Collision Detection Method Using Self Interference Cancelation for Random Access Multiuser MIMO

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1 elecronics Aricle Collision Deecion Mehod Using Self Inerference Cancelaion for Random Access Muliuser MIMO Kazuma Ando 1, Kenaro Nishimori 1, * ID, Ryochi Kaaoka 1, Takefumi Hiraguri 2, Yoshiaki Morino 2 and Tsuomu Misui 1 1 Faculy of Engineering, Niigaa Universiy, Ikarashi 2-nocho 85, Nishi-ku, Niigaa , Japan; ando@gis.ie.niigaa-u.ac.jp (K.A.); r.kaaoka@m.ieice.org (R.K.); ben321@oulook.jp (T.M.) 2 Faculy of Engineering, Nippon Insiue of Technology, 4-1 Gakuendai, Miyashiro-machi, Minamisaiama-gun, Saiama , Japan; hira@ni.ac.jp (T.H.); e112437@esu.ni.ac.jp (Y.M.) * Correspondence: nishimori@m.ieice.org; Tel.: Received: 3 November 217; Acceped: 2 December 217; Published: 22 December 217 Absrac: This paper proposes an inerference deecion mehod for muliuser-muliple inpu muliple oupu (MU-MIMO) ransmission, which uilizes periodical preamble signals in he frequency domain and he concep of full-duplex ransmission when assuming idle anennas a he access poin () in MU-MIMO. In he propose mehod, collision deecion (CD) of MU-MIMO is achieved by uilizing asynchronous MU-MIMO called random access MU-MIMO. In random access MU-MIMO, several anennas ha are no used for he ransmission exis, due o asynchronous MU-MIMO. Hence, idle anennas a he can receive preamble signals while he ransmi anennas a he ransmi he preamble signals: his procedure is regarded as full-duplex ransmission, which cancels he self-inerference beween anennas. The inerference can be deeced by subracing he shor preamble signal, which is muliplied by he esimaed channel response using he received signal afer he FFT processing. Moreover, we uilize dual polarizaion o reduce he muual coupling beween ransmi and receive anennas a he. Through a compuer simulaion, i is shown ha he proposed mehod can successfully deec collision from oher user erminals (UTs) wih OFDM signals when he inerfering power from he inerfering user erminal (IT) is greaer han he noise power. In addiion, he inerfering power from IT a he and he desired user erminal (DT) is measured in an acual indoor environmen, and he possibiliy of using he proposed mehod a he is discussed by using he measuremen resuls. Keywords: random access muliuser MIMO; collision deecion; shor preamble signal; self-inerference 1. Inroducion Access poins (s) are being increasingly deployed in wireless local area neworks (WLANs) as a means of fulfilling cellular sysem offload requiremens [1]. Since he service area covered by an in a WLAN sysem is considered o be a small cell, many s mus be deployed in areas wih heavy raffic. However, since individual s have limied frequency channels, collisions will occur beween user erminals (UTs) when many UTs aemp o connec wih an. To avoid such collisions, wired LAN sysems can implemen an access conrol scheme called carrier sense muliple access/collision deecion (CSMA/CD) in which, prior o packe ransmission, collisions are deeced from volage variaions occurring wihin he Eherne cable [2]. As his mehod for wired LAN ransmission involves packe deecion in advance and immediae reransmission, is efficiency is over 9% [3]. As a resul, he issues surrounding his mehod have been inensively sudied [4,5]. On he oher hand, anoher access conrol scheme called CSMA/collision avoidance (CA) is adoped in wireless LAN [6]. Unlike wired LAN, i is difficul o deec he packe collision wih wireless Elecronics 218, 7, 1; doi:1.339/elecronics711

2 Elecronics 218, 7, 1 2 of 11 ransmission. Therefore, he recepion characerisics are judged from he reply of acknowledgemen (ACK) sen by a receiving saion. Compared o wired LAN, he ransmission efficiency of wireless LAN is very small, and is value is less han 65%, because reransmission canno be sared unil ACK is judged [3]. Therefore, collision deecion, which is used in wired communicaion sysems, is essenial for achieving high ransmission efficiency. There are wo ypes of collision deecion schemes. The firs collision deecion mehod involves single-user MIMO ransmission [7,8]. For 2 2 MIMO, his mehod uilizes he fac ha he second anenna is idle when he firs anenna ransmi shor preamble signals, which are used for iming synchronizaion wih he UT. The