Course Summary

1
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)

2
Future Wireless NetworksThe Vision
Ubiquitous Communication Among People and Devices
Wireless Internet access Nth generation
Cellular Wireless Ad Hoc Networks Sensor Networks
Wireless Entertainment Smart Homes/Spaces Automat
ed Highways All this and more

• Hard Delay/Energy Constraints

• Hard Rate Requirements

3
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.

4
Design Challenges, contd

• Wireless channels are a difficult and
  capacity-limited 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)

5
Signal Propagation Main effects

• Path Loss
• Shadowing
• Multipath

6
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 Bc BD)

7
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

8
Linear Modulation in AWGN

• ML detection induces decision regions
• Example 8PSK
• Ps (symbol error rate) depends on
• of nearest neighbors
• Minimum distance dmin (depends on gs)
• Approximate expression
• ?M is of nearest neighbors ?M relates dmin
  and average symbol energy.

9
Linear Modulation in Fading

• In fading gs - and therefore Ps - is random
• Metrics outage probability, average Ps , or
  combined outage and average.

Ts
Ps
Outage
Ps(target)
10
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 .

11
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)

12
ISI Effects

• Delay spread exceeding one symbol time causes ISI
  (self-interference).
• ISI leads to irreducible error floor
• Increasing signal power increases ISI power
• ISI requires that TsgtgtTm (RsltltBc)

Tm
0
13
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

14
Variable-Rate Variable-Power MQAM
Goal Optimize S(g) and M(g) to maximize EM(g)
15
Optimal Adaptive Scheme

• Power Water-Filling
• Spectral Efficiency

g
Equals Shannon capacity with an effective power
loss of K.
16
Practical Adaptation Constraints

• Constellation restriction
• Constant power restriction
• Constellation updates.
• Estimation error.
• Estimation delay.
• Lead to practical adaptive modulation schemes
  (Ch. 9)

17
Diversity

• Send bits over independent fading paths
• Combine paths to mitigate fading effects.
• Independent fading paths - how to create?
• Space, time, frequency, polarization diversity.
• Combining techniques
• Selection combining (SC)
• Equal gain combining (EGC)
• Maximal ratio combining (MRC)
• ...

18
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.

19
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

20
Direct Sequence Spread Spectrum (DSSS)

• Bit sequence modulated by chip sequence
• Spreads bandwidth by large factor (K)
• Despread by multiplying by sc(t) again (sc(t)1)
• Mitigates ISI and narrowband interference
• ISI mitigation a function of code autocorrelation
• Must synchronize to incoming signal

S(f)
s(t)
sc(t)
Sc(f)
S(f)Sc(f)
1/Tb
1/Tc
TbKTc
2
21
RAKE Receiver

• Multibranch receiver
• Branches synchronized to different MP components
• These components can be coherently combined
• Use SC, MRC, or EGC

Demod
sc(t)
y(t)

dk
Diversity Combiner
Demod
sc(t-iTc)
Demod
sc(t-NTc)
22
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)

a
a
23
Bandwidth Sharing in general

• FDMA
• TDMA
• CDMA
• (Hybrid Schemes)

7C29822.033-Cimini-9/97
24
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 ...!

25
Cellular System Design

• 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.

26
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
27
Dynamic Resource AllocationAllocate resources as
user and network conditions change

• Resources
• Channels
• Bandwidth
• Power
• Rate
• Base stations
• Access
• Optimization criteria
• Minimize blocking (voice only systems)
• Maximize number of users
• Maximize revenue
• Subject to some minimum performance for each user

28
Higher LayerNetworking Issues
NETWORK ISSUES

• Architecture
• Mobility Management
• Identification/authentication
• Routing
• Handoff
• Control
• Reliability and Quality-of-Service

8C32810.53-Cimini-7/98
29
A final return to QoS...
Wireless Internet access Nth generation
Cellular Wireless Ad Hoc Networks Sensor Networks
Wireless Entertainment Smart Homes/Spaces Automat
ed 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
30
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 arent necessarily good
  metrics (e.g. in very slow fading, non-ergodic
  conditions).

31
Cross-layer Design (or IET meets ITEM)

• Application
• Network
• Access
• Link
• Hardware

Delay Constraints Rate Requirements Energy
Constraints Mobility
Optimize and adapt across design layers Provide
robustness to uncertainty Schedule dedicated
resources
32
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

33
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...).

34
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!