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Communication over Bidirectional Links: Enhancing Wireless Networks Through Knowledge Gain

Explore the advantages and challenges of bidirectional communication in wireless networks, focusing on learning unknown parameters, resource allocation algorithms, MAC with bidirectional links, and cooperative links. Discover how obtaining explicit and direct information enhances network utility, requires resources allocation, and boosts MAC capacity.

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Communication over Bidirectional Links: Enhancing Wireless Networks Through Knowledge Gain

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  1. Communication over Bidirectional Links A. Khoshnevis, D. Dash, C Steger, A. Sabharwal TAP/WARP retreat May 11, 2006

  2. Wireless Networks • Higher throughput • TAP: 400 Mbps • WiMax/Mesh • 4G

  3. Queue Network of Unknowns Interference Topology Channel Battery

  4. q1 S1 l1 D q2 l2 S2 Medium Access Example • If S1 knows q2 and S2 knows q1 • No need for handshaking • TDMA scheduling • No collision • As load increases • Probability of queue empty reduces • Network utility increases Having the “knowledge” about Queue states, increases the utilization

  5. W X Y + H Q(H) S1 D S2 How to learn about unknowns • There is gain in knowing unknown parameters • The information can be gathered • Directly • Feedback • Training • Dedicated link, information sharing • Indirectly • Overhearing • Passive sensing

  6. Need for Bidirectional links • Indirect • Limited • Highly depends on the topology and availability • Direct • Amount of information can be controlled An explicit sharing of information requires flow of information in both directions among all communicating nodes, hence Communication over Bidirectional Links

  7. Cost-Benefit of learning the unknowns • Catch • We don’t care about the unknown • Only care about sending data • Time varying in nature • Periodic measurements • Spend resources for non-data If considering the true cost of knowing the unknown, is there still any gain left?

  8. S1 D S2 S1 h D Our research • Unknown Channel • Chris, Farbod, Ashu, Behnaam • Allerton’05, ISIT’06, JSAC’06 • Resource allocation algorithm • Uncertainty of noise • Farbod, Dash, Ashu • CTW’06, Asilomar’06 • Coding scheme • Randomness of source • Upcoming NSF proposal • Access mechanism

  9. X1 Y X2 Multiple Access Channel: MAC • The system is modeled by • Information theory answers: What is the maximum rate (R1,R2) at which X1 and X2 can transmit with arbitrary small probability of error

  10. Standard solution method • Finding an achievable upper bound • Achievability proof • Converse proof • Typical solution to MAC R2 R1

  11. MAC with Bidirectional links • Time is slotted • Forward channel: multiple access • Reverse channel: feedback from receiver • Superposition coding Un-decoded New Information Tx Decodable From Feedback Decoded Un-decodable Rx

  12. Our model j,l I’,k’

  13. Contribution and results • Considering resources in feedback • Time • Power (Pf) • Coding scheme to compress the feedback information • Pf/ eP

  14. Interpretation of result • In second timeslot • Both user help to resolve uncertainty Co-operation induced by feedback

  15. X1 Y X2 Cooperative link • Anticipate the exponential feedback power is resolved • Under investigation • Rate region • Coding strategies

  16. What if… • Receiver has information for senders • Superimpose feedback information with its own information

  17. Achievable rate region • A:  = 0 • Only Broadcast • B:  = 1 • Only MAC B A R3

  18. h2 h1 h2 h1 Channel state vs. data feedback • So far, receiver sends back unresolved information • In fading environment using channel state • Power / rate control increases the throughput • Feedback can be used to send back channel state information

  19. X2 X1 X3 X4 Randomness of source • Challenges: • K is random • Under delay constraint • Access mechanism is required • Each node needs to know the number of active users

  20. Recap Ongoing work: • Gaining information about the unknowns increases the throughput • Obtaining information is best when it is explicit and direct • Requires resources (power and time) to be allocated to unknowns • Requires bidirectional communication link • Capacity of MAC increases with “realistic” feedback • Power in the feedback link is large Up coming: • Cooperative link • Channel state vs. data feedback • Randomness of the source

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