Transcript Systematic Design of Space-Time Trellis Codes for Wireless

ECE 4371, Fall, 2014
Introduction to Telecommunication
Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 23
Nov. 19nd, 2014
Outline

Review of OSI model

Duplexing

Multiple Access
– Coordinated access




FDMA
TDMA
CDMA
OFDMA
– Random acess



Aloha
CSMA, RTS/CTS
CSMA/CD
ISO/OSI Reference Model

The OSI RM contains seven protocol layers, starting with physical
media interconnections at Layer 1, through applications at Layer 7.

OSI model defines only the functions of each of the seven layers
and the interfaces between them.

Implementation
details are not part
of the model.
ISO/OSI Reference Model: Physical Layer

The Physical layer receives a stream of
bits from the Data Link layer above it,
encodes them and places them on the
communications medium.

The Physical layer conveys
transmission frames, called Physical
Protocol Data Units, or Physical PDUs.
Each physical PDU carries an address
and has delimiter signal patterns that
surround the payload, or contents, of
the PDU.

Issues:
– mechanical and electrical interfaces
– time per bit
– distances
Modulation

Process of varying a carrier signal
in order to use that signal to
convey information
– Carrier signal can transmit far
away, but information cannot
– Modem: amplitude, phase, and
frequency
– Analog: AM, amplitude, FM,
frequency, Vestigial sideband
modulation, TV
– Digital: mapping digital
information to different
constellation: Frequency-shift
key (FSK)
ISO/OSI Reference Model: Data Link

The Data Link layer negotiates frame sizes
and the speed at which they are sent with
the Data Link layer at the other end.
– The timing of frame transmission is
called flow control.



Data Link layers at both ends acknowledge
packets as they are exchanged. The sender
retransmits the packet if no
acknowledgement is received within a given
time interval. ARQ
Medium Access Control - needed by
mutiaccess networks.
Issues:
– framing (dividing data into chunks)

header & trailer bits
– addressing
01100010011
10110000001
Automatic Repeat-reQuest (ARQ)

Alice and Bob on their cell phones
– Both Alice and Bob are talking

What if Alice couldn’t understand Bob?
– Bob asks Alice to repeat what she said

What if Bob hasn’t heard Alice for a while?
– Is Alice just being quiet?
– Or, have Bob and Alice lost reception?
– How long should Bob just keep on talking?
– Maybe Alice should periodically say “uh huh”
– … or Bob should ask “Can you hear me now?” 
Time-Division Multiplexing
Figure Block diagram of TDM system.
ISO/OSI Reference Model: Network




At the originating computers, the
Network layer adds addressing
information to the Transport layer
PDUs.
The Network layer establishes the
route and ensures that the PDU size
is compatible with all of the equipment
between the source and the
destination.
Its most important job is in moving
PDUs across intermediate nodes.
Issues:
– packet headers
– virtual circuits
London Metro Map
ISO/OSI Reference Model: Transport

the OSI Transport layer provides end-toend acknowledgement and error correction
through its handshaking with the Transport
layer at the other end of the conversation.
– The Transport layer is the lowest layer
of the OSI model at which there is any
awareness of the network or its
protocols.


Transport layer assures the Session layer
that there are no network-induced errors in
the PDU.
Issues:
– headers
– error detection: CRC
– reliable communication
Parity Check

Add one bit so that xor of all bit is zero
– Send, correction, miss
– Add vertically or horizontally

Applications: ASCII, Serial port transmission
ISO/OSI Reference Model: Session

The Session layer arbitrates the dialogue
between two communicating nodes,
opening and closing that dialogue as
necessary.

It controls the direction and mode (half duplex or full-duplex).

It also supplies recovery checkpoints during
file transfers.

Checkpoints are issued each time a block
of data is acknowledged as being received
in good condition.
Responsibilities:
– establishes, manages, and terminates sessions
between applications.
– service location lookup

ISO/OSI Reference Model: Presetation

The Presentation layer provides
high-level data interpretation
services for the Application
layer above it, such as
EBCDIC-to-ASCII translation.

