Handover in

UMTS

Stud. Ing. Coroi Alexandru Mihai Prof. Coord. Dr. Ing. Balint Cornel

An2, Master of CN.

Content:

1. Introduction………………………………………………………….………………..……..3

2. UMTS Handover Types……………………………………………………………………...5

2.1. UMTS Handover Types……………………………………………………………5

2.2. Types of air Interface Measurements……………………………………………....7

3. UMTS Hard Handover………………………………………………………………………7

4. UMTS Soft Handover……………………………………………………………………….9

5. UMTS Softer Handover…………………………………………………………………….11

6. Inter-RAT / Intersystem UMTS / GSM handover………………………………………….13

7. Bibliography………………………………………………………………………………..15

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1.Introduction

Handover, which means the transfer of a ceellphone user from a cell to another is a

necessity of communication. This can be between different cells, or between two sectors of the

same cell.

The American term is often “handoff”, which is the most used within some American

organizations such 3GPP2 and in American originated technologies such as CDMA2000. In

Europe, the term is more common within international and European organizations such as ITU-

T, IETF, ETSI and 3GPP, and standardized within European originated standards such GSM and

UMTS. In literature most common is the term handover. At least, UMTS handover tends to be

used in Europe, while the UMTS handoff is more likely used within North America.

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.

The reasons that may lead to handover procedure:

When the phone is moving away from the area covered by one cell and entering the area

covered by another cell the call is transferred to the second cell in order to avoid call termination

when the phone gets outside the range of the first cell;

When the capacity for connecting new calls of a given cell is used up and an existing or

new call from a phone, which is located in an area overlapped by another cell, is transferred to

that cell in order to free-up some capacity in the first cell for other users, who can only be

connected to that cell.

In networks which are not using CDMA, when the channel used by the phone becomes

interfered by another phone using the same channel in a different cell, the call is transferred to a

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different channel in the same cell or to a different channel in another cell in order to avoid the

interference.

Another situation is still in a non-CDMA cell, for the call to be transferred to a larger

umbrella-type of cell in order to minimize the frequency of the handovers due to this movement),

when a fast-travelling user, connected to a large, umbrella-type of cell, stops then the call may be

transferred to a smaller macro cell or even to a micro cell in order to free capacity on the

umbrella cell for other fast-traveling users and to reduce the potential interference to other cells

or users (this works in reverse too, when a user is detected to be moving faster than a certain

threshold).

Cellular networks

The aim of network and service providers to offer a wide variety of – often bandwidth

extensive – services to a broad market of users via wireless networks, made the scarcity of the

radio frequency spectrum a hot political issue in the telecommunications market. To use the

spectrum more efficiently cellular systems were designed. In opposite to “old” communication

systems – using one transmitter transmitting at high power levels in a limited channel – a cellular

architecture uses many transmitters at low power what makes it able to reuse frequencies.

Traditional cellular systems are designed so that adjacent cells use different frequencies. As long

as the cells are separated and the signal strength calibrated, there will not be harmful inter-cell

interference. The picture on the next page shows the typical layout of a seven-way frequency

reuse system often used in GSM networks. Cell 1 makes use of frequency 1f , in cell 2 frequency

2f is transmitted. In this example every seventh cell reuses a certain frequency, hence this

architecture is said to have a frequency reuse factor of N = 7. Sometimes the D/R ratio is used to

characterize the frequency reuse. D represents the minimum distance between cells using the

same frequencies; R is the radius of the hexagonal cell. For N=7, D/R equals 4,6; following the

relation.

NR

D3

, (1)

In UMTS systems the radio spectrum can be used even more efficient by applying

CDMA as multiple access scheme, resulting in a frequency reuse factor of theoretically (1).

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Fig1 Seven-way frequency reuse cellular system representation

During the spectrum assignment process a service provider is usually given a portion of

the total spectrum band allocated to one technology. This spectrum band is further divided into

smaller slices dedicated to the different base stations in the cells. On top of this frequency

division architecture a suitable multiple access scheme is deployed. In GSM networks Time

Division Multiple Access (TDMA) is used to efficiently distribute the bandwidth inside a cell to

the users. The UMTS system uses CDMA as multiple access scheme to utilise the bandwidth as

efficient as possible.