self-inerference beween hese wo anennas a he can be canceled by using dual polarized anennas, which are generally used in MIMO sysems [9 11]. In his paper, his idea is applied for muli-user MIMO sysems. In he second mehod, an approximae CSMA/CD for WLANs is realized using a scheme called CSMA/collision noificaion (CN) [12]. In CSMA/CN, a receiver deecs he inerfering signal while receiving a packe and immediaely noifies he ransmier, which uilizes wo anennas: one for normal ransmission and he oher dedicaed o lisening for noificaions. Upon deecion of a noificaion signal, he ransmier abors is ransmission, freeing up he channel for oher ransmiers in he viciniy [12]. Since he ransmier mus use an addiional anenna o discriminae beween is own ransmi signal and he noificaion signal from he receiver, he CSMA/CN procedure uilizes a correlaion calculaion based on a prior (raining) signal received a he ransmier. When he noificaion signal arrives during he ransmission of he own ransmi signal, anoher anenna judges wheher collision is deeced, because he correlaion value beween he received signal (ideally only noificaion signal plus he noise) and he known raining signal becomes high afer canceling he own ransmi signal [12]. However, here are wo issues in he CSMA/CN procedure. Firsly, i is difficul o deec collision in a real propagaion environmen using only he correlaion value, because he correlaion value wih limied number of samples is acually very small as he power level of he noificaion signal is much lower han ha of he own ransmi signal a anoher anenna on he ransmier. Secondly, since he power of he own ransmi signal is much higher han ha of he noificaion signal from he receiver, he received power becomes sauraed a he amplifier and i causes he noise power o increase; [12] provided here is no concree counermeasure for his effec. In addiion o hese problems, feedback delay may sill occur even if he receiver immediaely noifies he ransmier of a collision. These wo problems have been improved by [7,8]. In his paper, a novel collision deecion mehod using asynchronous MU-MIMO ransmission called random access MU-MIMO [13] is proposed. In random access MU-MIMO, he ransmission from each user is sared immediaely afer preparing he daa packe for a cerain user. Hence, asynchronous downlink ransmission is employed, and null daa is ransmied for he user whose daa packe is no prepared. In his sudy, we uilize his feaure on random access MU-MIMO, and idle anennas, which do no ransmi any signal, can be prepared insead of ransmiing null daa. Thus, he collision deecion using idle anennas in random access MU-MIMO is realized similar o ha in single user MIMO. In random access MU-MIMO, he unused anennas can receive preamble signals, while ransmission is employed wih muliple anennas ha are used in random access MU-MIMO. The signals are mapped o several subcarriers in shor preamble signals of IEEE82.11 based OFDM signals, and he signals in he frequency domain are ransformed by IFFT a he ransmier. A he receiver, he correlaion is calculaed, and he iniial iming of shor preamble signals is deeced. Convenionally, he shor preamble signals are discarded afer iming synchronizaion. On he oher hand, in he proposed mehod, he shor preamble signals are ransformed ino he frequency domain by FFT processing. The inerference from he inerfering user erminals (IT) can be deeced by subracing he shor preamble signal, which is muliplied by he esimaed channel response using he received signal. The effeciveness of he proposed mehod is shown by compuer simulaion. In addiion, he inerfering power from he IT a he and desired user erminal (DT) is verified

3 Elecronics 218, 7, 1 3 of 11 by assuming packe collision in an acual indoor environmen. By analyzing he inerfering signal power from IT o and DT, he possibiliy of collision deecion using he proposed mehod is demonsraed. The remainder of his paper is organized as follows. Secion 2 shows he proposed mehod using random access MU-MIMO. The basic characerisics and effeciveness of he proposed mehod are demonsraed via compuer simulaion in Secion 3. In order o verify he possibiliy of using he proposed mehod a he, he characerisics of he inerference power in an acual indoor environmen is presened in Secion Proposed Mehod 2.1. Basic Concep of Proposed