The performance of current consumer-grade devices for 60 GHz wireless networks is limited. While such networks promise both high data rates and uncomplicated spatial reuse, we find that commercially available devices based on the WiHD and WiGig standards may suffer from their cost-effective design. Very similar mechanisms are used in upcoming devices based on the IEEE 802.11ad standard. Hence, understanding them well is crucial to improve the efficiency and performance of next generation millimeter wave networks. In this paper, we present the first in-depth beamforming, interference, and frame level protocol analysis of off-the-shelf millimeter wave systems with phased antenna arrays. We focus on (a) the interference due to the lack of directionality of consumer-grade antennas, and (b) the degree of data aggregation of current devices. Regarding (a), our beam pattern measurements show strong side lobes that challenge the common conception of high spatial reuse in 60 GHz networks. We also show that reflections in realistic settings worsen this effect. Further, we measure weak directionality when beamforming towards the boundary of the transmission area of an antenna array. Regarding (b), we observe that devices only aggregate data if connections require high bandwidth, thus increasing medium usage time otherwise.

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Boon and bane of 60 GHz networks: practical insights into beamforming, interference, and frame level operation

With the multi-antenna design of WiFi interfaces, phased array has become a promising mechanism for accurate WiFi localization. State-of-the-art WiFi-based solutions using Angle-of-Arrival (AoA), however, face a number of critical challenges. First, their localization accuracy degrades dramatically due to low Signal-to-Noise Ratio (SNR) and incoherent processing. Second, they tend to produce outliers when the available number of packets is low. Moreover, the prior phase calibration schemes are not multipath robust and accurate enough. All of the above degrade the robustness of localization systems. In this paper, we present ROArray, a RObust Array based system that accurately localizes a target even with low SNRs. The key insight of ROArray is to use sparse recovery and coherent processing across all available domains, including time, frequency, and spatial domains. Specifically, in the spatial domain, ROArray can produce sharp AoA spectrums by parameterizing the steering vector based on a sparse grid. Then, to expand into the frequency domain, it jointly estimates the Time-of-Arrival (ToAs) and AoAs of all the paths using multi-subcarrier OFDM measurements. Furthermore, through a novel multi-packet fusion scheme, ROArray is enabled to perform coherent estimation over multiple packets. Such coherent processing not only increases the virtual aperture size, which enlarges the number of maximum resolvable paths but also improves the system robustness to noise. In addition, ROArray includes an online phase calibration technique that can eliminate random phase offsets while keeping communication uninterrupted. Our implementation using off-the-shelf WiFi cards demonstrates that, with low SNRs, ROArray significantly outperforms state-of-the-art solutions in terms of localization accuracy; when medium or high SNRs are present, it achieves comparable accuracy.

RoArray: Towards More Robust Indoor Localization Using Sparse Recovery with Commodity WiFi

Increasing capacity demands in emerging wireless technologies are expected to be met by network densification and spectrum bands open to multiple technologies. These will, in turn, increase the level of interference and also result in more complex inter-technology interactions, which will need to be managed through spectrum sharing mechanisms. Consequently, novel spectrum sharing mechanisms should be designed to allow spectrum access for multiple technologies, while efficiently utilizing the spectrum resources overall. Importantly, it is not trivial to design such efficient mechanisms, not only due to technical aspects, but also due to regulatory and business model constraints. In this survey we address spectrum sharing mechanisms for wireless inter-technology coexistence by means of a technology circle that incorporates in a unified, system-level view the technical and non-technical aspects. We thus systematically explore the spectrum sharing design space consisting of parameters at different layers. Using this framework, we present a literature review on inter-technology coexistence with a focus on wireless technologies with equal spectrum access rights, i.e., (i) primary/primary, (ii) secondary/secondary, and (iii) technologies operating in a spectrum commons. Moreover, we reflect on our literature review to identify possible spectrum sharing design solutions and performance evaluation approaches useful for future coexistence cases. Finally, we discuss spectrum sharing design challenges and suggest future research directions.

