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What is UMTS ?

             Standing for "Universal Mobile Telecommunications System", UMTS represents an evolution in terms of capacity, data speeds and new service capabilities from second generation mobile networks.

               A key member of the global family of third generation (3G) mobile technologies identified by the ITU, 3G/UMTS offers mobile operators significant capacity and broadband capabilities to support greater numbers of voice and data customers - especially in urban centres - plus higher data rates at lower incremental cost than 2G. Making use of radio spectrum in bands identified by the ITU for Third Generation IMT-2000 mobile services and subsequently licensed to operators, 3G/UMTS employs a 5 MHz channel carrier width to deliver significantly higher data rates and increased capacity compared with second generation networks. This 5 MHz channel carrier provides optimum use of radio resources, especially for operators who have been granted large, contiguous blocks of spectrum - typically ranging from 2x10 MHz up to 2x20 MHz - to reduce the cost of deploying 3G networks.

Crucially, 3G/UMTS has been specified as an integrated solution for mobile voice and data with wide area coverage. Universally standardized via the Third Generation Partnership Project (www.3gpp.org) and using globally harmonized spectrum in paired and unpaired bands, 3G/UMTS in its initial phase offers theoretical bit rates of up to 384 kbps in high mobility situations, rising as high as 2 Mbps in stationary/nomadic user environments. Symmetry between uplink and downlink data rates when using paired (FDD) spectrum also means that 3G/UMTS is ideally suited for applications such as real-time video telephony - in contrast with other technologies such as ADSL where there is a pronounced asymmetry between uplink and downlink throughput rates.

Specified and implemented as an end-to-end mobile system, 3G/UMTS also features the additional benefits of automatic international roaming plus integral security and billing functions, allowing operators to migrate from 2G to 3G while retaining many of their existing back-office systems. Offering increased capacity and speed at lower incremental cost compared with second generation mobile systems, 3G/UMTS gives operators the flexibility to introduce new multimedia services to business users and consumers while providing an enhanced user experience. This in turn provides the opportunity for operators to build on the brand-based relationships they already enjoy with their customers - and drive new revenue opportunities by encouraging additional traffic, stimulating new usage patterns and strengthening customer loyalty.

Ongoing technical work within 3GPP will see further increases in throughput speeds of the WCDMA Radio Access Network (RAN). High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) technologies are already standardised and are undergoing network trials with operators in the Far East and North America. Promising theoretical downlink speeds as high as 14.4 Mbps (and respectively 5.8 Mbps uplink), these technologies will play an instrumental role in positioning 3G/UMTS as a key enabler for true 'mobile broadband'. Offering data transmission speeds of the same order of magnitude as today's Ethernet-based networks that are a ubiquitous feature of the fixed-line environment, 3G/UMTS will offer enterprise customers and consumers all the benefits of broadband connectivity whilst on the move

            UMTS, the Universal Mobile Telecommunications System is the third generation (3G) successor to the second generation GSM based technologies including GPRS, and EDGE. Although UMTS uses a totally different air interface, the core network elements have been migrating towards the UMTS requirements with the introduction of GPRS and EDGE. In this way the transition from GSM to the 3G UMTS architecture does not require such a large instantaneous investment.

UMTS uses Wideband CDMA (WCDMA or W-CDMA) to carry the radio transmissions, and often the system is referred to by the name WCDMA. It is also gaining a third name. Some are calling it 3GSM because it is a 3G migration for GSM.

Specifications and Management

In order to create and manage a system as complicated as UMTS or WCDMA it is necessary to develop and maintain a large number of documents and specifications. For UMTS or WCDMA, these are now managed by a group known as 3GPP - the Third Generation Partnership Program. This is a global co-operation between six organizational partners - ARIB, CCSA, ETSI, ATIS, TTA and TTC.

The scope of 3GPP was to produce globally applicable Technical Specifications and Technical Reports for a 3rd Generation Mobile Telecommunications System. This would be based upon the GSM core networks and the radio access technologies that they support (i.e., Universal Terrestrial Radio Access (UTRA) both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes).