Mehod In his paper, we consider an IEEE82.11n/ac-based wireless LAN sysem. When he number of anennas in he is N, and he number of anennas a he UTs is one, he makes a group wih N users. Hereinafer, we assume ha N is 4. In he case of wireless LAN, he process of carrier sense prohibis he ransmission by oher UTs. MIMO and MU-MIMO are inroduced o he IEEE82.11n/ac-based wireless LAN. We focus on he inerference signals due o packes ha are synchronously ransmied from each UT, afer each UT employs a carrier sense in he IEEE82.11n/ac-based wireless LAN. Figure 1 shows he ransmi sequence of convenional and random access MU-MIMO. In convenional MU-MIMO, all he packes of he synchronous ransmied muliuser group are generaed, and he channel sae informaion (CSI) beween he and UTs is esimaed. Afer acquisiion of CSI, he convenional MU-MIMO downlink ransmission sars. As shown in Figure 1, since he waiing ime of he firs generaed packe in convenional MU-MIMO ransmission is long, he ransmission efficiency of convenional MU-MIMO is very low. On he oher hand, random access MU-MIMO realizes asynchronous MU-MIMO. Afer one packe of he synchronous ransmission muliuser group is generaed, he CSI beween he and all he UTs in muliuser group is immediaely esimaed. The daa packes are (DATA1 and DATA2 in Figure 1) insananeously ransmied in random access MU-MIMO. Therefore, since his procedure reduces no only he overhead of CSI esimaion bu also he ransmission wai ime for each daa generaion, random access MU-MIMO improves he ransmission efficiency [13]. Creae iming of daa UT1 UT2 UT3 UT4 UT1 UT2 UT3 UT4 DATA1 DATA2 DATA3 DATA4 Waiing ime CSI feedback Waiing ime CSI feedback DATA1 DATA2 DATA3 DATA4 DATA1 DATA2 DATA3 DATA4 Convenional MU-MIMO Random access MU-MIMO Figure 1. Transmission sequence by convenional and asynchronous MU-MIMO. As can be seen in Figure 1, for he random access MU-MIMO, he proposed mehod uilizes he fac ha he number of daa packes for a cerain period is less han he number of idle anennas, which are no necessarily required for ransmission a he. Figure 2 shows an example of he

4 Elecronics 218, 7, 1 4 of 11 daa sequence in random access MU-MIMO ransmission and a basic idea of he proposed mehod. As can be seen in Figure 2a, he daa packes are ransmied afer gahering he downlink signals. Hence, he proposed mehod focuses on his feaure, and he receive anennas are prepared o deec self-inerference cancellaion beween he anennas. As can be seen in Figure 2, only Daa 1 and 2 for UT 1 and 2 are ransmied. Hence, a leas wo anennas are required for MU-MIMO ransmission. In he proposed mehod, he anennas #3 and #4 a he are used for collision deecion. Since a leas hree, wo, and one anenna are required a he in he periods 2, 3, and 4, respecively, as shown in Figure 2a, he unused anennas are prepared for realizing collision deecion. DATA 1 DATA 2 DATA 3 DATA 4 Period 2 Period 4 Period 1 Period 3 (a) UT1 UT3 UT2 UT4 (b) #1 #2 #3 #4 Inerference signal Inerference Terminal Figure 2. Concep of he proposed mehod Deailed Principle of Proposed Mehod Figure 3 shows he waveforms before and afer applying he proposed mehod. As can be seen in Figure 3, he preamble signals are ransmied for synchronizaion and esimaion of CSI before ransmiing he daa signals; he shor preamble signal is uilized for deecing he iniial iming of he daa packe. From Figure 4, i is seen ha he shor preamble signal is adoped in he IEEE82.11n-based wireless LAN sysem. A he ransmier, he signals are mapped for only welve subcarriers in order o generae a periodic signal in he ime domain [6]; following his IFFT processing, he signal is ransmied. A he receiver, iming synchronizaion based on he calculaed correlaion value beween he received signal, y(), and he shor preamble signal in he ime domain beween he known ransmied and received signals, s p (), is employed. Noe ha he correlaion calculaion is compleely differen from ha in [12], because his calculaion is employed for finding he iniial iming of he daa packe. The correlaion beween hese signals, ρ, is denoed as ρ = L =1 L =1 s p()y() sp () 2 L =1 y() 2, (1) where L is he number of symbols for which correlaion calculaion is conduced (for IEEE82.11n-based OFDM signals, L = 16) [6]. The correlaion value, ρ, is maximized a he iniial iming of he shor preamble signals.

