Post on 24-May-2018
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!
Antonio Capone: Wireless Networks 3
Network architecture
Access network
Backbone network
Antonio Capone: Wireless Networks 4
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
Antonio Capone: Wireless Networks 5
Broadcast channel
Centralized broadcast channelDistributed broadcast channel
Antonio Capone: Wireless Networks 6
Centralized broadcast channel
o Fixed access point (cellular systems, WLAN, WMAN)
Wired network
o Mobile-access point connection
Antonio Capone: Wireless Networks 7
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
Antonio Capone: Wireless Networks 8
Distributed broadcast channel
o Ad-hoc wireless networks (mesh networks, sensor networks)
o mobile- mobile connections
Antonio Capone: Wireless Networks 9
Distributed broadcast channel
o In multi-hop operation mobile stations can forward information
sourcedestinationrelay
relay
Antonio Capone: Wireless Networks 10
Wired-Wireless networks:Main differences
o Shared transmission mediumè Multiple access mechanismsè Radio resource reuse
Centralswith
cable
Radiochannel
Antonio Capone: Wireless Networks 11
Wired-Wireless networks:Main differences
o Radio channelè Variable channel characteristicsè Advanced modulation and coding
schemes
Antonio Capone: Wireless Networks 12
o User mobilityè Stand-by mobilityè Active session (conversation) mobility
Wired-Wireless networks:Main differences
Antonio Capone: Wireless Networks 13
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
Antonio Capone: Wireless Networks 14
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)
Antonio Capone: Wireless Networks 15
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
Antonio Capone: Wireless Networks 16
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
Antonio Capone: Wireless Networks 17
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
Antonio Capone: Wireless Networks 18
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
Antonio Capone: Wireless Networks 19
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
Antonio Capone: Wireless Networks 20
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
Antonio Capone: Wireless Networks 21
Multiple access
Antonio Capone: Wireless Networks 22
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
Antonio Capone: Wireless Networks 23
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
Antonio Capone: Wireless Networks 24
Wireless networks: Multiplexing
o One transmitting station
Typical problem indownlink (forward link) of cellular systems (base station – mobile users)
Antonio Capone: Wireless Networks 25
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
Antonio Capone: Wireless Networks 26
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
Antonio Capone: Wireless Networks 27
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
Antonio Capone: Wireless Networks 28
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
Antonio Capone: Wireless Networks 29
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
Antonio Capone: Wireless Networks 30
TDMA: Guard time
)2(max iigT t=
Antonio Capone: Wireless Networks 31
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
Antonio Capone: Wireless Networks 32
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)
Antonio Capone: Wireless Networks 33
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
Antonio Capone: Wireless Networks 34
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)
Antonio Capone: Wireless Networks 35
CDMA: spreading and despreading
o Different signals use the same radio band
sM1(t)
sM2(t)
+fnB
Antonio Capone: Wireless Networks 36
CDMA: spreading and despreading
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
Antonio Capone: Wireless Networks 37
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)