본 논문에는 초광대역 통신시스템을 위한 주파수 영역의 통계학적 채널 모델을 서술하고 있다. 채널 모델링은 3개의 사무실 환경, 46개의 송, 수신 위치에서 얻어진 23,000개의 채널응답함수로 부터 얻어졌다. 측정실험을 통해 얻어진 데이터를 바탕으로 주파수 변화에 따른 경로감쇄지수 변화에 대해 서술한 후 전파환경과 가시경로의 존재여부에 따른 수신신호의 확률분포모델을 연구하였다. 마지막으로는 수신된 주파수 톤에 해당하는 수신파워의 표준편차와 같은 통계적 특성들을 고찰하였는데, 가시경로가 존재하는 경우에는 송, 수신기 사이의 거리가 멀어지면서 표준편차 값이 커지고 그에 따라 수신 주파수 톤의 파워가 평균 수신파워에서 일정한 범위 안에 들어올 확률이 떨어지는 것을 알 수 있었다.이 논문은 두뇌한국 21 사업과 대학 IT 연구센터 사업(ITRC)의 지원으로 수행되었다

강노경

김성철

김정욱

최진원

The Journal of Korean Institute of Communications and Information Sciences

In this paper, we present the statistical characterization of ultra wideband (UWB) channel of office
environments in frequency domain. The 23,000 channel transfer functions of 46 transmitter-receiver location
pairs are obtained using the measurement system based on frequency sweep technique. From the measured
data, path loss exponent variation to the sub-frequency band is firstly reported. Then, distributions of
received signals are analyzed considering the propagation environments and existence of line-of-sight (LOS).
Finally, the statistical properties of received signal powers of frequency tones are reported. For LOS cases, as
the separation between transmitter and receiver increases, the standard deviation of signal powers of the
received frequency tones increases and probability, the received signal powers are located within specific
range from the received power mean values, decreases

