1

MODULE D2 – CELLULAR NETWORKS

mobnet.epfl.ch"

Some of the slides are adapted from Stallings, Wireless Communications & Networks, Second Edition, Chapter 10

© 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4

Some of the content is inspired by Rappaport, Wireless Communications, Second Edition, Chapters 3 and 9, 2002

2

Public Switched Telephone Network - PSTN (reminder)"

Local switch

Local switch

Transit switch

Outgoing call

Incoming call

Transit switch

Transit switch

Long distance network

- Transfer mode: circuit switching!- All the network (except part of the access network) is digital!- Each voice channel is usually 64kb/s!

3

PSTN Trunk Dimensioning (reminder)"

switch N channels

A

Assumptions!•  Loss system: if the N channels are busy, any additional call is dropped !•  Independent sources!

( )1 PrBlocking AN

ρ−

=

[ ] [Erlangs]A E Xλ= ⋅

where X = call duration [sec/call] Y = call arrival [calls/sec] ~ Poisson(λ)

0

Pr Pr("call dropped because line busy") Erlang-B( , )!

!

N

Blocking iN

i

AA NANi=

= = =⎛ ⎞⎜ ⎟⎝ ⎠

∑

Each channel N carries a traffic

switch offered load

4

Basic Call (reminder)"Calling terminal Network Called terminal

Off-hook

Dial tone

Dialing

Ring indication Alert signal

Off hook Remove ring indication

Bi-directional channel

On hook

Billing

On hook signal

Resource allocation

Translation + routing

Conversation

5

Architecture of Cellular Networks"

External Network

Cellular Network

Mobile Station Base

Station Mobile

Switching Center

Server (e.g., Home Location

Register)

6

7

Registration"

Tune on the strongest signal

Nr: 079/4154678

8

Service Request"

079/4154678 079/8132627 079/4154678

079/8132627

9

Paging broadcast"

079/8132627? 079/8132627?

079/8132627?

079/8132627?

Note: paging makes sense only over a small area

10

Response"

079/8132627

079/8132627

11

Channel Assignment"

Channel 47

Channel 47 Channel

68

Channel 68

12

Conversation"

13

Handover (or Handoff)"

14

Message Sequence Chart"

Caller Base

Station Switch Base

Station Callee

Periodic registration Periodic registration

Service request Service request

Ring indication Ring indication

Page request Page request Paging broadcast Paging broadcast

Paging response Paging response

Assign Ch. 47 Tune to Ch.47

Assign Ch. 68 Tune to Ch. 68

Alert tone

User response User response Stop ring indication Stop ring indication

15

Peculiarities of Cellular Networks"γ  Mobility!

i  User location => periodic registration and/or paging!i  Moving from a cell to another => handoff (US) or handover

(UK) procedures!i  Moving from one network to another => roaming!

γ  Ether!i  Multiple users per cell => access technology (e.g., SDMA,

FDMA, TDMA, CDMA)!i  Channel impairments => coding, error detection,

retransmission, forward error correction!i  Bandwidth => channel reuse, signal compression, efficient

modulation and coding!i  Privacy and security => encryption!

γ  Energy!i  Limited autonomy => power control, discontinuous

transmission!

16

Offered Services"γ  Telephony services (i.e., voice mail, call transfer,…)!γ  Short Message Services (SMS)!γ  Packet switched data (e.g., GPRS, EDGE, HSDPA, LTE),

notably for Web access!γ  Location-based services!γ  Application store (AppStore of Apple, Application Market of

Android,...)!γ  Entertainment (music, video,…); Mobile TV!γ  Mobile extension of online social networks (Facebook Mobile,…)!γ  Friend location (Foursquare, Google Latitude,

LocaliserMesAmis,…)!γ  Peer-to-peer wireless services (e.g., over Bluetooth and WiFi in

ad hoc mode); !

17

Relevant Service Features User Perspective"

γ  Terminal characteristics !i  Weight, size, robustness!i  Price !i  Battery life!i  User interface!

γ  Network characteristics!i  Coverage area (of home network + roaming agreements)!i  Call blocking/dropping!i  Transmission quality (error rate, signal to distortion ratio, delay)!

γ  Service characteristics!i  Price!i  Range of services!i  Confidentiality, Authentication and Privacy!

Relevant Service Features Operator Perspective"

γ  Efficiency!i  Spectrum efficiency!i  Frequency reuse!i  Cell radius!

γ  Cost!i  Infrastructure cost!i  Deployment time and adaptability!i  Roaming agreements!

