MSIT 413: Wireless Technologies - Northwestern...

N O R T H W E S T E R N MSIT | Master of Science in Information Technology

U N I V E R S I T Y

MSIT 413: Wireless Technologies Week 5

Michael L. Honig Department of EECS

Northwestern University

January 2016


U N I V E R S I T Y

Outline

•  MIMO •  Multiple Access techniques

–  FDMA, TDMA –  OFDMA (LTE) –  CDMA (3G, 802.11b, Bluetooth) –  Random Access


U N I V E R S I T Y

Diversity •  Idea: Obtain multiple independent copies of the received signal.

–  Improves the chances that at least one is not faded.

•  Macroscopic (space): copies of signal are received over distances spanning many wavelengths.

•  Microscopic (space): copies of signal are received over distances spanning a fraction of a wavelength

•  Different types…


U N I V E R S I T Y

Microscopic Space Diversity

•  Want signals s1 and s2 to experience independent fading (why?). –  distance between antennas should be ≥ ½ wavelength. –  Ex: 900 MHz, λ = c/f ≈ 1/3 meter

2 GHz, λ ≈ 0.15 meter

Antenna 1 Antenna 2

s1 s2


U N I V E R S I T Y

Multiple Antennas: Multi-Input/Multi-Output (MIMO) Channel

Multi-Channel Detector

Transmitted

Data (multiple data streams)

Estimated Data

•  Multiple (M) antennas at receiver and transmitter •  Channel has multiple inputs and multiple outputs.

5


U N I V E R S I T Y

Single Transmit Antenna


Transmitted

Data (single stream)

Estimated Data

•  Multiple receiver antennas provides spatial diversity •  Lowers error rate

•  “Single-Input/Multiple-Output (SIMO)” channel

6


U N I V E R S I T Y

Multi-Input/Single Output (MISO) Channel

Single-Channel Detector

Transmitted

Data (single or multiple streams)

Estimated Data

•  Transmitting the same symbol from all transmitters provides transmit spatial diversity (e.g., select the best antenna, turn the others off).

•  Practical for cellular downlink. 7


U N I V E R S I T Y

Downlink Beamforming

Different beams can use the same frequency!

Narrow “beam” focused on one user

8

•  M antennas at the base station (single or multiple antennas at mobiles)

•  Can support up to M data streams. •  Multi-user MIMO: multiple users on the same channel

–  Introduced in LTE, 802.11ac


U N I V E R S I T Y

OFDM Signal

Orthogonal Frequency Division Multiplexing (OFDM)

Split into M substreams source

bits

Modulate Carrier f1

+

Modulate Carrier f2

Modulate Carrier fM

substream 1

substream 2

substream M …


U N I V E R S I T Y

Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel

•  Multiple (M) antennas at receiver and transmitter.


Transmitted Data Estimated

Data

10


U N I V E R S I T Y


•  Multiple (M) antennas at receiver and transmitter. •  Transmitted data is divided into M substreams, one for each antenna.

•  Transmit antennas are used to multiplex multiple data streams.


Substream 1

Estimated Data Substream M

11


U N I V E R S I T Y



•  Transmit antennas are used to multiplex multiple data streams. •  Multiple receiver antennas (plus signal processing) are used to

remove interference from the different antennas.


Substream 1

Estimated Data Substream M

12


U N I V E R S I T Y



•  Transmit antennas are used to multiplex multiple data streams. •  Multiple receiver antennas (plus signal processing) are used to

remove interference from the different antennas. •  Data rate (Shannon capacity) is proportional to M!


Estimated Data

Substream 1

Substream M

13


U N I V E R S I T Y

WiFi Evolution: 802.11n

•  Technology based on OFDM with multiple antennas at the transmitter and receivers

•  Supports data rates up to 540 Mbps –  4 spatial streams, 40 MHz bandwidth –  Can replace USB 2.0 connections.

•  Also important part of 802.11ac (multi-user MIMO)

14


U N I V E R S I T Y

The Multiple Access Problem

•  Frequency-Division (AMPS)

•  Time-Division (GSM)

•  Code-Division (3G, Bluetooth) Direct Sequence/Frequency-Hopped

•  Orthogonal Frequency Division Multiple Access (OFDMA)

•  Random Access (Wireless Data)

How can multiple mobiles “access” (communicate with) the same base station?


