Universal Mobile Telecommunications SystemFrom Wikipedia, the free encyclopedia

"3GSM" redirects here. For the mobile exhibition, see Mobile World Congress.

UMTS Network Architecture

Universal Mobile Telecommunications System (UMTS) is one of the third-

generation (3G) mobile telecommunicationstechnologies, which is also being

developed into a 4G technology. The first deployment of the UMTS is the release99

(R99) architecture. It is specified by 3GPP and is part of the global ITU IMT-

2000 standard. The most common form of UMTS usesW-CDMA (IMT Direct Spread)

as the underlying air interface but the system also covers TD-CDMA and TD-

SCDMA (both IMT CDMA TDD). Being a complete network system, UMTS also

covers the radio access network (UMTS Terrestrial Radio Access Network, or

UTRAN) and the core network (Mobile Application Part, or MAP), as well as

authentication of users via USIM cards (Subscriber Identity Module).

Unlike EDGE (IMT Single-Carrier, based on GSM) and CDMA2000 (IMT Multi-

Carrier), UMTS requires new base stations and new frequency allocations. However,

it is closely related to GSM/EDGE as it borrows and builds upon concepts from GSM.

Further, most UMTS handsets also support GSM, allowing seamless dual-mode

operation. Therefore, UMTS is sometimes marketed as 3GSM, emphasizing the

close relationship with GSM and differentiating it from competing technologies.

The name UMTS, introduced by ETSI, is usually used in Europe. Outside of Europe,

the system is also known by other names such as FOMA [1]  or W-CDMA.[nb 1][1] In

marketing, it is often referred to as 3G or 3G+.

Interoperability and global roaming

UMTS phones (and data cards) are highly portable—they have been designed to roam easily

onto other UMTS networks (if the providers have roaming agreements in place). In addition,

almost all UMTS phones are UMTS/GSM dual-mode devices, so if a UMTS phone travels

outside of UMTS coverage during a call the call may be transparently handed off to available

GSM coverage. Roaming charges are usually significantly higher than regular usage charges.

Most UMTS licensees consider ubiquitous, transparent global roaming an important issue. To

enable a high degree of interoperability, UMTS phones usually support several different

frequencies in addition to their GSM fallback. Different countries support different UMTS

frequency bands – Europe initially used 2100 MHz while the most carriers in the USA use

850Mhz and 1900Mhz. T-mobile has launched a network in the US operating at 1700 MHz

(uplink) /2100 MHz (downlink), and these bands are also being adopted elsewhere in the

Americas. A UMTS phone and network must support a common frequency to work together.

Because of the frequencies used, early models of UMTS phones designated for the United

States will likely not be operable elsewhere and vice versa. There are now 11 different

frequency combinations used around the world—including frequencies formerly used solely

for 2G services.

UMTS phones can use a Universal Subscriber Identity Module, USIM (based on GSM's SIM)

and also work (including UMTS services) with GSM SIM cards. This is a global standard of

identification, and enables a network to identify and authenticate the (U)SIM in the phone.

Roaming agreements between networks allow for calls to a customer to be redirected to them

while roaming and determine the services (and prices) available to the user. In addition to

user subscriber information and authentication information, the (U)SIM provides storage

space for phone book contact. Handsets can store their data on their own memory or on the

(U)SIM card (which is usually more limited in its phone book contact information). A (U)SIM

can be moved to another UMTS or GSM phone, and the phone will take on the user details of

the (U)SIM, meaning it is the (U)SIM (not the phone) which determines the phone number of

the phone and the billing for calls made from the phone.

Japan was the first country to adopt 3G technologies, and since they had not used GSM

previously they had no need to build GSM compatibility into their handsets and their 3G

handsets were smaller than those available elsewhere. In 2002, NTT DoCoMo's FOMA 3G

network was the first commercial UMTS network—using a pre-release specification[9], it was

initially incompatible with the UMTS standard at the radio level but used standard USIM

cards, meaning USIM card based roaming was possible (transferring the USIM card into a

UMTS or GSM phone when travelling). Both NTT DoCoMo and SoftBank Mobile (which

launched 3G in December 2002) now use standard UMTS.

