Cross Layer PHY/MAC/Routing Protocols for MIMO and virtual MISO Ad-Hoc Networks

1. Cross Layer PHY/MAC/Routing Protocols for MIMO and virtual MISO Ad-Hoc Networks PI : Srikanth V. Krishnamurthy Students: Gentian Jakllari, Ece Gelal UC Riverside 1

2. Objectives • The goals of this project are to design cross layer solutions that span the PHY to the routing layers for facilitating MIMO operations in ad hoc networks. 2

3. Summary of Accomplishments • Designed and analyzed a PHY/MAC/Routing framework for exploiting Virtual MISO (VMISO) communications for unicasting in mobile ad-hoc networks • The work on unicasting was selected among the top four Infocom 2006 papers for fast tracked publication in IEEE TMC • Designed and analyzed centralized and distributed protocols for VMISO based network-wide broadcasting • This work was recently accepted for publication in IEEE JSAC • Designed and analyzed topology control algorithms for facilitating MAC/Routing cross layer interactions in antenna array equipped ad hoc networks (Poster) • This work will appear in IEEE SECON 2006 and IEEE MASS 2006 • Preliminary work on understanding the benefits of diversity gain with MIMO -- Paper in Asilomar conference on signals and systems. • Preliminary work on designing topology control/MAC protocols for achieving spatial multiplexing in multi-user MIMO based ad hoc networks 3

4. Roadmap • Our work on enabling and exploiting Virtual MISO links in ad hoc networks • Both unicast and broadcast solutions. • Our preliminary work on exploiting diversity gain in MIMO equipped ad hoc networks. • Our preliminary ideas on achieving spatial multiplexing in multi-user MIMO based ad hoc networks (joint work with Prof. Bhaskar Rao). 4

5. Premise • Space-Time (ST) communications can significantly improve signal quality on fading channels and hence can help improve capacity of ad hoc networks. • However, higher layer protocols are needed in order to enable and exploit ST communications • The layers 2 & 3 (MAC/Routing) protocols have to interact closely with the PHY in order to enable and exploit ST communications under a network setting. • We initially start out with virtual MISO -- nodes jointly use their omni-directional antenna elements to form an array. 5

6. Diversity Gain -- Range Increase • Node A cooperates with its neighbors to send five replicas of the same packet to B • The range is increased by a factor of 5.4 while the interference 1.7 • For diversity gain 15 dB, path loss 3, BER 10-3 • Transmission range higher than interference range ! B A C Note: B has CSI ! 6

7. Synchronization and Distributed ST Communications • The signals must arrive in phase at the receiver • The nodes are located at different distances relative to the receiver • Relative delay is comparable to a symbol duration • Maximum relative delay is1.67s • Symbol duration with 802.11 2Mbps ->1s; 11Mbps -> 0.727; OFDM->3.2 s • Relative delay is bigger (Frequency selective) • Time-reverse space time codes • Space-Time OFDM • Equalizer 7

8. Enabling VMISO Communications: Our work in a nutshell • Design of Multi-layer (PHY/MAC/Routing) framework to enable and exploit distributed ST communications for range extension • Physical layer exports appropriate information to the upper layers • The MAC layer handles all the coordination for realizing the cooperation • MAC/Routing work jointly to build shorter paths exploiting the extension in range • Significant improvement in throughput and end-to-end delay is possible 8

9. S D S I D Overview of our Framework • Discovery of the primary path • Use any existing source-based routing protocol • A new path based on virtual MISO links is built dynamically by the MAC layer • The packets are transmitted over the new path 9

10. Building Virtual MISO Links • S selects 4 nodes at random and elicits cooperation by sending an RTS • S and its neighbors send pilot tones to D on orthogonal channels • S and its neighbors send an MRTS to D simultaneously • Upon reception of an MCTS from D, S will transmit the data packet to its neighbors and then all of them will send the data simultaneously S D 10

11. Identifying the Next Hop • The transmitter anycasts an MRTS to potential candidates • All the candidates that receive the MRTS and are available set a timer • The timer depends on the distance of the candidate from the destination -- the closer it is, the smaller the timer • If the timer of a candidate expires and no transmission is sensed, the specific candidate replies with the MCTS 11

12. Simulation Parameters 12

13. Results • The throughput is increase by almost 100% • The end-to-end delay is decreased by almost 75% 13

14. Network Wide Broadcasting (NWB) • Transmission of information from one node to the entire network • Route discovery • Dissemination Services • Key performance parameters • Coverage: percentage of nodes receiving the broadcast • Latency • Cost incurred to perform the broadcast 14

15. A Distributed VMISO-Based NWB Protocol--1 Overview of our protocol YES Reception Rebroadcast? K SISO Neighbors? Idle/Receive Mode NO YES NO Transmit SISO Build/Transmit VMISO 15

