optimising mimo wi-fi performance

CDT Industrial mtg 27th July

Optimising MIMO Wi-Fi PerformanceEvangelos Mellios, Di Kong, Siming Zhang, Fai Tila, Geoffrey Hilton and Andrew NixPresented at:The Royal Society

Communication networks beyond the capacity crunchThursday 14 May 2015

Communication Systems & Networks CSN Group 2015

Communication Systems & Networks CSN Group 2015Presentation OutlineIntroductionTesting methodologyResultsConclusions

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Communication Systems & Networks CSN Group 2015IntroductionMore than 70% of todays data traffic passes over Wi-Fi and the demand for high data-rates is growing rapidly due to applications such as high definition video streaming.Modern Wi-Fi standards (802.11n/ac) try to meet these demands by employing multiple antennas systems (MIMO) to exploit diversity, beamforming and spatial multiplexing gains.

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Communication Systems & Networks CSN Group 2015IntroductionPoor antenna design/integration and current interpretation of EIRP regulations reduce significantly the benefits of MIMO technology. The aim of this presentation is:to introduce a comprehensive and repeatable methodology for testing MIMO WLANs;to discuss how to optimise MIMO antenna configurations while remaining within current EIRP limitations.

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Communication Systems & Networks CSN Group 2015Testing MethodologyStage 1: Antenna Characterisation

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 1Each antenna element is measured individually in-situ with the Device Under Test (DUT) mounted on a wooden board.3D radiation patterns for two orthogonal polarisations.Directivity.Total efficiency (relative to a reference antenna).

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 1Casing, PCB and wooden board distort the radiation patterns.Example of a patch antenna pattern in isolation and inside a DUT.

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 1At the client side:Three theoretical clients assumed for benchmarking (results averaged over all three client types).

An off-the-shelf client unit.

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Communication Systems & Networks CSN Group 2015Testing MethodologyStage 2: Propagation Modelling

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 2The measured antenna characteristics are convolved with 3D indoor ray-tracing propagation data (software tool developed at the University of Bristol) to derive the combined channel/antenna response.

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 2Virtual three-floors test-house:10 Access Point and 10 Client locations (one per room).12 Access Point and 12 Client rotations (azimuth) 30 degree steps.

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Communication Systems & Networks CSN Group 2015Testing MethodologyStage 3: Throughput Simulations

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Communication Systems & Networks CSN Group 2015Testing Methodology Stage 3802.11n/ac PHY-layer simulator executed on a High Performance Computing (HPC) platform to predict data rate for each link.Open-loop and closed-loop simulations.Selects optimal MCS mode and number of spatial streams.

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Communication Systems & Networks CSN Group 2015Want to know more?14https://youtu.be/dlcv7dHYzV4


Communication Systems & Networks CSN Group 2015Results and DiscussionIs there an optimal antenna configuration for 3x3 MIMO Access-Points?

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Communication Systems & Networks CSN Group 2015Results Antenna Configurations5 antenna configurations studied using three identical dipoles:AP1: Three verticalAP2: Three horizontalAP3: Two vertical One horizontal (-shape)AP4: One vertical Two horizontal (T-shape)AP5: All orthogonal to each other

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Communication Systems & Networks CSN Group 2015Results Antenna ConfigurationsTotal average throughput (for closed-loop 3x3 MIMO):

A multi-dimensional antenna configuration (AP5) increases performance by up to 26% for the theoretical clients and 33% for the real client with respect to single-dimensional antenna configurations (AP1 / AP2).

17Theoretical clientsReal client


Communication Systems & Networks CSN Group 2015Results EIRP RegulationsAccording to the current interpretation of regulations the EIRP is reduced by 10log(Number_of_active_Tx_Antennas).This is a reduction of 4.8dB (compared to a single antenna solution) for a 3x3 system. This is only sensible if all the antenna radiation patterns fully overlap (e.g. AP1).This level of back off is not valid or necessary for orthogonal antenna solutions (e.g. AP5). Our calculations show that for AP5 the power should be reduced by just 0.2dB in order to remain within the allowed EIRP limits.18



Results EIRP Regulations19AP1 (Three vertical identical dipoles).

AP5 (Three orthogonal identical dipoles).Max EIRPMax EIRP



Results EIRP Regulations20

AP1 (Three vertical identical dipoles).

AP5 (Three orthogonal identical dipoles).Max EIRP-3dBMax EIRP-0.1dB



Results EIRP Regulations21

AP1 (Three vertical identical dipoles).

AP5 (Three orthogonal identical dipoles).Max EIRP-3dB-4.8dBMax EIRP-0.1dB-0.2dB


Communication Systems & Networks CSN Group 2015Results EIRP RegulationsTotal average throughput (closed-loop 3x3 MIMO - theoretical clients):

The performance of AP5 with respect to AP1 is 26% better using the current EIRP reduction and 36% better using the proposed EIRP reduction.22Current EIRP reductionProposed EIRP reduction


Communication Systems & Networks CSN Group 2015Next steps Hardware-in-the-loop (two-stage MIMO Over-The-Air testing).Test chipset performance under controlled and repeatable conditions.

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Communication Systems & Networks CSN Group 2015ConclusionsWe have introduced a test methodology for MIMO WLANs that combines antenna measurements, in-home propagation modelling and system-level Wi-Fi performance evaluation.A multi-dimensional MIMO antenna configuration was shown to offer remarkable performance improvements, whereas current interpretation of EIRP regulations reduce MIMO performance significantly.As an example, we showed that the performance of a 3x3 MIMO system was increased by 26% / 36% with the current / proposed EIRP reductions using three orthogonal dipoles in comparison to three vertical dipoles. 24


Communication Systems & Networks CSN Group 201525@BristolCSN

Thank You

[email protected]@bristol.ac.uk



optimising mimo wi-fi performance

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