R ied extensively for the evaluation of different transmission

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1 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. 39. NO. 5. OCTOBER 1990 Measurement and Analysis of the Indoor Radio Channel in the Frequency Domain 75 I STEVEN J. HOWARD AND KAVEH PAHLAVAN, SENIOR MEMBER, IEEE Abstract-Using a network analyzer, several experiments for the frequency-domain characterization of the indoor radio channel in the GHz band are performed. In the experiments, the frequency response measurements are taken at spatially distributed locations throughout the test area by fixing the receiver in a central location and moving the transmitter to different locations. The experiments were performed in a high-rise office building and a three-story building with offices and laboratories. For each experiment, the exponent of the power-distance relationship and the statistics of the 3-dB width of the frequency correlation function are determined from the frequency-domain data. The approximation to the impulse response of the channel is obtained from the inverse Fourier transform of the frequency response. An empirical exponential relationship between the 3-dB width of the frequency correlation function and the inverse of the rms delay spread of the impulse response is derived. I. INTRODUCTION ECENTLY, indoor radio propagation has been stud- R ied extensively for the evaluation of different transmission systems for application in universal portable phones and wireless local area networks. Radio propagation studies can be performed either in the time domain or in the frequency domain. The reported wide-band measurements and modeling for the indoor radio channels have been performed almost exclusively in the time domain [ l]-[5]. These measurements determine the channel impulse response by sending a narrow pulse and by observing the effect of the channel on the received signal. The limited measurement in [4] reports the changes observed in the channel frequency response when an object moves close to the transmitter or the receiver. This paper presents the results of the frequency-domain measurements in two different buildings, compares the statistical behavior of the two, and relates the results to the timedomain statistics. Coherent wide-band frequency-domain measurements, presented in this paper, provide magnitude and phase of the frequency response of the channel. As a result, the exact time-domain response is also obtained by taking the inverse Fourier transform of the measured data. In applications such as channel modeling or performance calculation, the phase information is necessary. The wide-band Manuscript received December ; revised May 18, This work was supported in part by the National Science Foundation under Grant NCR and a Raytheon Company Fellowship. S. J. Howard is with Raytheon Company. Marlboro. MA K. Pahlavan is with the Department of Electrical Engineering. Worcester Polytechnic Institute, Worcester, MA IEEE Log Number time-domain measurements in [ 11-[5] provide only the magnitude of the time-domain response. The transmitted power in the frequency-domain measurement has a constant envelope, as opposed to the time-domain measurement where the ratio of the peak to average transmitted power is large. This fact allows a larger area to be measured and reduces the effects of nonlinearities. Also, the set up for the frequency measurements is easier and the measurement time is shorter when it is compared with the time-domain measurements explained in [ 11 and [2]. 11. MEASUREMENT SYSTEM The block diagram of the measurement system used for frequency-domain characterization of the indoor radio channel is shown in Fig. 1. The main component of the measurement system is a network analyzer that outputs a swept frequency signal and analyzes the received signal. The time to sweep the frequency band is 400 ms. The signal generated by the network analyzer is used as the input to a 45-dB transmitter RF amplifier. The output of the RF power amplifier is propagated by a dipole antenna. The signal from the receiver dipole antenna is passed through an attenuator and a series of amplifiers with a gain of 60 db. The output of the amplifiers is returned to the network analyzer to determine the frequency and time response of the channel. The measured data is then read and stored by the PC controller for further analysis. The choice of the 200-MHz band centered at 1 GHz is for a variety of reasons. The 200-MHz bandwidth in the frequency