Cellular Networks Principles of Cellular Networks First Generation - - PowerPoint PPT Presentation

CMPE 477

Cellular Networks

 Principles of Cellular Networks  First Generation Analog, AMPS  Second Generation TDMA, GSM

System Architecture Radio Interface Localization and Calling  Handover

CMPE 477 – Wireless and Mobile Networks

Cellular Concept and Organization

Motivation: Instead of using high-power transmitter/receiver (mobile radio telephone), lower-power systems with

 shorter radius  Numerous transmitters/receivers

Idea: Space division multiple access Areas divided into cells

 Each served by its own antenna  Served by base station consisting of transmitter,

receiver, and control unit

 Mobile stations communicate only via the base station

Cell Structure

A structure where adjacent antennas are equidistant?

 A hexagonal pattern  Simplifies the task of determining when to switch to the

adjacent antenna and which one to choose

 Cell radius R (radius of the circle that circumscribes the hexagon)  Distance between cells

In practice hexagonal pattern cannot be used due to

 topographical limitations  signal propagation conditions  limitations on the antennas

R d 3 

Cellular Network Organization

Advantages of cell structures:

 higher capacity, higher number of users  less transmission power needed  more robust, decentralized  base station deals with interference, transmission area etc.

locally

Challenges:

 fixed network needed for the base stations  handover (changing from one cell to another) necessary  interference with other cells

Frequency Reuse

Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells

 10 to 50 frequencies assigned to each cell  Transmission power controlled to limit power at that

frequency escaping to adjacent cells

 The issue is to determine how many cells must

intervene between two cells using the same frequency, reuse factor (N)

,... 3 , 2 , 1 , , ), * (

2 2

    J I J I J I N N d D 

D: distance between centers of cells that use the same frequency

Example Reuse Patterns

Approaches to Cope with Increasing Capacity

Adding new channels Frequency borrowing – frequencies are taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge- shaped sectors, each with their own set of channels Microcells – antennas move to buildings, hills, and lamp posts

Power Control

Design issues making it desirable to include dynamic power control in a cellular system

 Received power must be sufficiently above the

background noise for effective communication

 Desirable to minimize power in the transmitted signal

from the mobile

 Reduce cochannel interference, alleviate health concerns,

save battery power

 In SS systems using CDMA, it’s desirable to equalize the

received power level from all mobile units at the BS

Types of Power Control

Open-loop power control

 Depends solely on mobile unit  No feedback from BS  Not as accurate as closed-loop, but can react quicker to

fluctuations in signal strength

Closed-loop power control

 Adjusts signal strength in reverse channel based on

metric of performance

 BS makes power adjustment decision and

communicates to mobile on control channel

Development of mobile telecommunication systems

1G 2G 3G 2.5G IS-95 cdmaOne IS-136 TDMA D-AMPS GSM PDC GPRS IMT-DS UTRA FDD / W-CDMA EDGE IMT-TC UTRA TDD / TD-CDMA cdma2000 1X 1X EV-DV (3X) AMPS NMT IMT-SC IS-136HS UWC-136 IMT-TC TD-SCDMA CT0/1 CT2 IMT-FT DECT CDMA TDMA FDMA IMT-MC cdma2000 1X EV-DO

First-Generation Analog

Advanced Mobile Phone Service (AMPS)

 In North America, two 25-MHz bands allocated to AMPS

 One for transmission from base to mobile unit  One for transmission from mobile unit to base

 Each band split in two to encourage competition  Frequency reuse exploited

Operation:

Subscriber initiates call by keying in phone number and presses send key MTSO verifies number and authorizes user MTSO issues message to user’s cell phone indicating send and receive traffic channels MTSO sends ringing signal to called party Party answers; MTSO establishes circuit and initiates billing information Either party hangs up; MTSO releases circuit, frees channels, completes billing

