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the industry to develop high data rate wireless applications, which is unlikely to be achieved with current wireless tech- nologies. As newly emerging products ...
IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 1, NO. 2, APRIL 2012

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60 GHz PHY Performance Evaluation with 3D Ray Tracing under Human Shadowing Z¨ulk¨uf Genc¸, Wim Van Thillo, Andr´e Bourdoux, and Ertan Onur

Abstract—This work evaluates the system-level performance of 60 GHz channels by simulating a realistic living room scenario with various numbers of humans and three different antenna configurations. In the first step, a comprehensive ray tracing study with a 3D ray tracing tool is performed to determine the channel impulse responses. Feeding our PHY simulator with the collected channel responses, the performance figures are generated over received SNR. It was observed that for a satisfactory BER performance, a directional antenna with the minimum of 10 dB antenna gain should be used at least in the receiver side when there is human shadowing in the environment. Index Terms—60 GHz, BER, communications, directional antennas, home networks, human shadowing, indoor, performance evaluation, PHY, physical layer, wireless.

This paper investigates the 60 GHz channel characteristics in a typical living room scenario under the influence of various human shadowing and evaluates the system performance with several antenna types. In the first place, the channel impulse responses (CIR) are measured using three dimensional (3D) ray tracing methods with three different antenna configurations by varying the number of humans in the environment. The obtained CIRs are feeded to the PHY simulator to determine the bit error rate (BER) performance of the 60 GHz system. The simulation results show how the BER performance of a 60 GHz system in a living room environment is affected by increasing number of humans and different types of antennas. II. C HANNEL I MPULSE R ESPONSE G ENERATION

I. I NTRODUCTION

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HE 60 GHz technology has a great potential to provide wireless communication at multi-gigabit rates in future home networks with its 5 GHz block of globally overlapping bandwidth in the license-free spectrum. The earlier cost barrier in front of the millimeter-wave systems has now been breached with the recent advances in CMOS technology. The possibility of producing low-cost 60 GHz chips attracted the attention of the industry to develop high data rate wireless applications, which is unlikely to be achieved with current wireless technologies. As newly emerging products and various ongoing standardization efforts show [1]–[3], home networks are the primary target of the multi-gigabit wireless applications. One of the critical factors that may influence the widespread adoption of the millimeter-wave technology in home networks will be communication reliability. A 60 GHz system experiences 28 dB higher free-space path loss than a similar WLAN system operating at 2.4 GHz. 60 GHz signals are also heavily attenuated by the human body [4]. These propagation characteristics require the thorough investigation of the effects of the human shadowing on the 60 GHz indoor communications. The temporal effects of the human blocking of 60 GHz paths were analyzed by several former studies [4], [5]. The main focus of those studies was the time-variant characterization and modeling of the shadow fading events induced by humans. However, the impact of human presence on the system-level performance has not been addressed in-depth previously to the best of our knowledge. Manuscript received January 11, 2012. The associate editor coordinating the review of this letter and approving it for publication was A. Bletsas. This work was supported by the IOP GenCom program of the Dutch Ministry of Economic Affairs in the scope of Future Home Network project. The authors are with Delft University of Technology, the Netherlands, and Imec, Belgium (e-mail: [email protected], [email protected], [email protected], [email protected]). Digital Object Identifier 10.1109/WCL.2012.022012.120033

To evaluate the system performance of the 60 GHz living room network scenario, the channel impulse responses were generated with 3D ray tracing simulations [6], which is a reliable prediction method for 60 GHz [7]. The algorithm of the ray tracing simulator is based on the ray launching technique, in which the finite number of rays are launched into all directions in the three-dimensional space and traced until the given path loss (-120 dB) is exceeded. The rays are launched with the angular resolution of 1◦ so that all the possible paths could be swept with approximately 42000 rays from 0◦ to 360◦ in azimuth and from −90◦ to 90◦ in elevation. The path loss, reflections and diffractions are computed for each ray by considering the frequency, incident angle, permittivity, polarization, the thickness and the dielectric properties of the materials. The path loss model includes the free-space path loss calculated with Friis equation and the losses from reflections and diffractions. The diffraction losses are only by humans whereas the reflection losses arise from both humans and the walls. The maximum order of reflections was set to two because the contribution of 60 GHz signals to the received power becomes negligible after more than two reflections [8]. The empty indoor environment was designed in the standard living room dimensions with the length, width and height of 6 m, 6 m and 3 m, respectively. The humans in the scenario were modeled as rectangular prisms with uniform randomly chosen heights between 160 cm and 190 cm, lengths and widths between 40 cm and 60 cm. To achieve realistic propagation conditions, the dielectric properties (thickness, real and imaginary part of the permittivity) of the common building materials and the human were used as listed in Table I [6]. To perform the measurements, 25 transceiver positions were defined at the height of 1 m in a grid layout by considering the possible locations of wireless user devices as seen in Fig. 1. Additional to those 25 positions, an access point (AP) position

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IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 1, NO. 2, APRIL 2012

