basis of GSM/GPRS and can thus be compatible withthe existing GSM/GPRS networks. It can be based on the TDM, ATM and IP
technologies to evolve towards the all-IP network architecture. Based on the
ATM technology, the UTRAN uniformly processes voice and packet services
and evolves towards the IP network architecture.
The cdma2000 system is a 3G standard put forward on the basis of the IS-95
standard. Its standardization work is currently undertaken by 3GPP2. Circuit
Switched (CS) domain is adapted from the 2G IS95 CDMA network, Packet
Switched (PS) domain is A packet network based on the Mobile IP technology.
Radio Access Network (RAN) is based on the ATM switch platform, it provides
abundant adaptation layer interfaces.
The TD-SCDMA standard is put forward by the Chinese Wireless
Telecommunication Standard (CWTS) Group and now it has been merged into
the specifications related to the WCDMA-TDD of 3GPP. The core network
evolves on the basis of GSM/GPRS. The air interface adopts the TD-SCDMA
mode.
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In mobile communication systems, GSM adopts TDMA; WCDMA, cdma2000
and TD-SCDMA adopt CDMA.
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Frequency Division Multiple Access means dividing the whole available
spectrum into many single radio channels (transmit/receive carrier pair). Eachchannel can transmit one-way voice or control information. Analog cellular
system is a typical example of FDMA structure.
Time Division Multiple Access means that the wireless carrier of one
bandwidth is divided into multiple time division channels in terms of time (or
called timeslot). Each user occupies a timeslot and receives/transmits signals
within this specified timeslot. Therefore, it is called time division multiple
access. This multiple access mode is adopted in both digital cellular system
and GSM.
CDMA is a multiple access mode implemented by Spreading Modulation.
Unlike FDMA and TDMA, both of which separate the user information in
terms of time and frequency, CDMA can transmit the information of multiple
users on a channel at the same time. The key is that every information before
transmission should be modulated by different Spreading Code to broadband
signal, then all the signals should be mixed and send. The mixed signal would
be demodulated by different Spreading Code at the different receiver.
Because all the Spreading Code is orthogonal, only the information that was
be demodulated by same Spreading Code can be reverted in mixed signal.
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In third generation mobile communication systems, WCDMA and cdma2000
adopt frequency division duplex (FDD), TD-SCDMA adopts time divisionduplex (TDD).
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WCDMA including the RAN (Radio Access Network) and the CN (Core
Network). The RAN is used to process all the radio-related functions, whilethe CN is used to process all voice calls and data connections within the UMTS
system, and implements the function of external network switching and
routing.
Logically, the CN is divided into the CS (Circuit Switched) Domain and the PS
(Packet Switched) Domain. UTRAN, CN and UE (User Equipment) together
constitute the whole UMTS system
A RNS is composed of one RNC and one or several Node Bs. The Iu interface isused between RNC and CN while the Iub interface is adopted between RNC
and Node B. Within UTRAN, RNCs connect with one another through the Iur
interface. The Iur interface can connect RNCs via the direct physical
connections among them or connect them through the transport network.
RNC is used to allocate and control the radio resources of the connected or
related Node B. However, Node B serves to convert the data flows between
the Iub interface and the Uu interface, and at the same time, it also
participates in part of radio resource management.
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The overall structure of the WCDMA network is defined in 3GPP TS 23.002.
Now, there are the following three versions: R99, R4, R5.
3GPP began to formulate 3G specifications at the end of 1998 and beginning
of 1999. As scheduled, the R99 version would be completed at the end of
1999, but in fact it was not completed until March, 2000. To guarantee the
investment benefits of operators, the CS domain of R99 version do not
fundamentally change., so as to support the smooth transition of
GSM/GPRS/3G.
After R99, the version was no longer named by the year. At the same time,the functions of R2000 are implemented by the following two phases: R4 and
R5. In the R4 network, MSC as the CS domain of the CN is divided into the
MSC Server and the MGW, at the same time, a SGW is added, and HLR can be
replaced by HSS (not explicitly specified in the specification).
In the R5 network, the end-to-end VOIP is supported and the core network
adopts plentiful new function entities, which have thus changed the original
call procedures. With IMS (IP Multimedia Subsystem), the network can use
HSS instead of HLR. In the R5 network, HSDPA (High Speed Downlink PacketAccess) is also supported, it can support high speed data service.
In the R6 network, the HSUPA is supported which can provide UL service rate
up to 5.76Mbps. And MBMS (MultiMedia Broadcast Multicast Service) is also
supported.
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The layer 1 supports all functions required for the transmission of bit
streams on the physical medium. It is also in charge of measurementsfunction consisting in indicating to higher layers, for example, Frame
Error Rate (FER), Signal to Interference Ratio (SIR), interference power
and transmit power.
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
NodeB or the RNC.
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Protocol structures in UTRAN terrestrial interfaces are designed
according to the same general protocol model. This model is shown inabove slide. The structure is based on the principle that the layers and
planes are logically independent of each other and, if needed, parts of
the protocol structure may be changed in the future while other parts
remain intact.
Horizontal Layers
The protocol structure consists of two main layers, the Radio
Network Layer (RNL)and the Transport Network Layer (TNL). All
UTRAN-related issues are visible only in the Radio Network Layer,and the Transport Network Layer represents standard transport
technology that is selected to be used for UTRAN but without
any UTRAN-specific changes.
