UNIT II MAC, TELE COMMUNICATION AND SATELLITE SYSTEMS ... · PDF fileUNIT – II MAC, TELE COMMUNICATION AND SATELLITE SYSTEMS Medium access Control Techniques- SDMA-TDMA-FDMA- CDMA

CS601 Wireless Communication and Networks Unit - II

MTech CSE (PT, 2011-14) SRM, Ramapuram 1 hcr:innovationcse@gg

UNIT – II MAC, TELE COMMUNICATION AND

SATELLITE SYSTEMS Medium access Control Techniques- SDMA-TDMA-FDMA- CDMA- Comparison.

Tele communication systems- GSM-DECT and TETRA.

Satellite Systems- Routing, Localization and hand over.

MOTIVATION FOR A SPECIALIZED MAC

Problems in wireless networks

signal strength decreases proportional to the square of the distance

the sender would apply CS and CD, but the collisions happen at the receiver

it might be the case that a sender cannot “hear” the collision, i.e., CD does not work

furthermore, CS might not work if, e.g., a terminal is “hidden”

Hidden and exposed terminals

Hidden terminals

A sends to B, C cannot receive A

C wants to send to B, C senses a “free” medium (CS fails)

collision at B, A cannot receive the collision (CD fails)

A is “hidden” for C

Exposed terminals

B sends to A, C wants to send to another terminal (not A or B)

C has to wait, CS signals a medium in use

but A is outside the radio range of C, therefore waiting is not necessary

C is “exposed” to B

Near and far terminals

Terminals A and B send, C receives

signal strength decreases proportional to the square of the distance



the signal of terminal B therefore drowns out A’s signal

C cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the

physical layer

Also severe problem for CDMA-networks - precise power control needed!

Access methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access)

o segment space into sectors, use directed antennas

o cell structure

FDMA (Frequency Division Multiple Access)

o assign a certain frequency to a transmission channel between a sender and a receiver

o permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping

Spread Spectrum)

TDMA (Time Division Multiple Access)

o assign the fixed sending frequency to a transmission channel between a sender and a receiver for a

certain amount of time

Space Division Multiple Access (SDMA)

SDMA is used for allocating a separated space to users in wireless networks.

A typical application involves assigning an optimal base station to a mobile phone user.

The mobile phone may receive several base stations with different quality.

A MAC algorithm could now decide which base station is best, taking into account which frequencies

(FDM), time slots (TDM) or code (CDM) are still available (depending on the technology).

SDMA is never used in isolation but always in combination with one or more other schemes.

The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the

infrastructure implementing space division multiplexing (SDM)

Frequency Division Duplex - FDD/FDMA

General scheme, example GSM

All uplinks use the band between 890.2 and 915 MHz,

all downlinks use 935.2 to 960 MHz.



TIME DIVISION MULTIPLE ACCESS (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for

communication, i.e., controlling TDM.

Tuning in to a certain frequency is not necessary, i.e., the receiver can stay at the same frequency the

whole time.

Using only one frequency, and thus very simple receivers and transmitters

Fixed TDM

The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern.

This results in a fixed bandwidth and is the typical solution for wireless phone systems

Assigning different slots for uplink and downlink using the same frequency - time division duplex (TDD).

Time division multiplexing for multiple access and duplex

Classical Aloha

Aloha neither coordinates medium access nor does it resolve contention on the MAC layer.

each station can access the medium at any time

random, distributed (no central arbiter), time-multiplex

If two or more stations access the medium at the same time, a collision occurs and the data is destroyed

Slotted Aloha

Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries



Carrier Sense Multiple Access (CSMA)

sensing the carrier before accessing the medium

this decreases the probability of a collision.

o hidden terminals cannot be detected,

o if a hidden terminal transmits at the same time as another, a collision might occur at the receiver.

Types of CSMA

non-persistent CSMA

o stations sense the carrier and start sending immediately if the medium is idle.

o If the medium is busy, the station pauses a random amount of time before sensing the medium again

and repeating this pattern.

