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Using the TB-1 and GT-100 at a "Multipath Dojo" in the Major Metropolis of Osaka

Evening Sunlight Amid the High-Rises

When you're walking between tall buildings during the early morning or sunset hours, a flash of light might catch your eye at times, coming from a surprising direction. This is often due to sunlight reflecting off of a building's exterior into the smaller street or alley where you stand.

Interestingly enough, positioning signals from GNSS satellites reflect off of building surfaces in the same way that light does. As a result, signals that travel in a straight-line course from satellite to antenna may intersect with signals that have reflected off of a surface and taken a different course to that antenna, resulting in intermixing of the two signal types. In other cases, signals may be blocked from reaching their destination entirely by a building, reflected instead in various different directions. It goes without saying that both of these phenomena reduce the Accuracy of positioning and time synchronization operations.

Challenges Unique to Major Urban Centers

Groups of large buildings present unique challenges. Major cities are filled with great numbers of people who create intense demand for signal transmission, and as mobile phone networks evolve from 4G to 5G and 6G infrastructures, increasingly high-frequency signal waves are being used. High-frequency signals offer the advantage of faster communication speeds, but they also come with the disadvantage of being unable to travel long distances. Mobile phone network operators must build dense networks of base stations to counter this problem, which is why built-up areas of big cities see large number of new base station construction projects.
One challenge that arises in this situation is successfully acquiring reliable time-related information.

As a prerequisite, any equipment that transmits radio signals must employ a accurate oscillator to maintain signal frequencies within a prermitted range, and many mobile base stations use oscillators that rely on GNSS as their source in order to obtain stable frequencies and signal timings.

Why Accurate Time Information Is Important

Let's explore this in further detail. In addition to frequency accuracy, time accuracy down to not just the millisecond, but to the microsecond, is required due to the widely established use today of time-division duplexing, or TDD, approach. Outward and return signals the same, shared frequency to divide up time—in other words, they operate like trains traveling in opposing directions on a single track. In order to avoid figurative "collisions" between these trains, which operate at high frequencies, it's necessary to plan out intricate timetables and manage operations carefully. To this end, the key is the precision and accuracy of the clocks.
Returning from our train metaphor to base stations: It's necessary for each base station to maintain synchronized time with other, nearby stations. Just like an off-key vocalist in a choir, a single out-of-sync base station can have adverse effects on nearby stations. As touched upon earlier, mobile phone network operators today face the following tough cycle of challenges, which is even more daunting now than in the past:

1. High communication service demand in big cities
2. The necessity of correct time information for areas with high signal demand
3. The necessity of optimal GNSS signal receipt environmental conditions to achieve item 2
4. The decline of GNSS signal receipt conditions in large cities (despite the requirement in item 3)
5. Return to item 1

Behind the scenes, operators are dedicated to finding a solution to this ongoing challenge.

A Facility to Showcase the Capabilities of DSS

As a company that provides timing products for time synchronization operations, FURUNO ELECTRIC offers GNSS receiver product utilizing Dynamic Satellite Selection™ (DSS) in order to help mobile phone network operators overcome problems with their base stations. DSS makes use of a statistical approach developed by the NTT, which revolves around an algorithm designed to select satellites offering the best signal receipt conditions. I won't go into depth about the strengths of this technology in this article. However, it's important for FURUNO, as the company releasing this new technology to the world, to demonstrate its effectiveness in some way. In short, we must show people just what this impressive technology can do.

Multipath Lab Near the High-rise District of Nakanoshima, Osaka

To this end, FURUNO has selected a major business district in Osaka for the location of its Multipath Lab. "Multipath," or "multipath propagation," refers to the arrival of radio waves at an antenna after being reflected at random, which creates a challenging environment for GNSS receivers. At the Lab, we installed a dual-band GNSS antenna on the balcony, where it is exposed to no rain and hardly any sunlight, and has a view of nothing more than surrounding buildings and the sky. The antenna cable passes through a hole to the interior of the air conditioner duct, and from there into the adjacent room interior where it connects with our performance verification system. Compared with an open-sky situation, wherein nothing obstructs signal arrival, built-up urban areas like this present problems for signal receipt, yet the antenna is able to receive 'poor GNSS signals' with ease, at any time and any conditions. If we were discussing a new car model on a test track, it would be a high-quality rough road indeed. Likening the situation to severe martial arts and spiritual training, one might be compelled to call this facility a "multipath dojo" for the equipment in question, where it undergoes rigorous training to withstand the harsh conditions of signal reception in dense urban environments.

The Excruciating Search for a Suitable Lab Location

FURUNO is not located in the middle of any major city, where real estate prices tend to be high, so we needed to search for a suitable rental property for the Lab. In order to find a room with poor signal receipt conditions, we were on the lookout for a low-rise building sandwiched between numerous high-rises. As someone who lived through Japan's asset price bubble era, it reminded me of the kinds of properties a land shark might have looked for at that time. Chief in Charge of Overseas Sales Tomofumi Sumida recalls his efforts for the antenna project: "I had found a drinking establishment located in a building that seemed just right for the job. I visited a number of times and, after getting to know the staff, asked if I could be introduced to the building owner. They chased me out of there in a heartbeat."
Even if Sumida had been able to talk with the owner or a connected real estate agent, it is highly unlikely they would have understood or been receptive to his explanation of the plan to install a special type of antenna product on the balcony and utilize various types of verification-use electronic equipment inside the apartment.