Introduction to Wireless Networking
Communication without Wires
Overview
❒ Wireless and Mobile Network Architecture❒ Wireless Link Characteristics❒ IEEE 802.11 Wireless LAN (WLAN) ❒ Cellular Networks
Wireless and Mobile Network Architecture
Wireless Is Everyone’s Favorite
❒ Number of wireless (mobile) phone subscribers now exceeds number of wired phone subscribers (5-to-1)!
❒ Number of wireless Internet-connected devices exceeds number of people on Earth!
❍ Laptops, smartphones, tablets promise anytime untethered Internet access
❒ Two important (but different) challenges
❍ Wireless: Communication over wireless access link
❍ Mobility: Maintaining Internet connectivity for a mobile device that changes its point of attachment to the access network
Architecture of a wireless network
network infrastructure
Wireless Hosts Laptop, smartphone,
tablet Run applications Limited battery life if
mobile May be stationary
(non-mobile) or mobile Wireless does not
always mean mobility
Devices
network infrastructure
Base Station Typically connected
to wired network Or point to point
wireless “backhaul”
Relay - responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers,
802.11 access points
Infrastructure
network infrastructure
Wireless Link Typically used to
connect mobile(s) to base station Also used as
backbone link Multiple access
protocol coordinates link access
Various data rates, transmission distance
Radio Access Network Link
network infrastructure
Infrastructure Mode
Base station connects mobiles into wired network
Handoff: mobile changes base station providing connection into wired network May or may not
change IP subnet
Infrastructure Mode
network infrastructure
Ad hoc mode No base stations Nodes can only
transmit to other nodes within link coverage
Nodes organize themselves into a network: route among themselves
Ad hoc Mode
A Word about Ad hoc Routing❒ Latencies in mobile ad hoc
networks can get very high❍ Each hop adds more latency❍ Movement means routing tables
need continual adjustment❍ Peculiarities of wireless link (next
up) make maintaining connectivity challenging
❒ Ad hoc networks have limited application❍ One or two hop sensor networks❍ Military applications (drones, etc.)❍ Small sized fixed ad hoc meshes
❒ Maximum size of ad hoc network:❍ 3 – 8 hops depending on
application❍ Practical size is 2 hops for mobile❍ Fixed ad hoc backbones can extend
The Internet
Wireless Link Characteristics
Bandwidth of selected wireless links
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m – 4 Km
Long-rangeoutdoor
5Km – 20 Km
.056
.384
1
4
5-11
54
2G: IS-95, CDMA, GSM
2.5G: GSM-Edge, CDMA2000
802.11b
802.11a,g
3G: UMTS/WCDMA/HSPDA, CDMA2000-1xEVDO
4G: LTE
802.11a,g point-to-point
200 802.11n
Dat
a ra
te (
Mbp
s)
1000 802.11ac
cellular protocol in regulated spectrum
IEEE 802.11 WLANin unregulated spectrum
5G peakRate: 10-20 Gbps!
Wireless Network Taxonomy
single hop multiple hops
infrastructure(e.g., APs)
noinfrastructure
host connects to base station (WiFi,
cellular) which connects to
larger Internet
no base station, noconnection to larger Internet (Bluetooth,
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger
Internet: mesh net, cellularrelay
no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node
Signal propagation characteristics of wireless links makes maintaining connectivity a significant
challenge!
Basic Sources of Signal Loss on Wireless Link❒ Signal power decreases in proportion to the
distance from the sender:❍ For free space propagation:❍ True of wired signals as well but not as steep
❒ Interference from other sources ❍ Devices using the same band
• Other devices communicating• Microwave ovens for WLAN on 2.4 GHz band• Wireless phones, garage door openers, baby monitors
❍ Radio frequency interference (RFI) inadvertently radiated• Solar panel inverters and power supplies• AC motors and wiring
Controlling interference is the Number One taskof radio protocols!