16. A Distributed VMISO-Based NWB Protocol-2 • Every node that receives a broadcast packet performs the following steps • Decide whether its necessary to broadcast or not • Upon receiving a broadcast packet, the node sets a timer • If the nodes receives more than  copies of the broadcast packet the packet is discarded. If not, the node will rebroadcast • If the node has more than K SISO neighbors it will venture to dynamically create a VMISO link and broadcast • The node broadcasts the packet via SISO and lists a subset (k) of its one-hop neighbors for potential cooperation • The K neighbors and the node itself follow with sending pilot tones orthogonal in time to provide CSI at the receivers • All the K+1 transmit the broadcast packet simultaneously, right after the last pilot tone • If the node has less than K SISO neighbor it will resort to a SISO transmission 16

17. Simulations: Protocols Under Comparison • C-Greedy: A centralized VMISO based greedy heuristic • D-Coop: The distributed cooperative protocol • D-Coop:ANA are the results predicted for D-Coop by our analysis. • D-Non-Coop: A non-cooperative distributed algorithm • Flood: A typical flooding protocol • C-MCDS: The broadcast packets are transmitted by a set of nodes computed by an approximation to the minimum connected dominating set. 17

18. Results on Coverage and Cost • The VMISO based protocols offer superior performance over the SISO based schemes. • There is a good match between the results predicted from the analysis and those generated by the simulations. 18

19. Studying MIMO Gains At The Routing Layer • We have started some preliminary work on examining the benefits of MIMO at the routing layer. • In particular, the question that we ask is “Is it preferable to use the diversity gain to increase rate, range or reliability?” • Use fairly realistic channel models in the OPNET simulator. • Currently, we have generated some preliminary results -- mainly looking at range enhancements. • A comprehensive study will be done in the near future. 19

20. Routes in the Sample Topology • Simulation Setup: • - 802.11g MAC protocol used • - A diversity gain of 20dB simulated (Haykin et.al.) Routes found with DSR routing protocol using MIMO communications, exploiting the increase in range with the diversity gain Routes found with DSR routing protocol using SISO communications 20

21. Numerical Results • OMNI : SISO communications • P-MIMO : “Imitating” MIMO effect on a single antenna by increasing transmission power • Routing overhead per data traffic is least with MIMO • Number of hops per route is smaller with P-MIMO,however: • Packet delivery delay at MAC layer is very high with P-MIMO • Media access delay is boosted with P-MIMO • MAC layer throughput with MIMO is higher than SISO, and is almost twice of that with P-MIMO 21

22. Other studies • We have done some preliminary experiments with random topologies. • We are in the process of inserting realistic constructions including the exchange of pilot symbols. • The overarching design is to provide an assessment of how best to use diversity gain. 22

23. Spatial Multiplexing with MIMO • Challenges • Carrier sensing is not a viable option -- a node should not abort its transmission attempt simply because it senses the channel busy. • CSI is needed -- at least at the receiver. • The number of simultaneous transmissions in the vicinity of a receiver should be controlled/limited in order to ensure reliable information delivery. 23

24. Is CSI exchange possible ? • Even with vehicular speeds of 50 mph, the time taken for a node to move a distance that is approximately equal to the wavelength is of the order of milliseconds (assuming the Ghz bands). • At high data rates, perhaps four to five packet exchanges are possible within this time. • Receiver feedback seems possible. • We are currently examining this further. 24

25. Preliminary ideas • Selection diversity at transmitter • Successive Interference cancellation (SIC) at receiver. • In order to ensure the success of the SIC, it is important to ensure that the number of simultaneous transmissions is controlled. • Strategy: A combination of scheduled/random access. 25

26. Preliminary Ideas (continued) • Perform topology control to divide the network into sparser groups. • Nodes may belong to multiple groups. • At any given time instance, only one group is participating in the communications -- other groups are silent. • Provide statistical guarantees on successful reception of packets. • Within groups, the communication is via random access • Preliminary thoughts are to have the receiver invite transmitters based on prior knowledge of queued packets. 26

27. Looking into the future • We will examine the benefits of diversity gain in more detail -- in particular, we expect to articulate the trade-offs between exploiting the gain to achieve rate and/or range enhancements • We expect to have a completed design, with analysis and simulations of our joint topology/control and MAC layer framework for spatial multiplexing with MIMO. • We are also just beginning to take a look at the impact of jammers on the MAC layer functions -- we expect to make some progress during 2006-07.. 27

28. Thank you 28

29. Backup Slides 29

30. Probabilistic-Based Performance Evaluation--1 • A node that implements our distributed protocol will broadcast with probability: • Where P(EVMISO|R) is the probability that a node will decide to broadcast: • And P(DVMISO) is the probability that an additional copy is received before the timer expires • Finally, the probability that a node has K SISO neighbors is: 30

31. Probabilistic-Based Performance Evaluation--2 • The probability that a node will receive a broadcast packet is: • Where PR1 is the probability a broadcast packet is received from a specific neighbor: • Where qSISO/VMISO is the probability that a SISO/VMISO transmission is not corrupted due to Rayleigh fading is: 31