Received signal code powerFrom Wikipedia, the free encyclopedia

In the UMTS cellular communication system, Received Signal Code Power (RSCP)

denotes the power measured by a receiver on a particular physical communication

channel. It is used as an indication of signal strength, as a handover criterion, in

downlink power control, and to calculate path loss. In CDMA systems, a physical

channel corresponds to a particular spreading code, hence the name.

While RSCP can be defined generally for any CDMA system, it is more specifically

used in UMTS. Also, while RSCP can be measured in principle on the downlink as

well as on the uplink, it is only defined for the downlink and thus presumed to be

measured by the UE and reported to the Node B.[1]

3GPPFrom Wikipedia, the free encyclopedia

The 3rd Generation Partnership Project (3GPP) is a collaboration between groups

of telecommunications associations, to make a globally applicable third-generation

(3G) mobile phone system specification within the scope of the International Mobile

Telecommunications-2000 project of the International Telecommunication

Union (ITU). 3GPP specifications are based on evolved Global System for Mobile

Communications (GSM) specifications. 3GPP standardization encompasses Radio,

Core Network and Service architecture.[1]

The groups are the European Telecommunications Standards Institute, Association

of Radio Industries and Businesses/Telecommunication Technology

Committee (ARIB/TTC) (Japan), China Communications Standards

Association [2], Alliance for Telecommunications Industry Solutions (North America)

and Telecommunications Technology Association (South Korea).[1] The project was

established in December 1998.

3GPP should not be confused with 3rd Generation Partnership Project 2 (3GPP2),

which specifies standards for another 3G technology based on IS-95 (CDMA),

commonly known as CDMA2000.

Radio access networkMain article: UTRAN

UMTS also specifies the UMTS Terrestrial Radio Access Network (UTRAN), which

is composed of multiple base stations, possibly using different terrestrial air interface

standards and frequency bands.

UMTS and GSM/EDGE can share a Core Network (CN), making UTRAN an

alternative radio access network to GERAN (GSM/EDGE RAN), and allowing

(mostly) transparent switching between the RANs according to available coverage

and service needs. Because of that, UMTS' and GSM/EDGE's radio access networks

are sometimes collectively referred to as UTRAN/GERAN.

UMTS networks are often combined with GSM/EDGE, the later of which is also a part

of IMT-2000.

The UE (User Equipment) interface of the RAN (Radio Access Network) primarily

consists of RRC (Radio Resource Control), RLC (Radio Link Control) and MAC

(Media Access Control) protocols.

- RRC protocol handles connection establishment, measurements, radio bearer

services, security and handover decisions.

- RLC protocol primarily divides into three Modes - Transparent Mode (TM),

Unacknowledge Mode (UM), Acknowledge Mode (AM). The functionality of AM entity

resembles TCP operation where as UM operation resembles UDP operation. In TM

mode, data will be sent to lower layers without adding any header to SDU of higher

layers.

- MAC handles the scheduling of data on air interface depending on higher layer

(RRC) configured parameters.

Set of properties related to data transmission is called Radio Bearer (RB). This set of

properties will decide the maximum allowed data in a TTI (Transmission Time

Interval). RB includes RLC information and RB mapping. RB mapping decides the

mapping between RB<->logical channel<->transport channel. Signaling message will

be send on Signaling Radio Bearers (SRBs) and data packets (either CS or PS) will

be sent on data RBs. RRC and NAS messages will go on SRBs.

Security includes two procedures: integrity and ciphering. Integrity validates the

resource of message and also make sure that no one (third/unknown party) on radio

interface has not modified message. Ciphering make sure that no one listens your

data on air interface. Both integrity and ciphering will be applied for SRBs where as

only ciphering will be applied for data RBs.

Migrating from GPRS to UMTS

From GPRS network, the following network elements can be reused:

Home Location Register  (HLR)

Visitor Location Register (VLR)

Equipment Identity Register (EIR)

Mobile Switching Center  (MSC) (vendor dependent)

Authentication Center  (AUC)

Serving GPRS Support Node (SGSN) (vendor dependent)

Gateway GPRS Support Node (GGSN)

From Global Service for Mobile (GSM) communication radio network, the following

elements cannot be reused

Base station controller (BSC)

Base transceiver station (BTS)

They can remain in the network and be used in dual network operation where 2G and

3G networks co-exist while network migration and new 3G terminals become

available for use in the network.

