Lecture overview

History of cellular systems (1G)

GSM introduction

Basic architecture of GSM system

Basic radio transmission parameters of GSM

Analogue cellular systems 70s

In the early 70’s radio frequencies were a scarce resource

Used by police, taxis, etc.

Thus optimisation was essential

In 1971, the Bell telephone company proposed a new cellular network to meet the limited frequency bands.

Bell proposed the Advanced Mobile Phone System (AMPS) which was a cellular system

This was first demonstrated in Chicago, where it has been in operation since 1978

Analogue cellular systems 80s

AMPS was standardised in 1982 by the Federal Communications Commission (FCC)

This became the single radio telephony standard in North America

cellular

analogue

At the same time around the world, several different cellular radio networks came into operation.

Like AMPS these were analogue

Analogue cellular systems 80s

Ireland TACS (Total Access Communication System)

Britain TACS

Italy TACS

Germany C450

Spain NMT (Nodrdic Mobile Telephony)

France Radiocom 2000, NMT

Analogue cellular systems 80s

System Began operation

Channel width (kHz)

Frequency (MhZ)

No. Channels

AMPS 1983 30 825-845 m-b 870-890 b-m

660

NMT-450 1981 25 453-457.5 m-b463-467.5 b-m

180

C-450 1985 25 451-455.7 m-b461-465.7 b-m

1000

TACS plus 1985 25 890-915 m-b 935-960 b-m

1000

NMT-900 1986 12.5 890-915 m-b 935-960 b-m

1999

Downlink – from BS to MS

Uplink – from MS to BS

Analogue cellular systems 80s

The standards named on the previous slides were all independent and thus

The equipment was limited to operate within the boundaries of one country

The market for mobile equipment was limited

Lack of competition for components

Being analogue meant small

subscriber capacity

traffic capacity

Towards GSM

A new approach was necessary to overcome the shortfalls of the analogue system

initially it was proposed to use a greater part of the radio spectrum - this would only have been temporary measure

The new approach centred on two advances in technology:The development of time division multiplexing andChanging form analogue transmission to digital transmission

Towards GSMIn 1982 the Conference of European Posts and Telegraphs (CEPT) created the Group Special Mobile (GSM).

This group was charged with the responsibility for the creation of a mobile radio telecommunication standard for Europe

Firstly the bandwidths for mobile communication were established

890-915MHz for uplink transmission 935-960MHz for downlink transmission

Objectives of GSM

The GSM Standard’s aims include:

Large Subscriber base and wide availability

World wide compatibility

Efficient use of the radio spectrum

QoS comparable to that of the fixed network

The ability to access the network from either mobile or portable handsets

GSM’s Technical objectives

By 1987 the standard had decided on the various technical aspects of the standard.

Digital transmissionTime Division Multiplexing of radio channelsEncryption of radio channel transmissionNew compression algorithms for reduced data rate compared to the coding laws used in telecommunications.Compatibility with ISDN networks

Global System for Mobile CommunicationsBasic architecture

The area of GSM operation is divided into subareas managed by different Mobile Switching Centers (MSC)

Each MSC is connected to a Visitor’s Location Register (VLR) – data base containing necessary information on all MS temporarily located in the area served by particular MSC

GSM system has 3 additional databases:Home Location Register (HLR) – database of all MS permanently registered in the system

Authentication Center (AC) – allows checking if the user is allowed to perform a call

Equipment Identification Register (EIR) – contains serial no of mobile phones used in the system. Lost or stolen phones are placed on the black, which prevents them form being used in the system

HLR is central database storing information on:Current location of the MS and all the parameters of the permanently registered users that allows the system to establisha connection with the user, even if the user is temporarily in adifferent GSM network

The address of the VLR which is associated with the current location of the user

List of additional booked services

Encryption keys for digital signal transmission and user authentication

Other

HLR and VLR exchange information regarding the users currently located in the area served by the VLRThis allows to establish a current location of the called user by reading information from user’s HLR and routing the call to MSC serving the area in which the MS is locatedVLR stores the information necessary for initiation of mobile-originated callMSC are connected with each otherOne or more MSC are called Gateway MSC and connect the GSM system with external networks (PSTN, ISDN etc.)Each MSC controls at least one Base Station System (BSS)BSS consists of:

BS Controller (BSC)Base Transceiver Station (BTS) or Base Station (BS)

BS performs:Basic signal transmissionSignal receptionSimple control functionsGSM-specific speech coding/decoding and data rate adaptation

Main task of MSC is to coordinate the call set-up between 2 GSM users of GSM user and a user of an external network

Some of the function performed by MSC:Calling user, setting-up and maintaining the connectionDynamic resources management Rerouting of a call between different BSC during hand offInterface operation with external networks (GMSC)Encrypting user’s dataReassigning the carriers in order to redistribute networks resources

Operation and Maintenance Center (OMC) – supervises operation of particular GSM system blocks

OMC is connected to all switching blocks and performs management functions:

Traffic accounting

Traffic monitoring

Management in case of failure

HLR management

Communications between OMC and network blocks is implemented by leased telephone links or other fixed networks and is performed using the SS7 signalling protocol of X.25 protocol

Basic Radio Transmission Parameters of GSM SystemTwo 25 MHz wide bands

Uplink 890-915 MHz

Downlink 935-960 MHz

Frequency Division Duplex (FDD) is used – separate frequency band for opposite direction of communication

Both bands divided into 124 frequency intervals of 200 kHz

For each carrier time is divided into 8 slots

Multiple access is realised by assigning the connection a particular carrier frequency and a selected time slot

Thus GSM uses TDMA/FDMA scheme (Time/Frequency Division Multiple Access)

In TDMA/FDMA a physical channel is a sequence of time slots (denoted by the assigned slot no.) which are placed on a selected carrier

Physical channels are arranged in pairs – one physical channel in each direction – they are marked with the same time slot no and they frequency differs by 45 MHz

The time slots numbering in the downlink direction is delayed by 3

Thus mobile station never transmits and receives signals to and from BS at the same time.

This reduces requirement for RF and DSP blocks

Computation power can be shared between transmitter and receiver

SummaryHistory of cellular system Introduction to GSM

Objectives of GSMTechnological changes

Basic architectureMSCDatabases (HLR, VLR etc.)BSS

Basic radio transmission parametersTDMA/FDMAPhysical channel