domain gives an equivalent resolution in the time domain of 5 ns, which is what many of the timedomain systems are capable of producing. We want to cover the 900-MHz band because of allocation of this band for indoor transmission, and the many other reported timedomain measurements are for this band. The components used in the construction of the measurement system have a flat response in the 1-GHz range. Fig. 2 shows a plot of the magnitude and phase of a typical frequency response H (f, x) measured at a location x, and the corresponding magnitude of the time-domain response 1 h (7, x)l obtained from the inverse Fourier transform. The magnitude of the frequency response in decibels, the phase of the frequency response in degrees, and the magnitude of the time response on a linear scale are shown. The received signal level is adjusted by means of the attenuators to maintain an approximately 0-dB re /90/ $01.OO IEEE

2 ~ 152 RECEIVE ANTENNA IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. 39. NO. 5. OCTOBER 1990 TRANSMIT v I 3WBm Max ATTENUATOR 4MB 3WBm personalc7xmner WrmGplB rjtewple n U ki 5 0 j e -100 ceived signal power relative to the transmitted signal power. The attenuator settings and receiver-transmitter separation are recorded during the measurement process. The frequency response consists of 801 complex samples at a frequency spacing of 0.25 MHz for a frequency span of 200 MHz, which is centered at 1 GHz. From this frequency response, a time response of 4000-ns duration is derived. The time response is truncated to show only that portion with significant energy. The frequency selective nature of the channel is seen to result in deep nulls at certain frequencies. The phase is linear for most of the frequency band, except for those frequencies at deep nulls where a phase jump is observed. The time-domain response illustrates the multipath propagation which causes the frequency selectivity DESCRIPTION OF THE MEASUREMENTS In this paper, the frequency response measurements are spatially distributed throughout the test area, such as the floor of a building, by fixing the receiver in a central location and moving the transmitter to different locations. The locations are selected based on the existence or likely existence of communication equipment for wireless local area networks. As shown in [6], the maximum rate of variations in the channel characteristics is below 10 Hz (time variations of the order of 100 ms) which are caused by movements close to the transmitter or the receiver; the acquisition time in our measurement system is 400 ms. To avoid the variations of the channel during the acquisition time, the surrounding environment is kept stationary by preventing movements close to the transmitter and the receiver. The objective of experiments is to determine the effect of location and path obstructions on radio-wave propagation. The results from three experiments are reported in this paper Frequency (YHx) (b) (C) Fig. 2. An example of a measurement made with the network analyzer. (a) The magnitude of the frequency response in decibels. (b) The phase of the frequency response in degrees. (c) The magnitude of the inverse Fourier transform of the frequency response. The measurements were obtained from 128 different locations in two buildings. The area covered in each building is on the order of a picocell (less than 50-m radius) where picocell is the smallest cell size considered for the future digital cellular portable radio systems. The first set of measurements were obtained from an office, located at the 16th floor of a 32-story building in downtown Worcester, MA. The office consists of a central open area surrounded by small offices. A total of 70 frequency responses were collected. The receiver was placed in a central location and the transmitter was moved to different locations for each frequency response measurement. The measurements are divided into two groups, 46 mobile (GI) and 24 fixed (G2). Fixed measurements were taken from locations where data transmission devices such as a computer terminal existed. Mobile measurements were