U N I V E R S I T Y

Duplexing (Two-way calls)

Frequency-Division Duplex (FDD)

Time-Division Duplex (TDD)

Channel 1

Channel 2

Time slot (frame) 1

Time slot (frame) 2


U N I V E R S I T Y

Combinations

•  FDMA/FDD (AMPS) •  TDMA/FDD (GSM) •  TDMA/TDD (IS-136 or 2G in the U.S.) •  CDMA/FDD (IS-95, CDMA2000) •  CDMA/TDD (3G/UMTS) •  Frequency-Hopped CDMA/TDD (Bluetooth) •  OFDMA/TDD and FDD (WiMax, 4G)


U N I V E R S I T Y



•  Time-Division (IS-136, GSM)

•  Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped



How can multiple mobiles access (communicate with) the same base station?


U N I V E R S I T Y

Cellular Spectrum (50 MHz)

A* A B A* B*

uplink

downlink

824

869 870 880 890 891.5 894

825 835 845 846.5 849

AMPS (1G): 30 kHz Channels

416 FDD Channels (requires 12.5 MHz): •  395 FDD voice channels •  21 FDD control channels


U N I V E R S I T Y

Properties of FDMA •  Can be analog or digital (AMPS is analog).

•  Narrowband: channel contained within coherence

bandwidth – undergoes flat fading.

•  Low capacity

•  Best for circuit-switched (dedicated) connections.

•  Requires guard channels for adjacent channel interference.


U N I V E R S I T Y



•  Time-Division (GSM)






U N I V E R S I T Y

Frame

H 1 2 N H . . .

N time slots H: Frame Header

Time Division Multiple Access

Time slots

H 1 2 N H . . .

H 1 2 N H . . .

Channel f1

Channel f2

Channel fK

. . .


U N I V E R S I T Y

Frame

H 1 2 N H . . . N time slots H: Frame Header

Data to or from user K + control information

Time Slot

Preamble and synch

Guard time

Time Slot


U N I V E R S I T Y

H 1 2 H

TDMA/Time-Division Duplex

3 4 5 6

Uplink time slots Downlink time slots { {


U N I V E R S I T Y

Properties of TDMA •  Data transmission occurs in bursts.

–  Must ensure small delays for speech. –  High peak to average power on reverse link.

•  Can measure signal strength in idle time slots (e.g., for handoff).

•  Can assign multiple time slots for higher data rates.

•  Significant overhead/complexity for synchronization. –  Guard times needed between time slots for delay spread.

•  May require an equalizer to mitigate intersymbol interference.


U N I V E R S I T Y

Global System for Mobile Communications (GSM)

•  Originated in Europe –  Main objective: allow roaming across countries –  Incompatible with 1G systems

•  More than an air-interface standard: specifies wireline interfaces/functions

•  TDMA/FDMA, FDD •  Dynamic frequency assignment •  200 kHz channels •  270.833 kbps


U N I V E R S I T Y

GSM Frame Structure

•  200 kHz FDD channels divided into 8 time slots per frame •  Total number of available channels = (12.5 MHz – 2 X Guard Band)/200 kHz

100 kHz guard bands è 62 channels •  Total number of traffic channels = 8 X 62 = 496 •  Channel data rate = 270.839 kbps •  Without overhead, data rate/user = 24.7 kbps

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7

4.615 ms

156.25 bits 576.92 µs

T0 T1 T2 T22 T23 T24 I/S T10 T11 T12 S T13 T14 T15 ... … 120 ms

Tn: nth TCH frame S: Slow Associated Control Channel frame I: Idle frame

Speech Multiframe = 26 TDMA frames

Frame


U N I V E R S I T Y

GSM Frame Structure


100 kHz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = 270.839 kbps Without overhead, data rate/user = 24.7 kbps

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7

4.615 ms

156.25 bits 576.92 µs




Frame


U N I V E R S I T Y

GSM Frame Structure


100 kHz guard bands è 62 channels •  Total number of traffic channels = 8 X 62 = 496 •  Channel data rate = 270.839 kbps •  Without overhead, data rate/user = 24.7 kbps

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7

4.615 ms

156.25 bits 576.92 µs




Frame


U N I V E R S I T Y

GSM Time Slots

0 1 2 3 4 5 6 7

4.615 ms

120 ms

6.12 s

3 57 1 26 1 57 3 8.25

576.92 µs

“Normal Burst” Traffic Channel (TCH)

•  148 bits/time slot •  114 coded information bits •  Frame efficiency 74% (total bits – overhead bits)/(total bits)

