3G UMTS / WCDMA technologiesThere are several key areas of 3G UMTS / WCDMA. Within these there are several key

technologies that have been employed to enable UMTS / WCDMA to provide a leap in performance

over its 2G predecessors.

Some of these key areas include:

Radio interface:   The UMTS radio interface provides the basic definition of the radio signal.

W-CDMA occupies 5 MHz channels and has defined formats for elements such as

synchronisation, power control and the like Read more about the UMTS / W-CDMA radio

interface.

CDMA technology :   3G UMTS relies on a scheme known as CDMA or code divison

multiple access to enable multiple handsets or user equipments to have access to the base

station. Using a scheme known as direct sequence spread spectrum, different UEs have

different codes and can all talk to the base station even though they are all on the same

frequency Read more about the code division multiple access.

UMTS network architecture:   The architecture for a UMTS network was designed to

enable packet data to be carried over the network, whilst still enabling it to support circuit

switched voice. All the usual functions enabling access toth e network, roaming and the like

are also supported. Read more about the UMTS network architecture.

UMTS modulation schemes:   Within the CDMA signal format, a variety of forms of

modulation are used. These are typically forms of phase shift keying. Read more about

the modulation schemes.

UMTS channels:   As with any cellular system, different data channels are required for

passing payload data as well as control information and for enabling the required resources

to be allocated. A variety of different data channels are used to enable these facilities to be

accomplishedRead more about the physical & logical channels.

UMTS TDD:   There are two methods of providing duplex for 3G UMTS. One is what is

termed frequency division duplex, FDD. This uses two channels spaced sufficiently apart so

that the receiver can receive whilst the transmitter is also operating. Another method is to

use time vision duplex, TDD where short time blocks are allocated to transmissions in both

directions. Using this method, only a single channel is required Read more about the TDD

system.

Handover:   One key area of any cellular telecommunications system is the handover

(handoff) from one cell to the next. Using CDMA there are several forms of handover that are

implemented within the system. Read more about the Handover.

3G UMTS enhancements

The basic 3G UMTS cellular system enabled data rates up to 2048kbps to be achieved. However as

the use of data rapidly increased, these figures were no longer sufficient and further data rate

increases were required.

A scheme known as HSDPA, high speed packet download access was first introduced to enable the

downlink speed to be increased. This was followed with HSUPA, high speed packet uplink access

was introduced. The combined suite was then known as HSPA, high speed packet access.

Note on HSPA, High Speed Packet Access:

High speed packet access added additional channels and signalling to the downlink (HSDPA) and in the Uplink

(HSUPA). By adding these channels, significant increases in data rates and capacity have been achieved.

Click on the link for further information about High Speed Packet Access, HSPA

UMTS WCDMA specification summaryThe UMTS WCDMA system offered a significant improvement in capability over the previous 2G

services.

3G UMTS SPECIFICATION SUMMARY

PARAMETER SPECIFICATION

Maximum data rate 2048 kbps low range384 kbps urban and outdoor

RF channel bandwidth 5 MHz

Multiple access scheme CDMA

Duplex schemes FDD and also TDD

The basic 3G UMTS is able to provide a reasonable data transfer rate, although by the latest

standards it is relatively slow. Nevertheless 3G UMTS paved the way for mobile broadband,

providing data rates that were unequalled at the time.

The UMTS 3G architecture is required to provide a greater level of performance to that of the original

GSM network. However as many networks had migrated through the use of GPRS and EDGE, they

already had the ability to carry data. Accordingly many of the elements required for the WCDMA /

UMTS network architecture were seen as a migration. This considerably reduced the cost of

implementing the UMTS network as many elements were in place or needed upgrading.

With one of the major aims of UMTS being to be able to carry data, the UMTS network architecture

was designed to enable a considerable improvement in data performance over that provided for

GSM.

3G UMTS network constituentsThe UMTS network architecture can be divided into three main elements:

1. User Equipment (UE):   The User Equipment or UE is the name given to what was previous

termed the mobile, or cellphone. The new name was chosen because the considerably

greater functionality that the UE could have. It could also be anything between a mobile

phone used for talking to a data terminal attached to a computer with no voice capability.

2. Radio Network Subsystem (RNS):   The RNS also known as the UMTS Radio Access

Network, UTRAN, is the equivalent of the previous Base Station Subsystem or BSS in GSM.

It provides and manages the air interface fort he overall network.

3. Core Network:   The core network provides all the central processing and management for

the system. It is the equivalent of the GSM Network Switching Subsystem or NSS.

The core network is then the overall entity that interfaces to external networks including the public

phone network and other cellular telecommunications networks.

UMTS Network Architecture Overview

User Equipment, UEThe USER Equipment or UE is a major element of the overall 3G UMTS network architecture. It

forms the final interface with the user. In view of the far greater number of applications and facilities

that it can perform, the decision was made to call it a user equipment rather than a mobile. However

it is essentially the handset (in the broadest terminology), although having access to much higher

speed data communications, it can be much more versatile, containing many more applications. It

consists of a variety of different elements including RF circuitry, processing, antenna, battery, etc.

There are a number of elements within the UE that can be described separately:

UE RF circuitry:   The RF areas handle all elements of the signal, both for the receiver and

for the transmitter. One of the major challenges for the RF power amplifier was to reduce the

power consumption. The form of modulation used for W-CDMA requires the use of a linear

amplifier. These inherently take more current than non linear amplifiers which can be used

for the form of modulation used on GSM. Accordingly to maintain battery life, measures were

introduced into many of the designs to ensure the optimum efficiency.

Baseband processing:   The base-band signal processing consists mainly of digital

circuitry. This is considerably more complicated than that used in phones for previous

generations. Again this has been optimised to reduce the current consumption as far as

possible.

Battery:   While current consumption has been minimised as far as possible within the

circuitry of the phone, there has been an increase in current drain on the battery. With users

expecting the same lifetime between charging batteries as experienced on the previous

generation phones, this has necessitated the use of new and improved battery technology.

Now Lithium Ion (Li-ion) batteries are used. These phones to remain small and relatively light

while still retaining or even improving the overall life between charges.

Universal Subscriber Identity Module, USIM:   The UE also contains a SIM card, although

in the case of UMTS it is termed a USIM (Universal Subscriber Identity Module). This is a

more advanced version of the SIM card used in GSM and other systems, but embodies the

same types of information. It contains the International Mobile Subscriber Identity number

(IMSI) as well as the Mobile Station International ISDN Number (MSISDN). Other information

that the USIM holds includes the preferred language to enable the correct language

information to be displayed, especially when roaming, and a list of preferred and prohibited

Public Land Mobile Networks (PLMN).

