1. Performance Evaluation of Channel Access Mechanisms in 6 GHz Spectrum
IEEE Vienna, 17 July 2019
802.11 Coexistence Workshop
Performance Evaluation of Channel Access Mechanisms in 6 GHz Spectrum
Jiayin Zhang
Weiwei Fan
David Mazzarese
Mohamed Salem

2. Background
Potential Access Mechanisms in 20MHz operation bandwidth
Evaluation assumptions, methodologies and results
Narrow band vs. Wideband LBT in 6 GHz
Conclusions

3. Background
There are discussions to open 6 GHz for unlicensed usage in both USA and EU
In FCC, the potential spectrum is from 5.925GHz to 7.125GHz, further divided into 4 bands of UN-II-5/6/7/8.
In ETSI, new WI on 5.925GHz to 6.425GHz was approved in June 2019 by TC BRAN (BRAN(19)102016r2).
The 6 GHz unlicensed spectrum attracted interests from both IEEE and 3GPP NR-U
3GPP NR-U in Release 16 will support operations in 6 GHz (RAN1 freeze at the end of 2019) .
IEEE802.11ax extended design of 5 GHz into 6 GHz. The testing in WFA (WiFi6) for 6GHz is expected in Jan
IEEE802.11be will also support 6 GHz spectrum in the future.
In 3GPP NR-U Rel-16 Study item and Work Item
The coexistence between NR-U and ax in 6 GHz spectrum were evaluated by several companies. The schemes defined in 5 GHz were the starting points.
The support of wideband LBT (>20MHz) as well as narrow band LBT (per 20MHz) was also discussed to simplify the implementation.

4. Potential Access Mechanisms in 6GHz
Baseline: (existing scheme for 5GHz LAA)
Scheme 1: 11ax PD=-82dBm, ED=-62dBm; NR-U ED only=-72dBm
Common ED threshold between ax and NR-U
Scheme 2: 11ax PD=-82dBm, ED=-82dBm; NRU ED only=-82dBm
Scheme 3: 11ax PD=-82dBm, ED=-72dBm; NRU ED only=-72dBm
Scheme 4: 11ax PD=-82dBm, ED=-62dBm; NRU ED only=-62dBm
Common preamble :
Scheme 5:
Common 11a preamble (L-STF+L-LTF+L-SIG);
11ax/NRU PD=-82dBm, ED=-62dBm;
assuming –4 dB detection SINR of 11a preamble
Scheme 6:
Common 11ax preamble (L-STF+L-LTF+L-SIG+RL-SIG);
assuming –7 dB detection SINR of 11ax preamble without auto-detection.

5. Evaluation assumptions and methodology in TR38.889
Common configurations
Carrier Frequency
6GHz
Carrier Channel Bandwidth
20MHz baseline
Number of carriers 1
Number of users per operator
Exactly 5 per gNB per 20MHz
Channel Model
Indoor: NR InH Mixed Office model for all links
Outdoor:NR UMi street canyon for all links
BS/AP Tx Power
23dBm (total across all TX antennas)
UE/STA Tx Power
18dBm (total across all TX antennas)
BS/AP Antenna gain
0 dBi
UE/STA Antenna gain
BS/AP Noise Figure
5dB
UE/STA Receiver Noise Figure
9dB
UE receiver
MMSE-IRC
MIMO
DL/UL MU MIMO + DL/UL OFDMA, ideal CSI/CQI feedback
BS/AP antenna Array configuration
(M, N, P, Mg, Ng) = (1, 2, 2, 1, 1), dH = dV = 0.5 λ
UE/STA antenna Array configuration
Tx/Rx: (M, N, P, Mg, Ng) = (1, 1, 2, 1, 1), dH = dV = 0.5 λ
Traffic model
Use Table A.1.1.
Note: Results based on the mixed traffic models can be used to determine the design.
Indoor scenario
outdoor scenario 1/2
system specific configurations
NR-U
802.11ax
Numerology
60kHz SCS + ~1.2us CP
78.125kHz SCS + 0.8us CP
MCOT/TXOP
8ms
4ms
Response timing
K1 >= 0.75ms, K2 >= 0.75ms
16us SIFS
Max MCS
NR LDPC with 256QAM
802.11ac/ax LDPC with 256QAM
Other
CC-HARQ, 1 switching point within a COT.
MPDU: 1500B MSDU + 14 B header
RTS/CTS off; NAV on MAC header