Capabilities

UMTS uses Wideband CDMA - WCDMA - as the radio transmission standard. It employs a 5 MHz channel bandwidth. Using this bandwidth it has the capacity to carry over 100 simultaneous voice calls, or it is able to carry data at speeds up to 2 Mbps in its original format. However with the later enhancements of HSDPA and HSUPA  included in later releases of the standard the data transmission speeds have been increased to 14.4 Mbps.

Many of the ideas that were incorporated into GSM have been carried over and enhanced for UMTS. Elements such as the SIM have been transformed into a far more powerful USIM (Universal SIM). In addition to this, the network has been designed so that the enhancements employed for GPRS and EDGE can be used for UMTS. In this way the investment required is kept to a minimum.

A new introduction for UMTS is that there are specifications that allow both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes. The first modes to be employed are FDD modes where the uplink and downlink are on different frequencies. The spacing between them is 190 MHz for Band 1 networks being currently used and rolled out.

However the TDD mode where the uplink and downlink are split in time with the base stations and then the mobiles transmitting alternately on the same frequency is particularly suited to a variety of applications. Obviously where spectrum is limited and paired bands suitably spaced are not available. It also performs well where small cells are to be used. As a guard time is required between transmit and receive, this will be smaller when transit times are smaller as a result of the shorter distances being covered. A further advantage arises from the fact that it is found that far more data is carried in the downlink as a result of internet surfing, video downloads and the like. This means that it is often better to allocate more capacity to the downlink. Where paired spectrum is used this is not possible. However when a TDD system is used it is possible to alter the balance between downlink and uplink transmissions to accommodate this imbalance and thereby improve the efficiency. In this way TDD systems can be highly efficient when used in picocells for carrying Internet data. The TDD systems have not been widely deployed, but this may occur more in the future. In view of its character, it is often referred to as TD-CDMA (Time Division CDMA).

Constituents of UMTS

UMTS or as it is often termed, Wideband CDMA, WCDMA is being widely deployed. It offers many advantages over GSM, GPRS, and EDGE in terms of much higher data rates and greater flexibility. These basic technical abilities reflect as a much richer number of applications and features that the 3G phones can be used to perform. This not only gives the user a much more useful 'phone', but this also translates into higher revenues for the operator.

The network for UMTS can be split into three main constituents. These are the mobile station, called the User Equipment or UE, the base station subsystem known as the Radio Network Subsystem (RNS) and the core network.

User Equipment, UE

The UE for UMTS / WCDMA is equivalent to the mobile equipment used on GSM networks. Essentially it is the handset, 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.

For UMTS / WCDMA mobiles, as for any system, the circuitry used within the UE can be broadly split into the RF and base-band processing areas. 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.

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 optimized to reduce the current consumption as far as possible.

While current consumption has been minimized 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.

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. 

Radio Network Subsystem

This is the section of the UMTS / WCDMA network that interfaces to both the UE and the core network. It contains what are roughly equivalent to the Base Transceiver Station (BTS) and the Base Station Controller (BSC). Under UMTS terminology, the radio transceiver is known as the Node B. This communicates with the various UEs. It also communicates with the Radio Network Controller (RNC). This is undertaken over an interface known as the Iub. The overall radio access network is known as the UMTS Radio Access Network (UTRAN). The RNC component of the Radio Access Network (RAN) connects to the core network.

Core Network

The core network used for UMTS is based upon the combination of the circuit switched elements used for GSM plus the packet switched elements that are used for GPRS and EDGE. Thus for The Core The network is divided into circuit switched and packet switched domains. Some of the circuit switched elements are Mobile services Switching Centre (MSC), Visitor Location Register (VLR) and Gateway MSC. Packet switched elements are Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Some network elements, like EIR, HLR, VLR and AUC mentioned in Chapters 4 and 6 are shared by both domains and operate in the same manner that they did with GSM.

The Asynchronous Transfer Mode (ATM) is specified for UMTS core transmission. The architecture of the Core Network may change when new services and features are introduced. Number Portability Data Base (NPDB) will be used to enable the subscriber to change network provider while keeping their old phone number. Gateway Location Register (GLR) may be used to optimize the subscriber handling between network boundaries. MSC, VLR and SGSN can merge to become a UMTS MSC.

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.