CDMA also enables different data rates for different services in a more flexible way. The

next chapter deals with CDMA and the UMTS access part in more detail.

2. UMTS Handover Types

2.1. UMTS handover types

Within 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

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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 Node B.

- 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 with dependency

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

sections below.

Generally we can distinguish between intra-cell handover and inter-cell handover. For

UMTS the following types of handover are specified:

Handover 3G -3G (i.e. between UMTS and other 3G systems)

FDD soft/softer handover

FDD inter-frequency hard handover

FDD/TDD handover (change of cell)

TDD/FDD handover (change of cell)

TDD/TDD handover

Handover 3G - 2G (e.g. handover to GSM)

Handover 2G - 3G (e.g. handover from GSM)

The most obvious cause for performing a handover is that due to its movement a user can

be served in another cell more efficiently (like less power emission, less interference). It may

however also be performed for other reasons such as system load control.

- Active Set is defined as the set of Node-Bs the UE is simultaneously connected to (i.e.,

the UTRA cells currently assigning a downlink DPCH to the UE constitute the active set).

- Cells, which are not included in the active set, but are included in the CELL INFO LIST

belong to the Monitored Set.

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- Cells detected by the UE, which are neither in the CELL_INFO_LIST nor in the active

set belong to the Detected Set. Reporting of measurements of the detected set is only applicable

to intra-frequency measurements made by UEs in CELL_DCH state.

2.2. The different types of air interface measurements are:

- Intra-frequency measurements: measurements on downlink physical channels at the

same frequency as the active set. A measurement object corresponds to one cell.

- Inter-frequency measurements: measurements on downlink physical channels at

frequencies that differ from the frequency of the active set. A measurement object corresponds to

one cell.

- Inter-RAT measurements: measurements on downlink physical channels belonging to

another radio access technology than UTRAN, e.g. GSM. A measurement object corresponds to

one cell.

- Traffic volume measurements: measurements on uplink traffic volume. A measurement

object corresponds to one cell.

- Quality measurements: Measurements of downlink quality parameters, e.g. downlink

transport block error rate. A measurement object corresponds to one transport channel in case of

BLER. A measurement object corresponds to one timeslot in case of SIR (TDD only).

- UE-internal measurements: Measurements of UE transmission power and UE received

signal level.

- UE positioning measurements: Measurements of UE position. The UE supports a

number of measurements running in parallel. The UE also supports that each measurement is

controlled and reported independently of every other measurement.

3. UMTS Hard Handover

The 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.

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

number of basic stages of a hard handover:

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. The link between the

existing Node B and the UE is broken. 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.

As it is mentioned, a handover is generally performed when the quality of the link

(measured in terms of the power of the received pilot) between the Node B and the UE on the

move is decreasing and it is possible to hand over the connection to another cell with better radio

characteristics. In previous 2G systems like GSM, the handover process tears down (i.e. literarily

interrupts the connection for a short period of time, not noticeable by the end user) an existing

connection and replaces it with a new connection to a new cell where the user is handed over

with a different frequency (concept known as “hard handover”). This cell where the user is

handed over is so-called the “”target” cell. Since all cells in W-CDMA use the same frequency,

in 3G systems it is possible to make the connection to the new cell before leaving the current cell

and keeping always at least one radio link with a Node B. This concept is known as "soft"

handover. Hard Handover however, is also used in 3G systems when it is needed to change the

frequency of the carrier, either performing inter-frequency handover (i.e. change of UMTS

carrier frequency for balancing load purposes) or performing Inter-RAT (Radio Access

Technology).

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.

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Fig.2. Hard Handover Example

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.

3. UMTS Soft Handover

Soft 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 Node B undertake a soft handover, the UE receives signals from the

two Node Bs and combines them using the RAKE receiver capability available in the signal

processing of the UE.

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

accomplished in the Node B as more than one Node B 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.

Fig 3. Soft Handover Example

Once the soft handover has been completed, the links to the old Node B are dropped and

the UE continues to communicate with the new Node B.

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: 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.