Signal Loss from Motion and the Urban Environment❒ Multipath Interference
❍ Radio signal reflects off objects
❍ Arrives at a slightly different time
❍ Reduces signal strength
❒ Doppler Shift❍ As the mobile device
moves, the frequency of the signal changes slightly
❍ Changes channel response
Other Problems with Wireless ChannelsMultiple wireless senders and receivers create
additional problems (beyond multiple access):
AB
C
Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each
other means A, C unaware of their interference at B
A B C
A’s signalstrength
space
C’s signalstrength
Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each
other interfering at B
Signal to Noise Ratio (SNR) and Bit Error Ratio (BER)❒ SNR: signal-to-noise ratio
❍ larger SNR – easier to extract signal from noise (a “good thing”)
❒ SNR versus BER tradeoffs❍ Given physical layer: increase
power -> increase SNR->decrease BER
❍ Given SNR: choose physical layer that meets BER requirement, giving highest thruput• SNR may change with
mobility: dynamically adapt physical layer (modulation technique, rate)
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)B
ER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
HighLevelModulationScheme
Techniques for Controlling Interference❒ Geographical Separation
❍ Divide the geographic area into cells and use some property of radio to isolate devices in a cell• Frequency• Spreading Code
❒ Channel Multiplexing❍ Devices share the channel by:
• Either:– Each device gets a fine grained access identifier
(timeslot, spreading code, etc.)• Or:
– Each device is responsible for arranging its transmission such that the channel is contention-free
Geographical Separation❒ Break geographic coverage area into “cells”
❍ Power falls off quickly as distance
❒ Use channel multiplexing within and between cells to allow devices within a cell to share frequency❍ Within a cell, devices are distinguished by their multiplexing identifier❍ When the mobile moves to a new cell, change multiplexing identifier
❒ Handover❍ When signal strength goes below a certain point, switch over new cell’s
base stationFine grained technique
distinguishes
Access and Frequency❒ Most cellular wireless protocols use different frequencies for uplink
and downlink❍ GSM, 3G, FD-LTE❍ Exception: TD-LTE uses time division duplex to split the same channel
between uplink and downlink• For operators who don’t have licenses for paired spectrum
❒ Wireless LAN protocols use different frequencies to reduce interference between cells❍ 802.11, 802.11a, 802.11b, 802.11n, 802.11ac❍ CSMA/CA uses to share channel between uplink and downlink
Source: http://gnuradio.org/redmine/attachments/download/146/gsm900_arfcn1.jpg
Source: http://alpha.tmit.bme.hu/meresek/2_4_ghz_wi-fi_channels_802_11bg_wlan.gif
European 802.11g channels, 2.4 GHz Band (22 MHz channels, 5 MHz center separation)
GSM900 Uplink and Downlink
Channel Multiplexing❒ Time Division Multiple Access (TDMA)
❍ Each device gets a specific time slot
❒ Code Division Multiple Access (CDMA)❍ Each device gets a specific spreading code
❒ Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)❍ Check if someone is using channel first, random backoff if so
❒ Frequency Hopping Spread Spectrum (FHSS)❍ Device changes frequency according to a prenegotiated random
sequence❍ Likelihood of interference is low❍ Also slow frequency hopping
• Secondary technique
❒ Orthogonal Frequency Division Multiple Access (OFDMA)❍ Break bandwidth down into separate subcarriers❍ Assign each device a particular collection of subcarriers❍ Particularly well suited to MIMO (Multiple Input Multiple Output)❍ Best Bits per Hertz
MIMO – Efficient Utilization of Multiple Antennas❒ MIMO: Multiple Input Multiple Output❒ Around 2005, realization that multiple antennas could increase
throughput❍ Use multipath propagation to your advantage!❍ Maybe even signal steering to follow mobile
• Base station has lots of antennas• Device only has a couple
❍ Use of MIMO approximately doubles throughput!
