1. On standalone transmissions with short fixed LBT
IEEE Coexistence Workshop, July 2019

2. Introduction
Listen Before Talk or LBT is the procedure by which a device confirms the absence of other transmissions in the channel before starting its own transmission. LBT is a key procedure for transmission in an unlicensed channel.
In NR-U/LAA/Wi-Fi, independent transmissions use the following types of LBT:
CAT2 LBT where a device that intends to transmit checks the channel for a short fixed duration of time.
CAT4 LBT where a device that intends to transmit checks the channel for a random duration of time. The random duration is a function of the device’s immediate transmission history (whether previous transmissions were successful or not), the nature of data to be transmitted (voice/video/best effort/background) etc.
Wi-Fi uses CAT4 LBT for almost all its transmissions. CAT2 LBT is used in extremely rare cases, such as when abandoning the current channel due to DFS, which account for much less than 1% of transmissions.
LAA uses CAT4 LBT for the majority of its transmissions and CAT2 LBT for up to 5% of DL transmissions. NR-U proposes to do the same too.
This presentation discusses transmissions with short fixed duration (CAT2) LBT.

3. Considerations on CAT2 and CAT4 LBT
We discuss the following aspects:
Latency of transmissions with CAT2 LBT and CAT4 LBT, the latency being defined as the average time required by a device implementing the LBT scheme to correctly transmit a packet.
Fairness: Whether CAT2 LBT can degrade the performance of devices that use other LBT schemes, for example CAT4 LBT.
Please note that the CAT2 LBT being referred to here is the LBT performed for independent or standalone transmissions on the primary channel. These are transmissions that are outside any Channel Occupancy Time that is granted to an unlicensed device on successfully completing CAT4 LBT.

4. Latency of CAT2 and CAT4 LBT
The transmission latency is the time that a device implementing the LBT scheme requires to correctly transmit a packet.
In an unloaded channel with low/no collision, CAT2 and CAT4 LBT have the same latency.
In a loaded channel, the latency depends on the channel being busy due to the presence of transmissions from other devices, as well as on collisions.
Collisions occur in the following ways:
As an example, if 2 NR-U nodes start sensing the channel at the same time, find it idle and start transmitting:
For CAT2 LBT, the chance of collision is 100%
For CAT4 LBT, the chance of collision is 25% for the highest priority traffic class and <<1% for the lowest priority traffic class
If additionally, some nodes cannot hear each other (the hidden nodes):
Transmissions with CAT2 LBT can collide again in subsequent retransmissions.
CAT4 LBT randomizes the sensing duration of the retransmissions which reduces the probability of subsequent collisions.
So, CAT4 LBT adapts the duration that a device senses a channel to the collision observed in the channel.
On the contrary, CAT2 LBT, due to its fixed sensing duration, cannot adapt to collisions.

5. Fairness of CAT2 LBT
Fairness of an LBT scheme measures whether devices that implement it degrade the performance of devices that do not implement it.
We evaluate whether CAT2 LBT can increase the latency of devices that perform CAT4 LBT.
In the event of collisions between transmissions with CAT2 LBT and CAT4 LBT, CAT2 will sense the channel for the same fixed duration for its next transmission while CAT4 will double its random wait time (in order to reduce collisions) for its next transmission.
So, intuitively, CAT2 LBT can increase/degrade the latency of CAT4 LBT.
If the total % of CAT2 LBTs are low, there may not be any measurable degradation.
On the other hand, if the % of CAT2 LBTs is significant, it can degrade the performance of devices with CAT4 LBT.

6. Simulations on Latency and Fairness of CAT2 LBT (1)
It has been claimed, based on simulations, that there is no evidence CAT2 LBT degrades the performance of CAT4 LBT.
In this context, we note the following:
The simulations considered, a sparse topology, a 120m x 80m area, with 3 nodes transmitting with CAT2 LBT for up to 5% airtime per node, coexisting with transmissions with CAT4 LBT from 3 other nodes.
It is unlikely that CAT2 LBT in such a sparse configuration, can cause any significant degradation to CAT4 LBT.
To evaluate the effect of CAT2 LBT on CAT4 LBT , we designed the following more typical configuration:
A topology with 8 nodes, all of which can hear each other.
For each node, 1 high priority packet of 1ms duration is transmitted every 20 ms.
To transmit the high priority packet, two options are evaluated:
4 nodes use CAT2 LBT, while 4 other nodes use CAT4 LBT of the highest priority class. This models a 5% upper bound of CAT2 LBT per device.
All 8 nodes use CAT4 LBT of the highest priority class.
To load the network, we consider that half of the nodes have additional file transfer traffic that uses CAT4 LBT of a lower priority.

7. Simulations on Latency and Fairness of CAT2 LBT (2)
The modeled configuration represents an typical topology with a mix of NR-U and Wi-Fi devices, where in the presence of regular data traffic, NR-U nodes use CAT2 LBT to transmit 1ms of DRS every 20ms i.e. up to 5% of the airtime per device, while Wi-Fi uses CAT4 LBT of the highest priority to transmit beacons (and voice).
Mapping to the topology that we modeled, this means that the latency of Wi-Fi beacons and voice can increase significantly in the presence of DRS transmissions by NR-U
We compare the transmission latencies in the following 2 cases:
All 8 nodes use CAT4 LBT
4 nodes use CAT4 LBT, the other 4 nodes use CAT2 LBT
We observe that the latency of the 4 nodes that use CAT4 LBT of the highest priority class increases by 22% to 150% when the other 4 nodes do CAT2 LBT.
This confirms that CAT2 LBT with a 5% upper limit per device can significantly degrade the performance of CAT4 LBT, even when the network is moderately dense.

8. Conclusions and Recommendations
In NR-U/LAA/Wi-Fi, standalone transmissions use either CAT2 LBT which has a short fixed sensing duration, or CAT4 LBT which has a random sensing duration.
CAT4 LBT achieves equivalent performance as CAT2 LBT in an unloaded channel with low/no collision and optimal performance in a congested channel with more collisions.
CAT2 LBT can degrade the performance of nodes that do CAT4 LBT.
Such degradation can be observed even if CAT2 LBT is limited to 5% per device, as the total percentage of CAT2 LBTs in a network can scale with the number of devices performing CAT2 LBT.
Coexistence in a mixed-technology network will be improved by disallowing CAT2 LBT.
If CAT2 LBT is considered essential to the functioning of certain devices, the upper bound of CAT2 LBT should be reduced to much below 5% per device.

9. End