Hello, everyone!
Today, I would like to continue on the open challenges of IEEE 802.11ac amendment in WLAN.
It's still a work in progress to figure out how to make the most of IEEE 802.11ac's new DL-MU-MIMO capabilities. First, because frequent CSI exchanges between STAs and the AP are required, it is unclear when DL-MU-MIMO beats SU-MIMO, or whether MU-MIMO outperforms multi-user packet aggregation when the amount of data directed to each destination is not balanced.
Although it will always depend on the quantity of traffic delivered to each destination and the buffer capacity at the AP, packet aggregation can be a method to balance the duration of multi-user spatial streams.
A second open challenge, closely related to the previous point, is the development of efficient schedulers that take into account traffic priorities, buffer state, different MIMO strategies, TXOP sharing policies, STA grouping, and the availability of fresh CSI feedbacks to maximize throughput and guarantee the required QoS for each active traffic flow.
It's worth noting that the AP can reduce mutual interference between transmitted spatial streams by having updated CSI estimates from all STAs, resulting in lower packet error probability and higher transmission rates.
The cost of acquiring the CSI from all STAs, on the other hand, is high and grows linearly with the channel sounding rate and the number of STAs. Proposals to reduce CSI overhead are currently being developed. For example, wherever possible, the CSI overhead is decreased by limiting channel sounding. Another unsolved issue is how to group the STAs, as the goal is to find STA groups with compatible (i.e. orthogonal) channels.
TXOP sharing is considered by offering two alternative techniques for improving the discussed back-of procedure in terms of throughput and fairness.
The optimal utilization of spectrum when several channel widths are used in settings with numerous overlapping WLANs is a third major problem for IEEE 802.11ac.
Individual WLANs can possibly obtain higher capacity by increasing channel width. However, the presence of other WLANs in the area increases the likelihood of frequency overlapping, which may have the opposite impact as inter-WLAN contention appears.
As a result, adaptive mechanisms for selecting the channel center frequency and channel width, as well as MAC protocols for selecting the instantaneous channel width used for each transmission, are required.
Using a game theoretic framework, for example, focus on the channel selection problem when WLANs can employ several channel widths. A technique was presented to allow communication between nodes with partially overlapping channels, which could give better channel interference resilience.
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