in Multi-hop Wireless Networks

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Nov 24, 2013 (3 years and 4 months ago)

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Optimal Routing, Link
Scheduling and Power Control
in Multi
-
hop Wireless Networks


R. L. Cruz and
Arvind

V.
Santhanam

University of California, San Diego

IEEE INFOCOM 2003

Outline


Motivation


Objective


Optimal Scheduling and Power Control


Routing and Capacity Allocation


Resource Allocation Examples


Conclusion














1

Motivation


Minimizing the total power in broadband wireless networks is of
paramount importance :


Increase operational lifetime in the case of battery powered devices


Coexist symbiotically with other systems which share the same
frequency spectrum.



Most currently deployed networks that support high data rates use
only a “single hop” for wireless communication


However, use of multiple hops to transport data has been shown to
enhance network capacity and may be necessary due to cabling
limitations in many environments







2

Network Set Up




Network of base
-
stations interconnected to each other through
wireless links.


Each base
-
station serves as an ingress or egress for the aggregate
of traffic associated with the mobile users in its domain


Each base
-
station routes its data through other base stations via
multiple wireless hops to an access point connected to a wired
infrastructure.

3

Objective




Joint routing, link scheduling and power control to support high data
rates for broadband wireless multi
-
hop networks


Finding an optimal link scheduling and power control policy that
minimizes the total average transmission power in the wireless multi
-
hop network, subject to given constraints regarding the minimum
average data rate per link, as well as peak transmission power
constraints per node.


Incorporating a Globally optimum routing scheme.






4

Notations



N
stationary nodes (base stations)
1,2,…N


A set
ε

of L
ε

transmission links,
among the possible
N(N −1) links
between nodes, constitutes
a network topology.


Link
l(
i,j
) :
transmitter node
i

uses signal power
P(l)


G(
i,j
)


path gain from node
i

to node
j
(Assumed to be constant)


T(l) : Transmitting node of link
l


R(l) : Receiving node of link
l


η
j

: Ambient noise power at node
j



: SNR for link
l


X(l)
: Achieved data rate on link
l


W
: Frequency bandwidth





5

6


Shannon Capacity of link =


Assumption :
X(l)
is a linear function of (At small values of SIR)



All links share the same frequency band of width W.


Assuming the Gaussian approximation to compute the BER, the data
rate of a link
l with a tolerable BER of 10
−q
and using BPSK
modulation is given by




C(l)
: Desired data rate on link
l. Thus


Matrix form :









If F is a stable matrix, i.e. all
eigenvalues

of F are strictly inside the
unit circle, then (
I
− F
)
−1

exists and has nonnegative
elements. Thus,








7



can be seen as the Minimal power vector that
supports the network topology


Foschini
-
Miljanic

algorithm for cellular network


Constraints:


If
eigenvalues

of
F

are on or outside the unit circle, there does not exist
any power vector P which supports the required data rates.


If the
eigenvalues

of
F

are inside the unit circle but close to the
boundary, the minimal power vector will be very large.


Even if the
eigenvalues

of
F

are well inside the unit circle, using the
minimal power vector (
I


F)
-
1

b
may be inefficient.






8

Example


A square network of 4 nodes and 2 links
{(1, 2), (3, 4)}.


G(1, 2) = G(3, 4) = G
0

and G(1, 4) = G(3, 2) = G
0
/2


X({1, 2}) = X({3, 4}) = W
’/
2


n
0

= G
0

(at both receivers)



(
I


F)
-
1

b
= (0.67 0.67)
T


But, if time sharing is employed between the 2 transmitters, and
each transmitter active for 50% of the time
-
> the same average
data rate of W’/2 is achieved, with average power of 0.5 Watt.


It can be inefficient to operate with all links in a wireless network active
concurrently


However, in order to achieve high data rates with constraints on peak
transmission power, concurrent transmissions may be necessitated



9


C(l) are determined by the average rates at which traffic is
generated by users and the routing algorithm that is used.


In wireless networks it is possible to reconfigure the data rates of the
links on a fast time scale in response to changing traffic and channel
conditions.







10

Optimal Scheduling and Power Control (1)


Time divided into slots.