Vertical Planes
Control Plane
The Control Plane is used for all UMTS-specific control signaling.
It includes the Application Protocol (i.e. RANAP in Iu, RNSAP in
Iur and NBAP in Iub), and the Signaling Bearer for transporting
the Application Protocol messages. The Application Protocol is
used, among other things, for setting up bearers to the UE (i.e.
the Radio Access Bearer in Iu and subsequently the Radio Link in
Iur and Iub). In the three plane structure the bearer parameters
in the Application Protocol are not directly tied to the User Plane
technology, but rather are general bearer parameters. The
Si nalin Bearer for the A lication Protocol ma or ma not be
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Protocol Structure for Iu CS
The Iu CS overall protocol structure is depicted in above slide.
The three planes in the Iu interface share a common ATM
(Asynchronous Transfer Mode) transport which is used for all
planes. The physical layer is the interface to the physical medium:
optical fiber, radio link or copper cable. The physical layer
implementation can be selected from a variety of standard off-
the-shelf transmission technologies, such as SONET, STM1, or E1.
Iu CS Control Plane Protocol Stack The Control Plane protocol stack consists of RANAP, on top of
Broadband (BB) SS7 (Signaling System #7) protocols. The
applicable layers are the Signaling Connection Control Part
(SCCP), the Message Transfer Part (MTP3-b) and SAAL-NNI
(Signaling ATM Adaptation Layer for Network to Network
Interfaces).
Iu CS Transport Network Control Plane Protocol Stack
The Transport Network Control Plane protocol stack consists of
the Signaling Protocol for setting up AAL2 connections
(Q.2630.1 and adaptation layer Q.2150.1), on top of BB SS7
protocols. The applicable BB SS7 are those described above
without the SCCP layer.
Iu CS User Plane Protocol Stack
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Protocol Structure for Iu PS
The Iu PS protocol structure is represented in above slide. Again,
a common ATM transport is applied for both User and Control
Plane. Also the physical layer is as specified for Iu CS.
Iu PS Control Plane Protocol Stack
The Control Plane protocol stack consists of RANAP, on top of
Broadband (BB) SS7 (Signaling System #7) protocols. The
applicable layers are the Signaling Connection Control Part
(SCCP), the Message Transfer Part (MTP3-b) and SAAL-NNI
(Signaling ATM Adaptation Layer for Network to Network
Interfaces).
Iu PS Transport Network Control Plane Protocol Stack
The Transport Network Control Plane is not applied to Iu PS. The
setting up of the GTP tunnel requires only an identifier for the
tunnel, and the IP addresses for both directions, and these are
already included in the RANAP RAB Assignment messages.
Iu PS User Plane Protocol Stack
In the Iu PS User Plane, multiple packet data flows are
multiplexed on one or several AAL5 PVCs. The GTP-U (User Plane
part of the GPRS Tunneling Protocol) is the multiplexing layer
that provides identities for individual packet data flow. Each
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The Iub interface is the terrestrial interface between NodeB and RNC.
The Radio Network Layer defines procedures related to the operationof the NodeB. The Transport Network Layer defines procedures for
establishing physical connections between the NodeB and the RNC.
The Iub application protocol, NodeB application part ( NBAP ) initiates
the establishment of a signaling connection over Iub . It is divided into
two essential components, CCP and NCP.
NCP is used for signaling that initiates a UE context for a dedicated UE
or signals that is not related to specific UE. Example of NBAP-Cprocedure are cell configuration , handling of common channels and
radio link setup
CCP is used for signaling relating to a specific UE context.
SAAL is an ATM Adaptation Layer that supports communication
between signaling entities over an ATM link.
The user plane Iub Frame Protocol ( FP ), defined the structure of the
frames and the basic in band control procedure for every type of
transport channel. There are DCH-FP, RACH-FP, FACH-FP, HS-DSCH FP
and PCH FP.
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Iur interface connects two RNCs. The protocol stack for the Iur is
shown in above slide.
The RNSAP protocol is the signaling protocol defined for the Iur
interface.
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Source coding can increase the transmitting efficiency.
Channel coding can make the transmission more reliable.
Spreading can increase the capability of overcoming interference.
Through the modulation, the signals will transfer to radio signals from digital
signals.
Bit, Symbol, Chip
Bit : data after source coding
Symbol: data after channel coding and interleaving
Chip: data after spreading
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AMR is compatible with current mobile communication system (GSM, IS-95,
PDC and so on), thus, it will make multi-mode terminal design easier.
The AMR codec offers the possibility to adapt the coding scheme to the radio
channel conditions. The most robust codec mode is selected in bad
propagation conditions. The codec mode providing the highest source rate is
selected in good propagation conditions.
During an AMR communication, the receiver measures the radio link quality
and must return to the transmitter either the quality measurements or the
actual codec mode the transmitter should use during the next frame. Thatexchange has to be done as fast as possible in order to better follow the
evolution of the channels quality.
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Source coding can increase the transmitting efficiency.
Channel coding can make the transmission more reliable.
Spreading can increase the capability of overcoming interference.
Scrambling can make transmission in security.
