TELE4652 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

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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)

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Analogue Telephone

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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

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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)

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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.

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!)

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Remote Terminal (diagram)

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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.

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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

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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.

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Generic (Digital) Wireless Communication System

Transmitter

Receiver

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Input data

Source

Coder

Channel

Coder

Modulator

Tx

Antenna Channel…

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

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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

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AMPS - Overview

Forward Channels (Base Station to Mobile Station)

869 – 894 MHz

Reverse Channels (MS to BTS)

824 – 849 MHz

Channels always assigned as a duplex pair, separated by

45MHz

- allowed standard rejection filters to be built

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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 (870-890 MHz), and Reverse (825-845 MHz)

- Two Operators, with 10 MHz each (Telecom Australia & Primus)

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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!

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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

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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

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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

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OSI Model (Recap)

Open System Interconnection – standard 7 layer protocol

stacks

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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)

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AMPS - Mobile SS7

Lowest three layers changed for MS <-> BTS and

BTS <-> MTSO

Otherwise, identical to the SS7 protocol

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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

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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

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AMPS - RCC

-Initial 48 bits for synchronisation (bit sync – alternating 1s

and 0s; word sync – distinct pattern 11100010010)

- 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

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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, , so FM signal bandwidth

was about 30 kHz

- Signal Processing as below:

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( )ffBW data ∆+= 2

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.

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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

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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)

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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)

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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

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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

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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

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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

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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???

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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

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2G Mobile Networks

The most successful 2G network was GSM

-this was a European standard, which achieved world-wide

dominance

- Stood for Groupe Specialé Mobile (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

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Pulse Code Modulation (PCM)

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.

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Linear Predictive Coding

• Encoding – analysis by synthesis loop.

• Transmission – vocal tract filter coefficients and excitation

characteristics

• Decoding – synthesis model

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GSM – Speech Processing

Digital Processing also allowed Voice Activity Detection,

Noise Extrapolation, and Comfort Noise (Silence Descriptor)

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GSM – Digital Signal Processing

A GSM phone – functional description.

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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: 935 - 960 MHz

- Uplink: 890 – 915 MHz

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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)

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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.

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GSM – Network

The network is more developed, with its own backbone

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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

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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

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GSM – Authentication

The digital network allows more advanced security

techniques to be employed:

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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 0 1 1 1 0 0 1 0 1 0 0 0 1 1 1 0 0 1 1 0 1 …Ciphering Key 0 0 0 1 1 0 1 0 1 0 1 0 0 0 1 1 0 1 1 1 0 …

Ciphered Data 0 1 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0 0 0 1 1 …

Ciphered Data 0 1 1 0 1 0 0 0 0 0 1 0 1 1 0 1 0 0 0 1 1 …Ciphering Key 0 0 0 1 1 0 1 0 1 0 1 0 0 0 1 1 0 1 1 1 0 …

Recovered Data 0 1 1 1 0 0 1 0 1 0 0 0 1 1 1 0 0 1 1 0 1 …

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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

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GSM – SS7-based Protocol

Still a circuit-switched core, built on SS7

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GSM – Operations

Example of Signalling – mobile originated call

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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

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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

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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, email, internet,

etc, all provided mobile via a common standard...

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