6. Performance comparison with common ED
When one of operator #2 replace Wi-Fi AP with NR-U gNB, the UPT of remaining Wi-Fi operator (#1) increased when common ED threshold is adopted by both Wi-Fi and NR-U.
In most cases, adopting common ED threshold of -62dBm between Wi-Fi and NR-U could achieve best performance of mean and 5th percentile UPT.
The NR-U performance could also benefit from the increased common ED threshold in most cases
Gain of Wi-Fi OP#1 when OP#2 replacing Wi-Fi AP with NR-U gNB
Common ED=-82dBm
(scheme2)
Common ED=-72dBm
(scheme3)
Common ED=-62dBm
(scheme4)
Mean UPT (Wi-Fi in coex)
3.4%
21.4%
31.8%
5% UPT(Wi-Fi in coex)
3.1%
29.9%
73.4%
Gain of NRU OP#2 in coex with Wi-Fi OP#1
(compared with scheme1)
Common ED=-82dBm
(scheme2)
Common ED=-72dBm
(scheme3)
Common ED=-62dBm
(scheme4)
Mean UPT (NR-U in coex)
-11.6%
0.9%
7.2%
5% UPT(NR-U in coex)
-30.8%
2.1%
7.6%
Note: DL performance with single stream, medium traffic load in indoor scenario

7. Performance comparison with common preamble
11ax performance is degraded due to less spatial reuse opportunities when all NR-U bursts are treated same as Wi-Fi using -82dBm PD level.
NR-U performance is also degraded because of less spatial reuse (PD=-82dBm for all burst) and limited detection sensitivity of 11a and 11ax preamble.
Gain of 11ax by introducing common preamble to NRU
11a preamble
11ax preamble
Compared with scheme 1
Compared with scheme 4
Mean UPT (Wi-Fi in coex)
-17.2%
-18.4%
-16.9%
-18.2%
5% UPT(Wi-Fi in coex)
-26.9%
-40.9%
-24.6%
-39.1%
Gain of NR-U by introducing common preamble to NRU
11a preamble
11ax preamble
Compared with scheme 1
Compared with scheme 4
Mean UPT (Wi-Fi in coex)
-13.0%
-18.8%
-12.7%
-18.5%
5% UPT(Wi-Fi in coex)
-34.8%
-39.4%
-30.9%
-35.8%
Note: DL performance with single stream, medium traffic load in indoor scenario

8. Narrow band vs. Wideband LBT in 6 GHz
NR-U Rel-16 supports component carrier of at most 100MHz, as NR in Rel-15. Multiple such carriers (inter/intra band) can be aggregated.
The use of 320MHz channel bandwidth is also under discussion in be for higher peak throughput.
Transmitter should perform LBT at each Observation Slot of 9 μs on each of 20MHz operation channel, assuming similar multi-carrier channel access defined in 5GHz (option1 in ETSI , or type A in 3GPP TS37.213). The complexity is quite high when the operation bandwidth is wide.
Simplification can be achieved, if option 2 in ETSI or type B in 3GPP TS is adopted, at the cost of limitation on the channel bonding patterns.
Wi-Fi performs ED on P20/S20/S40/S80 hierarchically when 160MHz operating bandwidth is configured.
Wideband LBT could simplify the implementation of wideband operation when it can be guaranteed that the channel is free of narrow band interference.
limiting usage of narrow band signal (20MHz) on certain sub-band
By long/short term measurement and LBT bandwidth adaption.
FFS: ED threshold

9. Conclusions
Common ED threshold (e.g. -62dBm) can benefit both ax and NR-U due to additional spatial reuse gain.
A common preamble and PD/ED threshold, such 11a preamble with PD=-82dBm and ED=-62dBm, degrades performance of both NR-U and Wi-Fi as it prohibits spatial reuse
Wideband LBT is beneficial for system operating with wide bandwidth to simplify LBT implementation.
IEEE and 3GPP should explore the possibility to mandate a minimum bandwidth larger than 20 MHz in greenfield 6 GHz spectrum

11. Evaluation results – indoor scenario [TR38.889] (1 spatial stream)

12. Evaluation results – outdoor scenario 1 [TR38.889](1 spatial stream)

13. Evaluation results – outdoor scenario 2 [TR38.889] (1 spatial stream)

14. Evaluation results – indoor scenario [TR38
Evaluation results – indoor scenario [TR38.889] (<=4 spatial streams)

15. Evaluation results – outdoor scenario 1 [TR38
Evaluation results – outdoor scenario 1 [TR38.889] (<=4 spatial streams)

16. Evaluation results – outdoor scenario 2 [TR38
Evaluation results – outdoor scenario 2 [TR38.889] (<=4 spatial streams)