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Politecnico di MilanoFacoltà di Ingegneria dell’Informazione

3 – Basic concepts

Wireless NetworksProf. Antonio Capone

Antonio Capone: Wireless Networks 2

Wireless Networkso Is the transmission

medium the only difference? n The peculiar medium

characteristics have great impact on system characteristics

n Wireless networks allow users to move and naturally manage mobility

Wireless or wired, what is better?

wiredwire-less

Well, it depends on the situation!


Network architecture

Access network

Backbone network


Wireless access networkso Wireless networks are mainly access

networkso Backbone networks composed of radio

point-to-point links are usually not considered wireless networks

o Wireless access networks are more challenging and have many fundamental differences with respect to wired access networks

o The first main difference is that the transmission medium is broadcast


Broadcast channel

Centralized broadcast channelDistributed broadcast channel


Centralized broadcast channel

o Fixed access point (cellular systems, WLAN, WMAN)

Wired network

o Mobile-access point connection


Centralized broadcast channel

o Cellular coverage: The territory coverage is obtained by Base Stations–BS (or Access Points) that provide radio access to Mobile Stations–MS within a service area called CELL

BaseStation

MobileStation

Cell


Distributed broadcast channel

o Ad-hoc wireless networks (mesh networks, sensor networks)

o mobile- mobile connections


Distributed broadcast channel

o In multi-hop operation mobile stations can forward information

sourcedestinationrelay

relay


Wired-Wireless networks:Main differences

o Shared transmission mediumè Multiple access mechanismsè Radio resource reuse

Centralswith

cable

Radiochannel



o Radio channelè Variable channel characteristicsè Advanced modulation and coding

schemes


o User mobilityè Stand-by mobilityè Active session (conversation) mobility



Mobility managemento In wireless networks, users can roam in the

service area moving among cellso This require an adaptive routing of

information based on user position in the network

o All wireless networks have a set of mobility management mechanisms to track user position


Mobility management: cellular systems

o In cellular systems mobility management adopts different procedures based on user state IDLE (no active call) or ACTIVE (in conversation)n ACTIVE: dynamic call rerouting cell-by-

cell (Handover)n IDLE: user position tracking (Cell

selection, Location Update, Paging)


Mobility management: Cell selection

o Base stations transmit periodically general system information and cell identity on a broadcast channel

o User terminals scan all channels to received broadcast transmissions from nearby base stations

o User terminal select autonomously the best cell, usually based on signal strength


Mobility management: Location Update

o Location Area: set of cellso User position tracking is based on location areas

and not on cellso The currently visited LA is stored in a data base

LA 1LA 2

Data Base


Mobility management: Location Update

o When a mobile terminal in idle state move in a different LA a Location Update procedure is started

o Information in the data base is used to route incoming call to mobile terminals

LA 2

Data Base

LA 1


Mobility management: Paging

o When a call must be routed to a mobile terminal the currently visited LA is retrieved from the data base

o Then the paging procedure is startedo All base stations in the LA broadcast a paging

message with the ID of the called usero When the mobile terminal replies the call is

routed

Data Base

pagingpaging

reply


Mobility management: Handover

o Handover is always triggered by the network based on measurements performed by the mobile terminal (received powers, quality, etc.)

o Handover procedures must be fast to avoid quality degradation

Dt t

Handover TH

Receiver THh


Mobility management: Data networks (WLAN, WMAN, …)

o Within the same network, mobility is managed at layer two

o Among different networks mobility is managed at layer three (using e.g. Mobile IP)

A

RAP2AP1

A


Multiple access


Multiplexing and multiple access

o Different information flows sharing the same physical channel

o One transmitting station: multiplexingo Many transmitting stations (one per flow):

multiple access


Multiplexing and multiple access

AM

Node 1

AM

Node 4

AM

Node 3

AM

Node 2

Broadcast channel

MPX DMPX

Node 1 Node 2

MultipleAccess

Multiplexing


Wireless networks: Multiplexing

o One transmitting station

Typical problem indownlink (forward link) of cellular systems (base station – mobile users)


Wireless networks: Multiple access

o Several transmitting stations (coordination problem)

Typical uplink (reverse link) problem in cellular systems (from users to base station)

o A duplexing technique is also needed for sharing between uplink and downlink channels


Wireless networks: Frequency reuse

o The radio resource is limited and can not be exclusively dedicated to a channel in a cell

o The same radio resource is used in different cells sufficiently far apart to not interfere

o Critical problem with a trade off between number and quality of channels

o … see later on


Multiple access

o From now on “Multiple access” includes also multiplexing and duplexing

o Multiple access at the physical layer: A single channel is divided into subchannels using physical parameters (frequency, time, code) – static resource management

o Multiple access at logical layer: packet access with logical information in the packet header and distributed coordination mechanisms – dynamic resource management

o In real systems different multiple access techniques at physical and logical layers are usually combined together


FDM/FDMA (Frequency Division Multiplexing/Multiple Access)

o Available bandwidth is divided into sub-bands and assigned to different sub-channels

o Simple technique used basically in all systems

fmin fmax

mod.f


TDM/TDMA (Time Division Multiplexing/Multiple Access)

o Time is divided into slotso Groups of N consecutive slots are organized

into frameso A subchannel can use a given slot in all

frames

1 2 3 4 5 1 2 3 4 53 4 5 1

frame frame

... ...

slot


TDMA: Guard time

)2(max iigT t=


TDMA: reduced guard time

o Timing Advance:n If the propagation

delay t is known it can be compensated anticipating the transmission (centralized access only!)

n t must be dynamically estimated and signaled back to the mobile

1) First transm.

3) Delay signaled back

2) Delay estimation 4) Subsequent transm.with reduced guard time


CDM/CDMA (Code Division Multiplexing/Multiple Access)

o Symbols (bits) on the channels are multiplied by a code

o In CDM codes are orthogonal, while in CDMA they have limited correlation

0

0)()(

211

210

=×

=×

å

ò

=ii

N

i

T

cc

tCtCC1(t)

C2(t)


CDM/CDMA (Code Division Multiplexing/Multiple Access)

+

s1

sN

s2

sN

s2

s1

C1 C1

C2 C2

CN CN

ò å =×÷ø

öçè

æ -

=Tkk

N

iii sCCs

1

0


CDMA: spreading and despreading

o The code “expands” the radio bandwidth of the signal

S(f)f

SM(f)f

B

nB

Spreading ofthe radio spectrum

n number of chips in the code: “spreading factor” (SF)



o Different signals use the same radio band

sM1(t)

sM2(t)

+fnB



o At receiver the signal is multiplied by the code (de-spreading)

fnB fBDe-spreading

fB

o The interfence of the other signals is reduced by 1/n


Packet access

o At logical layer multiple access can be managed in a dynamic and distributed way using multiple access protocols

o First multiple access protocols have been designed for LANs

o Nowadays multiple access protocols are mainly used in wireless networks (no more shared medium wired LANs)

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