Through the modulation, the signals will transfer to radio signals from digital
signals.
Bit, Symbol, Chip
Bit : data after source coding
Symbol: data after channel coding and interleaving
Chip: data after spreading
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During the transmission, there are many interferences and fading. To
guarantee reliable transmission, system should overcome these influencethrough the channel coding which includes block coding, channel coding and
interleaving.
Block coding: The encoder adds some redundant bits to the block of bits and
the decoder uses them to determine whether an error has occurred during the
transmission. This is used to calculate Block Error Ratio (BLER) used in the
outer loop power control.
The CRC (Cyclic Redundancy Check) is used for error checking of the transportblocks at the receiving end. The CRC length that can be inserted has four
different values: 0, 8, 12, 16 and 24 bits. The more bits the CRC contains, the
lower is the probability of an undetected error in the transport block in the
receiver.
Note that certain types of block codes can also be used for error correction,
although these are not used in WCDMA.
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UTRAN employs two FEC schemes: convolutional codes and turbo codes. The
idea is to add redundancy to the transmitted bit stream, sO that occasional biterrors can be corrected in the receiving entity.
The first is convolution that is used for anti-interference. Through the
technology, many redundant bits will be inserted in original information.
When error code is caused by interference, the redundant bits can be used to
recover the original information. Convolutional codes are typically used when
the timing constraints are tight. The coded data must contain enough
redundant information to make it possible to correct some of the detected
errors without asking for repeats.
Turbo codes are found to be very efficient because they can perform close to
the theoretical limit set by the Shannons Law. Their efficiency is best with
high data rate services, but poor on low rate services. At higher bit rates,
turbo coding is more efficient than convolutional coding.
In WCDMA network, both Convolution code and Turbo code are used.
Convolution code applies to voice service while Turbo code applies to high
rate data service.
Note that both block codes and channel codes are used in the UTRAN. Theidea behind this arrangement is that the channel decoder (either a
convolutional or turbo decoder) tries to correct as many errors as possible,
and then the block decoder (CRC check) offers its judgment on whether the
resulting information is good enough to be used in the higher layers.
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Channel coding works well against random errors, but it is quite vulnerable to
bursts of errors, which are typical in mobile radio systems. The especially fastmoving UE in CDMA systems can cause consecutive errors if the power
control is not fast enough to manage the interference. Most coding schemes
perform better on random data errors than on blocks of errors. This problem
can be eased with interleaving, which spreads the erroneous bits over a
longer period of time. By interleaving, no two adjacent bits are transmitted
near to each other, and the data errors are randomized.
The longer the interleaving period, the better the protection provided by the
time diversity. However, longer interleaving increases transmission delays and
a balance must be found between the error resistance capabilities and the
delay introduced.
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Source coding can increase the transmitting efficiency.
Channel coding can make the transmission more reliable.
Spreading can increase the capability of overcoming interference.
Scrambling can make transmission in security.
Through the modulation, the signals will transfer to radio signals from digital
signals.
Bit, Symbol, Chip
Bit : data after source coding
Symbol: data after channel coding and interleaving
Chip: data after spreading
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Correlation is used to measure similarity of any two arbitrary signals. It is
computed by multiplying the two signals and then summing (integrating) theresult over a defined time windows. The two signals of figure (a) are identical
and therefore their correlation is 1 or 100 percent. In figure (b) , however, the
two signals are uncorrelated, and therefore knowing one of them does not
provide any information on the other.
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By spreading, each symbol is multiplied with all the chips in the orthogonal
sequence assigned to the user. The resulting sequence is processed and isthen transmitted over the physical channel along with other spread symbols.
In this figure, 4-digit codes are used. The product of the user symbols and the
spreading code is a sequence of digits that must be transmitted at 4 times the
rate of the original encoded binary signal.
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The receiver dispreads the chips by using the same code used in the
transmitter. Notice that under no-noise conditions, the symbols or digits arecompletely recovered without any error. In reality, the channel is not noise-
free, but CDMA system employ Forward Error Correction techniques to
combat the effects of noise and enhance the performance of the system.
When the wrong code is used for dispreading, the resulting correlation yields
an average of zero. This is a clear demonstration of the advantage of the
orthogonal property of the codes. Whether the wrong code is mistakenly used
by the target user or other users attempting to decode the received signal, the
resulting correlation is always zero because of the orthogonal property of
codes.
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Traditional radio communication systems transmit data using the minimum
bandwidth required to carry it as a narrowband signal. CDMA system mixtheir input data with a fast spreading sequence and transmit a wideband
signal. The spreading sequence is independently regenerated at the receiver
and mixed with the incoming wideband signal to recover the original data.
The dispreading gives substantial gain proportional to the bandwidth of the
spread-spectrum signal. The gain can be used to increase system performance
and range, or allow multiple coded users, or both. A digital bit stream sent
over a radio link requires a definite bandwidth to be successfully transmitted
and received.
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For common services, the bit rate of voice call is 12.2kbps, the bit rate of
video phone is 64kbps, and the highest packet service bit rate is384kbps(R99). After the spreading, the chip rate of different service all
become 3.84Mcps.
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Spreading means increasing the bandwidth of the signal beyond the
bandwidth normally required to accommodate the information. The spreadingprocess in UTRAN consists of two separate operations: channelization and
scrambling.