The UMTS network introduces new network elements that function as specified

by 3GPP:

Node B  (base transceiver station)

Radio Network Controller  (RNC)

Media Gateway (MGW)

The functionality of MSC and SGSN changes when going to UMTS. In a GSM

system the MSC handles all the circuit switched operations like connecting A- and B-

subscriber through the network. SGSN handles all the packet switched operations

and transfers all the data in the network. In UMTS the Media gateway (MGW) take

care of all data transfer in both circuit and packet switched networks. MSC and

SGSN control MGW operations. The nodes are renamed to MSC-server and GSN-

server.

High-Speed Downlink Packet AccessFrom Wikipedia, the free encyclopedia

  (Redirected from HSDPA)

High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third generation) mobile

telephony communications protocol in the High-Speed Packet Access (HSPA) family, also dubbed

3.5G, 3G+ or turbo 3G, which allows networks based on Universal Mobile Telecommunications

System (UMTS) to have higher data transfer speeds and capacity. Current HSDPA deployments

support down-link speeds of 1.8, 3.6, 7.2 and 14.0 Mbit/s. Further speed increases are available

with HSPA+, which provides speeds of up to 42 Mbit/s downlink and 84 Mbit/s with Release 9 of

the 3GPPstandards.[1]

High-Speed Uplink Packet Access

High-Speed Uplink Packet Access (HSUPA) is a 3G mobile telephony protocol in

the HSPA family with up-link speeds up to 5.76 Mbit/s. The name HSUPA was created

by Nokia. The 3GPP does not support the name 'HSUPA', but instead uses the

name Enhanced Uplink (EUL).[1]

The specifications for HSUPA are included in Universal Mobile Telecommunications

System Release 6 standard published by 3GPP. – "The technical purpose of the Enhanced

Uplink feature is to improve the performance of uplink dedicated transport channels, i.e. to

increase capacity and throughput and reduce delay."

Universal subscriber identity module

A 64K UICC in its larger carrier card

A Universal Subscriber Identity Module is an application for UMTS mobile telephony running

on a UICC smart card which is inserted in a 3G mobile phone. There is a common

misconception to call the UICC itself a USIM, but the USIM is merely a logical entity on the

physical card.

It stores user subscriber information, authentication information and provides storage space

for text messages and phone book contacts. The phone book on a UICC has been greatly

enhanced.

For authentication purposes, the USIM stores a long-term pre-shared secret key K, which is

shared with the Authentication Center (AuC) in the network. The USIM also verifies a

sequence number that must be within a range using a window mechanism to avoid replay

attacks, and is in charge of generating the session keys CK and IK to be used in the

confidentiality and integrity algorithms of the KASUMI block cipher in UMTS.

The equivalent of USIM on GSM networks is SIM, and on CDMA networks it is CSIM.

Orthogonal frequency-division multiplexingFrom Wikipedia, the free encyclopedia

  (Redirected from OFDM)

Passband modulation techniques

Analog modulation

AM · SSB  · QAM  · FM · PM · SM

Digital modulation

FSK · MFSK  · ASK · OOK · PSK · QAM

MSK · CPM · PPM · TCMOFDM · SC-FDE

Spread spectrum

CSS  · DSSS  · FHSS  · THSS

v • d • e

See also: Demodulation, modem,

line coding, PAM, PWM, PCM

Orthogonal frequency-division multiplexing (OFDM), essentially identical to coded

OFDM (COFDM) and discrete multi-tone modulation (DMT), is a frequency-division

multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of

closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several

parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a

conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying)

at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation

schemes in the same bandwidth.

OFDM has developed into a popular scheme for wideband digital communication,

whether wireless or over copper wires, used in applications such as digital television and audio

broadcasting, wireless networking and broadband internet access.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with

severe channel conditions (for example, attenuation of high frequencies in a long copper wire,

narrowband interference and frequency-selective fading due to multipath) without complex

equalization filters. Channelequalization is simplified because OFDM may be viewed as using

many slowly-modulated narrowband signals rather than one rapidly-modulated widebandsignal.

