M.S. Thesis Defense Ioannis Broustis broustis@cs.ucr Committee:

Achieving Efficient Spectrum Usage in Ultra Wide Band Ad Hoc Networks:The Case for a Multi-band Approach M.S. Thesis Defense Ioannis Broustis broustis@cs.ucr.edu Committee: Dr. Srikanth Krishnamurthy Dr. Michalis Faloutsos Dr. Mart Molle

Why deal with UWB? • The future is wireless • High Speed networks • Intra-home / office communication • Stream DVD content to HDTVs • Connect high-data rate peripherals • Current wireless solutions face problems • Main advantages offered by UWB • High data rates • Low-power operation and low cost • Low probability of detection • Low interference levels to existing services http://www.uwbplanet.com

Motivation & contribution • A lot of work has been done in the PHY layer of UWB • Only a few MAC schemes have been proposed for UWB • Most of them for master-slave deployments • Many assumptions - some of them cannot be implemented in the real world • We design and evaluate a novel multiband MAC protocol for UWB ad hoc networks that • Utilizes efficiently the available bandwidth • Achieves much better performance than other proposed MAC protocols for UWB • Conforms with FCC regulations

Roadmap • Overview of Impulse-Based UWB • What is UWB, Impulse-based PHY • What is the problem and how we address it • Our Multi-band MAC protocol • Simulation results • Conclusions - ongoing work • Questions

What is UWB? • 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 802.11a (0.1 GHz) EIRP EIRP limit: - 41.25 dBm/MHz Frequency (GHz) UWB Spectrum (7.5 GHz) 3.1 5.725 - 5.825 10.6

Impulse-based PHY • Band utilization • Single-Band Implementation • One sharp pulse occupies the whole BW at a time • Multi-Band Implementation • The 7.5 GHz are divided into multiple bands • The lowest limit of 500 MHz and the transmission power restrictions must be maintained • Multi-band: Beneficial • Reduces interference from co-located systems that share a portion of the BW • Parallel data transmissions in the different bands • Similar H/W cost 802.11a (5.725-5.825 GHz) EIRP 3.1 10.6

BPPM Tc frame time Signal transmission in impulse-based UWB • Time Hopping, based on Time Hopping Sequences (THS) • Pulse Position Modulation (PPM) • Many pulses transmitted per bit, to increase reliability • THS overlap is possible  pulse collisions THS1: 0, 3, 2, 6 Tf Tc 01234567 01234567 01234567 01234567 THS2: 4, 6, 3, 3 0 1

Roadmap • Overview of Impulse-Based UWB • What is the problem and how we address it • Our Multi-band MAC protocol • Simulation results • Conclusions - ongoing work • Questions

Power Delay Profile B A time What is the problem? • UWB pulses are subject to Multipath Delay Spread • Multiple time-shifted pulse copies appear at the Rx • Intersymbol Interference (ISI) • Tens of nanoseconds (~ 25-30nsec for indoor environments) obstacle obstacle

Potential solutions • Equalizers • Add considerable H/W complexity and overhead • CDMA + Rake receiver • Adds overhead and some H/W cost • Pulse spacing at least equal to the delay spread duration • The adoption of a multi-band mechanism does not reduce the data rate! Delay Spread Tc frame ~3nsec (10 bands) Multi-band Single-band Pulse width time ~0.3nsec

MAC overview • We divide the available BW into B bands • One band for requests and band information. The rest for data transmissions and ACKs • We conform with the FCC regulations • The maximum number of bands that we may have is 15, each of which will be 500 MHz wide • We obey to the maximum average EIRP of -41.25 dBm/MHz • Map of band availability • Superframes: Transmission of all control and data packets • Availability frames: Declare intention to keep using a band

MAC overview Data B-1 frequency ..….. ..….. Data 4 Data 3 Data 2 Data 1 ….. Control (REQ) time k1 k3 kB-1 k2 Superframe Availability frame Superframe

MAC details: protocol steps • Band selection and declare • Via the availability frame • Request initiation • In the control (REQ) band • THS of the Rx is used • Includes the selected band • Request acknowledgment and Data transfer • In the selected band, without Time Hopping • Data acknowledgment • In the selected band

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

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

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

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 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

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

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

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

REQ RACK DATA DACK READY Comparisons • 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

Simulation set-up • Simulator in C++

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

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

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

Simulations: average network throughput • Higher as much as 16.72% in our case

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

Simulations: band utilization • Poisson arrival traffic with very high arrival rate • Tendency for all bands to be used uniformly in the long run

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 • 2. Assume that in the single-band system: slot = pulse width • Determine the network capacity with and without an equalizer

Questions? (References available upon request) From: http://www.ida.gov.sg/idaweb/doc/download/I2887/UWB,_The_Standard_is_Set.pdf

EIRP • Effective Isotropic Radiated Power • Power emitted by an isotropic antenna, to produce the peak power density observed in the direction of maximum antenna gain • Antenna gain: the logarithm of the ratio of the antenna's radiation pattern to that of some ideal antenna • Often stands in decibels over a reference power level • EIRP(dBm) = power of transmitter - losses in transmission line + antenna gain • dBm: decibels per mWatt • EIRP is used to estimate the service area of the Tx definition csi.usc.edu/INTEL-USC/presentations/huang.ppt

Conformance with FCC regulations • Bandwidth: each of our bands is 500 MHz wide • Emission limits: • For the received SNR we have: • PT: Transmission power (-41.25 dBm/MHz) • N0: PSD of the thermal noise (114 dBm/MHz) • d: Tx-Rx distance • SNRR = 3 dB • 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 frequency is  1 MHz. • We conform with the peak power as well :) contribution

M.S. Thesis Defense Ioannis Broustis broustis@cs.ucr Committee: