ECE C392 Mobile Communication Concepts Apr 10, 13, 15, 2015

ECE F418 / EEE F418 Modern Communication Technologies

Topic # 10: Mobile Communication Network Concepts:

R2. Wireless Communications Priniciples and Practice. Theodore Rappaport, Pearson Education: Chapters 1,2 & 3

Apr 10, 13, 15 2015

1

History of Wireless Communications2

1897: Guglielmo Marconi firstly demonstrated radio's ability to provide continuous contact with ships sailing in the English channel.

Beginning of 20th century: Short wave radio communications were used in marine secure operation systems.

1921: The first car mounted radio telephone

1934 – Municipal police radio systems in the US (AM)

Going Further..

Motorola employee Martin Cooper

3

1946 – First commercial mobile radio-telephone service by Bell and AT&T in Saint Louis, USA. Half duplex (FM)

1968 – AT&T proposed the cellular concept to the FCC

1973 – First handheld cellular phone – Motorola.

1978 – First cellular net Bahrein

1980’s – 1G mobile communication systems(AMPS). Two 25 MHz bands are allocated. • Supports only speech• Low traffic capacity, unreliable handover, long call setup time and frequent call

drops• Inefficient use of bandwidth and poor battery life, poor voice qulity and bulky• Allows users to make voice calls in 1 country only.

Started Picking Up …4

1990’s: 2G (2nd generation mobile communication systems)

• 2G supports data, speech, FAX, SMS, and WAP services. • Frequency bands used by GSM are 890-960 MHz and 1710-1880 MHz• 890-915 MHz is dedicated to uplink communications and 935 – 960 MHz is

used for downlink communications.• 2G supports two standards- TDMA & CDMA• GSM uses TDMA/ Frequency division duplexing• IS-95 is commonly referred to as CDMA

Late 1990’s: 2.5G, based on the existing 2G systems, increase the data rate :

GPRS (up to 114kbps; for GSM networks), EDGE (up to 384kbps; for GSM and DCS-1800/1900 networks),

IS-95B (IS-95’s data-transmission extension)

In The New Millinium …5

2000’s: 3G (3rd generation mobile communication systems)

Initially intended to support multimedia communications; currently targeted to support Internet access, mobile TV, games etc

Employs all sorts of capacity enhancement techniques:

turbo codes (invented in 1993),

multiuser detection (invented in 1986),

transmit diversity (invented in ~1993),

fast power control (invented in ~1993),

smart antennas (long-known technique).

Data-transmission rate: From 144kbps (vehicular environments) to 2Mbps (indoor)

In to Future …4G

To Replace existing 3G systems with objectives:

– Data rates: 100 Mbit/s (outdoors) to 1 Gbit/s (indoors)

– High quality of service & security for next generation multimedia support.

– Spectrally Efficient: Challenge Shannon’s law of data communications ??

– Resources to support more simultaneous users per cell

– An all-IP packet switched network for integrated voice, data, and video

– Mobile IP to support VoIP, (ability to move around with the same IP address)

– Presence Awareness - Knowing who is on line and where

– Dynamically utilize and have smooth handovers across heterogeneous networks:

6

Technology Vs Mobile User Needs

Mobile browsing InternetCorporate Intranet

Personalized mobile servicesMultimedia messaging Entertainment / infotainment Location based services

Advanced mobile services Video messaging & streaming Interactive multimediaMultimedia conferencing

GSM GPRS EDGE

HE

AR

& S

EE

V

IEW

IN

TE

RA

CT

W-CDMA

Video telephony Video conferencingMultiparty gaming

WIMAX

/ WLAN

Text messaging (SMS) Internet browsing (WAP)

~64Kbps ~200 Kbps ~ 500 Kbps ~ 3 Mbps ~30 - 108 Mbps

7

Frequency Reuse: The need and the concept8

Fixed telephone network runs wires to every household

Suppose we give every household their own allocation of radio spectrum for analog speech of 4khz BW.