Superframe

Multiframe

Frame

Time slot

Hyperframe = 2048 superframes lasts ~3 hrs 28 min 54 sec

≈


U N I V E R S I T Y

GSM Capacity

•  Total bandwidth = 12.5 MHz, 200 kHz channels è 62 channels –  With cell cluster size N=3 (typical),

capacity is (62/3) x 8 ~ 165 users/cell

N O R T H W E S T E R N

U N I V E R S I T Y

MSIT | Master of Science in Information Technology





•  Orthogonal Frequency Division Multiple Access (OFDMA) (WiMax, LTE)




U N I V E R S I T Y


OFDM Signal

Orthogonal Frequency Division Multiplexing (OFDM)

Split into M

substreams source

bits

Modulate Carrier f1

+

Modulate Carrier f2

Modulate Carrier

fM

substream 1

substream 2

substream M


U N I V E R S I T Y


OFDM Spectrum

f4 frequency

Power

f1 ß 0 f2 f5 f6 f3

subchannels

Total available bandwidth

… …

M “subcarriers, or subchannels, or tones” “Orthogonal” subcarriers è no cross-channel interference.

Data spectrum for a single carrier


U N I V E R S I T Y


OFDM vs OFDMA

•  OFDM is a modulation technique for a particular user. •  OFDMA is a multiple access scheme

(allows many users to access a single receiver). •  Can OFDM be combined other multiple access

techniques?


U N I V E R S I T Y


OFDM vs OFDMA

•  OFDM is a modulation technique for a particular user. •  OFDMA is a multiple access scheme

(allows many users to access a single receiver). •  Can OFDM be combined other multiple access

techniques? –  Yes, e.g., FDMA and TDMA. –  OFDMA is different…


U N I V E R S I T Y


OFDM vs OFDMA Overall

User 1

User 2

User 3 User 4

Overall

User 1

User 2

User 3 User 4

OFDM with FDMA

OFDM users are assigned adjacent frequency bands.

Frequency diversity is determined by (BW of signal)/(coherence BW)

OFDMA

User subcarrier assignments are permuted across the entire available frequency band.

So what??


U N I V E R S I T Y


OFDM vs OFDMA Overall

User 1

User 2

User 3 User 4

Overall

User 1

User 2

User 3 User 4

OFDM (with FDMA)

OFDM users are assigned adjacent frequency bands.

Frequency diversity is determined by (BW of signal)/(coherence BW)

OFDMA

User subcarrier assignments are permuted across the entire available frequency band.

Each sub-carrier may experience independent fading. Frequency diversity is determined by the number of sub-carriers.

Also provides interference diversity.


U N I V E R S I T Y


•  Each user can be assigned a time/frequency slice. •  Requires a time/frequency scheduler.

OFDM/TDMA and OFDMA TDMA

TDMA\OFDMA

t

N

m

OFDM/TDMA:

subchannels

Each color represents a different user, which is assigned particular time slots.

Different sub-carriers can be assigned to different users.

time slot t


U N I V E R S I T Y


WiMax OFDMA Frame Structure (TDD example)

(downlink) (uplink)


U N I V E R S I T Y


Adaptive Rate Control

frequency

channel gain

f1 f2

large channel gain è higher data rate small channel gain

è lower data rate

•  How can we control the rate per subchannel? –  Change the modulation format (e.g., choose from QPSK/16-QAM/64 QAM) –  Change the code rate (i.e., change the number of redundant bits)

•  Requires feedback from receiver to transmitter


U N I V E R S I T Y


•  Use different frequencies (FDMA)

•  Use different time slots (TDMA)

•  Use different pulse shapes (CDMA)



U N I V E R S I T Y

Code Division Multiple Access •  Users transmit simultaneously over

the same frequency band •  Performance limited by interference


U N I V E R S I T Y

Two-User Example

time User 1:

T T/2

time T

T/2

User 2:

T 2T 3T 4T 5T

s2(t) 1 0 1 1 0

T 2T 3T 4T 5T s1(t)

1 1 0 1 0 bits:

T 2T 4T 5T

received signal r(t)= s1(t)+s2(t)

How to recover each users’ bits? 3T

2

-2

1

-1


U N I V E R S I T Y

Chip Sequence

time T

T/2

User 2:

T 2T 3T 4T 5T

s2(t) 1 0 1 1 0

chips

bits:

1 -1 1 1 -1 Transmitted chips: -1 1 -1 -1 1

chip duration Tc

symbol duration T=2Tc

User 2’s chip sequence (1, -1) is called a signature.


