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EE 6331, Spring, 2009 Advanced Telecommunication. Zhu Han Department of Electrical and Computer Engineering Class 23 Apr. 20 th , 2009. Outline. Review CDMA OFDM 2G-3G-4G Project 2 due on the exam Exam Coding is important: Linear and CRC CDMA Nyguist Three Criteria

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Ee 6331 spring 2009 advanced telecommunication

EE 6331, Spring, 2009Advanced Telecommunication

Zhu Han

Department of Electrical and Computer Engineering

Class 23

Apr. 20th, 2009


Outline

Outline

Review

CDMA

OFDM

2G-3G-4G

Project 2 due on the exam

Exam

Coding is important: Linear and CRC

CDMA

Nyguist Three Criteria

Term Presentation: 15 min., 13 slides, 2 questions, evaluation

Multiple Access: Chapter 5 of my book

Chapter 3 and 4 next week

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Spread spectrum transmission

Spread-spectrum transmission

  • Three advantages over fixed spectrum

    • Spread-spectrum signals are highly resistant to noise and interference. The process of re-collecting a spread signal spreads out noise and interference, causing them to recede into the background.

    • Spread-spectrum signals are difficult to intercept. A Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or simply an increase in the background noise for short Frequency-Hop codes on any narrowband receiver except a Frequency-Hop spread-spectrum receiver using the exact same channel sequence as was used by the transmitter.

    • Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

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Direct sequence spread spectrum

Direct Sequence Spread Spectrum

Unique code to differentiate all users

Sequence used for spreading have low cross-correlations

Allow many users to occupy all the frequency/bandwidth allocations at that same time

Processing gain is the system capacity

How many users the system can support

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Spreading despreading

Spreading & Despreading

  • Spreading

    • Source signal is multiplied by a PN signal: 6.134, 6.135

  • Processing Gain:

  • Despreading

    • Spread signal is multiplied by the spreading code

  • Polar {±1} signal representation

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Spreading despreading1

Spreading & Despreading

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Cdma example

CDMA Example

R

Receiver (a base station)

Data=1011…

Data=0010…

A

B

Transmitter

Transmitter (a mobile)

Codeword=101010

Codeword=010011

Data transmitted from A and B is multiplexed using CDMA and codewords.

The Receiver de-multiplexes the data using dispreading.

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Cdma example transmission from two sources

CDMA Example – transmission from two sources

1 0 1 1

A Data

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

0 1 0 0 1 1

A

Codeword

1 0 1 1 0 0

0 1 0 0 1 1

1 0 1 1 0 0

1 0 1 1 0 0

A Signal

0 0 1 0

B Data

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0

B

Codeword

1 0 1 0 1 0

0 1 0 1 0 1

1 0 1 0 1 0

1 0 1 0 1 0

B Signal

Transmitted

A+B

Signal

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Cdma example recovering signal a at the receiver

CDMA Example – recovering signal A at the receiver

A+B

Signal

received

A

Codeword

at

receiver

Integrator

Output

Comparator

Output

0 1 0 0

Take the inverse of this to obtain A

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Cdma example recovering signal b at the receiver

CDMA Example – recovering signal B at the receiver

A+B

Signal

received

B

Codeword

at

receiver

Integrator

Output

Comparator

Output

1 1 0 1

Take the inverse of this to obtain B

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Cdma example using wrong codeword at the receiver

CDMA Example – using wrong codeword at the receiver

A+B

Signal

received

Wrong

Codeword

Used at

receiver

Integrator

Output

Comparator

Output

X 0 1 1

Noise

Wrong codeword will not be able to decode the original data!

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Near far problem and power control

Near Far Problem and Power Control

  • At a receiver, the signals may come from various (multiple sources.

    • The strongest signal usually captures the modulator. The other signals are considered as noise

    • Each source may have different distances to the base station

  • In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time.

    • Therefore the receiver power at the base station for all mobile users should be close to eacother.

    • This requires power control at the mobiles.

  • Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.

B

pr(M)

M

M

M

M

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Frequency hopping spread spectrum

Frequency Hopping Spread Spectrum

  • Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

  • Military, bluetooth

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Orthogonal frequency division multiplexing

Orthogonal frequency-division multiplexing

  • Special form of Multi-Carrier Transmission.