In p-persistent CSMA

o systems nodes also sense the medium, but only transmit with a probability of p, with the station

deferring to the next slot with the probability 1-p, i.e., access is slotted in addition.

In 1-persistent CSMA systems,

o all stations wishing to transmit access the medium at the same time, as soon as it becomes idle. This

will cause many collisions if many stations wish to send and block each other.

Demand Assigned Multiple Access (DAMA)

Also called Reservation Aloha.

Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet

arrival and packet length)

Reservation can increase efficiency to 80% a sender reserves a future time-slot

o sending within this reserved time-slot is possible without collision

o reservation also causes higher delays

o typical scheme for satellite links

Examples for reservation algorithms:

o Explicit Reservation according to Roberts (Reservation-ALOHA)

o Implicit Reservation (PRMA)

o Reservation-TDMA

DAMA - Explicit Reservation (Reservation Aloha)

Two modes:

o ALOHA mode for reservation:

competition for small reservation slots, collisions possible

o Reserved mode for data transmission within successful reserved slots (no collisions possible)

it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all

stations have to synchronize from time to time

Packet Reservation Multiple Access

Implicit reservation

a certain number of slots form a frame, frames

are repeated

stations compete for empty slots according to

the slotted aloha principle

once a station reserves a slot successfully, this

slot is automatically assigned to this station in

all following frames as long as the station has

data to send



competition for this slots starts again as soon as the slot was empty in the last frame

Reservation TDMA

every frame consists of N mini-slots and x data-slots

every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).

other stations can send data in unused data-slots according to a round-robin sending scheme

(best-effort traffic)

Multiple Access with Collision Avoidance MACA

Uses short signaling packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it

sends a data packet

CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

Signaling packets contain

o sender address

o receiver address

o packet size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

MACA avoids the problem of hidden terminals

A and C want to send to B

A sends RTS first

C waits after receiving CTS from B

MACA avoids the problem of exposed terminals

B wants to send to A, C to another terminal

now C does not have to wait for it cannot receive CTS from A



Protocol machines for MACA

MACA variant: DFWMAC in IEEE802.11

Polling

If one terminal can be heard by all others, this “central” terminal (a.k.a. base station) can poll all other

terminals according to a certain scheme

The master can poll the slaves according to many schemes:

o round robin (only efficient if traffic patterns are similar over all stations),

o randomly,

o according to reservations (the classroom example with polite students) etc.

Example: Randomly Addressed Polling

base station signals readiness to all mobile terminals

terminals ready to send can now transmit a random number without collision with the help of CDMA or

FDMA (the random number can be seen as dynamic address)

the base station now chooses one address for polling from the list of all random numbers (collision if two

terminals choose the same address)

the base station acknowledges correct packets and continues polling the next terminal

this cycle starts again after polling all terminals of the list

Inhibit Sense Multiple Access (ISMA) / Digital Sense Multiple Access (DSMA).

Current state of the medium is signaled via a “busy tone”

the base station signals on the downlink (base station to terminals) if the medium is free or not

terminals must not send if the medium is busy

terminals can access the medium as soon as the busy tone stops

the base station signals collisions and successful transmissions via the busy tone and acknowledgements,

respectively (media access is not coordinated within this approach)

mechanism used for Cellular Digital Packet Data (CDPD) (USA, integrated into AMPS mobile system)



Code Division Multiple Access (CDMA)

all terminals send on the same frequency probably at the same time and can use the whole bandwidth of

the transmission channel

each sender has a unique random number, the sender XORs the signal with this random number

the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a

correlation function

Advantages:

o all terminals can use the same frequency, no planning needed

o huge code space (e.g. 232

) compared to frequency space

o interferences (e.g. white noise) is not coded

o forward error correction and encryption can be easily integrated

Disadvantages:

o higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is

a signal)

o all signals should have the same strength at a receiver

CDMA in theory

Sender A

sends Ad = 1, key Ak = 010011 (assign: “0“= -1, “1“= +1)

sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

Sender B

sends Bd = 0, key Bk = 110101 (assign: “0“= -1, “1“= +1)

sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

Both signals superimpose in space

interference neglected (noise etc.)

