Custom Coding Adaptive Rate Control and Distributed Detection

Custom Coding, Adaptive Rate Control, and Distributed Detection for Bluetooth Matthew C. Valenti Max Robert Assistant Professor Mobile & Portable Radio Research Group Lane Dept. of Comp. Sci. & Elect. Eng. Virginia Tech West Virginia University Blacksburgh, VA Morgantown, WV mvalenti@wvu. edu This work was supported by the Office of Naval Research under grant N 00014 -00 -0655, AOL, and the MPRG Affiliates Program copyright 2002

Motivation & Goals n Motivation Ø Ø n Bluetooth enables low cost/power wireless connectivity. However, range is restricted to ~10 m due to limited power, inefficient modulation, and modest error control capabilities. Goal of this study Ø Ø Develop strategies for improving the performance of Bluetooth in low SNR environments. Benefits: • Range extension. • Operate in noisy industrial environments. • Tolerate more interference. © 2002 Ø However, all proposed strategies comply with the standard. • We are not suggesting changes to the standard. 2/16

Features of Bluetooth n Radio layer Ø Gaussian frequency shift keying (BT=0. 5). • Nonorthogonal: 0. 28 h 0. 35 • 1 Megabaud over 1 MHz occupied bandwidth. n Baseband layer Ø Transmissions are broken into 625 sec slots. • A packet may be 1, 3, or 5 slots long. Ø Time-division duplexing (TDD). • Master/slave take turns transmitting. Ø Packet-by-packet frequency hopping. • 79 or 23 channels spaced 1 MHz apart. • Piconet synchronized to master’s clock. © 2002 Ø ACL Packets for data. • DHx (Data high rate): No FEC. • DMx (Data medium rate): (15, 10) Hamming FEC code. • ARQ used by both DMx & DHx (assisted by CRC). 3/16

ACL Packet Structure 72 bits Access Code 54 bits Packet Header Payload Header 8 or 16 bits n 0 -2744 bits Payload Data 0 -2712 bits CRC 16 bits Causes of frame error: Ø Failure to synchronize with access code. • Sufficient for T>65 bits of the 72 to be correct. © 2002 Ø Failure to decode the packet header. • Protected by triple redundancy code. Ø Failure to decode the payload. 4/16

Throughput over BSC Channel 800 Data Rate in kbps 700 600 DH 3 500 DM 5 400 DM 3 Slots per frame Average number of ARQ transmissions 300 200 100 © 2002 Data bits per frame DH 5 DH 1 DM 1 0 10 -5 10 -4 10 -3 e 10 -2 10 -1 5/16

Throughput in AWGN 800 DH 5 Data Rate in kbps 700 Performance of noncoherent & nonorthogonal FSK: 600 500 DM 5 400 DM 3 300 200 DH 1 DM 1 We assume h=0. 32 100 © 2002 DH 3 0 5 10 =E /N in d. B 15 s o 20 6/16

Throughput in Quasi-Static Rayleigh Fading Data Rate in kbps 800 Quasi-static Rayleigh fading: • SNR constant for entire frame. • Varies from frame to frame. • SNR is exponentially distributed. 700 600 DH 5 DH 3 500 DM 5 • Average throughput. 400 DM 3 300 200 DH 1 DM 1 © 2002 100 0 0 5 10 15 20 =Es/No in d. B 25 30 7/16

Custom Error Control n The AUX 1 packet Ø Ø Ø A seventh ACL packet type. Occupies one slot. CRC & ARQ are turned off. • Operates as a “noisy bit pipe”. • Whatever is received is passed up to application. Ø n Can use AUX 1 to transport a custom code Ø © 2002 29 bytes of payload data. Implement FEC & ARQ on host computers. • Sender: First CRC encode, then FEC encode. ü Any FEC code can be used: BCH, Reed Solomon, turbo. ü Some FEC codes can also perform error detection. • Receiver: Decode FEC code, then CRC code. ü If errors, must manually ask for retransmission. Ø No modification of Bluetooth standard is needed. 8/16

