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