  • Multi-Carrier Modulation.

    • Divide a high bit-rate digital stream into several low bit-rate schemes and transmit in parallel (using Sub-Carriers)

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Ofdm bit loading

OFDM bit loading

  • Map the rate with the sub-channel condition

  • Water-filling

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Road map

CDMA

(IS 95 A)

IS 95 B

GSM

GPRS

W-CDMA

3X

UWC-136

TDMA

EDGE

cdmaOne

IS-95A

1X

No 3X

Road Map

1XRTT/3XRTT

cdma2000

4G

1999

2000

2001

2002

IS-95B

3G Phase 1

3G Phase 2

2G

2.5G

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Multiple access

Multiple Access

How can we share a wireless channel:

Results in Wireless Media Access Control Protocols

How we can change base stations: Results in Handoff algorithms and protocols

How can we seamlessly support mobile applications over wireless links:

Results in mobility protocols like Mobile IP, Cellular IP, etc.

How can we design efficient transport protocols over wireless links:

Results in solutions like SNOOP, I-TCP, M-TCP, etc.

How different wireless networks/systems are designed?

Bluetooth, IEEE 802.11, GSM, etc.

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Wireless system architecture and functions

Wireless System Architecture and Functions

Applications

TCP/IP

Neighbor Discovery and Registration,

Multicasting, Power Saving Modes, Address

Translation (IP-MAC), Routing, Quality of Services,

Subnet Security

Wireless Subnet

Controller

Medium Access Control, MAC level Scheduling,

Link Layer Queueing, Link Layer Reliability – ACKs,

NACKs, ….

Wireless

Link Layer

(Layers 1 and 2 in ISO/OSI Network Reference Model)

Link Controller

Transceiver

Frame Controller

Framing and frame synchronization, error control,

CRC, bit scrambling, widening, ….

Carrier frequency, channel bandwidth, carrier detect,

Captude detect, channel data rate, modulation,

Received signal strength (RSSI), transmit power,

Power control, …

Physical Radio

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Medium access control

Medium Access Control

Wireless spectrum (frequency band) is a very precious and limited resource.

We need to use this resource very efficiently

We also want our wireless system to have high user capacity

A lot of (multiple) users should be able to use the system at the same time.

For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system.

The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…)

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Wireless media access control

Wireless Media Access Control

Random Schemes (Less-Coordinated)

Examples: MACA, MACAW, Aloha, 802.11 MAC,…

More suited for wireless networks that are designed to carry data: IEEE 802.11 Wireless LANs

Coordinated Schemes

Examples: TDMA, FDMA, CDMA

More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,…

Polling based Schemes

Examples: Bluetooth, BlueSky,…

Access is coordinated by a central node

Suitable for Systems that wants low-power, aims to carry voice and data at the same time.

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Duplexing

Duplexing

It is sharing the media between two parties.

If the communication between two parties is one way, the it is called simplex communication.

If the communication between two parties is two- way, then it is called duplex communication.

Simplex communication is achieved by default by using a single wireless channel (frequency band) to transmit from sender to receiver.

Duplex communication achieved by:

Time Division (TDD)

Frequency Division (FDD)

Some other method like a random access method

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Duplexing1

Duplexing

Usually the two parties that want to communication in a duplex manner (both send and receive) are:

A mobile station

A base station

Two famous methods for duplexing in cellular systems are:

TDD: Time Division Duplex

FDD: Frequency Division Duplex

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Duplexing fdd

Duplexing - FDD

  • A duplex channel consists of two simplex channel with different carrier frequencies

    • Forward band: carries traffic from base to mobile

    • Reverse band: carries traffic from mobile to base

F

M

B

R

Base

Station

Mobile

Station

Reverse

Channel

Forward

Channel

frequency

fc,,F

fc,R

Frequency separation

Frequency separation should be carefully decided

Frequency separation is constant

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Duplexing tdd

Duplexing - TDD

  • A single radio channel (carrier frequency) is shared in time in a deterministic manner.

