MURI Review Meeting, UCSD - September 14, 2006

1. MURI Review Meeting, UCSD - September 14, 2006 PHY-aware MAC protocol design for MIMO ad hoc networks Michele Zorzi University of California at San Diego, and University of Padova, Italy Contributors: Paolo Casari, Francesco Rossetto, Marco Levorato and Stefano Tomasin US Army Research Office Multi-University Research Initiative (MURI) Grant # W911NF-04-1-0224

2. Outline • Issues in ad hoc networks with multiple antennas • A new solution for the beamforming gain asymmetry • A cross-layer solution for MIMO ad hoc networks • Backoff and coordination of transmissions • Conclusions

3. Multiple antennas in ad hoc networks • Recently introduced as a means for • Improving parallelism (greater spatial reuse) • Achieving higher bit rates • Bridging longer distances • Directional communications • Exploit higher transmit/receive antenna gain andnull signal propagation toward some directions • MIMO links • Many (not necessarily independent) PDUs are superimposedin space (need for channel tracking and adaptive weighing) • Greater channel capacity • Higher complexity at the receiver

4. Random access and MIMO ad hoc nets • We focus on random access techniques • Best known is IEEE 802.11 • Main ingredients/features of 802.11 • CSMA and collision avoidance via RTS/CTS • Clear view of network activity • Backoff and retransmissions • These aspects need to be revisited if MIMO is used • Collisions need to be controlled rather than avoided • Signaling is trickier if directionality is exploited • Difficult to know who is active/available • How is backoff applied (source vs. destination)? • We deal with some of these issues and try to get insight on developing effective random access schemes

5. Beamforming gain asymmetry [1],[2] • Parallelism may be significantly improved using multiple antennas • Control packets need to be sent omnidirectionally • Otherwise, lack of control information may impair correct data transmissions between parties • Beamforming may extend the range of transmissions, but most of the neighborhood will not detect the signal • Extending signaling range through directional transmissions only covers a fraction of space • Solution by Tassiulas et al.: use sequential directional transmissions • Solution proposed here: use proper Space-Time Codesto achieve omnidirectionality and range extension

6. Circular RTS approach • Sweeping the horizon increases latency, causing a potential throughput reduction • Needs a LOS environment for beamforming to be effective • Tassiulas assumes the azimuth can be perfectly sliced • As many slices as antennas • Actually, this is not true • Considering the 3dB beamwidths of endfire and broadside beams, the number of needed beams is • 5 beams for 4 antennas • 10 beams for 8 antennas Broadside 3dB beamwidth

7. Proposed signaling solution • Extend range maintaining omnidirectionality through STC • Design a solution that works in a general Ricean env. • Note that transmit CSI is not available (one different channel per user) • Solution • Use RX maximal ratio combining for array gain (needs training) • Use STBC at the transmitter to get coding gain instead of beamforming gain • Codes should be full-rate (for delay) and full-diversity (for outage) • Fading improves STBC performance through greater diversity • Generalized ABBA codes used(2x2  Alamouti, 4x4, 8x8) Space Time

8. BER results with 4 antennas • 4x4 STBC approach compared to ideal Circular RTS (perfect slices) • Total Rayleigh vs. total LOS environment • STBC outperforms CRTS in Rayleigh fading at higher SNR and vice versa, but in either case the difference is at most 2 dB • If the BER loss is tolerable, the delay abatement can be fully exploited

9. Network results • In a simulated network, the delay has been reduced by 20% to 50% • STBC always provides the required level of performance at lower delay • STBC works in every channel condition • With 8 antennas, only a half-rate 8x8 STBC has comparable BER wrt CRTS • Introduces a delay of 2… • …which is still lower than the 10-fold delay of CRTS in this scenario

10. Gain asymmetry: conclusions • An STC-based scheme can achieve increased-range communications omnidirectionally without delays • Can be used in signaling phase of common protocols • The delay decrease positively impacts the overall networking performance • Publications: [1] F. Rossetto, M. Zorzi; "A Space-Time based Approach to Solving the Gain Asymmetry in MIMO ad hoc Networks"; Proc. of IEEE VTC Spring, Melbourne, Australia, May 7-10, 2006. [2] F. Rossetto, M. Zorzi; "On gain asymmetry and broadcast efficiency in MIMO ad hoc networks"; Proc. of IEEE ICC, Istanbul, Turkey, June 11-15, 2006.

