TELE4652 Mobile and Satellite Communications

Transcription

1 Mobile and Satellite Communications Lecture 1 Introduction to Cellular Mobile Communications Public Switched Telephone Networks (PSTN) Public Land Mobile Networks (PLMN) evolved from the PSTN - Aimed to introduce mobility -Essentially same network, but the Telephone becomes mobile -The structure of 1G Mobile Networks was very similar to the old PSTN

2 PSTN structure -Known as POTS -Twisted copper-wires to customer premises -Remote terminal used to allow separation and echo cancellation -Central Office performs switching (essentially a router) Analogue Telephone

3 Analogue Telephone Operation -Circuit Switched -the role of the CO was to create an electric circuit between two telephones -each microphone would produce a current on the line, which would then excite the earphone at the other end of the line -initially, an operator would physically make the connection -> evolved into digital, automated switching Routing/Switching -First Generation -tell the operator who you d like to be connected to! - Second Generation -rotary dialling the line is interrupted with 0.1 sec pulses, the number of pulses corresponding to the digit being called -Third Generation - Dual Tone Multi-Frequency (DTMF)

4 Dual-tone Multi-frequency (DTMF) a 4 4 matrix, with each row representing a low frequency, and each column representing a high frequency. Pressing a single key (such as '1' ) will send a sinusoidal tone of the two frequencies (697 and 1209 Hz). The multiple tones are the reason for calling the system multi-frequency. These tones are then decoded by the switching centre to determine which key was pressed. 8/2/ Remote Terminal Developed to allow amplification, and so increase the distance possible for communication -amplification required conversion of two-wire to four-wire -also allowed digitisation without altering customer premises equipment and pulling up copper-wire (we still have twisted pair copper wires to the home today!)

5 Remote Terminal (diagram) First Generation Mobile Networks Allow the CPE to be mobile -> Mobile Terminal Issues/Challenges: -Location: how do we know where it is if it can roam? - The radio-channel: Difficult and unpredictable. - Battery power vs. Range To achieve reasonable range, the first Mobile Phones were car mounted -> needed physically large batteries.

6 Cellular Model The Mobile Station(MS) must be physically close to a Base Station (radio tower), to ensure reasonable signal strength for communication -> Implies a high density of Base Stations Cell the geographical area over which a MS can receive an acceptable signal strength Allow Frequency Re-use -> Capacity Radio Communications The radio channel is difficult to communicate: - signal direction is unknown and variable (mobile) - channel characteristics unknown and time-varying - noise and interference -Multi-pathing - Relative motion Doppler Effect These challenges are common to all systems with communication over the wireless channel.

7 Generic (Digital) Wireless Communication System Transmitter Input data Source Coder Channel Coder Modulator Tx Antenna Channel Receiver Channel Rx antenna demodulator Channel Decoder Source Decoder Output data Channel Estimator AMPS Advanced Mobile Phone System -One of the first PLMN developed -Developed in the early 1980s by AT&T in the USA -There were similar systems developed around the world at the time (e.g. ETACS European Total Access Communication System) -Formed the basis for later 2G and 3G networks -Conceived as a Phone Network focussed only on the communication of speech -> extension of PSTN to radio - Analogue systems speech was transmitted as an analogue current

8 AMPS FDMA/FDD FDMA (Frequency Division Multiple Access) -Multiple users can access the system at the same time by using different frequencies. -Channel bandwidth was 30 khz FDD (Frequency Division Duplexing) -Communication possible in two directions simultaneously by using different frequencies -Forward and Reverse channels at different frequencies AMPS - Overview Forward Channels (Base Station to Mobile Station) MHz Reverse Channels (MS to BTS) MHz Channels always assigned as a duplex pair, separated by 45MHz - allowed standard rejection filters to be built

9 AMPS Channel Assignment -Channels are 30 khz, so 832 duplex channels available. - Assigned to a pair of network operators (competition) -Each operator had 416 channels (395 for voice data, and 21 for control signalling) -Slightly different in Australia -Forward ( MHz), and Reverse ( MHz) - Two Operators, with 10 MHz each (Telecom Australia & Primus) AMPS Cell Structure Adopted a N = 7 frequency re-use rate. Channels broken into 7 groups (A,B,C,D,E,F,G) A = {Channels 1, 8, 15, 22,...} B = {Channels 2, 9, 16, 23,...} A group of channels is assigned to a cell in such a way to minimise interference between co-channel cells Frequency re-use -> same channel used at different locations in the network -> Increase capacity!