Example: BCH Coding in AWGN © 2002 Data Rate in kbps 150 Notes: Used 16 bit CRC plus (232, k) shortened BCH code t is the error correction capability of the code up to 2 d. B gain by using custom coding 100 BCH t=10 BCH coding bound 1 t 43 50 DM 1 0 5 5. 5 6 6. 5 7 7. 5 8 Es/No in d. B DM 3 8. 5 9 9. 5 10 9/16

Adaptive Rate Control Optimal packet type depends on instantaneous SNR. n Can select the packet to match the current SNR. n Ø n Most of the benefit comes from selecting from among a small set of packets. Ø Ø © 2002 n If custom coding is used, then can also pick the code parameters (e. g. t). Set {DH 5, DM 5, and DM 1} gives most of the gain. CQDDR is a protocol from CSR (David Mc. Call) which operates under same principle. Problem is that the channel SNR must be known a priori (predicted). Ø An alternative approach is to use hybrid-ARQ with incremental redundancy (which is “blind”). 10/16

Adaptive Coding for Quasi-Static Fading 800 Adaptive BCH: n Use AUX 1 to transport custom code. n Adapt t to match instantaneous SNR Fully Adaptive: n Choose from among 6 standard packets. n Can also choose a custom coded AUX 1 packet. n Gain is 1. 5 d. B. n The set {DH 5, DM 1} yields almost same performance (within 0. 1 d. B). Data Rate in kbps 700 600 500 DH 5 400 DM 3 300 “Fully” Adaptive 200 Adaptive BCH 10 100 © 2002 DM 5 0 DM 1 0 5 10 Es/No in d. B 15 20 25 11/16

Antenna Diversity Performance in fading can be improved by using multiple (receive) antenna elements. n Best performance improvement is achieved using maximal ratio combining. n Ø n Instead, we perform post-detection combining on a packet level. Ø Ø Ø © 2002 However, this is too complex and requires coherent detection. Ø Use CRC to determine if packet is correct or not. If a packet is correct at any antenna, then it will be accepted by the system. Packet is only needs to be retransmitted if it is incorrect at all antennas. Note that this requires a separate receiver for each antenna. 12/16

800 Average Throughput (kbps) 700 M=6 600 M=1 Gain @500 kbps 3. 2 d. B for M=2 6 d. B for M=6 500 400 300 Performance of Bluetooth With M-antenna elements Using packet-level combining Of DH 5 packets In quasi-static Rayleigh fading 200 100 © 2002 M=2 0 5 10 15 20 25 Average Es/No in d. B 30 35 13/16

Distributed Detection n Packet-level combining required the M antennas to be attached to M transceivers. Ø Ø AP #3 AP #2 Ø © 2002 AP #4 MS MS location A B AP #5 AP #1 Ø Ø No reason why they must be colocated. The transceivers could be connected through a backbone as in an infrastructure-based WLAN. Detection is distributed over space. When the mobile is equidistant to the M transceivers, performance is as if they are connected to the same device. However, the diversity advantage diminishes if mobile not in center. AP #6 14/16

800 AP #3 Average Throughput (kbps) 700 AP #2 M=6 AP #4 600 500 AP #1 MS MS location A B AP #5 AP #6 400 200 100 © 2002 Gain @500 kbps 0. 4 d. B for M=2 1 d. B for M=6 Performance of Bluetooth With M-antenna elements Using packet-level combining Of DH 5 packets In quasi-static Rayleigh fading When mobile is at location B 300 0 M=1 5 10 15 20 25 Average Es/No in d. B 30 35 15/16

Conclusion n Several strategies can be used to improve performance of Bluetooth. Ø Ø Each strategies complies with standard. Custom coding: • Use AUX 1 to transport custom BCH code. Ø Adaptive rate control: • Match the frame type to prevailing channel condition. Ø Antenna diversity: • Use M antennas, but combine at packet level. • Antennas don’t need to be co-located. ü Multiple Bluetooth devices can mimic antenna array. n Future work: Ø Channel tracking and prediction. © 2002 • Hybrid ARQ with incremental redundancy. Ø Actual implementation of these strategies. • Validation of channel models. Ø Application of similar concepts to 802. 11 16/16