Department of Information Engineering University of Padova, ITALY

1. Department of Information EngineeringUniversity of Padova, ITALY Special Interest Group on NEtworking & Telecommunications Carrier–Sense ARQ: Squeezing Out Bluetooth Performance while Preserving Standard Compliancy Andrea Zanella andrea.zanella@dei.unipd.it IEEE ICC 2009 June 14-18 in Dresden, Germany June 14-18 in Dresden, Germany

2. Motivations • Bluetooth was designed to be integrated in portable battery driven electronic devices  Energy Saving is a key issue! • Units periodically scan radio channel for valid packets • Scanning takes just the time for a valid packet to be recognized • Units that are not addressed by any valid packet are active for less than 10% of the time • WPAN market is expanding and it aims at becoming the standard the facto for short range communications  High Throughput is very welcome! • Bluetooth v2.0 + EDR (Enhanced Data Rate) promise bit rates up to 3 Mbps and faster node connections June 14-18 in Dresden, Germany

3. A B B B B B H G F H Retransmissions NAK MASTER • Automatic Retransmission Query (ARQ): • Each data packet is transmitted and retransmitted until positive acknowledge is returned by the destination • Negative acknowledgement is implicitly assumed! • Errors on return packet determine transmission of duplicate packets (DUPCK) • Slave filters out DUPCKs by checking their sequence number • DUPCKs waste energy & throughput!!! • A. Zanella "A Mathematical Framework for the Performance Analysis of Bluetooth with Enhanced Data Rate" , IEEE Transactions on Communications, August 2009 • M. Valenti and M. Robert, “Custom coding, adaptive rate control, and distributed detection for bluetooth,” VTC 2002 ACK SLAVE X A DPCK B X DPCK June 14-18 in Dresden, Germany

4. Aims of the work • Goal: • Improve performance by reducing the number of DUPCKs!!! • Method • Enhance the native ARQ scheme with Carrier Sense to avoid useless retransmissions • Carrier sensing is generally provided by last generation chipset • Carrier-Sense ARQ  CS-ARQ June 14-18 in Dresden, Germany

5. Key idea • Carrier-sense can be used to infer transmission outcome in case slave’s reply is not received! • Idle channel: slave did not received master’s pck and, hence, did not reply • RTX is needed • Busy channel: slave sent back ACK or NAK that got lost • RTX may generate DUPCK!!! June 14-18 in Dresden, Germany

6. Key idea (cont) • To avoid tx of DUPCKs, any time the slave’s feedback id doubtful (busy channel with no valid packet) enter a soliciting phase • In the soliciting phase • Suspend the ARQ scheme • Keep sending POLL packets until a valid frame is returned by the slave • Resume the ARQ scheme using the ACK info carried by the slave’s frame June 14-18 in Dresden, Germany

7. CS-ARQ: state diagram June 14-18 in Dresden, Germany Fetch a new packet Tx packet & wait for feedback Slave’s feedback NAK ACK ACK No valid pck rcvd Carrier Sense Slave’s feedback BUSY NAK IDLE Tx POLL No valid pck rcvd

8. Mathematical model (I) • Assumptions: • Single slave piconet • Saturated links • Master and slave have always packets waiting for transmission • Unlimited retransmission attempts • Packets are transmitted over and over again until positive acknowledgement • Static Segmentation & Reassembly policy • Unique packet type per connection • Independent error events in successive transmissions • Justified by FH • Model • CS-ARQ behavior described by by means of a 2-State Markov Chain • Define appropriate reward functions • Data, Energy, Time • Apply renewal reward theorem to get system performance • Throughput, energy efficiency, energy balancing, … June 14-18 in Dresden, Germany

9. Mathematical Model (II) • State A  Standard ARQ phase • Master transmits packets that have never been correctly received by the slave • State S  Soliciting Phase • Master transmits POLLs waiting for a valid reply frame Reception Event Index Slaves rx Master rx • State transition probabilities depend on the reception events… • Ds = Data successful • AC ok, HEAD ok, CRC ok • Df = Data failure • AC ok, HEAD ok, CRC error • Hf = HEAD failure • AC ok, HEAD error • Af = AC failure • AC error June 14-18 in Dresden, Germany PAS 1-PAS 1-PSA PSA A S

10. Mathematical Model (III) • State A  Standard ARQ phase • Master transmits packets that have never been correctly received by the slave • State S  Soliciting Phase • Master transmits POLLs waiting for a valid reply frame • The steady-state probabilities are, then, June 14-18 in Dresden, Germany PAS 1-PAS 1-PSA PSA A S

11. Reward Functions • For each state j we define the following reward functions • Tj= Average amount of time spent in state j • Dj(x)= Average amount of data delivered by unit x{M,S} • Wj(x)= Average amount of energy consumed by unit x{M,S} • The average amount of reward earned in state j is given by • Performance indexes • Energy Efficiency:  • Goodput: G June 14-18 in Dresden, Germany

12. Performance Analysis Results June 14-18 in Dresden, Germany

13. Case study scenario • The model is general and can be applied to any other scenario • We here focus on a simple scenario as an example of the results that analysis enabled by the model • Case-study • Rayleigh fading • Same SNR at master & slave • Downlink traffic only June 14-18 in Dresden, Germany

14. Goodput Std ARQ goodput • CS-ARQ yields from 20 kb/s to 100 kb/s goodput gain, provided that the correct packet format is selected • Single-slot packet formats do not bring any improvement • 2EDR is preferbale for SNR<20 dB, 3EDR for SNR>20 dB June 14-18 in Dresden, Germany

15. Energy efficiency Std ARQ energy efficiency • CS-ARQ yields from 5% up to 40% energy efficiency gain • 2EDR is preferbale for SNR<20 dB, 3EDR for SNR>20 dB June 14-18 in Dresden, Germany

16. Conclusions • CS-ARQ yields some performance gain almost for free • Only requires CS capability and very limited software changes • Furthermore, CS-ARQ is • Fully Standard compliant • Complementary to other performance enhancement schemes • Stand-Alone (suffices to be implemented by the master) June 14-18 in Dresden, Germany