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Experimental Results of Network-Assisted Interference Suppression Scheme Using Adaptive Beam-Tilt Switching

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Experimental Results of Network-Assisted Interference Suppression Scheme Using Adaptive Beam-Tilt Switching Hindawi International Journal of Antennas and Propagation Volume 2017, Article ID 2164038, 10 pages https://doi.org/10.1155/2017/2164038 Research Article Experimental Results of Network-Assisted Interference Suppression Scheme Using Adaptive Beam-Tilt Switching Tomoki Murakami,1 Riichi Kudo,1 Koichi Ishihara,1 Masato Mizoguchi,1 and Naoki Honma2 1 NTT Access Network Service Systems Laboratories, Nippon Telegraph and Telephone Corporation, Yokosuka, Japan 2Faculty of Engineering, Iwate University, Morioka, Japan Correspondence should be addressed to Tomoki Murakami; [email protected] Received 4 October 2016; Accepted 18 April 2017; Published 14 May 2017 Academic Editor: Ahmed Toaha Mobashsher Copyright © 2017 Tomoki Murakami et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper introduces a network-assisted interference suppression scheme using beam-tilt switching per frame for wireless local area network systems and its effectiveness in an actual indoor environment. In the proposed scheme, two access points simultaneously transmit to their own desired station by adjusting angle of beam-tilt including transmit power assisted from network server for the improvement of system throughput. In the conventional researches, it is widely known that beam-tilt is effective for ICI suppression in the outdoor scenario. However, the indoor effectiveness of beam-tilt for ICI suppression has not yet been indicated from the experimental evaluation. Thus, this paper indicates the effectiveness of the proposed scheme by analyzing multiple-input multiple- output channel matrices from experimental measurements in an office environment. The experimental results clearly show that the proposed scheme offers higher system throughput than the conventional scheme using just transmit power control. 1. Introduction have been investigated [13–16]. They cooperatively suppress ICI without significantly degrading the received power at To handle the massive mobile traffic from smartphones each STA except for cases in which the correlation of prop- and tablet devices, data-offloading to small cells such as agation channels between AP and desired/nondesired STAs wireless local area network (WLAN) systems is progressing is high. Moreover, beamforming techniques including TPC in addition to the evolution of cellular systems [1–3]. For have been proposed and its effectiveness is clarified in [17]. the efficient operation of the above networks, heterogeneous Beamforming technologies on orthogonal frequency network (HetNet) with large macrocells in combination with division multiplexing (OFDM) are classified into two cat- small cells has been researched [4–6]. Small cell technologies egories: digital beamforming, which controlled amplitude intheHetNethavehigherpotentialoftheimprovementof and phase per subcarrier by digital signal processing, and spectrum efficiency by taking advantage of dense reuse of analog beamforming, which controlled them in common frequency channel [7–9]. However, as excess installations of for all subcarriers by using phase shifters connected to each small cell cause intercell interference (ICI), it is difficult to antenna. Digital beamforming offers higher capacity and obtain the maximum effect of frequency channel reuse [10]. greater flexibility because of its precise control per subcarrier To suppress ICI between small cells, network-assisted [13, 14]. However, it demands precise frequency/time syn- transmit power control (TPC) technologies, which cooper- chronizations and large computational load for transmission atively suppress ICI by adjusting each transmit power at weight calculation. On the other hand, analog beamforming multiple APs and improve system throughput, have been is done by using phase shifters [15, 16]. While it dispenses with investigated [11, 12]; unfortunately, they also degrade the digital signal processing units, its throughput performance is received power at each station (STA) because of TPC. More less than that of digital beamforming because beamforming recently, network-assisted beamforming technologies also is common to all subcarriers. More recently, techniques that 2 International Journal of Antennas and Propagation NW Z .. .. Z . .. .. .. M . Tx Tx M . Tx 1 . .. G2 . Tx 1 . G1 . H1 H2 .. N N . .. .. d Phase shifter N N Antenna Rx Rx Rx 1 Rx 1 AP-1 AP-2 STA-1 STA-2 Figure 1: System model. attempt to offer the features of both digital and analog steptowardpracticaluse,weevaluatetheproposedschemeby beamforming have been investigated [18–20]. Considering using