Transmissions begin and end on slot boundaries


P
m
(l)
: Transmission power for the transmitter
T(l)

for link
l
in slot
m




be the network power vector for
slot
m


P
max

(
i
)
: maximum transmission power of node
i



: links in
ε

that originate at node
i










11

Optimal Scheduling and Power Control (2)


Constraint on Peak Transmission Power :









(1)




Achieved data rate for link
l
in slot
m




Long term average rate of link l



Constraint on Minimum acceptable average data rate :








(2)







12

Optimal Scheduling and Power Control (3)


Required minimum average rate vector



Average power consumed by the transmitter for link
l




Average network power vector


There may or may not exist a sequence of network power vectors
P1, P2, . . . that satisfy (1) and (2).


If there does exist a sequence of such network power vectors, aim is
to minimize a linear function of


where
α
(l)
is a positive weight


13

Primal Problem






subject to constraints
(1) and (2)
(3)



This optimization involves choosing optimal power levels in each
slot for each transmitter

14

Duality Approach (1)


Let the optimal cost in
(3)
as a function of be denoted as



if there is no network power vector satisfying
(1)

&
(2)



is a convex function of the vector


Set of Dual variables


Potential function V as



Dual objective function is defined as







subject to
(1)


For any non
-
negative vector





subject to
(1)

&
(2)







subject to
(1)



15

Duality Approach (2)


Dual Optimization Problem






(4)











The components of the optimal dual variable vector represent a
sensitivity of the optimal cost with respect to a perturbation in the
minimum average data rate for a link. Thus,









(Optimization over a single slot)


where




16

Duality Approach (3)


Let
M
be the number of extreme points of
S
P




denote the extreme points of
S
P

.


Each point in
S
P

can be represented as a combination of


Upper bound on
M is 2
L
ε



It can be shown that




Complexity of computing is O(
M
)



is an affine function of . Thus,





Iterative ascent algorithm used to solve the dual problem
(4)


17

Feasibility check


If is feasible, then



If




is infeasible.





18

Computing the Optimal Policy


Solving
(4)
yields the optimal dual variable vector and extremal
power vectors
P
*,
i

such that









(5)




If is finite, an optimal schedule of network power vector
vectors exists that consists solely
P
*,
i



Thus, in the optimal policy, every node is transmitting at possible
peak power to exactly one receiver, or not transmitting at all.





where K
be the number of extremal network power vectors
P
*,
i

such
that
(5)
holds.



,



19

Optimal Policy


Assuming

is finite,
X
avg
(l)
is exactly equal to
C(l)
in an optimal
policy


Let
X
*,
i

(l)
: Rate of link

l

corresponding to the
P
*,
i



Assuming
is finite, there exists a “weight vector”






such that







indicates the relative frequency at which the extremal network
power vector
P
*,
i

is utilized in an optimal policy
















20

Reducing Complexity


Optimal schedule can have no more than (L
ε

+ 1) transmission modes


M can be as large as
2
L
ε



Complexity of minimizing dual objective function can be exponential


Reducing the set of possible transmission modes considered can
greatly reduce complexity.


No node transmits or receives data at the same time in the optimal
schedule


For multihop networks, transmission modes which consist of multiple
simultaneous transmissions to a receiver are eliminated

21

Hierarchical Link Scheduling and Power
Control


Hierarchical approach to minimize the total average transmission
power of all the links in a network with a large number of links


Links in the network partitioned into groups called clusters


Links in a cluster geographically close to each other


Links in one cluster scheduled somewhat independently of links in other
clusters


Cluster level scheduling is done at the top


Clusters geographically far way from each other impose negligible
mutual interference


If desired data rate on links are sufficiently low, the optimal policy
activates all the clusters in the network simultaneously.








22

Routing and Capacity Allocation


Optimal values for the dual variables .


Can estimate the cost of supporting additional traffic on each link


If an additional
ε

units of traffic to be routed along route
r,
then




where
r(l)
= 1 if link
l
is included in route
r

and
r(l)
= 0 otherwise


Shortest path algorithm with weights for link
l
to the sensibility


Since H(.) is convex, increases after additional traffic is
allocated on link
l.
Thus, routes initially unattractive may become
more attractive after traffic is added on other links.


