Joint Research and Case Study Using the Radio Wave Propagation Analysis Software, RapLab

Tokyo Institute of Technology - Takada Laboratory

Tokyo Institute of Technology - Takada Laboratory

Tokyo Institute of Technology, Graduate School of Science and Engineering, Department of International Development Engineering, Professor Jun-ichi Takada

"Kozo Keikaku Engineering's contribution to our lab is limitless. I hope to continuously cooperate with Kozo Keikaku Engineering and spread radio wave propagation research from Japan to the world."

The Takada Laboratory at the Tokyo Institute of Technology is renowned for its research on radio wave propagation and radio channel models. In order to achieve highly accurate propagation simulation, the lab utilizes RapLab, a radio propagation analysis software that uses 3D ray tracing method. As a RapLab user and one who made great contributions developing the software, professor Takada shared us about his most recent research, and his opinion of the software and Kozo Keikaku Engineering Inc.

1. Takada Laboratory, Renowned for its Radio Wave Propagation Research and Radio Channel Model Development

― Please tell us about of International Development Engineering and Takada Laboratory's research topics.

In order to cultivate brilliant engineers, the government during the Meiji period founded the Tokyo Institute of Technology in 1881 as the first technological educational institution in Japan. After its transition to a new-system university in 1949, it became a national university corporation in 2004.

My laboratory belongs to the Department of International Development Engineering which has a multidisciplinary engineering curriculum. It also encompasses the fields of economics, policy, environment, and social sciences. The department was established under the aim to solve global problems that cannot be easily addressed at the national or regional level by integrating a wide range of international development-related knowledge into expertise in existing engineering technological fields.

Takada Laboratory, as a part of the Department of International Development Engineering, conducts research from the perspective of electromagnetic wave engineering in fields such as wave propagation, electromagnetic theory and measurement, radio channel models, radio signal processing, and applications for information technology in international development. Through appropriate technologies, we also conduct international cooperative activities together with local government organizations in developing countries such as Mongolia, Laos, and Cambodia.

Research topics at Takada Laboratory

Research topics at Takada Laboratory

2. Research in Mobile Communications Using Microwaves

― Please explain a little about your recent research topics.

Recently, with the penetration of mobile communications and wireless LANs, there is a demand for higher capacity of wireless broadband communication. But as the demand grows, the limitation of frequency bands is rising as a problem. To respond to this increasing demand, multi-user/multi-site MIMO (Multi Input Multi Output) technology which realizes effective use of high frequency radio waves is gradually put into practice. We are engaged in radio wave propagation research to investigate the possibilities of using MIMO technology in the microwave band, which is currently focused as a new frequency resource for mobile communications.

Mr. Junichi Takada

"With the proliferation
of mobile communications
and wireless LANs, there
is a demand for increased
capacity beyond conventional
broadband communication."

Since high frequency radio waves cannot travel far, it was long thought that microwaves could not be used for mobile communications. Recently, however, the idea of securing transmission of low frequency control signals and base transmission capacity while opportunistically using high frequencies is becoming mainstream by a method known in the mobile communications field as "carrier aggregation".

For example, in a research and development project on the expansion of radio frequency resources headed by the Ministry of Internal Affairs and Communications (MIC), it is attempted to develop high-frequency microwaves to realize ultra-high bit rate mobile communications. Furthermore, in LTE Advanced (4th generation), which was recommended in the IMT Advanced as the evolved version of LTE, a variety of research into the use of microwaves began after the new allotment of 3.5 GHz band at the 2007 World Radiocommunication Conference (WRC-07) of the International Telecommunication Union Radiocommunication Sector (ITU-R).

Another focus is research into MIMO channel measurement. MIMO is a technology which spatially multiplexes and transmits several radio signals with multiple antennas. By evolving further, the number of channels capable of simultaneous parallel transmission has increased. Furthermore, multiuser technologies allow several users to multiplex and share one frequency channel, and the base station cooperation technologies, to link multiple base stations and increase data rate for cell-edge users, are becoming commonplace.

