A Multiband MAC Protocol for Impulse-based UWB Ad Hoc Networks

1. A Multiband MAC Protocol for Impulse-based UWB Ad Hoc Networks Ioannis Broustis, Srikanth V. Krishnamurthy, Michalis Faloutsos, Mart Molle and Jeffrey R. Foerster {broustis, krish, michalis, mart} @ cs.ucr.edu jeffrey.r.foerster @ intel.com

2. The context • UWB pros • High data rates • Low-power operation and low cost • Low probability of detection • Low interference levels • Wireless needs • High Speed networking • Low cost, low power transport • Home, enterprise environments • Current wireless solutions • Low data rates, high power consumption Picture from http://kom.aau.dk/group/03gr1096/thesis.pdf

3. Motivation & contribution • A lot of work has been done in the PHY layer of UWB • Only a few MAC proposals for UWB • Most of them for master-slave deployments • Many assumptions - some of them cannot be implemented in the real world • Some do not take into account the PHY characteristics • We design and evaluate a novel multiband MAC protocol for UWB ad hoc networks • Utilizes efficiently the available bandwidth • Achieves much better performance than other MAC protocols for Ad Hoc UWB • Conforms with FCC regulations

4. Roadmap UWB Overview The problem Our MAC protocol Conclusions Simulation Results

5. UWB definitions • Any signal that occupies: • At least 500 MHz of bandwidth, or • More than 25% of a fractional bandwidth: • Available bandwidth: 7500 MHz • FCC has allocated the band from 3.1 GHz to 10.6 GHz for UWB communications • Emission levels must fall under max limits (average -41.25 dBm/MHz) • Traditionally: pulse transmissions • Range: 0 to 15m PSD 802.11a (0.1 GHz) EIRP FCC limit: - 41.25 dBm/MHz Frequency (GHz) UWB Spectrum (7.5 GHz) 3.1 5.725 - 5.825 10.6

6. Bandwidth utilization 802.11a (5.725-5.825 GHz) EIRP 3.1 10.6 • Single-Band Implementation • One transmission occupies the whole BW at a time • Multi-Band Implementation • The 7.5 GHz are divided into multiple bands • FCC regulations must be obeyed • Benefits from multiband approach • Low interference from/to systems that share a portion of the BW • Parallel data transmissions in the different bands • Similar H/W cost with single-band implementations

7. Impulse-based UWB Tc frame • Time Hopping, as per Time Hopping Sequences (THS) • Binary Pulse Position Modulation • Many pulses per bit, to increase reliability • THS overlap  Pulse collisions • Tx, Rx based THS • PAM also possible time 0 1 THS1: 0, 3, 2, 6 Tf Tc 01234567 01234567 01234567 01234567 THS2: 4, 6, 3, 3

8. Roadmap UWB Overview The problem Our MAC protocol Conclusions Simulation Results

9. What is the problem? Power Delay Profile time • UWB pulses are subject to Multipath Delay Spread • Multiple time-shifted pulse copies appear at the receiver • Intersymbol Interference (ISI) • Tens of nanoseconds (~ 25 to 30nsec for indoor environments) • Collisions at the receiver, with subsequent pulse transmissions • From the same or different transmitter obstacle B A A obstacle

10. Potential solutions Delay Spread Tc frame ~3nsec (10 bands) Multi-band Single-band Pulse width time ~0.3nsec • Equalizers, CDMA + Rake receiver • Add overhead and Hardware cost • Pulse spacing at least equal to the delay spread duration • The adoption of a multi-band mechanism does not reduce the data rate • A set of carriers modulate the pulse in each band and determine the pulse shape

11. Roadmap UWB Overview The problem Our MAC protocol Conclusions Simulation Results

12. MAC overview Data B-1 frequency ..….. ..….. Data 4 Data 3 Data 2 Data 1 Control (REQ) ….. time k1 k2 k3 kB-1 Superframe Availability frame Superframe • We divide the available BW into B bands • One band for requests and band information. The rest for data transmissions and ACKs • Map of Band availability • Superframes: Transmission of all control and data packets • Availability frames: Declare intention to keep using a band

13. Conformance with FCC regulations • Bandwidth: each of our bands is 500 MHz wide • Emission limits : -41.25 dBm/MHz • For the received SNR we have: • Attenuation for each band • PT: Transmitter PSD (-41.25 dBm/MHz) • N0: PSD of the thermal noise (-114 dBm/MHz) • d: Tx-Rx distance • SNRR = 3 dB • fc for the upper band • For the last band: fc = 10.35 GHz  distance ~ 7 meters • We set this distance as the maximum distance for all bands • We use lower transmission powers for the other (lower) bands • We conform with the average power and the pulse frequency is  1 MHz.  We conform with the peak power constraint as well :) contribution

