1. On a common preamble between Wi-Fi and NR-U
IEEE Coexistence Workshop, July 2019

2. Introduction (1)
Wi-Fi and NR-U be deployed in close proximity. So, it is important that they coexist well.
Similarity in channel access is conducive to good coexistence.
However, the channel access mechanisms of Wi-Fi and NR-U are different. This presentation considers one of the key differences and its consequence.
A basic step in channel access is for a device to determine if the channel is idle i.e. there are no ongoing transmissions, before it decides to transmit.
NR-U checks for an idle channel by measuring the total received energy in that channel and comparing it to a predetermined threshold. This is known as Energy Detection or ED.
Wi-Fi checks this by looking for a known signature (called a “preamble”) that is prefixed to all Wi-Fi transmissions. This is referred to as Preamble Detection or PD.
If Wi-Fi doesn’t detect the preamble, it uses ED as a fallback.
The above applies to both intra and inter technology channel sensing.
This key difference in channel access makes coexistence between Wi-Fi and NR-U unpredictable (i.e. dependent on the network topology and RSSI distributions of the nodes) and can make both Wi-Fi and NR-U unfair to each other.
We consider 2 possible solutions: a common Energy Detection scheme and a common Preamble Detection scheme.

3. Energy Detection Pros: Can ensure equal airtime between Wi-Fi and NR-U
Simple to implement
Cons:
Cannot be set much lower than -72dBm
In the presence of elevated and spurious noise, can lead to false “busy” detects and lowers the chances of a device accessing the channel.
Hence, cannot protect links that are heard below -72dBm.
The ED mechanism detects only energy and has no other information about neighboring transmissions.
Cannot be varied dynamically for informed tradeoff between spectral efficiency and fairness.
Does not enable power save

4. Preamble Detection Pros:
Can ensure mutual fairness between Wi-Fi and NR-U.
The PD threshold can be adapted dynamically and if required, set significantly lower than -72dBm
Is very suitable for protecting weak links and/or extending coverage
The preamble includes (at a minimum) the duration of transmission.
Decoding the duration of the current “on-air” transmission enables significant power save at the sensing device.
The preamble lets a device identify valid RLAN transmission from channel noise. This allows:
Preamble detection to function in presence of elevated channel noise.
Informed tuning of the PD threshold to tradeoff spectral efficiency and fairness.
Cons:
More difficult to implement than a common ED,

5. Common Detection Mechanism and Thresholds
Based on the previous discussion, the following can be inferred:
Fair coexistence between Wi-Fi and NR-U requires either a common ED or a common PD.
A common ED is simpler to implement than a common PD.
A common PD has significant performance advantages over a common ED.
Given the above, the following aspects need to be considered:
Can a common ED along with other associated procedures provide performance similar to a common PD?
What should be the nature of a common PD and can it be implemented by Wi-Fi and NR-U?

6. Can a common ED provide performance similar to a common PD?
Coexistence performance of Common ED + technology specific PD
Setting the ED threshold lower results in many false “busy” detects in an idle channel.
To alleviate this, it has been proposed that Wi-Fi and NR-U can select a common ED at -72dBm and each technology can implement its own technology-specific PD which can then be varied dynamically.
However, this solution has limited utility in a mixed NR-U + Wi-Fi environment as the mutual deferral zone for Wi-Fi and NR-U will still be limited by the common ED threshold at -72dBm.
No power saving opportunity in a common ED
It has been claimed that other technology-specific procedures such as DRX offer sufficient opportunities for power save, and so PD enabled power save is not necessary.
However, power save via DRX requires an environment where a gNB is able to opportunistically transition a UE in and out of DRX. This in turn requires the ability to transmit/receive at predictable times. It is not effective in an unlicensed environment where the transmit/receive opportunities are unpredictable.

7. What should be the structure of a common preamble?
The general structure of a common preamble can be as shown below:
The nature of the common preamble will depend on the band of operation:
5 GHz: due to the presence of legacy Wi-Fi, the common preamble has to be a signature that can be understood by legacy Wi-Fi. Keeping this in mind, the 20us a preamble is the only candidate for common preamble prefix.
6 GHz: Coexistence is primarily between ax/11be and NR-U. So, the a preamble need not be used. Considering the standardization roadmap and the expected rapid uptake of ax capable devices in 6 GHz, Broadcom prefers the ax preamble (or a part of it) to be used as the common preamble prefix.
This structure includes a common preamble as a prefix, while allowing each technology to follow the common preamble with a technology specific preamble as suffix.

8. Implementation of a common preamble via co-located Wi-Fi
The proposal is as follows:
A Wi-Fi module colocated with NR-U manages CCA on behalf of NR-U, including Energy Detection and Tx/Rx of the a preamble.
NR-U informs Wi-Fi of the channel access parameters and Tx time instants.
Wi-Fi indicates channel access status and any parsed a preamble to NR-U.
Handsets already implement front-end sharing and signaling between LTE and Wi-Fi. NR-U can use this signaling interface to instruct Wi-Fi to Tx/Rx the preamble.
Base stations can use an interface similar to handsets
Minimal BOM and testing required to integrate NR-U with a Wi-Fi module with limited functionality
Some LAA small cells may already use a Wi-Fi IC solely to Tx/Rx the a preamble
Broadcom has verified that its Wi-Fi module and the existing LTE - Wi-Fi interface in handsets is capable of performing the above functionality.
Sample interface exchanges are illustrated for 4 typical NR-U scenarios.