U N I V E R S I T Y

Chip Sequence

time T T/2

User 1:

chips

1 1 -1 1 -1 Transmitted chips: 1 1 -1 1 -1

chip duration Tc

symbol duration T=2Tc

T 2T 3T 4T 5T s1(t)

1 1 0 1 0 bits: 1

-1

User 1’s signature is (1, 1).


U N I V E R S I T Y

Two-User Example

T 2T 3T 4T 5T

s2(t)

T 2T 3T 4T 5T s1(t)

T 2T 4T 5T

r(t)= s1(t)+s2(t)

3T

2

-2

1

-1

1 -1 1 1 -1 Transmitted chips: -1 1 -1 -1 1

2 0 0 2 -2 Received chips: 0 2 -2 0 0

1 1 -1 1 -1 Transmitted chips: 1 1 -1 1 -1

1 0 1 1 0

1 1 0 1 0


U N I V E R S I T Y

Correlation

Given two sequences: a1, a2, a3, …, aN

b1, b2, b3, …, bN

The correlation between the sequences is defined as:

(a1 x b1) + (a2 x b2) + (a3 x b3) + … + (aN x bN)

Examples: 1 1 1 1 1 correlated with 1 1 1 1 1 = 5 1 1 1 1 1 correlated with 1 -1 1 -1 1 = 1 2 4 1 3 1 correlated with -2 1 8 2 0 = -4 + 4 + 8 +6 + 0 = 14

If the correlation between two sequences is zero, they are said to be orthogonal.


U N I V E R S I T Y

Correlator Receiver

r(t) Sample Chips

Correlate with desired user’s signature

Bit Decision < 0 à 0 > 0 à 1

estimated bits


U N I V E R S I T Y

Why Does This Work?

Correlate with User 1’s signature

A1 s1 + A2 s2

signature (1,1) amplitude


A2 s2 0

The user signatures are orthogonal.

Now observe that:


A1 s1

2A1

2A1


U N I V E R S I T Y

Correlator, or “Matched Filter” Receiver


A1s1+ A2s2 User 1’s bits


User 2’s bits

The correlator is “matched” to user 1’s signature s1, and rejects s2 (and vice versa).




U N I V E R S I T Y

Observations •  Users transmit simultaneously (not TDMA). •  Users overlap in frequency (not FDMA).

frequency

Spectrum: User 1

0

signal bandwidth is roughly 1/T

frequency 0

signal bandwidth is roughly 1/Tc = 2/T Spectrum: User 2

Bandwidth expansion (factor of 2) è “spread spectrum” signaling.


U N I V E R S I T Y

Users and Bandwidth Expansion To guarantee orthogonal signatures (no interference), the length of the signatures must be ≥ the number of users.

Example (4 users):

time T/2 T/4 User 3: 3T/4

T time T/2 T/4 User 4: 3T/4

T

time T/2 T/4 User 2: 3T/4

T time T/2 T/4 User 1: 3T/4

T

The chip rate is 4 times the symbol rate, hence the bandwidth expansion is a factor of 4.

signature: 1 1 1 1 signature: 1 1 -1 -1

signature: 1 -1 -1 1 signature: 1 -1 1 -1


U N I V E R S I T Y

Correlator Receiver (4 users)


s1+ s2 + s3 + s4 User 1’s bits


User 2’s bits




User 3’s bits Bit Decision < 0 à 0 > 0 à 1


User 4’s bits Bit Decision < 0 à 0 > 0 à 1


U N I V E R S I T Y

DS-CDMA Transmitter

Spreader (generate chips)

Modulator (e.g., QPSK)

Source bits chips RF signal

Ex: 100 source symbols, processing gain = 10 è 1000 chips

X

sin 2πfct

Baseband signal Passband (RF) signal

time

Nyquist chip shape

Tc


U N I V E R S I T Y

Orthogonality and Asynchronous Users

•  Orthogonality among users requires: –  Synchronous transmissions –  No multipath

T 2T 3T 4T 5T s1(t)

1

-1

1 1 0 1 0

T 2T 3T 4T 5T

s2(t) 1 0 1 1 0

time

Asynchronous users can start transmissions at different times.

Chips are misaligned è signatures are no longer orthogonal!