10 Lakh household (say Hyderabad) * 4Khz= 4 GHz ! Clearly impractical

Frequency Reuse is a method used by service providers to improve the efficiency of a cellular network and to serve millions of subscribers using a limited radio spectrum

Is based on the fact that after a distance a radio wave gets attenuated and the signal falls bellow a point where it can no longer be used or cause any interference

Thus, a transmitter transmitting in a specific frequency will have only a limited coverage area. Beyond this coverage area, that frequency can be reused by another transmitter

Network Cells

9

The entire network coverage area is divided into cells based on the principle of frequency reuse

CELL :

Basic geographical unit of a cellular network;

is the area around an antenna where a specific frequency is used;

is represented graphically as a hexagonal shape, but in reality it is irregular in shape

Handover: When a subscriber moves to another cell, the antenna of the new cell takes over the communication process

CLUSTER: is a group of adjacent cells, usually 7 cells; no frequency reuse is done within a cluster

The frequency spectrum is divided into sub bands and each sub band is used within one cell of the cluster

Types of cells10

Macrocell –

Coverage is large (aprox. 6 miles in diameter);

Used in remote areas,

High-power transmitters and receivers are used

Microcell –

Coverage is small (half a mile in diameter)

Mostly used in urban zones;

Low-powered transmitters and receivers are used to avoid interference with cells in another clusters

Picocell – covers areas such as building or a tunnel

Cellular services

• Voice communication

• Short Messaging Service (SMS)

• Multimedia Messaging Service (MMS)

• Global Positioning System (GPS)

• Wireless Application Protocol (WAP) – to access the Internet

11

12

BTS (Base Transceiver Station)

• Main component of a cell • Connects the subscribers to the cellular

network; • for transmission/reception of information

it uses several antennas spread across the cell

BSC (Basic Station Controller)

• it is an interface between BTSs • it is linked to BTSs by cable or

microwave links; • it routes calls between BTSs; • it is also connected to the MSC

MSC (Mobile Switching Center)

• Coordinator of a cellular network, • It is connected to several BSCs, • Routes calls between BSCs; • Links the cellular network with

other networks like PSTN through fiber optics, microwave or copper cable

Cellular Network

Mobile Identification numbers13

SIM – a memory card holding identity information, phone book etc.GSM Uses SIM.

CDMA does not support SIM cards, but uses “Re-Usable Identification Module” (RUIM)

IMEI – A unique 15 digit number identifying each phone, is incorporated in the cellular phone by the manufacturer

When a phone tries to access a network, the service provider verifies its IMEI with a database of stolen phone numbers; if it is found in the database, the service provider denies the connection

the IMEI is located on a white sticker/label under the battery, but it can also be displayed by typing *#06# on the phone

IMSI – A 15-digit unique number provided by the service provider and incorporated in the SIM card which identifies the subscriber

IMSI enables a service provider to link a phone number with a subscriber

TMSI – is a temporary number, shorter than the IMSI, assigned by the service provider to the phone on a temporary basis

TMSI key identifies the phone and its owner in the cell it is located; when the phone moves to a different cell it gets a new TMSI key

TMSI key are used for securing GSM networks

Components of a Cellular Phone (MSU – Mobile Subscriber Unit)

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Radio transceiver – low power radio transmitter and receiver

Antenna, usually located inside the phone

Control circuitry – formats the data sent to and from the BTS; controls signal transmission and reception

Man-machine interface – consists from a keypad and a display; is managed by the control circuitry

Subscriber Identity Module (SIM) – integrated circuit card that stores the identity information of subscriber

Battery, usually Li-ion, the power unit of the phone

How a call is Handled ? Registering the Mobile to a network

• When powered on, the phone does not have a frequency / time slot /code assigned to it yet;

• So it scans for the control channel of the BTS and picks the strongest signal

• Then it sends a message (including its identification number) to the BTS to indicate its presence

• The BTS sends an acknowledgement message back to the cell phone

• The phone then registers with the BTS and informs the BTS of its exact location