9. Scenario 1: NR-U starts sensing the channel in order to Tx
Scenario 1: NR-U starts sensing the channel in order to Tx. The channel is sensed to be idle. However, due to the slotted nature of NR-U, Tx doesn’t start immediately. So, NR-U performs extended defer and Tx starts at the next valid NR-U Tx instance t0
NR-U sends Wi-Fi the CCA parameters (SIFS + AIFS + random number) t1
Wi-Fi starts CCA t2
NR-U sends Wi-Fi information confirming data availability and the time t5 when the next Tx can start. t3
Wi-Fi completes CCA in the last 9us slot of the backoff timer. Preamble Tx may start at t4, but is not permitted until t5, due to the slotted nature of NR-U Tx t4
Wi-Fi ends additional single-slot CCA. Repeats until t5 t5
Wi-Fi starts Tx of a preamble t6
Wi-Fi ends Tx of a preamble t7
NR-U starts Tx within 16us of t6. The time gap has to be <= 16 us for the transmission to qualify as a single burst.

10. Scenario 2: NR-U receives data on the DL and transmits a response after a fixed time gap with CCA
NR-U informs Wi-Fi that CCA must be performed for a fixed duration of 25us starting t1 and if successful, UL Tx must start at t2. t1
NR-U DL burst ends and UL Tx must start at t2 which is 25us or more later, if CCA is successful. There can be a gap between the end of DL burst and the start of CCA, if the start is more than 25us after the end of DL burst.
Wi-Fi starts CCA in 25us (ED + PD) t2
Wi-Fi completes CCA and doesn’t detect preamble or energy in the CCA period
Wi-Fi starts Tx of a preamble t3
Wi-Fi ends Tx of a preamble t4
NR-U starts Tx within 16us of t3

11. Scenario 3: NR-U starts sensing the channel (CCA) in order to transmit
Scenario 3: NR-U starts sensing the channel (CCA) in order to transmit. While doing CCA, it receives DL preamble/data for itself. So, it aborts CCA and starts receiving the data t0
NR-U sends Wi-Fi the CCA parameters (SIFS + AIFS + random number) t1
Wi-Fi starts CCA (ED + PD) t2
Wi-Fi detects preamble and/or energy in the last 9us CCA slot. (The timing diagram considers preamble/energy detection in the last CCA slot as it provides the strictest timing constraint.) t3
Wi-Fi aborts a preamble Tx since the channel is sensed to be busy t4
Wi-Fi completes Rx of a preamble and NR-U- specific Rx starts t5
Wi-Fi decodes the L-SIG and sends the duration value to NR-U. However, NR-U does not use this information as it is receiving data meant for itself.

12. Scenario 4: NR-U starts sensing the channel (CCA) in order to transmit
Scenario 4: NR-U starts sensing the channel (CCA) in order to transmit. While doing CCA, it receives DL preamble/data not intended for itself. So, it aborts CCA and moves to Power Save mode till the end of the current transmission t0
NR-U sends Wi-Fi the CCA parameters (SIFS + AIFS + random number) t1
Wi-Fi starts CCA (ED + PD) t2
Wi-Fi detects preamble and/or energy in the last 9us CCA slot (The timing diagram considers preamble/energy detection in the last CCA slot as it provides the strictest timing constraint). t3
Wi-Fi aborts a preamble Tx since the channel is sensed to be busy t4
Wi-Fi completes Rx of a preamble, decodes the L-SIG and sends the duration value to NR-U. t5
NR-U decides to sleep till the end of the current Tx t6
NR-U wakes up before the end of the current Tx and resumes channel sensing

13. Implementation of a common preamble directly by NR-U
Tx/Rx of the a preamble can also be implemented directly in the NR-U module.
802.11a preamble consists of STF (8us), LTF (8us) and L-SIG (4us).
Tx:
STF/LTF can be treated as fixed time-domain signals; precomputed and stored in memory.
Don’t need frequency domain processing or IFFT.
L-SIG is BPSK rate ½.
Doesn’t require scrambling or puncturing.
Simple interleaver and mapper designs.
Rx:
STF/LTF needed to decode BPSK rate ½ L-SIG. Coarse channel estimation suffices.
L-SIG:
Contains only 24 bits including tail bits.
Traceback design is simple as tail bits are included
Convolution decoder design (via Viterbi) is simple (unlike classic Viterbi).
As L-SIG is BPSK rate ½, de-mapper and de-interleaver designs are simple.

14. Conclusion and Way Forward (1)
A common preamble will lead to better coexistence between NR-U and Wi-Fi than a common energy detection threshold.
For 5 GHz, the a preamble can be the only candidate for a common preamble.
For 6GHz, the a preamble need not be used.
Considering the standardization roadmap and the expected rapid uptake of ax capable devices in 6 GHz, Broadcom prefers the ax preamble (or a part of it) to be used as the common preamble.
It is technically feasible for NR-U to implement Tx/Rx of the a preamble, either via a co-located Wi-Fi module or directly in NR-U.
Broadcom has verified that its Wi-Fi module and the existing LTE - Wi-Fi interface in handsets is capable of performing the above functionality within the required timing constraints.

15. Conclusion and Way Forward (2)
It has been argued that NR-U using the preamble will make it dependent on the Wi-Fi standard.
However, an interdependent and common design across standards can sometimes provide the most optimal industry solution.
This won’t be the first time an inter-standard solution has been devised in 3GPP. A proposal from the 3GPP TR “Study on further enhancements to LTE D2D, UE to network relays for IoT and wearables” is an example where the 3GPP specification will depend on Wi-Fi MAC/PHY.
The case for adopting the preamble is also similar and should be considered as an optional feature, since it achieves better coexistence and performance for NR-U and Wi-Fi, contributing to successful market adoption of both these technologies.

16. End