11. … Decision Feedback Multiuser Detection(V-BLAST) in ad hoc networks [3],[4] • Great advantage can be derived from the use of BLAST in MIMO ad hoc networks • Incoming signals have highly different attenuations (better case for multiuser detection) • More packets can be simultaneously sent (spatial multiplexing, SM) • Upon reception of multiple signals: • Select the one with highest SNR • Obtain ZF weights by (pseudo)-inverting the correlation matrix • Detect the signal • Cancel the detected contribution • Restart with the 2° highest SNR • …and so on Canceled Under detection SNR ZF nulling

12. Network design – Frame structure • The major drawback of BLAST is that receivers may easily become overwhelmed if too many signals • How to limit transmissions w/o hampering throughput? • Choice: organize transmissions in frames • RTS slots used to declare transmit needs (heard by all receivers) • Receivers use the CTS to limit own traffic and avoid overload • Incoming traffic estimates can be derived from the RTSs • DATA packets are sent according to info in CTSs • In the ACK phase, the receivers confirm any correct detection • NO NEED for symbol synchronization (it’s a complexity issue…)

13. Network design – CTS policy • In high traffic scenario, nodes cannot be allowed to send everything they request • Need for a policy to limit the number of TX granted • Reception and interference cancellation capabilities are limited by the maximum number of trackable channels • Choice: Follow Traffic (FT) policy • From RTSs, understand which packets are wanted and which are interferers • Always grant at least one wanted request • Take other requests in order of decreasing power, and grant them if they are wanted • Otherwise, account for the fact that they are interferers to cancel • Terminate when the maximum number of trackable channels is reached

14. Example of FT operations • Three wanted requests (4+4+2 packets in total) • Five interfering requests (14 pkt) • The red one isgranted first (greatest RX power) • Other are either granted (if wanted) or estimated and canceled (if interfering) • Some of them (lightest yellow) are neglected Transmission requests SNR Expected interference Mapped into channel estimation resources Result: a tradeoff betweenthroughput and protection from interference From RTSs

15. Other details on CTS policies • Balance between the needs for • High reuse and superposition of transmissions • Protection from stronger (highest power) interference • Acceptable receiver load (wanted + unwanted estimated PDUs) • Acceptable unestimated interference • Other examples of CTS policies • Partially Follow Traffic (PFT): according to the previous figure • If possible, the receiver grants all transmission requests first • If there are some estimation resources left, as many interferers as possible are accommodated, in order of decreasing RX power • Expected to be unable to sufficiently suppress interference • Do Not Follow Traffic (NFT): according to the previous figure • As many wanted transmissions as possible are granted • No interfering PDU is estimated and canceled • Expected to perform poorly, since it does not exploit MIMO benefits

16. Network simulation and parameters

17. PHY level approximation [5],[6] • A fully detailed bit-level simulation of a MIMO ad hoc network is overly complex • Need to reproduce the behavior of a multiuser detector • Need to transmit a large number of packets to achieve confidence • Solution: evaluate BER through a Gaussian approximation of the power of residual errors after cancellation • One-shot calculation of the SINR of the current detection Extracted signal power Space-filtered noise power UnestimatedinterferingPDUs’ power Error signal power(due to imperfectcancellations) Power of still uncanceled PDUs

18. Throughput performance • Throughput = # of correctly received wanted PDUs per frame • NFT shows the poorest performance • PFT sustains more traffic but soon shows congestion (insufficient interference protection) • FT does not experience deadlock because of its good balance between throughput and interference cancellation • Analysis and fully detailed simulation are in very good agreement

19. PDU reception success performance • % of correctly received PDUs • Performance agrees with the throughput results • The poor PFT and NFT behavior is mainly explained with low success in detecting incoming signals • Message: the tradeoff between throughput and interference protection is crucial in a MIMO ad hoc network

