Ruizhi Liao

MU-MIMO MAC Protocols for Wireless Local Area Networks: A Survey

As wireless devices boom and bandwidth-hungry applications (e.g., video and cloud uploading) get popular, todays wireless local area networks (WLANs) become not only crowded but also stressed at throughput. Multiuser multiple-input-multiple-output (MU-MIMO), an advanced form of MIMO, has gained attention due to its huge potential in improving the performance of WLANs. This paper surveys random access-based medium access control (MAC) protocols for MU-MIMO-enabled WLANs. It first provides background information about the evolution and the fundamental MAC schemes of IEEE 802.11 Standards and Amendments, and then identifies the key requirements of designing MU-MIMO MAC protocols for WLANs. After this, the most representative MU-MIMO MAC proposals in the literature are overviewed by benchmarking their MAC procedures and examining the key components, such as the channel state information acquisition, decoding/precoding, and scheduling schemes. Classifications and discussions on important findings of the surveyed MAC protocols are provided, based on which, the research challenges for designing effective MU-MIMO MAC protocols, as well as the envisaged MACs role in the future heterogeneous networks, are highlighted.

Performance analysis of IEEE 802.11ac wireless backhaul networks in saturated conditions

According to the ongoing IEEE 802.11ac amendment, the wireless network is about to embrace the gigabit-per-second raw data rate. Compared with previous IEEE standards, this significant performance improvement can be attributed to the novel physical and medium access control (MAC) features, such as multi-user multiple-input multiple-output transmissions, the frame aggregation, and the channel bonding. In this paper, we first briefly survey the main features of IEEE 802.11ac, and then, we evaluate these new features in a fully connected wireless mesh network using an analytic model and simulations. More specifically, the performance of the MAC scheme defined by IEEE 802.11ac, which employs the explicit compressed feedback (ECFB) mechanism for the channel sounding, is evaluated. In addition, we propose an extended request-to-send/clear-to-send scheme that integrates the ECFB operation to compare with the IEEE 802.11ac-defined one in saturated conditions. The comparison of the two MAC schemes is conducted through three spatial stream allocation algorithms. A simple but accurate analytical model is derived for the two MAC schemes, the results of which are validated with simulations. The observations of the results not only reveal the importance of spatial stream allocations but also provide insight into how the newly introduced features could affect the performance of IEEE 802.11ac-based wireless mesh networks.

DCF/USDMA: Enhanced DCF for uplink SDMA transmissions in WLANs

The traditional IEEE 802.11 DCF allows only one node to access the channel each time. In order to take the benefit of multiple simultaneous uplink transmissions when the AP is equipped with multiple antennas, a new MAC protocol called DCF/USDMA is presented in this paper. DCF/USDMA extends the IEEE 802.11 Distributed Coordination Function (DCF) by introducing a second contention round, which is used to accommodate the multiple simultaneous transmissions from multiple stations. Results show that the DCF/USDMA protocol is able to achieve a significant performance gain compared to that of the conventional DCF, the analysis of which also provides some insights about how the different parameters that control the DCF/USDMA operation can be optimized.

DCF/DSDMA: Enhanced DCF with SDMA downlink transmissions for WLANs

Multi-user MIMO is a promising technology to enhance the performance of WLANs. However, the MAC protocol of the IEEE 802.11 standard (the DCF MAC protocol) is not able to solve the new coordination and synchronization issues that are needed to enable multiple simultaneous transmissions. This paper presents an enhanced MAC protocol for Downlink SDMA in WLANs where only the Access Point is equipped with multiple antennas. Compared to the traditional WLANs, results show that the proposed protocol is able to exploit the benefit of multiple antennas at the Access Point, providing significant performance gains in terms of throughput and delay as well as the most suitable number of active users that a WLAN can support.

Modelling and Enhancing Full-Duplex MAC for Single-Hop 802.11 Wireless Networks

Theoretically, full-duplex (FD) transmissions hold the promise of doubling the channel capacity. However, this capacity increase can not directly translate to the useful throughput at the medium access control (MAC) layer due to overheads. In this letter, we propose an enhanced full-duplex MAC (FD+) for improving the performance of wireless networks. We also present a simple analytic model for investigating what gains can be achieved at the MAC layer. The simple model for FD transmissions can accurately capture the new sub-scenarios of two-node simultaneous transmissions that are traditionally handled as collisions by IEEE 802.11 distributed coordination function (DCF). The performance of FD+ is compared with DCF, a plain FD (FD-Plain) scheme and an FD MAC (FD-MAC) scheme from the literature. The analytic and simulation results show that FD+ is able to provide an average factor of 2.3 and 1.3 throughput increase against FD-Plain and FD-MAC, respectively.

A queueing model for SDMA downlink transmissions

This paper presents a simple queuing model to explore the benefits of simultaneous downlink transmissions by utilizing the Multiple Input Multiple Output (MIMO) technology when a finite-buffer queue is considered. Results show that the system performance, in terms of frame losses and delay, can not be properly explored without considering the impact of the queueing process and its interactions with the number of available spatial streams.

Analysis of fixed and mobile sensor systems for parking space detection

This paper analyses the relative merits of fixed and mobile sensor solutions for monitoring street car parking availability. A fixed sensor solution requires placement of sensors under each allocated parking space. The mobile approach uses a ranger mounted on a moving vehicle which continuously monitors the presence of parked cars and spaces. One of the challenges of the mobile sensor solution is to account for the variability of the position of the vehicle across the road. This paper proposes a dual detector solution using sonar and lidar together with differential detection system to mitigate the uncertainty.