The USIM also contains a short message storage area that allows messages to stay with the

user even when the phone is changed. Similarly "phone book" numbers and call information

of the numbers of incoming and outgoing calls are stored.

The UE can take a variety of forms, although the most common format is still a version of a "mobile

phone" although having many data capabilities. Other broadband dongles are also being widely

used.

3G UMTS Radio Network SubsystemThis is the section of the 3G UMTS / WCDMA network that interfaces to both the UE and the core

network. The overall radio access network, i.e. collectively all the Radio Network Subsystem is

known as the UTRAN UMTS Radio Access Network.

The radio network subsystem is also known as the UMTS Radio Access Network or UTRAN.

Read more about the UMTS Radio Access Network.

3G UMTS Core NetworkThe 3G UMTS core network architecture is a migration of that used for GSM with further elements

overlaid to enable the additional functionality demanded by UMTS.

In view of the different ways in which data may be carried, the UMTS core network may be split into

two different areas:

Circuit switched elements:   These elements are primarily based on the GSM network

entities and carry data in a circuit switched manner, i.e. a permanent channel for the duration

of the call.

Packet switched elements:   These network entities are designed to carry packet data. This

enables much higher network usage as the capacity can be shared and data is carried as

packets which are routed according to their destination.

Some network elements, particularly those that are associated with registration are shared by both

domains and operate in the same way that they did with GSM.

UMTS Core Network

Circuit switched elements

The circuit switched elements of the UMTS core network architecture include the following network

entities:

Mobile switching centre (MSC):   This is essentially the same as that within GSM, and it

manages the circuit switched calls under way.

Gateway MSC (GMSC):   This is effectively the interface to the external networks.

Packet switched elements

The packet switched elements of the 3G UMTS core network architecture include the following

network entities:

Serving GPRS Support Node (SGSN):   As the name implies, this entity was first

developed when GPRS was introduced, and its use has been carried over into the UMTS

network architecture. The SGSN provides a number of functions within the UMTS network

architecture.

o Mobility management    When a UE attaches to the Packet Switched domain of the

UMTS Core Network, the SGSN generates MM information based on the mobile's

current location.

o Session management:    The SGSN manages the data sessions providing the

required quality of service and also managing what are termed the PDP (Packet data

Protocol) contexts, i.e. the pipes over which the data is sent.

o Interaction with other areas of the network:    The SGSN is able to manage its

elements within the network only by communicating with other areas of the network,

e.g. MSC and other circuit switched areas.

o Billing:    The SGSN is also responsible billing. It achieves this by monitoring the flow

of user data across the GPRS network. CDRs (Call Detail Records) are generated by

the SGSN before being transferred to the charging entities (Charging Gateway

Function, CGF).

Gateway GPRS Support Node (GGSN):   Like the SGSN, this entity was also first

introduced into the GPRS network. The Gateway GPRS Support Node (GGSN) is the central

element within the UMTS packet switched network. It handles inter-working between the

UMTS packet switched network and external packet switched networks, and can be

considered as a very sophisticated router. In operation, when the GGSN receives data

addressed to a specific user, it checks if the user is active and then forwards the data to the

SGSN serving the particular UE.

Shared elements

The shared elements of the 3G UMTS core network architecture include the following network

entities:

Home location register (HLR):   This database contains all the administrative information

about each subscriber along with their last known location. In this way, the UMTS network is

able to route calls to the relevant RNC / Node B. When a user switches on their UE, it

registers with the network and from this it is possible to determine which Node B it

communicates with so that incoming calls can be routed appropriately. Even when the UE is

not active (but switched on) it re-registers periodically to ensure that the network (HLR) is

aware of its latest position with their current or last known location on the network.

Equipment identity register (EIR):   The EIR is the entity that decides whether a given UE

equipment may be allowed onto the network. Each UE equipment has a number known as

the International Mobile Equipment Identity. This number, as mentioned above, is installed in

the equipment and is checked by the network during registration.

Authentication centre (AuC) :   The AuC is a protected database that contains the secret

key also contained in the user's USIM card.

Physical layer within UMTS / WCDMA is totally different to that employed by GSM. It employs a

spread spectrum transmission in the form of CDMA rather than the TDMA transmissions used for

GSM. Additionally it currently uses different frequencies to those allocated for GSM.

The UTRA, UMTS radio access is the technology that is the radio interface, and the network, or

UMTS Radio Access Network is known as the UTRAN. Sometimes the UTRAN may also be known

as the Radio Network Subsystem, or RNS.

UMTS radio access network, UTRAN

The UMTS Radio Access Network, UTRAN, or Radio Network Subsystem, RNS comprises two main

components:

Radio Network Controller, RNC:   This element of the UTRAN / radio network subsystem

controls the Node Bs that are connected to it, i.e. the radio resources in its domain.. The

RNC undertakes the radio resource management and some of the mobility management

functions, although not all. It is also the point at which the data encryption / decryption is

performed to protect the user data from eavesdropping.

Node B:   Node B is the term used within UMTS to denote the base station transceiver. This

part of the UTRAN contains the transmitter and receiver to communicate with the UEs within

the cell. It participates with the RNC in the resource management. NodeB is the 3GPP term

for base station, and often the terms are used interchangeably.

In order to facilitate effective handover between Node Bs under the control of different RNCs, the

RNC not only communicates with the Core Network, but also with neighbouring RNCs.

3G UMTS UTRAN Architecture

UTRAN interfacesThe UMTS standards are structured in a way that the internal functionality of the different network

elements is not defined. Instead, the interfaces between the network elements is defined and in this

way, so too is the element functionality.

There are several interfaces that are defined for the UTRAN elements:

Iub :   The Iub connects the NodeB and the RNC within the UTRAN. Although when it was

launched, a standardisation of the interface between the controller and base station in the

UTRAN was revolutionary, the aim was to stimulate competition between suppliers, allowing

opportunities like some manufacturers who might concentrate just on base stations rather

than the controller and other network entities.

Iur :   The Iur interface allows communication between different RNCs within the UTRAN.

The open Iur interface enables capabilities like soft handover to occur as well as helping to

stimulate competition between equipment manufacturers.

Iu :   The Iu interface connects the UTRAN to the core network.

Having standardised interfaces within various areas of the network including the UTRAN allows

network operators to select different network entities from different suppliers.

UTRA uplink & downlinkWhen looking at the radio air interface and its associated properties, it is necessary to define the

directions in which the transmissions are occurring. Being a full duplex system, i.e. transmitting

simultaneously in both directions, it is necessary to be able to define which direction is which.

Uplink;   This may also sometimes be known as the reverse link, and it is the link from the

User Equipment (UE) to the Node B or base station.