Differences Between First and Second Generation Systems

 Digital traffic channels – first-generation systems are

almost purely analog; second-generation systems are digital

 Encryption – all second generation systems provide

encryption to prevent eavesdropping

 Error detection and correction – second-generation digital

traffic allows for detection and correction, giving clear voice reception

 Channel access – second-generation systems allow

channels to be dynamically shared by a number of users

2nd Generation TDMA: GSM

formerly: Groupe Spéciale Mobile (founded 1982)

 now: Global System for Mobile Communication

Pan-European standard (ETSI, European Telecommunications Standardisation Institute) simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations  seamless roaming within Europe possible

 today many providers all over the world use GSM (more than

184 countries in Asia, Africa, Europe, Australia, America)

 around 800 million subscribers  GSM900, GSM1800, GSM1900

Performance characteristics of GSM (wrt. analog systems)

Total mobility

 international access, chip-card enables use of access points of different

providers

Worldwide connectivity

 one number, the network handles localization

High capacity

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

High transmission quality

 high audio quality and reliability for wireless, uninterrupted phone calls at

higher speeds (e.g., from cars, trains)

Security functions

 access control, authentication via chip-card and PIN

GSM: Mobile Services

GSM offers

 several types of connections

 voice connections, data connections, short message service

 multi-service options (combination of basic services)

Three service domains

 Bearer Services  Telematic Services  Supplementary Services

Bearer Services

 Services to transfer data between access points  Transparent Bearer Services use the physical layer to

transmit data

 Forward error correction is used

 Non-Transparent Bearer Services use layer 2 and 3

protocols for error correction and flow control Specification of services up to the terminal interface (OSI layers 1-3)

 Different data rates for voice and data (original standard)

 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

 Offered services

 mobile telephony

primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz

 Emergency number

common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption

• f other connections possible)

 Short Message Service (SMS)

alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS

 Group-3 fax:

Fax data is transmitted as digital data over the analog phone network

Supplementary services

 Services in addition to the basic services, cannot be

• ffered stand-alone

 May differ between different service providers,

countries and protocol versions

 Important services

 identification: forwarding of caller number  suppression of number forwarding  automatic call-back  conferencing with up to 7 participants  locking of the mobile terminal (incoming or outgoing

calls)

Architecture of the GSM system

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

 components

 MS (mobile station)  BS (base station)  MSC (mobile switching center)  LR (location register)

 subsystems

 RSS (radio subsystem): covers all radio aspects  NSS (network and switching subsystem): call forwarding,

handover, switching

 OSS (operation subsystem): management of the network

GSM: overview

fixed network BSC BSC MSC MSC GMSC OMC, EIR, AUC VLR HLR NSS with OSS RSS VLR

GSM: elements and interfaces

NSS

MS MS BTS BSC GMSC IWF OMC BTS BSC MSC MSC Abis Um EIR HLR VLR VLR A BSS PDN ISDN, PSTN

RSS

radio cell radio cell MS AUC

OSS

signaling O

System architecture: radio subsystem

Components

 MS (Mobile Station)  BSS (Base Station Subsystem):

consisting of

 BTS (Base Transceiver Station):

sender and receiver

 BSC (Base Station Controller):

controlling several transceivers

Interfaces

 Um : radio interface  Abis : standardized, open interface with

16 kbit/s user channels

 A: standardized, open interface with

64 kbit/s user channels

Um Abis A BSS radio subsystem network and switching subsystem MS MS BTS BSC MSC BTS BTS BSC BTS MSC

System architecture: network and switching subsystem

Components

 MSC (Mobile Services Switching Center):  IWF (Interworking Functions)  ISDN (Integrated Services Digital Network)  PSTN (Public Switched Telephone Network)  PSPDN (Packet Switched Public Data Net.)  CSPDN (Circuit Switched Public Data Net.)

