Cross-link RTS/CTS for MLO mm-Wave WLANs

TL;DR

This work tackles the hidden-terminal problem in mm-wave WLANs using directional RTS/CTS by introducing cross-link RTS/CTS under multi-link operation (MLO) and formalizing a generalized RTS/CTS hard-core process (G-HCP) to model spatial interactions. The authors develop a stochastic-geometry framework with two thinning schemes (Type I and II) to capture synchronous and asynchronous access, derive node-intensity and mean-interference expressions, and extend ASAPPP with MISR-based gains for Nakagami-$M$ fading to approximate success probability. They also provide an exact expression for the expected number of hidden nodes under directional RTS/CTS and present a cross-link protocol design alongside a unified benchmark for fair comparison with conventional directional RTS/CTS. The results reveal a fundamental trade-off: cross-link RTS/CTS improves link reliability at the cost of throughput, while directional RTS/CTS increases throughput but worsens reliability and increases hidden-node risk, guiding protocol selection for future WLAN standards.

Abstract

The directional RTS/CTS mechanism of mm-wave Wi-Fi hardly resolves the hidden terminal problem perfectly. This paper proposes cross-link RTS/CTS under multi-link operation (MLO) to address this problem and introduces a novel point process, named the generalized RTS/CTS hard-core process (G-HCP), to model the spatial transceiver relationships under the RTS/CTS mechanism, including the directional case and the omnidirectional case. Analytical expressions are derived for the intensity, the mean interference, an approximation of the success probability, and the expected number of hidden nodes for the directional RTS/CTS mechanism. Theoretical and numerical results demonstrate the performance difference between two RTS/CTS mechanisms. The cross-link RTS/CTS mechanism ensures higher link quality at the cost of reduced network throughput. In contrast, the directional RTS/CTS sacrifices the link quality for higher throughput. Our study reveals a fundamental trade-off between link reliability and network throughput, providing critical insights into the selection and optimization of RTS/CTS mechanisms in next-generation WLAN standards.

Figures (13)

• Figure 1: Illustration of the hidden terminal problem under omnidirectional and directional RTS/CTS mechanisms. Three types of APs are shown: (i) black APs that are actively communicating with their associated Stations (STAs), (ii) green APs that are silenced by the RTS/CTS mechanism and thus prohibited from transmitting, and (iii) the remaining APs that act as hidden terminals and may cause collisions. In the omnidirectional case, nearby APs detect the RTS/CTS frames and defer transmissions. In the directional case, the beamformed transmission prevents AP1 and AP2 from overhearing the RTS/CTS frames, even though they are located near the STA. As a result, AP1 and AP2 may initiate transmissions during the ongoing communication, leading to collisions.
• Figure 2: Illustration of the mm-wave Wi-Fi supporting MLO. This mm-wave Wi-Fi devices are equipped with multiple independent physical layer for each link, enabling simultaneous communication on different frequency bands or channels. The connection between AP and STA includes multiple links including at least one link in the Sub-7 GHz band and another in the mm-wave band. If there is any blockage in the mm-wave link, the Sub-7 GHz link can ensure communication quality.
• Figure 3: The comparisons between the different gains.
• Figure 4: Illustration of the exclusion region. (a) is the case of omnidirectional case, i.e., the cross-link RTS/CTS mechanism. (b) is the case of directional case with $N_{\rm r}$=16 and $N_{\rm r}$=8. The exclusion region consists of two parts, i.e. the physical carrier sensing cleaned region and the RTS/CTS cleaned region.
• Figure 5: SIR ccdf for the type i@ hard-core process and MISR-based approximation for $\lambda_p=4\times10^{-4} {\rm m}^{-2}$ and $R=300$m under the directional RTS/CTS mechanism.

Theorems & Definitions (10)

• Theorem 1
• proof
• Corollary 1
• Lemma 1
• proof
• Theorem 2
• proof
• Definition 1
• Theorem 3
• proof