Transcript M(g)

ECE 6332, Spring, 2014
Wireless Communication
Zhu Han
Department of Electrical and Computer Engineering
Class 25
April. 23rd, 2014
Outline

Adaptive Modulation and Coding

Diversity

Network Basics

OSI Model; TCP/IP Model
Adaptive Modulation

Change modulation relative to fading

Parameters to adapt:
– Constellation size
– Transmit power
– Instantaneous BER
– Symbol time
– Coding rate/scheme
Only 1-2 degrees of freedom needed for good performance

Optimization criterion:
– Maximize throughput
– Minimize average power
– Minimize average BER
Variable-Rate Variable-Power MQAM
One of the
M(g) Points
log2 M(g) Bits
Uncoded
Data Bits
M(g)-QAM
Modulator
Power: P(g)
Point
Selector
Delay
To Channel
g(t)
g(t)
BSPK
4-QAM
16-QAM
Goal: Optimize P(g) and M(g) to maximize R=Elog[M(g)]
Optimization Formulation

Adaptive MQAM: Rate for fixed BER
1.5g
P(g )
P(g )
M (g )  1 
 1  Kg
 ln( 5BER ) P
P

Rate and Power Optimization
P(g ) 

max E log 2 [ M (g )]  max E log 2 1  Kg
P (g )
P (g )
P 

Same maximization as for capacity, except for K=-1.5/ln(5BER).
Optimal Adaptive Scheme

Power Adaptation
g
P(g )  g10  g1K g  K0  g K

P  0
else

1
g
0
Spectral Efficiency
g
R
  log 
B g
g

2
K
K

 p(g )dg .

1
gK
gk
g
g
Equals capacity with effective power loss K=-1.5/ln(5BER).
Spectral Efficiency
K2
K1
K=-1.5/ln(5BER)
Can reduce gap by superimposing a trellis code
Constellation Restriction

Restrict MD(g) to {M0=0,…,MN}.

Let M(g)=g/gK*, where gK* is later optimized.

Set MD(g) to maxj Mj: Mj  M(g).

Region boundaries are gj=MjgK*, j=0,…,N

Power control maintains target BER
M3
M(g)=g/gK*
MD(g)
M3
M2
M2
M1
0
Outage
g0
M1
g1=M1gK*
g2
g3
g
Power Adaptation and Average Rate

Power adaptation:
– Fixed BER within each region


Es/N0=(Mj-1)/K
Channel inversion within a region
– Requires power increase when increasing M(g)
Pj (g )
P

( M j  1) /(gK ) g j  g  g j 1 , j  0

0
g  g1

Average Rate
R N
  log 2 M j p(g j  g  g j 1 )
B j 1
Efficiency in Rayleigh Fading
Practical Constraints

Constellation updates: fade region duration
j
tj 
 T  TM
N j 1  N j
t j  AFRD
TM  delay spread
N j  level crossing rate at min fade in region
N j 1  level crossing rate at max fade in region

Error floor from estimation error
– Estimation error at RX can cause error in absence of noise
(e.g. for MQAM)
– Estimation error at TX causes mismatch of adaptive power
and rate to actual channel

Error floor from delay: let r(t,t)=g(t-t)/g(t).
– Feedback delay causes mismatch of adaptive power and
rate to actual channel
Main Points

Adaptive modulation leverages fast fading to improve
performance (throughput, BER, etc.)

Adaptive MQAM uses capacity-achieving power and
rate adaptation, with power penalty K.
– Comes within 5-6 dB of capacity

Discretizing the constellation size results in negligible
performance loss.

Constellations cannot be updated faster than 10s to 100s
of symbol times: OK for most dopplers.

Estimation error/delay causes error floor
Diversity

Send bits over independent fading paths
– Combine paths to mitigate fading effects.

Independent fading paths
– Space, time, frequency, polarization diversity.