U N I V E R S I T Y

Correlator, or “Matched Filter” Receiver


s1(t) + s2(t-τ)


The multiple access interference adds to the background noise and can cause errors. For this reason, CDMA is said to be interference-limited. Because CDMA users are typically asynchronous, and because of multipath, it is difficult to maintain orthogonal signatures at the receiver. Consequently, in practice, the signatures at the transmitter are randomly generated.



delay Signal 1 + multiple acess interference (MAI) From user 2

Signal 2 + multiple acess interference (MAI) From user 1


U N I V E R S I T Y

Processing Gain (PG)

The performance of CDMA depends crucially on the Processing Gain:

Bandwidth of spread signal / Symbol rate (minimum bandwidth needed)

or equivalently, Number of chips per symbol


U N I V E R S I T Y

Processing Gain (PG)

The performance of CDMA depends crucially on the Processing Gain:

Bandwidth of spread signal / Symbol rate (minimum bandwidth needed)

or equivalently, Number of chips per symbol

Fundamental tradeoff: increasing the PG

•  decreases the correlation between random signatures. •  decreases interference. •  increases the bandwidth of the signal.


U N I V E R S I T Y

Example

•  IS-95 (2G CDMA) –  Total bandwidth = 1.25 MHz –  Data rate = 9.6 kbps –  PG ≈ 130

•  3G/CDMA2000

–  Total bandwidth = 1.25 MHz –  Data rate varies between 14.4. kbps (voice)

up to 2 Mbps (1X-DO) –  PG varies from 1 to 130


U N I V E R S I T Y

Properties of CDMA •  Robust with respect to interference

•  No frequency assignments (eases frequency planning)

•  Asynchronous

•  High capacity with power control –  Power control needed to solve near-far problem.

•  Wideband: benefits from frequency/path diversity.

•  Benefits from voice inactivity and sectorization. •  No loss in trunking efficiency.

•  Soft capacity: performance degrades gradually as more users are added.

•  Soft handoff


U N I V E R S I T Y

Near-Far Problem SO… THEN THE THIRD

TIME I CALLED CUSTOMER SERVICE, I SAID &%$#%^…


Near-Far Problem

User 1

User 2

amplitude A1

amplitude A2

Output of correlator receiver is signal + interference. As the interferer moves closer to the base station, the interference increases. In practice, power variations can be up to 80 dB!

Conclusion: User 1’s signal is overwhelmed by interference from user 2!


U N I V E R S I T Y


Closed-Loop Power Control

User 1

User 2

•  Base station gives explicit instructions to mobiles to raise/lower power. •  Needed to solve near-far problem (equalizes received powers). •  Introduced by Qualcomm in the late 80’s.

raise power

lower power


U N I V E R S I T Y


Closed-Loop Power Control: Properties

User 1

User 2

•  Crucial part of CDMA cellular systems (IS-95, 3G). •  Minimizes battery drain. •  Complicated (increases cost) •  Requires overhead: control bits in feedback channel to tell transmitter

to lower/raise power

raise power

lower power


U N I V E R S I T Y


Properties of CDMA •  Robust with respect to interference

•  No frequency assignments (eases RF planning).

•  Asynchronous

•  High capacity with power control. –  Power control needed to solve near-far problem.

•  Wideband: benefits from frequency/path diversity.

•  Benefits from voice inactivity and sectorization. –  No loss in trunking efficiency.

•  Soft handoff


U N I V E R S I T Y


Soft Handoff (CDMA) ”Make before break”

MSC

BSC BSC

MSC

BSC BSC

MSC

BSC BSC

Hard Handoff (TDMA)

MSC

BSC BSC

MSC

BSC BSC

MSC

BSC BSC

BEFORE DURING AFTER


U N I V E R S I T Y


Applications of Spread-Spectrum

•  Cellular •  Military (preceded cellular applications) •  Wireless LANs (overlay)


U N I V E R S I T Y


Military Spread Spectrum

•  Can “hide” a signal by “spreading it out” in the frequency domain. •  Requires a very large PG (several 100 – 1000). •  Enemy must know spreading code (the “key” containing 100’s of

bits) to demodulate – too complicated for simple search. •  Spread spectrum signals have the “LPI/LPD” property:

low probability of intercept / low probability of detect.

