001 wcdma ran fundamental

WCDMA RAN Fundamental

Huawei Technologies Co., Ltd.

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Revision Record

Date Version Change description Author

04-07-2007 1C Victor Toledo



Table of Contents

1 3G Overview ........................................................................................................ 9

3G Evolution ................................................................................................ 11 3G Objectives .............................................................................................. 12 3G Spectrum Allocation ............................................................................... 13 WCDMA Bands............................................................................................ 14

2 CDMA Fundamentals ....................................................................................... 15

Multiple Access Technology and Duplex Technology.................................. 15 Characteristics of CDMA system ................................................................. 18

3 WCDMA Key Technologies .............................................................................. 20

Communication Model of WCDMA system.................................................. 20 WCDMA Source Coding .............................................................................. 21 WCDMA Channel Coding ............................................................................ 22 WCDMA Interleaving .................................................................................... 22 Spreading and Despreading......................................................................... 23 Correlation.................................................................................................... 27 WCDMA Spreading codes............................................................................ 27 WCDMA Scrambling codes: Gold sequence ................................................ 29 WCDMA Modulation..................................................................................... 30 Fading ......................................................................................................... 31 Principle of RAKE receiver .......................................................................... 33 WCDMA Power control................................................................................. 35

4 Performance Enhancement Methods.............................................................. 36

Smart antenna features ............................................................................... 36 Introduction to diversity technique ............................................................... 37



Objectives

Upon completion of this module, you will be able to:

� Know the development of 3G

� Outline the advantage of CDMA technology

� Outline the fundamentals of UTRAN

� Outline the key technologies of UTRAN



1 3G Overview

Figure 1.- Evolution of cellular standards.

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 NTT. 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

� Large volume and big weight

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 (Personal

Communication System) networks.



Since the 2G mobile communication systems focus on the transmission of voice and

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 data services.

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.

3G Evolution

� Proposal of 3G

� IMT-2000: the general name of third generation mobile

communication system

� The third generation mobile communication was first proposed in

1985，and was renamed as IMT-2000 in the year of 1996

− Commercialization: around the year of 2000

− Work band : around 2000MHz

− The highest service rate :up to 2000Kbps



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 (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.

3G Objectives

3G is developed to achieve:

� Universal frequency band for standard and seamless global coverage

� High spectral efficiency

� High quality of service with complete security and reliability

� Easy and smoothly transition from 2G to 3G, compatible with 2G

� Provide multimedia services, with the rates:

� Vehicle environment: 144kbps

� Walking environment: 384kbps

� Indoor environment: 2Mbps



1. Capable of roaming globally: users can roam within the whole system,

even in the whole world, and can be provided with guaranteed quality of service at

different rates and in different statuses of motion.

2. Providing diversified services: providing voice, data with variable rates,

active video non-voice services, especially multimedia services.

3. Capable of adapting to many kinds of environment: can integrate the

existing Public Switched Telephone Network (PSTN), Integrated Service Digital

Network (ISDN), cordless system, land mobile communication system and

satellite communication system to provide seamless coverage.

4. Sufficient system capacity, powerful management capability of multiple

users, high security performance and quality of service.

3G Spectrum Allocation

Figure 2.- Spectrum allocation for 3G system.



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 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.

WCDMA Bands

� Main bands

� 1920 ~ 1980MHz / 2110 ~ 2170MHz

� Supplementary bands: different country maybe different

� 1850 ~ 1910 MHz / 1930 MHz ~ 1990 MHz (USA)

� 1710 ~ 1785MHz / 1805 ~ 1880MHz (Japan)

� 890 ~ 915MHz / 935 ~ 960MHz (Australia)

� ……

� Frequency channel number＝central frequency×5, for main band:

− UL frequency channel number ：9612～9888

− DL frequency channel number : 10562～10838

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 the duplex spacing is 80 MHz.



2 CDMA Fundamental

Multiple Access Technology and Duplex Technology

The possibility to operate in either FDD or TDD mode is allowed for efficient

utilization of available spectrum according to frequency allocation in different regions.