Databases

 HLR (Home Location Register)  VLR (Visitor Location Register)  EIR (Equipment Identity Register)

network subsystem MSC MSC fixed partner networks IWF ISDN PSTN PSPDN CSPDN SS7 EIR HLR VLR ISDN PSTN

Radio subsystem

The Radio Subsystem (RSS) comprises the cellular mobile network up to the switching centers

 Components Base Station Subsystem (BSS):

 Base Transceiver Station (BTS): radio components including

sender, receiver, antenna - if directed antennas are used

• ne BTS can cover several cells

 Base Station Controller (BSC): switching between BTSs,

controlling BTSs, managing of network resources, mapping

• f radio channels (Um) onto terrestrial channels (A interface)

 BSS = BSC + sum(BTS) + interconnection

Mobile Stations (MS)

Base Transceiver Station and Base Station Controller

Tasks of a BSS are distributed over BSC and BTS

 BTS comprises radio specific functions  BSC is the switching center for radio channels

Functions BTS BSC Management of radio channels X Frequency hopping (FH) X X Management of terrestrial channels X Mapping of terrestrial onto radio channels X Channel coding and decoding X Rate adaptation X Encryption and decryption X X Paging X X Uplink signal measurements X Traffic measurement X Authentication X Location registry, location update X Handover management X

Mobile station

Terminal for the use of GSM services Comprises all user equipment and software for communication with a GSM Network Transmits with a power of 2W(GSM 900), 1W (GSM 1800)

 User independent, device-specific hardware and software

 IMEI (International Mobile Equipment Identity): Identification of MS,

theft protection, etc.

 SIM (Subscriber Identity Module):

 personalization of the mobile terminal, stores all user specific data  without the SIM, only emergency calls are possible  Card type, serial number, a list of subscribed services, personal identity

number (PIN), pin unblocking key (PUK), authentication key, international mobile subscriber identity (IMSI).

 Dynamic information: Cipher key, temporary mobile subscriber identity

(TMSI) and location area identification (LAI)

Network and switching subsystem

NSS is the main component of the public mobile network GSM

 switching, mobility management, interconnection to other

networks, system control

 Components

 Mobile Services Switching Center (MSC)

 controls all connections via a separated network to/from a mobile

terminal within the domain of the MSC

 several BSC can belong to a MSC  forms the fixed backbone network of the GSM system  A gateway MSC (GMSC) has additional connections to other

networks

 Handles all signaling for connection setup, connection release,

handover of connections to other MSCs.

 Databases (important: scalability, high capacity, low delay)

 Home Location Register (HLR)

 central master database containing user data, permanent and

semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)

 Static data: Mobile Subscriber ISDN Number (MSISDN), IMSI  Dynamic data: Location area, mobile subscriber roaming number,

current VLR and MSC.

 Visitor Location Register (VLR)

 local database for a subset of user data, including data about all

user currently in the domain of the VLR

 data such as IMSI, MSISDN and HLR is stored  When an MS enters an LA the VLR is responsible for, VLR copies all

relevant information from user’s HLR

`

Network and switching subsystem

Operation subsystem

The OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystems

 Authentication Center (AUC)

 generates user specific authentication parameters on request of a

VLR

 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)

 registers GSM mobile stations and user rights, all IMEI’s  stolen or malfunctioning mobile stations can be locked and

sometimes even localized

 List of valid IMEI’s (white list), list of malfunctioning IMEI’s (gray

list)

 Operation and Maintenance Center (OMC)

 different control capabilities for the radio subsystem and the

network subsystem via the O interface

 Traffic monitoring, subscriber and security management,

accounting and billing

Operation subsystem

Radio Interface

GSM implements SDMA using cells with BTS and assigns an MS to a BTS Media Access combines TDMA and FDMA

 Two 25 MHz bands for uplink and downlink (FDMA)  124 channels each way, 1 and 124 are not used, 32 for

• rganizational data, 90 are left for users

f t

124 1 124 1 20 MHz

200 kHz 890.2 MHz 935.2 MHz 915 MHz 960 MHz

Physical Channels

Each channel is divided into frames that are repeated continuously (TDMA)

 Each frame is 4.615 ms  Frames are divided into 8 timeslots  Each slot represents a physical channel and lasts for 577 µs.  Data is transmitted in small portions called bursts in a slot,

guard spaces are used before and after the bursts

 Each physical channel has a raw data rate of 33.8kbps, thus

each radio carrier transmits approximately 270kbps.

