# Future Is Unlicensed: Private 5G Unlicensed Network for Connecting Industries of Future
## 4 Challenges and Research Directions
### Challenge 1: LBT Is Mandatory to Operate in the Unlicensed or Shared Band  
\"Listen before talk\" has been designed to ensure a better neighbor to WiFi while coexisting in the unlicensed band [41]. However, the presence of LBT in channel access, characterized by a decentralizedand asynchronous random access to the radio channel for uplink data transmission, can highly degrade NR-U performance in terms of channel access delay and latency due to the LBT requirement for each new transmission. In this scenario, the UEs' system has a disadvantage in accessing the channel as they need to go through the following contentions for uplink in the NR-U system [13; 34]:  
1. To access UL service for data transmission, the UE must first send a scheduling request (SR) to the serving eNodeB (eNB). To do this, the UE must first conduct LBT to obtain the medium before transmitting the SR.
2. Once the SR is received, the eNB prepares a UL grant for a specific sub-frame(s) to the UE, and the eNB will again obtain the channel by LBT. 3. After receiving a UL grant, the UE again performs LBT to acquire the channel for UL data transmission. Furthermore, in the case of a UE having received a UL grant and LBT being needed but failing (time constraint), i.e., the UE loses its opportunity to transmit, the associated frequency/time domain resources for the SR and UL grant transmission are wasted. The UE can perform a transmission for the same data only after the eNB detects that the expected transmission has failed and re-schedules the same data transmission, resulting in an increase in overhead and a delay in transmitting data packets through the uplink. Hence, the uplink LBT procedure needs to be redesigned to improve the UL performance in standalone NR-U, and it must deliver downlink (DL) and uplink (UL) throughputs comparable to or better than other unlicensed technologies. We already know that the use of LBT in the uplink transmission significantly increases channel access delay due to the grant and scheduling request (SR) required in LTE mode. The main issue observed here is that the UE suffers from long delays before it can attempt to access the channel, which is caused by three LBTs needing to succeed before the UE can transmit. Most ongoing works have proposed to eliminate the granting of access in uplink transmission as in [34; 42; 43]. However, another simple approach to reduce the overhead of the UL grant transmission mode would be to reduce the grant and LBT transmissions as much as possible as presented in [13]. Multiple LBT requirement can be reduced by the concept of shared channel occupancy time (COT). In LAA, the maximum COT (MCOT) defines the maximum time allowed to share the channel among an end and its served nodes. This means a node/eNB that performs an LBT can occupy and share the channel for a specific period with served nodes/eNB. Nonetheless, a gap of 16 \(\upmu\)s [44] is allocated between the transmissions of different nodes. If a gap is sensed over this limit, the node is further allowed to occupy the additional channel time without channel sensing. Most importantly, the rules require that the total channel occupancy by the initiating node and serving eNB, i.e., DL and UL transmissions, shall not exceed the corresponding MCOT. Thus, the concept of shared MCOT between the eNB and its UEs reduces the occurrence of situations where two exponential back-offs are needed before a UL transmission can happen. In this case, an eNB can initiate the transmit opportunity with the grant transmission (without LBT), and UEs can benefit from using reduced LBT as long as their transmission falls within the same MCOT, as shown in Figure 3b. Figure 3: NR-U listen before talk (LBT) and shared maximum channel occupancy time (MCOT). SR, scheduling request. **(a)** NR-U LBT channel access procedure; **(b)** Shared MCOT. For example, in Figure 3b, an MCOT length of 4 ms is used. At the starting point, eNB begins DL transmission after an exponential random back-off is successfully achieved.