As + Bs = (-2, 0, 0, -2, +2, 0)

Receiver wants to receive signal from sender A

apply key Ak bitwise (inner product)

Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

result greater than 0, therefore, original bit was “1“

receiving B

Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0“

Coding and spreading of data from sender A



Coding and spreading of data from sender B

Reconstruction of A’s data

Reconstruction of B’s data



Spread Aloha Multiple Access (SAMA)

Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders

with individual codes at the same time

use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing

according to aloha

each sender uses the same spreading code

combination of CDMA and TDMA

Comparison of SDMA, TDMA, FDMA, and CDMA mechanisms

Approach SDMA TDMA FDMA CDMA

Idea segment space into

cells/sectors

segment sending time

into disjoint time-slots,

demand driven or fixed

patterns

segment the

frequency band into

disjoint sub-bands

spread the

spectrum using

orthogonal codes

Terminals

only one terminal

can be active in one

cell/one sector

all terminals are active for

short periods of time on

the same frequency

every terminal has

its own frequency,

uninterrupted

all terminals can be

active at the same

place at the same

moment,

uninterrupted

Signal

separation

cell structure,

directed antennas

synchronization in the

time domain

filtering in the

frequency domain

code plus special

receivers

Advantages

very simple,

increases capacity

per km²

established, fully digital,

flexible

simple, established,

robust

flexible, less

frequency planning

needed, soft

handover

Dis-

advantages

inflexible, antennas

typically fixed

guard space needed

(multipath propagation),

synchronization difficult

inflexible,

frequencies are a

scarce resource

complex receivers,

needs more

complicated power

control for senders

Comment

only in combination

with TDMA, FDMA

or CDMA useful

standard in fixed

networks, together with

FDMA/SDMA used in

many mobile networks

typically combined

with TDMA

(frequency hopping

patterns) and

SDMA (frequency

reuse)

still faces some

problems, higher

complexity, lowered

expectations; will be

integrated with

TDMA/FDMA



GSM Groupe Spéciale Mobile => Global System for Mobile Communication

Performance characteristics of GSM

Communication

o mobile, wireless communication; support for voice and data services

Total mobility

o international access, chip-card enables use of access points of different providers

Worldwide connectivity

o one number, the network handles localization

High capacity

o better frequency efficiency, smaller cells, more customers per cell

High transmission quality

o high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (cars, trains)

Security functions

o access control, authentication via chip-card and PIN

Disadvantages of GSM

no end-to-end encryption of user data

no full ISDN bandwidth of 64 kbit/s to the user, no transparent B-channel

reduced concentration while driving

electromagnetic radiation

abuse of private data possible

roaming profiles accessible

high complexity of the system

several incompatibilities within the GSM standards

Mobile Services

GSM offers

o several types of connections

voice connections, data connections, short message service

o multi-service options (combination of basic services)

Three service domains

o Bearer Services

o Telematic Services

o Supplementary Services

Bearer and Tele Services Reference Model

Bearer Services

Telecommunication services to transfer data between access points

Specification of services up to the terminal interface (OSI layers 1-3)

Different data rates for voice and data (original standard)

o data service (circuit switched)

synchronous: 2.4, 4.8 or 9.6 kbit/s

asynchronous: 300 - 1200 bit/s



o data service (packet switched)

synchronous: 2.4, 4.8 or 9.6 kbit/s

asynchronous: 300 - 9600 bit/s

Tele Services

Telecommunication services that enable voice communication via mobile phones

All these basic services have to obey cellular functions, security measurements etc.

mobile telephony

Emergency number

o common number throughout Europe (112); mandatory for all service providers; free of charge;

connection with the highest priority (preemption of other connections possible)

