AP cooperation in Wi-Fi: Joint transmission with a novel precoding scheme, resilient to phase offsets between transmitters

Multi Access-Point (M-AP) cooperation is expected to play a key role in the next-generation Wi-Fi standard (namely the upcoming IEEE WLAN 802.11bn/UHR, dubbed Wi-Fi-8), particularly in dense deployments where inter-cell interference hinders further increase in network capacity. Among the various considered AP-cooperation techniques, coherent Joint Transmission (JT) is the most ambitious, aiming at deploying an M-AP cluster as a single ‘super-AP’ serving multiple stations in a downlink Multi-User Multiple-Input Multiple-Output (DL MU-MIMO) fashion. Ideally, in the absence of practical impairments – predominantly inter-AP phase misalignment – prior-art JT schemes exhibit superior performance relative to simpler M-AP precoding schemes such as Coordinated Beam-Forming (Co-BF), equivalent to ∼10 dB Signal-to-Interference-plus-Noise-Ratio gain in certain scenarios. However, the Wi-Fi standard – being based on low-cost implementation and deployment strategies as a major driver for success – cannot guarantee the required tight inter-AP synchronization levels, thus jeopardizing the JT advantage in practice (essentially wiping out the gain relative to Co-BF, in the above scenarios). As a cure, we develop a novel MIMO-precoding scheme, which preserves most of the JT gain (∼7 out of the ∼10 dB above) while being inherently resilient to phase offsets between the APs, thus paving the way to practical incorporation of JT into Wi-Fi-8. •Multi Access-Point cooperation schemes are explored for (next-generation) Wi-Fi 8.•The most ambitious scheme, Joint Transmission (JT), is very sensitive to impairments.•Practical over-the-air synchronization mechanisms lead to inter-AP phase offsets.•These offsets ruin the advantage of prior-art JT MIMO precoding schemes.•A novel robust JT precoding technique recovers most of the lost performance gains.