# Physical Layer Latency Management Mechanisms: A Study for Millimeter-Wave Wi-Fi
## 1 Introduction
### Related Work on Latency Reduction  
WiGig's contribution to latency has already been debated in several other works in the past. For example, in mapping the E2E latency between the application layers of two devices, the authors of [18] established that 10-15 ms time delays are typically encountered in indoor scenarios with a direct line-of-sight between the two communicating devices. The results obtained using the ns-3 network simulator are roughly similar to the experimental findings in [19; 20; 21; 22]. All of these studies elaborate on the appropriateness of WiGig for serving XR applications, receiving high-resolution videos streams over the network. Their approach to latency reduction includes combining WiGig with sub-6 GHz Wi-Fi [22], dynamically tuning the video encoder [20], synchronising data transmission with application-specific events [21], and leveraging user pose information for quicker beam- and access point (AP) switching [19]. As a result of these latency mitigation strategies, the range of expected latency values of individual video frames gets extended to 5-50 ms. Regardless of how successful such top-down approaches are in latency reduction, they are all limited by the underlying network layers. As demonstrated in [23], even sub-10 ms session transfer time delays in the MAC layer generate up to two orders of magnitude higher delays in the transport layer. Hence, optimizing the lower network layers is crucial both for reducing overall latency and understanding the recorded time delays, when looking from the top down. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Industry** & **Application** & **Max. Latency (ms)** & **Latency Type** & **Ref.** \\ \hline \multirow{3}{*}{XR} & VR entertainment & 20 & RTT and E2E & [4; 5] \\  & Professional AR/MR & 10 & RTT and E2E & [6; 7] \\  & usage & & & \\ \hline V2X, UVs & Platooning & 25 & RTT & [6; 8; 9] \\  & Remote control & 10 & E2E & [6; 7; 8; 9] \\ drones & Cooperative & 10 & RTT & [6; 7; 8; 9] \\  & driving/flight & & & \\ \hline \multirow{3}{*}{i4.0} & Remote control and & \multirow{3}{*}{50} & E2E & [10] \\  & monitoring & Cooperative robots & 1 & RTT & [10] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Use cases imposing strict latency constraints. The above-mentioned AP hand-offs and session transfers between WiGig and Wi-Fi are one of the more commonly-optimized IEEE 802.11 MAC layer mechanisms, influencing transmission time delays. Others include augmenting the automatic repeat request (ARQ) scheme [24], leveraging aggregation of frames and of the already aggregated data to decrease the overhead [25], and using multiple distributed APs for even higher data rates [5]. However, except for associating the finite physical layer (PHY) data rates with transmission delays [26], there is a general lack of IEEE 802.11 PHY latency models and understanding of the underlying latency management mechanisms. Broader studies discussing the implications of the PHY on transmission latency [27] note the importance of adaptively setting the MCS based on the latest channel state information (CSI). Doing so increases the average throughput and, therefore, reduces transmission times. The authors also elaborate on the importance of short packets in view of timely data delivery, although, the approach may not be entirely applicable to the likes of XR video streaming applications because of their high throughput demands. Among other radio access technology (RAT) options, 5G new radio (5G NR) has been putting increased emphasis on the PHY and its role in ultra-reliable and low-latency communications (URLLC). It aims for a 1 ms small packet latency while keeping bit error ratio (BER) values below \(10^{-5}\)[28]. The main challenges 5G NR URLLC faces are reducing the transmission time of a single packet and achieving better control over individual packet processing times [29]. Although 5G NR URLLC tackles latency reduction by introducing new packet formats, it has several things in common with IEEE 802.11 ad.