Spread spectrum used for covertness, not multiple access.

frequency 0 frequency 0

spread

noise level


U N I V E R S I T Y


Applications of Spread-Spectrum

•  Cellular •  Military (preceded cellular applications) •  Wireless LANs (WiFi)


U N I V E R S I T Y


Spread Spectrum Underlay •  FCC requirements on spectrum sharing in the unlicensed

(Industrial, Scientific, Medical (ISM)) bands: –  “Listen before talk” –  Transmit power is proportional to the square root of the

bandwidth. •  Spread spectrum signaling is robust with respect to a narrowband

interferer. •  To a narrowband signal, a spread spectrum signal appears as

low-level background noise.

frequency

spread spectrum signal telemetry hospital monitor


U N I V E R S I T Y


Frequency-Hopped CDMA

Idea: “Hop” from channel to channel during each transmission.

time

frequ

ency

f1

f4

f3

f2

f5

User 1: blue

time slots


U N I V E R S I T Y


Frequency-Hopped CDMA

Idea: “Hop” from channel to channel during each transmission.

time

frequ

ency

f1

f4

f3

f2

f5

User 1: blue User 2: red

time slots

collision bits are lost


U N I V E R S I T Y


Hop Rate •  Can make synchronous users orthogonal by assigning

hopping patterns that avoid collisions. •  “Fast” hopping generally means that the hopping

period is less than a single symbol period. •  “Slow” hopping means the hopping period spans a few

symbols. •  The hopping rate should be faster than the fade rate

(why?).


U N I V E R S I T Y


Hop Rate •  Can make synchronous users orthogonal by assigning

hopping patterns that avoid collisions. •  “Fast” hopping generally means that the hopping

period is less than a single symbol period. •  “Slow” hopping means the hopping period spans a few

symbols. •  The hopping rate should be faster than the fade rate

so that the channel is stationary within each hop.


U N I V E R S I T Y


Properties of FH-CDMA •  Exploits frequency diversity (can hop in/out of fades) •  Can avoid narrowband interference (hop around) •  No near-far problem (Can operate without power control) •  Low Probability of Detect/Intercept •  Spread spectrum technique – can overlay •  Cost of frequency synthesizer increases with hop rate •  Must use error correction to compensate for erasures due

to fading and collisions. •  Applications

–  Military (army) –  Part of original 802.11 standard –  Enhancement to GSM –  Bluetooth


U N I V E R S I T Y


Bluetooth: A Global Specification for Wireless Connectivity

•  Wireless Personal Area Network (WPAN).

•  Provides wireless voice and data over short-range radio links via low-cost, low-power radios (“wireless” cable).

•  Initiated by a consortium of companies (IBM, Ericsson, Nokia, Intel)

•  Standard has been developed (IEEE 802.15.1 ).


U N I V E R S I T Y


Bluetooth Specifications •  Allows small portable devices to communicate together in an ad-hoc

“piconet” (up to eight connected devices).

•  Frequency-hopped spread-spectrum in the 2.4 GHz UNII band.

•  Interferes with 802.11b/g/n

•  1600 hops/sec over 79 channels (1 MHz channels)

•  Range set at 10m.

•  Gross data rate of 1 Mbps (TDD). –  64 kbps voice channels

•  Second generation (Bluetooth 2.0+) supports rates up to 3 Mbps. Competes with Wireless USB.


U N I V E R S I T Y






•  Orthogonal Frequency Division (WiMax, 4G)

•  Random Access (802.11, wireless data)



U N I V E R S I T Y


802.11 Random Access

Terminals send/receive messages (packets) to/from the AP at random times (i.e., when they appear).

Station A

Station B

Station C

Access Point (AP)


U N I V E R S I T Y


Cellular Call Setup (Random Access) 1. Call Request 2. Send numbers to switch

4. Request Channel/Time slot/Code 3. Page Receiver


U N I V E R S I T Y


Medium Access Control (MAC)

•  Fixed assignment access –  Each user is assigned a dedicated channel, time slot, or code –  Appropriate for circuit-switched traffic, transferring long data

files

•  Random access: users contend for access to the channel –  Users may collide, losing packets. –  Sometimes can negotiate rate (bandwidth, time slots, codes)

and power –  Widely used in wired networks –  Used in wireless networks for requesting

channel/time slot/code, WiFi


U N I V E R S I T Y


Carrier Sense Multiple Access (CSMA)

Packet arrives

Sense channel Busy?

Delay transmission (non-persistent)

no

yes

Transmit packet

•  “Listen before talk” (LBT) protocol •  How do collisions occur?