❒ MIMO incorporated into all recent wireless standards❍ HSPA, LTE, 802.11n, 802.11ab
Source: http://www.proxim.com/images/technology/Mimo_01.png
IEEE 802.11 Wireless LAN (WLAN)
*
*
*
Notes:• * indicates uses MIMO• DSSS = Direct Sequence Spread Spectrum, SC = Single Carrier• all use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) for multiple
access• all have base-station and ad-hoc network versions
802.11 Wireless LAN Standard
Source: http://www.mpdigest.com/issue/Articles/2012/Aug/Aeroflex/Default.asp
802.11 LAN architecture
wireless host communicates with base station base station = access
point (AP) Basic Service Set
(BSS) (aka “cell”) in infrastructure mode contains: wireless hosts access point (AP): base
station ad hoc mode: hosts
only
BSS 1
BSS 2
Internet
hub, switchor router
802.11: Channels, association❒ Example 802.11b/g:
❍ 2.4GHz-2.485GHz spectrum in US❍ Divided into 11 channels in US, 13 in Europe, 14 in Japan❍ Access Point (AP) admin chooses channel for AP❍ Interference with neighboring AP
• Possible if both on same channel• Even if not on same channel because only one group of 3 channels
do not overlap• For 802.11a, 24 channels do not overlap
❒ Host must associate with an AP❍ Host scans channels, listening for beacon frames
containing AP’s name (Service Set IDentifier, SSID) and MAC address
❍ Selects AP to associate with❍ May perform authentication❍ Runs DHCP to get IP address in AP’s subnet
• If AP on same subnet as previous, no need to change address
802.11: passive/active scanning
AP 2AP 1
H1
BBS 2BBS 1
1
23
1
Passive Scanning: (1)Beacon frames sent
from APs(2)Association Request
frame sent: H1 to selected AP
(3)Association Response frame sent from selected AP to H1
AP 2AP 1
H1
BBS 2BBS 1
122
3 4
Active Scanning: (1)Probe Request frame
broadcast from H1(2)Probe Response frames
sent from APs(3)Association Request
frame sent: H1 to selected AP
(4)Association Response frame sent from selected AP to H1
IEEE 802.11: Multiple Access❒ Avoid collisions: 2+ nodes transmitting at same
time❒ 802.11: CSMA - sense before transmitting
❍ Don’t collide with ongoing transmission by other node
❒ 802.11: no collision detection❍ Difficult to receive (sense collisions) when transmitting
due to weak received signals (fading)❍ Can’t sense all collisions in any case: hidden terminal,
fading❍ Goal is to avoid collisions: CSMA/C(ollision)A(voidance)
space
AB
CA B C
A’s signalstrength
C’s signalstrength
IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender
1 If sense channel idle for DIFS then
transmit entire frame (no CD)
2 If sense channel busy then
start random backoff time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff interval, repeat 2
802.11 receiver
- If frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
DIFS: Distributed Interframe SpacingSIFS: Short Interframe Spacing
Avoiding Collisions for Long Data Frames❒ Basic idea:
❍ Allow sender to “reserve” channel rather than random access of data frames
❍ Avoids collisions for long data frames
❒ Sender first transmits small request-to-send (RTS) packets to BS using CSMA
❍ RTSs may still collide with each other (but they’re short)
❒ BS broadcasts clear-to-send CTS in response to RTS
❒ CTS heard by all nodes❍ sender transmits data frame❍ other stations defer transmissions avoid data frame collisions completely
using small reservation packets!
