Wireless cellular networks basics
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Wireless Cellular Networks (basics). Part 1 – Propagation for dummies . l. f. The Radio Spectrum. Radio wave Wavelength l = c/f Speed of light c=3x10 8 m/s Frequency: f. [V|U|S|E]HF = [Very|Ultra|Super|Extra] High Frequency. f = 900 MHz  l = 33 cm. The radio spectrum.

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Wireless cellular networks basics

Wireless Cellular Networks(basics)

Part 1 – Propagation for dummies 


The radio spectrum

l

f

The Radio Spectrum

  • Radio wave

    • Wavelength l = c/f

    • Speed of light c=3x108 m/s

    • Frequency: f

[V|U|S|E]HF = [Very|Ultra|Super|Extra] High Frequency

f = 900 MHz  l = 33 cm


The radio spectrum1

The radio spectrum


Spectrum allocation

Spectrum Allocation

  • Cellular Systems

    • 400-2200 MHz range (VHF-UHF)

    • Simple, small antenna (few cm)

    • With less than 1W transmit power, can cover several floors within a building or several miles outside

  • wireless data systems

    • 2.4, 5 GHz zones (ISN band)

    • Main interference from microwave ovens

    • limitations due to absorption by water and oxygen - weather dependent fading, signal loss due to by heavy rainfall etc.

  • Higher frequencies:

    • more bandwidth

    • less crowded spectrum

    • but greater attenuation through walls

    • Current target: 60 GHz ultra high throughput WLANs/WPANs

  • Lower frequencies

    • bandwidth limited

    • longer antennas required

    • greater antenna separation required

    • several sources of man-made noise


Antennas ideal free space

r

Antennas (ideal, free space)

  • Isotropic (omnidirectional) tx antenna in free space

    • Transmitted power: Pt

    • Power attenuation Pa at distance d:down with sphere superficies

  • Power received by isotropic rx antenna

    • Planar wave

    • Ae = Effective Area

  • In two-way communication Same antenna may be used for tx and rx

Idealization: Isotropic antennas cannot be practically built


Antennas real free space

r

Antennas (real, free space)

  • Non isotropic tx antenna

    • Antenna gain Gt

    • Gain = Power output, in a particular direction, compared to that produced in any direction by a perfect omni-directional (isotropic) antenna

  • Non isotropic rx antenna

    • Antenna gain Gr


Friis free space model summarizing all previous considerations

Friis Free-Space Modelsummarizing all previous considerations

  • Pt = transmitter power

    • (W or mW)

  • Gt = transmitter antenna gain

  • Gr = transmitter antenna gain

    • (dimensionless)

  • l = c/f = RF wavelength (m)

    • c = speed of light (3x108 m/s)

    • f = RF frequency (Hz)

  • Pt Gt = Equivalent Isotropic Radiated Power (EIRP)

  • L = other system losses (hardware)

    • L >=1 (dimensionless)

  • d = distance between transmitter and receiver (m)

Summary: in free space,


Power units db and dbm

Power units – dB and dBm

  • Decibel (dB): log unit of intensity; indicates power lost or gained between two signals

  • Named after Alexander Graham Bell

  • dBm: absolute value (reference = 1 mW)

    • Versus dB = relative value = ratio

    • Power in dBm = 10 log(power/1mW)

PA = 1 Watt

PB = 50 milliWatt

 PA = 13 dB greater than PB


Decibels dbm

Decibels - dBm

  • Examples

    • 10 mW = 10 log10(0.01/0.001) = 10 dBm

    • 10 mW = 10 log10(0.00001/0.001) = -20 dBm

    • 26 dBm = ___2W= ___ dBm?

    • S/N ratio = -3dB  S = ___ X N?

  • Transmit power

    • Measured in dBm

      • Es. 33 dBm

  • Receive Power

    • Measured in dBm

      • Es. –10 dBm

  • Path Loss

    • Receive power / transmit power

    • Measured in dB

    • Loss (dB) = transmit (dBm) – receive (dBm)

      • Es. 43 dB = attenuation by factor 20.000


Friis example

Friis Example


Reference distance

Reference distance

  • If known received power at a reference distance dofrom tx

    • can calculate Pr(d) for any d

  • Must be smaller than typical distances encountered in wireless communication systems;

  • Must fall in the far-field region of the antenna

    • So that losses beyond this point are purely distance-dependent

  • Typical d0 selection: 100-1000m


More realistic propagation models

More realistic propagation models

  • Inverse square power law

    • Way too optimistic (ideal case); valid only for very short distances

    • Real world: h-th power law

    • with h ranging up to as much as h=7

      • If tough environment (e.g., lots of foliage),

    • typical values:

      • h=2 for small distances (20 dB/decade)

      • h=3 to h=4 (40 dB/decade) for mobile telephone distances

    • h higher in cities and urban areas; h lower in suburban or rural areas.