The first operation 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). Channelization codes are orthogonal codes, meaning that in ideal
environment they do not interfere each other.
The second operation is the scrambling operation. Scrambling is used on top
of spreading, so it does not change the signal bandwidth but only makes the
signals from different sources separable from each other. As the chip rate is
already achieved in channelization by the channelization codes, the chip rate
is not affected by the scrambling.
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Orthogonal codes are easily generated by starting with a seed of 1, repeating
the 1 horizontally and vertically, and then complementing the -1 diagonally.This process is to be continued with the newly generated block until the
desired codes with the proper length are generated. Sequences created in this
way are referred as Walsh code.
Channelization uses OVSF code, for keeping the orthogonality of different
subscriber physical channels. OVSF can be defined as the code tree illustrated
in the following diagram.
Channelization code is defined as Cch SF, k,, where, SF is the spreading factorof the code, and k is the sequence of code, 0kSF-1. Each level definition
length of code tree is SF channelization code, and the left most value of each
spreading code character is corresponding to the chip which is transmitted
earliest.
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N-42
The channelization codes are Orthogonal Variable Spreading Factor(OVSF)
codes. They are used to preserve orthogonality between different physicalchannels. They also increase the clock rate to 3.84 Mcps. The OVSF codesare 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 users bit. Because the chip rate isconstant, the different lengths of codes enable to have different user datarates. Low SFs are reserved for high rate services while high SFs are for lowrate services.
The length of an OVSF code is an even number of chips and the number ofcodes (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 orthogonalcodes. Furthermore, any two codes of different layers are orthogonal exceptwhen one of the two codes is a mother code of the other. For example C4,3 isnot orthogonal with C1,0 and C2,1, but is orthogonal with C2,0.
SF in uplink is from 4 to 256.
SF in downlink is from 4 to 512.
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N-43
For voice service (AMR), downlink SF is 128, it means there are 128 voice
services maximum can be supported in one WCDMA carrier;
For Video Phone (64k packet data) service, downlink SF is 32, it means there
are 32 voice services maximum can be supported in one WCDMA carrier.
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N-44
In addition to spreading, part of the process in the transmitter is the
scrambling operation. This is needed to separate terminals or base stationsfrom each other.
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N-45
Different scrambling codes will be planned to different cells in downlink.
Different scrambling codes will be allocated to different UEs in uplink.
The scrambling code is always applied to one 10 ms frame.
In UMTS, Gold codes are chosen for their very low peak cross-correlation.
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There are totally 512 primary scrambling codes defined by 3GPP. They are
further divided into 64 primary scrambling code groups. There are 8 primaryscrambling codes in every group. Each cell is allocated with only one primary
scrambling code.
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N-47
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N-49
Source coding can increase the transmitting efficiency.
Channel coding can make the transmission more reliable.
Spreading can increase the capability of overcoming interference.
Scrambling can make transmission in security.
Through the modulation, the signals will transfer to radio signals from digital
signals.
Bit, Symbol, Chip
Bit : data after source coding
Symbol: data after channel coding and interleaving
Chip: data after spreading
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N-50
A data-modulation scheme defines how the data bits are mixed with the
carrier signal, which is always a sine wave. There are three basic ways tomodulate a carrier signal in a digital sense: amplitude shift keying (ASK),
frequency shift keying (FSK), and phase shift keying (PSK).
In ASK the amplitude of the carrier signal is modified by the digital signal.
In FSK the frequency of the carrier signal is modified by the digital signal.
The PSK family is the most widely used modulation scheme in modern cellular
systems. There are many variants in this family, and only a few of them are
mentioned here.
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N-51
In binary phase shift keying (BPSK) modulation, each data bit is transformed
into a separate data symbol. The mapping rule is 1 >+ 1 and 0 > 1.There are only two possible phase shifts in BPSK, 0 and radians.
NRZ means none return zero.
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N-52
The quadrature phase shift keying (QPSK) modulation has four phases: 0, /2,
, and 3/2 radians. Two data bits are transformed into one complex datasymbol; A symbol is any change (keying) of the carrier.
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N-55
The UTRAN air interface uses QPSK modulation in the downlink, although
HSDPA may also employ 16 Quadrature Amplitude Modulation (16QAM).16QAM requires good radio conditions to work well. As seen, with 16QAM
also the amplitude of the signal matters.
As explained, in QPSK one symbol carries two data bits; in 16QAM each
symbol includes four bits. Thus, a QPSK system with a chip rate of 3.84Mcps
could theoretically transfer 2 3.84 = 7.68 Mbps, and a 16QAM system
could transfer 4 3.84 Mbps = 15.36 Mbps. In 3GPP also the usage of
64QAM with HSDPA has been studied.
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N-56
Source coding can increase the transmitting efficiency.
Channel coding can make the transmission more reliable.
Spreading can increase the capability of overcoming interference.
Scrambling can make transmission in security.
Through the modulation, the signals will transfer to radio signals from digital
signals.
Bit, Symbol, Chip
Bit : data after source coding
Symbol: data after channel coding and interleaving
Chip: data after spreading
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N-57
A mobile communication channel is a multi-path fading channel and any
transmitted signal reaches a receive end by means of multiple transmissionpaths, such as direct transmission, reflection, scatter, etc.