1 2 3 4 5 6 7 8 higher GSM frame structures

935-960 MHz 124 channels (200 kHz) downlink 890-915 MHz 124 channels (200 kHz) uplink

time

GSM TDMA frame GSM time-slot (normal burst) 4.615 ms 546.5 µs 577 µs

tail user data Training S guard space S user data tail guard space

3 bits 57 bits 26 bits 57 bits 1 1 3

GSM - TDMA/FDMA

Physical Channel (Timeslot) Structure

 Normal burst carries user data traffic, other burst formats are used

for control signaling

 Guard space: To avoid overlapping with other bursts due to different

path delays and to give transmitter time to turn on and off

 Tail or trail bits: Set to 0, used to enhance the receiver performance,

i.e. allow synchronization of transmissions from MSs located at different distances

 Training sequence: Used to adapt the parameters of the receiver to

the current propagation characteristics and to select the strongest signal in case of multipath propagation

 Flag S: indicates whether data field contains user or network control

data

GSM time-slot (normal burst) 546.5 µs

tail user data Training S guard space S user data tail guard space

3 bits 57 bits 26 bits 57 bits 1 1 3

Logical Channels

 The logical channel is the information which goes through the

physical channel

 Traffic Channels (TCH): used to transmit user data (e.g.: voice)  Full-rate TCH (TCH/F): 22.8kbps  Half-rate TCH (TCH/H): 11.4 kbps  Standard codecs for voice  Originally, linear prediction coding with pulse

excitation (260 bits to represent 20ms voice), 13kbps

 Full rate, 13kbps  Half rate, 5.6kbps  Enhanced full rate, 12.2 kbps

Logical Channels-control channels

 Control medium access, allocation of traffic channels, mobility

management

 Broadcast Control Channel:

 BTS uses this channel to signal MSs within a cell  Cell identifier, cell options (i.e., frequency hopping), available

frequencies, etc.

 Frequency correction channel, synchronization channel are

subchannels.

 Common Control Channel:

 Connection setup between BTS and MS  Paging channel: For calls toward an MS  Random Access Channel: If an MS wants to set up a call  Access Grant Channel: to signal an MS to use a TCH

Logical Channels-control channels II

 Dedicated Control Channel (Bidirectional):

 Standalone Dedicated Control Channel: If an MS has not established

a TCH it uses this channel for signaling, i.e., authentication, registration.

 Slow associated dedicated control channel (SACCH): Associated with

TCH, used to exchange system information, i.e., the channel quality

 Fast associated dedicated control channel (FACCH): Associated with

TCH, if more signaling information needs to be exchanged

 Control Channels cannot be used arbitrarily but follows a hierarchy  TCH/F is associated with SACCH for slow signaling  TCH/F is associated with FACCH for fast signaling

TTTTTTTTTTTTSTTTTTTTTTTTTx: 26 slots

24 out of 26 slots: TCH/F, each burst:114bit every 4.615ms = 24.7kbps 24/26: 22.8kbps (As specified by the TCH/F)

GSM hierarchy of frames

1 2 2045 2046 2047 ... hyperframe 1 2 48 49 50 ... 1 24 25 ... superframe 1 24 25 ... 1 2 48 49 50 ... 1 6 7 ... multiframe frame burst slot 577 µs 4.615 ms 120 ms 235.4 ms 6.12 s 3 h 28 min 53.76 s

Slow Frequency Hopping

 GSM specifies a slow frequency hopping mechanism to reduce the

vulnerability of the signals to transmission impairments, also to reduce co-channel interference between nearby cells

 A common hopping sequence is used  All of the signals in a cell have to be coordinated so that two of

them do not use the same frequency simultaneously

 All GSM terminals are capable of hopping but hopping is optional,

network operators decide

Two FH carriers

Localization

 GSM performs periodic location updates unless the MS is turned

• ff

 HLR always contains information about the current location (area)  VLR currently responsible for the MS informs HLR, as soon as the

MS moves to the range of a new VLR, HLR sends all the information

 Roaming: Changing VLRs with uninterrupted availability of all

services

 Within the network  Between different providers (national or international)

 National roaming is often not supported due to competing

• perators

 One device in 190 countries!