Downlink;   This may also sometimes be known as the forward link, and it is the link from

the Node B or base station to the User Equipment (UE).

The terms Uplink and Downlink are the terms that are used with UMTS, and especially within

Europe. The terms forward link and reverse link are more commonly used with the CDMA2000

technologies and also within North America.

Uplink and downlink transmission directions

UTRA FDD & TDDIn view of the fact that transmissions have to be made in both directions, i.e. in both uplink and

downlink. It is necessary to organise the way these transmissions are made. Two techniques are

used to ensure concurrent or near concurrent transmissions in both directions: frequency division

duplex and time division duplex.

UTRA-FDD:   The frequency division duplex version of UTRA uses a scheme whereby

transmissions in the uplink and downlink occur on different frequencies. Although this

requires double the bandwidth to accommodate the two transmissions, and filters to prevent

the transmitted signal from interfering with the receiver. Even though there is a defined split

between uplink and downlink, effective filters are required.

UTRA-TDD:   The time division version of the UTRA uses uplink and downlink transmissions

that use the same frequency but are timed to occur at different intervals.

Both UTRA-FDD and UTRA-TDD have their own advantages and disadvantages and therefore tend

to be used in different areas.

While the UTRA-FDD and UTRA-TDD both belong to 3G UMTS and are contained within the 3GPP

standards, they may have some slightly different parameters for their transmissions.

KEY SPECIFICATIONS FOR UTRAN OPERATION FOR FDD & TDD

PARAMETER UTRA FDD UTRA TDD

Multiple access method CDMA TDMA, CDMA

Channel spacing 5 MHz 5 MHz (and 1.6MHz for TD-SCDMA)

Carrier chip rate 3.84 Mcps 3.84 Mcps

Spreading factors 4 .. 512 1 .. 16

Time slot structure 15 slots / frame 15 / 14 slots / frame

Frame length (ms) 10 10

Multirate concept Multicode, and OVSF[1] Multicode, multislot and OVSF[1]

Burst types N/A (1) traffic bursts(2) random access burst(3) synchronisation burst

Detection Coherent based on pilot symbols

Coherent based on mid-amble

Dedicated channel power control

Fast closed loop 1500 Hz rate Uplink: open loop 100 Hz or 200 Hz rateDownlink: closed loop max 800 Hz rate

Notes

[1] OVSF = Orthogonal Variable Spreading Factor

The Physical layer is one of the key elements that differentiates the various cellular systems that

operate. Although there is very much more to a cellular system than just the physical layer, it is one

of the aspects that gains a high level of visibility.

The UMTS physical layer is totally different to that employed by GSM. It employs a spread spectrum

transmission in the form of CDMA rather than the TDMA transmissions used for GSM. Additionally it

currently uses different frequencies to those allocated for GSM.

UMTS physical layer signal formatOne of the chief elements of the UMTS physical layer or radio interface is the signal format that has

been adopted.

The UMTS physical layer utilises direct sequence spread spectrum format to enable a multiple

access scheme called Code Division Multiple Access, CDMA to be used.

Using CDMA, multiple users share the same channel, but different users are allocated different

codes, and in this way the system is able to distinguish between the different users.

      Read more about UMTS CDMA

The CDMA signal is 5MHz and in view of this, the UMTS physical layer is often referred to as

Wideband CDMA, W-CDMA. This compares to the US based cdmaOne and cdma2000 systems that

use a 1.25 MHz bandwidth.

UMTS transmitted signal characteristicsOne key element of the UMTS physical layer is the definition of the transmitted signal

characteristics. It is necessary to define the overall signal bandwidth and shape so that interference

is minimised for adjacent channels and users.The pulse shaping applied to the transmitted signals is

root raised cosine filtering with a roll-off-factor of 0.22.

The nominal carrier spacing is 5MHz, and the carrier centre frequencies are normally divisible by 5,

but the carrier frequency can be adjusted in increments of 200kHz. Accordingly the centre frequency

of UMTS carriers are indicated with an accuracy of 200kHz. This adjustment can be used to provide

operators with a more flexible use of their available spectrum.

One important characteristic of the signal is the way in which the signal spreads out either side of the

central area, and affecting other channels. It is never possible to have complete isolation or infinite

filtering and therefore spectral masks are defined showing elves that must be achieved for

compliance with the standard.

UMTS physical layer spectral mask

Shown in the diagram of the UMTS physical layer signal is the Adjacent Channel Leakage Ratio.

This is a measure of the signal level that appears in adjacent channels. ACLR 1 is the level in the

channel one up or down from the signal, and ACLR2 is two channels up or down.

The requirements are not surprisingly more stringent for base stations / NodeBs than for the

handsets or UEs.

ACLR REQUIREMENTS FOR UMTS PHYSICAL LAYER

ACLR1 ACLR2

UE / handset* 33dB 43dB

Base station 45dB 50dB

* ACLR values for handsets with power classes of 21dBm and 24dBm.

SynchronisationThe level of synchronisation required for the WCDMA system to operate is provided from the

Primary Synchronisation Channel (P-SCH) and the Secondary Synchronisation Channel (S-SCH).

These channels are treated in a different manner to the normal channels and as a result they are not

spread using the OVSFs and PN codes. Instead they are spread using synchronisation codes. There

are two types that are used. The first is called the primary code and is used on the P-SCH, and the

second is named a secondary code and is used on the S-SCH.

The primary code is the same for all cells and is a 256 chip sequence that is transmitted during the

first 256 chips of each time slot. This allows the UE to synchronise with the base station for the time

slot.

Once the UE has gained time slot synchronisation it only knows the start and stop of the time slot,

but it does not know information about the particular time slot, or the frame. This is gained using the

secondary synchronisation codes.

There is a total of sixteen different secondary synchronisation codes. One code is sent at the

beginning of the time slot, i.e. the first 256 chips. It consists of 15 synchronisation codes and there

are 64 different scrambling code groups. When received, the UE is able to determine before which

synchronisation code the overall frame begins. In this way the UE is able to gain complete

synchronisation.

The scrambling codes in the S-SCH also enable the UE to identify which scrambling code is being

used and hence it can identify the base station. The scrambling codes are divided into 64 code

groups, each having eight codes. This means that after achieving frame synchronisation, the UE

only has a choice of one in eight codes and it can therefore try to decode the CPICH channel. Once

it has achieved this it is able to read the BCH information and achieve better timing and it is able to

monitor the P-CCPCH.

UMTS power control

As with any CDMA system it is essential that the base station receives all the UEs at approximately

the same power level. If not, the UEs that are further away will be lower in strength than those closer

to the node B and they will not be heard. This effect is often referred to as the near-far effect. To

overcome this the node B instructs those stations closer in, to reduce their transmitted power, and

those further away to increase theirs. In this way all stations will be received at approximately the

same strength.