Frequencies

There are currently six bands that are specified for use for UMTS / WCDMA although operation on other frequencies is not precluded. However much of the focus for UMTS is currently on frequency allocations around 2 GHz. At the World Administrative radio Conference in 1992, the bands 1885 - 2025 and 2110 - 2200 MHz were set aside for use on a world wide basis by administrations wishing to implement International Mobile Telecommunications-2000 (IMT-2000). The aim was that allocating spectrum on a world wide basis would facilitate easy roaming for UMTS / WCDMA users.

Within these bands the portions have been reserved for different uses:

• 1920-1980 and 2110-2170 MHz Frequency Division Duplex (FDD, W-CDMA) Paired uplink and downlink, channel spacing is 5 MHz and raster is 200 kHz. An Operator needs 3 - 4 channels (2x15 MHz or 2x20 MHz) to be able to build a high-speed, high-capacity network.
• 1900-1920 and 2010-2025 MHz Time Division Duplex (TDD, TD/CDMA) Unpaired, channel spacing is 5 MHz and raster is 200 kHz. Transmit and receive transmissions are not separated in frequency.
• 1980-2010 and 2170-2200 MHz Satellite uplink and downlink.

Carrier frequencies are designated by a UTRA Absolute Radio Frequency Channel Number (UARFCN). This can be calculated from:

UARFCN = 5 x (frequency in MHz)

UMTS uses wideband CDMA as the radio transport mechanism. The channels are spaced by 5 MHz. The modulation that is used is different on the uplink and downlink. The downlink uses quadrature phase shift keying (QPSK) for all transport channels. However 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.

Spreading

The data to be transmitted is encoded using a spreading code particular to a given user. In this way only the desired recipient is able to correlate and decode the signal, all other signals appearing as noise. This allows the physical RF channel to be used by several users simultaneously.

The data of a CDMA signal is multiplied with a chip or spreading code to increase the bandwidth of the signal. For WCDMA, each physical channel is spread with a unique and variable spreading sequence. The overall degree of spreading varies to enable the final signal to fill the required channel bandwidth. As the input data rate may vary from one application to the next, so the degree of spreading needs to be varied accordingly.

For the downlink the transmitted symbol rate is 3.84 M symbols per second. As the form of modulation used is QPSK this enables two bits of information to be transmitted for every symbol, thereby enabling a maximum data rate of twice the symbol rate or 7.68 Mbps. Therefore if the actual rate of the data to be transmitted is 15 kbps then a spreading factor of 512 is required to bring the signal up to the required chip rate for transmission in the required bandwidth. If the data to be carried has a higher data rate then a lower spreading rate is required to balance this out. It is worth remembering that altering the chip rate does alter the processing gain of the overall system and this needs to be accommodated in the signal processing as well. Higher spreading factors are more easily correlated by the receiver and therefore a lower transmit power can be used for the same symbol error rate.

The codes required to spread the signal must be orthogonal if they are to enable multiple users and channels to operate without mutual interference. The codes used in W-CDMA are Orthogonal Variable Spreading Factor (OVSF) codes, and they must remain synchronous to operate. As it is not possible to retain exact synchronization for this, a second set of scrambling codes is used to ensure that interference does not result. This scrambling code is a pseudo random number (PN) code. Thus there are two stages of spreading. The first using the OSVF code and the second using a scrambling PN code. These codes are used to provide different levels of separation. The OVSF spreading codes are used to identify the user services in the uplink and user channels in the downlink whereas the PN code is used to identify the individual node B or UE.

On the uplink there is a choice of millions of different PN codes. These are processed to include a masked individual code to identify the UE. As a result there are more than sufficient codes to accommodate the number of different UEs likely to access a network. For the downlink a short code is used. There are a total of 512 different codes that can be used, one of which will be assigned to each node B.

Synchronization

The level of synchronization required for the WCDMA system to operate is provided from the Primary Synchronization Channel (P-SCH) and the Secondary Synchronization 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 synchronization 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 synchronize with the base station for the time slot.

Once the UE has gained time slot synchronization 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 synchronization codes.

There is a total of sixteen different secondary synchronization codes. One code is sent at the beginning of the time slot, i.e. the first 256 chips. It consists of 15 synchronization codes and there are 64 different scrambling code groups. When received, the UE is able to determine before which synchronization code the overall frame begins. In this way the UE is able to gain complete synchronization.

