EE360: Lecture 8 Outline Intro to Ad Hoc Networks
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Transcript of EE360: Lecture 8 Outline Intro to Ad Hoc Networks
EE360: Lecture 8 OutlineIntro to Ad Hoc
Networks Announcements
Makeup lecture this Friday, 2/7, 12-1:15pm in Packard 312
Paper summary 1 due todayProposal feedback sent, revision due
Monday 2/10HW 1 posted, due 2/19
Overview of Ad-hoc Networks Design Issues MAC Protocols Routing Relay Techniques Adaptive Techniques Power Control
Ad-Hoc Networks
Peer-to-peer communications No backbone infrastructure or centralized
control Routing can be multihop. Topology is dynamic. Fully connected with different link SINRs Open questions
Fundamental capacity Optimal routing Resource allocation (power, rate, spectrum,
etc.) to meet QoS
Ad-Hoc NetworkDesign Issues
Ad-hoc networks provide a flexible network infrastructure for many emerging applications.
The capacity of such networks is generally unknown.
Transmission, access, and routing strategies for ad-hoc networks are generally ad-hoc.
Crosslayer design critical and very challenging.
Energy constraints impose interesting design tradeoffs for communication and networking.
Medium Access Control
Centralized access entails significant overhead
Decentralized channel access more commonMinimize packet collisions and insure
channel not wastedCollisions entail significant delay
Aloha w/ CSMA/CD have hidden/exposed terminals
802.11 uses four-way handshakeCreates inefficiencies, especially in multihop
setting
HiddenTerminal
ExposedTerminal
1 2 3 4 5
Frequency ReuseMore bandwidth-efficientDistributed methods needed.Dynamic channel allocation
hard for packet data.Mostly an unsolved problemCDMA most common
No overhead required
DS Spread Spectrum Reuse:
Code Assignment Common spreading code for all nodes
Collisions occur whenever receiver can “hear” two or more transmissions.
Near-far effect improves capture.Broadcasting easy
Receiver-orientedEach receiver assigned a spreading
sequence.All transmissions to that receiver use the
sequence.Collisions occur if 2 signals destined for
same receiver arrive at same time (can randomize transmission time.)
Little time needed to synchronize. Transmitters must know code of
destination receiver Complicates route discovery. Multiple transmissions for broadcasting.
Transmitter-oriented
Each transmitter uses a unique spreading sequence
No collisionsReceiver must determine sequence of
incoming packet Complicates route discovery. Good broadcasting properties
Poor acquisition performance Preamble vs. Data assignment
Preamble may use common code that contains information about data code
Data may use specific codeAdvantages of common and specific codes:
Easy acquisition of preamble Few collisions on short preamble New transmissions don’t interfere with the data
block
Introduction to Routing
Routing establishes the mechanism by which a packet traverses the network
A “route” is the sequence of relays through which a packet travels from its source to its destination
Many factors dictate the “best” route
Typically uses “store-and-forward” relayingNetwork coding breaks this paradigm
SourceDestination
Routing Techniques Flooding
Broadcast packet to all neighbors
Point-to-point routingRoutes follow a sequence of linksConnection-oriented or connectionless
Table-drivenNodes exchange information to develop
routing tables
On-Demand RoutingRoutes formed “on-demand”
“A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols”: Broch, Maltz, Johnson, Hu, Jetcheva, 1998.
Relay nodes in a route
Intermediate nodes (relays) in a route help to forward the packet to its final destination (minimize total TX power)
Decode-and-forward (store-and-forward) most common: Packet decoded, then re-encoded for
transmission Removes noise at the expense of complexity
Amplify-and-forward: relay just amplifies received packet Also amplifies noise: works poorly for long
routes; low SNR. Compress-and-forward: relay compresses
received packet Used when Source-relay link good, relay-
destination link weak
SourceRelay Destination
Often evaluated via capacity analysis
Route dessemination Route computed at centralized node
Most efficient route computation.Can’t adapt to fast topology changes.BW required to collect and desseminate
information
Distributed route computationNodes send connectivity information to local
nodes.Nodes determine routes based on this local
information.Adapts locally but not globally.
Nodes exchange local routing tablesNode determines next hop based on some
metric.Deals well with connectivity dynamics.Routing loops common.
Reliability Packet acknowledgements needed
May be lost on reverse linkShould negative ACKs be used.
Combined ARQ and codingRetransmissions cause delayCoding may reduce data rateBalance may be adaptive
Hop-by-hop acknowledgementsExplicit acknowledgementsEcho acknowledgements
Transmitter listens for forwarded packet More likely to experience collisions than a
short acknowledgement.Hop-by-hop or end-to-end or both.
Adaptive Techniques forWireless Ad-Hoc
Networks
Network is dynamic (links change, nodes move around)
Adaptive techniques can adjust to and exploit variations
Adaptivity can take place at all levels of the protocol stack
Negative interactions between layer adaptation can occur
What to adapt, and to what?
QoSAdapts to application needs, network/link
conditions, energy/power constraints, …
RoutingAdapts to topology changes, link changes,
user demands, congestion, …
Transmission scheme (power, rate, coding, …)Adapts to channel, interference,
application requirements, throughput/delay constraints, …
Adapting requires information exchange across layers and should happen on different time scales
Bottom-Up View:Link Layer Impact
“Connectivity” determines everything (MAC, routing, etc.)Link SINR and the transmit/receive strategy
determine connectivity
Can change connectivity via link adaptation
Link layer techniques (MUD, SIC, smart antennas) can improve MAC and overall capacity by reducing interference
Link layer techniques enable new throughput/delay tradeoffsHierarchical coding removes the effect of
burstiness on throughputPower control can be used to meet delay
constraints
Power Control Adaptation
Each node generates independent data. Source-destination pairs are chosen at random.