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N-58
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N-59
Furthermore, with the moving of a mobile station, the signal amplitude, delay
and phase on various transmission paths vary with time and place. Therefore,the levels of received signals are fluctuating and unstable and these multi-
path signals, if overlaid, will lead to fast fading. Fast fading conforms to
Rayleigh distribution. The mid-value field strength of fast fading has relatively
gentle change and is called slow fading. Slow fading conforms to lognormal
distribution.
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N-60
Diversity technology means that after receiving two or more input signals
with mutually uncorrelated fading at the same time, the system demodulatesthese signals and adds them up. Thus, the system can receive more useful
signals and overcome fading.
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N-61
Diversity technology is an effective way to overcome overlaid fading. Because
it can be selected in terms of frequency, time and space, diversity technologyincludes frequency diversity, time diversity and space diversity.
Time diversity: Channel coding
Frequency diversity: WCDMA is a kind of frequency diversity. The signal
energy is distributed on the whole bandwidth.
Space diversity: using two antennas
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N-62
The RAKE receiver is a technique which uses several baseband correlators to
individually process multipath signal components. The outputs from thedifferent correlators are combined to achieve improved reliability and
performance.
When WCDMA system is designed for cellular system, the inherent wide-
bandwidth signals with their orthogonal Walsh functions were natural for
implementing a RAKE receiver. In WCDMA system, the bandwidth is wider
than the coherence bandwidth of the cellular. Thus, when the multi-path
components are resolved in the receiver, the signals from different paths are
uncorrelated with each other. The receiver can then combine them using
some combining schemes. So with RAKE receiver WCDMA system can use the
multi-path characteristics of the channel to get signal with better quality.
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N-63
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N-64
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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N-66
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 User Equipment and Serving RNC.
RL: A "radio link" is a logical association between single User
Equipment and a single UTRAN access point. Its physical realization
comprises one or more radio bearer transmissions.
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N-67
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N-68
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 carried services, 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 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 dividedinto dedicated channel and common channel.
Physical channel: It is the ultimate embodiment of all kinds of
information when they are transmitted 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 a physical
channel.
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N-69
As in GSM, UMTS uses the concept of logical channels.
A logical channel is characterized by the type of information that istransferred.
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
sub-layer 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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N-71
In order to carry logical channels, several transport channels are
defined. They are: Broadcast Channel (BCH): a downlink channel used for
broadcast of system information into the entire cell.
Paging Channel (PCH): a downlink channel used for broadcast of
control information into the entire cell, such as paging.
Random Access Channel (RACH): a contention based uplink
channel used for initial access or for transmission of relatively
small amounts of data (non real-time dedicated control or traffic
data).
Forward Access Channel (FACH): a common downlink channel
used for dedicated signaling (answer to a RACH typically), or for
transmission of relatively small amounts of data.
Dedicated Channel (DCH): a channel dedicated to one UE used in
uplink or downlink.
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N-72
Now we will begin to discuss the physical channel. Physical channel is
the most important and complex channel, and a physical channel isdefined by a specific carrier frequency, code and relative phase. In
CDMA system, the different code (scrambling code 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
two types of physical channel: UL and DL.
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The different physical channels are:
Synchronization Channel (SCH): used for cell search procedure.There is the primary and the secondary SCHs.
Common Control Physical Channel (CCPCH): used to carrycommon control information such as the scrambling code usedin DL (there is a primary CCPCH and additional secondaryCCPCH).
Common Pilot Channels (P-CPICH and S-CPICH): used forcoherent detection of common channels. They indicate thephase reference.
Dedicated Physical Data Channel (DPDCH): used to carrydedicated data coming from layer 2 and above (coming fromDCH).
Dedicated Physical Control Channel (DPCCH): used to carrydedicated control information generated in layer 1 (such aspilot, TPC and TFCI bits).
Page Indicator Channel (PICH): carries indication to inform theUE that paging information is available on the S-CCPCH.
Acquisition Indicator Channel (AICH): it is used to inform a UEthat the network has received its access request.
High Speed Physical Downlink Shared Channel (HS-PDSCH): it isused to carry subscribers BE service data (mapping on HSDPA)coming from layer 2.
High Speed Shared Control Channel (HS-SCCH): it is used to carrycontrol message to HS-PDSCH such as modulation scheme, UEID etc.
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The different physical channels are:
Dedicated Physical Data Channel (DPDCH): used to carrydedicated data coming from layer 2 and above (coming from
DCH).
Dedicated Physical Control Channel (DPCCH): used to carry
dedicated control information generated in layer 1 (such as
pilot, TPC and TFCI bits).
Physical Random Access Channel (PRACH): used to carry random
access information when a UE wants to access the network.
High Speed Dedicated Physical Control Channel (HS-DPCCH): itis used to carry feedback message to HS-PDSCH such
CQI,ACK/NACK.
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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 primaryscrambling code used by that cell in order to be able to decode the
BCCH (for system information). This is done with the help of 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 is transmitted unscrambled during the first 256 chips of
each time slot, in time multiplex with 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 as the 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 time slot. P-SCH always transmits primary
synchronization code. S-SCH transmits secondary synchronization codes.