Presentation layer services are
also called into play if we use
encryption or certain types of
data compression.
Responsibilities:

– data encryption
– data compression
– data conversion
Substitution Method

Shift Cipher (Caesar’s Cipher)
I CAME I SAW I CONQUERED
H BZLD H TZV H BNMPTDSDC
Julius Caesar to communicate with his army
Language, wind talker
ISO/OSI Reference Model

The Application layer supplies
meaningful information and services to
users at one end of the communication
and interfaces with system resources
(programs and data files) at the other
end of the communication.

All that applications need to do is to send
messages to the Presentation layer, and
the lower layers take care of the hard
part.

Issues:
– application level protocols
– appropriate selection of “type of service”

Responsibilities:
– anything not provided by any of the other
layers
TCP/IP Architecture
• TCP/IP is the de facto
global data
communications standard.
• It has a lean 3-layer
protocol stack that can be
mapped to five of the
seven in the OSI model.
• TCP/IP can be used with
any type of network, even
different types of networks
within a single session.
TCP/IP Architecture

The concept of the datagram was fundamental to the
robustness of ARPAnet, and now, the Internet.

Datagrams can take any route available to them
without human intervention.
Layering & Headers

Each layer needs to add some control information to the data to do it’s job.

This information is typically pre-pended to the data before being given to the
lower layer.

Once the lower layers deliver the data and control information - the peer layer
uses the control information.
DATA
Process
H
DATA
Transport
Network
H H
DATA
Network
Data Link
H H H
DATA
Data Link
Process
Transport
Protocols and networks in the TCP/IP model

How a call is made?
Summary





Physical: Language between two machines
Data-Link: communication between machines on the same
network.
Network: communication between machines on possibly
different networks.
Transport: communication between processes (running on
machines on possibly different networks).
Connecting Networks
– Repeater:
physical layer
– Bridge:
data link layer
– Router:
network layer
– Gateway:
network layer and above.
IEEE 802 Standards
The 802 working groups. The important ones are marked with *. The ones
marked with  are hibernating. The one marked with † gave up.
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.
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
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
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
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
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
Ack/Nack
Data
M3
M1
M2
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)
During this 2T period of time, the packet may collide
with someone else packet.
Contention for Aloha
Throughput of Aloha
Normalized
Throughput
0.2
~0.185
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0
0.2
0.4
0.5
0.6
0.8
1
Normalized
Channel Occupancy
Slotted Aloha
Reservation Protocols

Reservation Aloha

Packet Reservation 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.
ECE6331
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.
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.
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.
Exposed Terminal Problem
B’s cell
A
B
C’s cell
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.
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
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.
Solution for Hidden Terminal Problem
A is transmitting to B.
C’s cell
A’s cell
X
RTS(n)
X defers transmission
until expected CTS
reception time by RTS
sender.
A
RTS(n)
CTS(n)
Data(n)
B
CTS(n)
C
C defers transmission
for duration of n bytes of
data transmission. Node A
is no longer hidden from C
effectively.
Waiting time of node X is much smaller than waiting time of node C.
Solution for Exposed Terminal Problem
B is transmitting to A
B’s cell
A
RTS(n)
CTS(n)
Data(n)
B
C’s cell
RTS(n)
C
RTS(m)
D
CTS(m)
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.
CSMA/CA Collision Avoidance
RTS/CTS is used to reserve channel for
the duration of the packet transmission. This prevents
Access
Point hidden and exposed terminal
problems
Mobile
RTS
CTS
DATA
ACK
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
Ethernet: CSMA/CD
Figure 6-11: CSMA/CD process
Homework 5

Three questions
– Due 12/1

Exam review class 12/1

Next class 4.5G
ECE 4371 Fall 2008