FDD and TDD are defined as follows:

� FDD: A duplex method whereby the Uplink and the Downlink transmissions use 2

separate frequency bands:

� Uplink 1920 MHz - 1980 MHz; Downlink 2110 MHz - 2170 MHz.

� Bandwidth: each carrier is located on the center of a 5 MHz wide band.

� Channel separation: nominal value of 5 MHz that can be adjusted.

� Channel raster: 200 kHz (center frequency must be a multiple of 200 kHz).

� Tx-Rx frequency separation: nominal value of 190 MHz. This value can be

either fixed or variable (minimum of 134.8 and maximum of 245.2 MHz).

� Channel number: the carrier frequency is designated by the UTRA Absolute

Radio Frequency Channel Number (UARFCN). This number is sent by the network

(for the uplink and downlink) on the BCCH logical channel and is defined by Nu= 5 *

(Fuplink MHz) and ND= 5 * (Fdownlink MHz).

� TDD: A duplex method by which the Uplink and the Downlink transmissions are

carried over the same frequency using synchronized time intervals. The carrier uses a 5

MHz band, although there is a low chip rate solution under study by the 3GPP (1.28

Mcps). The available frequency bands for TDD will be:

1900-1920 MHz and 2010-2025 MHz.

The transmission medium is a resource that can be divided into individual channels

according to different criteria depending on the technology used:



Here’s how the three most popular multiple access techniques divide their channels:

� FDMA Frequency Division Multiple Access

• Each user on a different frequency,

• One channel uses one frequency.

� TDMA Time Division Multiple Access

• Each user on a different window period in time (“time slot”),

• TDMA usually uses FDMA to divide the frequency band into smaller

frequency channels, which are then divided in a time division fashion

(GSM),

• a channel is a specific time slot on a specific frequency.

� W-CDMA Wideband Code Division Multiple Access

• Each user uses the same frequency all the time, but mixed with

different distinguishing code patterns,

• W-CDMA usually uses FDMA to divide the frequency band into smaller

frequency channels, which are then divided in a code division fashion

(UMTS),

• a channel is a unique set of codes, and a specific frequency carrier.



Multiple Access Technology

Figure 3.- Multiple access technologies.

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.

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 all 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.



Characteristics of CDMA System

� High Spectral Efficiency

� Frequency multiplex coefficient is 1.

� Soft capacity

� Quality

� Coverage

� Interference

� Self-interference system

� A UE transmission power is interference for another UE.

In CDMA system, mutual interference between users or cells is permitted, so

adjacent cells can be distributed with same frequency. That is why the spectrum

efficiency is very high and the capacity is also very large in CDMA system. But it also

causes self-interference, if the interference is out of control, the capacity and quality of

CDMA system will be worse, so many technologies were invented to control the

interference, and it is not easy.

The second feature of CDMA is security. After spreading, the narrowband signal of

the user will be changed to broadband signal, is close to noise, only people who use the

same spreading code can revert it. Of course, it causes the other shortcoming: more

frequency band needed.

The third feature of CDMA is soft capacity. Because all of the carrier resource (the

main resource is power) is “shared” by all of the users, if some user occupy more power,

it will cause the capacity lower. Soft capacity will cause network planning more complex,

emulation is necessary.



Characteristics of WCDMA FDD

� Channel bandwidth: 5MHz

� Chip rate: 3.84Mcps

� Frame length: 10ms

� Voice coding: AMR (Adaptive Multi-Rate)

� Uplink and downlink modulation: QPSK/QPSK (R99)

� Coherence demodulation aided with pilot

� Fast closed loop power control: 1500Hz

� Handover: soft/hard handover

� Support synchronous and asynchronous NodeB operation

� Compatible with GSM-MAP core network

� Support open loop and closed loop transmit diversity mode



3 WCDMA Key Technologies

Communication Model of WCDMA System

Figure 4.- WCDMA Communication Model. Source coding can increase the transmitting efficiency.

Channel coding can make the transmission more reliably.

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.

Terms

� Bit, Symbol, Chip

� Bit : data after source coding

� Symbol: data after channel coding and interleaving

� Chip: data after spreading

� Process Gain: 10log (cps/bps); for different service, the Gain is

different

− Process Gain is smaller, UE need more power for this service

− Process Gain is smaller, the coverage of the service is smaller



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.

