3g Umts.docx

3G UMTS / WCDMA Basics Tutorial 3GPP UMTS, the Universal Mobile Telecommunications System is the third generation (3G) successor to the second generation GSM based cellular technologies which also include 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 did not require such a large instantaneous investment. UMTS uses Wideband CDMA (WCDMA / 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.

3GPP UMTS Specifications and Management 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). Since it was originally formed, 3GPP has also taken over responsibility for the GSM standards as well as looking at future developments including LTE (Long Term Evolution) and the 4G technology known as LTE Advanced.

3G UMTS 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 (described in other articles accessible from the cellular telecommunications menu page ) 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).

3G UMTS / WCDMA technologies There 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.

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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.

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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.

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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.

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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.

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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.

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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 SPECIFICATION SUMMARY PARAMETER

SPECIFICATION

Maximum data rate

2048 kbps low 384 kbps urban and outdoor

RF channel bandwidth

5 MHz

Multiple access scheme

CDMA

Duplex schemes

FDD and also TDD

range

3G UMTS / WCDMA Network Architecture 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 constituents The 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

CDMA for UMTS The choice of CDMA for use with the third generation, 3G UMTS telecommunications system arose from a variety of technical reasons. It offers significant advantages over the schemes used in the previous 2G systems that were predominantly TDMA based schemes. The main benefits of the use of CDMA as a multiple access scheme are: 

Improved spectral efficiency: The use of CDMA as the multiple access technology, combined with the QPSK modulation format used provides significant improvements in terms of the spectral efficiency. Figures for the performance improvements gained vary considerably dependent upon the conditions, but the scheme gives some significant benefits.

Some calculated estimates give figures as high as three or four times that of technologies such as GSM, although in reality the benefits may be a bit less. 

Adjacent cells may use the same channel frequency: As a result of the way in which spread spectrum signals such as CDMA operate.

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Improved handover: Within CDMA it is possible to do what is termed a "soft handover" where the UE communicates with two base stations at the same time. This significantly improves handover reliability.

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Enhanced security: The use of spread spectrum and the multiple spreading codes for CDMA significantly reduces the possibility of eavesdropping, although within GSM eavesdropping of the transmitted signal was not the problem it was for the original analogue systems where anyone with a scanner radio receiver could listen to telephone conversations.

UMTS / WCDMA modulation UMTS modulation schemes There 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.

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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.

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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.

Downlink modulation The 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 modulation 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.

3G UMTS channel structures 3G 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: transferred

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Transport Channels: The 3G transport channels along with the logical channel again defines the way in which the data is transferred

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Physical channels: The physical channels carry the payload data and govern the physical characteristics of the signal.

The logical channels define the way in which the data will be

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.

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Paging Control Channel (PCCH) (downlink). This channel is associated with the PICH and is used for paging messages and notification information.

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Dedicated Control Channel (DCCH) (up and downlinks) This channel is used to carry dedicated control information in both directions.

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

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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.

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Dedicated Traffic Channel (DTCH) used to carry user data or traffic.

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Common Traffic Channel (CTCH) (downlink) A unidirectional channel used to transfer dedicated user information to a group of UEs.

(up and downlinks). This is a bidirectional channel

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.

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Broadcast Channel (BCH) (downlink). This channel broadcasts information to the UEs in the cell to enable them to identify the network and the cell.

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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.

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Paging Channel (PCH) (downlink). This channel carries messages that alert the UE to incoming calls, SMS messages, data sessions or required maintenance such as reregistration.

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Random Access Channel (RACH) (uplink). This channel carries requests for service from UEs trying to access the system

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Uplink Common Packet Channel (CPCH) (uplink). This channel provides additional capability beyond that of the RACH and for fast power control.

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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.

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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.

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Physical Random Access Channel (PRACH) (uplink). This channel enables the UE to transmit random access bursts in an attempt to access a network.

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

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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.

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Physical Downlink Shared Channel (PDSCH) (downlink). This channel shares control information to UEs within the coverage area of the node B.

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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.

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Synchronisation Channel (SCH) The synchronisation channel is used in allowing UEs to synchronise with the network.

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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.

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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.

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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.

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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.