TABLE I D IELECTRIC P ROPERTIES IN 60 GHZ Material Thickness[m] Real Name Human Body 0.02 4 Human Concrete 0.2 6.14 Outer Wall Plasterboard 0.1 2.81 Inner Wall Concrete 0.3 6.14 Ceiling Wood 0.02 1.57 Ground Soda-Borosilicate Glass 0.02 4.58 Window Wood 0.04 1.57 Door

Imaginary -0.2 -0.3015 -0.0461 -0.3015 -0.0964 -0.0458 -0.0964

ray with the maximum power [8]. In this configuration, the space-time realization of the propagation channel is assumed to be known. Even if this assumption is not practical, it can be useful to provide an upper-bound for the antenna performance. III. R ESULTS AND D ISCUSSIONS A. Power Delay Profile

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Fig. 1. The 2D view of the living room simulation scenario in which the circles represent the possible locations for the transmitter and the receiver with the (x,y) coordinates below.

was defined in the middle of the ceiling at the height of 3 m. The total 26 positions enabled the simulation of 325 bidirectional channels for a single simulation study by placing the transmitter and the receiver in every combination of 26 positions. Simulation runs were repeated 50 times by repositioning the static humans randomly in each run. Since replacement of humans in the simulations may change the channel characteristics, each channel response to particular human replacement was recorded separately in the CIR matrices. This approach leaded to creation of CIR matrices comprising 50 × 325 channel columns as seen in (1) in each simulation study with a particular number of humans. ⎞ ⎛ Channel .. .. Channel CIR M atrix = ⎝

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(1)

Three types of antenna configurations with linear vertical polarization were studied in accordance with the TGad evaluation methodology of IEEE P802.11 Wireless LANs [9]: • Omni-directional configuration, both transmitter Tx and receiver Rx antennas are omni-directional with ideal isotropic radiation pattern, which assumes a spherical antenna that transmits and combines all signal rays equally from every direction. • Directional configuration, the transmitter antenna is omni-directional whereas the receiver has a directional antenna with phased antenna array which has one steerable main lobe into the particular direction [8]. The antenna is assumed to receive the signals only in the direction of the main lobe. The elevation (θ) and azimuth (φ) angles of the main lobe were set to 60◦ and 30◦ , respectively. • Maximum Ray Beamforming configuration, both transmitter and receiver antennas are directional and steering directions of antennas are adjusted along the channel

In the extraction of the multipath power delay characteristics, a single channel was considered to imitate the device-todevice (DD) communication case. To realize DD channel, the transmitter antenna was mounted at (3,5) and the receiver was placed at (3,1) as shown in Fig. 1. The multipath components recorded through 50 simulations of the channel with various human replacements were plotted in Fig. 2. Although the increasing number of humans did not increase the number of multipath components, in many cases it even reduced them, the channel was observed to encounter more diverse multipath signals with larger inter-arrival times. The use of directional antennas substantially reduced the number and diversity of the multipath components as indicated by the results. To evaluate the time dispersion introduced by multipath channels, the rms (root mean square) delay spread was calculated from the ray tracing simulations of 16250 channels for omni-directional and directional antenna configurations in the empty, 3-human and 6-human living room scenarios separately as seen in Fig. 3. The threshold of -30 dB below the strongest path was used to consider the paths for calculations. The maximum rms delays were recorded during the blockage of the LOS path and strong reflections, which occurs more frequently with increasing number of humans in the environment. Especially, when a diffraction by human body takes place in the close proximity of the receiver, the adverse impact on the delay spread becomes more noticeable. In this case, the diffracted rays have a larger intensity because of the limited dispersion. On the other hand, when the LOS path is present, the increasing number of humans restricts the multipath reception by blocking the reflections with their bodies. The use of directional antennas decreases the delay spread by approximately 50% as seen in Fig. 3. They also minimize the influence of the human shadowing on the delay characteristics. B. PHY Layer Performance All PHY processing was carried out in compliance with the IEEE 802.15.3c standard for 60 GHz communications [10]. The required PHY blocks were modeled in our PHY simulator implemented in MATLAB as seen in Fig. 4. An uncoded QPSK modulation scheme was simulated at 1.760 Gsymbol/sec. In the transmitter side, the standard-compliant preamble bits are first scrambled to whiten the data. Then they are spread and mapped onto complex constellation symbols. The payload bits are also scrambled, spread and mapped onto complex constellation symbols. Then, payload and preamble are combined in a frame, which is digitally filtered and sent to the analog Tx front end.