measured MIMO channel matrices in an indoor office the future wireless system, it is necessary to reevaluate each environment. The experimental results clearly show that the technology of both analog and digital beamforming. This proposed scheme offers higher system throughput than the paper focuses on beam-tilt technologies as analog beamform- conventional power control. ing for ICI suppression. This paper is organized as follows. Section 2 introduces In the conventional researches, it is widely known that the system model. Section 3 describes our proposed inter- beam-tilt is effective for ICI suppression in the outdoor ference suppression scheme. Section 4 details a performance scenario such as macrocells. In the outdoor scenario, since analysis that uses the measured MIMO channel matrices. We theinfluenceofdirectwavepathisrelativelystrong,ICI conclude with a summary of key points in Section 5. is expected to be greatly suppressed by avoiding direct Notations. Vectors and matrices are in boldface. Superscript wave path [15]. Also in the indoor scenario, although the −1 effectofICIsuppressionbecomessmallerthanthatinthe A denotes the Hermitian transpose operator of A, A denotes the inverse of A, ‖A‖ denotes the Frobenius norm outdoor scenario because of multipath environment, there × are reports which indicate the effectiveness of beam-tilt of A, I is an identity matrix of size (×), C denotes using the frequency band of WLAN systems. For example, the set of complex matrix of size (×), det(A) is the [21] presents the multiple-input multiple-output (MIMO) determinant of A,and{A} is expectation of A. antenna design including beam-tilt and its effectiveness for indoor base stations but does not mention the effect of ICI 2. System Model suppression from experimental evaluations. In this paper, we introduce a network-assisted interfer- Figure 1 shows the system model in this paper; two APs ence suppression scheme using adaptive beam-tilt switching simultaneously transmit to their own desired STA on down- including TPC for WLAN systems. In the proposed scheme, link MIMO-OFDM transmission. APs and STAs utilize the two APs simultaneously transmit to their own desired STA same frequency channel and bandwidth. It is also assumed by adjusting angle of beam-tilt and transmit power per that each AP has an array of ×antenna elements frame according to STA locations for the improvement of (: the number of horizontal rows, :thenumberof system throughput. As an additional technique, the proposed vertical columns) for communication with its own desired scheme uses a simple setting approach, which calculates STA equipped with antennas (≥). antenna angle of beam-tilt from channel state information (CSI) elements control angle of beam-tilt via analog beamforming between the selected horizontal row of AP antenna elements by using phase shifters connected to each antenna element. and STA, given that there are fewer radio frequency (RF) On the other hand, antenna elements perform MIMO front ends than antenna elements. In order to clarify the transmission via digital beamforming. It is assumed that effectiveness of our adaptive beam-tilt switching in an indoor transmit time and center frequency are synchronized at both environment, we have evaluated the proposed scheme by APs. Figure 2 shows the conceptual design of beam-tilt when computer simulations based on raytracing [22]. For the next antenna elements are spaced by . When angle of beam-tilt International Journal of Antennas and Propagation 3 is ,phasedifference between antenna elements is ex- dz sin pressed by #1 2 sin dz = , (1) Angle of beam-tilt #2 1→1( ) 2→2( )∈C× where is wavelength. H 1 , H 2 denote #3 MIMO channel matrices of the th (=1,...,) subcarrier . between AP-1 and STA-1, AP-2, and STA-2 after adjusting . angle of beam-tilt and are expressed by #Z 1→1 H (1) Antenna element 1(−1) 1(−1) Beam direction =[∑ h1,1,, ⋅⋅⋅ ∑ h1,,,], =1 √ =1 √ (2) Figure 2: Conceptual design of beam-tilt. 2→2 H (2) (−1) (−1) 2 2 =[∑ ⋅⋅⋅ ∑ ], √ h2,1,, √ h2,,, =1 =1 3. Proposed Scheme ×1 where h1,,,, h2,,, ∈ C denote channel vectors This section explains the network-assisted interference sup- of the th subcarrier between (th ( = 1,...,), th pression scheme using adaptive beam-tilt switching including ( = 1,...,)) antenna element of AP-1 and STA-1 and TPC. In the proposed scheme, two APs simultaneously (th, th) antenna element of AP-2 and STA-2, respectively. transmit to their own desired STA on MIMO-OFDM trans- Also, 1, 2 are parameters of beam-tilt of AP-1 and AP-2, mission while adjusting parameters (angle of beam-tilt and 1→2 2→1 × respectively. G (1), G (2)∈C denote MIMO transmit power) calculated to suit STA locations. Moreover, channel matrices between AP-1 and STA-2 and AP-2 and theproposedschemecanbeappliedtoenvironmentswith
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