Lecture overview. History of cellular systems (1G) GSM introduction. Basic architecture of GSM system. Basic radio transmission parameters of GSM

Transcription

1 Lecture overview History of cellular systems (1G) GSM introduction Basic architecture of GSM system Basic radio transmission parameters of GSM

2 Analogue cellular systems 70 s 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

3 Analogue cellular systems 80 s 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

4 Analogue cellular systems 80 s Ireland Britain Italy Germany Spain France TACS (Total Access Communication System) TACS TACS C450 NMT (Nodrdic Mobile Telephony) Radiocom 2000, NMT

5 Analogue cellular systems 80 s System Began Channel Frequency operation width (khz) (MhZ) AMPS m-b b-m NMT m-b b-m C m-b b-m TACS plus m-b b-m NMT m-b b-m No. Channels Downlink from BS to MS Uplink from MS to BS

6 Analogue cellular systems 80 s 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

7 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 and Changing form analogue transmission to digital transmission

8 Towards GSM In 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 MHz for uplink transmission MHz for downlink transmission

9 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

10 GSM s Technical objectives By 1987 the standard had decided on the various technical aspects of the standard. Digital transmission Time Division Multiplexing of radio channels Encryption of radio channel transmission New compression algorithms for reduced data rate compared to the coding laws used in telecommunications. Compatibility with ISDN networks

11 Global System for Mobile Communications Basic architecture

12 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) permanently registered in the system database of all MS 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

13 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 establish a connection with the user, even if the user is temporarily in a different 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

14 HLR and VLR exchange information regarding the users currently located in the area served by the VLR This 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 located VLR stores the information necessary for initiation of mobileoriginated call MSC are connected with each other One 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)

15 BS performs: Basic signal transmission Signal reception Simple control functions GSM-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 connection Dynamic resources management Rerouting of a call between different BSC during hand off Interface operation with external networks (GMSC) Encrypting user s data Reassigning the carriers in order to redistribute networks resources

16 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

17 Basic Radio Transmission Parameters of GSM System Two 25 MHz wide bands Uplink MHz Downlink 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)

18 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

19 Summary History of cellular system Introduction to GSM Objectives of GSM Technological changes Basic architecture MSC Databases (HLR, VLR etc.) BSS Basic radio transmission parameters TDMA/FDMA Physical channel