W SS C OMMUNICATIONS C OMMUNICATIONS Aditya K. Jagannatham Indian - - PowerPoint PPT Presentation

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MOBILE WIRELESS COMMUNICATIONS COMMUNICATIONS

Aditya K. Jagannatham Indian Institute of Technology Kanpur Indian Institute of Technology Kanpur Commonwealth of Learning Vancouver

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Wireless Communications Wireless Communications

• Channel is the air medium.
• Multiple users can

simultaneously transmit y

• ver the air medium
• For instance, different cell

phone users in a cell are phone users in a cell are trying to transmit to the Base Station.

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Base Station.

Wireless Communications Wireless Communications

The answer is M l i l A How to allocate h di Multiple Access (MA) technology! the medium to a certain user?

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Multiple Access Technologies Multiple Access Technologies

• FDMA

“Frequency Division for Multiple

• FDMA – Frequency Division for Multiple

Access” E h i ll t d diff t f

• Each user is allocated a different frequency

band.

– Forms the 1st Generation or 1G Mobile Technology

User 1 User 2 User 3 User 4 User 5

Frequency (Hz)

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q y ( )

An FDMA Phone An FDMA Phone

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Multiple Access Technologies Multiple Access Technologies

• TDMA – Time Division for Multiple Access.

p

• Each user is allocated a certain time “slot” for

information transmission information transmission.

User 1 User 2 User 3 User 4 User 5

Time (seconds) GSM TDMA!

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GSM uses TDMA!

Introduction to GSM Introduction to GSM

GSM (Gl b l S t f M bil

• GSM (Global System for Mobile

Communication)is an ETSI (European Telecommunication Standards Institute) standard

– For 2G pan‐European digital cellular with international roaming international roaming.

• Formed in 1982 by allocating the bands

890‐915 MHz and 935‐960 MHz for Pan‐ European PLMN (Public Land Mobile

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Network).

Introduction to GSM Introduction to GSM

• Main Charter

– To develop a unified 2G standard to resolve the p roaming problem in Europe, with six 1G standards.

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Introduction Introduction

• GSM went beyond the air‐interface and

defined a system that complied with ISDN y p (Integrated Services Digital Network) like services services.

– ISDN provides data services over traditional telephone network or PSTN (Public Switched telephone network or PSTN (Public Switched Telephone Network)

• Hence, GSM is a robust digital cellular

standard.

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Timeline – Brief History of GSM

1982 Frequency bands allocated for Pan‐European PLMN (Public Land Mobile Network). 1986 GSM Task Force formed 1987 Memorandum of understanding signed. ETSI officially included GSM in its domain. Name of the group was changed to Special 1989 Mobile Group (SMG). Hence, the resulting standard was named GSM (Groupe Spécial Mobile) Mobile). 1991 Specification completed. 1992 First deployment 1993 32 Operators in 22 countries.

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2001 Deployed in close to 150 countries.

GSM Services GSM Services

• Analog cellular systems were designed for the

sole purpose of voice traffic similar to PSTN. p p

• GSM is an integrated voice‐data service that

provides a number of services beyond voice provides a number of services beyond voice.

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GSM Reference Architecture GSM Reference Architecture

i l d d d i i i l

• Wireless standard description involves

– Detailed terminal specs. – Fixed hardware (H/W) backbone. – Software (S/W) databases for operational ( / ) p support.

• GSM is organized into three major segments.

GSM is organized into three major segments.

– Mobile station (MS). Base station subsystem (BSS) – Base station subsystem (BSS). – Network and switching subsystem (NSS).

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GSM Reference Architecture GSM Reference Architecture

Mobile Station (MS) Networking and Switching Subsystem (NSS) SIM Base Station subsystem (BSS) VLR HLR PSTN User BTS BSC UE MSC PSTN User EIR AUC Other MSCs MOOC on M4D 2013

Mobile Station (MS)

Mobile Station (MS)

Mobile Station (MS)

SIM

• Functionality

– Communicates information with user.

