Transcript EE359 – Lecture 20 Outline

Course Summary

Overview/history of wireless communications (Ch. 1)

Signal Propagation and Channel Models (Ch. 2 + 3)

Fundamental Capacity Limits (Ch. 4)

Modulation and Performance Metrics (Ch. 5)

Impact of Channel on Performance (Ch. 6)

Adaptive Modulation (Ch. 9)

Diversity (Ch. 7)

Spread Spectrum (Ch. 13)

Cellular Networks (Ch. 15)
Future Wireless Networks:
The Vision
Ubiquitous Communication Among People and Devices
Wireless Internet access
Nth generation Cellular
Wireless Ad Hoc Networks
Sensor Networks
Wireless Entertainment
Smart Homes/Spaces
Automated Highways
All this and more…
• Hard Delay/Energy Constraints
• Hard Rate Requirements
• +++
“Mega-themes” of TTT4160-1

The wireless vision poses great technical challenges

The wireless channel greatly impedes performance





Low fundamental capacity.
Channel is randomly time-varying
ISI and other interference must be compensated for
...
Hard to provide performance guarantees (needed for multimedia!).

We can compensate for flat fading using diversity or adaptation.

(MIMO channels promise a great capacity increase.)

A plethora of ISI compensation techniques exist

Various tradeoffs in performance, complexity, and implementation.
Design Challenges, cont’d

Wireless channels are a difficult and capacitylimited broadcast communications medium

Traffic patterns, user locations, and network
conditions are constantly changing

Applications are heterogeneous - with hard
constraints that must be met by the network(s)

Energy, delay, and rate constraints change design
principles across all layers of the protocol stack
(cross-layer design)
Signal Propagation:
Main effects

Path Loss

Shadowing

Multipath
d
Pr/Pt
d=vt
Statistical Multipath Model


Random # of multipath components, each with varying
amplitude, phase, doppler, and delay
Narrowband channel (signal BW smaller than coherence
BW): FLAT fading



Signal amplitude varies randomly (complex Gaussian).
Characterized by 2nd order statistics (Bessel function), average fade
duration, etc.
Wideband channel: FREQUENCY-SELECTIVE

Characterized in general by channel scattering function (simplified:
Modulation Considerations

We want: high rates, high spectral efficiency, high
power efficiency, robustness to channel variations,
cheap implementations... Trade-off required!

Linear Modulation (MPAM, MPSK, MQAM)
Information encoded in amplitude/phase
 More spectrally efficient than nonlinear
 Easier to adapt to channel conditions.
 Issues: differential encoding, pulse shaping, bit mapping.


Nonlinear modulation (FSK)
Information encoded in frequency
 More robust to channel and amplifier nonlinearities

Linear Modulation in AWGN

ML detection induces decision regions

Example: 8PSK
dmin

Ps (symbol error rate) depends on



# of nearest neighbors
Minimum distance dmin (depends on gs)
Approximate expression:

Ps   M Q  M g s


M is # of nearest neighbors; M relates dmin and average
symbol energy.
Linear Modulation in Fading
In fading gs - and therefore Ps - is random
 Metrics: outage probability, average Ps , or
combined outage and average.

Ts
Outage
Ps
Ps(target)
Ps
Ts
Ps   Ps (g s ) p(g s )dg s
Moment Generating
Function (MGF) Approach

Simplifies average Ps calculation

Uses alternate Q function representation

Ps reduces to MGF of gs-distribution
Closed form, or simple numerical calculation
for general fading distributions
 In general: Fading greatly increases average
Ps .

Doppler Effects

High Doppler causes channel phase to
decorrelate between symbols

Leads to an irreducible error floor for
differential modulation


Increasing power does not reduce error
Error floor depends on BDTs product (higher the
larger it is)
ISI Effects

Delay spread exceeding one symbol time
causes ISI (self-interference).
0

ISI leads to irreducible error floor


Tm
Increasing signal power increases ISI power
ISI requires that Ts>>Tm (Rs<<Bc)
Capacity of Flat Fading
Channels

Three cases
 Fading statistics known
 Fade value known at receiver
 Fade value known at receiver

and transmitter
Optimal Adaptation
 Vary rate and power relative to channel
 Optimal power adaptation is water-filling
 Exceeds AWGN channel capacity at low SNRs
 Suboptimal techniques come close to capacity
Variable-Rate Variable-Power MQAM
One of the
M(g) Points
log2 M(g) Bits
Uncoded
Data Bits
M(g)-QAM
Modulator
Power: S(g)
Point
Selector
Delay
To Channel
g(t)
g(t)
BSPK
4-QAM
16-QAM
Goal: Optimize S(g) and M(g) to maximize EM(g)
Optimal Adaptive Scheme

Power Water-Filling
S (g )  g  g

S
 0
1
1
K
0

1
g
g  g
0
g0
K
1
gK
K
else
gk
g
Spectral Efficiency
g 
R
  log   p(g )dg .
B g
g 

g
2
K
K
Equals Shannon capacity with an effective power loss of K.
Practical Adaptation Constraints

Constellation restriction

Constant power restriction

Constellation updates.

Estimation error.

Estimation delay.