It is also important for node Bs to control their power levels effectively. As the signals transmitted by

the different node Bs are not orthogonal to one another it is possible that signals from different ones

will interfere. Accordingly their power is also kept to the minimum required by the UEs being served.

To achieve the power control there are two techniques that are employed: open loop; and closed

loop.

Open loop techniques are used during the initial access before communication between the UE and

node B has been fully established. It simply operates by making a measurement of the received

signal strength and thereby estimating the transmitter power required. As the transmit and receive

frequencies are different, the path losses in either direction will be different and therefore this

method cannot be any more than a good estimate.

Once the UE has accessed the system and is in communication with the node B, closed loop

techniques are used. A measurement of the signal strength is taken in each time slot. As a result of

this a power control bit is sent requesting the power to be stepped up or down. This process is

undertaken on both the up and downlinks. The fact that only one bit is assigned to power control

means that the power will be continually changing. Once it has reached approximately the right level

then it would step up and then down by one level. In practice the position of the mobile would

change, or the path would change as a result of other movements and this would cause the signal

level to move, so the continual change is not a problem.

There are very many frequency bands that are used to carry the 3G UMTS transmissions.

These frequency bands are allocated on an international basis to enable roaming and also to

allocate bands internationally to minimise interference.

UMTS frequency bands backgroundAs the use of 3G UMTS has grown, so too has the requirement for frequency allocations. Initially

bands in the region of 1885 - 2025 and 2110 - 2200 MHz were set aside.

These frequency bands were originally set aside at the World Administrative radio Conference in

1992, to enable use on a worldwide basis by administrations wishing to implement International

Mobile Telecommunications-2000, IMT-2000.

As the requirement for additional spectrum grew with the increased use of 3G UMTS, more

allocations were set aside.

Although not all bands are available in all countries, all bands are managed on an international

basis. In this way roaming is possible.

3G UMTS bandwidthUMTS uses wideband CDMA as the radio transport mechanism and the UMTS channels are spaced

by 5 MHz.

The UMTS signal bandwidth is normally considered to be 5 MHz but this figure includes the 0.58

MHz guard bands either side.

Therefore when the two guard bands, one either side, are excluded this leaves and effective signal

bandwidth of 3.84 MHz within the flat response area of the signal for the transmission itself.

It is also necessary to consider the roll-off factor for the signal of 0.22. This roll-off factor is

determined by the Root Raised Cosine filter specified by 3GPP. This means that the total signal

bandwidth increasing the skirts is 4.68 MHz.

It is also worth noting that the bandwidth used for the TD-SCDMA variant of 3G UMTS used in China

is 1.6 MHz.

UARFCN channel numbersUMTS carrier frequencies are designated by a UTRA Absolute Radio Frequency Channel Number,

UARFCN. The UARFCN is used to define channel numbers in an easy and unambiguous fashion.

The UARFCN can be easily calculated from the following equation or formula:

The UARFCN is only able to represent channels that are centred on a multiple of 200 kHz and these

do not always align with licensing in North America. Accordingly 3GPP added several special values

for the common North American channels.

3G UMTS frequency bands - FDDAs FDD, frequency division duplex requires bands for uplink and downlink, the bands for FDD are

different to those required for TDD time division duplex.

The main UMTS / WCDMA frequency bands for FDD operation are summarised below:

3G UMTS FREQUENCY BANDS - FDD

BAND NUMBER BAND COMMON NAME UL FREQUENCIES DL FREQUENCUES

1 2100 IMT 1920 - 1980 2120 - 2170

2 1900 PCS A-F 1850 - 1910 1930 - 1990

3 1800 DCS 1710 - 1785 1805 - 1880

4 1700 AWS A-F 1710 - 1755 2110 - 2155

5 850 CLR 824 - 849 869 - 894

6 800 830 - 840 875 - 885

7 2600 IMT-E 2500 - 2570 2620 - 2690

8 900 E-GSM 880 - 915 925 - 960

9 1700 1749.9 - 1784.9 1844.9 - 1879.9

10 1700 EAWS A-G 1710 - 1770 2110 - 2170

11 1500 LPDC 1427.9 - 1447.9 1475.9 - 1495.9

12 700 LSMH 699 - 716 729 - 746

13 700 USMH C 777 - 787 746 - 756

14 700 USMH D 788 - 798 758 - 768

19 800 832.4 - 842.6 877.4 - 887.6

20 800 EUDD 832 - 862 791 - 821

21 1500 UPDC 1447.9 - 1462.9 1495 - 1510.9

22 3500 3410 - 3490 3510 - 3590

25 1900 EPCS A-G 1850 - 1915 1930 - 1995

26 850 ECLR 814 - 849 859 - 894

Frequency bands 15, 16, 17, 18, 23 and 24 are now reserved frequency bands.

3G UMTS frequency bands - TDDThe main UMTS frequency bands for TDD operation are summarised below.

3G UMTS FREQUENCY BANDS - TDD

BAND REFERENCE BAND NAME FREQUENCIES

A Lower IMT 1900 - 1920

A Upper IMT 2010 - 2025

B Lower PCS 1850 - 1910

B Upper PCS 1930 - 1990

C PCS duplex gap 1910 - 1930

D IMT-E 2570 - 2620

E 2300 - 2400

F 1880 - 1920

It is also noted that several of the UMTS frequency bands overlap or share similar frequencies. This

is because the allocations are different in different areas, and each frequency band definition is

given a new band number for that particular band.

UMTS modulation schemesThere are several considerations that were taken into account when making the choice for the

overall format for the UMTS WCDMA modulation formats. Some of the considerations were:

It is necessary to ensure that the data is carried efficiently over the available spectrum, and

therefore maximum use is made of the available spectrum, and hence the capacity of the

system is maximised.

The modulation scheme should be chosen to ensure that the efficiency of the RF power

amplifier in the handset or UE is made as high as possible. By enabling the power amplifier

to be maximised, less battery power is consumed for the same transmitted power. As battery

power is of particular importance to users, this is a key requirement.

The modulation format should be chosen to avoid the audio interference caused to many

nearby electronics equipment resulting from the pulsed transmission format used on many

2G systems such as GSM

As the uplink and downlink have different requirements, the exact format for the modulation format

used on either direction is slightly different.

UMTS modulation schemes for both uplink and downlink, although somewhat different are both

based around phase shift keying formats. This provides many advantages over other schemes that

could be used in terms of spectral efficiency and other requirements.