이창훈

SNU Open Repository and Archive

 사무실 환경에서 초광대역 채널의 통계학적 특성모델 최진원 O, 강노경, 이창훈, 김성철 서울대학교 전기, 컴퓨터공학부  Statistical Characterization of UWB channel in Office Environments Jinwon ChoiO, Noh-Gyoung Kang, Chang-Hoon Lee and Seong-Cheol Kim School of Electronics Engineering & INMC, Seoul National University {caesar, peterpan, lchjsa, sckim}@maxwell.snu.ac.kr   Abstract  In this paper, we present the statistical characterization of ultra wideband (UWB) channel of office environments in frequency domain. The 23,000 channel transfer functions of 46 transmitter-receiver location pairs are obtained using the measurement system based on frequency sweep technique. From the measured data, path loss exponent variation to the sub-frequency band is firstly reported. Then, distributions of received signals are analyzed considering the propagation environments and existence of line-of-sight (LOS). Finally, the statistical properties of received signal powers of frequency tones are reported. For LOS cases, as the separation between transmitter and receiver increases, the standard deviation of signal powers of the received frequency tones increases and probability, the received signal powers are located within specific range from the received power mean values, decreases.      1. Introduction  For prospective high data rate and short range communication system, the ultra wideband (UWB) technique has attracted increasing interest in recent years. Especially, after Federal Communication Commission (FCC) approved the regulation for UWB transmission, discussions about the commercial UWB systems have been very active [1]. In order to develop the efficient UWB system and to predict its effects on other communication systems, it is required to understand the accurate UWB communication channel properties at first. The literature has reported the experimental campaign and results about UWB channel models including suggested standard model [2] ~ [7]. However, the channel characterization works in frequency domain are insufficient even though the wide frequency band is the most distinguishable feature of UWB signal and some researches were dealt with in [8][9]. So, we studied the UWB channel in office environments based on the measurement results obtained from the frequency sweep measurement system. The office is selected as measurement environment because it is the environment where huge data are transferred frequently and many devices coexist. In this work, we investigated the statistical characteristics of UWB signal in frequency domain. At first, the path loss exponent variation to the frequency is analyzed. Path loss exponent is the essential parameter of log distance path loss law which is the most widely used path behavior describing formula. The parameters are analyzed according to the receiver conditions and smaller frequency band with 500MHz bandwidth. Then, distribution of received signal power of each receiver condition is reported. Finally, we investigate the standard deviation of received signal power and existence probabilities of the received signal power within specific range from the received power mean values. The rest of the paper is organized as follows: Section 2 describes the channel measurement system and scenario. Section 3 gives the measured results and analysis. Finally, the conclusion is described in section 4.    2. Measurement Campaign A. Measurement system Among UWB channel measurement techniques, we selected the frequency domain channel sounding system for measurements [10]. In this measurement technique, wide frequency bands are swept using a set of narrow-band signals and channel frequency responses are recorded using a vector network analyzer (VNA). The constructed measurement system is pictured in Figure1. A VNA transmits 801 continuous frequency tones uniformly distributed from 5GHz ~ 6.6GHz with the frequency separation of 2MHz. This frequency interval allows us to capture multi-paths with the maximum excess delay of 500 nano-seconds and the bandwidth of 1.6GHz gives the time resolution of 0.625 nano-seconds. The transmitting and receiving antennas are omni-directional with 2.1dBi gain and mounted on the 1.6m tripods. The sample of measured channel transfer function is illustrated in Figure 2.  a)                    b)  Figure 1. Measurement system in a) anechoic chamber and b) measurement environment   Figure 2. Measured channel transfer function at a LOS receiver position over 100 seconds  B. Measurement Scenario  We carried out the measurements at three office environments of SNU, Korea. The locations of transmitting and receiving antennas are illustrated in Figure 3 with the floor and wall-type description of environments. The Office 1 is located on the 5th floor of ferroconcrete building, Office 2 is located on the 2nd floor and Office 3 is on the 4th floor of the other ferroconcrete building. In Office 1 and 2, there is a metal wall at the center dividing the office into two small laboratories while the last environment consists of small offices and corridor. During measurements, all doors are kept opened and pedestrians are restricted. Out of 46 receiver positions, 21 positions have line-of-sight (LOS) path from transmitter to receiver and 25 positions do not. The each receiver position had 5 spatial positions for sector average and 100 frequency responses were collected in each spatial position (500 responses in single receiver positions). The time interval between conjunctive responses is one second which is much larger than the general maximum excess delay in indoor environments [11].    