5 Elecronics 218, 7, 1 5 of 11 Desired signal #1 #1 Preamble Daa #2 Preamble Daa si #2 #3 #4 Power Time domain FFT Desired + Inerference Inerference Power f Frequency domain Calc. Power f Frequency domain Inerference signal Inerference Terminal Figure 3. Waveforms before and afer applying he proposed mehod. 5 4 Power [W] Subcarrier number Figure 4. Shor preamble signal. In a ypical wireless LAN sysem, he shor preamble is no used afer iming synchronizaion. In conras, in he proposed mehod, he shor preamble is used afer FFT processing. When he inerference signals arrive a he, he received signal a he k-h subcarrier afer FFT, r p (k), is denoed as r p (k) = M h si (k)x i (k) + h I (k)s I (k) + n(k), (2) i=1 where h si (k) is he self-inerference channel response beween he i-h ransmi and he receive anenna of he a he k-h subcarrier. h I (k) is he channel response beween IT and he receive anenna of he a he k-h subcarrier. x i (k) is he produc of he ransmied signal of he i-h ransmi anenna of he and he weigh vecor, which is generaed by block diagonalizaion algorihm. s I (k) and n(k) denoe he inerference signal, which assumes he shor preamble signal, and he hermal noise a he k-h subcarrier, respecively. M is he number of self-inerference signals a he receive anennas. Here, he ransmied signals, which are shor preamble signals, are already known, and h si denoes he esimaed channel response of h si. The channel response, h si (k), can be assumed o be obained beween he idle ime a he in advance, because he propagaion characerisic beween he Anenna #1 and #2 a he could be no changed and h si (k) will be regarded as he saic channel. The channel response of he inerference signal, h I (k), is esimaed as r p (k) M i=1 h si (k)x i (k) h I (k)s I (k) + n(k). (3) The inerference deecion can be realized by using Equaion (3) while he ransmission is employed.