Note on PSK:

Phase shift Keying, PSK is a form of modulation used particularly for data transmissions. If offers an effective way of

transmitting data. By altering the number of different phase states which can be adopted, the data speeds that can be

achieved within a given channel can be increased, but at the cost of lower resilience to noise an interference.

Click on the link for a PSK tutorial

Downlink modulationThe UMTS modulation format for the downlink is more straightforward than that used in the uplink.

The downlink uses quadrature phase shift keying, QPSK.

The QPSK modulation used in the downlink is used with time-multiplexed control and data streams.

While time multiplexing would be a problem in the uplink, where the transmission in this format

would give rise to interference in local audio systems, this is not relevant for the downlink where the

NodeB is sufficiently remote from any local audio related equipment to ensure that interference is not

a problem.

Uplink modulationHowever the uplink uses two separate channels so that the cycling of the transmitter on and off does

not cause interference on the audio lines, a problem that was experienced on GSM. The dual

channels (dual channel phase shift keying) are achieved by applying the coded user data to the I or

In-phase input to the DQPSK modulator, and control data which has been encoded using a different

code to the Q or quadrature input to the modulator.

There are many 3G UMTS channels that are used within the UMTS system. The data carried by the

UMTS / WCDMA transmissions is organised into frames, slots and channels.

In this way all the payload data as well as the control and status data can be carried in an efficient

manner.

3G UMTS channel structures3G UMTS uses CDMA techniques (as WCDMA) as its multiple access technology, but it additionally

uses time division techniques with a slot and frame structure to provide the full channel structure.

A channel is divided into 10 ms frames, each of which has fifteen time slots each of 666

microseconds length. On the downlink the time is further subdivided so that the time slots contain

fields that contain either user data or control messages.

On the uplink dual channel modulation is used so that both data and control are transmitted

simultaneously. Here the control elements contain a pilot signal, Transport Format Combination

Identifier (TFCI), FeedBack Information (FBI) and Transmission Power Control (TPC).

The channels carried are categorised into three:

Logical Channels:   The logical channels define the way in which the data will be

transferred

Transport Channels:   The 3G transport channels along with the logical channel again

defines the way in which the data is transferred

Physical channels:   The physical channels carry the payload data and govern the physical

characteristics of the signal.

The channels are organised such that the logical channels are related to what is transported,

whereas the physical layer transport channels deal with how, and with what characteristics. The

MAC layer provides data transfer services on logical channels. A set of logical channel types is

defined for different kinds of data transfer services.

3G UMTS Logical Channels:The 3G logical channels include:

Broadcast Control Channel (BCCH)   (downlink). This channel broadcasts information to

UEs relevant to the cell, such as radio channels of neighbouring cells, etc.

Paging Control Channel (PCCH)   (downlink). This channel is associated with the PICH

and is used for paging messages and notification information.

Dedicated Control Channel (DCCH)   (up and downlinks) This channel is used to carry

dedicated control information in both directions.

Common Control Channel (CCCH)   (up and downlinks). This bi-directional channel is used

to transfer control information.

Shared Channel Control Channel (SHCCH)   (bi-directional). This channel is bi-directional

and only found in the TDD form of WCDMA / UMTS, where it is used to transport shared

channel control information.

Dedicated Traffic Channel (DTCH)   (up and downlinks). This is a bidirectional channel

used to carry user data or traffic.

Common Traffic Channel (CTCH)   (downlink) A unidirectional channel used to transfer

dedicated user information to a group of UEs.

3G UMTS Transport Channels:The 3G UMTS transport channels include:

Dedicated Transport Channel (DCH)   (up and downlink). This is used to transfer data to a

particular UE. Each UE has its own DCH in each direction.

Broadcast Channel (BCH)   (downlink). This channel broadcasts information to the UEs in

the cell to enable them to identify the network and the cell.

Forward Access Channel (FACH)   (down link). This is channel carries data or information

to the UEs that are registered on the system. There may be more than one FACH per cell as

they may carry packet data.

Paging Channel (PCH)   (downlink). This channel carries messages that alert the UE to

incoming calls, SMS messages, data sessions or required maintenance such as re-

registration.

Random Access Channel (RACH)   (uplink). This channel carries requests for service from

UEs trying to access the system

Uplink Common Packet Channel (CPCH)   (uplink). This channel provides additional

capability beyond that of the RACH and for fast power control.

Downlink Shared Channel (DSCH)   (downlink).This channel can be shared by several

users and is used for data that is "bursty" in nature such as that obtained from web browsing

etc.

3G UMTS Physical Channels:The 3G UMTS physical channels include:

Primary Common Control Physical Channel (PCCPCH)   (downlink). This channel

continuously broadcasts system identification and access control information.

Secondary Common Control Physical Channel (SCCPCH)   (downlink) This channel

carries the Forward Access Channel (FACH) providing control information, and the Paging

Channel (PACH) with messages for UEs that are registered on the network.

Physical Random Access Channel (PRACH)   (uplink). This channel enables the UE to

transmit random access bursts in an attempt to access a network.

Dedicated Physical Data Channel (DPDCH)   (up and downlink). This channel is used to

transfer user data.

Dedicated Physical Control Channel (DPCCH)   (up and downlink). This channel carries

control information to and from the UE. In both directions the channel carries pilot bits and

the Transport Format Combination Identifier (TFCI). The downlink channel also includes the

Transmit Power Control and FeedBack Information (FBI) bits.

Physical Downlink Shared Channel (PDSCH)   (downlink). This channel shares control

information to UEs within the coverage area of the node B.

Physical Common Packet Channel (PCPCH)   This channel is specifically intended to

carry packet data. In operation the UE monitors the system to check if it is busy, and if not it

then transmits a brief access burst. This is retransmitted if no acknowledgement is gained

with a slight increase in power each time. Once the node B acknowledges the request, the

data is transmitted on the channel.

Synchronisation Channel (SCH)   The synchronisation channel is used in allowing UEs to

synchronise with the network.

Common Pilot Channel (CPICH)   This channel is transmitted by every node B so that the

UEs are able estimate the timing for signal demodulation. Additionally they can be used as a

beacon for the UE to determine the best cell with which to communicate.

Acquisition Indicator Channel (AICH)   The AICH is used to inform a UE about the Data

Channel (DCH) it can use to communicate with the node B. This channel assignment occurs

as a result of a successful random access service request from the UE.

Paging Indication Channel (PICH)   This channel provides the information to the UE to be

able to operate its sleep mode to conserve its battery when listening on the Paging Channel

(PCH). As the UE needs to know when to monitor the PCH, data is provided on the PICH to

assign a UE a paging repetition ratio to enable it to determine how often it needs to 'wake up'

and listen to the PCH.