We have conducted analyses of MIMO propagation measurement in "Research and Development of Adaptive Antenna Systems for Next Generation Mobile Device," one of the R&D projects promoting private sectors funded by Telecommunications Advancement Organization of Japan. We were also involved in developing an 11 GHz MIMO channel sounder under the research and development project for expansion of radio spectrum resources, entitled "Research and Development of Elemental Technology for Ultra-High Bit Rate Mobile Communication Systems."

Furthermore, we worked on the standardization of dynamic channel models in wireless BAN (Body Area Network) for connecting in-body, on-body and off-body wireless terminals. We also implemented electromagnetic simulations on human body models.

As a measure to prevent radio frequency resources for radio communications from drying up, we also investigated spectrum sensing technology to detect frequency white space (unused frequency bands) and examined interference protection criteria.

3. RapLab, Researcher-oriented Tool which is not a "black box"

― Please let us know your reason for using this radio wave propagation analysis tool.

Radio wave propagation is a natural phenomenon that cannot be controlled or adjusted artificially, different from devices and circuits. The mainstream of radio communication system used to be a simple point-to-point topology with good line-of-sight, such as fixed satellite radio and terrestrial microwave link. But today, multiple access systems such as cellular phones and wireless LANs, where radio waves propagate through multiple paths in a complex environment and line-of-sight is obstructed or total shadowed.

To design better radio communication systems, it is extremely important to reveal the mechanism of this uncontrollable radio wave propagation and clarify the propagation paths that radio waves travel.

I began my research on radio wave propagation after I was appointed as associate professor at Tokyo Institute of Technology in 1994. In order to further the research of radio wave propagation, a large-scale experimental equipment called channel sounder was needed to transmit and receive radio waves in actual environments. However, at the time, as a newly appointed associate professor, there was no budget to conduct experimental research of radio wave propagation, so I decided to start by first setting up simulations.

That is when my group developed an outdoor propagation simulator using the ray tracing method. We then conducted an experiment using a vector network analyzer which is originally designed for RF circuit measurement, but was unable to predict the real environment from the simulation. We spent a long time trying to figure out why it didn't work well, and how to realize highly accurate radio propagation predictions of real environments. Although it was difficult, we fortunately gained many opportunities to conduct joint research, and continuously made experiments to answer these questions as a core of my research. Then one day, a member from Kozo Keikaku Engineering introduced me to RapLab, a radio wave propagation analysis software that used 3D ray tracing method.

When I first knew about RapLab in 2004, there were also other radio wave propagation simulators being sold in the market. However, I appreciated RapLab for how it discloses its calculation process as compared to other simulators, whose algorithms are like black boxes. As a researcher, it is difficult to use programs whose logic and methods of calculation are not open, but since the intermediate results of RapLab are also viewable, it is easy to pinpoint what is happening in the program.

Furthermore, other simulators were designed for telecom operators who design locations of cellular base stations and wireless LAN access points. Their priority is on high-speed calculation more than accuracy. On the other hand, RapLab is developed especially for researchers, and it has more consideration on calculation accuracy than design efficiency. And yet the software price is extremely reasonable. Therefore I felt I could afford using it continuously for my research.

Screen capture of RapLab, radio wave propagation analysis tool that employs 3D ray tracing method

Screen capture of RapLab, radio wave propagation analysis tool that employs 3D ray tracing method

4. Competitiveness of RapLab; the Imaging Method based Simulator

― Could you briefly explain the ray tracing method?

The ray tracing method is a method that searches for propagation routes by treating radio waves as if being optical rays. By tracking the rays from the transmitter point to the receiver point based on geometrical optics theory, it calculates the propagation loss (reception level), delay time, angle of departure, and angle of arrival.

How to search for a propagation path with the ray tracing method

How to search for a propagation path with the ray tracing method

There are two techniques for the ray tracing method.

1)Imaging Method
This is the method of tracing the reflection, diffraction, and transmission rays from the combination of transmitter point, receiver point, and all reflective surfaces. Through this method it is possible to solve for arriving rays at the receiver point with high accuracy.