Lead to practical adaptive modulation schemes
(Ch. 9)
Diversity

Send bits over independent fading paths
 Combine

Independent fading paths - how to create?
 Space,

paths to mitigate fading effects.
time, frequency, polarization diversity.
Combining techniques
 Selection combining (SC)
 Equal gain combining (EGC)
 Maximal ratio combining (MRC)
 ...
Diversity Performance

Maximal Ratio Combining (MRC)






Optimal technique (maximizes output SNR)
Combiner SNR is the sum of the branch SNRs.
Distribution of SNR hard to obtain.
Can use MGF approach for simplified analysis.
Exhibits 10-40 dB gains in Rayleigh fading.
Selection Combining (SC)




Combiner SNR is the maximum of the branch SNRs.
Diminishing returns with # of antennas.
CDF easy to obtain, pdf found by differentiating.
Can get up to about 20 dB of gain.
Spread Spectrum

Signal occupies channel bandwidth much
larger than actual signal bandwidth

Two main types:
 Direct Sequence Spread Spectrum (DSSS)
 Frequency Hopping Spread Spectrum

Focus on DSSS here

Basis for CDMA
Direct Sequence
Spread Spectrum (DSSS)

Bit sequence modulated by chip sequence
s(t)
S(f)
sc(t)
Sc(f)
S(f)*Sc(f)
1/Tb
Tc
Tb=KTc
1/Tc

Spreads bandwidth by large factor (K)

Despread by multiplying by sc(t) again (sc(t)=1)
2

Mitigates ISI and narrowband interference


ISI mitigation a function of code autocorrelation
Must synchronize to incoming signal
RAKE Receiver

Multibranch receiver

y(t)
Branches synchronized to different MP components
x
Demod
sc(t)
x
Demod
sc(t-iTc)
x
Diversity
Combiner
d^k
Demod
sc(t-NTc)

These components can be coherently combined

Use SC, MRC, or EGC
CDMA: Multiple Access SS

Interference between users mitigated by
code cross correlation

In downlink, signal and interference
have same received power

In uplink, “close” users drown out “far”
users (near-far problem)


Bandwidth Sharing in general

FDMA
Code Space
Time
Frequency

Code Space
TDMA
Time
Frequency
Code Space


7C29822.033-Cimini-9/97
CDMA
(Hybrid Schemes)
Time
Frequency
Multiuser Detection

In all CDMA systems and cellular systems in general,
users interfere with each other.

In most of these systems the interference is treated as
noise.



Systems become interference-limited
Often uses complex mechanisms to minimize impact of
interference (power control, smart antennas, etc.)
Multiuser detection exploits the fact that the structure
of the interference is known


Interference can be detected and subtracted out
Must however have a good estimate of the interference ...!
Cellular System Design
BASE
STATION



Frequencies, timeslots, or codes reused at
spatially-separate locations
Efficient system design is interference-limited
Base stations perform centralized control functions

Call setup, handoff, routing, adaptive schemes, etc.
Design Issues

Reuse distance

Cell size

Channel assignment strategy

Interference management
 Power adaptation
 Smart antennas
 Multiuser detection
 Dynamic resource allocation
8C32810.44-Cimini-7/98
Dynamic Resource Allocation
Allocate resources as user and network conditions change

Resources:







Channels
Bandwidth
Power
Rate
Base stations
Access
BASE
STATION
Optimization criteria



Minimize blocking (voice only systems)
Maximize number of users
Maximize “revenue”

Subject to some minimum performance for each user
Higher
Layer
NETWORK ISSUES
Networking Issues

Architecture

Mobility Management
 Identification/authentication
 Routing
 Handoff

Control

Reliability and Quality-of-Service
8C32810.53-Cimini-7/98
A final return to QoS...
Wireless Internet access
Nth generation Cellular
Wireless Ad Hoc Networks
Sensor Networks
Wireless Entertainment
Smart Homes/Spaces
Automated Highways
All this and more…
Applications have hard delay constraints, rate requirements,
and energy constraints that must be met
These requirements are collectively called QoS
Challenges to meeting QoS

No single layer in the protocol stack can
guarantee QoS: cross-layer design needed

It is impossible to guarantee that hard constraints
are always met
Average constraints aren’t necessarily good
metrics (e.g. in very slow fading, non-ergodic
conditions).

Cross-layer Design
(or “IET meets ITEM”)

Application
Network
 Access
 Link


Delay Constraints
Rate Requirements
Energy Constraints
Mobility
Hardware
Optimize and adapt across design layers
Provide robustness to uncertainty
Schedule dedicated resources
The Exam: Practical stuff

Time: Saturday, June 2nd, 09.00 - 13.00

Tools/aids allowed: Calculator only

List/sheet containing important/relevant formulas
will be provided as part of the exam

Mostly: Expect same “style” of questions as in
exercises
Exam preparations

For exercises, and solutions to exercises:
Consult course web page.

For questions to exercises: Consult the teaching
assistant, Changmian Wang (Sébastien de la
Kethulle has graduated and has a new job)

For questions to book: Consult Changmian
Wang or Geir Øien (in that order ;-) ).

For questions to lecture notes: Consult Geir
Øien or Changmian Wang (in that order...).
Course curriculum

All curriculum can be found in course textbook,
”Wireless Communications” by Andrea
Goldsmith

See list of chapters/sections in separate
handout (can also be found on web page)

In general ”lectures and exercises define the
curriculum”

Details not covered either in lectures or
exercises will not be emphasized at exam!