WCDMA Source Coding

� AMR (Adaptive Multi-Rate) voice coding

� Multi-rate:

− 8 kinds of coding rates

− Benefit multi-mode terminal design

� Adaptation: when cell load increases, the system will decrease

speech rate of part of subscribers automatically so as to support more

subscribers.

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.



WCDMA Channel Coding

� Purpose:

� Enhance the correlation among symbols so as to recover the signal

when interference occurs.

� Types

� Speech service: Convolution code（1/2、1/3）

� Data service: Turbo code

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

reliable transmission, system should overcome these influences through the channel

coding which includes convolution and interleaving.

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.

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.

WCDMA Interleaving

� Interleaving is used for continuous bit error correction

Figure 5.- Interleaving.



In channel coding, there is another technology named interleaving. Communications

over radio channel are characterized by fast fading that can cause large numbers of

consecutive errors. Most coding schemes perform better on random data errors than on

blocks of errors. By interleaving the data, no two adjacent bits are transmitted near to

each other, and the data errors are randomized.

Spreading and Despreading

Figure 6.- Spreading and despreading techniques.

Suppose bit sequence modulated with BPSK is adopted for the subscriber data, with

a rate of R, and then ±1 value is adopted for the bit of subscriber data.

The spreading here means to multiply each subscriber data bit with the spreading

code chip including N bits. Assume N=8, then data rate after spreading will be 8×R, with

same random attribute as the spreading code. We name its spreading factor as 8. And

the broad band signal obtained after spreading will be sent to the receiving end via the

radio channel.



As the product of signal rate and factor 8 equals to the bandwidth spreading of

subscriber data signal, CDMA system is also called the spreading system.

During Despreading, the spread subscriber data will be multiplied, bit duration by bit

duration, with the same 8 code chips that are used during the spreading of these bits. If

only excellent synchronization can be realized between the spread subscriber signal and

the Despreading code, the subscriber bit sequence can be retrieved. The Despreading

operation restores the signal bandwidth to the original value R.

Spreading

Figure 7.- Spreading example. 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 the original encoded

binary signal.



Despreading

Figure 8. - Despreading example. 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 CDMA system employs

Forward Error Correction techniques to combat the effects of noise and enhance the

performance of the system.

When the wrong code is used for Despreading, 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.



Spectrum Analysis of Spreading & Despreading

Figure 9.- Spreading and Despreading Spectrum analysis. Traditional radio communication systems transmit data using the minimum

bandwidth required to carry it as a narrowband signal. CDMA systems 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 Despreading 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.



Correlation

� Correlation is a measure of similarity of between any two arbitrary signals.

� EXAMPLE:

Figure 10.- Correlation.

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 any information on the other.

WCDMA Spreading Code: OVSF（（（（Walsh））））

� OVSF: Orthogonal Variable Spreading Factor, generated by Walsh matrix

Figure 11.- Walsh codes.



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.

Spreading code 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.

Spreading 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 spreading code, and the left most value of each spreading code character is

corresponding to the chip which is transmitted earliest.

Purpose of OVSF

� For uplink, OVSF is used to separate different services of one connection

� For downlink, OVSF is used to separate different connections

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.



Why we need scrambling code?

� Distinguishing cells or users

� Downlink

� Scrambling code is used for distinguishing cells

� OVSF code is used for distinguishing users

� Uplink

� Scrambling code is used for distinguishing users

� OVSF code is used for distinguishing channels of one user

In one network, usually a UE is surrounded by many base stations with cells. If UE

wants to get service from system, first it should distinguish different signals from different

cells. In WCDMA system, most of the cells use the same frequency, so UE can not get

any information thought the frequency. Here scrambling codes are used. Different cells

will be allocated different scrambling code.

WCDMA Scrambling Code: Gold Sequence

� Gold sequence is made by two m sequence.

� Advantage: No need to use GPS as the system clock, NodeB can

work in asynchronous mode, and it is also convenient for indoor coverage

� Disadvantage: the interference between code is larger than m

sequence

For uplink, 224 long scrambles and 224 short scrambles.

