1. Chapter-7 Special topics in ranging Prof. Jae-Young Pyun Presentation By Amod Pudasaini WHYNET lab Date: 5 th May 2015 1

2. Contents Interference mitigation techniques Clear Channel Assessment Privacy and security for ranging signals 2

3. Interference mitigationtechniques Interference Scenarios Narrowband interference mitigation Multiuser interference in IEEE 802.15.4a networks 3

4. Interference scenarios(1/3) 4

5. Interference scenarios(2/3) CaseDescription A Some preamble symbols may get corrupted and those symbol cannot be used for SNR improvement. It impacts range accuracy because the achievable processing gain decreases. B SFD part is affected and frame synchronization is lost. PHR and payload data cannot be decoded. Retransmission is required. 5

6. Interference scenarios(3/3) CaseDescription C If data rate field of PHR gets corrupted irrecoverably then payload data use is almost impossible. Source of the frame cannot be identified and TOA will be of no importance Retransmission is required. D If interference damage MAC header, the sender cannot be identified. Even after successful acquisition and synchronization MAC payload becomes useless. 6

7. Narrowband Interference Mitigation(1/3) 7

8. Narrowband Interference Mitigation(2/3) Divide UWB spectrum into sub bands Deactivate the Sub band that corresponds to frequency band of narrowband device 8

9. Narrowband Interference Mitigation(3/3) A multicarrier template waveform that consists of several sub band pulses is correlated with the received signal. 9

10. Multiuser interference in IEEE 802.15.4a networks Users of UWB share a common spectrum and management of multiple access within the same spectrum is important. IEEE 802.15.4a specifies 16 frequency channels and only 2 different preamble sequence is allowed within each band, one for each simultaneously Operating Pico nets (SOP). Multiuser Preamble are constructed from either of two symbols. 10

11. Multiuser interference in IEEE 802.15.4a networks Different cases of MUI coming from same code User 11 Fig a) With no MUI present the desired output is obtained Fig b) tm<td and Em<Ed, MUI is subjected to be seen as leading path Fig c) tm>td and Em<Ed, MUI would look like another multipath component Fig d) tm>td and Em>Ed, receiver locks in MUI peaks failing the ranging

12. Multiuser interference in IEEE 802.15.4a networks Relative position of preamble pulse with respect to data burst for T sym =T f 12 T sym : Data Symbol interval T f : Preamble frame interval Fig a) Data Symbol Fig b) preamble pulse falls into guard interval Fig c) Collision of Preamble Pulse and Data burst Fig d) Preamble Pulse and data burst arrive at different half and they do not collide

13. Multiuser interference in IEEE 802.15.4a networks R pp : Ratio of preamble length to payload length T sym : Data Symbol interval T f : Preamble frame interval 13

14. Multiuser interference in IEEE 802.15.4a networks 14 Fig a) Interference free Co-relator output Fig b) Co-relator output with transient interference Fig c) Co-relator output with median filtering

15. Clear Channel Assessment(1/4) Clear Channel Assessment (CCA) is one of the mechanism to sense the channel. The channel is supervised whether it is free or busy to receive a packet from next node. It determines the current state of a wireless medium for collision avoidance. Eg, CSMA/CA In UWB preamble with minimum or near zero autocorrelation are used for time synchronization. The correlation peaks are used to detect preamble. 15

16. Clear Channel Assessment(2/4) Preamble symbols are multiplexed with the entire PPDU(PHY Protocol Data Unit). Insert preamble into header and payload part of IEEE 802.15.4a packet after every k-symbol long interval. The inserted preamble do not interfere with the data, but reduces data carrying capability.  TDMA-type multiplexed preamble scheme 16 * Si: Preamble Symbol

17. Clear Channel Assessment(3/4) Periodically repeated preamble symbols are superimposed onto the entire PPDU(PHY Protocol Data Unit) Power level of overlaid preamble is set to lower than data symbols, consequently the circuit complexity is increased.  CDMA-type multiplexed preamble scheme 17

18. Clear Channel Assessment(4/4) 18

19. Private Ranging(1/6) An intruder intends to figure out location of sensor and tries to tamper and disable them. Prevent ranging devices from obtaining correct range information A malicious device can behave as a snooper, imposter or jammer Snooper observes signals in secret to obtain information on whereabouts of devices. Imposter engages in deception under an assumed name or identity to cause error in victims range calculations. A jammer simply emits interference to stall communications in a network. 19

20. Private Ranging(2/6)  Challenge-Response 20

21. Private Ranging(3/6) Assuming Secret key K is shared A sends a random challenge nonce(i-bit) N j to B B responds with Nj and another random nonce(i-bit) M j A sets timer t j time elapse between challenge and response B sends to A a number v=h k (nj | |m j ), where h k is fast hash function A checks if h k (nj | |m j )=v or not If it is not equal then A considers it as a bogus. 21

22. Private Ranging(4/6)  Cyclic Shift of perfectly balanced ternary sequences IEEE 802.15.4a signal structure publicly specifies eight length-31 PBTSs for the preamble for ranging. Imposter can acquire a transmitted IEEE 802.15.4a sequence out of a set of eight, especially running 8 correlators in parallel. Cyclic Shift property of ternary sequences is employed to disguise imposter. Assume a ternary symbol waveform expressed as The transmitted ranging waveform is expressed as 22

23. Private Ranging(5/6)  Cyclic Shift of perfectly balanced ternary sequences 23

24. Private Ranging(6/6) Cyclic Shift operation preserves the perfect periodic auto correlation property, but it circularly shifts the correlation peaks. Imposters unaware of the cyclic shift index k can still observe ranging waveform, but with ckT s meters error.  Cyclic Shift of perfectly balanced ternary sequences 24

25. Private Ranging(6/6)  Dynamic Preamble Selection IEEE 802.15.4a Dynamic Preamble Selection(DPS) is used as privacy mechanism for ranging. Authentication message is used between ranging parties to use preamble symbol for ranging In the message, originator defines the target of the ranging symbol to be used in RFRAME req and RFRAME rep frames. Then TW-TOA ranging protocol is run 25

26. Thank You 26