Topology is dynamic (link gain Gijs time-varying) Different link SIRs based on channel gains Gij
Power control used to maintain a target Ri value
jijiji
iiii PG
PGR
Gij
Gii
Pi
Pj
Power Control for Fixed Channels
Seminal work by Foschini/Miljanic [1993]
Assume each node has an SIR constraint
Write the set of constraints in matrix form
jiGG
jiF
ii
ijiij ,
,0
i
jijiji
iiii PG
PGR
0P0,uPFI
T
NN
NN
11
11
Gηγ,,
Gηγu
Scaled Interferer Gain Scaled Noise
Optimality and Stability
Then if rF <1 then a unique solution to
P* is the global optimal solution
Iterative power control algorithms PP*
uFIP -1*
uFP(k)1)P(k :dCentralize
(k)P)(R
γ1)(kP :dDistribute ii
ii k
What if the Channel is Random?
Can define performance based on distribution of Ri: Average SIROutage ProbabilityAverage BER
The standard F-M algorithm overshoots on average
How to define optimality if network is time-varying?
ii γERγlog]R[logE ii
Can Consider A New SIR Constraint
Original constrainti
jijiji
iiii γ
PGηPGR
0PGηγPGEji
jijiiii
Multiply out and take expectations
0uPFI Matrix form
ji,GEGEγ
ji0,F
ii
ijiij
T
u
]E[Gηγ,,
]E[Gηγ
NN
NN
11
11
Same form as SIR constraint in F-M for fixed channels
New Criterion for Optimality
If rF<1 then exists a global optimal solution
For the SIR constraint
Can find P* in a distributed manner using stochastic approximation (Robbins-Monro)
uFIP -1*
i
ijjiji
iii γPGηE
PGE
Robbins-Monro algorithm
Where ek is a noise term
Under appropriate conditions on
kkk εaP(k)ga-P(k)1)P(k
k
n 1
2k
k
1nkk aa0a :size Step
u(k)uP(k)F(k)Fεk
kε*PP(k)
Admission ControlWhat happens when a new user
powers up?More interference added to the systemThe optimal power vector will moveSystem may become infeasible
Admission control objectivesProtect current user’s with a
“protection margin”Reject the new user if the system is
unstableMaintain distributed nature of the
algorithm
Tracking problem, not an equilibrium problem
Fixed Step Size Algorithm Properties
Have non-stationary equilibria So cannot allow ak 0
A fixed step size algorithm will not converge to the optimal power allocation
This error is cost of tracking a moving target
O(a)||P~*P||EwhereP~P(k)
k
n 1
2k
k
1nkk aaaa :size Step
Example: i.i.d. Fading Channel
Suppose the network consists of 3 nodes
Each link in the network is an independent exponential random variable
Note that rF=.33 so we should expect this network to be fairly stable
104.02.04.10375.02.0375.1
E[G] i1η5γ ii
Power Control + … Power control impacts multiple
layers of the protocol stack Power control affects
interference/SINR, which other users react to
Useful to combine power control with other adaptive protocolsAdaptive routing and/or scheduling
(Haleh)Adaptive modulation and codingAdaptive retransmissionsEnd-to-end QoS…
Multiuser Adaptation
TrafficGenerator
DataBuffer
SourceCoder
ChannelCoder
Modulator(Power)
ReceiverChannel
Cross-Layer Adaptation
Channel interference is responsive to the cross-layer adaptation of each user
Multiuser Problem Formulation
Optimize cross-layer adaptation in a multi-user setting
Users interact through interferenceCreates a “Chicken and Egg” control
problemWant an optimal and stable equilibrium
state and adaptation for the system of users
The key is to find a tractable stochastic process to describe the interference
Linear Multi-User Receiver
Assume each of K mobiles has interference reduced by ci (may be via an N-length random spreading sequence)
The attenuation ci takes different values for different structures (MMSE, de-correlator, etc.)
ijjjj
Tii
Ti
iiiTi
K
iNii
tztaSccc
tztaSctiSIR
VVN
S
)()()()(
)()()(),(
,,1
22
2
1
Interference term
Interference Models Jointly model the state space of every
mobile in the systemProblem: State space grows exponentially
Assume unresponsive interferenceAvoids the “Chicken and Egg” control issueProblem: Unresponsive interference
models provide misleading results
Approximations use mean-field approach Model aggregate behavior as an averageCan prove this is optimal in some cases
Distributed Power Control for Time-Varying Wireless Networks: Optimality and ConvergenceT. Holliday, N. Bambos, P. W. Glynn, and A. Goldsmith, 2003 Allerton Conference
Summary Ad-hoc networks provide a flexible
network infrastructure for many emerging applications
Commercial ad-hoc networks to date have experienced poor performance: designs are ad-hoc
Design issues traverse all layers of the protocol stack, and cross layer designs are needed
Protocol design in one layer can have unexpected interactions with protocols at other layers.
The dynamic nature of ad-hoc networks indicate that adaptation techniques are necessary and powerful
Today’s presentation
Jeff will present “Principles and Protocols for Power Control in Wireless Ad Hoc Networks”
Authors: Vikas Kawadia and P.R. Kumar
Published in IEEE Journal on Selected Areas in Communications, January 2005