γ  Security!i  Resistance to fraud!i  Non-repudiability!

18

MHzcellsonsconversatiE

×=

For telephony:

19

Air Interface"

Messages

Logical channels

Radio link

Messages

Logical channels

Radio link

Packets

Messages

Bits

Users’ data

Packet structure, error detection/retransmission Topology: one to one

one to many (e.g., synch signals) many to one (e.g., service request)

Multiple access (e.g., CDMA, TDMA, FDMA) Duplex (e.g., Frequency Division Duplex - FDD) Modulation, source coding, channel coding, interleaving, diversity, channel equalization

Terminal Base Station

20

Cellular Networks"g  Covered area tesselated in cells!

•  One antenna per cell!•  Cells are controlled by Mobile

Switching Centers!

g  A mobile communicates with one (or sometimes two) antennas!

g  Cells are modeled as hexagons!g  Cells interfere with each other!

g  To increase the capacity of the network, increase the number of cells!

Generations of Cellular Networks…"

γ  1G: analog systems à not in use anymore!g  2G: GSM (introduced in 1992): FDMA/TDMA (900 and

1800MHz)!i  2.5G: with GPRS: packet switching, extended to E-GPRS (nicknamed

EDGE)!

g  3G: UMTS (introduced in 2002): CDMA (2100 MHz)!i  3.5G: with HSPDA (up to 14.4Mb/s); with HSPA+ (up to 84Mb/s) !

g  4G: LTE (introduced in 2013): OFDMA (800 and 2600MHz, then technology neutrality); up to 100Mb/s!

!GPRS: General Packet Radio Service!HSPDA: High Speed Downlink Packet Access!LTE: «Long Term Evolution»!For more information: see the 3GPP standards!

22

21.5 3RArea of the hexagon: Distance between adjacent cells:

3d R=

23

Frequency Reuse"

F3 F4

F5

F2 F7

F6 F1

F3 F4

F5

F2 F7

F6 F1

F3 F4

F5

F2 F7

F6 F1

g  Channel assignment strategies •  Fixed: each cell is allocated a predetermined set of channels •  Dynamic: each time a call request is made, the serving base station requests a channel from the MSC

g  Cells with the same name use the same set of frequencies g  Cells are organized into clusters In this example, the cluster size N = 7 g  In order to tesselate, the geometry of hexagons is such that N can only have values which satisfy N = i2 + ij + j2

with i = 0,1,2,… and j = 0,1,2,…

24

N: cluster size

i=2, j=0 i=2, j=1

i=3, j=2

25

26

Example: system of 32 cells with cell radius of 1.6km Total frequency bandwidth supporting 336 traffic channels Reuse factor (or cluster size) = 7 What geographic area is covered? Total number of supported channels? Solution: Cell area = 6.65km2 Covered area: 32*6.65=213km2 Channels/cell = 336/7=48 Total channel capacity: 32*48=1536 channels

Same question for a system of 128 cells with cell radius of 0.8km. As before: - total frequency bandwidth supporting 336 traffic channels -  reuse factor (or cluster size) = 7

Solution: Cell area: 1.66km2 Covered area: 128*1.66=213km2 Total channel capacity: 128*48=6144

27

Rate of calls per minute: 97/60 Average holding time per call: 294/97 Offered traffic: 294/60= 4.9 Erlangs

28

29

Interference & System Capacity"

γ  Sources of interference!i  Co-channel interference (same frequency)!

–  A call in a neighboring cell !–  Other base stations operating in the same frequency band!–  Non-cellular system leaking energy into the frequency band!

i  Adjacent channel interference (adjacent frequency)!–  Another mobile in the same cell!

γ  Consequences of interference!i  On data channel: !

–  Crosstalk (voice) !–  Erroneous data (data transmission)!

i  On control channel: !–  Missed/dropped calls!

30

Decibels (reminder)"

10110log 20

100dB⎛ ⎞ = −⎜ ⎟

⎝ ⎠

g  The decibel is a dimensionless unit used to express a power ratio!!!!where P0 is the reference power level and P is the considered power level!!g  Decibel (dB) !

•  express the magnitude of a physical quantity relative to a reference level. !•  represent very large range of ratios!•  are easy to manipulate (e.g., consecutive amplifiers)!

g  A ratio!•  can be expressed in decibels relative to 1 Watt (dBW) !•  is more frequently expressed in decibels relative to 1mW (dBm)!

!!!Example: !If the transmission power P0 is 10W and the received power P is 0.1W, the loss is !!