Multinumbering

o several ISDN phone numbers per user possible

Non-Voice-Teleservices

o group 3 fax, voice mailbox, electronic mail, Short Message Service (SMS)

Supplementary services

Services in addition to the basic services, cannot be offered stand-alone

Similar to ISDN services besides lower bandwidth due to the radio link

May differ between different service providers, countries and protocol versions

Important services

o identification: forwarding of caller number

o suppression of number forwarding

o automatic call-back

o conferencing with up to 7 participants

o locking of the mobile terminal (incoming or outgoing calls)

GSM SYSTEM ARCHITECTURE GSM is a PLMN (Public Land Mobile Network)

several providers setup mobile networks following the GSM standard within each country

components

o MS (mobile station)

o BS (base station)

o MSC (mobile switching center)

o LR (location register)

subsystems

o RSS (radio subsystem): covers all radio aspects

o NSS (network and switching subsystem): call forwarding, handover, switching

o OSS (operation subsystem): management of the network

Radio Sub System (RSS)

All radio specific entities

Mobile Stations (MS)

o all user equipment and software needed for communication with a GSM network

o Subscriber Identity Module (SIM)

all user-specific data that is relevant to GSM

o International Mobile Equipment Identity (IMEI)

o Personal Identity Number (PIN)

o PIN Unblocking Key (PUK)

o Authentication Key Ki

o Cipher Key Kc

o International Mobile Subscriber Identity (IMSI)

o Temporary Mobile Subscriber Identity (TMSI)

o Location Area Identification (LAI)

Base Station Subsystem (BSS)

o controlled by a base station controller (BSC)



o functions necessary to maintain radio connections to an MS,

o coding/decoding of voice

o rate adaptation to/from the wireless network part

o contains several BTSs

Base Transceiver Station (BTS)

o can form a radio cell or, several cells using sectorized antennas

o connected to MS via the Um interface (ISDN U interface for mobile use)

o connected to the BSC via the Abis interface.

Base Station Controller (BSC)

o manages the BTSs

o reserves radio frequencies

o handles the handover from one BTS to another within the BSS

o performs paging of the MS

o multiplexes the radio channels onto the fixed network connections at the A interface

O interface

o Signalling System No. 7 (SS7) based on X.25 (dashed lines)

A interface

o circuit-switched PCM-30 systems

Network and Switching Subsystem (NSS)

Mobile services switching center (MSC)

o Gateway MSC (GMSC)

additional connections to other fixed networks, such as PSTN and ISDN

o Interworking Functions (IWF)

Used to connect to public data networks (PDN) such as X.25.

o Standard Signaling System No. 7 (SS7)

o The MSC (mobile switching center) plays a central role in GSM

switching functions

additional functions for mobility support

management of network resources

interworking functions via Gateway MSC (GMSC)

integration of several databases

Functions of a MSC

specific functions for paging and call forwarding

termination of SS7 (signaling system no. 7)

mobility specific signaling

location registration and forwarding of location information

provision of new services (fax, data calls)

support of short message service (SMS)

generation and forwarding of accounting and billing information

Home Location Register (HLR)

o Mobile Subscriber ISDN number (MSISDN)

o International Mobile Subscriber Identity (IMSI)

o Location Area (LA)

o Mobile Subscriber Roaming Number (MSRN)

Visitor Location Register (VLR)

o local database for a subset of user data, data about all user currently in the domain of the VLR



Operation SubSystem (OSS)

enables centralized operation, management, and maintenance of all GSM subsystems

Authentication Center (AUC)

o generates user specific authentication parameters on request of a VLR

o authentication parameters used for authentication of mobile terminals and encryption of user data on

the air interface within the GSM system

Equipment Identity Register (EIR)

o registers GSM mobile stations and user rights

o stolen or malfunctioning mobile stations can be locked and sometimes even localized

Operation and Maintenance Center (OMC)

o different control capabilities for the radio subsystem and the network subsystem



Radio interface

GSM implements SDMA using cells with BTS and assigns an MS to a BTS.