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 synchronization, 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.

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  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.

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 data can be carried in an efficient manner.

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, transport and physical channels. The logical channels define the way in which the data will be transferred, the transport channel along with the logical channel again defines the way in which the data is transferred, the physical channel carries 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.

Logical Channels:

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.

Transport Channels:

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.

Physical Channels:

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.

Synchronization Channel (SCH) The synchronization channel is used in allowing UEs to synchronize 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.

This final page of the UMTS / WCDMA tutorial looks at three elements of the system, namely the way packet data is carried, the way speech coding is accomplished and handover, including hard, soft and softer handover.

Packet data
Packet data is an increasingly important element within mobile phone applications. WCDMA is able to carry data in this format in two ways. The first is for short data packets to be appended directly to a random access burst. This method is called common channel packet transmission and it is used for short infrequent packets. It is preferable to transmit short packets in this manner because the link maintenance needed for a dedicated channel would lead to an unacceptable overhead. Additionally the delay in setting up a packet data channel and transferring the operational mode to this format is avoided.

Larger or more frequent packets have to be transmitted on a dedicated channel. A large single packet is transmitted using a single-packet scheme where the dedicated channel is released immediately after the packet has been transmitted. In a multipacket scheme the dedicated channel is maintained by transmitting power control and synchronization information between subsequent packets.

Speech coding

Speech coding in UMTS uses a variety of source rates. As a result, a variety of vocoders are employed including the GSM EFR vocoder. When a variety of rates are available, a system known as Adaptive Multi-Rate (AMR) may be employed where rate is chosen according to the system capacity and requirements. This scheme is the same as that used on GSM. The actual vocoder that is chosen is governed by the system.

The speech coding process can be combined with a voice activity detector. This is particularly useful because during normal conversations there are long periods of inactivity. In the same way that discontinuous transmission is applied to GSM, the same is also true for UMTS. It employs the same technique of inserting background noise when there is no speech as when the discontinuous transmission cuts out the transmission no background noise would otherwise be heard and this can be very disconcerting for the listener.

Discontinuous reception

One of the big issues with mobile phones in general is that of battery life. It is one of the key differentiators that people take into account when buying a phone and this gives a measure of its importance. Taking this into consideration when developing the UMTS / WCDMA standard a discontinuous reception or sleep mode was introduced. This mode allows several non-essential segments of the phone circuitry to power down during periods when paging messages will not be received.

To enable this facility to be introduced into the UMTS UE circuitry the paging channel is divided into groups or subchannels. The actual number of the paging subchannel to be used by a particular UE is assigned by the network. In this way the UE only has to listen for part of the time. To achieve this the Paging Indicator Channel (PICH) is split into 10 ms frames, each of which comprises 300 bits - 288 for paging data and 12 idle bits. At the beginning of each paging channel frame there is a Paging Indicator (PI) that identifies the paging group being transmitted. By synchronising with the paging channels being transmitted it is able to turn the receiver on only when it needs to monitor the paging channel. As the receiver, with its RF circuitry, will consume power, savings can be made by switching it off.

Handover

        Within UMTS, handover follows many of the similar concepts to those used for other CDMA systems. There are three basic types of handover: hard, soft and softer. All three types are used but under different circumstances.

            Hard handover is like that used for the previous generations of systems. Here, as the UE moves out of range of one node B, the call has to be handed over to another frequency channel. In this instance simultaneous reception of both channels is not possible and there must be a physical break.

                 Soft handover is a technique that was not available on the previous generations of mobile phone systems. With CDMA systems it is possible to have adjacent cell sites on the same frequency, and as a result it is possible for the UE to receive the signals from two adjacent cells at once, and they are also able to receive the signals from the UE. When this occurs and handover is affected it is known as soft handover.

    The decisions about handover are generally handled by the RNC. It continually monitors information regarding the signals being received by both the UE and node B 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.

If a hard handover is required then the RNC will instruct the UE to adopt a compressed mode, allowing short time intervals in which the UE is able to measure the channel quality of other radio channels

Softer Handover

Fig 1: Softer Handover

Strictly speaking softer handover is not really a handover. In this case the UE combines more than one radio link to improve the reception quality. On the other hand the Node B combines the data from more than one cell to obtain good quality data from the UE. [1] Specifies the maximum number of Radio Links that a UE can simultaneously support as 8. In practice this would be limited to 4 as it is very difficult to make the receiver with 8 fingers.