10 AMPS Cell Structure MS power limited to 3W - limitations of battery power - minimise interference Cell radii typically 2 to 20 km -cell size and geometry depends on local radio environment - base station power level set depending on coverage Three control channels assigned per cell Later, channel assignment was made more flexible - high demand cells use more channels AMPS Network Topology Mobile link onto network was BTS BTS links to Mobile Telecommunication Switching Office (MTSO) Role of MTSO was to adapt signal for transmission over PSTN PSTN was the backbone performed the network routing

11 AMPS Circuit Switching Like the PSTN it was built on, AMPS was a circuit switched network Circuit Switching aim of network is to establish a continuous electronic circuit between two parties Built on the availability of a separate control channel for signalling information/call establishment/etc. Communication Protocol adopted was an evolution of Signalling System No. 7 (or SS7) a protocol widely used on the PSTN with a digital backbone OSI Model (Recap) Open System Interconnection standard 7 layer protocol stacks

12 SS7 Digital Telephony Telephone User Part (TUP) ISDN User Part (ISUP) Transaction Capabilities Application Part (TCAP) Signalling Connection Control Part (SCCP) Message Transfer Part (MTP) AMPS -Mobile SS7 Lowest three layers changed for MS <-> BTS and BTS <-> MTSO Otherwise, identical to the SS7 protocol

13 AMPS Control Channels -Dedicated control channels, distinct from traffic channels (SS7) Digital signalling -Used for call establishment, page response, network registration, location updating, and channel assignment -Control channels were 10 kbps, using binary FSK with frequency separation of 8 khz (allocated bandwidth 30 khz) -Manchester NRZ line coding very little DC energy, so control channels are spectrally distinct from voice channels -Forward (FCC) and Reverse (RCC) control channels used different data formats AMPS -FCC -56 data bits were encoded into 400 bits for transmission (frame length 400 bits) -56 bits broken into two, and (40, 28) BCH channel code is used. Each resulting 40 bit encoded word is repeated 5 times in the frame. -A 21 bit pre-amble was inserted for bit and word synchronisation

14 AMPS -RCC -Initial 48 bits for synchronisation (bit sync alternating 1s and 0s; word sync distinct pattern ) -Variable packet length. 1 to 6 words -Each data word is 36 bits, protected by (48,36) BCH code, and then repeated five times - Digital Colour Code, 7 bit base station address AMPS Voice Channels -Analogue voice data, band-limited to 3 khz (general telephone quality speech) -Frequency Modulation used, with frequency deviation 12kHz -Carson s rule, BW = 2( fdata+ f), so FM signal bandwidth was about 30 khz - Signal Processing as below:

15 AMPS Signal Processing Companding here used reduce distortion (not related to non-uniform quantisation of speech!) Emphasis improve noise performance of FM, by amplifying the high frequency parts Deviation Limiter reduce adjacent channel interference Supervisor Auditory Tone (SAT) inserted onto the voice signal to allow in-call signalling. -at either 5970, 6000, or 6030 Hz (above the human auditory band, so undetectable to users. AMPS -SAT Supervisor Auditory Tone needed for in-call signalling (not needed in PSTN) Allows two important functions of a Mobile Network to be carried out: -Hand-offs (or hand-overs). When a user moves into a neighbouring cell -Power Control. Need to manage the interference level in a cellular network

16 AMPS Hand-offs -MTSO monitors received strength of SAT, and that of neighbouring cells -If neighbouring BTS has a stronger signal, MTSO may decide to change the servicing base station -> hand-off -In AMPS, this implies a change of channel -Channel must be changed in a way that is undetectable to the user (no break in conversation) AMPS Hand-offs -There are many ways the network can decide to instigate a hand-off (study later...) -In many networks, spare capacity is maintained in cells to allow hand-offs (the axiom is, it is better to have a call attempt refused than a call dropped ) -In AMPS, a voice channel would be interrupted for 100ms to allow transfer of control information for a hand-off (format is the same as control channels)

17 Power Control Near-far Effect in Cellular Networks -Distant user can be swamped by signal from nearby user -network must direct near users to lower their power, and far users to increase their power -aim to have every user s signal received at BTS with the same power level -> control interference AMPS Network Registration 1. Phone is turned on. Scans for strongest Base Channel (FCC for each base station, 21 possibilities). Each base station continually transmits on its Base Channel. 2. Selects strongest (and this tells the MS the frequency set for this cell (A,B,C,D,E,F, or G) 3. Transmits a packet on the corresponding duplex RCC. This packet contains the user s Mobile Identification Number (MIN) and the Equipment Serial Number (ESN) 4. MS waits for an ACK on the FCC. If it doesn t get one, it will transmit the packet again -> ALOHA

18 AMPS Mobile Initiated Call 1. User keys in destination number and hits send. 2. MS scans for the strongest Base Channel. 3. MS sends a packet on the RCC duplex to this Base Channel. This message contains the MIN, ESN, and the destination number. 4. MTSO verifies that the MS has sent a valid set of parameters, and then sends a channel assignment message. 5. MSTO sends out signal to try to connect to the destination, using the SS7 protocol over the PSTN AMPS Mobile Initiated Call 6. If the destination was another mobile, a paging message is sent over the air to this MS. This paging message is sent out on all BTS that are serviced by the MTSO the MS was last registered to (course location information). 7. When called party responds, the MTSO establishes a circuit between the two parties. 8. When either party hangs up, the MTSO will free the radio channel and complete billing information