CPCH Status Indication Channel (CSICH)   This channel, which only appears in the

downlink carries the status of the CPCH and may also be used to carry some intermittent, or

"bursty" data. It works in a similar fashion to PICH.

Collision Detection/Channel Assignment Indication Channel (CD/CA-ICH)   This

channel, present in the downlink is used to indicate whether the channel assignment is

active or inactive to the UE.

By using the logical, physical and transport channels it is possible to carry the data for the control

and payload in a structured manner and provide efficient effective communications. The 3G UMTS

channels are thus an essential element of the overall system.

UMTS TDD (Universal mobile telecommunications system - time division duplex) is a growing

cellular technology. Although UMTS TDD or TD WCDMA is not as widely deployed as the more

popular UMTS FDD which is being deployed for the 3G mobile phone systems, UMTS TDD is

nevertheless being widely used and providing a viable service for many applications. In particular it

is being used to provide mobile broadband data services, and other applications may include its use

in providing mobile TV applications. In this way, UMTS is a growing cellular technology which will be

far more widely used in the years to come

TDD - time division duplexA communications system requires that communication is possible in both directions: to and from the

base station to the remote station. There are a number of ways in which this can be achieved. The

most obvious is to transmit on one frequency and receive on another. The frequency difference

between the two transmissions being such that the two signals do not interfere. This is known as

frequency division duplex (FDD) and it is one of the most commonly used schemes, and it is used by

most cellular schemes.

It is also possible to use a single frequency and rather than using different frequency allocations, use

different time allocations. If the transmission times are split into slots, then transmissions in one

direction take place in one time slot, and those in the other direction take place in another. It is this

scheme that is known as time division duplex, TDD, and it is used for UMTS-TDD.

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is necessary to have what

is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they

can transmit and receive. There are several methods that can be adopted. For applications including wireless and

cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously,

two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and

the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates

different time slots for transmission and reception.

Click on the link for more information on TDD FDD duplex schemes

When using a TDD system, there are a number of characteristics that are pertinent for TDD

systems. These characteristics need to be accommodated when developing or using TDD systems.

Utilisation of unpaired bands:   Typically there is more traffic in the downlink (network to

the mobile) than in the uplink (mobile to network). Accordingly the operator is able to allocate

more time to the downlink transmission than the uplink. This is not possible with the paired

spectrum required for FDD systems where it is not possible to re-allocate the use of the

different bands. As a result of this, it is possible to make very efficient use of the available

spectrum.

Discontinuous transmission:   In any TDD system it is necessary to switch between transit

and receive. This takes a certain amount of time. Not only does it take time for the mobile

and the base station to change between transmit and receive in terms of ramping up or down

the power, along with the settling of any transients. In addition to this the time is required

between transmit and receive to accommodate the transmission time between the mobile

and the base station. As a result a guard band is required.

Uplink / downlink interference:   As both uplink and downlink share the same channel

there can be interference between the two transmission directions. To overcome this, base

stations are synchronised to ensure that they do not transmit when an adjacent base station

is receiving, otherwise the better siting and possible higher power level will cause

interference.

Equivalent conditions for uplink and downlink:   As both uplink and downlink use the

same channel, they are subject to the same propagation conditions. With FDD systems

using different frequencies for the uplink and downlink there are significant differences. By

using the same frequency fading conditions can be counteracted more effectively.

UMTS TDD / FDD comparisonWhile UMTS TDD and UMTS FDD are both specified in the same standard and share very many

properties, there are naturally some differences.

PARAMETER UMTS TDD UMTS FDD

Multiple access method TDMA, CDMA CDMA

Duplex method TDD FDD

Channel spacing 5 MHz[1] 5 MHz

PARAMETER UMTS TDD UMTS FDD

Carrier chip rate 3.84 Mcps 3.84 Mcps

Time slot structure 15 / 14 slots / frame 15 slots / frame

Frame length (ms) 10 10

Multirate concept Multicode, multislot and OVSF[2] Multicode, and OVSF[2]

Burst types (1) traffic bursts(2) random access burst(3) synchronisation burst

N/A

Detection Coherent based on midamble Coherent based on pilot symbols

Dedicated channel power control

Uplink: open loop 100 Hz or 200 Hz rateDownlink: closed loop max 800 Hz rate

Fast closed loop 1500 Hz rate

Spreading factors 1 .. 16 4 .. 512

Notes

[1] for TD-SCDMA the channel spacing is 1.6 MHz

[2] OVSF = Orthogonal variable Spreading Factor

UMTS TDD within 3GPPAll the standards for UMTS 3G systems have been defined under the auspices of 3GPP - the third

generation partnership project. The standards not only define the FDD systems, but also the TDD

system.

In these specifications, it was the original intent of UMTS that the TDD spectrum would be used to

provide high data rates in selected areas forming what could be termed 3G hot zones.

UMTS TDD detailsUMTS TDD uses many of the same basic parameters as UMTS FDD. The same 5 MHz channel

bandwidths are used. UMTS TDD also uses direct sequence spread spectrum and different users

and what can be termed "logical channels" are separated using different spreading codes. Only

when the receiver uses the same code in the correlation process, is the data recovered. In W-CDMA

all other logical channels using different spreading codes appear as noise on the channel and

ultimately limit the capacity of the system. In UMTS TDD, a scheme known as multi user detection

(MUD) is employed in the receiver and improves the removal of the interfering codes, allowing

higher data rates and capacity.

In addition to the separation of users by using different logical channels as a result of the different

spreading codes, further separation between users may be provided by allocating different time

slots. There are 15 time slots in UMTS TDD. Of these, three are used for overhead such as

signalling, etc and this leaves twelve time slots for user traffic. In each timeslot there can be 16

codes. Capacity is allocated to users on demand, using a two dimensional matrix of timeslots and

codes.

In order for UMTS TDD to achieve the best overall performance, the transport format, i.e. the

modulation and forward error correction can be altered for each user. The schemes are chosen by

the network, and will depend on the signal characteristics in both directions. Higher order forms of

modulation enable higher data speeds to be accommodated, but they are less resilient to noise and

interference, and this means that the higher data rate modulation schemes are only used when

signal strengths are high. Additionally the levels of forward error correction can be changed. When

errors are likely, i.e. when signal strengths are low or interference levels are high, Similarly higher

levels of forward error correction are needed under low require additional data to be sent and this

slows the payload transfer rate. Thus it is possible to achieve much higher data transfer rates when

signals are strong and interference levels are low.

Spectrum allocations for UMTS TDDStandard allocations of radio spectrum have been made for 3G telecommunications systems in most

countries around the globe. In Europe and many other areas spectrum has been allocated for UMTS

FDD between 1920MHz to 1980MHz and 2110MHz to 2170MHz. For UMTS TDD spectrum is

primarily located between 1900MHz and 1920MHz and between 2010MHz and 2025MHz. In

addition to this there are some other allocations around 3 GHz.

