610

(Introduction The wireless channel

- (Definition of Free Space Propagation) (Generic Communication System) , (Noise source types

Multiplicative, Additive) (The Electromagnetic Spectrum) : , Megacells, Macrocells,

Microcells, Picocells (Aims of Cellular Systems) (Typical Cellular Network

Architecture) (Key elements of a standard cellular

network) (Cell Coverage General path

loss model) (Cell Pattern) - / (Cellular Reuse

concept), (Interference), / (Sectorisation), (Handover), / (Traffic)

(Multiple Access Schemes), duplexing

(Free Space Propagation)

(Free Space Propagation) ;

()

.

Base Station

-

(Propagation Mechanisms)

- Reflection (A Propagating wave impinges on an object which is large compared to its wavelength) E.g., the surface of the Earth, buildings, walls, etc.

- Diffraction: Caused when a a radio path between transmitter and receiver is obstructed by a surface with sharp edges. Waves bend around the obstacle edges in Non line of site areas

- Scattering: Caused when propagating wave impinges on objects smaller than its wavelength E.g., foliage, street signs, lamp posts

. Claude Shannon.

( , ) .

(Generic Communication System)

Source

Noise

Source

Transmitter Receiver Destination

The Channel

Base StationVoice-Data

( ) .

( ).

,

(Generic Communication System)

The channel - Noise source types

(Multiplicative) (Antenna directionality) - (Reflection - smooth surfaces) (Absorption - walls, trees, atmosphere) - ) (Scattering - rough surfaces) (Diffraction - edges of buildings and hills) (Refraction - atmosphere)

(Additive) (Thermal and shot noise in receiver) & (Atmospheric & Cosmic noise) (Interference intentional & otherwise)

Multiplicative

Noise

Additive

Noise

Noise Source

Three scales of Multiplicative Noise

Path

Loss

Transmit

Antenna

Receive

Antenna

Additive

NoiseFast

FadingShadowing

Fading processes

:

- Path loss

- - Shadowing

- Fast Fading

.

Path loss: , Tx Rx.

- Shadowing: Rx Tx (.. , .)

Fast Fading: () () .

VLFBand: LF MF HF VHF UHF SHF EHF

3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz

100 km 10 km 1 km 100 m 10 m 1 m 10 cm 1 cm 1 mm

Frequency

Free-space Wavelength

L, S, C, X, Ku, K, Ka

3kHz - 300GHz

()

: . : . SHF EHF.

: . : ( ) . (VHF - EHF).

Megacells: . : ( ).

()

Macrocells: , , . .

Microcells: , . .

Picocells: ( ) , . , .

: . System must provide a wide area outdoor coverage depth of coverage must be extended to include indoor areas interworking between different systems

(Capacity) blocked dropped calls

: :

PSTNMSC

HLR VLR

BSC BTS

BSS

BSC BTS

BSS

MS

AI

(Cellular Network Architecture)

GSM system terminology

MSC , (PSTN) .

(Home) () home location register (HLR).

() (Home) visitor location register (VLR).

, , .

.

The base station subsystem (BSS) is composed by the base station controller (BSC) handling the logical functionality and by several base transceiver stations (BTS, also known as basestation (BS) ) containing the RF and baseband parts of the BSS.

(BSS) (BSC) (BTS, - BS)

(BTSs) (Mobile Stations - MS) .

rhb

hm

/ (Cell Coverage)

(general path loss model):

cbm

T

R frhhkfLP

P,,,,

1

PR:Power received at the mobile

PT:BS transmit Power

r: Distance between BS and mobile

(BS) /

/ ( ), .

fc: Frequency of operation

hb: BS antenna height

hm: Mobile antenna height

k: constant of proportionality

To L (path loss)

(, , .)

, .

, (, / )

/ .

.

?

.

/

.

(Cell pattern)

Fiction Ideal Real

7-cell cluster

7

3

1

2 6

5

4

7

3

1

2 6

5

4

7

3

1

2 6

5

4

7

3

1

2 6

5

4

7

3

1

2 6

5

4

Coverage area tiled with 7-cell clusters

(Cellular Reuse Concept)

A 7 cell cluster

/

(/ ) / o ()

(C/I), (carrier C) (interfering I). C/I (modulation) , (coding) , (synchronisation schemes) .

(Interference)

46

144

6

111

R

D

DRI

C

k

NR

D3

R

23

6

1N

I

C

D

24

R: cell radius, D: reuse distance, N: cluster size

(BS) (.. )

: Cluster size 7 for C/I = 19dB Cluster size 4 for C/I = 14dB C/I

.

() (sectorisation). .

(Interference)

(Sectorisation)

. 2 ( 6 2) ( 3). (Increased Reuse). 30.5=1.7

A 3 sectored cell (120o)

7

3

1

2 6

5

4

7

3

1

2 6

5

4

7

3

1

2 6

5

4

7

3

1

2 6

5

4

Omni cells

Sectored cells

.

( ) , (Handover)

(Trunking Efficiency).

-H , , ( ). ,

(Sectorisation)

- Handover

(, Handover / Handoff)

(thresholds) (hysteresis)

mobile Assisted Handover .

Traffic

Trunking / .

Trunking Pool of channels allocated to users after request at the start of a call. Returned to pool at the end.

(blocked)

(grade of service)

Traffic Erlangs

Erlang.

Multiple Access Schemes and duplexing

:

FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) CDMA (Code Division Multiple Access) SDMA (Space Division Multiple Access)

Multiple Access Duplexing ( ) : Simplex Half-Duplex Full-Duplex

FDD (Frequency Division Duplexing) TDD (Time Division Duplexing)

, .. TDMA/FDD (GSM)

FDMA / FDD

-

(downlink, uplink). .

( uplink, downlink). .

Frequency

1 2 3 N ...

1 2 3 N ...

Uplink Downlink

Guard

FDMA / FDD FDMA:

. RF duplexers

(carrier) . equalizers.

H

(RF filters)

Guard Bands

TDMA / FDD

1 2 3 4

3 4 1 2

1

3

Frame

Burst

f1

f2

TDMA FDD duplex (frames)

(slots) time slots, uplink

downlink.

TDMA / FDD TDMA Features:

Transmission & Reception not simultaneous at the mobile so duplexers are not required

Wide Bandwidth needed to accommodate several users. Need for Equalisation

Due to Guard Time needed some bits are wasted

Signalling Overhead

TDMA :

duplexers

. equalization.

Guard Time bits

CDMA

CDMA / spread spectrum .

(chip rate) .

Source x

Spreading

Code

1

2

3

4

... N

Code

Frequency

CDMA

CDMA :

low mutual interference. .

(equalisers) Rake. multipath diversity gain

despreading.

Baseband FDMA TDMA.

, , .