100

10 log PBP

⎛ ⎞= ⋅ ⎜ ⎟

⎝ ⎠

1010 log1PPmW

⎛ ⎞= ⋅ ⎜ ⎟⎝ ⎠

31

Co-channel Interference (1/4)"

0 00 0

or (dBm) (dBm) 10 logr rd dP P P Pd d

α

α−

⎛ ⎞ ⎛ ⎞= = −⎜ ⎟ ⎜ ⎟

⎝ ⎠ ⎝ ⎠

g  Co-channel reuse ratio Q" where D = distance to the center of the nearest co-channel cell

R = radius of a cell N = cluster size (or “reuse factor”)

g  Signal-to-interference ratio (SIR)" where S = desired signal power

Ii = interference power caused by the ith interfering co-channel base station i0 = number of co-channel interfering cells

g  Average received power Pr at a distance d from the transmitting antenna! where P0 = power received at a small distance d0 from the transmitting antenna

α = path loss exponent

3DQ NR

= =

0

1

i

ii

S SSIRI

I=

= =

∑

F5

F5

R D

32

Co-channel Interference (2/4)"

( )0 0

3( ) NS D RI i i

αα

= =

If the transmit power of each base station is equal and α is the same throughout the coverage area, in a corner of a cell (most remote place from the base station in the cell) we have:!!!!!!!Considering only the first layer of interfering cells and assuming that they are equidistant from the desired base station (all at distance D):!

0

1

i

ii

S RI D

α

α

−

−

=

=

∑

33

Co-channel Interference (3/4)"

A

R

D-R

D-R

D

D+R

D+R

D

First tier of co-channel cells for a cluster size of N=7!Note: the marked distances are approximations!

34

Co-channel Interference (4/4)"

12( 1) 2 2( 1)

SI Q Q Qα α α− − −=

− + + +

Approximation of the SIR at point A!!!!!!Using the co-channel ratio!!!!!!Numerical example: If N=7, alpha = 4, then Q~4.6 and !

2( ) 2 2( )S RI D R D D R

α

α α α

−

− − −=

− + + +

49.56 17.8 S dBI≈ ≈

35

Capacity of Cellular Networks (1/2) FDMA/TDMA"

0min

1

16i

ii

S R R SI D ID

αα

α

−−

−

=

⎛ ⎞ ⎛ ⎞= = ≥⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠∑

1/

min

6 SQI

α⎛ ⎞⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

g  FDMA/TDMA capacity is bandwidth limited!!g  Consider the downlink channel interference. Assume that the mobile is located at the edge of the cell. Consider only the interference from the first tier of co-channel cells (6 cells if N = 7).!! We want the SIR to be greater than a given minimum SIRmin!!!!!!!!!Using the co-channel reuse ratio and because Q=D/R:!3Q N=

36

Capacity of Cellular Networks (2/2)FDMA/TDMA"

2 2/

/2min

63 3

t t

c c

B BmQ SB B

I

α

α

= =⎛ ⎞⎛ ⎞

⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

Techniques to improve capacity •  Cell splitting •  Sectoring

Radio capacity of cellular network !!!!where Bt is the total allocated spectrum for the system

Bc is the channel bandwidth !Using the co-channel reuse ratio!

t

c

BmB N

= radio channels/cell

37

Capacity of Cellular Networks CDMA"

γ  CDMA capacity is interference limited!γ  Techniques to reduce interference!

i  Multi-sectorized antennas!i  Discontinuous transmission mode (takes advantage of

intermittent nature of speech); duty factor between 3/8 and ½.!

γ  Power control: for a single cell, all uplink signals should be received approximately with the same power at the base station!

γ  Pilot signal: transmitted by the base station; used by each mobile to set its own power (for the uplink)!

38

CDMA Capacity: single cell case (1/2)"

Let N = number of users S = power of the signal received at the base station from a single user

Bit energy to noise ratio where R = bitrate

W = available bandwidth N0 = noise spectral density

Taking the thermal noise η into account Thus, the number of users that can access the system is

1( 1) 1SSNR

N S N= =

− −

0

/ /( 1)( / ) 1

bE S R W RN N S W N

= =− −

0

/( 1) ( / )

bE W RN N Sη

=− +

( )0

/1 - /S/b

W RNE N

η= +

39

CDMA Capacity: single cell case (2/2)"