20. Cross-layer MAC/PHY: conclusions • Potential of MIMO PHY can be exploited via careful PHY-aware MAC design • Interesting tradeoffs to be explored • Important to have effective simulation tools • Publications: [3] P. Casari, M. Levorato, M. Zorzi; "On the Implications of Layered Space-Time Multiuser Detection on the Design of MAC Protocols for Ad Hoc Networks"; Proc. of IEEE PIMRC 2005. [4] P. Casari, M. Levorato, M. Zorzi; "Some Issues Concerning MAC Design in Ad Hoc Networks with MIMO Communications"; WPMC Symposium, Aalborg, Denmark, September 2005. [5] M. Levorato, S. Tomasin, P. Casari, M. Zorzi; "Analysis of Spatial Multiplexing for Cross-Layer Design of MIMO Ad Hoc Networks"; IEEE VTC Spring, Melbourne, Australia, May 7-10, 2006. [6] M. Levorato, S. Tomasin, P. Casari, M. Zorzi; "An Approximate Approach for Layered Space-Time Multiuser Detection Performance and its Application to MIMO Ad Hoc Networks"; Proc. of IEEE ICC, Istanbul, Turkey, June 11-15, 2006. [7] M. Zorzi, J. Zeidler, A. Anderson, A. L. Swindlehurst, M. Jensen, S. Krishnamurthy, B. Rao, J. Proakis; "Cross-Layer Issues in MAC Protocol Design for MIMO Ad Hoc Networks"; IEEE Wireless Communications Magazine (special issue on smart antennas), August, 2006.

21. On channel access persistency and coordination among nodes [8,9] • The previous scenarios comprise frame-structured transmissions, a CTS policy, and a random exponential backoff in case a CTS is not received • The rationale is to limit persistency in transmission attempts until local overload has passed • Persistency reduction techniques, such as backoff • Reduce local traffic, avoiding congestion • Allow FT and similar policies to work on a smaller number of requests, thus being more efficient • Backoff policies have a great impact on network performance and deserve a deeper understanding • More effective concept: try to avoid TX to receivers who can’t hear • New challenge: how to know receivers’ availability?

22. Exponential backoff schemes • In order to understand which backoff configuration achieves the best performance, we compare • Destination-wise backoff (DEST–LOCK) which blocks transmission toward a single destination each time • Node-wise backoff (NODE–LOCK), which blocks any transmission • In either case, the silence time lasts for a number of frames randomly chosen in the interval [1,Bmax], with Window growth parameter Number of consecutive failures

23. NODE–LOCK throughput • Node-Lock prevents all transmissions upon unheard response to RTS • Result: channel access is more limited • Tuning the window growth parameter W changes the saturation throughput value • W = 1 is the best choice here (balances silence time and need to transmit) Increasing W

24. DEST–LOCK throughput • Dest-Lock only blocks transmissions toward unavailable receivers • Thus on average, nodes are left more free to transmit • Unlike Node-Lock, here we need to increase W, so that dest-wise silences are longer • Best performance is obtained with W = 16 (best transmission/silence balance) • Node-Lock outperforms the best Dest-Lock Increasing W

25. Beyond backoff: DSMA [8] • Distributed Scheduling for MIMO Ad hoc networks • Better results are expected if nodes could know exactly who is free to transmit to (impossible in random access) • Receivers: piggy-back in the ACK packet a special reservation message directed to wanted destinations • Transmitters and idle nodes: receive this ACK and, if reserved, refrain from transmissions in the following frame • This way, there is a higher probability that a wanted party is not engaged in something else, thus more links activated • Better parallelism and spatial reuse • More unlikely that no CTS response because of a busy receiver

26. DSMA: further details • Idle nodes’ probability of transmitting is constrained, so that they are somehow forced to listen to reservation even in high traffic situations: this prob is called ptx • Receivers do not always reserve a node, but do this with a tunable probability, called preserve • The maximum number of different nodes that can be reserved is dmax. • Thus, upon reservation any node scans its queue until it finds at most dmax different nodes to reserve • We tested this protocol and compared it to the previous schemes based on random access with backoff

27. DSMA throughput (ptx = 0.1) • The reservation scheme used in DSMA achieves a good coordination • In turn, this means better throughput… • …with almost no further overhead (reservations are included inside ACKs) • Drawback: the parameters (e.g., preserve) have different optimal values depending on traffic Increasing pr