The primary synchronization code is repeated at the beginning of each
time slot. The same code is used by all the cells and enables themobiles to detect the existence of the UMTS cell and to synchronize
itself on the time slot boundaries. This is normally done with a single
matched filter or any similar device. The slot timing of the cell is
obtained by detecting peaks in the matched filter output.
This is the first step of the cell search procedure. The second step is
done using the secondary synchronization channel.
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The S-SCH also consists of a code, the Secondary Synchronization Code
(SSC) that indicates which of the 64 scrambling code groups the cellsdownlink scrambling code belongs to. 16 different SSCs are defined.
Each SSC is a 256 chip long sequence.
There is one specific SSC transmitted in each time slot, giving us a
sequence of 15 SSCs. There is a total of 64 different sequences of 15
SSCs, corresponding to the 64 primary scrambling code groups. These
64 sequences are constructed so that one sequence is different from
any other one, and different from any rotated version of any sequence.
The UE correlates the received signal with the 16 SSCs and identifies
the maximum correlation value.
The S-SCH provides the information required to find the frame
boundaries and the downlink scrambling code group (one out of 64
groups). The scrambling code (one out of 8) can be determined
afterwards by decoding the P-CPICH. The mobile will then be able to
decode the BCH.
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The Common Pilot Channel (CPICH) is a pure physical control channel
broadcasted over the entire cell. It is not linked to any transportchannel. It consists of a sequence of known bits that are transmittedin parallel with the primary and secondary CCPCH.
The PCPICH is used by the mobile to determine which of the 8 possibleprimary scrambling codes is used by the cell, and to provide the phasereference for common channels.
Finding the primary scrambling code is done during the cell searchprocedure through a symbol-by-symbol correlation with all the codeswithin the code group. After the primary scrambling code has beenidentified, the UE can decode system information on the P-CCPCH.
The P-CPICH is the phase reference for the SCH, P-CCPCH, AICH andPICH. It is broadcasted over the entire cell. The channelization codeused to spread the P-CPICH is always Cch,256,0 (all ones). Thus, the P-CPICH is a fixed rate channel. Also, it is always scrambled with theprimary scrambling code of the cell.
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The Primary Common Control Physical Channel (P-CCPCH) is a fixed
rate (SF=256) downlink physical channel used to carry the BCHtransport channel. It is broadcasted continuously 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 one fix predefined transport format
combination, and the only bits transmitted are data bits from the BCH
transport channel. It is important to note that the P-CCPCH is not
transmitted 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 time multiplexed on every time slot.
Channelization code Cch,256,1 is always used to spread the P-CCPCH.
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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 soon be transmitted on the PCH. Thus,
the mobile only decodes the S-CCPCH when it is informed to do so by
the PICH. This enables to do other processing and to save the mobiles
battery.
The PICH carries Paging Indicators (PI), which are user specific and
calculated by higher 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 always contains 300 bits (SF=256). 288 of these bits are used to
carry paging indicators, while the remaining 12 are not formally part of
the PICH and shall not be transmitted. That part 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 be multiplexed on one PICH radio
frame. Thus, the number of bits reserved for each PI depends 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 all identical. If the PI in a certain frame is 1, it is an
indication that the UE associated with that PI should read the
corresponding frame of the S-CCPCH.
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The Secondary Common Control Physical Channel (S-CCPCH) is used to
carry the FACH and PCH transport channels. Unlike the P-CCPCH, it isnot broadcasted continuously. 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 useful message on the
PCH or FACH.
A UE will expect a message on the PCH after indication from the PICH
(page indicator channel), and it will expect a message on the FACH
after it has transmitted something on the RACH.
The FACH and the PCH can be mapped on the same or on separate S-
CCPCHs. If they are mapped on the same S-CCPCH, TFCI bits have to be
sent to support multiple transport formats
The figure above shows the frame structure of the S-CCPCH. There are
18 different slot formats determining the exact number of data, pilot
and TFCI bits. The data bits correspond to the PCH and/or FACH bits
coming from the transport sub-layer. Pilot bit are typically used when
beamforming techniques are used.
The SF ranges from 4 to 256. The channelization code is assigned by
the RRC layer as is the scrambling code, and they are fixed during thecommunication. They are sent on the BCCH so that every UE can
decode the channel.
As said before, FACH can be used to carry user data. The difference
with the dedicated channel is that it cannot use fast power control, nor
soft handover. The advantage is that it is a fast access channel.
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The Physical Random Access Channel (PRACH) is used by the UE to
access the network and to carry small data packets. It carries the RACHtransport channel. The PRACH 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 part that carries the RACH message. The preamble is an
identifier which consists of 256 repetitions of a 16 chip long signature
(total of 4096 chips). There are 16 possible signatures, basically, the UE
randomly selects one of the 16 possible preambles and transmits 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 high enough and that the UE is authorized to
transmit, which it will do after acknowledgment from the network. If
the UE doesnt get a response from the network, it has to select a new
signature to transmit.
The message part is 10 or 20 ms long (split into 15 or 30 time slots)
and is made of the RACH data and the layer 1 control information.
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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 thecontrol part is always 256. The control part consists of 8 pilot bits for
channel estimation and 2 TFCI bits to indicate the transport 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 the signature that was used for the preamble (for signatures s=0 to
s=15: OVSFcontrol= Cch,256,m, where m=16s + 15; OVSFdata= Cch,SF,m, where
m=SF*s/16.