UE

– Demodulates radio signals, extracts digital voice Modifies user info for transmission over the air – Modifies user info for transmission over the air‐ interface to communicate with the BS.

• MS has two elements

– Mobile Equipment (ME) q p ( )

• Purchased from equipment vendor.
• Components include speaker/microphone and the

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p p / p radio modem (modulation‐demodulation).

Mobile Station (MS)

Mobile Station (MS)

Mobile Station (MS)

SIM

– Subscriber Identity Module (SIM)

• Smart card issued at the subscription time

UE

• Smart card issued at the subscription time

identifying the user specs such as operator, service type service type. –Identity of user in the mobile network

• Calls in GSM are directed to the SIM rather

than the terminal

• SMS (Short Message Service) messages are

also stored in the SIM.

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Mobile Station (MS)

Mobile Station (MS)

Mobile Station (MS)

SIM

• SIM has a significant impact on the way that a

UE

g p y user transacts with the service provider. –For instance, determines charging, roaming , g g, g etc.

• SIM carries the user personal information,

p , which enables a number of useful applications.

• SIM is identified with an IMSI (International

Mobile Subscriber Identity) for the internal

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network.

Base Station Subsystem (BSS) Base Station Subsystem (BSS)

Base Station subsystem (BSS) BTS BSC y ( )

Wired Infrastructure Wireless Signaling

• BSS communicates with the user through the

wireless air‐interface (through ME).

• Communicates with the wired infrastructure

through a different set of wired protocols. g p

• BSS provides for the translation from air‐

interface protocols to the wired medium interface protocols to the wired medium protocols.

• Separates packet data from PSTN traffic

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Separates packet data from PSTN traffic.

– To implement packet data services such as GPRS.

Base Station Subsystem (BSS)

S SC Base Station subsystem (BSS)

• BSS has two architectural elements

BTS BSC

• BSS has two architectural elements

– Base Transceiver Station (BTS)

• Counterpart of MS for physical communication.
• Includes Tx, Rx and signaling equipment for Demod
• One BSS may have several BTSs in its domain.

– Base Station Controller (BSC)

• Small switch inside the BSS that is in charge of

frequency administration.

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• Also in charge of handover among the BTSs inside a

BSS.

Network and Switching Subsystem

• NSS is master system

responsible for network

• peration
• peration.
• It is responsible for

C i ti ith th i d

Networking and Switching Subsystem (NSS)

– Communication with other wired and wireless networks. – Also support for registration and

VLR HLR PSTN

Also support for registration and maintenance of the connection with the MSs.

(

MSC PSTN

• Connects to the PSTN (Public

Switched Telephone Network) through ISDN protocols

EIR AUC

through ISDN protocols.

• It has one H/W element i.e.

MSC and four S/W elements –

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MSC and four S/W elements – VLR, HLR, EIR and AUC.

NSS Architectural Elements

• Mobile Station Controller (MSC)

– The H/W part of the NSS. – Communicates with other MSCs in the coverage area of the service provider

Networking and Switching Subsystem (NSS)

provider. – Also communicates with the PSTN switches.

VLR HLR PSTN

• This is the Gateway MSC (GMSC)
• Home Location Register (HLR)

MSC PSTN

– Database S/W that handles management of the mobile subscriber account

EIR AUC

subscriber account. – Stores the subscriber’s address, service type, current location,

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forwarding address etc.

NSS Architectural Elements

• Visitor Location Register

(VLR)

– Temporary database S/W in

Networking and Switching Subsystem (NSS)

Temporary database S/W in Visiting Cell, similar to the HLR.

VLR HLR PSTN

HLR. – Identifies the subscribers visiting inside the coverage

MSC PSTN

visiting inside the coverage area of the MSC. Th ll f H MSC

EIR AUC

– Thus, calls from Home MSC can be forwarded to visiting MSC

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MSC.