For downlink, 262,143 (218 - 1) scrambles, but only 8192 scrambles from 0 to 8191)

are adopted at present.

The length of scrambling code is 38400 chips.



WCDMA Modulation

� Different modulation methods corresponding to different transmitting

abilities in air interface

� R99/R4: adopt QPSK

� DL max data rate is 2.7Mbps

QPSK: Quadrature Phase Shift Keying. Phase shift keying in which four different

phase angles are used.

16QAM: 16 Quadrature Amplitude Modulation

Multi-path Environment

Figure 12.- Multi-path effects on UE.

Radio propagation in the land mobile channel is characterized by multiple

reflections, diffractions and attenuation of the signal energy. These are caused by natural

obstacles such as buildings, hills, and so on, resulting in so-called multipath propagation.

There are two effects resulting from multipath propagation:

1. The signal energy (pertaining, for example, to a single chip of a CDMA

waveform) may arrive at the receiver across clearly distinguishable time instants.



2. Also, for a certain time delay position there are usually many paths nearly equal

in length along which the radio signal travels.

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. The received signals are different in signal energy

and time delay.

Fading

Figure 13.- Slow and Fast fading effects.

Slow fading

In case shadow effect is caused by obstacles, and the receiving signal strength

decreases but the field strength mid-value changes slowly with the change of the

topography, the strength decrease is called “slow fading” or “shadow fading”. The field

strength mid-value of slow fading takes on a logarithmic normal distribution, and is

related to location/locale. The fading speed is dependent on the speed of the mobile

station.



Fast fading

In case the amplitude and phase of the combined wave change sharply with the

motion of the mobile station, the change is called “fast fading”. The spatial distribution of

deep fading points is similar to interval of half of wavelength. Since its field strength

takes on Rayleigh distribution, the fading is also called Rayleigh fading.

The amplitude, phase and angle of the fading are random.

Fast fading is subdivided into the following three categories:

Time-selective fading: In case the user moves quickly and causes Doppler effect on

the frequency domain, and thus results in frequency diffusion, time-selective fading will

occur.

Space-selective fading: The fading features vary between different places and

different transmission paths.

Frequency-selective fading: The fading features vary between different frequencies,

which results in delay diffusion and frequency-selective fading.

In order to mitigate the influence of fast fading on wireless communication, typical

methods are: space diversity, frequency diversity, and time diversity.

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 fading. The mid-value field strength of Rayleigh fading has relatively gentle

change and is called “Slow fading”. And it conforms to lognormal distribution.



Principle of RAKE Receiver

Figure 14.- The RAKE receiver architecture.

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.

Figure above shows a simplified block diagram of a Rake receiver. As you can see,

a number of Rake fingers containing correlators are used to track the different multipath

reflections from one scrambling code. The outputs from the fingers are fed into a

combiner. One of three different types of combining processes is employed to produce

an output that is the sum of the individual multipath components. In order to achieve this

tracking, each finger simply correlates the signal with the same scrambling code but at a

different phase shift. Since this is similar to using a different code, a finger could quite

easily be used to track another base station. This is exactly what happens in the case of

Soft or Softer handovers, which are explained later. The output from one finger is not fed

into the combiner. This finger correlates the received signal with the scrambling code of

known neighboring base stations in order to measure their power. This information is

used to determine when to perform handovers. This finger is known as the “Searcher

Finger”.



To make it possible for the Rake receiver to track these various components it must

have some way of measuring the signal levels and phases. This is achieved by the base

station transmitting known pilot symbols in the transmitted data. The Rake receiver looks

for these bits and uses them to determine the phase and signal strength of each

component.

When WCDMA systems were designed for cellular systems, 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 each tap on the delay line are uncorrelated with each other.

The receiver can then combine them using any of the combining schemes. The WCDMA

system then uses the multi-path characteristics of the channel to its advantage to

improve the operation of the system.

Structure of RAKE Receiver

Figure 15.- RAKE Receiver Structure.