• After the phone is registered to the BTS, the BTS assigns a channel to the phone and the phone is ready to receive or make calls

15

How a call is Handled ? – Making a Call

• The subscriber dials the receiver’s number and sends it to the BTS

• The BTS sends to its BSC the ID, location and number of the caller and also the number of the receiver

• The BSC forwards this information to its MSC

• The MSC routes the call to the receiver’s MSC which is then sent to the receiver’s BSC and then to its BTS

• The communication with the receiver’s cell phone is established

16

How a call is Handled ? – Receiving a call

• When the receiver’ phone is in an idle state it listens for the control channel of its BTS

• If there is an incoming call, the BSC and BTS send a message to the cells in the area where the receiver’s phone is located

• The phone monitors its message and compares the number from the message with its own

• If the number matches, the cell phone sends an acknowledgement to the BTS

• After authentication, the communication is established between the caller and the receiver

17

Number of Channels & System Capacity in Cellular Systems

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Channels & System Capacity in Cellular System

For a Cluster size of N , Frequency Reuse Factor = 1/N

Due to hexagonal geometry, N can only take values that satisfies

19

If the Number of channels allocated to each cell is k, If the number of Cells (that use k channels, called a Cluster) is N, the Total number of Duplex channels S = kN

.

N = i2 + ij + j2

(with non negative integers i & j)

If Cluster is replicated M times within the system, then the capacity of the system is C = MkN = MS

The cells A,B,C,D,E,F form a cluster of size 7

All the cells “ A” will use the same frequency.

19-cell reuse example (N=19)

N = i2 + ij + j2

1

12

3

4

56

79

8

1011

1213

14

15

1617

18

19

1

20

N = 32 + 3X2 + 22 = 19

Procedure for locating the nearest co channel neighbour

1. Move along any chain of Hexagonal chain by “ i“ cells

2. Take anti-clock wise turn by 60 degrees.

3. Move along the chain of Hexagonal chain by “ j“ cells

i = 3, j = 2;

Example21

If a total of 33 MHz of BW is allocated to a particular FDD cellular telephone system, which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse.

2. Total available BW = 33 MHz.

4. For 4 – cell reuse, number of channels per cell = 660/4 = 165

1. Duplex Channel BW = 2 X 25 = 50 kHz

3. Total available channels = 33 MHz/ 50 Khz = 660



Channel Assignment Strategy

• Fixed assignment – Each cell is allocated pre determined number of channels (Ex: 165

channels)– Any new call, within the cell, can be served only if unused channel is

available.(Ex: out of 165, if any of them is free)– Otherwise call is blocked– Borrowing strategy (borrowing from un used frequencies of the adjacent

cells) is possible with supervision from MSC.

• Dynamic assignment– Voice channels are allocated dynamically(even from the set allocated to

neighbouring cells)– MSC coordinates the allocation– MSC needs to collect real time data on channel occupancy, traffic

distribution

22

Handoffs – the basics23

Hand off threshold level > minimum acceptable signal level to maintain the call.

Signal level measurements should take instantaneous fading into consideration

Handoffs – the basics24

Handoff could be Base Station Assisted Hand offMobile Assisted Hand off

Intersystem hand off – Roaming Scenario

Time needed for hand off is also a function of the speed of the vehicle

Dwell time, (time over which the call is maintained without hand off) statistics are key to design of handoff algorithms

Interference• Co Channel & Adjacent Channel Interference• Co Channel:

– To reduce, have sufficient distance between the cells that use same frequency

– R : Radius of the cell– D : Distance between the centres of the nearest co channel cells– Co Channel Reuse Ratio : Q = D/R = (3N) ½

– Larger Q => smaller levels of co channel interference

25

Signal to Interference Ratio26

If the number of co channel interfering cells is i0 then the Signal to Interference ratio at a mobile receiver can be expressed as

S is the signal power from desired base station and Ii is the interference power from i-th interfering co channel