Bookshelf BookshelfTableTableBookshelfDeskTableTx2Rx26Rx25Rx27Rx28Rx29Rx30Rx31Rx32Rx34Rx33Bed11mDoorDeskDoor a) Office 1       b) Office 2  c) Office 3 Figure 3. Ground plans and wall types of three offices where measurements performed  3. Measurement Results and Analysis A. Path loss exponent variation  Generally used log-distance path loss formula is given in Eq. 1 where PL(d0) means path loss at the reference distance d0(=1m), d is separation of the transmitter-receiver, n is the path loss exponent and Xσ is the standard deviation of shadowing factor [9].  )()/log(10)()( 00 dBXddndPLdPL σ++=   (1)  From the measured data, the optimized parameters are obtained in Table 1 using minimum mean square error algorithm. In general, path loss exponent is 2 in free space and smaller than 2 when the waveguide effect works on for indoor LOS environments [12]. For LOS positions of Office 1 and Office 2, the path loss exponent values are much smaller than 2 because a metal wall dividing offices made the waveguide effect. Especially, in Office 2, as the upraised desks, bookshelves and metal blinds of glass window enhanced the waveguide effect, the path loss exponent value is very small. And the standard deviation of shadowing factor is about 1 dB in LOS cases and it is increased to the 2.5 ~ 4dB in NLOS cases [9].  However, as indicated in FCC regulation, UWB system could use only 500MHz bandwidth of the assigned frequency band and widely suggested UWB system, multi-band orthogonal frequency division multiplexing scheme, is based on this UWB property [14]. According to this scheme, UWB system is operated with divided frequency band of 500MHz bandwidth.  Table 1. The empirical path loss parameters according to receiver condition  PL(d0) [dB] n Xσ  [dB] LOS (Office 1) -35.596 1.58 1.063 NLOS (Office 1) -35.596 2.13 2.656 LOS (Office 2) -37.913 1.32 1.101 NLOS (Office 2) -37.913 2.10 4.546 NLOS (Office 3) -43.786 2.85 4.441 n: path loss exponent     Xσ :the standard deviation of shadowing factor  Hence, we swept our measurement 1.6GHz bandwidth by sliding the 500MHz window with the discrete interval of 100MHz and analyzed the path loss exponent variation to the each 500MHz bandwidth window. The path loss exponent variation is illustrated in Figure 4 where x-axis means the center frequency of the smaller 500MHz band. From the results, we found out that the variation is related to the measurement environment not to the existence of LOS. In Office 1 and 3, path loss exponent increases as frequency band becomes higher while it has the convex shape in Office 2 while path loss exponent variation has the same tendency in LOS and NLOS cases in same office.  B. Distribution of received signal power  To understand the path loss properties of UWB signal properly, not only the average path loss characteristics but also the distribution of the received signal power has to be analyzed. Figure 5 illustrates the typical cumulative distribution functions (CDFs) of received signal of LOS and NLOS locations. The CDFs are based on the 500 frequency responses in each receiver location. In Figure 5, x-axis indicates the normalized received signal and y-axis means the cumulated probability. The CDF of LOS locations is similar to the Rician distribution with K=1 and those of NLOS locations are almost to the Rayleigh distribution.  1.11.51.92.32.73.13.53.95.25 5.35 5.45 5.55 5.65 5.75 5.85 5.95 6.05 6.15 6.25 6.35Center frequency [GHz]Pathloss exponentLOS of Office 1NLOS of Office 1LOS of Office 2NLOS of Office 2NLOS of Office 3Figure 4. Path loss exponent (n) variation with respect to the center frequency of smaller frequency band in each propagation condition Figure 5. Comparison between received signal CDFs of LOS and NLOS locations  As the FCC regulates the transmission of UWB system, the low power transmit signal suffers the relatively bigger noise effect. As a result, CDFs of LOS locations are similar to the Rician distribution with low K value.   C. Statistical analysis of received frequency tones  In this section, we analyzed the statistical feature of received frequency tones. The STDEV of received signal power of frequency tones indicates the signal power difference of received frequency tones and probability is the systematic parameter which can be used for predicting the signal strength variation. In LOS, the STDEV of received frequency tones increases as transmitter-receiver separation increases in near distance and saturates over the separation of 6m. This phenomenon is illustrated in Figure 6. This implies that the in near distance difference of received power to frequency is small and this value grows to the separation. But, over the certain distance (6m in this campaign), this parameter has the saturated value.    