The low symbol rate makes the use of a guard interval between symbols affordable, making it

possible to handle time-spreading and eliminateintersymbol interference (ISI). This mechanism

also facilitates the design of single frequency networks (SFNs), where several adjacent

transmitters send the same signal simultaneously at the same frequency, as the signals from

multiple distant transmitters may be combined constructively, rather than interfering as would

typically occur in a traditional single-carrier system.

UMTS-FDD

UMTS-FDD is designed to operate in the following paired bands:

Operating

Band

Frequency

Band

Common

Name

ULFrequenciesUE transmit (MHz)

DLFrequencies UE receive (MHz)

Channel

Number

(UARFCN) UL

Channel

Number

(UARFCN) DL

Region

I 2100 IMT 1920 - 19802110 - 2170

9612 - 9888

10562 - 10838

Europe, Asia, Africa, Oceania (Telstra),

Brazil

II 1900 PCS 1850 - 19101930 - 1990

9262 - 9538

additional 12,

37, 62, 87, 112,

137, 162, 187, 212, 237, 262, 287

9662 - 9938

additional 412, 437, 462, 487, 512, 537, 562, 587, 612, 637, 662, 687

North America (AT&T, Bell

Mobility, Telus,Rogers), Latin America

III 1800 DCS 1710 - 1785 1805 - 937 - 1162 - Europe, Asia,

1880 1288 1513 Oceania

IV 1700 AWS 1710 - 17552110 - 2155

1312 - 1513

additional 1662, 1687, 1712, 1737, 1762, 1787, 1812, 1837, 1862

1537 - 1738

additional 1887, 1912, 1937, 1962, 1987, 2012, 2037, 2062, 2087

USA (T-Mobile, Cincinnati

Bell Wireless), Canada (WIND

Mobile, Mobilicity, Videotron)

V 850 CLR 824 - 849 869 - 894

4132 - 4233

additional 782, 787, 807, 812, 837, 862

4357 - 4458

additional 1007, 1012, 1032, 1037, 1062, 1087

Americas (AT&T, Bell Mobility, Telus,

Rogers), Oceania (Telstra, Telecom

NZ)

VI 800 830 - 840 875 - 885

4162 - 4188

additional 812,

837

4387 - 4413

additional 1037,

1062

Japan (NTT docomo)

VII 2600 IMT-E 2500 - 25702620 - 2690

2012 - 2338

additional 2362, 2387, 2412, 2437, 2462, 2487, 2512, 2537, 2562, 2587, 2612, 2637, 2662, 2687

2237 - 2563

additional 2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912

Europe (future)

VIII 900 GSM 880 - 915 925 - 960 2712 - 2937 - Europe[1], Asia, Oceania

2863 3088

(Optus, VodafoneAU, Vodafone NZ),

Dominican Republic (Orange), Venezuela

(Digitel GSM)

IX 1700 1749.9 - 1784.91844.9 - 1879.9

8762 - 8912

9237 - 9387

Japan (E Mobile, NTT docomo)

X 1700 1710 - 17702110 - 2170

2887 - 3163

additional 3187, 3212, 3237, 3262, 3287, 3312, 3337, 3362, 3387, 3412, 3437, 3462

3112 - 3388

additional 3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687

XI 1500 1427.9 - 1447.91475.9 - 1495.9

3487 - 3562

3712 - 3787

Japan (Softbank)

XII 700 SMH 698 - 716 728 - 746

3612–3678

additional 3702, 3707, 3732, 3737, 3762, 3767

3837–3903

additional 3927, 3932, 3957, 3962, 3987, 3992

USA (future) (lower SMH blocks A/B/C)

XIII 700 SMH 777 - 787 746 - 756

3792–3818

additional 3842,

3867

4017–4043

additional 4067,

4092

USA (future) (upper SMH block C)

XIV 700 SMH 788 - 798 758 - 768

3892–3918

additional 3942,

3967

4117–4143

additional 4167,

4192

USA (future) (upper SMH block D)

Band

TS 25.101 DL to UL Frequency Separation

(MHz)

TS 25.101 Center

Frequency Range (MHz)

TS 25.101 UARFCNEquation

TS 25.101 UARFCN

Range

Test Set "DL Channel"