Generally speaking when RRC connection is established, it would always be established on one cell. The network initiates Intra-Frequency measurements to check if there are any other cells the UE can connect simultaneously to improve the quality of the data being transferred between the RNC and the UE. If a suitable cell is found then Active Set Update procedure is initiated. Using this Active Set Update message, the network adds or deletes more than one radio link to the UE. The only requirement is that from the start till the end of this Active Set Update procedure, one Radio Link should remain common.

Soft Handover

Fig 2: Soft Handover

Soft Handover is the same as softer handover but in this case the cells belong to more than one node B. In this case the combining is done in the RNC. It is possible to simultaneously have soft and softer handovers.

Fig 3: Soft Handover with Iur connection

A more complicated soft handover would include a cell that belongs to a Node B in different RNC. In this case an Iur connection is established with the drift RNC (RNC 2) and the data would be transferred to the Serving RNC (RNC 1) via Iur connection.

In a typical UMTS system, the UE is in soft/softer handover around 50% of the time. One of the very important requirements for the soft/softer handover is that the frames from different cells should be within 50ms of each other or this would not work.

The last thing one needs to remember is that the soft/softer handover is initiated from the RNC and the core network is not involved in this procedure.

Hard Handover

Hard handover occurs when the radio links for UE change and there are no radio links that are common before the procedure is initiated and after the procedure is completed. There are two types of hard handover. First is Intra-frequency hard handover and the second is Inter-frequency hard handover.

Intra-frequency hard handover will not occur for the FDD system. It would happen in TDD system. In this case the code spreading/scrambling code for UE will change but the frequency remains the same.

Inter-frequency hard handover generally occurs when hierarchical cells are present. In this case the frequency at which the UE is working changes. This happens when the current cell is congested, etc. Have a look at the Inter-Frequency Measurement primer for more information.

Hard handover procedure can be initiated by the network or by the UE. Generally it would be initiated by the network using one of the Radio Bearer Control messages. In case of UE initiated, it would happen if the UE performs a Cell Update procedure and that Cell Update reaches the RNC on a different frequency. The Core Network is not involved in this procedure.

APPLICATIONS

While 3G is generally considered applicable mainly to mobile wireless, it is also relevant to fixed wireless and portable wireless. The ultimate 3G system might be operational from any location on, or over, the earth's surface, including use in or by:

    ·         Homes

    ·         Businesses

    ·         Government offices

    ·         Medical establishments

    ·         The military

    ·         Personal and commercial land vehicles

    ·         Private and commercial watercraft and marine craft

    ·         Private and commercial aircraft (except where passenger use  restrictions apply)

    ·         Portable (pedestrians, hikers, cyclists, campers)

    ·         Space stations and spacecraft

    Proponents of 3G technology promise that it will "keep people connected at all times and in all places." Researchers, engineers, and marketeers are faced with the challenge of accurately predicting how much technology consumers will actually be willing to pay for. (Recent trends suggest that people sometimes prefer to be disconnected, especially when on vacation.) Another concern involves privacy and security issues. As technology becomes more sophisticated and bandwidth increases, systems become increasingly vulnerable to attack by malicious hackers (known as crackers) unless countermeasures are implemented to protect against such activity.

    One can hardly doubt, in any event, that 3G technology will provide plenty of jobs for talented engineers, programmers, and sales people, as well as widespread opportunities for entrepreneurs interested in wireless communications.

UMTS is the European standard for 3G mobile communication systems which provide an enhanced range of multimedia services. It has evolved from its basic format through developments such as HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access) to provide very high bandwidth capabilities to support the next generation of telecommunication services.

GLOSSARY

 1XEV-DO      Data Only

REFERENCES

1.  www.umtsforum.com

2.  www.radioelectronics.com

3.  www.nms.com

4.  www.wikipedia.com

5.  www.wirelessfuture.it

6.  www.bitpipe.com

7.  www.comarco.com

8.  www.juni.com

9.  www.accuris.com

10. www.wisegeek.com