UMTS TDD performanceUMTS TDD is able to support high peak data rates. Release 5 of the UMTS standard provides

HSDPA (high-speed downlink packet access). The scheme allows the use of a higher order

modulation scheme called 16-QAM (16 point quadrature amplitude modulation), which enables peak

rates of 10 Mbps per sector in commercial deployments. The next release increases the modulation

to 64-QAM, and introduces intercell interference cancellation (called Generalized MUD) and MIMO

(multiple in, multiple out). In combination, these increase the peak rate to 31 Mbps per sector

TD-SCDMA is an additional TDD version of UMTS. Devised in China, the system provides a number

of advantages in several applications. TD-SCDMA has been adopted as a 3G standard by the

International Telecommunications Union (ITU), and it is part of the 3GPP UMTS system being

defined in the 3GPP standards.

Much of the initial work for the system was undertaken by the China Academy of

Telecommunications Technology (CATT). Apart from the advantages of the basoc TDD approach,

TD-SCDMA is able to support IP services, and it has been designed to incorporate new technologies

such as joint detection, adaptive antennas, and dynamic channel allocation

While similar in many was to UMTS TDD, TD-SCDMA is has a number of differences and handsets

for the two systems would not be compatible unless the capability for both systems was specifically

built in to them.

TD-SCDMA basicsOne of the key elements of TD-SCDMA is the fact that it uses a TDD, Time Division Duplex

approach. As seen with UMTS TDD this has advantages in a number of areas, enabling the balance

to be changed between uplink and downlink to accommodate the different levels of data transfer. It

also has advantages in terms of using unpaired spectrum, spectrum efficiency for certain loads and

it does not require expensive diplexers in the handsets to enable simultaneous transmission on the

uplink and downlink, although transmit / receive switching times must be accommodated and can

reduce the efficiency of the system.

Note on TDD and FDD duplex schemes:

In order for radio communications systems to be able to communicate in both directions it is necessary to have what

is termed a duplex scheme. A duplex scheme provides a way of organizing the transmitter and receiver so that they

can transmit and receive. There are several methods that can be adopted. For applications including wireless and

cellular telecommunications, where it is required that the transmitter and receiver are able to operate simultaneously,

two schemes are in use. One known as FDD or frequency division duplex uses two channels, one for transmit and

the other for receiver. Another scheme known as TDD, time division duplex uses one frequency, but allocates

different time slots for transmission and reception.

Click on the link for more information on TDD FDD duplex schemes

As a further advantage, TD-SCDMA uses the same RAN as that used for UMTS. In this way it is

possible to run TD-SCDMA alongside UMTS, and thereby simplifying multi-system designs.

Although UMTS (W-CDMA) and cdma2000 are widely recognized as 3G cellular standards, TD-

SCDMA is equally valid. In fact it has been adopted as the low chip rate (LCR) version of the 3GPP

TDD standard.

TD-SDCMA specification overviewThe TD-SCDMA standard provides many advantages. As already mentioned it has many similarities

to W-CDMA, although a summary of the basic features and specification is given below:

CHARACTERISTIC FIGURE

Bandwidth 1.6 MHz

Chip rate per carrier 1.28 Mcps

Frame Rate 10ms

Spectrum spreading mode DS SF=1/2/4/8/16

Modulation QPSK / 8PSK / 16QAM

Channel coding Convolutional codes: R=1/2,1/3 Turbo implemented

Interleaving 10/20/40/80 ms

Frame structure Super frame 720ms,Radio frame 10msSubframe 5 ms

Uplink synchronisation 1/2 chip

Number of voice channels per carrier 48

Spectrum Efficiency 25Erl./MHz

Total transmission rate provided by each carrier

1.971Mbps

TD-SCDMA operationThe UMTS TD-SCDMA system has adopted a number of advanced techniques and technologies to

optimise the operation. These are often above and beyond those that have been catered for in the

more widely used standard forms of FDD and TDD UMTS. Some of these result from the fact that

TD-SCDMA uses the same frequency for both uplink and downlink, and as a result of the higher

processing levels now available.

These include:

Smart antennas:   Smart antenna technology is incorporated into the base station. This

enables beams to be formed and this is able to reduce interference between terminals and

concentrate transmitted power at active terminals. This technique is implemented using

smart antenna arrays that incorporate advanced DSP algorithms. The base station is able to

locate the mobile terminals and to steer transmit beams to specific terminals. In this way

spatial beamforming is able to reduce interference within a given channel with a resulting

improvement in the downlink capacity.

Joint detection technology:   Within CDMA, multiple users all occupy the same frequency

band, accessing he base station using different codes. In this way, multiple-access

interference results and this is a major problem in CDMA-based systems. A technique

referred to as joint detection technology treats signals from all users as useful and processes

them in parallel. As the maximum number of users in any time slot is 16, the processing

complexity to separate users is kept within manageable limits.

User terminals and base station synchronisation:   The synchronisation of the network

enables precise adjustment of the timing advances for transmission from terminals so that

signals from different users arrive at the base station together, and not overlapping in time

into the transmit time frames making detection much simpler. This synchronisation enables

faster search for neighbouring cells during handover and it also removes the need for soft

handover.

Handover or handoff is as important for UMTS as any other form of cellular telecommunications

system. As with any other cellular telecommunications system it is essential that UMTS handover is

performed seamlessly so that the user is not aware of any change. Any failures within the UMTS

handover (or UMTS handoff) procedure will lead to dropped calls which will in turn result in user

dissatisfaction and ultimately it may lead to users changing networks, thereby increasing the churn

rate.

It is worth noting that the two terms UMTS handover and UMTS handoff have the same meaning.

UMTS handover tends is the terminology that tends to be used within Europe, whereas UMTS

handoff is more likely to be used within North America.

UMTS handover typesWithin UMTS it is possible to define a number of different types of UMTS handover or handoff. With

the advent of generic CDMA technology, new possibilities for effecting more reliable forms of

handover became possible, and as a result one of a variety of different forms of handover are

available depending upon the different circumstances.

For purely inter W-CDMA technology, there are three basic types of handover:

Hard handover:   This form of handover is essentially the same as that used for 2G

networks where one link is broken and another established.

Soft handover:   This form of handover is a more gradual and the UE communicates

simultaneously with more than one Node B or base station during the handover process.

Softer handover:   Not a full form of UMTS handover, but the UE communicates with more

than one sector managed by the same NodeB.

UMTS GSM inter RAT handover:   This form of handover occurs when mobiles have to

change between Radio Access Technologies.

