owa010010 wcdma ran overview issue 1.12 [

8/2/2019 Owa010010 Wcdma Ran Overview Issue 1.12 [

http://slidepdf.com/reader/full/owa010010-wcdma-ran-overview-issue-112- 1/65

WCDMA RAN Fundamental N-0

Confidential Information of Huawei. No Spreading Without Permission




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 most important

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




To solve these fundamental technical defects of the analog systems, the digital

mobile communication technologies emerged and the second generation mobile

communication systems represented by GSM and IS-95 came into being in the

middle of 1980s. The typical examples of the second generation cellular mobile

communication systems are the DAMPS of USA, the IS-95 and the European GSM

system.

The GSM (Global System for Mobile Communications) is originated from Europe.

Designed as the TDMA standard for mobile digital cellular communications, it

supports the 64 kbps data rate and can interconnect with the ISDN. It uses the

900 MHz band while the DCS1800 system uses the 1800 MHz band. The GSM

system uses the FDD and TDMA modes and each carrier supports eight channels

with the signal bandwidth of 200 kHz.

The DAMPS (Digital Advanced Mobile Phone System) is also called the IS-54

(North America Digital Cellular System). Using the 800 MHz bandwidth, it is the

earlier of the two North America digital cellular standards and specifies the use of

the TDMA mode.

The IS-95 standard is another digital cellular standard of North America. Using

the 800 MHz or 1900 MHz band, it specifies the use of the CDMA mode and has

already become the first choice among the technologies of American PCS


.

Since the 2G mobile communication systems focus on the transmission of voiceand low-speed data services, the 2.5G mobile communication systems emerged in

1996 to address the medium-rate data transmission needs. These systems include

GPRS and IS-95B.

The CDMA system has a very large capacity that is equivalent to ten or even

twenty times that of the analog systems. But the narrowband CDMA technologies

come into maturity at a time later than the GSM technologies, their application

far lags behind the GSM ones and currently they have only found large-scale

commercial applications in North America, Korea and China. The major services of

mobile communications are currently still voice services and low-speed dataservices.

With the development of networks, data and multimedia communications have

also witnessed rapid development; therefore, the target of the 3G mobile

communication is to implement broadband multimedia communication.

The 3G mobile communication systems are a kind of communication system that

can provide multiple kinds of high quality multimedia services and implement

global seamless coverage and global roaming. They are compatible with the fixed

networks and can implement any kind of communication at any time and any

place with portable terminals.




Put forward in 1985 by the ITU (International Telecommunication Union), the 3G


mobile communication system was called the FPLMTS (Future Public Land Mobile

Telecommunication System) and was later renamed as IMT-2000 (InternationalMobile 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

application of 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 air

interface 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.




ITU has allocated 230 MHz frequency for the 3G mobile communication system


IMT-2000: 1885 ~ 2025MHz in the uplink and 2110~ 2200 MHz in the downlink.

Of them, the frequency range of 1980 MHz ~ 2010 MHz (uplink) and that of 2170MHz ~ 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.




The WCDMA system uses the following frequency spectrum (bands other than


those specified by 3GPP may also be used): Uplink 1920 MHz ~ 1980 MHz and

downlink 2110 MHz ~ 2170 MHz. Each carrier frequency has the 5M band andthe 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.




Compatible with abundant services and applications of 2G, 3G system has an


open integrated service platform to provide a wide prospect for various 3G

services.

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 servicegenerally have the QoS requirement.

The concept of multimedia service (streaming, interactive,

background) is introduced.




Formulated by the European standardization organization 3GPP, the core


network evolves on the basis of GSM/GPRS and can thus be compatible with the

existing GSM/GPRS networks. It can be based on the TDM, ATM and IPtechnologies 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.




In mobile communication systems, GSM adopts TDMA; WCDMA, cdma2000 and


TD-SCDMA adopt CDMA.




Frequency Division Multiple Access means dividing the whole available spectrum


into many single radio channels (transmit/receive carrier pair). Each channel can

transmit one-way voice or control information. Analog cellular system is a typicalexample 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 timeand 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.




In third generation mobile communication systems, WCDMA and cdma2000


adopt frequency division duplex (FDD), TD-SCDMA adopts time division duplex

(TDD).




WCDMA including the RAN (Radio Access Network) and the CN (Core Network).


The RAN is used to process all the radio-related functions, while the CN is used to

process all voice calls and data connections within the UMTS system, andimplements 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 is

used 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 amongthem 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.




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 byHSS (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 Packet Access)

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.




MIMO: multiple input multiple output


DC: Dual Carrier




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 ErrorRate (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.




Protocol structures in UTRAN terrestrial interfaces are designed according


to t e same genera protoco mo e . T is mo e is s own in a ove s i e.The structure is based on the principle that the layers and planes arelogically independent of each other and, if needed, parts of the protocolstructure 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). AllUTRAN-related issues are visible only in the Radio Network Layer,and the Transport Network Layer represents standard transporttechnology that is selected to be used for UTRAN but without anyUTRAN-specific changes.

Vertical Planes

Control Plane

The Control Plane is used for all UMTS-specific control signaling. Itincludes the Application Protocol (i.e. RANAP in Iu, RNSAP in Iurand 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 ApplicationProtocol are not directly tied to the User Plane technology, butrather are general bearer parameters. The Signaling Bearer for the

Application Protocol may or may not be of the same type as theSignaling Bearer for the ALCAP. It is always set up by O&M actions.




User Plane

All information sent and received by the user, such as the coded

voice in a voice call or the packets in an Internet connection, are

transported via the User Plane. The User Plane includes the Data

Stream(s), and the Data Bearer (s) for the Data Stream(s). Each Data

Stream is characterized by one or more frame protocols specified for

that interface.

