Legacy Coexistence – A Better Way?

1. Legacy Coexistence – A Better Way? • Authors: • Date: 2007-11-26 Hart et al (Cisco)

2. Summary • Some say VHT should steer clear of 5 GHz because of the coexistence problems • 11n has taken a long time to resolve just 20/40 coexistence • We argue that this is mainly because of limitations in early implementations leading to standards compromise • If we start with solid PHY coexistence via additional modest RX requirements, many coexistence problems disappear Hart et al (Cisco)

3. Coexistence of 11n with 11abg is a standards compromise • A collection of protections • some for sound technical reasons • others based on what vendors had (or had not) implemented some time in the past Hart et al (Cisco)

4. Some coexistence mechanisms are disconnected from legacy behavior • In 20/40, CCA protection via ED at -62 dBm on the secondary • More hidden nodes on the secondary • With GF, CCA protection via ED at -72 dBm • More hidden nodes with GF • In 20/40, no virtual carrier sense on the secondary • RTS, CTS, CTS2self on secondary not respected • Duration/ID field on secondary not respected • Duration/ID field in 40 MHz frames not respected by legacy and non-40MHz devices on either channel (weak ACK protection, should start a TXOP with a 20 MHz frame) • In 20/40 exponential backoff using medium busy measured on the primary only, with a brief (PIFS) CCA inspection on the secondary • Less responsive to congestion on the secondary channel Hart et al (Cisco)

5. Can We Devise An Improved CCA? • ED at -82 dBm is challenging: • Requires either a low noise floor or causes a higher false-busy rate • Preamble detection at -82 dBm is well understood for 20 MHz • For 40/80/160 MHz, this requires 2/4/8 parallel filters for each 20 MHz sub-channel, and parallel short symbol detectors • Preamble detection is ineffective after a transmission, for in-progress frames on other 20 MHz channels • We need a mid-packet CCA Hart et al (Cisco)

6. Mid-Packet CCA • Packet detection without a preamble • No carrier frequency recovery • No timing recovery • No channel estimation • OFDM looks like Gaussian noise yet can be identified by its regular cyclic extension • Note: mid-packet CCA is possible for DSSS and CCK also • DSSS cross-correlation • CCK is composed of QPSK chips, so x4/|x|3 looks like DC • Obscured by carrier frequency offsets and delay spread Hart et al (Cisco)

7. Example Scheme for Mid-Packet CCA for OFDM • Many further improvements are possible (e.g. short GI) Hart et al (Cisco)

8. Mid-Packet CCA Performance – Channel Type & Detection Duration Hart et al (Cisco)

9. Mid-Packet CCA Performance – SNR Hart et al (Cisco)

10. Mid-Packet CCA Performance – Carrier Frequency Offset Hart et al (Cisco)

11. Mid-Packet CCA Performance – Number of RX Antennas Hart et al (Cisco)

12. How to Interop Test for Mid-Packet CCA Compliance • Test 1: 40 MHz BSS enabled. 20 MHz BSS on primary alternates (a) periods of near-100% duty cycle long-packet SIFS-spaced traffic with (b) periods of no traffic. Record PER. • Test 2: 40 MHz BSS enabled. 20 MHz BSS on secondary alternates (a) periods of near-100% duty cycle long-packet SIFS-spaced traffic with (b) periods of no traffic. Verify 40 MHz frames are transmitted from B to A (e.g. via MAC stats). Record PER. • Any excess PER from test @ to test 1 is due to disallowed transmissions from DUT B Hart et al (Cisco)

13. Summary of Coexistence Mechanisms • Parallel start-of-packet detection on each 20 MHz channel is feasible • Parallel filters and short symbol detectors • Parallel mid-packet detection on each 20 MHz channel is feasible • Parallel filters and cyclic extension detectors • Behavior at packet end is TBD: EIFS backoff? Something else? • Parallel PLCP decoding on each 20 MHz channel is tougher but is not so hard it can be ruled out yet • Parallel receivers, but for the brief BPSK PLCP only • Parallel virtual carrier sense on each 20 and 40 MHz channel is infeasible • Fully parallel receivers • Ability to receive on one channel even when transmitting on a nearby channel Hart et al (Cisco)

14. Questions? • ? Hart et al (Cisco)

15. Strawpoll • If VHT produced a PAR for 5 GHz operation, would you support further investigation into improved legacy coexistence methods such as are described on slide 13? • Yes • No • Abstain Hart et al (Cisco)

16. Backup Slides Hart et al (Cisco)

17. Open Problems • What about false alarms from other wireless systems? • What about false alarms from adjacent-channel WiFi? • How well does this work with short-GI? Hart et al (Cisco)

18. False Alarms from Non-WiFi Signals • Previous scheme was optimized for OFDM vs noise • What about false alarms from other wireless systems? • Sinusoids that positively autocorrelate at any delay • Narrowband signal that approximate sinusoids • etc • Target the cyclic extension specifically: • We have a positive autocorrelation for 800ns then “noise” for 3.2us • Replace 800ns moving average impulse response with an impulse response with zero mean Hart et al (Cisco)

19. False Alarms from Non-WiFi Signals – Still OK against noise Hart et al (Cisco)