    • The time is slotted with fixed slot length (sec)

    • Some slots are used for forward channel (traffic from base to mobile)

    • Some slots are used for reverse channel (traffic from mobile to base)

M

B

Mobile

Station

Base

Station

Slot number

0 1 2 3 4 5 6 7 …

F

R

F

R

F

R

F

R

….

channel

Reverse

Channel

Forward

Channel

time

Ti+1

Ti

Time separation

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Duplexing tdd versus fdd

Duplexing – TDD versus FDD

FDD

FDD is used in radio systems that can allocate individual radio frequencies for each user.

For example analog systems: AMPS

In FDD channels are allocated by a base station.

A channel for a mobile is allocated dynamically

All channels that a base station will use are allocated usually statically.

More suitable for wide-area cellular networks: GSM, AMPS all use FDD

TDD

Can only be used in digital wireless systems (digital modulation).

Requires rigid timing and synchronization

Mostly used in short-range and fixed wireless systems so that propagation delay between base and mobile do not change much with respect to location of the mobile.

Such as cordless phones…

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Multiple access coordinated

Multiple Access - Coordinated

We will look now sharing the media by more than two users.

Three major multiple access schemes

Time Division Multiple Access (TDMA)

Could be used in narrowband or wideband systems

Frequency Division Multiple Access (FDMA)

Usually used narrowband systems

Code Division Multiple Access

Used in wideband systems.

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Narrow and wideband systems

Narrow- and Wideband Systems

Narrowband System

The channel bandwidth (frequency band allocated for the channel is small)

More precisely, the channel bandwidth is large compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel)

AMPS is a narrowband system (channel bandwidth is 30kHz in one-way)

Wideband Systems

The channel bandwidth is large

More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel.

A large number of users can access the same channel (frequency band) at the same time.

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Narrow and wideband systems1

Narrow- and Wideband Systems

Narrowband Systems

Could be employing one of the following multiple access and duplexing schems

FDMA/FDD

TDMA/FDD

TDMA/TDD

Wideband systems

Could be employing of the following multiple access and duplexing schemes

TDMA/FDD

TDMA/TDD

CDMA/FDDCDMA/TDD

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Cellular systems and mac

Cellular Systems and MAC

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Frequency division multiple access

Frequency Division Multiple Access

  • Individual radio channels are assigned to individual users

  • Each user is allocated a frequency band (channel)

    • During this time, no other user can share the channel

  • Base station allocates channels to the users

B

fN,F

f1,F

f2,F

f2,R

f1,R

fN,R

M

M

M

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Features of fdma

Features of FDMA

An FDMA channel carriesone phone circuit at a time

If channel allocated to a user is idle, then it is not used by someone else: waste of resource.

Mobile and base can transmit and receive simultaneously

Bandwidth of FDMA channels are relatively low.

Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)

Lower complexity systems that TDMA systems.

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Capacity of fdma systems

Capacity of FDMA Systems

Frequency spectrum allocated for FDMA system

Guard

Band

channel

Guard

Band

Bt : Total spectrum allocation

Bguard: Guard band allocated at the edge of the spectrum band

Bc : Bandwidth of a channel

AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz

channel bandwidth (simplex): Number of channels is 416.

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Time division multiple access

Time Division Multiple Access

The allocated radio spectrum for the system is divided into time slots

In each slot a user can transmit or receive

A user occupiess a cyclically repeating slots.

A channel is logically defined as a particular time slot that repeats with some period.

TDMA systems buffer the data, until its turn (time slot) comes to transmit.

This is called buffer-and-burst method.

Leaky bucket

Requires digital modulation

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Tdma concept

TDMA Concept

Downstream Traffic: Forward Channels: (from base to mobiles)

1

2

3

N

1

2

3

….

N

Logical forward channel to a mobile

Base station broadcasts to mobiles on each slot

Upstream Traffic: Reverse Channels: (from mobile to base)

1

2

3

N

1

2

3

….

N

Logical reverse channel from a mobile

A mobile transmits to the base station in its allocated slot

Upstream and downstream traffic uses of the two different carrier frequencies.

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Tdma frames

TDMA Frames

Multiple, fixed number of slots are put together into a frame.

A frame repeats.

In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels.

In TDMA/FDD: a different carrier frequency is used for a reverse or forward

Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa).

Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame.