Collision Avoidance: RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 Frame: Addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 4: used only in ad hoc mode
InternetrouterH1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr
dest. address source address
802.3 frame
802.11 Frame: Addressing
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
duration of reserved transmission time (RTS/CTS)
frame seq #(for RDT)
frame type(RTS, CTS, ACK, data)
802.11 Frame: More
802.11: Mobility within same Subnet
❒ H1 remains in same IP subnet: IP address can remain same❍ If the subnet changes
then some IP mobility technique must be used
❒ switch: which AP is associated with H1?❍802.1 MAC learning:
switch will see frame from H1 and “remember” which switch port can be used to reach H1
H1 BBS 2BBS 1
Rate adaptation❒ base station, mobile
dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
QAM256 (8 Mbps)QAM16 (4 Mbps)
BPSK (1 Mbps)
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1. SNR decreases, BER increase as node moves away from base station
2. When BER becomes too high, switch to lower transmission rate but with lower BER
802.11: advanced capabilities
Power Management Node-to-AP: “I am going to sleep until
next beacon frame” AP knows not to transmit frames to this
node node wakes up before next beacon frame
Beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent node will stay awake if AP-to-mobile
frames to be sent; otherwise sleep again until next beacon frame
802.11: advanced capabilities
Cellular Networks
“The Gs”:Most Commonly Deployed
TheG
Approximate
DeploymentDate
Protocol Modulation/Access
MaximumDataRate
MaximumLatency
1G 1980’s US,AUS – AMPSSE,NO,FI,CH,NE,RU – NMTUK,DE- TACS, C-450JP – TZ-80x, JTACS,Many others
Analog, later TDMA for US (IS-136)
Predigital: noneUS IS-136: 48.6 kbps
1-5 s
2G 1990 for voice2000 for data
EU,rest of the world ex. JP and US: GSMUS: IS-95Japan: PDCA few others
GSM/GPRS: TDMAIS-95: CDMAPDC: TDMA
GPRS: 56–114 kbpsIS-95: 1.8-14.4 kbpsPDC: 9.6-28.8 kbps
1-5 s
3G 2000 World : UMTS, EDGEUS, Korea, few others: IS-2000
EDGE: TDMAUMTS: WCDMAIS-2000: CDMA
EDGE: 59.2-236.8 kbpsUMTS: 5.76-42 MbpsIS-2000: 153 kbps-14.7 Mbps
100-300 ms
4G 2010 World: LTEUS, Korea, few others: WiMax
LTE: OFDMA downlink SC-FDMA uplinkWiMax: OFDMA
LTE-Advanced: 500 Mbps-1GbpsWiMax: 56-128 Mbps
20-50 ms
5G Planned for 2022
Unknown Probably some version of LTE due to best b/Hz
Likely 1-10 Gbps <1 ms
Comparing WiFi with Cellular
❒ Standardized by 3GPP❒ Licensed Spectrum
❍ Varies depending on G
❒ Mobile data rate up to 100 Mbps for trains
❒ Cell size to 10’s of km❒ Expensive RBS
❍ Pico/microcell BS available
❒ Radio resource management co-ordinates radio use between cells
❒ Extensive access network with mobility management
❒ Support for policy, QoS, charging❍ Inherited from legacy telephone
network
❒ Standardized by IEEE❒ Unlicensed Spectrum
❍ ISM Bands: 2.4 GHz, 5 GHz, 60 GHz
❒ Fixed data rate up to 1 Gbps walking speed
❒ Cell size to 100 m
❒ Cheap APs❒ No radio resource
management between APs
❒ Access network is 802.1 Ethernet w. L2 mobility only
❒ No support for policy and charging❍ QoS support in 802.1
Cellular WiFi
Evolved Packet “Core”:Access Network for Cellular❒ Link types EPC supports:
❍ 2-4G radio access networks• Through 3G UTRAN Packet Core for 2-3G• 4G Packet Core is new
❍ WiFi, WiMax in “trusted” and “untrusted” forms• “trusted” means it uses the operator’s AAA system
❍ IS-2000 US CDMA 2K standard❍ Wired Broadband
• Probably not much deployment
❒ Radio Resource Management❍ Cells co-ordinate use of spectrum between themselves
• Share information about:– Spreading codes for CDMA/WCDMA– Subcarriers for OFDMA
❍ Reduces interference, improves reception quality
What Cellular Has That WiFi Doesn’t❒ Mobility management
❍ Control plane• GTP-C
– officially “GPRS Tunnelling Protocol” but long ago left GSM behind
❍ Data plane• GTP-U• PMIP Proxy Mobile IP• Dual Stack Mobile IP
❒ Policy, QoS, Charging❍ Policy control points❍ Ability to establish different QoS at radio, MAC, and
IP level❍ Charging and billing control points
For WiFi and WiMax
Detailed EPC Architecture2&3G
WiredBroadband
IS-2000
WiFi & WiMax
We focushere
Source: Olsson, et. al., “EPC and 4G Packet Networks”, Academic Press, 2013
Basic EPC For LTE
Operator WAN/Internet
eNode B
LTE Radio Access
Serving Gateway (SGW)
PacketData
Network Gateway (PGW)
Mobility Management
Entity(MME)
HomeSubscriber
Serivce(HSS)
Policy ControlAnd RatingFunction(PCRF)
OfflineChargingSystem(OFCS)
OnlineChargingSystem(OCS)
Rx
S5/S8
S1-U
X2
S1-MME
S6a
S11
S10
SGi
Gz Gy
Gx
GxcS9
We focus onmobility management
UserEquipment(UE)
EPC Mobility Management: More than Just IP Mobility❒ Terminals that are not actively
communicating are notified about incoming traffic❍ Allows terminals to go into power saving Idle
Mode, paged when traffic comes in
❒ Terminal can initiate traffic towards services on the Internet and other users
❒ Ongoing sessions are maintained as the user moves within or between access technologies❍ Through anchored IP mobility management
This is IP MobilityManagement
This is Idle Modepacket delivery
IP Mobility Management Basics❒ If an end host moves, its IP address must remain the
same to maintain session connectivity❍ IP addresses are bound into a topology
❒ Choice #1: Use /32 host routes, change routing in routers & switches as the host moves❍ Nobody does this❍ Requires too much bandwidth on control plane and makes all
the RIBs/FIBs too big
❒ Choice #2: Anchor host IP address back in the network, tunnel packets to host, move wireless end as host moves❍ Everybody does this❍ Minimizes control update messages and confines large RIB
size to the anchor
❒ EPC uses bearers to tunnel packets as host moves
Bearers❒ 3GPP’s term for a virtual network slice dedicated to a single IP
address❍ But there can be multiple TCP & UDP flows running over a single bearer
❒ Comes with a traffic classification for IP network and radio access QoS❍ Every UE gets a default bearer with Best Effort traffic classification❍ If a UE accesses a media service like voice calling, an enhanced bearer with
Enhanced QoS is brought up.
❒ Single direction only❍ Uplink or downlink
Source: http://www.artizanetworks.com/img/lte_sol_epc_fig04.jpg
RadioChannel GTP-U
GTP-U ❒ GTP-U is the tunnel protocol
for EPC❍ Like VxLAN or GRE
❒ UPD protocol:❍ The GTP-U port is 2152
❍ Version - Version number, 1❍ PT – Protocol type, differentiates
GTP (1) from GTP’ (0)❍ Sp – reserved, set to 0❍ E – set to 1 if there is an
extension header❍ S – set to 1 if there is a
sequence number❍ PN – set to 1 if there is an N-PDU
Source: http://flylib.com/books/en/4.215.1.86/1/
Encapsulation Header Fields
❍ Message type – indicates type of GTP message
❍ Length – length of payload❍ Tunnel Endpoint IDentifier (TEID) –
Identifies the endpoint of the tunnel❍ Sequence number –Number
increasing in value❍ N-PDU – used by the SGW for Radio
Access Network handover❍ Extension Header Type – Type of the
next extension header
Paging for LTE: Packet Delivery to Idle Mode UE
SGW
PGW
MME
Operator WAN/Internet
xy
Packet for xyand he’s asleep!
Hmm. He lastreported in inthe Blue TrackingArea
Broadcast onPaging Channelfor xy!xy where are you?
xy where are you?
xy where are you? xy where are you?
Please reactivatemy default bearer!