If power received at a distance d from the base station antenna is expressed as

P0 is the power received at a close in point in the far field at a reference distance d0 from the base station antenna and n is path loss exponent ( ranges between 2 & 4 for urban case)

If R is the radius of the cell and all transmitters transmit same power, and Di is the distance of the i-th transmitter from mobile,

Considering only the interference from the first layer of cells and the mobile is equidistant from all interfering towers,

Example27

a) Start with 7 cell reuse pattern. N = 7 (i = 2, j = 1). For path loss exponent n = 4,

= 73.5 = 18.66 dB

Since minimum required S/I should be > 15 dB, N=7 can be used for n = 4 case.

b) With 7 cell reuse pattern. N = 7. For path loss exponent n = 3,

= 16.0 = 12.05 dB

Since minimum required S/I should be > 15 dB, N=7 can not be used for n = 3 case.

N = i2 + ij + j2

Let us try (i = 2, j = 2, N = 12 ). n = 3,

= 36.0 = 15.56 dB

Signal to Interference RatioIn the Exact Geometry layout case: Co-channel cells for 7-cell reuse

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Key Definitions for Trunked Radio29

a) Setup Time: Time required to allocate a trunked radio channel to a requesting User

b) Blocked Call: Call which can not be completed at the time of request due to congestion. (Lost Call)

c) Holding Time: Average duration of a typical call. Denoted by H (in seconds)

d) Traffic intensity: Measure of channel time utilization, Average channel occupancy measured in Erlangs. Dimension less quantity, Denoted by A

f) Load: Traffic Intensity across the entire trunked radio system, measured in Erlangs

g) Grade of Service (GoS): Measure of congestion: Probabilty of Blocking (Erlang B) or probability of Delay (Erlang C)

e) Request rate: Average number of call requests per unit time. ( l /s)

User generated traffic Intensity

“U” Users offered traffic Intensity

Traffic intensity per channel

Average delay In the System

Erlang B: Blocked calls are dropped.

Erlang C: Blocked calls are qued and delayed:

Erlang B System30

Erlang B Chart31C= 5, GOS = 0.005, A = 1.13 (from Erlang B Chart)

Erlang C Chart32

Example

C= 5, GOS = 0.005, A = 1.13 (from Erlang B Chart)

C= 20, GOS = 0.005, A = 11.10 (from Erlang B Chart)

33

Example

C= 19, GOS = 0.02, A = 12 (from Erlang B Chart)

34

Example


35

Example


36

Example37

Example

Area of Hexagon = 2.5981 R2

38

Example


39

Example

Number of unique channels = 95 * 7 = 665

40

Global System for Mobile Communication (GSM)

41

GSM architecture

42

GSM characteristics

• previous standard in cellular communication were restrictive

• GSM – global digital standard for cellular phones that offered roaming facility

• first named Groupe Special Mobile and used in Europe; then usage extended to other continents

• GSM operate in frequency bands: 900MHz, 1800 MHz, 1900 MHz

• GSM provides voice and data services

43

Base Station Subsystem (BSS)

44

HLR, VLR and EIR registers

• Home Location Register (HLR) - is a database maintained by the service provider containing permanent data about each subscriber (i.e. location, activity status, account status, call forwarding preference, caller identification preference)

• Visitor Location Register (VLR) – database that stores temporary data about a subscriber; it is kept in the MSC of the of the area the subscriber is located in; when the subscriber moves to a new area the new MSC requests this VLR from the HLR of the old MSC

• Equipment Identity Register (EIR) – database located near the MSC and containing information identifying cell phones

45

Authentication Center (AuC)

• 1st level security mechanism for a GSM cellular network• is a database that stores the list of authorized

subscribers of a GSM network• it is linked to the MSC and checks the identity of each

user trying to connect• also provides encryption parameters to secure a call

made in the network

46

GSM Mobile Switching Center (MSC)