For the digitized signals input to the baseband, Despreading and integration of

subscriber data symbols is completed via the correlator and local Code Generator,

specifics are as follows: Channel Estimator uses the pilot signal to estimate the channel

status; Phase Rotator deletes the phase affection caused by the channel from the

received signal according to the estimated channel status. The function of Delay



Equalizer is to obtain the signal energy distribution at different delay positions via the

matching filter, identify the multipaths with large energy, and allocate their time values to

different receive paths of the RAKE receiver. The delay equalizer is to compensate the

difference of symbol arriving time for each path. At last, the RAKE combiner adds the

symbols after channel compensation to provide multipath diversity to withstand fading.

From the aspect of realization, the processing of RAKE receiver can be based on either

chip level or symbol level. The correlator, local code generator and matching filter belong

to the chip level processing, and this is generally realized via ASIC device; Channel

estimation, phase spinning and combination belong to symbol level processing, and this

is realized via DSP. Though the realization methods and functions of the RAKE receiver

between UE and BTS are different, the principles are complete the same.

WCDMA Fast Power Control – Control Fast Fading

Figure 16.- Power control.

The rate of power control can be up to1500 times per second, which is faster than

that of fading, thus, it can overcome shadow fading and fast fading effectively

Power control also decreases interference of system, and increase system capacity

and quality.

For UE, Power control can save power, and expand conversational time.



4 Performance Enhancement Methods

HSDPA Key Techniques - Overview

Figure 17 . - HSDPA Key Techniques.

GENERAL PRINCIPLES

HSDPA concept is based on a set of basic principles:

1 Shared channel transmission based on multi-code transmission where resources

are shared in time & code domain

2 16QAM Modulation to increase the peak bit rate

3 Short TTI in order to support fast retransmission, fast link adaptation, fast

scheduling and short round trip delay (Transmission Time Interval)(500 times/sec!)

4 Fast scheduling selects the user with best radio conditions

5 Fast link adaptations adapts for fast changes in radio conditions

6 Fast re-transmission with Soft combining if packets are corrupted.

7 Dynamic Power Allocations



Figure 18 . - HSDPA Key Techniques – AMC.

Figure 19 . - HSDPA Key Techniques – HARQ.

Fast hybrid Automatic Repeat ReQuest allows UEs to rapidly request

retransmissions of erroneously received transport blocks.

• The UE attempts to decode each transport block it receives, reporting to

RBS its success or failure 5 ms after the reception of the transport block.

The hybrid ARQ mechanism in RBS can rapidly respond to retransmissions

requests.

This leads to shorter Round Trip Times.



• The UE employs soft combining, that is: it combines soft information from

previous transmission attempts with the current transmission to increase the

probability of soft combining.

This reduces error rates for retransmissions.

This functionality is mainly sort of fine tuning the effective code rate and

compensating for errors made by link adaptation mechanism.

Figure 20. - HSDPA Key Techniques - Fast scheduling.

Fast Scheduling is about to decide to which terminal the shared channel

transmission should be directed at any given moment.

It’s called channel-dependent scheduling because it’s dependent on the

instantaneous channel condition.

The basic idea is to transmit at the fading peaks of the channel in order to increase

the capacity and to use the resources more efficiently.

But this might lead to large variations in data rate of the users.



The trade-off is between the cell throughput and fairness against users. In some

cases, there might be a particular user who is perhaps on the cell border which might not

be allocated the radio resources because he does not have good enough C/I.

Remember that we don’t have SHO for dedicated shared channel.

So there are a number of scheduling algorithms which takes into consideration

the trade-of between throughput and fairness. And the next slide will be about them.

The scheduling strategy has a large impact on the system capacity and the end user

performance. The variations in the radio-conditions should be utilized by a good

scheduler. We have a couple of different schedulers.

Round Robin scheduler allocates the channel to users sequentially. Quite simple

but offers rather poor performance.

Proportional Fair is the one which allocates the channel to the user with relatively

best channel quality. It gives rather high throughput and rather fair, whereas Max C/I

allocates the channel with absolutely best channel quality. It does not care about fairness

at all.

A practical scheduler should operate somewhere between the RR and the max C/I

scheduler and exactly where it should operate is dependent on the traffic load and traffic

type among the other things. The higher the system load, the more visible the difference

between the different scheduling algorithm. But Proportional Fair scheduler is proposed

for Interactive/Best Effort traffic and also to avoid that some users get no throughput.