14. MAC details: band selection • Nodes that intend to keep occupying a data band, transmit a short beacon during the availability frame • The rest of the nodes “listen” to the whole availability frame • Information about which bands will be occupied during the upcoming superframe Data band k slot k REQ band Superframe Superframe Availability frame

15. MAC details: request (REQ) initiation • The REQ packet is transmitted in the Req-band • It includes the selected band of the Tx • The receiver’s THS is used • Nodes are allowed to initiate a REQ transmission only at the beginning of a superframe Data band Data band Free Free Free REQ band REQ (Receiver’s THS) Availability frame Superframe

16. MAC details: REQ acknowledgment • 4 possible cases • 1. Everything goes fine • The receiver decodes the request • Both nodes switch to the selected band • The receiver sends the RACK packet (consecutive pulses) • The Data and DACK packet transmissions follow Data band RACK DATA DACK Data band REQ band REQ Availability frame Superframe

17. MAC details: REQ acknowledgment • 4 possible cases • 2. Two or more requests towards the same receiver collide • The receiver cannot decode the request • The transmitters switch to their selected bands, waiting for the RACK • After a specific time interval they will assume that their request did not reach the receiver • Backoff timers are initiated (decreased by one per superframe) • When backoff=0 the node retransmits the request Data band Response not received Back-off countdown Data band REQ (same THS) REQ band REQ (same THS) REQ (same THS) Availability frame Availability frame Superframe … … … Superframe

18. MAC details: REQ acknowledgment • 4 possible cases • 3. The intended receiver is currently busy • The receiver will not hear the request • The transmitter however will switch to its selected band • The transmitter initiates a backoff timer and retransmits the request as soon as this timer becomes zero Data-band DACK DATA chunk from C to D Response not received Back-off countdown Data band REQ towards node C REQ band REQ Availability frame Superframe … … … Superframe

19. MAC details: REQ acknowledgment • 4 possible cases • 4. Two or more RACKs collide • If two or more transmitters select the same band, a RACK collision is likely to occur in that data band • Further actions are temporarily aborted, until the upcoming availability frame • The requests are retransmitted after the end of the upcoming availability frame • With our policy, Data packet collisions are avoided RACK Data band RACK Abort Temporarily Data band REQ REQ band REQ Availability frame Superframe

20. MAC details: DATA and DACK • The RACK, DATA and DACK packets are transmitted with consecutive pulses • After the end of the session, transmitter and receiver switch to the REQ band • If they don’t have packets to send, they stay idle listening to their own THSs Data band RACK DATA DACK Data band REQ band REQ Availability frame Superframe

21. Roadmap UWB Overview The problem Our MAC protocol Conclusions Simulation Results

22. Comparisons REQ RACK DATA DACK READY • We compare our scheme with a single-band approach, in which: • THSs are used for all kinds of packets. • Each pair of nodes has a predetermined common - unique THS A B Steps: The Tx sends a request to the Rx as per the Rx’s THS Both Tx and Rx switch to their common THS. The Rx sends a reply back The Tx further transmits the data packet The Rx sends an ACK as soon as it receives the data packet Both Tx and Rx switch to their own THSs. They further transmit a short beacon to indicate their availability

23. Simulation set-up • Simulator in C++

24. Simulations: pulse collisions • Decreased by an order of magnitude • Data packets in our case are collision-free

25. Simulations: BER • The bit error rate is decreased by more than 4 times in our case

26. Simulations: average packet delay • Time from: packet arrival in the queue until completion of its transmission • Decreased by a factor of 6 for low densities

27. Simulations: average network throughput • Higher as much as 16.7% in our case • Light traffic  beneficial for the single-band case • Would observe larger difference with heavier traffic 

28. Simulations: average throughput for high loads • High CBR arrival rate • More than an order of magnitude better throughput in our case

29. Roadmap UWB Overview The problem Our MAC protocol Conclusions Simulation Results

30. Conclusions • We propose a novel multiband MAC protocol for UWB ad hoc networks • Better network performance than previous impulse-UWB MAC • No equalizer or CDMA required to address the delay spread effects • Utilizes efficiently the 7.5 GHz bandwidth • Adopts all the advantages of a multiband UWB approach • Respects the FCC regulations • Our ongoing work with UWB: • 1. New multiband MAC that employs binary conflict resolution • Applicable for home, office and wearable ad hoc networks • Demonstrates much better performance in terms of throughput and delay

31. Questions? (References available upon request)