• is a switching center of the GSM network; coordinates BSCs linked to it

47

GSM Channels

48

GSM Access Scheme and Channel Structure

• GSM uses FDMA and TDMA to transmit voice and data• the uplink channel between the cell phone and the BTS

uses FDMA and a specific frequency band• the downlink channel between the BTS and the cell

phone uses a different frequency band and the TDMA technique

• there is sufficient frequency separation between the uplink freq. band and the downlink freq. band to avoid interference

• each uplink and downlink frequency bands is further split up as Control Channel (used to set up and manage calls) and Traffic Channel (used to carry voice)

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GSM uplink/downlink frequency bands used

GSM Frequency band

Uplink/BTS Transmit

Downlink/BTS Receive

900 MHz 935-960 MHz 890-915 MHz

1800 MHz 1805-1880 MHz 1710-1785 MHz

1900 MHz 1930-1990 MHz 1850-1910 MHz

50

GSM uplink/downlink frequency bands

• uplink and downlink take place in different time slots using TDMA

• uplink and downlink channels have a bandwidth of 25 MHz

• these channels are further split up in a 124 carrier frequencies (1 control channels and the rest as traffic channels); each carrier frequency is spaced 200 KHz apart to avoid interference

• these carrier frequencies are further devided by time using TDMA and each time slot lasts for 0.577 ms.

51

GSM Control Channel

• is used to communicate management data (setting up calls, location) between BTS and the cell phone within a GSM cell

• only data is exchanged through the control channel (no voice)

• a specific frequency from the frequency band allocated to a cell and a specific time slot are allocated for the control channel (beacon frequency); a single control channel for a cell

• GSM control channels can have the following types:– broadcast channel– common control channel– dedicated control channel

52

Broadcast Channel

• type of control channel used for the initial synchronization between the cell phone and the BTS

• is composed from:– Frequency Correction Channel (FCCH) – is composed from a

sequence of 148 zeros transmitted by the BTS– Synchronization Channel (SCH) – follows the FCCH and contains

BTS identification and location information– Broadcast Control Channel (BCCH) – contains the frequency

allocation information used by cell phones to adjust their frequency to that of the network; is continuously broadcasted by the BTS

53

Common Control Channels

• type of control chan. used for call initiation• is composed of:

– Paging Channel (PCH) – the BTS uses this channel to inform the cell phone about an incoming call; the cell phone periodically monitors this channel

– Random Access Channel (RACH) – is an uplink channel used by the cell phone to initiate a call; the cell phone uses this channel only when required; if 2 phones try to access the RACH at the same time, they cause interference and will wait a random time before they try again; once a cell phone correctly accesses the RACH, BTS send an acknowledgement

– Access Grant Channel (AGCH) – channel used to set up a call; once the cell phone has used PCH or RACH to receive or initiate a call, it uses AGCH to communicate to the BTS

54

Dedicated Control Channels

• control channel sed to manage calls• is comprised from:

– Standalone Dedicated Control Channel (SDCCH) – used along with SACCH to send and receive messages; relays signalling information

– Slow Associated Control Channel (SACCH) – on the downlink BTS broadcasts messages of the beacon frequency of neighboring cells to the cell phones; on the uplink BTS receives acknowledgement messages from the cell phone

– Fast Associated Control Channel (FACCH) – used to transmit unscheduled urgent messages; FACCH is faster than SACCH as it can carry 50 messages per second, while SACCH an caryy only 4.

55

Traffic Channel

• is used to carry voice data• based on the TDMA the traffic (voice channel) is divided

in 8 different time slots numbered from 0 to 7• the BTS sends signals to a particular cell phone in a

specific time slot (from those 8 time slots) and the cell phone replies in a different time slot

56

GSM Call Processing

57

Initializing a call1. when the cell phone is turned on it scans all the available frequencies for the

control channel2. all the BTS in the area transmit the FCCH, SCH and BCCH that contain the

BTS identification and location 3. out of available beacon frequencies from the neighboring BTSs, the cell

phone chooses the strongest signal4. based on the FCCH of the strongest signal, the cell phone tunes itself to the

frequency of the network5. the phone send a registration request to the BTS6. the BTS sends this registration request to the MSC via the BSC7. the MSC queries the AUC and EIR databases and based on the reply it

authenticates the cell phone8. the MSC also queries the HLR and VLR databases to check whether the

cell is in its home area or outside9. if the cell phone is in its home area the MSC gets all the necessary

information from the HLR if it is not in its home area, the VLR gets the information from the corresponding HLR via MSCs

10. then the cell phone is ready to receive or make calls.

