OWA010010 WCDMA RAN Fundamental ISSUE 1.111 [Compatibility Mode]

8/13/2019 OWA010010 WCDMA RAN Fundamental ISSUE 1.111 [Compatibility Mode]

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WCDMA RAN Fundamental

Confidential Information of Huawei. No Spreading Without Permission

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l 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.

l The typical examples of the f irst 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.

l 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.

l However, their defects are also obvious:

p Low utilization of the frequency spectrum

p Limited types of services

p No high-speed data services

p Poor confidentiality and high vulnerability to interception and number

embezzlement

p High equipment cost


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l 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 mobilecommunication systems are the DAMPS of USA, the IS-95 and the European

GSM system.

l 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.

l 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.

l 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

(Personal Communication System) networks.

l

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.

l 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.

l 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.

l 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


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l 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 (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 application of the 3G mobile communication systems would

enter a new and essential phase.

l 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.

l 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.


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l 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 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 thesingle-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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l 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 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 theduplex spacing is 80 MHz.


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l 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.

l Features of 3G Services

l 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:

p Compatible backward with all the services provided by GSM.

p The real-time services (conversational) such as voice service

generally have the QoS requirement.

p The concept of multimedia service (streaming, interactive,

background) is introduced.


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l 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 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 IPnetwork architecture.

l 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.

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

TD-SCDMA adopt CDMA.


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l 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 typical

example of FDMA structure.

l 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.

l 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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l In third generation mobile communication systems, WCDMA and cdma2000 adopt

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


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l The overall structure of the WCDMA network is defined in 3GPP TS 23.002. Now,

there are the following three versions: R99, R4, R5.

l 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.

l 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).

l

In the R5 network, the end-to-end VOIP is supported and the core network adoptsplentiful 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.

l 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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l Source coding can increase the transmitting efficiency.

l Channel coding can make the transmission more reliable.

l Spreading can increase the capability of overcoming interference.

l Through the modulation, the signals will transfer to radio signals from digitalsignals.

l Bit, Symbol, Chip

p Bit : data after source coding

p Symbol: data after channel coding and interleaving

p Chip: data after spreading


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l AMR is compatible with current mobile communication system (GSM, IS-95, PDC

and so on), thus, it will make multi-mode terminal design easier.

l 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.

l 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 channels

quality.


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l Scrambling can make transmission in security.

l Through the modulation, the signals will transfer to radio signals from digital

signals.

l Bit, Symbol, Chip





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l 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.

l 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.

l 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.

l 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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l 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 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 withinterleaving, 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.

l 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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signals.

l Bit, Symbol, Chip





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l 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 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 anyinformation on the other.


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l 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 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 theoriginal encoded binary signal.


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l 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. The

spreading sequence is independently regenerated at the receiver and mixed with

the incoming wideband signal to recover the original data. The dispreading givessubstantial 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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l 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.


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l 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.

l 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.

l 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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l 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.

l 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.

l 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 spreading

code character is corresponding to the chip which is transmitted earliest.


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

codes. They are used to preserve orthogonality between different physical

channels. They also increase the clock rate to 3.84 Mcps. The OVSF codes are

defined using a code tree.

l 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.

l A channelization sequence modulates one users bit. Because the chip rate is

constant, the different lengths of codes enable to have different user data rates.

Low SFs are reserved for high rate services while high SFs are for low rate

services.

l 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.

l The generated codes within the same layer constitute a set of orthogonal codes.

Furthermore, any two codes of different layers are orthogonal except when one of

the two codes is a mother code of the other. For example C4,3 is not orthogonal

with C1,0 and C2,1, but is orthogonal with C2,0.

l SF in uplink is from 4 to 256.

l SF in downlink is from 4 to 512.


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l For voice service (AMR), downlink SF is 128, it means there are 128 voice

services maximum can be supported in one WCDMA carrier;

l 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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l In addition to spreading, part of the process in the transmitter is the scrambling

operation. This is needed to separate terminals or base stations from each other.


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l Different scrambling codes will be planned to different cells in downlink.

l Different scrambling codes will be allocated to different UEs in uplink.

l The scrambling code is always applied to one 10 ms frame.

l In UMTS, Gold codes are chosen for their very low peak cross-correlation.


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l 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 primary scrambling code.


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l 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 shift

keying (FSK), and phase shift keying (PSK).

l In ASK the amplitude of the carrier signal is modified by the digital signal.

l In FSK the frequency of the carrier signal is modified by the digital signal.

l 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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l 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.

l NRZ means none return zero.


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

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

symbol is any change (keying) of the carrier.


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l 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.

l 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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signals.

l Bit, Symbol, Chip





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l 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.


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l 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 iscalled !slow fading". Slow fading conforms to lognormal distribution.


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l 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 and

overcome fading.


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l 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.

l 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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OWA010010 WCDMA RAN Fundamental ISSUE 1.111 [Compatibility Mode]

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Transcript of OWA010010 WCDMA RAN Fundamental ISSUE 1.111 [Compatibility Mode]