6 Elecronics 218, 7, 1 6 of Effeciveness of he Proposed Mehod by Compuer Simulaion Through his compuer simulaion, in which IEEE82.11n/ac-based OFDM signal is assumed, he effeciveness of he proposed mehod is verified by evaluaing he characerisics of self-inerference reducion and deecion of inerference from ITs. Table 1 shows he simulaion parameers. As can be seen in Table 1, when considering he self-inerference signal beween anennas, he average signal o noise power raio (average SNR) is se o be 45 db from he previous measuremen [7], and we confirmed ha he fading caused by self-inerference beween anennas can be negligible due o is large power, even if people work around he [8]. On he oher hand, he propagaion of inerference signal is assumed o be Rayleigh fading. In he simulaion, he signal o inerference power raio (SIR) is changed from o 5 db. The oher basic parameers are he same as ha for he IEEE82.11n/ac sandard wih 2 MHz mode. When h si (k) denoes he esimaed channel response beween he anennas and ρ h () denoes he ime variaion of channel response, h si, he channel response beween he i-h ransmi and a receive anenna of he is denoed as h si (k) = ρ h () h si (k) + 1 ρ 2 h ()h iid(k). (4) From he resuls in [8], ρ h () is se o be.9999 in he evaluaion. The simulaion process is summarized as follows: [Sep (1)] The desired signal and he inerference signal wih he OFDM signal forma are creaed and added. [Sep (2)] The shor preamble signal is deeced from he signal by he sliding correlaion. [Sep (3)] The inerference signal is esimaed and deeced. Table 1. Simulaion parameers. Number of Trial 11 Number of ransmi anennas (N T ) 2 Number of receive anennas (N R ) 1 Number of desired user erminals (N U ) 2 Number of inerference user erminals (N IT ) 1 Average SNR 45 db Average SIR 5 db Propagaion pah of inerference signal Rayleigh fading Bandwidh 2 MHz Number of FFT poins 64 Figure 5 and 6 show he received power of inerference from IT in he frequency domain, and hese are esimaed by he proposed mehod by assuming ha he SIR is 1 and 45 db. The received power is no ransmied when here is self-inerference beween he anennas; only he inerference signal from he IT arrives a he receiver of he. The esimaed and ideal inerfering powers are ploed in hese figures. When considering low SIR, as shown in Figure 5, he inerference power versus subcarrier number for he esimaed inerfering power is almos idenical o ha for he ideal inerfering power. Alhough he esimaion seems o be possible when he SIR is 1 db (in Figure 5), we can observe ha he esimaion error wih SIR = 45 db is much larger han ha wih SIR = 1 db.

7 Elecronics 218, 7, 1 7 of 11 Power [db] 2 1 Inerference Esimaion Subcarrier number Figure 5. Power versus subcarrier number (SIR = 1 [db]). Power [db] Inerference Esimaion Subcarrier number Figure 6. Power versus subcarrier number (SIR = 45 [db]). In order o evaluae how o esimae he inerference signal when considering low SIR, he inerference channel response is esimaed by using Equaions (2) and (4). If he muliplicaion of inerference signal and shor preamble signal is denoed as h I (k) s p (k), he esimaed equaion is denoed as h I (k) s p (k) r p (k) M i=1 h si (k)s i (k). (5) The esimaion error on he inerference power by he proposed mehod is evaluaed. The esimaion error is denoed as Esimaion Error = 2 log h I (k)s p (k) 1 h I (k) s p (k), (6) where h I (k)s p (k) 2 and h I (k) s p (k) 2 denoe he ideal and esimaed inerference power, respecively. In his sudy, he number of rials was 1,1. Figure 7 shows he esimaion error versus SIR. The median values are ploed for each SIR for all he rials by changing he channel response. As can be seen in Figure 7, he esimaion error is less han 1 db, when he SIR is less han 3 db. Hence, he proposed mehod can successfully cancel he self-inerference beween anennas and esimae he inerference wih small power.

8 Elecronics 218, 7, 1 8 of 11 Esimaion Error [db] 15 SNR = 45 db 1 5 Esimaion Error Threshold value SIR [db] 4 5 Figure 7. Esimaion error versus SIR. 4. Inerference Power Characerisics in an Acual Indoor Environmen We carried ou indoor measuremen using IEEE82.11n/ac-based OFDM signals, in order o verify how serious he problem of inerference is for he DT, when considering he collision deecion a he. Figure 8 and Table 2 show he measuremen environmen and parameers, respecively. In his measuremen, he inerfering power from IT o and DTs is verified by assuming packe collision in an acual indoor environmen. Hence, in his measuremen, IT is a ransmi anenna, and and DT are receive anennas. The OFDM signal has been measured in an acual indoor environmen. The received power was analyzed by he measured OFDM signal. The proposed mehod is employed by he measured received signals in Secion 2. DT number N Area number D Average disance [m] Sairs D N = 5 D = 4.65 N = 4 D = N = 3 D = N = 2 D = N = 1 D = 4.57 Figure 8. Measuremen environmen. The size of he room is m and and DTs are locaed in a corridor (ouside he room). The cener frequency and ransmi power are 2.55 GHz and 21 dbm, respecively. The oher basic measuremen parameers are he same as ha for he IEEE82.11ac sandard signal forma. The shor preamble signals are obained by sliding correlaion a each measuremen poin. In order o avoid specific characerisics in he measuremen, he ransmier is moved wih an inerval of.5 wavelenghs by using a posiion conroller, and 17 measuremen poins are obained for each ransmi locaion.