Each of the different types of handover is used on different occasions dependent upon the

conditions. Further details of each type of UMTS handover are given in the individual sections below.

UMTS hard handoverThe name hard handover indicates that there is a "hard" change during the handover process. For

hard handover the radio links are broken and then re-established. Although hard handover should

appear seamless to the user, there is always the possibility that a short break in the connection may

be noticed by the user.

The basic methodology behind a hard handover is relatively straightforward. There are a number of

basic stages of a hard handover:

1. The network decides a handover is required dependent upon the signal strengths of the

existing link, and the strengths of broadcast channels of adjacent cells.

2. The link between the existing NodeB and the UE is broken.

3. A new link is established between the new NodeB and the UE.

Although this is a simplification of the process, it is basically what happens. The major problem is

that any difficulties in re-establishing the link will cause the handover to fail and the call or

connection to be dropped.

UMTS hard handovers may be used in a number of instances:

When moving from one cell to an adjacent cell that may be on a different frequency.

When implementing a mode change, e.g. from FDD to TDD mode, for example.

When moving from one cell to another where there is no capacity on the existing channel,

and a change to a new frequency is required.

One of the issues facing UMTS hard handovers was also experienced in GSM. When usage levels

are high, the capacity of a particular cell that a UE is trying to enter may be insufficient to support a

new user. To overcome this, it may be necessary to reserve some capacity for new users. This may

be achieved by spreading the loading wherever possible - for example UEs that can receive a

sufficiently strong signal from a neighbouring cell may be transferred out as the original cell nears its

capacity level.

3G UMTS soft handoverSoft handover is a form of handover that was enabled by the introduction of CDMA. Soft handover

occurs when a UE is in the overlapping coverage area of two cells. Links to the two base stations

can be established simultaneously and in this way the UE can communicate with two base stations.

By having more than one link active during the handover process, this provides a more reliable and

seamless way in which to perform handover.

In view of the fact that soft handover uses several simultaneous links, it means that the adjacent

cells must be operating on the same frequency or channel as UEs do not have multiple transmitters

and receivers that would be necessary if they were on different frequencies.

When the UE and NodeB undertake a soft handover, the UE receives signals from the two NodeBs

and combines them using the RAKE receiver capability available in the signal processing of the UE.

In the uplink the situation is more complicated as the signal combining cannot be accomplished in

the NodeB as more than one NodeB is involved. Instead, combining is accomplished on a frame by

frame basis. The best frames are selected after each interleaving period. The selection is

accomplished by using the outer loop power control algorithm which measures the signal to noise

ratio (SNR) of the received uplink signals. This information is then used to select the best quality

frame.

Once the soft handover has been completed, the links to the old NodeB are dropped and the UE

continues to communicate with the new NodeB.

As can be imagined, soft handover uses a higher degree of the network resources than a normal

link, or even a hard handover. However this is compensated by the improved reliability and

performance of the handover process. However with around 5 to 10% of handovers falling into this

category, network operators need to account for it. 

Note on the RAKE receiver

A RAKE receiver is a form of radio receiver that has been made feasible in many areas by the use of digital signal

processing, DSP. It is often used to overcome the effects of multipath propagation. It achieves this by using several

sub-receivers known as "fingers" which are given a particular multipath component. Each finger then processes its

component and decodes it. The resultant outputs from the fingers are then combined to provide the maximum

contribution from each path. In this way rake receivers and multipath propagation can be used to improve the signal

to noise performance.

3G UMTS softer handoverA form of handover referred to as softer handover is really a special form of soft handover. It is a

form of soft handover that occurs when the new radio links that are added are from the same

NodeB. This occurs when several sectors may be served from the same NodeB, thereby simplifying

the combining as it can be achieved within the NodeB and not require linking further back into the

network.

UMTS softer handover is only possible when a UE can hear the signals from two sectors served by

the same NodeB. This may occur as a result of the sectors overlapping, or more commonly as a

result of multipath propagation resulting from reflections from buildings, etc.

In the uplink, the signals received by the NodeB, the signals from the two sectors can be routed to

the same RAKE receiver and then combined to provide an enhanced signal.

In the downlink, it is a little more complicated because the different sectors of the NodeB use

different scrambling codes. To overcome this, different fingers of the RAKE receiver apply the

appropriate de-spreading or de-scrambling codes to the received signals. Once this has been done,

they can be combined as before.

In view of the fact that a single transmitter is used within the UE, only one power control loop is

active. This may not be optimal for all instances but it simplifies the hardware and general operation.

Inter-RAT / Intersystem or iRAT handoverIn many instances it is necessary for the UMTS radio access network to handover to the 2G GSM

network. These handovers are given a variety of names including Inter-RAT handover as they are

handing over between different forms of Radio Access Technology, Intersystem Handover, and

UMTS / GSM Handover. These handovers may be required for one of a variety of reasons including:

Limited UMTS coverage

UMTS network busy whereas spare capacity is available on GSM network

The most common form of intersystem or inter-RAT handover is between UMTS and GSM. There

are two different types of inter-RAT handover or iRAT handover:

UMTS to GSM handover:   There are two further divisions of this category of handover:

o Compressed mode handover:    Using compressed mode handover the UE uses the

gaps in transmission that occur to analyse the reception of local GSM base stations.

The UE uses the neighbour list provided by the UMTS network to monitor and select

a suitable candidate base station. Having selected a suitable base station the

handover takes place, but without any time synchronisation having occurred.

o Blind handover:    This form of handover occurs when the base station hands off the

UE by passing it the details of the new cell to the UE without linking to it and setting

the timing, etc of the mobile for the new cell. In this mode, the network selects what it

believes to be the optimum GSM based station. The UE first locates the broadcast

channel of the new cell, gains timing synchronisation and then carries out non-

synchronised intercell handover.

Handover from GSM to UMTS :   This form of handover is supported within GSM and a

"neighbour list" was established to enable this occur easily. As the GSM / 2G network is

normally more extensive than the 3G network, this type of handover does not normally occur

when the UE leaves a coverage area and must quickly find a new base station to maintain

contact. The handover from GSM to UMTS occurs to provide an improvement in

performance and can normally take place only when the conditions are right. The neighbour

list will inform the UE when this may happen.

UMTS handover methodologyThe decisions about handover are generally handled by the RNC. It continually monitors information

regarding the signals being received by both the UE and NodeB and when a particular link has fallen

below a given level and another better radio channel is available, it initiates a handover. As part of

this monitoring process, the UE measures the Received Signal Code Power (RSCP) and Received

Signal Strength Indicator (RSSI) and the information is then returned to the node B and hence to the

RNC on the uplink control channel.