FDD is used to separate downlink and uplink

GSM TDMA frame, GSM time slots

Bursts, normal burst, guard space, GSM TDMA frame, slots, and bursts

Frame Hierarchy



Protocols

channel coding and error detection/correction

forward error correction (FEC)

voice activity detection (VAD)

radio resource management (RR)

BTS management (BTSM)

Mobility management (MM)

temporary mobile subscriber identity (TMSI)

international mobile subscriber identity (IMSI)

call management (CM)

o call control (CC)

o short message service (SMS)

o supplementary service (SS)

dual tone multiple frequency (DTMF)

pulse code modulation (PCM)

Signaling system No. 7 (SS7)

BSS application part (BSSAP)

Localization

automatic, worldwide localization of users.

system always knows where a user currently is, and the same phone number is valid worldwide

The HLR always contains information about the current location

VLR currently responsible for the MS informs the HLR about location changes

Roaming

Changing VLRs with uninterrupted availability of all services is also called roaming

Three types

o within the network of one provider,

o between two providers in one country (national roaming)

o between different providers in different countries (international roaming).



To locate an MS and to address the MS, several numbers are needed

Mobile station international ISDN number (MSISDN)

o follows the ITU-T standard E.164 for addresses as it is also used in fixed ISDN networks

o Consists of

country code (CC)

national destination code (NDC)

subscriber number (SN)

o e.g., +49 179 1234567

49 for Germany

179 for the address of the network provider

International mobile subscriber identity (IMSI)

o Consists of

a mobile country code (MCC)

the mobile network code (MNC)

the mobile subscriber identification number (MSIN)

Temporary mobile subscriber identity (TMSI)

o To hide the IMSI

o GSM uses the 4 byte TMSI for local subscriber identification

o a VLR may change the TMSI periodically

Mobile station roaming number (MSRN)

o hides the identity and location of a subscriber

o contains

the current visitor country code (VCC),

the visitor national destination code (VNDC)

the identification of the current MSC together with the subscriber number

o helps the HLR to find a subscriber for an incoming call

Calling

Mobile Terminated Call (MTC)



Mobile Originated Call (MOC)

Message flow for MTC and MOC

Handover

two basic reasons for a handover

The mobile station moves out of the range of a BTS

o signal level decreases

o error rate may grow

o diminish the quality of the radio link

load balancing

o (MSC, BSC) may decide that the traffic in one cell is too high and shift some MS to other cells



Four Types of handover in GSM

Intra-cell handover

o Within a cell, narrow-band interference could make transmission at a certain frequency impossible.

o The BSC could then decide to change the carrier frequency

Inter-cell, intra-BSC handover

o The mobile station moves from one cell to another, but stays within the control of the same BSC.

o The BSC then performs

a handover,

assigns a new radio channel in the new cell

releases the old one

Inter-BSC, intra-MSC handover

o As a BSC only controls a limited number of cells; GSM also has to perform handovers between cells

controlled by different BSCs.

o This handover then has to be controlled by the MSC

Inter MSC handover

o A handover could be required between two cells belonging to different MSCs

o Now both MSCs perform the handover together

Handover decision depending on receive level



Inter BSC, Intra-MSC handover

Security

Security services offered by GSM

Access control and authentication

o authentication of a valid user for the SIM with a secret PIN

o next step is the subscriber authentication based on a challenge-response scheme

Confidentiality

o voice and signaling encrypted on the wireless link (after successful authentication)

o exists only between MS and BTS

Anonymity

o temporary identity TMSI (Temporary Mobile Subscriber Identity)

o newly assigned at each new location update (LUP)

o encrypted transmission

Algorithms

Algorithm A3 is used for authentication,

A5 for encryption; implemented in the devices, identical for all providers

A8 for the generation of a cipher key

Authentication

Based on the SIM, which stores

o the individual authentication key Ki,

o the user identification IMSI

o the algorithm used for authentication A3.

uses a challenge-response method:

the access control AC generates a random number RAND as challenge

the SIM within the MS answers with SRES (signed response) as response

The AuC (Authentication centre) performs the basic generation of random values RAND, signed responses

SRES, and cipher keys Kc for each IMSI

forwards this information to the HLR

The current VLR requests the appropriate values for RAND, SRES, and Kc from the HLR.

the VLR sends the random value RAND to the SIM.