Formulae with ref distance

Formulae with ref. distance


Two ray ground propagation model

Two-Ray Ground Propagation Model

  • Theoretical foundation for h=4

    • Two-ray model assumes one direct LOS path and one reflection path reach receiver with significant power

    • Easy to solve

Line-Of-Sight ray

ht

hr

reflected ray

Transmit and receive antennas at different height (in general)


Two ray model geometry

Two-ray model – geometry

ddirect

ht

hr

q

q

(

)

dreflect


Two ray model path analysis

Two ray model – path analysis

  • EM waves travel for different distance

    • Sum up with different phase!

    • A = attenuation along direct path

    • B = attenuation along reflected path (reflection not ideal, in general)


Two ray model field strength

Two ray model – field strength

  • Phase difference

  • Received field strength

    • Let Edirect be the field strength given by direct ray.

    • Then

  • Assume ideal reflection (r=-1)


Two ray model power computation

Two ray model – power computation

  • Received power

    • Proportional to |E|2


Two ray model conclusion

Two ray model - conclusion

  • Typical values:

    • ht ~ few tens of m

    • hr ~ couple of meters

    • l ~ few tens of cm

    • d ~ hundred meters – few km

i.e. attenuation follows a 40 dB/decade rule! Versus 20 dB/decade of the free-space model


Propagation impairments

Propagation impairments

Diffraction

  • When the surface encountered has sharp edges  bending the wave

    Scattering

  • When the wave encounters objects smaller than the wavelength (vegetation, clouds, street signs)

Line of sight

Reflection

Shadowing

BS

MS

BS


Multipath characteristics not just attenuation

Multipath Characteristics(not just attenuation)

  • A signal may arrive at a receiver

    • many different times

    • From many different directions

  • due to vector addition, signal may

    • Reinforce

    • Cancel

  • signal strength differs

    • from place to place

    • from time to time!

  • (slow/fast fading)


Attenuation fading

Attenuation + fading

Fast fading

Signal power

Distance BS  MS (m)

Slow fading

Distance BS  MS (km)


Statistical nature of received power

Statistical nature of received power

Signal strength (dB)

Short term fading

Mean value predictedby attenuation model (constant at given d)

Long term fading

Time (or movement)

Different (statistical) mathematical models for slow and fast fading

(details out of the goals of this lecture)


Cell radius

Cell radius

  • How do we determine cell radius?

  • Seems very simple: given

    • Pt = transmitted power (dBm)

    • Pth = threshold power (dBm)

      • Sensitivity of the receiver, i.e. minimum amount of received power for acceptable performance

  • Path loss computed as

    • Lp = Pt - Pth

  • Radius computed from Lp

    • Via h-law propagation formula

    • Via Okumura-Hata formula (or other empirical model)

  • But…


Example part 1

Example (part 1)

  • Received power at 10 mt: 0.1 W

  • Threshold power: Pth = -50 dBm

  • h = 3.7

Result: because of statistical power fluctuation (fading)

outage probability at cell border will be about 50%!!!


Fading margin

prob

Mean path

loss

M

Power

at cell

border

1% - 2%

Fading Margin

  • Previouscomputationdoesnot account forlong-termfading

    • Needtokeepit in count, asitdoesnotreduce when the MS makessmallmoves

    • IDEA: reduce cellradiusto account fora “fading margin” M

  • Fading Margindefinition:

    • M = averagereceivedpower at cellborder (dBm) – thresholdpower (dBm)

      • M=0 meansthat the powerreceived at cellborderisequalto the threshold

      • M=6 (dB) powerreceived at cellborderis 4 x the powerthreshold

  • Fading Margincomputation

    • Through appropriate statisticalmodeloflong-term fading (typicallylognormal)


Example part 2

Example (part 2)

  • Received power at 10 mt: 0.1 W

  • Threshold power: Pth = -50 dBm

  • h = 3.7

  • If we use a fading margin M=6 

What is the experienced outage at cell border?