0

1 /1/s

b

W RNE Nδ⎡ ⎤

= + ⎢ ⎥⎣ ⎦

To increase this number, 2 main techniques: - Leverage on the sporadicity of users’ activity (e.g., switch off a user while he does not talk) - Antenna sectorization Let δ = duty cycle (or factor) of voice (typically between 3/8 and ½)

Ns = number of users per sector If the number of users is large and thermal noise is neglected:

0

/( 1) ( / )

b

s

E W RN N Sδ η

=− +

CDMA Capacity: multiple cells case (1/3) "

40

B0 controls the transmit power of its in-cell users, but not that of users in neighboring cells Frequency reuse factor on the uplink where N0 = total interference power received from N-1 in-cell users

Ui = number of users in the ith adjacent cell Nai = average interference power from a user located in the ith adjacent cell

Average received power from users in adjacent cell is computed as where Nij = power received at the base station of interest from the jth user in

the ith cell

0

0 i aii

NfN U N

=+∑

/ai ij ij

N N U=∑

B0

B6

B5

B4

B3

B2

B1

41

CDMA Capacity: multiple cells case (2/3) "Concentric circular geometry

d0

Considered cell

R

2R+d0 2R-d0

3R

2d0

Adjacent cell

q1

M1 : number of wedge- shaped cells of the first surrounding layer of cells A1 : area of the first surrounding layer A1 = M1 A To let all cells have the same size A, we must have: M1 = 8 q1 = 450

By recursion, for the ith layer: Ai = i8A qi = p/4i

First surrounding layer

42

CDMA Capacity: multiple cells case (3/3) "

d0

R

2R+d0

2R-d0

3R

q

Inner sublayer

Outer sublayer

d

d’

( )

( )

0

0

0

22 2

0

22 2

For the inner sublayer, namely for (2 1) (2 ) (case depicted in the figure):

' sin 2 cos

For the outer sublayer, namely for (2 ) (2 1) :

' sin cos 2

Inter

i R d i R d

d d Ri d d

i R d d i R

d d d Ri d

θ θ

θ θ

− ≤ ≤ −

= + − −

+ ≤ ≤ +

= + − −

0

0, , 0 0 0 0

ference power at B from the th subscriber of the th cell :

( , , ) ( '/ ) ( / )

In practice, the frequency reuse factor for CDMA is in the order of 0.3 to 0.7 (as a comparison, in the ca

i j

j iP r d P d d d d

f

α αθ =

se of FDMA with cluster size = 7, = 1/7). f

Note: i is the layer number (i=1 if we consider only the first layer)

Interfering cells

Hands-on Radio Planning"

γ  Making use of CloudRF, a radio-frequency planning software, you will be in the shoes of a telecommunication engineer deploying radio antennas!

γ  Two real-world exercises that we will finalise at next week’s lecture; please try to start it beforehand!!

43

Hands-on Radio Planning"γ  Tasks to be completed before April 1st:!1.  Create a personal account:

http://cloudrf.com/account/register!2.  Sign in with your personal account:

http://cloudrf.com/account/login!3.  Order a free Region 1 Amateur plan:

http://cloudrf.com/collections/plans/products/itu-1-amateur-plan-for-europe-africa-middle-east-and-russia!

4.  Start playing with the software using the Web interface: https://web.cloudrf.com/web/!

5.  Do the exercises mentioned in the homework called HW-D2-CloudRF!

For any problem, please contact Mathias Humbert:mathias.humbert@epfl.ch!! 44

45

Conclusion"

γ  In this Module D2, we have addressed essentially network capacity and planning!

γ  Cellular networks: many base stations!γ  Capacity can be increased notably by cell splitting

and cell sectoring!γ  Reminder: Division multiple access used in cellular

network generations (all with SDMA, of course):!i  2G: GSM: FDMA/TDMA!i  3G: UMTS: CDMA!i  4G: LTE: OFDMA (Orthogonal Frequency-Division Multiple

Access) for the downlink and SC-FDMA (Single-carrier Frequency Division Multiple Access) for the uplink!

!

!

46

References"

γ  Agrawal & Zeng: Chapter 5!γ  T. Rappaport: Wireless Communications, 2nd edition, Prentice

Hall, 2001!γ  M. Schwartz: Mobile Wireless Communications, Cambridge

University Press, 2005!γ  W. Stallings: Wireless Communications and Networks, 2nd

edition, Prentice Hall, 2005, Chapter 10!

γ  Schiller, Chapter 4!

γ  3GPP and LTE specifications – www.3gpp.org

(even more difficult to navigate than IETF specifications!)!!

47