For streaming, traffic priorities can be taken into account. Streaming services before

background services can be prioritized.

Short term variations in the channel such as multipath fading and variations in

interference level can be acceptable and go unnoticed for many packet-data applications.

Long term variations such as the distance between terminal and the RBS are more

restrictive. So the practical scheduler should be fair to long-term variations and should

exploit short term variations.



Figure 21 . - HSDPA Key Techniques – CDM and TDM.

Shared-channel transmission

In HSDPA, a new DL transport channel is introduced called high speed DL shared

channel. The idea is that a part of the total downlink code resource is dynamically shared

between a set of packet-data users, primarily in the time domain. The codes are

allocated to a user only when they are actually to be used for transmission which leads

to efficient code and power utilization.

In HSDPA, maximum 15 channelization codes with Spreading Factor (SF) = 16 can

be used for this new DL channel. Channelization codes are used enabling user data

rates up to 4.32 Mbps (the system is capable of enabling 4.32 Mbps).

The main benefit with DL shared channel transmission is to reduce the risk for code-

limited capacity. Sharing codes in the code domain, in other words, code multiplexing, is

also possible by employing different subsets of the complete channelization code set for

different users.

Sharing in the code domain is useful for providing efficient support of small payloads

when the transmitted data does not require the full set of HS-DSCH codes configured in

the cell. Useful when supporting terminals cannot despread the full set of codes.



Number of codes which will be used in each cell is configured or slowly adapted by

RNC according to # of resources needed for packet data services on HS channel and

other services such as voice. It’s the RBS which dynamically allocates the codes to the

users every 2 ms.

Figure 22 . - HSDPA Key Techniques – 16QAM.

HSDPA supports both QPSK and 16QAM.

• 16 QAM is optional in RBS,

• but mandatory in the UE except for 2 UE categories.

• It gives approximately double data rate as QPSK given a fixed code rate.

• It is more bandwidth efficient but of course requires better channel

conditions.

16QAM – 16 Quadrature Amplitude Modulation

16QAM (High Order Modulation) is more bandwidth efficient, i.e. can carry more bits

per Hertz But is also less robust and typically requires higher energy per bit for a given

error rate.



Higher-order modulation can be used together with shared-channel transmission to

support higher data rates and achieve higher capacity, assuming it is used only when the

radio-channel conditions so allow.

16 QAM is good in bandwidth limited scenarios but not in power limited scenarios.

It’s basically good near to base station and low dispersive environments. A good

example is micro and indoor cells.

• With the introduction of G-Rakes or dual antennas in UEs, higher

modulation will be chosen more often by link adaptation.

So in the future, higher modulation will be utilized more by means of advanced

receivers in the terminals.

Smart Antenna

Figure 23.- Omni, directional and smart antenna coverages.

� Reduce interference

� Increase coverage and capacity

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.



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 fading. The mid-value field strength of Rayleigh

fading has relatively gentle change and is called “Slow fading”. And it conforms to

lognormal distribution.

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.

Figure 24.- Smart antenna features.

Introduction to Diversity Technique

� Diversity technique is used to obtain uncorrelated signals for combining

� Reduce the effects of fading

� fast fading caused by multi-path

� Slow fading caused by shadowing



� Improve the reliability of communication

� Increase the coverage and capacity

� Macroscopic diversity

� Soft handover and softer handover

� Reduce large-scale fading

� Microscopic diversity

� Time diversity

� Rake Receiver, Block interleaving

� Frequency diversity

� The user signal is distributed on the whole bandwidth

frequency spectrum

� Space diversity

� Receive diversity

� Transmit diversity

� Polarization diversity

� Vertical polarization

� Horizontal polarization

� Diversity methods

� Time diversity

� Frequency diversity

� Space diversity

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.

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: block interleaving, error-correction

Frequency diversity: frequency hopping, CDMA is also a kind of frequency diversity;

the signal energy is distributed on the whole bandwidth.

Space diversity: using twin receive antennas, RAKE receivers

During a handover, the mobile station contacts multiple base stations and searches

for the strongest frame, it is called macro diversity.

001 wcdma ran fundamental

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