Localization II

 Numbers needed for localization  Mobile Station International ISDN Number (MSISDN):

Phone number: Country code (+90)+national destination code (address of the nw provider, 532)+subscriber number

 International Mobile Subscriber Identity (IMSI): International

unique identification

 Temporary Mobile Subscriber Identity (TMSI): Hides the IMSI,

is valid within the location area of VLR

 Mobile Station Roaming Number: Hides the IMSI, Visitor

country code, visitor national destination code

 Is the address to the serving MSC/VLR

Location Based Registration

 When a terminal enters in a new cell and detects a location

area identifier on the broadcast control channel

 If the location are a of the new cell is different than the

location of the previous cell, the terminal registers its location

 Request a stand-alone dedicated control channel (SDCCH) on

RACH

 The networks sends an immediate assignment message on

Access Grant Channel and move the terminal to SDCCH

 Terminal sends the IMSI/TMSI and network verifies the

identity (on SDCCH)

 The networks assigns a new TMSI (on SDCCH)  Terminal releases the channel (on SDCCH)

Mobile Terminated Call

PSTN calling station GMSC HLR VLR BSS BSS BSS MSC MS

1 2 3 4 5 6 7 8 9 10 11 12 13 16 10 10 11 11 11 14 15 17

1: calling a GSM subscriber 2: forwarding call to GMSC 3: signal call setup to HLR 4, 5: request MSRN from VLR 6: forward responsible MSC to GMSC 7: forward call to current MSC 8, 9: get current status of MS 10, 11: paging of MS 12, 13: MS answers 14, 15: security checks 16, 17: set up connection

Mobile Originated Call

PSTN GMSC VLR BSS MSC MS

1 2 6 5 3 4 9 10 7 8

1, 2: connection request 3, 4: security check 5-8: check resources (free circuit) 9-10: set up call

MTC/MOC

BTS MS paging request channel request immediate assignment paging response authentication request authentication response ciphering command ciphering complete setup call confirmed assignment command assignment complete alerting connect connect acknowledge data/speech exchange BTS MS channel request immediate assignment service request authentication request authentication response ciphering command ciphering complete setup call confirmed assignment command assignment complete alerting connect connect acknowledge data/speech exchange

MTC MOC

PCH RACH AGCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH SDCCH FACCH TCH OACSU: Off-air Call Setup: Assigning TCH after the subscriber accepts the call, OACSU is optional in GSM

Handover

 Cellular systems require handover as single cells do not cover the

whole area (up to 35 km)

 Reasons:  MS moves out of the range of a BTS

Signal level decreases and falls below a minimum threshold for communication, error rate may grow due to interference

 Load balancing

MSC, BSC may decide to shift some MSs to other cells if traffic in a cell is too high

4 types of handover

MSC MSC BSC BSC BSC BTS BTS BTS BTS MS MS MS MS 1 2 3 4

1: Intra-cell: BSC can change the frequency due to increasing narrowband interference 2: Inter-call, intra-BSC: MS moves to another cell but stays within the control of same BSC 3: Inter-BSC, intra-MSC: Between cells controlled by different BSCs, MSC coordinates 4:Inter-MSC: between cells controlled by different MSCs, both MSCs perform the handover

Handover decision

receive level BTSold receive level BTSnew MS MS HO_MARGIN BTSold BTSnew

Both MS and BTS perform periodic measurements of uplink and downlink quality BSC collects all values (error rate, signal level), calculates average values Values are compared to thresholds, HO_MARGIN to avoid ping-pong effect

Handover procedure

HO access

BTSold BSCnew measurement result BSCold Link establishment MSC MS measurement report HO decision HO required BTSnew HO request resource allocation

• ch. activation
• ch. activation ack

HO request ack HO command HO command HO command HO complete HO complete clear command clear command clear complete clear complete

Review Terms

AMPS GSM BTS BSC MS MSC GMSC HLR VLR EIR TCH Frame/Multiframe/Superframe Physical Channel Logical Channel IMEI IMSI TMSI MSRN MSISDN Handover Abis U_m PIN PUK