19 Evolution to 2G Networks Demand for cellular networks grew rapidly -> push to increase the capacity of cellular networks Radio spectrum was limited, so the next stage in evolution became to make wholly digital networks -> Convert voice into digital form for transmission -> This resulted in an increase in network capacity, but how??? Advantages of Digital Communications -Better signal fidelity, through use of channel coding and digital signal processing - Ability to use source coding for data compression - Encryption of data, for security -Access to advanced multiplexing and multiple access techniques -Equalisation, Channel Estimation, and diversity techniques could be employed - Availability of silicon-based electronics

20 2G Mobile Networks The most successful 2G network was GSM -this was a European standard, which achieved world-wide dominance -Stood for Groupe SpecialéMobile (French for Special Interest group in Mobile Communications), but was later changed to Global Standard for Mobile communications - Differed fundamentally from AMPS: - Voice data was digitised - Network grew away from PSTN backbone Pulse Code Modulation (PCM) 8/2/

21 Speech Compression The great increase in network capacity came largely from speech compression: -GSM used a sampling rate of 8 khz, with 13-bit raw PCM encoding -> data rate to 104 kbps -GSM standardised a voice-codec, a type of Linear Predictive Coding (LPC) called RPE-LTP (Regular Pulse Excitation with Long-Term Prediction) -the resulting data rate was 13 kps! -the speech quality was indistinguishable from the original. Linear Predictive Coding Encoding analysis by synthesis loop. Transmission vocal tract filter coefficients and excitation characteristics Decoding synthesis model

22 GSM Speech Processing Digital Processing also allowed Voice Activity Detection, Noise Extrapolation, and Comfort Noise (Silence Descriptor) GSM Digital Signal Processing A GSM phone functional description.

23 GSM Overview - Introduced in 1990 in Europe (Germany, France, Scandinavia) - FDD inherited from 1G networks - Separation of 45 MHz between duplex channel pair - Spectral Allocation (example varied between countries) - Downlink: MHz - Uplink: MHz GSM TDMA Developed as a Time Division Multiple Access (TDMA) system -each user is assigned a repeating time-slot in an 8- timeslot frame. -each channel has a bandwidth of 200 khz (also partly FDMA)

24 GSM No duplexer required Can offset Forward and Reverse channels for a user, so no need to transmit and receive at the same time: This is really Time Division Duplexing -just the 1G legacy for also FDD. GSM Network The network is more developed, with its own backbone

25 GSM Network Elements Mobile Station (MS) holds the SIM (Subscriber Identity Module), with the IMSI, the phone number on the local network, TMSI, and Authentication Keys and algorithms Base Transceiver Station (BTS) effectively the antenna for the network. Radio coverage area defines the cell. Base Station Controller(BSC) centralising the processing for the radio link. Handles channel assignment and frequency administration. Mobile Switching Centre(MSC) the major routing controller for the network GSM Network Elements Operations and Maintenance Centre(OMC) monitors the network, traffic loads, channel usage, and billing Implemented as parts of the MSC: Home Location Register (HLR) database of users at this network location (IMSI, authentication keys, and phone number). Also stores each user s last know location (serving MSC) Visitor Location Register (VLR) database of equivalent information of user currently in the cell Authentication Centre (AuC) handles authentication and ciphering Equipment Identity Register (EIR) used for tracking lost, stolen, and faulty equipment

26 GSM Authentication The digital network allows more advanced security techniques to be employed: GSM Ciphering Ciphering is a technique of providing security in digital transmissions. Requires a Ciphering Key to be known at the source and destination Example: Plain Data Ciphering Key Ciphered Data Ciphered Data Ciphering Key Recovered Data

27 GSM In-call signalling Digital data enables easy multiplexing of control data into signal stream (no more SAT) -> SACCH (Slow Associated Control Channel) up to two out of every 26 data timeslots is left for control signalling Allows: - Mobile Assisted Hand-offs (MAHO) -Open and Closed Loop power control GSM SS7-based Protocol Still a circuit-switched core, built on SS7

28 GSM Operations Example of Signalling mobile originated call Evolution to 3G networks What lead to the development of 3G networks? - the growth in popularity of: - the sms (short message service) - the internet 3G networks are no longer conceived to be voice communication networks, but are data networks -aim to communicate a range of different data types

29 3G network characteristics There are three main differences in 3G networks 1. IP backbone packet switching is not suited to variable data types and data rates 2. Adaptable Air-data rate data ranges from sms-messages, voice, up to video and multi-media streaming -> CDMA based 3. Better QoS (Quality of Service) speech is relatively robust to error and distortion, but internet pages are not 4G Networks? What s next in the evolution of Cellular Networks? Maybe: - Ever expanding data rates and QoS. - Seamless global roaming same User Terminal used anywhere in the world but primarily - Integration of Services telephone, TV, , internet, etc, all provided mobile via a common standard...