Range

I (IMT-2000) 190

2112.4 - 2167.6,

increment = 0.2

5 * (center freq in MHz)10562 - 10838

10562 - 10838

II(U.S. PCS) 80

1932.4 - 1987.6,

increment = 0.2

5 * (center freq in MHz) 9662 - 9938 9662 - 9938

1932.5 - 1987.5,

increment = 5

5 * ((center freq in MHz) - 1850.1 MHz)

412, 437, 462, 487, 512, 537, 562, 587, 612, 637, 662, 687

412, 437, 462, 487, 512, 537, 562, 587, 612, 637, 662, 687

III(DCS/PCS) 95

1807.4 - 1877.6,

increment = 0.2

5 * ((center freq in MHz) - 1575 MHz)

1162 - 1513 1162 - 1513

IV 400

2112.4 - 2152.6,

increment = 0.2

5 * ((center freq in MHz) - 1805 MHz)

1537 - 1738 1537 - 1738 *

2112.5 - 2152.5,

increment = 5

5 * ((center freq in MHz) - 1735.1 MHz)

1887, 1912, 1937, 1962, 1987, 2012, 2037, 2062,

1887, 1912, 1937, 1962, 1987, 2012, 2037, 2062,

2087 2087 *

V(US Cellular)

45

871.4 - 891.6,

increment = 0.2

5 * (center freq in MHz) 4357 - 4458 4357 - 4458 #

871.5, 872.5, 876.5, 877.5,

882.5, 887.5

5* ((center freq in MHz) - 670.1 MHz)

1007, 1012, 1032, 1037, 1062, 1087

1007, 1012, 1032, 1037, 1062, 1087 #

VI(Japan 800)

45

877.4 - 882.6,

increment = 0.2

5 * (center freq in MHz) 4387 - 44134387 - 4413

+

877.5, 882.55 * ((center freq in MHz) -

670.1 MHz)1037, 1062 1037, 1062 +

VII 120

2622.4 - 2687.6,

increment = 0.2

5 * ((center freq in MHz) - 2175 MHz)

2237 - 2563 2237 - 2563

2622.5 - 2687.5,

increment = 5

5 * ((center freq in MHz) - 2105.1 MHz)

2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912

2587, 2612, 2637, 2662, 2687, 2712, 2737, 2762, 2787, 2812, 2837, 2862, 2887, 2912

VIII 45

927.4 - 957.6,

increment = 0.2

5 * ((center freq in MHz) - 340 MHz)

2937 - 3088 2937 - 3088

IX 95

1847.4 - 1877.4,

increment = 0.2

5 * (center freq in MHz) 9237 - 9387 9237 - 9387 *

X 400

2112.4 - 2167.6,

increment = 0.2

5 * ((center freq in MHz) - 1490 MHz)

3112 - 3388 3112 - 3388 *

2112.5 - 2167.5,

increment = 5

5 * ((center freq in MHz) - 1430.1 MHz)

3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687

3412, 3437, 3462, 3487, 3512, 3537, 3562, 3587, 3612, 3637, 3662, 3687 *

Deployment in other frequency bands is not precluded.

UMTS-TDD

UMTS-TDD is designed to operate in the following bands:

Frequencies (MHz) Channel Number (UARFCN)

1900 - 1920 9512 - 9588

2010 - 2025 10062 - 10113

1850 - 1910 9262 - 9538

1930 - 1990 9662 - 9938

1910 - 1930 9562 - 9638

2570 - 2620 12862 - 13088

Common pilot channelFrom Wikipedia, the free encyclopedia

CPICH stands for Common Pilot CHannel in UMTS and some other CDMA communications

systems.

In WCDMA FDD cellular systems, CPICH is a downlink channel broadcast by Node Bs with

constant power and of a known bit sequence. Its power is usually between 5% and 15% of

the total Node B transmit power. A common the CPICH power is 10% of the typical total

transmit power of 43 dBm.

The Primary Common Pilot Channel is used by the UEs to first complete identification of the

Primary Scrambling Code used for scrambling Primary Common Control Physical Channel

(P-CCPCH) transmissions from the Node B. Later CPICH channels provide allow phase and

power estimations to be made, as well as aiding discovery of other radio paths. There is one

primary CPICH (P-CPICH), which is transmitted using spreading code 0 with a spreading

factor of 256, notationally written as Cch,256,0[1]. Optionally a Node B may broadcast one or more

secondary common pilot channels (S-CPICH), which use arbitrarily chosen 256 codes, written

as Cch,256,n where 0 < n < 256.