GENC ¸ et al.: 60 GHZ PHY PERFORMANCE EVALUATION WITH 3D RAY TRACING UNDER HUMAN SHADOWING

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The wireless channel and analog front ends were simulated with 3D ray tracing as explained in Section II. The channel responses, which include received power, time of arrival, angles of arrival and phase information of the signal, were recorded for every human scenario and antenna configuration separately, that resulted in 9x16250 channel realizations. On every channel, 50 frame transmissions were simulated with random additive white Gaussian noise (AWGN). The received frame was sampled at twice the symbol rate by the analog-to-digital convertor (ADC). After sampling, the received frame is filtered with a square root raised cosine (SRRC) filter and sampling clock offset (SCO) is compensated using the Gardner algorithm. Then, the signal is downsampled by a factor two. The timing of the preamble of the PHY frame is recovered using a cross-correlation of the received frame with the known Golay sequences in the PHY preamble. The preamble is then separated from the payload data. Using the preamble, carrier frequency offset (CFO) and IQ imbalance are jointly estimated using an EM algorithm. Next the channel impulse response is estimated along with the noise power using the properties of complementary Golay sequences [10]. Finally the minimum mean square error equalizer (MMSE) coefficients are calculated based on this channel estimate. The payload data of the frame is then translated into the frequency domain using an FFT, equalized, and then converted back to the time domain with an IFFT. CFO and IQ imbalance are compensated, the received constellation is derotated, despreaded and demapped. Finally, the received bits are descrambled.

By comparing the received bits to the transmitted bits, the BER performance figures have been generated over Eb /N0 as seen in Fig. 5. The figures represent the average uncoded BER performance of all the channels with the particular antenna configuration and human number. As the results indicate, the presence of humans in the environment increases the required signal level for a satisfactory BER (10−4 ) performance when only omni-directional antennas are used in the system. The deployment of directional antennas not only improves the required signal levels but it also minimizes the adverse effect of increasing human shadowing on the general system performance. C. Link Budget Analysis The link budget analysis of the 60 GHz channels in the living room scenario was performed by considering the actual received power and the antenna gains. For this analysis, the SNR levels obtained in the ray tracing simulations were mapped to BER values using the performance figures in Fig. 5 and averaged. This analysis takes into account the received power (Pr ), receiver and transmitter antenna gains (Gr , Gt ), 10 dBm transmitted power (Pt ), 4 dB noise figure (F ), 4 dB interconnect loss (I) and −81.52 dBm thermal noise (Pn ) which is Pn = −174(dBm/Hz) + 10 log10 B.

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IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 1, NO. 2, APRIL 2012

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As can be seen in Fig. 6, a 60 GHz system using only omnidirectional antennas cannot provide a satisfactory home networking performance in the presence of humans. To achieve 10−4 BER, a directional antenna with the minimum of 10 dB antenna gain should be used at least in the receiver side. In the denser environments which are open to more severe shadowing, the directional antennas should be used at both sides of the channel to minimize the required antenna gain. IV. C ONCLUSIONS In this study, the physical layer performance of the 60 GHz channels was investigated in a living room scenario under various levels of human shadowing. Using the model environment specified with realistic geometrical layout and the dielectric properties of the surface materials, channel responses were obtained through comprehensive 3D ray tracing simulations. These channel responses were used by the PHY simulator in the generation of performance figures which illustrate the impact of human shadowing and antenna configurations. The results showed that the obstruction of the LOS and strong reflection paths makes it eminently challenging to maintain the link connectivity with omni-directional antennas in the presence of humans. It was observed that to achieve the satisfactory BER performance for home networking applications, a directional antenna with the minimum of 10 dB antenna gain should be used at least in the receiver side. In the

denser environments, the directional antennas should be used at both sides of the channel. These finding can be useful in the development of future 60 GHz communication systems targeting the home networks. R EFERENCES [1] “IEEE 802.15.3c WPAN Millimeter Wave Alternative PHY Task Group.” Available: www.ieee802.org/15/pub/TG3c.html [2] “WirelessHD Specification Version 1.0a,” Aug. 2009. [3] ECMA-387, “High Rate 60GHz PHY, MAC and HDMI PAL Standard, 2nd Edition,” ECMA International, Dec. 2010. [4] S. Collonge, G. Zaharia, and G. E. Zein, “Influence of the human activity on wide-band characteristics of the 60 GHz indoor radio channel,” IEEE Trans. Wireless Commun., vol. 3, no. 6, pp. 2396–2406, Nov. 2004. [5] M. Jacob, C. Mbianke, and T. Kurner, “A dynamic 60 GHz radio channel model for system level simulations with MAC protocols for IEEE 802.11ad,” in Proc. 2010 IEEE International Symposium on Consumer Electronics, pp. 1–5. [6] J. Deissner, J. Hubner, D. Hunold, and J. Voigt, RPS Radiowave Propagation Simulator User Manual-Version 5.4. Actix GmbH, 2008. [7] H. Xu, V. Kukshya, and T. Rappaport, “Spatial and temporal characteristics of 60-GHz indoor channels,” IEEE J. Sel. Areas Commun., vol. 20, no. 3, pp. 620–630, Apr. 2002. [8] A. Maltsev, “Channel models for 60 GHz WLAN systems,” IEEE documents, 802.11-09/0334r8, Mar. 2010. [9] E. Perahia, “TGad evaluation methodology,” IEEE documents, 802.1109/0296r16, Jan. 2010. [10] “IEEE Standard for Information technology - Part 15.3, Amendment 2: Millimeter-wave-based Alternative Physical Layer Extension,” IEEE Std 802.15.3c-2009, pp. c1–187, Dec. 2009.