3 ~ HOWARD AND PAHLAVAN: INDOOR RADIO CHANNEL IN FREQUENCY DOMAIN 153 taken from locations where data transmission by portable phone would occur, such as the middle of a room or areas close to desks. The third group of measurements (G3) was performed on the second floor of the three-story Atwater Kent Laboratories at the Worcester Polytechnic Institute. The receiver was placed in the central computer terminal room. Measurements from 58 locations close to computer terminals in the same room, the adjacent laboratories, the power systems laboratory across a hallway, and offices across another hallway were taken. IV. MEASUREMENT ANALYSIS The main objective of the indoor radio wave propagation measurements is to determine the radio coverage and data rate limitations in various buildings [ 1]-[5], [71-[8]. The radio coverage is related to the distance-power relationship in the area, and the data rate is limited by the frequency selective fading multipath characteristics of the channel. In the time-domain measurements, the data rate limitations are studied by examining the statistics of the rms delay spread [ 11-[5]. In this section, the statistics of the 3-dB width of the autocorrelation function of the frequency response are derived and an empirical relation between the 3-dB width of the frequency correlation function and the rms delay spread of the channel is developed. A. Received Power versus Distance For a fixed transmitter power (P,), the received power (P,) decreases with distance (d), as P,(d) = Ad-" (1) where a is the exponent of the power-distance relationship and A is a constant set by the transmitted power and the measurement system gain. When the logarithm of (1) is taken, the linear relationship 10 log,o[pr(d)] = 10 loglop] - loa log,o[d] (2) between power in decibels and 10 logio of the distance results. For free space, a = 2; values of a obtained from time-domain measurements are given in [ 11, [2]. The values of a reported for office environments are normally between 2 and 3, but lower and higher values have been reported for other indoor environments. In this paper, the average power for each measurement is calculated by averaging the power over all sample points of the measured frequency response. Using linear regression analysis [9], the minimum mean square error (MMSE) line is calculated for the dependence of average power (db) on 10 loglo of the distance for each global experiment. The slope of the regression line gives the experimental value of - a. Fig. 3 shows a scatter plot of received power (db) versus distance on a log scale for G 1 and the MMSE line fitted to the data. The linear regression analysis gives a = 2.599, and the standard deviation of the average received powers from the regression estimates is d = Fig. 3. A scatter piot of the power (db) versus the distance on a log scale for experiment GI. Also shown is the line with the minimum mean square error fit to the data. TABLE I VALUES OF 01, d (db), r, AND A (db) DERIVED FROM LINEAR REGRESSION OF POWER (db) ON 10 log,, OF DISTANCE FOR EACH OF THE THREE GLOBAL EXPERIMENTS EXPERIMENT Q d(db) G G G r A(&) db. The correlation coefficient, r = -0.93, indicates that decreasing received power is highly correlated with increasing distance. Table 1 gives the value of a, d (db), r, and A (db) determined for the three global experiments. The a values are all very close to 2.5 with a high degree of correlation between the average received power and the distance. B. Correlation in Frequency Domain The complex autocorrelation function of the frequency response W R(Af,.) = j H(f,.) H*(f + Af,.) df (3) -w is computed for all frequency responses. The 3-dB width of 1 R(Af, x)l is a measure of the similarity or coherence of the channel in the frequency domain, which is inversely proportional to the delay spread of the channel. Fig. 4 shows the cumulative distribution function (CDF) of the 3-dB widths B,. for each of the three global experiments. The CDF shows the sample probability that B,. is greater than the value given on the abscissa. The results for the high rise building, G 2 and G 1, are very close. The experiment at WPI (G3) has slightly smaller 3-dB widths. Although the external structure of the buildings are immensely different and the floor plans are also quite different, the CDF's and their medians are very similar. Table I1 gives the statistics of each experiment.

