October 11 2006IEEE MASS 2006A. A. Abouzeid
Cross-layer Optimal Decision Policies for Spatial Diversity Forwarding in
Wireless Ad Hoc Networks
Prof. Alhussein Abouzeid
Rensselaer Polytechnic Institute
Joint work with Jing Ai & Zhenzhen Ye
October 11 2006IEEE MASS 2006A. A. Abouzeid
Hussein Abouzeid
Jing Ai
(if we were here)
Zhenzhen Ye
October 11 2006IEEE MASS 2006A. A. Abouzeid
Outline
• Spatial Diversity Forwarding– Motivation
– Related work
• Problem formulation of Optimal Stopping Relaying
• Implementation of OSR
• Evaluations of OSR– Analytical results
– Simulation results
• Summary
October 11 2006IEEE MASS 2006A. A. Abouzeid
Spatial Diversity Forwarding: Motivation & Related Work
• Motivation: Exploiting spatial diversity inherent in multi-hop wireless networks– Observation: There naturally exist alternate next-hop relays with high
probability that one of them has favorable channel condition at any given instant!
• The key challenge is design of strategy for next-hop selection. Prior work can be classified into two classes:
1. FSR (First Stopping Relaying) [JD05, WZF04, etc.]– Selects the first relay that replies to the forwarding node. (Problem: though
minimum overheads incurred, the selected relay may be poor in quality.)2. LSR (Last Stopping Relaying) [SM05]
– Collecting CSI from all candidate relays and then selecting the best one. (Problem: though the selected relay might be (if CSI is not outdated) the best, it incurs the maximum delay.)
We propose OSR: Exploiting the range between these extreme cases.
October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of optimal stopping relaying (OSR)
• Optimal stopping relaying: investigate and design spatial-diversity forwarding policies based on a formally defined stochastic decision framework.
• Mapping next-hop selection problem to a sequential optimal stopping problem– The Decision maker: the forwarding node which has a packet to be
forwarded– Every time step, the decision maker observes a random variable which
is the state (e.g. channel quality) of the next available candidate. It also computes the reward up to that point in time.
– Action: whether to “stop” at a candidate next-hop relay and forward the packet to it, or “continue” observing other candidates
– Policy: The goal of the problem is to derive an optimal policy (i.e. a rule for deciding which action to take at every decision instant) in order to maximize the expected reward.
October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of OSR (cont.)• A “conceptual” decision making procedure of OSR
Channel Quality: 2
Channel Quality: 7
Channel Quality: 8
Channel Quality?
2
2 is low, continue!
Channel Quality?
7
7 is good enough, stop!
AB
October 11 2006IEEE MASS 2006A. A. Abouzeid
Problem formulation of OSR (cont.)
• Given: a forwarding node ns which intends to forward a packet toward its destination and a set of L candidate next-hop relays {n1, n2, . . . , nL} known at the routing layer, which can be characterized by L independent discrete random variables (rewards) {Θ1,Θ2, . . . ,ΘL}.
• Problem: what is the optimal policy at the forwarding node ns to select the next-hop relay to which the packet is to be forwarded so as to maximize the expected reward E{Θ}?
• Θ is defined as d*Rdata, a generalization of Information Efficiency (IE)
• Solution: the optimal policy is a threshold-based policy– easy to implement
October 11 2006IEEE MASS 2006A. A. Abouzeid
Implementation of OSR
• Physical layer: rayleigh fading channels– determined the channel quality statistics if known long-
term average
• MAC layer: an extended MAC anycast scheme based on IEEE 802.11– perform the “optimal stopping” decision-making procedure
• Network layer: Greedy geographic routing [BMSU99,BK00]– take charge of selecting a set of candidate next-hop relays
at a forwarding node
October 11 2006IEEE MASS 2006A. A. Abouzeid
MAC layer anycast
• Motivation– transform “costly” sequential decision-making procedure at the
forwarding node to a relatively “cheap” parallel decision-making procedure on the relay side.
