Ece 4331 fall 2009
Download
1 / 45

ECE 4331, Fall, 2009 - PowerPoint PPT Presentation


  • 149 Views
  • Uploaded on

ECE 4331, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 26 Nov. 19 th , 2009. Outline. Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right?  General Wireless System Architecture

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'ECE 4331, Fall, 2009' - tevin


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Ece 4331 fall 2009 l.jpg

ECE 4331, Fall, 2009

Zhu Han

Department of Electrical and Computer Engineering

Class 26

Nov. 19th, 2009


Outline l.jpg
Outline

Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right? 

General Wireless System Architecture

Media Access Control

Classes of MAC protocols

Simplex and Duplex Channels

Coordinated MAC Schemes

FDMA

TDMA

Capacity of TDMA systems and which factors affect the capacity.

Spread Spectrum Access Methods

FHMA

Case study: Bluetooth

CDMA

Hybrid Spread Spectrum Schemes.

Case Study


Multiple access l.jpg
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.


Wireless system architecture and functions l.jpg
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


Medium access control l.jpg
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…)


Wireless media access control l.jpg
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.


Duplexing l.jpg
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


Duplexing8 l.jpg
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


Duplexing fdd l.jpg
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


Duplexing tdd l.jpg
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


Duplexing tdd versus fdd l.jpg
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…


Multiple access coordinated l.jpg
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.


Narrow and wideband systems l.jpg
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.


Narrow and wideband systems14 l.jpg
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



Frequency division multiple access l.jpg
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


Features of fdma l.jpg
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.


Capacity of fdma systems l.jpg
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.


Time division multiple access l.jpg
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


Tdma concept l.jpg
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.


Tdma frames l.jpg
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.


General frame and time slot structure in tdma systems l.jpg
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


A tdma frame l.jpg
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.


Efficiency of a frame tdma system l.jpg
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


Slide25 l.jpg
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.



Features of tdma l.jpg
Features of TDMA

Enables the sharing of a single radio channel among N users

Requires high data-rate per radio channel to support N users simultaneously.

High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread s parameter)

Transmission occurs in bursts (not continues)

Enables power saving by going to sleep modes in unrelated slots

Discontinues transmission also enables mobile assisted handoff

Requires synchronization of the receivers.

Need guard bits, sync bits. large overhead per slot.

Allocation of slots to mobile users should not be uniform.

It may depend on the traffic requirement of mobiles.

This brings extra flexibility and efficiency compared to FDMA systems.


Capacity of tdma systems l.jpg
Capacity of TDMA Systems

Capacity can be expressed as

System Capacity (the capacity of the overall system covering a region)

Depends on:

Range of cells

Whether the system can support macro-cells, micro-cells or pico-cells.

Cell Capacity

Depends on the radio link performance between a base-station and mobiles:

The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc.

Data-rate over the channel which dependsmodulation efficiency (bits_per_second/Hz) and channel bandwidth.

The frequency re-use factor


Spread spectrum access l.jpg
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


Frequency hopping fhma l.jpg
Frequency Hopping (FHMA)

Digital muliple access technique

A wideband radio channel is used.

Same wideband spectrum is used

The carrier frequency of users are varied in a pseudo-random fashion.

Each user is using a narrowband channel (spectrum) at a specific instance of time.

The random change in frequency make the change of using the same narrowband channel very low.

The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.

Rate of hopping versus Symbol rate

If hopping rate is greather: Called Fast Frequency Hopping

One bit transmitted in multiple hops.

If symbol rate is greater: Called Slow Frequency Hopping

Multiple bits are transmitted in a hopping period

GSM and Bluetooth are example systems


Capacity of cdma systems l.jpg
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


Capacity of cdma systems32 l.jpg
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:


Case study bluetooth l.jpg
Case Study - Bluetooth

Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel.

Uses 79 narrowband channels (carrier frequencies) to hop through.

Freq (f) = 2402+k MHz, k = 0,...,78

Channel spacing is 1 MHz (narrowband channel bandwidth)

Wideband spectrum width = 79 MHz.

Hopping Rate = 1600 Hops/Second

Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver

79 MHZ

0

1

2

3

.....

77

78

79-Hop System

1 MHZ

A hop sequence could be: 7,1,78,67,0, 56,39,.......


Case study bluetooth p iconet and fhss l.jpg
Case Study: Bluetooth – Piconet and FHSS

Each node is classified as master or slave.

Master defines a piconet (a cell). Maximum 7 slaves can be connected to

a master. Master coordinates access to the the media.

All traffic has to go over master.

Slaves can not talk to each-other

directly.

Picocell

S

Range = 10m

Raw Data-rate: 1 Mbps/piconet

Radio channel used by devices in

a piconet is 79MHz channel, which

Is frequency hopped: hopping

though 789 channels.

Hoprate = 1600 hops/sec

FHSS

M

S

S

All slaves and the master hops according to the same hopping sequence.

The hopping sequence is determined by the clock and BT_address of the master.


Case study bluetooth scatternet and fhss l.jpg
Case Study: Bluetooth – Scatternet and FHSS

Piconet

S

S

Piconet can be combined

into scatternets.

Red slave acts as a

bridge between two

piconets.

M2

Piconet

S

FHSS

S

FHSS

M1

Each piconet uses FHSS with different

hopping sequences (masters are different).

This prevents interference between piconets.

S

S


Case study bluetooth media access in a piconet l.jpg
Case Study: Bluetooth - Media access in a piconet

Inside a piconet, access to the

frequency hopped radio channel

is coordinated using time

division multiple access: TDMA/TDD.

Slot duration = 1/1600 sec = 625ms

Piconet

S1

FHSS

M

In an even slot, master transmits to a

slave.

In an odd slot, the slave that is addressed

in the previous master-to-slave slot transmits.

S3

S2

0 1 2 3 4 5 6 7 …..

M-S1

S1-M

M-S2

S2-M

M-S3

S3-M

M-S1

S1-M

……

slot time=625ms


802 11 l.jpg
802.11

  • 2.4G-2.4835G, 5.725-5.825G

  • 802.11a/g, OFDM, 802.11b: CDMA


Channel l.jpg
Channel

  • 11, 5.5, 2, 1Mbps



802 1140 l.jpg

Application

Presentation

ISO

OSI

7-layer

model

Session

IEEE 802

standards

Transport

Network

Logical Link Control

Data Link

Medium Access (MAC)

Physical

Physical (PHY)

802.11

  • 802.11a/g: 54, 48, 36, 24, 18, 6Mbps

  • 802.11e -MAC Enhancements-Security/QoS

  • 802.11f- Inter-Access Point Protocol

  • 802.11h- Spectrum Managed 5Ghz

  • 802.11i- Enhanced Security (TKIP and 802.1x)

  • 802. 11p- vehicular

  • 802. 11n- MIMO


Wireless hotpot planner l.jpg
Wireless hotpot planner

  • Wireless valley





Fail of iridium satellite system l.jpg
Fail of Iridium Satellite System

  • The system was originally to have 77 active satellites, and as such was named for the element iridium, which has atomic number 77.

  • Too few users per square miles, cost too much.

  • Chapter 11bankruptcy on August 13, 1999

  • In 70s, 12 calls per NYC.