Transport Network Control Plane

The Transport Network Control Plane is used for all control

signaling within the Transport Layer. It does not include any Radio

Network Layer information. It includes the ALCAP protocol that is

needed to set up the transport bearers (Data Bearer) for the User

Plane. It also includes the Signaling Bearer needed for the ALCAP.

The Transport Network Control Plane is a plane that acts between

the Control Plane and the User Plane. The introduction of the

Transport Network Control Plane makes it possible for the

Application Protocol in the Radio Network Control Plane to be

completely independent of the technology selected for the Data

Bearer in the User Plane.

About AAl2 and AAL5


ove e ayer we usua y n an a ap a on ayer .

Its function is to process the data from higher layers for ATM

transmission.

This means segmenting the data into 48-byte chunks and

reassembling the original data frames on the receiving side. There

are five different AALs (0, 1, 2, 3/4, and 5). AAL0 means that no

adaptation is needed. The other adaptation layers have different

properties based on three parameters:

Real-time requirements;

Constant or variable bit rate;

Connection-oriented or connectionless data transfer. The usage of ATM is promoted by the ATM Forum. The Iu interface

uses two AALs: AAL2 and AAL5.

AAL2 is designed for the transmission of connection oriented, real-

time data streams with variable bit rates.

AAL5 is designed for the transmission of connectionless data

streams with variable bit rates.




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 (AsynchronousTransfer 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

A dedicated AAL2 connection is reserved for each individual CS

service.




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 flow uses UDP

connectionless transport and IP addressing.




The Iub interface is the terrestrial interface between NodeB and RNC. The


Radio Network Layer defines procedures related to the operation of the

NodeB. The Transport Network Layer defines procedures for establishingphysical 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-C procedure 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.




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.




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




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. That exchange has

to be done as fast as possible in order to better follow the evolution of the

channel’s quality.








Scrambling can make transmission in security.


signals.

Bit, Symbol, Chip

Bit : data after source coding Symbol: data after channel coding and interleaving

Chip: data after spreading




During the transmission, there are many interferences and fading. To guarantee


reliable transmission, system should overcome these influence through 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 transport

blocks 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.




UTRAN employs two FEC schemes: convolutional codes and turbo codes. The idea


is to add redundancy to the transmitted bit stream, sO that occasional bit errors

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 Shannon’s Law. Their efficiency is best with high data

rate services, but poor on low rate services. At higher bit rates, turbo coding ismore 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. The idea

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.




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 fast

moving UE in CDMA systems can cause consecutive errors if the power control isnot 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.










signals.

Bit, Symbol, Chip






Correlation is used to measure similarity of any two arbitrary signals. It is


computed by multiplying the two signals and then summing (integrating) the

result over a defined time windows. The two signals of figure (a) are identical andtherefore 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.




By spreading, each symbol is multiplied with all the chips in the orthogonal


sequence assigned to the user. The resulting sequence is processed and is then

transmitted over the physical channel along with other spread symbols. In thisfigure, 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.




The receiver dispreads the chips by using the same code used in the transmitter.


Notice that under no-noise conditions, the symbols or digits are completely

recovered without any error. In reality, the channel is not noise-free, but CDMAsystem 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.




Traditional radio communication systems transmit data using the minimum


bandwidth required to carry it as a narrowband signal. CDMA system mix their

input data with a fast spreading sequence and transmit a wideband signal. Thespreading 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.




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 is 384kbps(R99). After

the spreading, the chip rate of different service all become 3.84Mcps.




Spreading means increasing the bandwidth of the signal beyond the bandwidth


normally required to accommodate the information. The spreading process 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 alreadyachieved in channelization by the channelization codes, the chip rate is not

affected by the scrambling.




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 codeswith 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 factor of

the code, and k is the sequence of code, 0≤k≤SF-1. Each level definition length of

code tree is SF channelization code, and the left most value of each spreadingcode character is corresponding to the chip which is transmitted earliest.




The channelization codes are Orthogonal Variable Spreading Factor (OVSF) codes.


T ey are use to preserve ort ogona ity etween i erent p ysica c anne s.They also increase the clock rate to 3.84 Mcps. The OVSF codes are defined usinga 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 isconstant, 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 rateservices.

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 oneof the two codes is a mother code of the other. For example C4,3 is notorthogonal 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.




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.




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.




There are totally 512 primary scrambling codes defined by 3GPP. They are


further divided into 64 primary scrambling code groups. There are 8 primary

scrambling codes in every group. Each cell is allocated with only one primaryscrambling code.










signals.

Bit, Symbol, Chip






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 to modulate a

carrier signal in a digital sense: amplitude shift keying (ASK), frequency shiftkeying (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.




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 π radians.

NRZ means none return zero.




The quadrature phase shift keying (QPSK) modulation has four phases: 0, π/2, π,


and 3π/2 radians. Two data bits are transformed into one complex data symbol; A

symbol is any change (keying) of the carrier.




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 theamplitude 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.










signals.

Bit, Symbol, Chip






A mobile communication channel is a multi-path fading channel and any


transmitted signal reaches a receive end by means of multiple transmission paths,

such as direct transmission, reflection, scatter, etc.




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.




Diversity technology means that after receiving two or more input signals with


mutually uncorrelated fading at the same time, the system demodulates these

signals and adds them up. Thus, the system can receive more useful signals andovercome fading.




Diversity technology is an effective way to overcome overlaid fading. Because it


can be selected in terms of frequency, time and space, diversity technology

includes 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




The RAKE receiver is a technique which uses several baseband correlators to


individually process multipath signal components. The outputs from the different

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.

owa010010 wcdma ran overview issue 1.12 [

Documents