Simulation and Analysis of 37 GHz Millimeter Wave Propagation Characteristics in Indoor Office Environment

In this paper, the method of shooting and bounding ray tracing/image (SBR/IM) is used in the simulation of indoor office environment, with which the study on millimeter-wave propagation characteristics can be processed well. Through simulation parameters such as path loss, received power and root mean square (RMS) delay spread is obtained and analyzed. All the results of this paper could be used as a reference when setting the position of transmitter and selecting parameters in indoor office environments.

With the rapid development of economy in recent years, the proportion of global urbanization is increasing, and the main places where people live and work are more and more concentrated in indoor environments [3]. Statistics show that for the time people spend, 80% to 90% is indoors, 70% is on mobile phone use, for data transmission, 80% of them is occurred indoors. Nowadays, indoor short-range wireless interconnection (Wireless Local Area Network, WLAN for short) technology is developing rapidly [4], and people are in urgent need of high-quality indoor communication [5].
The most outstanding feature of millimeter wave is that the bandwidth it can provide reach up to hundreds of megabytes. However, due to the serious attenuation cause by obstacles, the apply of millimeter wave may be limited in indoor environment. Therefore, it is necessary to analyze millimeter-wave channel propagation characteristics in indoor radio environment [6][7] In this paper, millimeter-wave propagation of 37 GHz are simulated in indoor office environment.

SIMULATION ENVIRONMENT
The material for study of indoor office environment is from literature [8]. The office space modeled for measurement and the simulation is showed in Figure 1. The measurement environment is a office which width is much larger than depth (49.8m 16m) and its height is 2.7m. The height of partition and desk is 1.0m and 0.7m respectively. There are two transmitter antennas Tx1 and Tx2, for position of the two transmitters in the room, Tx1 is placed in the center and Tx2 is on the left side, which is marked in Figure 1. Both two transmitters are set at the height of 2.6m. The receiver antenna are set at the height of 1.5m, which is just above the desk and the partition. .

Path Loss
Path loss, or propagation loss, refers to the loss caused by the propagation of radio waves in space. It is caused by the radiation diffusion of transmitting power and the propagation characteristics of channels. It reflects the change of the average received signal power in macro scope. In theory, the path loss is the same for the same transceiver distance. But in practice, it is often found that the receiving power at different receiving points of the same receiving distance varies greatly, and even the receiving power at the same receiving point fluctuates greatly at different time points. The path loss function for indoor scenario is definited by the difference value between transmitter and receiver, which function can be shown as following.
PL (d 0 ) is the path loss determined by measurement. The value of d 0 is 1m in indoor environment commonly. d is the distance between transmitter and receiver, n is the path loss exponent and S is the shadow fading. The following two figure is result obtained from simulation. It shows that the path loss cecome bigger while the distanse become longer, and in the scenario of indoor office, path loss is smaller than that of free space under the same condition. For office the path loss exponent is 1.95 and for free space it is 2.

Received Power
The values of received power of 37.1 GHz through simulation are illustrated as shown in Figure 3. Obviously, the power become smaller while Tx-Rx distance become bigger. The received power from two transmitters are both range from -65 dBm to -45 dBm. With Tx2 as the transmitting antenna, the receiving antenna moves on the nine route. Taking three receiving point(P1 to P3 in Figure 1), each point has twenty five paths of received power. For the point P1 the total received power is -6 10 × 9.32 mW, for the total twenty five receiving paths there is a path with no reflection, four paths with once time reflection, nine paths with twice reflection and eleven paths with three times of reflection. The received power of the path with no reflection is  The distribution of received power for point P2 and P3 are shown in Table II. It can be seen in the indoor office environment that the received power of the path with no reflection is the largest, followed by the path with once reflection, the path with twice reflection and the path with three times of reflection contribute little to the total power.

RMS Delay Spread
RMS delay spread is caused by the different arrival time of different path of signal, which is defined as and the definition of the average arrival time is Where i p and i t is parameter of path i, means received power and arrival time.

CONCLUTION
In this paper, three different parameters are discussed: path loss, receiving power and RMS delay spread. In terms of path loss, it is observed that path loss component is smaller than that of free space with reflections from walls and desks and chairs. Although there is less interference in free space, there is no reflection wave, so the path loss index is higher than that of office environment. This point is also reflected in the received power. It can be seen that although the contribution of reflected wave becomes less and less as the number of reflections increases, it is the negligible reflected wave that makes the path loss of indoor environment lower. For the part of RMS delay spread, it is observed that the closer the receiving point is to the transmitting point, the smaller the delay spread is. This paper provides a material for channel research of indoor office. The shortcoming is that there is a lack of NLOS environment, in addition, the research objectives are somewhat single. More detailed research is needed on the study of 37 Ghz millimeter wave propagation.