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Wireless Cellular Networks (basics)

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Wireless Cellular Networks(basics)

Part 1 – Propagation for dummies

l

f

- 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

- 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

r

- 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

r

- 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

- 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,

- 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

- 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

- Measured in dBm
- Receive Power
- Measured in dBm
- Es. –10 dBm

- Measured in dBm
- Path Loss
- Receive power / transmit power
- Measured in dB
- Loss (dB) = transmit (dBm) – receive (dBm)
- Es. 43 dB = attenuation by factor 20.000

- 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

- 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.

- 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)

ddirect

ht

hr

q

q

(

)

dreflect

- 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)

- Phase difference
- Received field strength
- Let Edirect be the field strength given by direct ray.
- Then

- Assume ideal reflection (r=-1)

- Received power
- Proportional to |E|2

- 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

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

- 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)

Fast fading

Signal power

Distance BS MS (m)

Slow fading

Distance BS MS (km)

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)

- 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…

- 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%!!!

prob

Mean path

loss

M

Power

at cell

border

1% - 2%

- 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

- M = averagereceivedpower at cellborder (dBm) – thresholdpower (dBm)
- Fading Margincomputation
- Through appropriate statisticalmodeloflong-term fading (typicallylognormal)

- 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…

- 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

- 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

- 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)

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

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

- 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(basics)

Part 2 – Cellular Coverage Concepts

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

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

many BS

Very low power!!

Unlimited capacity!!

Usage of same spectrum

(12 frequencies)

(4 freq/cell)

Disadvantage:

Location

mobility management

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

MSC

LOCATION AREA

BSC

BTS

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

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

- 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

- overall number of frequencies

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

A

D

B

C

B

A

D

A

C

C

D

A

B

A

D

C

D

B

- Example case:
- Reuse pattern = 4

Shaped by terrain, shadowing, etc

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

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 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,…

- General formula
- Valid for hexagonal geometry
- K = cluster size
- D = reuse distance
- R = cell radius
- q = D/R =frequency reuse factor

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°

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)

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

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

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

- Frequency reuse implies that remote cells interfere with tagged one
- Co-Channel Interference (CCI)
- sum of interference from remote cells

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

Dint~ D

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

target conditions:

S/I=9 dB

h=4

Solution:

target conditions:

S= 18dB

h=4.2

Solution:

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

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B

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B

D

A

D

A

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B

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B

C

A

D

A

D

B

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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

In CCI computation, contribute of additional interferers is marginal

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

Sector 3

Sector 1

Sector 2

CELL a

Inferference from 2 cells, only

Instead of 6 cells

A

A

A

A

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

60o Directional antennas

CCI reduction:

1 interfereer only

6 x S/I in the omni case

Improvement: 7.78 dB