Survey of Spectrum Sharing for Inter-Technology Coexistence

With the multi-antenna design of WiFi interfaces, phased array has become a promising mechanism for accurateWiFi localization. State-of-the-art WiFi-based solutions using AoA (Angle-of-Arrival), however, face a number of critical challenges. First, their localization accuracy degrades dramatically when the Signal-to-Noise Ratio (SNR) becomes low. Second, they do not fully utilize coherent processing across all available domains. In this paper, we present ROArray, a Robust Array based system that accurately localizes a target even with low SNRs. In the spatial domain, ROArray can produce sharp AoA spectrums by parameterizing the steering vector based on a sparse grid. Then, to expand into the frequency domain, it jointly estimates the ToAs (Time-of-Arrival) and AoAs of all the paths using multi-subcarrier OFDM measurements. Furthermore, through multi-packet fusion, ROArray is enabled to perform coherent estimation across the spatial, frequency, and time domains. Such coherent processing not only increases the virtual aperture size, which enlarges the number of maximum resolvable paths, but also improves the system robustness to noise. Our implementation using off-the-shelf WiFi cards demonstrates that, with low SNRs, ROArray significantly outperforms state-of-the-art solutions in terms of localization accuracy; when medium or high SNRs are present, it achieves comparable accuracy.

Robust Indoor Wireless Localization Using Sparse Recovery

The use of directional antennas in millimeter-wave communication promises high spatial reuse at multi-gigabit-per-second data rates in dense wireless networks. Existing work studies such networks using commercial hardware but is limited to individual links. Moreover, such hardware typically allows for little or no control of the lower layers of the protocol stack. In this paper, We study the performance of dense millimeterwave deployments featuring up to eight stations. To this end, we use a practical IEEE 802.11ad millimeter-wave testbed that allows access to the lower layer parameters of each station. This enables us to analyze the impact of these parameters on upper layer performance. We study, for first time to our best knowledge, issues such as the impact of channel contention on the buffer size at the transport layer, the effect of frame aggregation, and the efficiency of spatial sharing. Our results show that using large buffer sizes with TCP is harmful due to channel contention despite the multi-gigabit-per-second data rates. Further, frame aggregation is only beneficial up to a certain level due to higher error rates for large frames. Finally, we also study delay, showing that the regular beacon transmission time can degrade performance.

Medium Access and Transport Protocol Aspects in Practical 802.11 ad Networks

IEEE 802.11n WLAN supports frame aggregation called aggregate MAC protocol data unit (A-MPDU) as a key MAC technology to achieve high throughput. While it has been generally accepted that aggregating more subframes results in higher throughput by reducing protocol overheads, our measurements reveal various situations where the use of long A-MPDU frames frequently leads to poor performance in time-varying environments. Especially, since mobility intensifies the time-varying nature of the wireless channel, the current method of channel estimation conducted only at the beginning of a frame reception is insufficient to ensure robust delivery of long A-MPDU frames. Based on extensive experiments, we develop MoFA, a standard-compliant mobility-aware A-MPDU length adaptation scheme with ease of implementation. Our prototype implementation in commercial 802.11n devices shows that MoFA achieves the throughput 1.8x higher than a fixed duration setting (i.e., 10 ms, the maximum frame duration according to IEEE 802.11n standard). To our best knowledge, this is the first effort to optimize the A-MPDU length for commercial 802.11n.

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MoFA: Mobility-aware Frame Aggregation in Wi-Fi

IEEE 802.11n/ac wireless local area network (WLAN) supports frame aggregation, called aggregate medium access control (MAC) protocol data unit (A-MPDU), to enhance MAC efficiency by reducing protocol overhead. However, the current channel estimation process conducted only once during the preamble reception is known to be insufficient to ensure robust delivery of long A-MPDU frames in mobile environments. To cope with this problem, we first build a model which represents the impact of mobility with a noise vector in the I-Q plane, and then analyze how the mobility affects the A-MPDU reception performance. Based on our analysis, we develop STRALE, a standard-compliant and mobility-aware PHY rate and A-MPDU length adaptation scheme with ease of implementation. Through extensive simulations with 802.11ac using ns-3 and prototype implementation with commercial 802.11n devices, we demonstrate that STRALE achieves up to 2.9x higher throughput, compared to a fixed duration setting according to IEEE 802.11 standard.