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General frame and time slot structure in tdma systems

General Frame and Time Slot Structure in TDMA Systems

One TDMA Frame

Preamble

Information

Trail Bits

Slot 1

Slot 2

Slot 3

Slot N

Guard

Bits

Sync

Bits

Control

Bits

Information

CRC

One TDMA Slot

A Frame repeats in time

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A tdma frame

A TDMA Frame

Preamble contains address and synchronization info to identify base station and mobiles to each other

Guard times are used to allow synchronization of the receivers between different slots and frames

Different mobiles may have different propagation delays to a base station because of different distances.

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Efficiency of a frame tdma system

Efficiency of a Frame/TDMA-System

  • Each frame contains overhead bits and data bits.

    • Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits.

bT: total number of bits in a frame

Tf: frame duration (seconds)

bOH: number of overhead bits

Number of channels in a TDMA cell:

m: maximum number of TDMA users supported in a radio channel

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Ee 6331 spring 2009 advanced telecommunication

TDMA

TDMA Efficiency

GSM: 30% overhead

DECT: 30% overhead

IS-54: 20% overhead.

TDMA is usually combined with FDMA

Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.

There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels)

For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA.

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Contemporary tdma systems

Contemporary TDMA Systems

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Spread spectrum access

Spread Spectrum Access

SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth.

Provides

Immunity to multipath interference

Robust multiple access.

Two techniques

Frequency Hopped Multiple Access (FHMA)

Direct Sequence Multiple Access (DSMA)

Also called Code Division Multiple Access – CDMA

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Capacity of cdma systems

Capacity of CDMA Systems

  • Uplink Single-cell System Model

  • Assumptions

  • Total active users Ku

  • The intra-cell MAI can be

  • modeled as AWGN

  • Perfect power control is assumed

  • Random sequences

User 2

...

User 1

User k

.

.

.

.

.

.

...

User n

User Ku

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Capacity of cdma systems1

Capacity of CDMA Systems

Coarse estimate of the reverse link (uplink) capacity

Assumptions:

Single Cell.

The interference caused by other users in the cell can be modeled as AWGN.

Perfect power control is used, i.e. the received power of each user at the base station is the same.

If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is

where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as:

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Scheduling and spectrum allocation

Scheduling and Spectrum Allocation

For TDMA system: whose information to transmit, Scheduling Problem

Round Robin

Opportunistic scheduling: channel good? transmits

Fairness: Max-min fair and proportional fair

Cross-layer design: delay issue

For FDMA system: where to load the bits

Bit loading problem

For OFDMA system

Time and frequency slots are assigned

Complicated assignment problem

Single Cell without interference case, or multicell interference case

Channel Allocation problem

Cognitive radio

Wireless ad hoc/sensor networks

Admission control

Reject the users if there is no more resources

Handoff has higher priority

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Random access

Random Access

Packet Radio Protocols

Multihop radio network that carries packets

Not circuit oriented like GSM, CDMA, etc.

Example Protocols

Pure Aloha

Slotted Aloha

CSMA Protocols

1-persistent CSMA

non-persistent CSMA

p-persistent CSMA

CSMA/CD

Reservation Protocols

Reservation Aloha

PRMA

Others

MACA, MACAW

IEEE 802.11 MAC

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Pure aloha

Pure Aloha

Algorithm:

A mobile station transmits immediately whenever is has data.

It then waits for ACK or NACK.

If ACK is not received, it waits a random amount of time and retransmits.

Ignoring the propagation delay between mobiles

and base station:

B

The time difference between the time

a mobile send the first bit of packet and the

time the base station receives the last bit of

the packet is given by 2T.

T = C/P

T: packet time.

C: channel data rate (bps)

P: packet length (bits)

Ack/Nack

Data

M3

M1

M2

During this 2T period of time, the packet may collide

with someone else packet.

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Contention for aloha

Contention for Aloha

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Throughput of aloha

Throughput of Aloha

Normalized

Throughput

~0.185

0.5

Normalized

Channel Occupancy

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Slotted aloha

Slotted Aloha

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Reservation protocols

Reservation Protocols

Reservation Aloha

Packet Reservation Multiple Access

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Csma carrier sen s e multiple access

CSMA: Carrier Sense Multiple Access

Aloha does not listen to the carrier before transmission.

CSMA listen to the carrier before transmission and transmits if channel is idle.

Detection delay and propagation delay are two important parameters for CSMA

Detection delay: time required to sense the carrier and decide if it is idle or busy

Propagation delay: distance/speed_of_ligth. The time required for bit to travel from transmitter to the receiver.