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The PRACH transmission is based on the access frame structure. Theaccess frame is access of 15 access slots and lasts 20 ms (2 radioframes).
To avoid too many collisions and to limit interference, a UE must waitat least 3 or 4 access slots between two consecutive preambles.
The PRACH resources (access slots and preamble signatures) can bedivided between different Access Service Classes (ASC) in order toprovide different priorities of RACH usage. The ASC number rangesfrom 0 (highest priority) to 7 (lowest priority).
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The Acquisition Indicator Channel (AICH) is a common downlink
channel used to control the uplink random accesses. It carries theAcquisition Indicators (AI), each corresponding to a signature on the
PRACH (uplink). When the NodeB receives the random access from a
mobile, it sends back the signature of the mobile to grant its access. If
the NodeB receives multiple signatures, it can sent all these signatures
back by adding the together. At reception, the UE can apply its
signature to check if the NodeB sent an acknowledgement (taking
advantage of the orthogonality of the signatures).
The AICH consists of a burst of data transmitted regularly every access
slot frame. One access slot frame is formed of 15 access slots, andlasts 2 radio frames (20 ms). Each access slot consists of two parts, an
acquisition indicator part of 32 real-valued symbols and a long part
during which nothing is transmitted to avoid overlapping due to
propagation delays.
s (with values 0, +1 and -1, corresponding to the answer from the
network to a specific user) and the 32 chip long sequence is
given by a predefined table. There are 16 sequences , each
corresponding to one PRACH signatures. A maximum of 16 AIs can be
sent in each access slot. The user can multiply the received multi-level
signal by the signature it used to know if its access was granted.
The SF used is always 256 and the OVSF code used by the cell is
indicated in system information type 5.
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There are two kinds of uplink dedicated physical channels, the
Dedicated Physical Data Channel (DPDCH) and the Dedicated PhysicalControl Channel (DPCCH). The DPDCH is used to carry the DCH
transport channel. The DPCCH is used to carry the physical sub-layer
control bits.
Each DPCCH time slot consists of Pilot, TFCIFBITPC
Pilot is used to help demodulation
TFCI: transport format control indicator
FBI:used for the FBTD. (feedback TX diversity)
TPC: used to transport power control command.
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On the figure above, we can see the DPDCH and DPCCH time slot
constitution. The parameter k determines the number of symbols perslot. It is related to the spreading factor (SF) of the DPDCH by this
simple equation: SF=256/2k. The DPDCH SF ranges from 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 that symbols are transmitted during all
slots for the DPDCH
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The uplink DPDCH and DPCCH are I/Q code multiplexed. But the
downlink DPDCH and DPCCH is time multiplexed. This is maindifference.
Basically, there are two types of downlink DPCH. They are
distinguished by the use or non use of the TFCI field. TFCI bits are not
used for fixed rate services or when the TFC doesnt change.
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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. The parameter k in the figure above determines
the total number of bits per time slot. It is related 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, data or signaling, as well as layer 1 control bits. Like it
was mentioned before, the payload from the DPDCH and the control
bits from the DPCCH are time multiplexed on every time slot. The
figure above shows how these two channels are multiplexed. There is
only one DPCCH in downlink for one user.
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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 several HS-PDSCH and transferred in 2ms subframe using one or
several channelization code with fixed SF=16.
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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 ofHS-SCCH starts two slot prior to 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-DSCH transmission to multiple users in parallel
(code multiplex), multiplex HS-SCCH may be needed in a cell. The
specification allows for up to four HS-SCCHs as seen from a UE point of
view .i.e. UE must be able to decode four HS-SCCH.
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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 HARQ acknowledgement is transmitted in the first
slot of the subframe while the channel quality indication is transmitted
in the rest slot.
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This page indicates how the mapping can be done between logical,
transport and physical channels. Not all physical channels arerepresented because not all physical channels 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, to DCH 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, to DCH 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.
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The purpose of the Cell Search Procedureis to give the UE the
possibility of finding a cell and of determining the downlink scramblingcode and frame synchronization of that cell. This is typically performedin 3 steps:
PSCH(Slot synchronization): The UE uses the SCHs primarysynchronization code to acquire slot synchronization to a cell.The primary synchronization code is used by the UE to detectthe existence of a cell and to synchronize the mobile on the TSboundaries.This is typically done with a single filter (or anysimilar device) matched to the primary synchronization codewhich is common to all cells. The slot timing of the cell can be
obtained by detecting peaks in the matched filter output.
SSCH (Frame synchronization and code-group identification):The secondary synchronization codes provide the informationrequired to find the frame boundaries and the group number.Each group number corresponds to a unique set of 8 primaryscrambling codes. The frame boundary and the group numberare provided indirectly by selecting a suite of 15 secondarycodes. 16 secondary codes have been defined C1, C2, .C16. 64possible suites have been defined, each suite corresponds to oneof the 64 groups. Each suite of secondary codes is composed of15 secondary codes (chosen in the set of 16), each of which willbe transmitted in one time slot. When the received codesmatches one of the possible suites, the UE has both determinedthe frame boundary and the group number.