NSS Architectural Elements NSS Architectural Elements

• Authentication Center (AUC)

– Holds different algorithms

Networking and Switching Subsystem (NSS)

g that are used for authentication and

VLR HLR PSTN

encryption of subscribers. – Different SIM cards have

MSC PSTN

Different SIM cards have different algorithms and the AUC collects all of these

EIR AUC

AUC collects all of these algorithms.

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NSS Architectural Elements NSS Architectural Elements

• Equipment Identification

q p Register (EIR)

• Keeps the IMEI (International

Networking and Switching Subsystem (NSS)

• Keeps the IMEI (International

Mobile Equipment Identity) h l h f

VLR HLR PSTN

that reveals the manufacturer, country of production,

MSC PSTN

terminal type.

– Used to report stolen phones

EIR AUC

Used to report stolen phones and to check if the phone is

• perating according to the

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• perating according to the

service type.

GSM BASIC GSM BASIC OPERATION

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What happens in a GSM phone? What happens in a GSM phone?

• GSM (Global System for Mobile) uses TDMA,

( y ) ie Time Division for Multiple Access technology.

• Each user is allocated a time “slot” on a

frame of data bits.

• The raw data rate of GSM is 270 Kbps.
• Each user transmits for 577 micro seconds
• Each user transmits for 577 micro seconds

– This corresponds roughly to 156 bits of information information.

• 8 users use the same frequency band

Which implies that a frame size is 8 x 577 micro

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– Which implies that a frame size is 8 x 577 micro ‐ secs or 4.615 ms.

What happens in a GSM phone? What happens in a GSM phone?

• Each segment of user data of 156 bits is known as

a “Burst”.

• Each user burst has the following structure.

Encrypted Training Encrypted TB GP TB yp Bits (58) g Sequence (26) yp Bits (58) (3) (8.25) (3)

• The “Bursts” of 8 users together form a frame.

The Bursts of 8 users together form a frame.

• These frames are hierarchically organized in a

frame structure.

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GSM Frame Hierarchy GSM Frame Hierarchy

20 8 S f f (3 2048 Superframe: Hyperframe (3 Hr 28 min 53.76 s) 51 traffic or 26 control frames : Superframe (6.12s) 26 Traffic frames: Multiframe (120ms) 51 Control frames: Multiframe (235.4ms) 8 Slots: Frame (4.615ms) 156.25 bits: Burst (0.577ms) MOOC on M4D 2013

Handoff (Handover) in GSM Handoff (Handover) in GSM

• Transfer from one BTS/BSS to another
• Transfer from one BTS/BSS to another
• Two types of handover

– Internal

• Between two BTSs of the same BSS.

– External

• Between two BSSs controlled by same MSC.
• Sometimes between BSSs controlled by

different MSC, where old MSC handles call management.

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Handoff (Handover) in GSM Handoff (Handover) in GSM

• Handover is initiated for different
• Handover is initiated for different

reasons.

– Most common is signal strength deterioration. – Traffic balancing, to ease traffic congestion by moving calls to a lightly loaded cell. y g g y

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Handover Procedure in GSM Handover Procedure in GSM

• Outline of the Handover procedure

p

– BTS provides the MS with a list of available channels in the neighboring cells via BCCH channels in the neighboring cells via BCCH (Broadcast Channel). MS monitors the RSS (Received Signal Strength) – MS monitors the RSS (Received Signal Strength) from the BCCHs of the neighboring cells and reports values to BSC reports values to BSC. – This is termed as mobile‐assisted handover.

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Handover Procedure in GSM Handover Procedure in GSM

• BTS also monitors RSS from the MS to make a

handover decision.

– BSC negotiates a new channel with the new BSS and indicates to the MS. BSS and indicates to the MS. – Upon completion, MS indicate this with a handoff complete message to the BSC handoff complete message to the BSC.

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