4 154 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL 39. NO. 5. OCTOBER dB Width (Wiz) Fig. 4. The cumulative distribution function of the 3-dB widths for each of the three global experiments. TABLE 11 STATISTICS OF 3-dB WIDTH AND RMS DELAY SPREAD FOR EACH EXl'ERIMENT I GI G2 G3 EXPERIMENT Fig. 5. A scatter plot of the 3-dB widths versus the rms Delay spreads for each global experiment in a log-log scale. Also shown are the lines with NO. 3dB 3dB 3dB RMS RMS the MMSE fit to the data. WIDTH WIDTH WIDTH DELAY DELAY 1 MEAN 1 S.D. I SPREAD 1 SPREAD LIRE- MENIS (MHz) (MHz) (MHz) '.D. (ns) TABLE I VALUES OF 0. d (LOG). r, AND c DERIVED FROM LINEAREGRESSION OF THE LOG 3-dB WIDTH ON THE LOG RMS DELAY SPREAD zl 1 MEDIAN C. Relation Between 3-dB Width and RMS Delay Spread For the time responses derived from frequency-domain measurements, the rms delay spread rrma is computed by where nm m j -m jm.)i2.)i2 Tlh(r, dr - r= (5) -m (h(r, dr Analysis and simulation shows that the maximum data rate without diversity or equalization is few percent of ; ; 7 [7], [8]. Using the inverse relationship between width in the frequency domain and duration in the time domain, a relationship of the form B, = Cr;: between the 3-dB width (MHz) of the frequency correlation function and the rms delay spread (ns) of the channel in time domain is determined from a linear regression of the logarithms. Fig. 5 shows the scatter plot of 3-dB width of the frequency correlation function versus the corresponding rms delay spread of all three global experiments on a log-log scale and the lines with the MMSE fit to the logarithm of the data. For G1, the empirical relationship is determined as B, = 1974~;:~. Table I11 gives the values of 0, C, the standard deviation of the logarithm of the 3-dB widths from the regression estimates, and the correlation coeffi Delay Spread (ns) EXPERIMENT I B I d(log) I I 1 C G1 I I I G2 I I I 4.89 I G3 I I I I cients for each experiments. For the experiments, the 0's are around 1.9, which is similar to results for the outdoor mobile radio channels [ 101. The correlation coefficients for these experiments are more than 89 %. V. SUMMARY AND CONCLUSIONS Indoor radio propagation measurements in the GHz frequency band using a network analyzer were reported. The objective of the experiments were to determine the effect of transmitter location on the received signal in a central station. These experiments were performed in an office in the 16th floor of a 32-story building, and the 2nd floor of the three-story Atwater Kent laboratory building at the Worcester Polytechnic Institute, Worcester, MA. The measurements in the high rise building were further broken down to distinguish locations where fixed equipment such as PC's might reside, and mobile locations where portable equipment such as cordless phones might be used. From the frequency-domain data base, the exponent of the power-distance relationship was found to be around 2.5 for the all three global experiments. The 3- db width of the frequency correlation function was always less than SO MHz. The empirical relationship between the 3-dB width of the frequency correlation function was shown to be exponentially related to the inverse of the rms delay spread with the exponent ranging from 1.8 to 2.0. Within a picocell in the buildings with vast difference in exterior, the characteristics of the channel did not vary significantly when the interiors were similar.

5 HOWARD AND PAHLAVAN: INDOOR RADIO CHANNEL IN FREQUENCY DOMAIN 755 ACKNOWLEDGMENT The authors would like to thank R. Ganesh for his help during the measurements. REFERENCES [l] K. Pahlavan, R. Ganesh. and T. Hotaling, Multipath propagation measurements on manufacturing floors at 910 MHz, Electron. Lett., vol. 3, pp , Feb [2] A. M. Saleh and R. A. Valenzuela, A statistical model for indoor multipath propagation, IEEE J. Select. Areas Cotnmun.. vol. SAC- 5, pp , Feb [3] D. M. J. Devasirvatham, Time delay spread measurements of wideband radio systems within a building, Electron. Lett., pp Nov [4] A. A. M. Saleh, A. J. Rustako, Jr., and R. S. Roman, Distributed antennas for indoor radio communications. I Trans. Comrnunications. vol. COM-35, pp Dec R. Ganesh and K. Pahlavan. On the modeling of fading multipath indoor radio channels. in Proc. I GLOBECOM 89. Dallas. TX, NOV. 30, 1989, pp [6] S. J. Howard and K. Pahlavan, Doppler spread measurements of the indoor radio channels, Electron. Lett., vol. 26, pp Jan [7] T. A. Sexton and K. Pahlavan, Channel modeling and adaptive equalization of indoor radio channels. leee J. Selecr. Areas Commun., vol. SAC-5, pp , Feb [8] S. J. Howard and K. Pahlavan. Performance of a DFE modem evaluated from measured indoor radio multipath profiles, in Proc. ICC 90. Atlanta, GA. Apr , G. W. Snedecor and W. G. Cochran. Statisrical Methods. Ames, IA: Iowa State University Press, [IO] A. S. Bajwa and J. D. Parsons, Large area characterisation of urban UHF multipath propagation and its relevance to the performance bounds of mobile radio systems, Inst. Elerr. Eng. Proc., vol pt. F. no. 2, pp Apr