58

Initializing a call (2)

59

Making a call

1. when thee phone needs to make a call it sends an access request (containing phone identification, number) using RACH to the BTS; if another cell phone tries to send an access request at the same time the messages might get corrupted, in this case both cell phones wait a random time interval before trying to send again

2. then the BTS authenticates the cell phone and sends an acknowledgement to the cell phone

3. the BTS assigns a specific voice channel and time slot to the cell phone and transmits the cell phone request to the MSC via BSC

4. the MSC queries HLR and VLR and based on the information obtained it routes the call to the receiver’s BSC and BTS

5. the cell phone uses the voice channel and time slot assigned to it by the BTS to communicate with the receiver

60

Making a call (2)

61

Receiving a call

1. when a request to deliver a call is made in the network, the MSC or the receiver’s home area queries the HLR; if the cell phone is located in its home area the call is transferred to the receiver; if the cell phone is located outside its home area, the HLR maintains a record of the VLR attached to the cell phone

2. based on this record, the MSC notes the location of the VLR and indicated the corresponding BSC about the incoming call

3. the BSC routes the call to the particular BTS which uses the paging channel to alert the phone

4. the receiver cell phone monitors the paging channel periodically and once it receives the call alert from the BTS it responds to the BTS

5. the BTS communicates a channel and a time slot for the cell phone to communicate

6. now the call is established

62

Receiving a call (2)

63

GSM Security

• Personal Identification Number (PIN)• User Authentication• TMSI-based Security

64

Personal Identification Number (PIN)

• the PIN is stored on the SIM card of the cell phone• when the cell phone is turned on, the SIM checks the

PIN; in case of 3 consecutive faulty PIN inputs a PUK (Personal Unblocking Key) is asked for

• in case of 10 faulty PUK inputs, the SIM is locked and the subscriber must ask a new SIM

• this security measure is within the cell phone and the service provider is not involved

65

User Authentication

• a mechanism for encrypting messages in a GSM network

• the network sends random data to the cell phone (RAND)

• each cell phone is allocated a secret key (KI)• using RAND and KI and the A3 encryption algorithm the

cell phone generates a signed result (SRES) which is then sent to the network

• a similar process takes place in the network which generates a signed result specific to the cell phone

• the network compares its SRES with the SRES generated by the phone and in case of a match the cell phone is connected to the network

66

TMSI-Key Based Security

• is most used in a GSM cellular network• a TMSI key provides a temporary identification to a cell

phone and is provided by the network upon authentication

• a TMSI key keeps changing according to the location of the cell phone this way preventing unauthorized access to a channel and preventing intruder from tracing location

• the mapping between IMSI and TMSI keys is handled by the VLR

• ISMI are used only when the SIM is used for the first time

67

Multiple access schemes

Frequency Division Multiple Access

- when the subscriber enters another cell a unique frequency is assigned to him; used in analog systems

Time Division Multiple Access

- each subscriber is assigned a time slot to send/receive a data burst; is used in digital systems

Code Division Multiple Access

- each subscriber is assigned a code which is used to multiply the signal sent or received by the subscriber

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Other cellular concepts

• handover = moving a call from one zone (from the transmitter-receiver from one zone) to another zone due to subscriber’s mobility

• roaming = allowing the subscriber to send/receive calls outside the service provider’s coverage area

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The control channel

• this channel is used by a cellular phone to indicate its presence before a frequency/time slot/code is allocated to him

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ECE C392 Mobile Communication Concepts Apr 10, 13, 15, 2015

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Transcript of ECE C392 Mobile Communication Concepts Apr 10, 13, 15, 2015