If we assume lognormal slow fading, with sdB=4 dB…


Empirical attenuation models

Empirical attenuation models

  • Consider specific scenarios

    • Urban area (large-medium-small city), rural area

    • Models generated by combining most likely ray traces (LOS, reflected, diffracted, scattered)

    • Based on large amount of empirical measurements

  • Account for parameters

    • Frequency; antenna heights; distance

  • Account for correction factors

    • (diffraction due to mountains, lakes, road shapes, hills, etc)

  • Many models for distance ranges, frequency ranges, indoor vs outdoor

    • Okumura-Hata ; Lee’, others  cellular frequencies, large distances > 1km

    • Walfish-Ikegami  800-2000 MHz , microcellular distances (20m – 5 km)

      • Adopted by European Cooperation in the field of Scientific and Technical (COST) research as reference model for 3G systems

    • Indoor propagation models


Example okumura hata model

Example: Okumura-Hata model

  • Hata (1980): very simple model to fit Okumura results

  • Provide formulas to evaluate path loss versus distance for various scenarios

    • Large cities; Small and medium cities; Rural areas

    • Limit: d>=1km

Parameters:

  • f = carrier frequency (MHz)

  • d = distance BS  MS (Km)

  • hbs = (effective) heigh of base station antenna (m)

  • hms = height of mobile antenna (m)

Effective BS

Antenna height


Okumura hata urban area

Okumura-Hata: urban area

  • a(hms) = correction factor to differentiate large from medium-small cities;

  • depends on MS antenna height

Very small correction difference between large and small cities (about 1 dB)


Okumura hata suburban rural areas

Okumura-Hata: suburban & rural areas

  • Start from path loss Lp computed for small and medium cities


Okumura hata examples

Okumura-Hata: examples

F=900MHz, hbs=80m, hms=3m


Okumura hata and h

Okumura-Hata and h

  • Coefficient of Log(d) depends only on hbs

  • 10h = attenuation (dB) in a decade

    • (d=1  d=10)

  • The higher the BS, the lower the coefficient h


Wireless cellular networks basics1

Wireless Cellular Networks(basics)

Part 2 – Cellular Coverage Concepts


Coverage for a terrestrial zone

Coverage for a terrestrial zone

Signal OK if Prx > -X dBm

Prx = c Ptx d-4

greater Ptx greater d

d

  • 1 Base Station

  • N=12 channels

  • (e.g. 1 channel = 1 frequency)

  • N=12 simultaneous calls

BS


Cellular coverage target cover the same area with a larger number of bss

Cellular coveragetarget: cover the same area with a larger number of BSs

19 Base Station

12 frequencies

4 frequencies/cell

Worst case:

4 calls (all users in same cell)

Best case:

76 calls (4 users per cell)

Average case >> 12

Low transmit power

  • Key advantages:

  • Increased capacity (freq. reuse)

  • Decreased tx power


Cellular coverage microcells

Cellular coverage (microcells)

many BS

Very low power!!

Unlimited capacity!!

Usage of same spectrum

(12 frequencies)

(4 freq/cell)

Disadvantage:

Location

mobility management


Cellular system architecture the gsm network example high level view

Cellular system architecture: the GSM Network examplehigh-level view

MSC = Mobile Switching Center

= administrative region

PSTN

Public switched telephone network

MSC

MSC

Base Station

Base Station

PLMN

Public Land

Mobile Network

MSC role: telephone switching central with special mobility management capabilities


Gsm system hierarchy

MSC

LOCATION AREA

BSC

BTS

GSM system hierarchy

MSC region

MSC: Mobile Switching Center

LA: Location Area

BSC: Base Station Controller

BTS: Base Transceiver Station

Hierarchy: MSC region  n x Location Areas  m x BSC  k x BTS


Gsm essential components

GSM essential components

OMC

To fixed network

(PSTN, ISDN, PDN)

GMSC

EIR

AUC

HLR

VLR

MSC

BSC

BTS

MSMobile Station

BTSBase Transceiver Station

BSC Base Station Controller

MSC Mobile Switching Center

GMSCGateway MSC

OMCOperation and Maintenance Center

EIREquipment Identity Register

AUCAuthentication Center

HLRHome Location Register

VLRVisitor Location Register

BTS

BTS

BSC

BTS

BTS

MS


Cellular capacity

Cellular capacity

  • Increased via frequency reuse

    • Frequency reuse depends on interference

    • need to sufficiently separate cells

      • reuse pattern = cluster size (7  4  3): discussed later

  • Cellular system capacity: depends on

    • overall number of frequencies

      • Larger spectrum occupation

    • frequency reuse pattern

    • Cell size

      • Smaller cell (cell  microcell  picocell  femtocell) = greater capacity

      • Smaller cell = lower transmission power

      • Smaller cell = increased handover management burden


Hexagonal cells

Hexagon:

Good approximation for circle

Ideal coverage pattern

no “holes”

no cell superposition

B

B

B

A

A

A

D

D

D

C

C

C

hexagonal cells

A

D

B

C

B

A

D

A

C

C

D

A

B

A

D

C

D

B

  • Example case:

    • Reuse pattern = 4


Cells in real world

Cells in real world

Shaped by terrain, shadowing, etc

Cell border: local threshold, beyond which neighboring BS signal is received stronger than current one


Reuse patterns

R

D

7

7

6

6

2

2

1

1

1

1

4

4

4

4

2

2

2

2

1

1

5

5

3

3

3

3

3

3

4

4

Cluster: K = 7

D

K = 4

Reuse patterns

  • Reuse distance:

    • Key concept

    • In the real world depends on

      • Territorial patterns (hills, etc)

      • Transmitted power

        • and other propagation issues such as antenna directivity, height of transmission antenna, etc

  • Simplified hexagonal cells model:

    • reuse distance depends on reuse pattern (cluster size)

    • Possible clusters:

      • 3,4,7,9,12,13,16,19,…


Reuse distance

Reuse distance

  • General formula

  • Valid for hexagonal geometry

  • K = cluster size

  • D = reuse distance

  • R = cell radius

  • q = D/R =frequency reuse factor


Proof

Distance between two cell centers:

(u1,v1)  (u2,v2)

Simplifies to:

Distance of cell (i,j) from (0,0):

Cluster: easy to see that

hence:

v

(3,2)

u

(1,1)

30°

Proof


Clusters

Clusters

Clusters:

  • Number of BSs comprised in a circle of diameter D

  • Number of BSs whose inter-distance is lower than D

K=13(i=3,j=1)

K=7(i=2,j=1)

K=4 (i=2,j=0)


Clusters dim

D

D

D

D

D

D

D

C

C

C

C

C

C

C

B

B

B

B

B

B

B

A

A

A

A

A

A

A

Clusters (dim)

B

A

D

B

C

B

A

D

A

D

B

C

B

C

B

D

A

D

A

C

B

C

B

C

A

D

A

D

B

C

B

C

B

D

A

D

A

C

B

C

B

C

A

D

A

D

B

C

B

C

B

D

A

D

A

B

C

B

D

A

B


Possible clusters all integer i j values

Possible clustersall integer i,j values


Co channel interference

A

G

E

A

C

F

G

E

D

C

F

A

B

D

C

E

A

B

F

G

E

A

C

F

G

B

D

C

F

A

B

D

E

A

Co-Channel Interference

  • Frequency reuse implies that remote cells interfere with tagged one

  • Co-Channel Interference (CCI)

    • sum of interference from remote cells


Cci computation assumptions

Assumptions

NI=6 interfering cells

NI=6: first ring interferers only

we neglect second-ring interferers

Negligible Noise NS

S/N ~ S/I

d-h propagation law

h=4 (in general)

Same parameters for all BSs

Same Ptx, antenna gains, etc

Key simplification

Signal for MS at distance R

Signal from BS interferers at distance D

R

Dint

Power

Po

Power

Po

R

D

CCI Computation - assumptions

Dint~ D


Cci computation

CCI computation

By using the assumptions of same cost and same D:

Results depend

on ratio q=D/R (q=frequency reuse factor)

Alternative expression: recalling that

NI=6,m=4 

USAGE: Given an S/I target, cluster size K is obtained


Examples

target conditions:

S/I=9 dB

h=4

Solution:

target conditions:

S= 18dB

h=4.2

Solution:

Examples


S i computation assuming 4 6 interferers only first ring

S/I computationassumingη=4 & 6 interferersonly (first ring)


Additional interferers

case K=4

note that for each cluster there are always NI=6 first-ring interferers

B

A

D

B

C

B

A

D

A

D

B

C

B

C

B

D

A

D

A

C

B

C

B

C

A

D

A

D

B

C

B

C

B

D

A

D

A

C

B

C

B

C

A

D

A

D

B

C

B

C

B

D

A

D

A

B

C

B

D

A

B

Additional interferers

In CCI computation, contribute of additional interferers is marginal


Sectorization

Directional antennas

Cell divided into sectors

Each sector uses different frequencies

To avoid interference at sector borders

PROS:

CCI reduction

CONS:

Increased handover rate

Less effective “trunking” leads to performnce impairments

sectorization

Sector 3

Sector 1

Sector 2

CELL a


Cci reduction via sectorization three sectors case

Inferference from 2 cells, only

Instead of 6 cells

A

A

A

A

CCI reduction via sectorizationthree sectors case

G

E

A

C

F

G

E

D

C

F

B

D

C

With usual approxs

(specifically, Dint~ D)

E

B

F

G

E

A

C

F

G

B

D

C

F

B

D

E

A

G

F

Conclusion: 3 sectors = 4.77 dB improvement


6 sectors

60o Directional antennas

CCI reduction:

1 interfereer only

6 x S/I in the omni case

Improvement: 7.78 dB

6 sectors


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