The CPICH contains 20 bits of data, which are either all zeros, or in the case that Space-Time

Transmit Diversity (STTD) is employed, is a pattern of alternating 1's and 0's for transmissions

on the Node B's second antenna[2]. The first antenna of a base station always transmits all

zeros for CPICH.

A UE searching for a WCDMA Node B will first use the primary and secondary

synchronisation channels (P-SCH and S-SCH respectively) to determine the slot and frame

timing of a candidate P-CCPCH, whether STTD is in use, as well as identifying which one of

64 code groups is being used by the cell. Crucially this allows to UE to reduce the set of

possible Primary Scrambling Codes being used for P-CPICH to only 8 from 512 choices. At

this point the correct PSC can be determined through the use of a matched filter, configured

with the fixed channelisation code Cch,256,0, looking for the known CPICH bit sequence, while

trying each of the possible 8 PSCs in turn. The results of each run of the matched filter can be

compared, the correct PSC being identified by the greatest correlation result.

Once the scrambling code for a CPICH is known, the channel can be used for measurements

of signal quality, usually comprising of RSCP and Ec/I0. Timing and phase estimations can

also be made, providing a reference that helps to improve reliability when decoding other

channels from the same Node B.

Pilot signals are not a requirement of CDMA, however, they do make

the UE's receiver simpler and improve the reliability of the system.

Antenna diversity

Antenna diversity, also known as space diversity (micro-diversity as well as macro-diversity,

i.e. soft handover, see below), is any one of several wireless diversity schemes that use two

or more antennas to improve the quality and reliability of a wireless link. Often, especially in

urban and indoor environments, there is not a clear line-of-sight (LOS) between transmitter

and receiver. Instead the signal is reflected along multiple paths before finally being received.

Each of these bounces can introduce phase shifts, time delays, attenuations, and even

distortions that can destructively interfere with one another at the aperture of the receiving

antenna. Antenna diversity is especially effective at mitigating these multipath propagation

situations. This is because multiple antennas afford a receiver several observations of the

same signal. Each antenna will experience a different interference environment. Thus, if one

antenna is experiencing a deep fade, it is likely that another has a sufficient signal.

Collectively such a system can provide a robust link. While this is primarily seen in receiving

systems (diversity reception), the analog has also proven valuable for transmitting systems

(transmit diversity) as well.

Inherently an antenna diversity scheme requires additional hardware and integration versus a

single antenna system but due to the commonality of the signal paths a fair amount of

circuitry can be shared. Also with the multiple signals there is a greater processing demand

placed on the receiver, which can lead to tighter design requirements. Typically, however,

signal reliability is paramount and using multiple antennas is an effective way to decrease the

number of drop-outs and lost connections.

Types of handover

In addition to the above classification of inter-cell and intra-cell classification of handovers,

they also can be divided into hard and soft handovers:

A hard handover is one in which the channel in the source cell is released and only

then the channel in the target cell is engaged. Thus the connection to the source is

broken before the connection to the target is made—for this reason such handovers are

also known as break-before-make. Hard handovers are intended to be instantaneous in

order to minimize the disruption to the call. A hard handover is perceived by network

engineers as an event during the call.

A soft handover is one in which the channel in the source cell is retained and used for

a while in parallel with the channel in the target cell. In this case the connection to the

target is established before the connection to the source is broken, hence this handovers

is called make-before-break. The interval, during which the two connections are used in

parallel, may be brief or substantial. For this reason the soft handovers is perceived by

network engineers as a state of the call, rather than a brief event. Soft handovers may

involve using connections to more than two cells, e.g. connections to three, four or more

cells can be maintained by one phone at the same time. When a call is in a state of soft

handovers the signal of the best of all used channels can be utilised for the call at a given

moment or all the signals can be combined to produce a clearer copy of the signal. The

latter is more advantageous, and when such combining is performed both in

the downlink(forward link) and the uplink (reverse link) the handover is termed as softer.

Softer handovers are possible when the cells involved in the handovers have a single cell

site .