1. Soft-in/ Soft-out Noncoherent Sequence Detection for Bluetooth: Capacity, Error Rate and Throughput Analysis Rohit Iyer Seshadri and Matthew C. Valenti Lane Dept. of Computer Science and Electrical Engineering West Virginia University iyerr, mvalenti @csee.wvu.edu

2. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 2/22 Achieve dramatic improvements in energy efficiency and throughput for Bluetooth with a minimal increase in complexity by using: –Sequence based, noncoherent demodulator –Bit-interleaving –Soft-decision decoding –Feedback from channel decoder to demodulator Obtain an information theoretic bound on the minimum signal to noise ratio required for reliable signaling –Bit-wise log-likelihood ratios used to compute Shannon capacity under modulation, channel and receiver design constraints Demonstrate performance improvements over popular receivers using an extensive simulation campaign –Evaluate packet error rate (PER) and throughput performance for data medium (DM) - rate packet types Objectives

3. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 3/22 Bluetooth Low cost/ low power connectivity for wireless personal area networks Operates in the license free 2.4 GHz ISM band Band divided into 79 channels, each 1 MHz wide. Channels changed up to 1600 times per second Channel symbol rate of 1 Mbps Uses Gaussian frequency shift keying (GFSK) –M =2 –B g T =0.5 –0.28 ≤h ≤0.35

4. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 4/22 Benchmark Bluetooth System Detector: Limiter discriminator integrator (LDI) Baseband GFSK signal during kT ≤ t ≤ (k+1)T GFSK phase Encoder: (15, 10) Shortened Hamming Code (SHC), single error correction code

5. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 5/22 Bluetooth System with Sequence Detection GFSK pulse shape causes adjacent symbol interference Detector: Soft-Decision differential phase detector with Viterbi decoding (SDDPD-VD), [Fonseka, 2001] Viterbi decoding can produce burst errors, which could be mitigated by bit-interleaving

6. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 6/22 Bluetooth System with SISO-SDDPD SDDPD-VD forms hard estimates on code bits Bit-interleaved coded modulation (BICM) LLRs from detector passed to decoder, which performs soft-decision decoding Additionally, soft-information can be also be fed from decoder to detector: BICM with iterative decoding (BICM-ID) SISO-SDDPD generates bit-wise LLRs for the code bits No gains over BICM Behavior explained using EXIT curves

7. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 7/22 SISO-Soft-Decision Differential Phase Detection Received signal at the output of a frequency nonselective, Rician channel, before filtering r’(t, a) = c(t) x(t, a) + n’(t) Received signal after filtering r(t, a) = c(t) x(t, a) + n(t) Received signal phase (t, a) = (t, a) +

8. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 8/22 SISO-Soft-Decision Differential Phase Detection Detector finds the phase difference between successive symbol intervals The GFSK pulse shape causes adjacent symbol interference The phase difference space from 0 to 2  is divided into R sub-regions Detector selects the sub-region D k in which lies The sequence of phase regions (D 0, D I, …) is sent to a branch metric calculator

9. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 9/22 SISO-Soft-Decision Differential Phase Detection Let be the phase differences corresponding to any transmitted sequence A branch metric calculator finds the conditional probabilities Branch metrics sent to a 4-state MAP decoder whose state transition is from to The SISO-SDDPD estimates the LLR z k for a k as

10. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 10/22 FEC for Bluetooth Bluetooth specifies 7 types of ACL packets for data transfer 6 out of the 7 packet types use cyclic redundancy check (CRC) and ARQ 3 out of these 6, i.e. data medium (DM1, DM3, DM5) also use a (15, 10) shortened Hamming code (SHC) for forward error correction (FEC) The (15, 10) SHC is cyclic and described by the generator polynomial The cyclic code can hence be expressed using a 2 5 = 32 state trellis and decoded by running either a Viterbi or MAP algorithm over the trellis

11. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 11/22 Capacity Under Modulation, Channel And Receiver Design Constraints Channel capacity denotes maximum allowable data rate for reliable communication over noisy channels In any practical system, the input distribution is constrained by the choice of modulation –Capacity is mutual information between the bit at modulator input and LLR at detector output Constrained capacity in nats is; [Caire, 1998]

12. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 12/22 Capacity Under Modulation, Channel And Receiver Design Constraints Constrained capacity for the proposed system is now In bits per channel use Constrained capacity hence influenced by –Modulation parameters (M, h and B g T) –Channel –Detector design Computed using Monte-Carlo simulations

13. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 13/22 Performance Evaluation and Comparisons Performance of proposed SISO-SDDPD with BICM compared against –Limiter discriminator integrator detector with hard decision channel decoding, with and without bit-interleaving: LDI-HDD –SDDPD-VD, followed by hard decision channel decoding, with and without bit- interleaving: SDDPD-HDD –SISO-SDDPD followed by soft decision channel decoding, without bit-interleaving: SISO-SDDPD-SDD Comparisons made on the basis of –Bit error rate –Packet error rate –Throughput

14. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 14/22 Bit Error Rate Comparison Scenario: Minimum E b /N o to achieve BER= 10 -4. Six simulated points from top to bottom are 1) LDI-HDD 2) LDI-HDD with interleaving 3) SDDPD-HDD 4) SDDPD-HDD with interleaving 5) SISO-SDDPD-SDD 6) SISO-SDDPD with BICM Information theoretic bound for SISO-SDDPD based BICM SDDPD specifications: R=24 uniform sub-regions Channel parameters: Nonselective, Rician fading, K =2 dB Bluetooth specifications : h =0.315, DM1 packet types SISO-SDDPD with BICM gives the best BER performance

15. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 15/22 Packet Error Rate Comparison Scenario: Packet error rate for DM1 packet types. SDDPD specifications: R=24 uniform sub-regions Channel parameters: Nonselective, Rician fading, K =2 dB Bluetooth specifications : h =0.315 DM1 packet types SISO-SDDPD with BICM gives the best packet error rate performance. Gain over LDI based systems = 9 dB Gain over SDDPD-HDD based systems =4 dB

16. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 16/22 Throughput Comparison Scenario: Throughput for DM1, DM3 and DM5 packet types Solid curve: Systems without interleaving Dotted curve: Systems with interleaving SDDPD specifications: R=24 uniform sub-regions Channel parameters: Nonselective, Rician fading, K =2 dB Bluetooth specifications : h =0.315 SISO-SDDPD with BICM gives the best throughput performance For maximal throughput, packet type should be adaptively selected to match SNR

17. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 17/22 Conclusions An energy efficient, noncoherent receiver design investigated for Bluetooth –Soft-in/ soft-out, soft decision differential phase detector developed –BICM paradigm applied to Bluetooth Error rate and throughput compared against LDI detector and Fonseka’s SDDPD with Viterbi decoding –SISO-SDDPD-SDD shown to outperform LDI-HDD and SDDPD- HDD –Additional gains possible with interleaving Constrained capacity found using Monte Carlo simulations

18. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 18/22 Future Work An algorithm that designs nonuniform phase regions using received phase differences and adapts itself to varying channel conditions and GFSK parameters –Nonunifrom regions can perform better than uniformly phase regions [Fonseka, 1999] –Results in a smaller look-up table Estimating the Rician K factor and E b /N o at the receiver using the Expectation-Maximization algorithm

19. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 19/22 Complexity Branch metric calculations in SISO-SDDPD –Metric calculations involve nonlinear functions –Pre-calculated and stored in a look-up table –Table needs to be updated once at each E b /N o Number of states in the detector –SISO-SDDPD operates on a M 2 - state trellis Number for states in the channel decoder, with soft-decision decoding –ML/ MAP decoding performed on a 32- state trellis

20. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 20/22 Sensitivity to h estimation errors Scenario: Effect of incorrect estimates of h on SISO- SDDPD and LDI detectors SDDPD specifications: R=24 uniform sub-regions Channel parameters: Nonselective, Rician fading, K =2 dB Bluetooth specifications : Correct value of h =0.315 Values assumed at detector =0.28, 0.35 DM1 packet types SISO-SDDPD more robust to incorrect estimates of h

21. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 21/22 EXIT Chart Scenario: EXIT chart for the SISO-SDDPD based BICM receiver SD-DPD specifications: R=24 uniform sub-regions Channel parameters: Nonselective, Rician fading, K =2 dB Bluetooth specifications : h =0.315, BgT =0.5 Detector EXIT curve predicts no improvement with BICM-ID

22. 4/6/2006 SISO-Noncoherent Sequence Detection for Bluetooth 22/22 Throughput Calculations Throughput: Maximum achievable, one way data rate [Valenti, 2002] N t : Total number of times a given packet must be transmitted (on an average) until it is successfully decoded N s : Number of slots occupied per round trip, including one return slot Duration of each slot: 625 µsec K u : Number of data bits in the packet type