9 Elecronics 218, 7, 1 9 of 11 Table 2. Measuremen condiions. Transmiing Signal Cener frequency Transmi power High of High of IT and DT IEEE82.11 Based OFDM Signal 2.55 GHz 21 dbm 2.4 m 1.2 m Figure 9 shows he inerference o noise power raio (INR) versus he disance from IT o a he posiion. The INR is obained by averaging 17 measuremen poins. The broken line indicaes he hreshold value, which is obained by he esimaion error wih 1 db in Figure 7, because he INR is 15 db when SIR and SNR are 3 and 45 db, respecively. When he INR is greaer han he hreshold value, he proposed mehod can deec he inerference while cancelling he self-inerference beween anennas. Moreover, Figure 1 shows ha he INR versus disance from IT o is idenical in his measuremen. As can be seen in Figure 1, he inerference can be deeced a a disance of 16 m from he. Since he endency of INR among and DTs are similar, he insead of DTs can successfully esimae he inerfering power from IT when considering a room wih small size. On he oher hand, i is verified from Figure 1 ha DT receives he inerference bu canno receive inerference when he disance is greaer han 16 m. Figure 11 shows he SIR of DTs versus disance from IT o. As can be seen in Figure 11, in such a scenario, collision canno be deeced a he bu he received power from o DT is much higher han ha from IT and DT. Therefore, he communicaion beween DT and is no a serious problem. INR [db] Disribuion Threshold Disance from IT o [m] Figure 9. INR versus disance from IT o. Average INR [db] Max 5 Min Disance from IT o [m] Figure 1. Average INR versus disance from IT o.

10 Elecronics 218, 7, 1 1 of 11 SIR of DTs [db] 5 4 Max 3 Median 2 1 Min Disance from IT o [m] Figure 11. Range of SIR a DTs. 5. Conclusions In his paper, a collision deecion mehod using asynchronous MU-MIMO called random access MU-MIMO is proposed. This mehod uilizes he fac ha idle anennas, which do no ransmi signals, can be employed o receive and deec inerference when considering random access MU-MIMO. Hence, idle anennas a he can receive preamble signals while ransmi anennas a he ransmis he preamble signals: his procedure is regarded as full-duplex ransmission, which cancels self-inerference beween anennas. The inerference can be deeced by subracing he shor preamble signal, which is muliplied by he esimaed channel response using he received signal afer he FFT processing. Through he compuer simulaion, in which IEEE82.11n/ac-based OFDM signal is assumed, he effeciveness of he proposed mehod is verified by evaluaing he characerisics of self-inerference reducion and deecion of inerference from ITs. I is verified ha he esimaion error on he inerference power is less han 1 db, when he SIR is less han 3 db. Hence, he proposed mehod can successfully cancel he self-inerference beween anennas and esimae inerference wih small power. In addiion, we carried ou indoor measuremen using IEEE82.11n/ac-based OFDM signals, in order o verify how serious he problem of inerference is for he DT, when considering he collision deecion a he. I is shown ha he proposed mehod accuraely esimaes he inerference power even if he inerference power is small, and ha collision deecion a he is realized from he measuremen. As he basic sudy in he proposed