Both sides, perform the same operation with RAND and the key Ki, called A3.

The MS sends back the SRES generated by the SIM;

the VLR can now compare both values.

If they are the same, the VLR accepts the subscriber, otherwise the subscriber is rejected.

Encryption

MS and BSS can start using encryption by applying the cipher key Kc



Kc is generated using the individual key Ki and a random value by applying the algorithm A8.

the SIM in the MS and the network both calculate the same Kc

The key Kc itself is not transmitted over the air interface

MS and BTS can now encrypt and decrypt data using the algorithm A5 and the cipher key Kc.

New data services

High Speed Circuit Switched Data (HSCSD)

higher data rates are achieved by bundling several TCHs

air interface user rate (AIUR)

For n channels, HSCSD requires n times signaling during handover, connection

setup and release. Each channel is treated separately

General Packet Radio Service (GPRS)

packet mode transfer for applications that exhibit traffic patterns such as frequent transmission of small

volumes (e.g., typical web requests) or infrequent transmissions of small or medium volumes (e.g., typical

web responses)

point-to-point (PTP) packet transfer service

PTP connection oriented network service (PTP-CONS)

PTP connectionless network service (PTP-CLNS)

QoS-profile

o service precedence (high, normal, low),

o reliability class and delay class of the transmission,

o user data throughput.

GPRS architecture

o GPRS support nodes (GSN)

o gateway GPRS support node (GGSN)

o packet data networks (PDN)

o serving GPRS support node (SGSN)

o GPRS register (GR)

o GPRS tunnelling protocol (GTP).

o subnetwork dependent convergence protocol (SNDCP)

o base station subsystem GPRS protocol (BSSGP)

o radio link protocol (RLC)

o packet data traffic channels (PDTCHs).



DECT Digital Enhanced Cordless Telecommunications (DECT)

digital European cordless telephone and digital European cordless telecommunications

standard describes air interface between base-station and mobile phone

GSM is designed for outdoor use with a cell diameter of up to 70 km,

DECT can offer its service to some 10,000 people within one km2

Characteristics

frequency: 1880-1990 MHz

channels: 120 full duplex

duplex mechanism: TDD (Time Division Duplex) with 10 ms frame length

multplexing scheme: FDMA with 10 carrier frequencies,

TDMA with 2x 12 slots

modulation: digital, Gaußian

Minimum Shift Key (GMSK)

power: 10 mW average (max.

250 mW)

range: ca 50 m in buildings,

300 m open space

System

Architecture

global network

Local networks

home data base (HDB)

visitor data base (VDB)

fixed radio termination (FT)

portable radio termination (PT)

Protocol Architecture

Physical layer



Physical layer

modulation/demodulation

generation of the physical channel structure with a guaranteed throughput

controlling of radio transmission

channel assignment on request of the MAC layer

detection of incoming signals

sender/receiver synchronization

collecting status information for the management plane

MAC layer

maintaining basic services, activating/deactivating physical channels

multiplexing of logical channels

e.g., C: signaling, I: user data, P: paging, Q: broadcast

segmentation/reassembly

error control/error correction

Data link control layer

creation and keeping up reliable connections between the mobile terminal and basestation

two DLC protocols for the control plane (C-Plane)

o connectionless broadcast service:

paging functionality

o Lc+LAPC protocol:

in-call signaling (similar to LAPD within ISDN), adapted to the underlying MAC service

several services specified for the user plane (U-Plane)

o null-service: offers unmodified MAC services

o frame relay: simple packet transmission

o frame switching: time-bounded packet transmission

o error correcting transmission: uses FEC, for delay critical, time-bounded services

o bandwidth adaptive transmission

o „Escape“ service: for further enhancements of the standard

Network layer

similar to ISDN (Q.931) and GSM (04.08)

offers services to request, check, reserve, control, and release

resources

o necessary for a wireless connection

o necessary for the connection of the DECT system to the fixed network

main tasks

o call control: setup, release, negotiation, control

o call independent services: call forwarding, accounting, call redirecting

o mobility management: identity management, authentication, management of the location register