STRALE: Mobility-aware PHY rate and frame aggregation length adaptation in WLANs

IEEE 802.11 has evolved from 802.11a/b/g/n to 802.11ac in order to meet ever-increasing high throughput demand. The newest standard supports various channel widths up to 160 MHz by bonding multiple 20 MHz channels. In order to efficiently utilize multiple channel widths, 802.11ac defines two operations, namely, dynamic channel access (DCA) and dynamic bandwidth operation (DBO). In this paper, we reveal that the use of DCA improves channel utilization significantly, and DBO not only partly overcomes secondary channel hidden interference problems but also achieves better channel utilization. However, when a transmitter attempts to send an enhanced RTS frame before data transmission as part of DBO, the station has no way to know how much bandwidth will be used, thus leading to malfunction of virtual carrier sensing at neighboring stations. On the other hand, the use of enhanced RTS/CTS without hidden traffic wastes airtime significantly. To address these problems, we first define how to calculate an appropriate value of duration field for enhanced RTS/CTS, and then develop an algorithm which adaptively enables/disables DBO considering the (secondary) hidden interference. Through ns-3 simulations, we demonstrate that the proposed scheme achieves up to 2x higher throughput compared to the baseline of 802.11ac.

Enhancement of wide bandwidth operation in IEEE 802.11ac networks

LTE operation in unlicensed spectrum is emerging as a promising technology in achieving higher data rate with LTE since ultra-wide unlicensed spectrum, e.g., about 500 MHz at 5–6 GHz range, is available in most countries. Recently, 3GPP has finalized standardization of licensed assisted access (LAA) for LTE operation in 5 GHz unlicensed spectrum, which has been a playground only for Wi-Fi. LAA defines downlink transmission over 5 GHz spectrum, where channel sensing, i.e., listen before talk (LBT) operation similar to Wi-Fi medium access control (MAC) protocol, is performed before each transmission of eNodeB. LAA has a fixed 8 ms maximum channel occupancy time (COT), which is the maximum continuous transmission time after channel sensing, while Wi-Fi may transmit for much shorter time duration. As a result, when Wi-Fi coexists with LAA, Wi-Fi airtime and throughput can be much less than those achieved when Wi-Fi coexists with another Wi-Fi. To guarantee fair airtime and improve throughput of Wi-Fi, we propose a COT adaptation (COTA) algorithm, which observes Wi-Fi aggregate MAC protocol data unit (A-MPDU) frames and matches LAA's COT to the duration of A-MPDU frames. Moreover, COTA detects saturation of Wi-Fi traffic and adjusts COT only if Wi-Fi traffic is saturated. We prototype saturation detection algorithm of COTA with commercial off-the-shelf Wi-Fi device and show that COTA detects saturation of Wi-Fi networks accurately. Through ns-3 simulations, we demonstrate that COTA achieves up to 153% Wi-Fi throughput gain while providing airtime fairness between LAA and Wi-Fi.

COTA: Channel occupancy time adaptation for LTE in unlicensed spectrum

Recently, 3GPP has introduced licensed-assisted access (LAA) for long-term evolution (LTE) operation in 5 GHz unlicensed band to meet ever-increasing data traffic in cellular networks. However, the link adaptation scheme of the conventional LTE, adaptive modulation and coding (AMC), cannot operate well in unlicensed band due to intermittent collisions. Intermittent collisions make LAA eNB lower modulation and coding scheme (MCS) for the subsequent transmission and such unnecessarily lowered MCS significantly degrades spectral efficiency. To address this problem, we propose a collision-aware link adaptation algorithm (COALA). COALA exploits k- means unsupervised clustering algorithm to discriminate channel quality indicator (CQI) reports which are measured with collision interference and selects the most suitable MCS for the next transmission. By prototype-based experiments, we demonstrate that COALA detects collisions accurately, and by conducting ns-3 simulations in various scenarios, we also show that COALA achieves up to 74.9% higher user perceived throughput than AMC.

Yoon Kangjin

Papers

MoFA: Mobility-aware Frame Aggregation in Wi-Fi

STRALE: Mobility-aware PHY rate and frame aggregation length adaptation in WLANs

Enhancement of wide bandwidth operation in IEEE 802.11ac networks

COTA: Channel occupancy time adaptation for LTE in unlicensed spectrum

COALA: Collision-Aware Link Adaptation for LTE in Unlicensed Band