28. Backoff and DSMA: conclusions • Different levels of aggressiveness: nodelock vs destlock • Interesting tradeoffs can be studied • Distributed scheduling shows promise for improvements • Actual behavior depends on various parameters • These are preliminary results, more under way • Publications: [8] M. Levorato, P. Casari, M. Zorzi, "On the Performance of Access Strategies forMIMO Ad Hoc Networks", accepted to IEEE GLOBECOM, San Francisco, CA, USA, Nov 27 - Dec 1, 2006. [9] P. Casari, M. Levorato, M. Zorzi; "DSMA: an Access Method for MIMO Ad Hoc Networks Based on Distributed Scheduling"; Proc. of ACM IWCMC, Vancouver, Canada, July 3-6, 2006.

29. Conclusions • Multiple antennas in ad hoc networks are a very promising enabling technology for next generation wireless systems • The new challenges posed call for protocol design strategies that explicitly account for the special PHY • Cross-layer design is very helpful – information exchange among layers is essential • A mere stacking of an old MAC on a (though higher-capacity) MIMO PHY cannot do the job • Our study focused on the effects and consequences, pros and cons of such interactions, toward a deeper understanding of the use of MIMO in ad hoc networks

30. Current and future work • Include multihop operation in the study • Evaluate the performance of such schemes • Identify possible design tradeoffs, and MAC/routing strategies • Include waveform design in the study • We are evaluating LASTMUD with CDMA – other possibilities • Study the effect of power control • Study the effect of imperfect channel estimates • Look for improvements of DSMA • Study dependencies on traffic and other parameters • Compare random access and scheduled schemes

31. Published papers Effective use of Space-Time and Directional Communications [1] F. Rossetto, M. Zorzi; "A Space-Time based Approach to Solving the Gain Asymmetry in MIMO ad hoc Networks"; Proc. of IEEE VTC Spring, Melbourne, Australia, May 7-10, 2006. [2] F. Rossetto, M. Zorzi; "On gain asymmetry and broadcast efficiency in MIMO ad hoc networks"; Proc. of IEEE ICC, Istanbul, Turkey, June 11-15, 2006. Decision-Feedback Multiuser Detection in ad hoc nets: PHY evaluation and MAC design [3] P. Casari, M. Levorato, M. Zorzi; "On the Implications of Layered Space-Time Multiuser Detection on the Design of MAC Protocols for Ad Hoc Networks"; Proc. of IEEE PIMRC 2005. [4] P. Casari, M. Levorato, M. Zorzi; "Some Issues Concerning MAC Design in Ad Hoc Networks with MIMO Communications"; WPMC Symposium, Aalborg, Denmark, September 2005. [5] M. Levorato, S. Tomasin, P. Casari, M. Zorzi; "Analysis of Spatial Multiplexing for Cross-Layer Design of MIMO Ad Hoc Networks"; IEEE VTC Spring, Melbourne, Australia, May 7-10, 2006. [6] M. Levorato, S. Tomasin, P. Casari, M. Zorzi; "An Approximate Approach for Layered Space-Time Multiuser Detection Performance and its Application to MIMO Ad Hoc Networks"; Proc. of IEEE ICC, Istanbul, Turkey, June 11-15, 2006. [7] M. Zorzi, J. Zeidler, A. Anderson, A. L. Swindlehurst, M. Jensen, S. Krishnamurthy, B. Rao, J. Proakis; "Cross-Layer Issues in MAC Protocol Design for MIMO Ad Hoc Networks"; IEEE Wireless Communications Magazine (special issue on smart antennas), August, 2006. [8] M. Levorato, P. Casari, M. Zorzi, "On the Performance of Access Strategies forMIMO Ad Hoc Networks", accepted to IEEE GLOBECOM, San Francisco, CA, USA, Nov 27 - Dec 1, 2006. [9] P. Casari, M. Levorato, M. Zorzi; "DSMA: an Access Method for MIMO Ad Hoc Networks Based on Distributed Scheduling"; Proc. of ACM IWCMC, Vancouver, Canada, July 3-6, 2006.