• MRTS-CTS dialogue– Multicast RTS (MRTS) carries the threshold-based policy– CTS comes from the relay selected by the optimal stopping policy
• Feedbacks collision resolution– it is possible that more than one candidate next-hop relay qualified to
relay the packet by performing the threshold-based policy issued by the forwarding node
– prioritized the responses of relays in an order pre-assigned by the forwarding node
– with CSMA, the response of a higher-priority relay can suppress the responses of lower-priority relays
October 11 2006IEEE MASS 2006A. A. Abouzeid
MAC layer anycast (cont.)
• A sample timeline of the OSR scheme for three candidate next-hop relays
CTS1
NAV(MRTS)
NAV(CTS2)
MRTS
CTS2
DATA
ACK
Tx
Rx1
Rx2
Rx3
Others
CTS3
NAV(DATA)
SIFS
2SIFS
SIFS
SIFS
SIFS
Random back-offafter DIFS
October 11 2006IEEE MASS 2006A. A. Abouzeid
OSR v.s. FSR: Analytical Results
• Scenario– X-axis represents the average
SNR– Y-axis represents the gain
OSR over FSR in terms of IE– L homogenous candidate
next-hop relays
• Main observation– FSR can not utilize more than
two candidate relays as efficiently as OSR, especially when channels quality is average
October 11 2006IEEE MASS 2006A. A. Abouzeid
LSR v.s. OSR: Simulation Results (Qualnet)
• Comparison of end-to-end performance metrics in Qualnet [QUA]– Grid topology (8X8 grid), a single TCP flow
• throughput
– Random topology, multiple TCP flows (not included due to space limit)
– Static random topology, multiple UDP flows• packet delivery ratio• end-to-end delay• jitter
– Mobile topology, multiple UDP flows (not included due to space limit)
October 11 2006IEEE MASS 2006A. A. Abouzeid
Impact of Fading Velocity (vm) on Average FTP Throughput
• Note: M is a protocol parameter in geographic routing that specifies the maximum order of spatial diversity that can be utilized by a forwarding node • Observation: In OSR, larger M means better performance (which is what we want). Not true for LSR (due to overheads)
October 11 2006IEEE MASS 2006A. A. Abouzeid
Impact of Traffic Load on CBR flows
October 11 2006IEEE MASS 2006A. A. Abouzeid
Summary & Future Work
• Formulated the next-hop relay selection problem as a sequential decision problem and derived the Optimal Stopping Relaying (OSR) policies for improving spatial diversity gain in wireless ad hoc networks
• Implement OSR in a realistic protocol stack• Both analytical and simulation results reveal that OSR
outperforms FSR and LSR in terms of IE/end-to-end performance metrics
• Enable nodes to learn the fading channel characteristics online instead of relying on a channel model
• Extend such a decision framework to flow/service differentiation schemes.
October 11 2006IEEE MASS 2006A. A. Abouzeid
References
• [BK00] B. Karp and H T. Kung, “GPSR: greedy perimeter stateless routing for wireless networks,” in Proc. ACM/IEEE Mobicom 2000.
• [BMSU99] Prosenjit Bose, Pat Morin, Ivan Stojmenovic and Jorge Urrutia, “Routing with guaranteed delivery in ad hoc wireless networks,” in Proc. DIALM, 1999.
• [JD05] S. Jain and S. R. Das, “Exploiting path diversity in the link layer in wireless ad hoc networks,” in Proc. IEEE WoWMoM, 2005.
• [JZF04] J. Wang, H. Zhai and Y. Fang, “Reliable and efficient packet forwarding by utilizing path diversity in wireless ad hoc networks,” in Proc. IEEE Milcom, 2004.
• [QUA] “Qualnet 3.7 user’s guide.” http://www.scalable-networks.com/.
• [SM05] M. R. Souryal and N. Moayeri, “Channel-adaptive relaying in mobile ad hoc networks with fading,” in Proc. IEEE SECON, 2005.
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