Combining techniques
– Selection combining (SC)
– Equal gain combining (EGC)
– Maximal ratio combining (MRC)

Can have diversity at TX or RX
– In TX diversity, weights constrained by TX power
Multiple Input Multiple Output (MIMO)Systems

MIMO systems have multiple (M) transmit and
receiver antennas

With perfect channel estimates at TX and RX,
decomposes to M indep. channels
– M-fold capacity increase over SISO system
– Demodulation complexity reduction

Beamforming alternative:
– Send same symbol on each antenna (diversity gain)
Beamforming

Scalar codes with transmit precoding
v1
x
vM t
v2
x1
x2
xM t
u2
u1
y
uM r
y=uHHvx+uHn
• Transforms system into a SISO system with diversity.
•Array and diversity gain
•Greatly simplifies encoding and decoding.
•Channel indicates the best direction to beamform
•Need “sufficient” knowledge for optimality of beamforming
• Precoding transmits more than 1 and less than RH streams
•Transmits along some number of dominant singular values
Diversity vs. Multiplexing

Use antennas for multiplexing or diversity
Error Prone
Low Pe
Diversity/Multiplexing tradeoffs (Zheng/Tse)
log Pe ( SNR)
lim
 d
SNR 
log SNR
R(SNR)
lim
r
SNR   log SNR

d (r)  (M t  r)(M r  r)
*
How should antennas be used?

Use antennas for multiplexing:
High-Rate
Quantizer

ST Code
High Rate
Use antennas for diversity
Low-Rate
Quantizer
ST Code
High
Diversity
Decoder
Error Prone
Decoder
Low Pe
Depends on end-to-end metric: Solve by optimizing app. metric
Multiaccess vs. Point-to-point

Multiaccess means shared medium.
– many end-systems share the same physical
communication resources (wire, frequency, ...)
– There must be some arbitration mechanism.

Point-to-point
– only 2 systems involved
– no doubt about where data came from !
Internetwork

Connection of 2 or more distinct (possibly dissimilar) networks.

Requires some kind of network device to facilitate the
connection.
Net A
Net B
Comparison

Speed and Range
ISO/OSI Reference Model

To address the growing tangle of incompatible
proprietary network protocols, in 1984 the ISO formed a
committee to devise a unified protocol standard.

The result of this effort is the ISO Open Systems
Interconnect Reference Model (ISO/OSI RM).

The ISO’s work is called a reference model because
virtually no commercial system uses all of the features
precisely as specified in the model.

The ISO/OSI model does, however, lend itself to
understanding the concept of a unified communications
architecture.
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
Dijkstra's algorithm
Dijkstra's algorithm - is a solution to the single-source shortest
path problem in graph theory.
Works on both directed and undirected graphs. However, all
edges must have nonnegative weights.
Approach: Greedy
Input: Weighted graph G={E,V} and source vertex v∈V, such
that all edge weights are nonnegative
Output: Lengths of shortest paths (or the shortest paths
themselves) from a given source vertex v∈V to all other vertices
Dijkstra's algorithm - Pseudocode
dist[s] ←0
(distance to source vertex is zero)
for all v ∈ V–{s}
do dist[v] ←∞
(set all other distances to infinity)
S←∅
(S, the set of visited vertices is initially empty)
Q←V
(Q, the queue initially contains all
vertices)
while Q ≠∅
(while the queue is not empty)
do u ← mindistance(Q,dist)
(select the element of Q with the min.
distance)
S←S∪{u}
(add u to list of visited vertices)
for all v ∈ neighbors[u]
do if dist[v] > dist[u] + w(u, v)
(if new shortest path found)
then d[v] ←d[u] + w(u, v) (set new value of shortest path)
(if desired, add traceback code)
return dist
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
Dijkstra Animated Example
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
Public-Key Cryptography
RSA

by Rivest, Shamir & Adleman of MIT in 1977

best known & widely used public-key scheme

based on exponentiation in a finite (Galois) field over
integers modulo a prime
– nb. exponentiation takes O((log n)3) operations (easy)

uses large integers (eg. 1024 bits)

security due to cost of factoring large numbers
– nb. factorization takes O(e
log n log log n)
operations (hard)
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?
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.
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.
Final Words

The world has been changed, have you?