TETRA - TERRESTRIAL TRUNKED RADIO many different radio carriers

assign single carrier for a short period to one user/group of users

taxi service, fleet management, rescue teams

interfaces to public networks, voice and data services

very reliable, fast call setup, local operation

TETRA - ETSI standard

o formerly: Trans European Trunked Radio

o offers Voice+Data and Packet Data Optimized service

o point-to-point and point-to-multipoint

o ad-hoc and infrastructure networks

o several frequencies: 380-400 MHz, 410-430 MHz

o FDD, DQPSK

o group call, broadcast, sub-second group-call setup



SATELLITE SYSTEMS

Applications

Traditional

Weather forecasting

o infra red or visible light

Radio and TV broadcast satellites

o it is cheaper to install and, in most cases, no extra fees have to be paid for this service

Military satellites

o much safer from attack by enemies

Satellites for navigation

o global positioning system (GPS)

Mobile Communication

Global telephone backbones

o big cable in the sky

o being replaced by fiber optical cables crossing the oceans

o one-way, single-hop time delay of 0.25 s

Connections for remote or developing areas

o many places all over the world do not have direct wired connection to internet

Global mobile communication

Satellites using lower orbits

extend the area of coverage

mobile user link (MUL)

gateway link (GWL)

intersatellite links (ISL)

Typical satellite system for global mobile telecommunications

vertical handover

o between a cellular network and a satellite system

horizontal handover

o within cellular

Users should not notice the switching from, e.g., GSM, to a satellite network during conversation



Basics

Depending on the application, these orbits can be circular or elliptical

Satellites in circular orbits always keep the same distance to the earth’s surface following a simple law

m is the mass of the satellite;

R is the radius of earth with R = 6,370 km;

r is the distance of the satellite to the centre of the earth

f is the frequency of the rotation.

To keep the satellite in a stable circular orbit, both forces must be equal Fg = Fc

distance of a satellite to the earth’s surface depends on its rotation frequency.

Dependency of satellite period and distance to earth

Important parameters in satellite communication

inclination angle δ

o angle between the equatorial plane and the plane described by the satellite orbit

o inclination angle of 0 degrees means that the satellite is exactly above the equator

elevation angle ε

o angle between the center of the satellite beam and the plane tangential to the earth’s surface

footprint

o area on earth where the signals of the satellite can be received

propagation loss of the signals

o

Signal attenuation due to atmospheric absorption

Refer below



Four different types of orbits

Geostationary (or geosynchronous) earth orbit (GEO)

o distance of almost 36,000 km to the earth.

o Examples: all TV and radio broadcast satellites, many

weather satellites and satellites

o operating as backbones for the telephone network

Medium earth orbit (MEO)

o distance of about 5,000–12,000 km

o upcoming systems (e.g., ICO) use this class for various

reasons

Low earth orbit (LEO)

o were mainly used for espionage

o using altitudes of 500–1,500 km

Highly elliptical orbit (HEO)

o all satellites with noncircular orbits.

o few commercial communication systems

o perigee over large cities to improve communication quality.

Van Allen radiation belts,

o belts consisting of ionized particles

o inner Van Allen belt

heights of about 2,000–6,000 km

o outer Van Allen belt

about 15,000–30,000 km

o make satellite communication very difficult in these orbits.