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4. UMTS Softer Handover

A 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 Node B. This occurs when several sectors may be served from the same Node B,

thereby simplifying the combining as it can be achieved within the Node B and not require

linking further back into the network.

Fig.4. Softer Handover Example

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

served by the same Node B. 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 Node B, 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 Node

B 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 summary, in 3G systems there are two new handover concepts: Soft and Softer

handover, and they basically mean that it is possible to keep two or more concurrent connections

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with different Node Bs (Soft handover) or with the same Node B (e.g. when multi-path

propagation between the UE and Node B makes the Node B to receive the signal sent from the

UE from two different sectors). In both soft/softer handover, the UE always keeps at least one

radio link to the UTRAN. Both concepts are illustrated in the next Figure.

Fig.5. A comparison between Softer Handover and Soft Handover

To keep track of the number of connections, the concept of the Active Set is required.

The Active Set, as it is defined in Ericsson handover, is the set of cells used for a particular UE

connection. The UE has a radio link established to each of the cells present in its Active Set. This

set is updated dynamically (event based) during all the time that a connection is alive, based on

the measurements of the strength of the Primary Common Pilot Channel (P-CPICH) Ec/Io or the

Primary Common Pilot Channel (PCPICH) RSCP (Received Signal Code Power).

Ec/Io can be defined in terms of RSCP in the following way:

, (2)

where RSCP represents the power (measured in the UE) carried by the decoded pilot channel and

RSSI (Received Signal Strength Indicator) is the total wideband received power (measured in the

UE) within the channel bandwidth. So basically, during a user service session, there are these

possible events related to Active set updating (using the name of the events described in):

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• Event 1A : add new cell

• Event 1B : remove cell

• Event 1C : replace cell (if the active set is full)

• Event 1D : change best cell

Best Cell, according to Ericsson handover, is defined as the cell, among the ones in the

Active Set, having a measured P-CPICH with the highest quality Ec/No. From a Node B’s point

of view, an incoming handover request is similar to an incoming call, although the RRM

algorithms can differentiate whether the request comes from a Handover Connection or not, as it

is the case in the Ericsson RRM algorithms. Given that using handover in an appropriate way

leads to an improvement in capacity because of the soft hand over gain, the handover

connections have less probability to be blocked than new incoming non-handover calls. This

feature can be seen clearly in all the Ericsson Diagrams where different thresholds for blocking

are set-depending if the connection is guaranteed (e.g. voice) or not guaranteed (e.g. Web),

handover or non handover (i.e. new request) call. The inability to establish a new connection in

the target cell is referred to as a “handover failure” and it occurs when no new resources are

available in the target cells or when the radio link quality has decreased below acceptable levels

before the call could be handed-over (Handover thesis). The first reason leads to Handover

Blocked attempts and the last one leads to Handover Dropped attempts and both are good

measures of the Handover Performance in the network.

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.

5. Inter-RAT / Intersystem UMTS / GSM Handover.

In 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 RATs (Radio Access Technology),

Intersystem Handover, and UMTS / GSM Handover. These handovers may be required for one

of a variety of reasons including:

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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:

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

a) Compressed mode handover:   Using compressed mode handover the UE uses

the gaps in transmission that occur to analyze 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 synchronization having occurred.

b) 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 synchronization and then carries out non-synchronized 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.

As expanding markets demand increasing capacity there is a trend towards reducing the

size of cells in mobile communications systems (besides increasing the used frequency band).

This results in more frequent handovers and it is important to remark that this makes a reliable

handover mechanism more than ever desirable for efficient operation of any future cellular

mobile network.

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Bibliography:

[1] Stijn Van Cauwenberge, “Study of Soft Handover in UMTS”, Technical University of

Denmark, 2003.

[2] Rukhsar Ahmad Cheema, “Issues and Optimization of UMTS Handover”, Blekinge Institute

of Technology, feb. 2008 Sweden, (Superviser: Doru Constantinescu).

[3] A. F. Cosme, “UMTS Capacity Simulation Study”, 2003.

[4] http://www.umtsworld.com/

[5] http://www.radio-electronics.com

[6] http://www.wikipedia.com

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