2)Ray Launching Method
This is the method of launching out rays at particular angles from the transmitter point and searching for the arriving rays at the receiver point. It is a high-speed method where a capture area is set up surrounding the receiver point, and rays which arrive in the capture area are considered to have arrived at the receiver point.

Comparison of Imaging Method and Ray Launching Method

Comparison of Imaging Method and Ray Launching Method

RapLab uses the imaging method, which is rare in this field. I suppose this is why the software has become an extremely popular simulator among university laboratories and corporate laboratories that conduct radio wave propagation research and development.

5. Utilizing Calculation Results of RapLab in order to Produce Other Output Values

― Please tell us about how RapLab is used, its efficiency, and the cooperative relationship with Kozo Keikaku Engineering.

RapLab's calculation method follows geometric optics, an approximation method of electromagnetic wave theory at higher frequency. It calculates the propagation loss through the basic scattering mechnism of the imaging method: reflection, diffraction, and transmission, and makes it easy to visualize the radio wave propagation routes. Furthermore, since it precisely conducts ray trace calculations for the inputted buildings and terrain data, it is possible to accurately ascertain the arriving waves at the receiver point, making it extremely easy to use as an auxiliary research tool.

Visualization of radio wave propagation by RapLab

Visualization of radio wave propagation by RapLab

For example, simulation results are used as a reference to compare with measured parameters such as angle of arrival and field distribution. The lab is currently using RapLab as a reference for 11GHz indoor measurements. Its output is also used to make a reference graph to be compared with measurement results. The purpose of RapLab in this case is not to obtain the final prediction of propagation mechanism, but rather to identify whether the modeled mechanism are sufficient in this frequency range.

On the other hand, Kozo Keikaku Engineering was also trying to enhance its technology for prediction of radio wave propagation. To find out whether the ray tracing method was truly suitable for radio wave propagation research, I decided to welcome a talented engineer from Kozo Keikaku Engineering to my laboratory as a doctoral student for 3 years. The doctoral student was a person of excellent talent, made enormous contributions to the laboratory, and produced excellent results such as co-authored papers. Kozo Keikaku Engineering has the technological skills to put ideas into shape, and their staff's contribution is limitless.

6. Significance of the University to Continue its Research, Financial Benefit is not always the Highest Priority

― Please let us know about Takada Laboratory's future research.

We used to conducted radio wave propagation simulations with CAD databases, but with the recent development of laser measurement technology, we are currently focusing on a method called "point cloud," which reconstructs a structure from 3D coordinated point group data.

Now we are temporarily post-processing by hand to make surface profiles, and then running simulations with RapLab, but we are conducting joint research with Aalto University in Finland to make a method for directly simulating the propagation using point clouds. It seems to work more effective than I expected, and I look forward to continue its development.

― Finally, what is your expectation for RapLab and Kozo Keikaku Engineering?

The current ray trace method basically simulates flat surfaces in analyzing radio wave propagation. However, even after finely dividing up the scattering objects, there are the cases when simulations do not match with measurement results. If RapLab were also able to conduct simulations based on Physical Optics (PO) for scattering phenomena caused by curved surfaces and small objects that are difficult for the ray tracing to handle, it could be applied to a variety of uses in the world. I hope they will rise to the challenge.

I highly value Kozo Keikaku Engineering for its long contribution as a supportive presence in Japanese radio wave propagation research, through activities such as administrative support for the Japanese representatives to the SG3 (Radiowave Propagation) in the ITU-R (International Telecommunication Union Radiocommunication Sector).

Japan has always led the way in radio wave propagation research, but the number of researchers in modern Japan is gradually declining. It is difficult to enter the field since it requires very expensive large-scale experimental equipment and acquiring licenses for radio communication. In addition, it is hard to gain large profits or patent. However, Takada Laboratory is conducting research with the belief that it is significant to keep working on this research at the university, especially because it is one of the few special places where earning profit is not always the top priority.

I hope to continuously cooperate with Kozo Keikaku Engineering to spread radio wave propagation research from Japan to the world.

Interview date: November 2012
May, 1881 Established
In Meguro-ku, Tokyo (Ookayama Campus)
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Joint Research and Case Study Using the Radio Wave Propagation Analysis Software, RapLab