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Csma variations

CSMA Variations

1-persistent CSMA:

A station waits until a channel is idle. When it detects that the channel is idle, it immediately starts transmission

Non-persistent CSMA:

When a station receives a negative acknowledgement, it waits a random amount of time before retransmission of the packet altough the carrier is idle.

P-persistent CSMA

P-persistent CSMA is applied to slotted channels. When a station detects that a channel is idle, it starts transmission with probability p in the first available timeslot.

CSMA/CD

Same with CSMA, however a station also listen to the carrier while transmitting to see if the transmission collides with someone else transmission.

Can be used in listen-while-talk capable channels (full duplex)

In single radio channels, the transmission need to be interrupted in order to sense the channel.

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Maca medium access with collision avoidance

MACA – Medium Access with Collision Avoidance

CSMA protocols sense the carrier, but sensing the carrier does not always releases true information about the status of the wireless channel

There are two problems that are unique to wireless channels (different than wireline channels), that makes CSMA useless in some cases. These problems are:

Hidden terminal problem

Exposed terminal problem.

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Hidden terminal problem

Hidden Terminal Problem

C’s cell

A’s cell

A

B

C

Hidden

terminal

  • A is transmitting to B.

  • C is sensing the carrier and detects that it is idle (It can not hear A’s transmission).

  • C also transmits and collision occurs at B.

  • A is hidden from C.

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Exposed terminal problem

Exposed Terminal Problem

B’s cell

C’s cell

A

B

C

D

Exposed

terminal

  • B is transmitting to A. C is hearing this transmission.

  • C now wants to transmit to D. It senses the existence of carrier signal and

  • defers transmission to D.

  • However, C can actually start transmitting to D while B is transmitting to A,

  • Since A is out of range of C and C’s signals can not be heard at A.

  • C is exposed to B’s transmission.

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Maca solution concept

MACA Solution Concept

Ali, lets talk! I am available.

Can

Can, I want to talk to you!

Can, I want to talk to you!

Biltepe

Mountain

Ali

Veli

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Maca protocol

MACA Protocol

When a station wants to transmit data

It sends an RTS (Ready-to-Send) packet to the intended receiver

The RTS packet contains the length of the data that needs to be transmitted

Any station other than the intended recipient hearing RTS defers transmission for a time duration equal to the end of the corresponding CTS reception

The receiver sends back CTS (Clear-to-Send) packet back to sender if it is available to receive.

The CTS packet contains the length of the data that original sender wants to transmit

Any station other than the original RTS sender, hearing CTS defers transmission until the data is sent.

The original sender upon reception of the CTS, starts transmitting.

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Solution for hidden terminal problem

Solution for Hidden Terminal Problem

A is transmitting to B.

C’s cell

A’s cell

CTS(n)

RTS(n)

RTS(n)

X

A

B

C

CTS(n)

C defers transmission

for duration of n bytes of

data transmission. Node A

is no longer hidden from C effectively.

X defers transmission

until expected CTS

reception time by RTS

sender.

Data(n)

Waiting time of node X is much smaller than waiting time of node C.

ECE6331


Solution for exposed terminal problem

Solution for Exposed Terminal Problem

B is transmitting to A

B’s cell

C’s cell

RTS(n)

RTS(n)

A

B

C

D

RTS(m)

CTS(n)

CTS(m)

Data(n)

Data(m)

  • C defers transmission upon hearing B’s RTS until B could get CTS from A.

  • After that C can start transmission to D. For that it first sends an RTS.

  • C is not longer exposed to the data transmission of B.

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Csma ca collision avoidance

CSMA/CA Collision Avoidance

RTS/CTS is used to reserve channel forthe duration of the packet transmission. This prevents hidden and exposed terminalproblems

ACK is required to understand if the packet is correctly received (without any collisions ) at the receiver. Ethernet does not require ACK to be sent, since the transmitter can detect the collision on the channel (cable) without requiring an explicit feedback from the receiver.

A wireless transmitter can not detect collision, because:1) Transmit power is much larger than the received power: received signal is regarded as noise (not collision). 2) There could be a hidden terminal

Access Point

Mobile

RTS

CTS

DATA

ACK

ECE6331


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