For xy
Idle Mode Packet Delivery❒ UE goes into Idle Mode when not in use
❍ Still listening to control channels but not transmitting❍ Saves power
❒ Tracking area❍ Collection of cells in which network is tracking a UE as it moves❍ Independent of L2 LAN and L3 subnet❍ Size and configuration depends on radio reception and geographic
area
❒ eNodeBs advertise tracking area ID in their control channel messages
❒ When a UE moves from one tracking area to another, updates network about its location
❒ Paging❍ When a packet arrives at the PGW for an idle mode UE, a locating
procedure is instituted in the UE’s last reported tracking area❍ Utilizes a special paging channel to which all idle mode UEs listen
Paging for LTE: Packet Delivery to Idle Mode UE
SGW
PGW
MME
Operator WAN/Internet
xy
Zzzz!Packet for xyand he’s asleep!
Hmm. He lastreported in inthe Blue TrackingArea
Broadcast onPaging Channelfor xy!xy where are you?
xy where are you?
xy where are you? xy where are you?
Huh?Oh, a packetmust have arrived!
Please reactivatemy default bearer!
For xy
Paging for LTE:Tracking Area Update (TAU)
Note: Paging systems are Radio Access Technology (RAT) dependent, 2 & 3G are slightly different
BlueTrackingArea (TA)
GreenTrackingArea (TA)
eNodeB: TA BlueUE: No need to change
eNodeB: TA GreenUE: Whoops! Need to change
MME
OldMME
HSS
I’ve changed toGreen!
Can you send meUE xy’s context?
xy
Sure, here it is.
Cancel xy’s contextin Old MME!
Cancel xy’s context!
Done.
Done. Here’s xy’ssubscriber data,
TAU update succeeded
Summary
❒ Wireless is a particularly challenging medium❍ Interference❍ Multi-path
❒ Clever coding tricks have increased channel capacity steadily in the last 20 years❍ From less than 100Kbps to over 6 Gbps
❒ 802.11 is a cheap and popular link technology❍ Uses CSMA/CA for controlling device access
• With RTS/CTS as a secondary technique
❍ Variety of different modulation techniques• FHSS, DSSS, OFDM, SC
❍ Connection into wired link is through 802.1 Ethernet• MAC learning handles mobility management at link layer
Acknowledgements
❒ Kurose and Ross, “Computer Networking: A Top Down Approach”, University of Massachusetts
Additional Slides
Wireless Channel Model
S RR = F(S,N,I)C = G(S,N,B)
The Wireless Channel
S = sent signal amplitudeR = received signal amplitudeN = noiseI = interference
C = channel capacityB = channel bandwidth
C
Statistical Model for Channel Response: Rayleigh Fading❒ Rayleigh distribution used to
model channel in urban environments❍ Lots of scattering❍ Lots of multipath
❒ where:❍ r = envelope amplitude of
received signal❍ 2σ2 = predetection mean
power of multipath signalSource: http://www.gaussianwaves.com/gaussianwaves/wp-content/uploads/2012/05/Rayleigh_PDF_envelope.jpg
Information Theory: Shannon-Hartley Theorem❒ Maximum rate at which information can be
transmitted over a channel of a specific bandwidth in the presence of noise❍ Establishes the channel capacity
❒ Where❍ C = channel capacity in bits / second❍ B = bandwidth of channel in hertz❍ S = average signal power in watts❍ N = the noise power in watts
❒ If channel rate R < C, then intelligent coding techniques can improve the rate
6-60
Code Division Multiple Access (CDMA)
❒ Unique “code” assigned to each user; i.e., code set partitioning❍ All users share same frequency in each cell,
but each user has own “chipping” sequence (i.e., code) to encode data
❍ Allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
❒ Encoded signal = (original data) X (chipping sequence)
❒ Decoding: inner-product of encoded signal and chipping sequence
CDMA encode/decode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zi,m= di.cmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zi,m
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Σ Zi,m.cmm=1
M
M
CDMA: two-sender interference
using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!
Sender 1
Sender 2
channel sums together transmissions by sender 1 and 2