U N I V E R S I T Y


Carrier Sense Multiple Access (CSMA)

Packet arrives

Sense channel Busy?

Delay transmission (non-persistent)

no

yes

Transmit packet

•  “Listen before talk” (LBT) protocol •  Collisions occur if transmitters cannot sense the other

transmission (e.g., due to large propagation delay) –  Lower probability of collision/higher throughput than ALOHA

•  Long propagation times è more collisions –  ALOHA preferred for wide area applications


U N I V E R S I T Y


CSMA Example


U N I V E R S I T Y


CSMA with Collision Detection (CSMA/CD)

•  Nodes detect a collision in progress, and stop transmitting before the entire packet is transmitted.

•  Assumes nodes can hear each other when they are transmitting.

•  Appropriate for wired channels. •  Problems with wireless channels:

–  Nodes cannot transmit and receive at the same frequency at the same time.

–  Not all nodes may be in range of each other.


U N I V E R S I T Y


Hidden Terminal Problem

•  A is transmitting to B. •  C wants to transmit to D.

Station A Station B

Station C

Station D


U N I V E R S I T Y


Hidden Terminal Problem

•  A is transmitting to B. •  C wants to transmit to D. •  C may not sense A’s transmission, causing a collision at B.

Station A Station B

Station C

Station D

Coverage area for station C.


U N I V E R S I T Y


Exposed Terminal Problem

•  B is transmitting to A. •  C wants to transmit to D

Station A Station B

Station C

Station D


U N I V E R S I T Y


Exposed Terminal Problem

•  B is transmitting to A. •  C wants to transmit to D. •  C senses B’s transmission, and does not transmit even though it

would not cause interference at A.

Station A Station B

Station C

Station D



U N I V E R S I T Y


Basic Problem

Carrier sensing determines whether or not there are interfering sources near the transmitter, not the receiver.


U N I V E R S I T Y


Solutions •  Busy-tone multiple access (BTMA)

–  Separate control channel used to indicate that the channel is idle or busy.

–  An active station transmits a busy tone on the control channel. –  Each receiver that senses a busy tone turns on its own busy tone. –  Used in ad hoc networks.

•  Digital or Data Sense Multiple Access (DSMA)

–  Used in FDD cellular mobile data networks –  Forward channel periodically broadcasts a busy/idle bit for the

reverse link. –  Mobile transmits if bit is in idle state; base station sets bit to busy. –  Not carrier sensing: sensing is performed after demodulation.

•  Multiple Access with Collision Avoidance (MACA)


U N I V E R S I T Y


Revealing the Hidden Terminal

•  A sends a “Request to Send (RTS)” packet to B.

Station A Station B

Station C

Station D

Coverage area for station C. RTS


U N I V E R S I T Y



•  A sends a “Request to Send (RTS)” packet to B. •  B sends a “Clear to Send (CTS)” packet to A; heard by C!

Station A Station B

Station C

Station D

Coverage area for station C. CTS


U N I V E R S I T Y



•  A sends a “Request to Send (RTS)” packet to B. •  B sends a “Clear to Send (CTS)” packet to A; heard by C! •  C is silent for duration of A’s transmission (specified in CTS)

Station A Station B

Station C

Station D

Coverage area for station C. data


U N I V E R S I T Y



What if C hears RTS, but not CTS?

Station A Station B

Station C

Station D



U N I V E R S I T Y


Exposed Terminal

C will not hear the CTS from A.

Station A Station B

Station C

Station D

Coverage area for station C. RTS


U N I V E R S I T Y


RTS Collision

RTS messages from E and B collide à exponential backoff

Station A Station B

Station C

RTS

Station E

RTS


U N I V E R S I T Y


Corrupted CTS

CTS message from A is corrupted due to interference from E à exponential backoff by B

Station A Station B

Station C

CTS

Station E

Data, RTS, or CTS


U N I V E R S I T Y


MACA Protocol (RTS/CTS)

•  Terminals receiving either an RTS or CTS must not transmit for the duration of the packet. (What if the terminal hears RTS but not CTS?)

•  Collision occurs if multiple nodes transmit an RTS, or the CTS is not heard due to other interference.

•  Collision è binary exponential back-off

Transmitter Receiver

Request to Send (RTS), packet length

Clear to Send (CTS), packet length

Data

Ack

MSIT 413: Wireless Technologies - Northwestern...

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