Figure 6. Standard deviation of received frequency tones to separation between transmitter and receiver in LOS condition   Figure 7. Probabilities of the power level of received signal tones is located in the specified range from the mean value in LOS of Office 1  The probabilities which the received powers of frequency tones are located within specific range from the mean value are drawn in Figure 7. The 1dB means the probability of the received powers of frequency tones are located within the range [(mean)-1,(mean)+1]. And 90% probability means the 90% among the total frequency tones (801 frequency tones * 500 snapshots) is being within the specific range. The probabilities get down as separation gets bigger and saturated as the STDEV. The saturated numerical values in LOS and NLOS locations are reported in Table. 2. As shown in Figure 6,7 and Table 2, the probabilities are in inverse proportion to the STDEV of received frequency tones.  Table 2. The saturated values of STDEV of received frequency tones and probabilities within the specific rage from the mean value  STDEV Pr1dB Pr3dB Pr5dB Pr7dB Pr10dB LOS 5.643 0.1529 0.4406 0.6692 0.8216 0.9294 NLOS 6.811 0.1205 0.3569 0.5602 0.7203 0.8730 NLOSLOS 0.829 1.269 1.235 1.195 1.141 1.065   4. Conclusion  For efficient deployment of UWB communication system, the accurate channel has to be preceded. So, we analyzed the frequency dependent behavior of UWB channel in office environments. The measurement system uses the frequency sweep method for channel characterization. From the results measured at 3 offices, the path loss exponent is under 1.6 when the LOS path is guaranteed and is above 2 in NLOS case. And the path loss exponent of each smaller frequency band depends on the measurement environment not on the LOS condition. In distribution analysis we find that the measured signal levels of LOS sites follow the Rician fading and that of NLOS do Rayleigh. Finally, in LOS case, as separation between transmitter and receiver grows, the STDEV of received power of frequency tones increases and saturates to the similar value of NLOS. And, probabilities which received signal power located within specific range from the mean received power value are in inverse proportion to the STDEV..      5. References  [1] Federal Communications Commission, First Order and Report, Revision of Part 15 of the Commission’s Rules Regarding UWB Transmission Systems, Federal Communications Commission, FCC 02-48, Apr. 22, 2002 [2] Cassioli, D., Win, M.Z. and Molisch, A.F., “The ultra-wide bandwidth indoor channel: from statistical model to simulations,” IEEE Journal on Selected Areas in Communications, vol. 20,  Issue 6,  pp.1247–1257, Aug. 2002 [3] K. Siwiak, “A path link model for ultra wide band pulse transmissions” VTC 2001 Spring, vol. 2, pp.1173-1175, May 2001. [4] S.S Ghassemzadeh, L.J. Greenstein, A. Kavcic, T. Sveinsson and V. Tarokh, “UWB Indoor Path Loss Model for Residental and Commercial buildings,” in Proc. IEEE Veh. Technol. Conf. (VTC 200-Fall), vol. 5, pp.3115-3119, Oct 2003. [5] R.A. Scholtz et al, “UWB radio deployment challenges.” PIMRC 2000, vol. 1, pp.620-625, Sep. 2000 [6] Uguen, B., Plouhinec, E., Lostanlen, Y., Chassay, G., “A deterministic ultra wideband channel modeling,” IEEE Conference on Ultra Wideband Systems and Technologies, pp. 1-5, May 2002. [7] Ramirez-Mireles, F., “On the capacity of UWB over multipath channels,” IEEE Communications Letters,  vol  9,  Issue 6,  pp. 523-525, Jun 2005  [8] Chong, C.-C., Yong, S.K., “A Generic Statistical-Based UWB Channel Model for High-Rise Apartments,” IEEE Transactions on Antennas and Propagation, vol. 53,  Issue 8,  Part 1,  pp. 2389-2399, Aug. 2005    [9] Irahhauten, Z., Nikookar, H., Janssen, G.J.M., “An overview of ultra wide band indoor channel measurements and modeling,” IEEE Microwave and Wireless Components Letters, vol. 14,  Issue 8,  pp.386-388, Aug. 2004 This paper is partly supported by Brain Korea 21 project and University IT Research Center project [10] I. Oppermann. et al, UWB Theory and Applications, Wiley, England, 2004. [11] A. Saleh, R. A. Valenzuela, “A Statistical Model for Indoor Multipath Propagation”, IEEE Journal on Selected Areas in Communications, vol 5,  Issue 2,  pp.128 – 137, Feb 1987 [12] Theodore S. Rappaport, Wireless communications: principles and practice 2nd edition, Prentice Hall PTR, Upper Saddle River, NJ, USA, 2002 [13] H. L. Bertoni, Radio propagation for modern wireless systems, Prentice Hall PTR, New Jersey, 2000. [14] http://www.multibandofdm.org  

Statistical Characterization of UWB channel in Office Environments

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Statistical Characterization of UWB channel in Office Environments

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