Politecnico di Milano Facoltà di Ingegneria dell Informazione 3 Basic concepts Wireless Networks Prof. Antonio Capone
Wireless Networks Wireless or wired, what is better? Well, it depends on the situation! wireless wired o 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 Antonio Capone: Wireless Networks 2
Network architecture Backbone network Access network Antonio Capone: Wireless Networks 3
Wireless access networks o Wireless networks are mainly access networks o 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 4
Broadcast channel Centralized broadcast channel Distributed broadcast channel Antonio Capone: Wireless Networks 5
Centralized broadcast channel o Fixed access point (cellular systems, WLAN, WMAN) Wired network o Mobile-access point connection Antonio Capone: Wireless Networks 6
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 Base Station Mobile Station Cell Antonio Capone: Wireless Networks 7
Distributed broadcast channel o Ad-hoc wireless networks (mesh networks, sensor networks) o mobile- mobile connections Antonio Capone: Wireless Networks 8
Distributed broadcast channel o In multi-hop operation mobile stations can forward information relay source destination relay Antonio Capone: Wireless Networks 9
Wired-Wireless networks: Main differences o Shared transmission medium è Multiple access mechanisms è Radio resource reuse Central swith cable Radio channel Antonio Capone: Wireless Networks 10
Wired-Wireless networks: Main differences o Radio channel è Variable channel characteristics è Advanced modulation and coding schemes Antonio Capone: Wireless Networks 11
Wired-Wireless networks: Main differences o User mobility è Stand-by mobility è Active session (conversation) mobility Antonio Capone: Wireless Networks 12
Mobility management o In wireless networks, users can roam in the service area moving among cells o 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 13
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-bycell (Handover) n IDLE: user position tracking (Cell selection, Location Update, Paging) Antonio Capone: Wireless Networks 14
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 15
Mobility management: Location Update o Location Area: set of cells o User position tracking is based on location areas and not on cells o The currently visited LA is stored in a data base Data Base LA 1 LA 2 Antonio Capone: Wireless Networks 16
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 1 Data Base LA 2 Antonio Capone: Wireless Networks 17
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 started o All base stations in the LA broadcast a paging message with the ID of the called user o When the mobile terminal replies the call is routed paging paging reply Data Base Antonio Capone: Wireless Networks 18
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 h Handover TH Receiver TH Dt t Antonio Capone: Wireless Networks 19
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) AP1 AP2 R A A Antonio Capone: Wireless Networks 20
Multiple access Antonio Capone: Wireless Networks 21
Multiplexing and multiple access o Different information flows sharing the same physical channel o One transmitting station: multiplexing o Many transmitting stations (one per flow): multiple access Antonio Capone: Wireless Networks 22
Multiplexing and multiple access Node 1 Node 2 Multiplexing MPX DMPX Multiple Access Node 1 Node 2 Node 3 Node 4 AM AM AM AM Broadcast channel Antonio Capone: Wireless Networks 23
Wireless networks: Multiplexing o One transmitting station Typical problem in downlink (forward link) of cellular systems (base station mobile users) Antonio Capone: Wireless Networks 24
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 25
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 26
Multiple access o o o o From now on Multiple access includes also multiplexing and duplexing Multiple access at the physical layer: A single channel is divided into subchannels using physical parameters (frequency, time, code) static resource management Multiple access at logical layer: packet access with logical information in the packet header and distributed coordination mechanisms dynamic resource management In real systems different multiple access techniques at physical and logical layers are usually combined together Antonio Capone: Wireless Networks 27
FDM/FDMA (Frequency Division Multiplexing/Multiple Access) o Available bandwidth is divided into subbands and assigned to different subchannels o Simple technique used basically in all systems f mod. f min f max Antonio Capone: Wireless Networks 28
TDM/TDMA (Time Division Multiplexing/Multiple Access) o Time is divided into slots o Groups of N consecutive slots are organized into frames o A subchannel can use a given slot in all frames slot... 3 4 5 1 2 3 4 5 1 2 3 4 5 1... frame frame Antonio Capone: Wireless Networks 29
TDMA: Guard time Tg = max(2 i t i ) Antonio Capone: Wireless Networks 30
TDMA: reduced guard time o Timing Advance: n n If the propagation delay t is known it can be compensated anticipating the transmission (centralized access only!) t must be dynamically estimated and signaled back to the mobile 2) Delay estimation 3) Delay signaled back 4) Subsequent transm. with reduced guard time 1) First transm. Antonio Capone: Wireless Networks 31
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 C 1 (t) T ò 0 C 1 ( t) C 2 ( t) = 0 C 2 (t) N å i= 1 c 1i c 2i = 0 Antonio Capone: Wireless Networks 32
CDM/CDMA (Code Division Multiplexing/Multiple Access) s 1 s 1 s 2 C 1 C 1 C + 2 C 2 s 2 s N s N C N N æ - 1 ö òçå sici Ck = T è i= 0 ø s k C N Antonio Capone: Wireless Networks 33
CDMA: spreading and despreading o The code expands the radio bandwidth of the signal S(f) B f Spreading of the radio spectrum S M (f) nb f n number of chips in the code: spreading factor (SF) Antonio Capone: Wireless Networks 34
CDMA: spreading and despreading o Different signals use the same radio band s M1 (t) s M2 (t) + nb f Antonio Capone: Wireless Networks 35
CDMA: spreading and despreading o At receiver the signal is multiplied by the code (de-spreading) nb f De-spreading B f o The interfence of the other signals is reduced by 1/n B f Antonio Capone: Wireless Networks 36
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) Antonio Capone: Wireless Networks 37