Optimal paths does not always correspond to minimum energy
paths.





23

Resource Allocation Examples









24

String Topology






1 1


Source of data







5
-

Sink










Each node has
omni
-
directional antenna with peak power constraint
of 1 Watt.





Ambient nose power at all nodes assumed to be constant








25

String Topology (2)



W’ = 10
7




















Optimal policy schedules concurrent transmissions even though they are in
close geographic proximity











26

String Topology (3)












Below 4
Mbits
/sec, scheduling policy
reducesto

TDMA.


Between 4
Mbits
/sec and 4.98
Mbits
/sec, [
{(1, 2)},{(2, 3)}, {(3, 4)} and {(1, 2), (4, 5)}].


above 4.98
Mbits
/sec : Optimal transmission modes are[
{(1, 2)}, {(2, 3)}, {(3, 4)}, {(1,
2), (3, 4)} and {(1, 2), (4, 5)}].


27

Diamond Topology









Node 1 only source of data, and node 4 is the sink


Peak transmission power each node is fixed at 1 Watt


G(1,4) = 1/ d(1,4)
4

. Other path losses are given by inverse square law of
distance.



Minimum energy path 1
-
> 2
-
> 4


W’ = 10
7

28

Diamond Topology (2)












Traffic split over multiple paths even for moderate levels of ambient noise

29

Diamond Topology (3)










Traffic loads below 3.65
Mbits
/sec, optimal policy essentially TDMA


Beyond 3.65
Mbits
/sec, optimal policy is by scheduling {(1, 2), (3, 4)} and


{(1, 3), (2, 4)} for a dominant fraction of time and scheduling transmission modes


{(1, 2)}, {(2, 4)} for the remaining time.


30

Hierarchical Topology










Nodes 1,2 and 3 are the sources of data and node 6 is the sink


G(1, 6) = 1 / d(1,6)
4

and G(2, 6) =
1 /
d(2,6)
4



The minimum energy paths for the source nodes are:
{1 3 6}, {2 3 6} and {3 6}






31

Hierarchical Topology (2)










For low ambient noise, optimal transmission modes
{(1, 3)}, {(2, 3)} and {(3, 6)}
according to TDMA but also allows modes{(1, 4), (5, 6)} and {(2, 5), (4, 6)} to be
active for reasonable
fractions of time.


For sufficiently high ambient noise, dominant transmission modes are
{(1, 4), (5, 6),
(2, 3)} and {(2, 5), (4, 6), (1, 3)} and {(1, 4), (2, 5), (3, 6)}.



32

Minimizing Total Average Transmitter
and Receiver Power


New cost function to minimize :




where

θ
(l) is the energy expended per
bit that is constant for each receiver node
R(l).



Accounting for receiver power in formulation changes the choice of routes
and the rate allocations on links in each route.



Using multiple hops in such topologies would consume higher receiver
energy than using a single hop, making multihop routing inefficient.

33

Conclusion


The integrated routing, scheduling and power control framework
supports higher throughputs at the expense of decreased energy
efficiency.


Framework well suited for slow fading wireless channels which are
relatively constant for long durations of time.


Time synchronization needed between transmitters


Framework applicable for providing a means for wireless
interconnection of fixed stationary nodes, like wireless access nodes


In optimal policy, each nodes is either transmitting at peak power to
a single receiver or not transmitting at all, which it simple to
implement.


Energy could be conserved by “sleep” schedules


The optimal link schedule time
-
shares a small number of
optimal
subsets of
links (
L
ε

+ 1) in order to achieve the required data rates.








34

Conclusion (2)


Optimal policy for low required data rates or in low ambient noise
regimes schedules links in a TDMA sequence.


In moderate noise regimes, or to achieve higher data rates, optimal
strategy involves scheduling multiple simultaneous transmissions
even though they may be in close geographic proximity.


Minimal required average power is a convex function of the required
data rates on each link.


As the level of ambient noise increases, non
-
minimum energy paths
can be exploited to increase throughput, despite the fact that all links
share a common bandwidth.

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