PCPICH (Scrambling-code identification): The UE determines theexact primary scrambling code used by the found cell. Theprimary scrambling code is typically identified through symbol-by-symbol correlation over the PCPICH with all the codes within
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Physical random access procedure
1. Derive the available uplink access slots, in the next full access slot set, forthe set of available RACH sub-channels within the given ASC. Randomly select
one access slot among the ones previously determined. If there is no access
slot available in the selected set, randomly select one uplink access slot
corresponding to the set of available RACH sub-channels within the given ASC
from the next access slot set. The random function shall be such that each of
the allowed selections is chosen with equal probability
2. Randomly select a signature from the set of available signatures within the
given ASC.
3. Set the Preamble Retransmission Counter to Preamble_ Retrans_ Max
4. Set the parameter Commanded Preamble Power to Preamble_Initial_Power 5. Transmit a preamble using the selected uplink access slot, signature, and
preamble transmission power.
6. If no positive or negative acquisition indicator (AI +1 nor 1)
corresponding to the selected signature is detected in the downlink access slot
corresponding to the selected uplink access slot:
A: Select the next available access slot in the set of available RACHsub-channels within the given ASC;
B: select a signature;
C: Increase the Commanded Preamble Power;
D: Decrease the Preamble Retransmission Counter by one. If thePreamble Retransmission Counter > 0 then repeat from step 6.Otherwise exit the physical random access procedure.
7. If a negative acquisition indicator corresponding to the selected signature is
detected in the downlink access slot corresponding to the selected uplink
access slot, exit the physical random access procedure Signature
8. If a positive acquisition indicator corresponding to the selected signature is
detected , Transmit the random access message three or four uplink access
slots after the uplink access slot of the last transmitted preamble
9. exit the physical random access procedure
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Transmitter-antenna diversity can be used to generate multi-path
diversity in places where it would not otherwise exist. Multi-pathdiversity is a useful phenomenon, especially if it can be controlled. It
can protect the UE against fading and shadowing. TX diversity is
designed for downlink usage. Transmitter diversity needs two antennas,
which would be an expensive solution for the UEs.
The UTRA specifications divide the transmitter diversity modes into
two categories: (1) open-loop mode and (2) closed-loop mode. In the
open-loop mode no feedback information from the UE to the NodeB is
available. Thus the UTRAN has to determine by itself the appropriate
parameters for the TX diversity. In the closed-loop mode the UE sends
feedback information up to the NodeB in order to optimize the
transmissions from the diversity antennas.
Thus it is quite natural that the open-loop mode is used for the
common channels, as they typically do not provide an uplink return
channel for the feedback information. Even if there was a feedback
channel, the NodeB cannot really optimize its common channel
transmissions according to measurements made by one particular UE.
Common channels are common for everyone; what is good for one UE
may be bad for another. The closed-loop mode is used for dedicated
physical channels, as they have an existing uplink channel for feedback
information. Note that shared channels can also employ closed loop
power control, as they are allocated for only one user at a time, and
they also have a return channel in the uplink. There are two specified
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The TX diversity methods in the open-loop mode are
space time-block coding-based transmit-antenna diversity (STTD)
time-switched transmit diversity (TSTD).
In STTD the data to be transmitted is divided between two
transmission antennas at the base station site and transmitted
simultaneously. The channel-coded data is processed in blocks of four
bits. The bits are time reversed and complex conjugated, as shown in
above slide. The STTD method, in fact, provides two brands of diversity.
The physical separation of the antennas provides the space diversity,and the time difference derived from the bit-reversing process provides
the time diversity.
These features together make the decoding process in the receiver
more reliable. In addition to data signals, pilot signals are also
transmitted via both antennas. The normal pilot is sent via the first
antenna and the diversity pilot via the second antenna.
The two pilot sequences are orthogonal, which enables the receivingUE to extract the phase information for both antennas.
The STTD encoding is optional in the UTRAN, but its support is
mandatory for the UEs receiver.
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Time-switched transmit diversity (TSTD) can be applied to the SCH. Just
like STTD, the support of TSTD is optional in the UTRAN, butmandatory in the UE. The principle of TSTD is to transmit the
synchronization channels via the two base station antennas in turn. In
even-numbered time slots the SCHs are transmitted via antenna 1, and
in odd-numbered slots via antenna 2. This is depicted in above Figure.
Note that SCH channels only use the first 256 chips of each time slot
(i.e., one-tenth of each slot).
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The closed-loop-mode transmit diversity can only be applied to the
downlink channel if there is an associated uplink channel. Thus thismode can only be used with dedicated channels. The chief operating
principle of the closed loop mode is that the UE can control the
transmit diversity in the base station by sending adjustment commands
in FBI bits on the uplink DPCCH. The UE uses the base stations
common pilot channels to estimate the channels separately. Based on
this estimation, it generates the adjustment information and sends it to
the UTRAN to maximize the UEs received power.
There are actually two modes in the closed-loop method. In mode 1
only the phase can be adjusted; in mode 2 the amplitude is adjustable
as well as the phase. Each uplink time slot has one FBI bit for closed-
loop-diversity control. In mode 1 each bit forms a separate adjustment
command, but in mode 2 four bits are needed to compose a command.
This functions can be configured by LMT command ADD CELLSETUP.
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