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20/40 MHz 11n Interference on Bluetooth

20/40 MHz 11n Interference on Bluetooth. Authors:. Date: 2008-09-03. Abstract.

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20/40 MHz 11n Interference on Bluetooth

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  1. 20/40 MHz 11n Interference on Bluetooth Authors: Date: 2008-09-03 John R. Barr, Motorola Inc.

  2. Abstract Over the air measurements of interference experienced by shipping Bluetooth devices in the presence of 40 MHz 802.11n signals in 2.4 GHz spectrum. Measurements document that 40 MHz 802.11n signals from AP/STA seriously degrade performance of Bluetooth applications at distance of 4 ft, and degrades performance even as distance is increased to 12 ft. 40 MHz 802.11n signals (13% duty cycle) increase number of retransmissions sent by a factor of 15 (2% to 30%) and reduce the number of error free packets by a factor of two (94% to 55%) while running the same Bluetooth application. John R. Barr, Motorola Inc.

  3. d1 Motorola S9 Agilent signal generator E4438C Frontline Bluetooth Sniffer Bluetooth link Motorola Z8 Laptop running Agilent N7617B Agilent WLAN signal studio Test setup • 802.11n 40 MHz signal is generated using Agilent N7617B. Modulation is 64-QAM, 2 spatial streams, 5/6 coding rate, guard interval is 800nsec. Different duty cycles (% - ON) are used to emulate different traffic patterns (25% - low, 50% - medium and 75% - high). • Tests modified for 13% low at 40 MHz and 25% low for 20 MHz to compare equal traffic. • Motorola S9 is worn on the head by tester. Motorola Z8 was in the front pants pocket (display out and front side up). Distance between Motorola S9 and Motorola Z8 is ~3ft. Tester is sitting while doing the tests. This orientation is used to emulate a typical link margin between phone and headset. • Tests were performed for Bluetooth SCO, Bluetooth A2DP and Bluetooth sniff modes. • Frontline Bluetooth sniffer is used to collect the following packet statistics: CRC error in payload, Header Errors, number of retransmissions and number of packets in no error. • The packet statistics are measured at different times after the 802.11n 40MHz interference has been turned ON. This is to understand the effect of interference before and after AFH removes the affected channels. John R. Barr, Motorola Inc.

  4. Transmit Power Variations for Distance • The transmit power is varied on the signal generator to emulate varying distances between signal generator and Bluetooth devices. The antenna used on the signal generator was a monopole antenna. • The maximum transmit power from our signal generator is +17dBm. Including the antenna gain (~2.2dB) the output power will be ~19dBm. Our signal generator was ~2 ft from the headset and transmitting at power levels of +19dBm, +13dBm, +7dBm, +3.4dBm, +1dBm to represent separation distances of 2ft, 4ft, 8ft, 12ft and 16ft respectively. • This correlates with typical free space path loss measurements. • Other power level values can be used for respective distances if required. John R. Barr, Motorola Inc.

  5. Bluetooth SCO link802.11n interference with AFH turned OFF for phone and headset 1 KHz recording played with 802.11n 20 MHz (25% duty cycle) 1 KHz recording played with 802.11n 40 MHz (25% duty cycle) 1 KHz recording played with 802.11n 40 MHz (13% duty cycle) • Difficult to quantify SCO performance in terms of packet error statistics (no CRC on payload as in eSCO packets) • Thus, qualitative audio samples were taken. A call was placed to a 1 KHz test line. Audio samples were then recorded with AFH disabled for varying 802.11n parameters. • Numerous crackles can be heard in the 1 KHz tone in the presence of 802.11n interference John R. Barr, Motorola Inc.

  6. Bluetooth eSCO link802.11n 40 MHz low duty cycle (13%): 4ft from headset Packet statistics after interference is present for 120s Packet statistics after interference is present for 30s Packet statistics with no interference Packet statistics after interference is present for 10s • Minimal impact to eSCO due to 802.11n 40MHz (mid and high duty cycle) interference. • Minimal clicks and pops heard in audio in presence of 802.11n 40MHz low duty cycle interference with these specific Bluetooth devices since AFH keeps releasing the “affected” channels due to low duty cycle of interference. John R. Barr, Motorola Inc.

  7. Bluetooth eSCO link802.11n 20 MHz low duty cycle (25%): 4ft from headset Packet statistics after interference is present for 120s Packet statistics after interference is present for 30s Packet statistics with no interference Packet statistics after interference is present for 10s • Minimal impact to eSCO due to 802.11n 20MHz (mid and high duty cycle) interference. • Minimal clicks and pops heard in audio in presence of 802.11n 40MHz low duty cycle interference with these specific Bluetooth devices since AFH keeps releasing the “affected” channels due to low duty cycle of interference. John R. Barr, Motorola Inc.

  8. Bluetooth A2DP link802.11n 40 MHz low duty cycle (13%): 4ft from headset Packet statistics after interference is present for 30s Packet statistics after interference is present for 120s Packet statistics with no interference Packet statistics after interference is present for 10s • When the distance between 802.11n and Bluetooth devices is less than 12ft, irrespective of the duty cycle of the interference, there are numerous silence periods until the time AFH detects the bad channels. Even after AFH detects the interference, due to the high interference level in the remaining channels, we still have audible silence periods in the music. • A significant amount of retransmissions, CRC errors in payload and header errors are observed. John R. Barr, Motorola Inc.

  9. Packet statistics after interference is present for 120s Packet statistics after interference is present for 30s Packet statistics with no interference Packet statistics after interference is present for 10s Bluetooth A2DP link802.11n 40 MHz low duty cycle (25%): 4ft from headset • As the duty cycle of the interference increases, there are more retransmissions. • Music quality is very poor due to audible pauses till the time interference is not detected. Interference detection time will vary depending on the AFH implementation. • After 120s, this is the worst case scenario for A2DP with maximum retransmissions. John R. Barr, Motorola Inc.

  10. Packet statistics after interference is present for 120s Packet statistics after interference is present for 30s Packet statistics with no interference Packet statistics after interference is present for 10s Bluetooth A2DP link802.11n 20 MHz low duty cycle (25%): 4ft from headset • Minimal impact to Bluetooth A2DP performance since AFH detects the bad channels and removes them from the channel map John R. Barr, Motorola Inc.

  11. Bluetooth link in sniff mode • For tests conducted when the Bluetooth devices are in idle mode (in sniff), it was observed that at 2ft with high and medium 802.11n 40 MHz traffic, it is possible to drop the Bluetooth connection John R. Barr, Motorola Inc.

  12. Backup Slides John R. Barr, Motorola Inc.

  13. Bluetooth SCO link with 802.11n 40 MHz signal John R. Barr, Motorola Inc.

  14. Bluetooth A2DP link with 802.11n 40 MHz signal John R. Barr, Motorola Inc.

  15. References • https://mentor.ieee.org/802.11/file/08/11-08-0984-00-000n-bluetooth-test-cases-for-802-11n-40-mhz.ppt John R. Barr, Motorola Inc.

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