GEO

Orbit

o 35.786 km distance to earth surface, equatorial plane (inclination 0°)

complete rotation exactly one day, satellite is synchronous to earth rotation

Advantages

o can use fixed antenna positions, no adjusting is needed.

o ideal for TV and radio broadcasting.

o do not need a handover due to the large footprint

o do not exhibit any Doppler shift because the relative movement is zero.

Disadvantages:

o Northern or southern regions of the earth have more problems receiving these satellites due to the low

elevation above a latitude of 60°, i.e., larger antennas are needed in this case.

o Shading of the signals in cities due to high buildings and the low elevation further away from the

equator limit transmission quality.

o The transmit power needed is relatively high (some 10 W), problems for battery powered devices.

o Cannot be used for small mobile phones.

o high latency of over 0.25 s one-way – biggest problem for voice and also data communication

o many retransmission schemes which are known from fixed networks fail.

o Due to the large footprint, either frequencies cannot be reused or the GEO satellite needs special

antennas focusing on a smaller footprint.

o Transferring a GEO into orbit is very expensive

LEO

Orbit ca. 500 - 1500 km above earth surface

typical duration of LEO periods are 95 to 120 minutes

provide a high quality communication link by ensuring a high elevation for every spot on earth

will only be visible from the earth for around ten minutes

Further classification into

o Little LEOs with low bandwidth services (some 100 bit/s)

o big LEOs (some 1,000 bit/s)

o broadband LEOs with plans reaching into the Mbit/s range

Advantages:

o using low transmit power in the range of 1W

o The delay for packets delivered via a LEO is relatively low (approx 10 ms)

o Smaller footprints of LEOs allow for better frequency reuse

o LEOs can provide a much higher elevation in polar regions and so better global coverage.

Disadvantages

o handover necessary from one satellite to another

o many satellites necessary for global coverage

o more complex systems due to moving satellites

MEO

Orbit ca. 5000 - 12000 km above earth surface

Advantages:

o the system only requires a dozen satellites for global coverage

o move more slowly relative to the earth’s rotation allowing a simpler system design

o satellite periods are about six hours

o Depending on the inclination, a MEO can cover larger populations, so requiring fewer handovers.

Disadvantages:

o delay increases to about 70–80 ms.

o need higher transmit power

o need special antennas for smaller footprints.



Routing

Satellites offering Inter Satellite Link (ISL)

o traffic can be routed between the satellites.

o one user sends data up to a satellite

o the satellite forwards it to the one responsible for the receiver via other satellites.

o The last satellite now sends the data down to the earth

o only one uplink and one downlink per direction is needed

o The ability of routing within the satellite network reduces the number of gateways needed on earth.

Without ISL

o all traffic is relayed to earth, routed there relayed back to a satellite.

Localization

similar to that of terrestrial cellular networks

Home Location Register (HLR)

o stores all static information about a user with current location

Visitor Location Register (VLR)

o The last known location of a mobile user

Satellite User Mapping Register (SUMR)

o stores the current position of satellites

o a mapping of each user to the current satellite

Registration of a mobile station

o initially sends a signal which one or several satellites can receive.

o Satellites receiving such a signal report this event to a gateway.

o The gateway can now determine the location of the user via the location of the satellites.

o User data is requested from the user’s HLR, VLR and SUMR are updated

Calling a mobile station

o localization using HLR/VLR similar to GSM

o connection setup using the appropriate satellite

Handover in satellite systems

caused by the movement of the satellites

Intra satellite handover

o handover from one spot beam to another

o mobile station still in the footprint of the satellite, but in another cell

Inter satellite handover

o handover from one satellite to another satellite

o mobile station leaves the footprint of one satellite

Gateway handover

o Handover from one gateway to another

o mobile station still in the footprint of a satellite, but gateway leaves the footprint

Inter system handover

o Handover from the satellite network to a terrestrial cellular network

o mobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc

Comments & Feedback

Thanks to my family members who supported me while I spent hours and hours to prepare this.

Your feedback is welcome at [email protected]

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