mehod, inerference deecion performance is evaluaed in his paper. However, o evaluae he oal sysem performance, he bi error rae should be evaluaed. As he fuure sudy, he evaluaion wih he BER will be sudied when considering he oal sysem performance. Acknowledgmens: Par of his work was suppored by KAKENHI, Gran-in-Aid for Scienific Research (B) (17H3262, 17H1738). Auhor Conribuions: This sudy was led by K.A. while K.N., R.K., T.H., Y.M. and T.M. assised wih he compuer simulaions. Conflics of Ineres: The auhors declare no conflics of ineres. References 1. Nakamura, T.; Nagaa, S.; Benjebbour, A.; Kishiyama, Y. Trends in Small Cell Enhancemens in LTE Advanced. IEEE Commun. Mag. 213, 51, IEEE. IEEE 82.3 Sandard for Informaion Technology-Specific Requiremens Par 3: Carrier Sense Muliple Access wih Collision Deecion (CSMA/CD) Access Mehod and Physical Layer Specificaions; IEEE: Piscaaway, NJ, USA, Hiraguri, T.; Nishimori, K.; Ogawa, T.; Kaaoka, R.; Yoshino, H.; Makio, H. Access conrol scheme for collision deecion uilizing MIMO ransmission. IEICE Commun. Express 213, 2,

11 Elecronics 218, 7, 1 11 of Abeysekera, B.A.H.S.; Masuda, T.; Takine, T. Dynamic conenion window conrol mechanism o achieve fairness beween uplink and downlink flows in IEEE wireless LANs. IEEE Trans. Wirel. Commun. 28, 7, Hiraguri, T.; Ogawa, T.; Nagaa, K.; Ueno, T.; Jinno, K.; Nishimori, K. Queuing Schemes for Improving Downlink Throughpu on WLANs. J. Springer Wirel. Pers. Commun. 212, doi:1.17/s IEEE. IEEE Sandard for Local and Meropolian Area Neworks Par 11: Wireless LAN Medium Access Conrol (MAC) and Physical Layer (PHY) Specificaions; IEEE: Piscaaway, NJ, USA, Kawahara, M.; Nishimori, K.; Kaaoka, R.; Hiraguri, T.; Hideo, H. Inerference deecion using wireless LAN based MIMO ransmission. In Proceedings of he 213 Asia-Pacific Microwave Conference (MC), Seoul, Korea, 5 8 November 213; pp Kawahara, M.; Nishimori, K.; Hiraguri, T.; Makino, H. A new propagaion model for collision deecion using MIMO ransmission in wireless LAN sysems. In Proceedings of he 214 IEEE Inernaional Workshop on Elecromagneics, POS1.17, Sapporo, Japan, 4 6 Augus Erceg, V.; Soma, P.; Baum, D.S.; Careux, S. Muliple-Inpu Muliple-Oupu Fixed Wireless Radio Channel Measuremens and Modeling Using Dual-Polarized Anennas a 2.5 GHz. IEEE Trans. Wirel. Commun. 24, 3, Nishimori, K.; Honma, N.; Murakami, T.; Hiraguri, T. Effeciveness of relay MIMO ransmission by measured oudoor channel sae informaion. IEEE Trans. Anennas Propag. 212, 6, Kaaoka, R.; Nishimori, K.; Kawahara, M.; Hiraguri, T.; Makino, H. Inerference deecion mehod using wireless LAN based MIMO ransmission. IEICE Commun. Express 213, 2, Sen, S.; Choudhury, R.R.; Nelakudii, S. CSMA/CN: Carrier sense muliple access wih collision noificaion. IEEE/ACM Trans. New. 212, 2, Morino, Y.; Ogawa, T.; Nishimori, K.; Hiraguri, T. A Sudy of Synchronous Random Access Sysem for MU-MIMO; IEICE Technical Repor, CS214-33; IEICE: Kagoshima, Japan, 214. c 217 by he auhors. Licensee MDPI, Basel, Swizerland. This aricle is an open access aricle disribued under he erms and condiions of he Creaive Commons Aribuion (CC BY) license (hp://creaivecommons.org/licenses/by/4./).

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