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Critters Who Revitalize Forests Through the Spreading of Food Caches

The Japanese squirrel (Sciurus lis) can be found in the southwest portion of Japan's Honshu Island. In the fall, these squirrels gather and hide nuts, hoarding them away as food caches to survive until spring. For a research project, scientists attached tiny tracking devices to walnuts to keep track of their stashes and the numbers of nuts eaten by squirrels, stolen by mice and so forth. The data showed that squirrels hid nuts as far as 168 meters (551 feet) away from the original gathering location, and approximately 7 percent of them germinated before being eaten. This research provided valuable statistics showing how these creatures, who sometimes forget where they hid their nuts, have contributed to reforestation.

Concealment By Spreading Around

Just as secrecy is important to squirrels who hide their stashes to get through the winter, privacy is vital in the world of wireless communications. In the latter case, this is often accomplished using a technique known as spread spectrum, which entails spreading information-containing signals over a wide frequency band rather than transmitting them in one specified band with the same transmission power output.

One common application of spread spectrum technology is CDMA, which is short for code-division multiple access. That method will be the focus of this article.

The Advantages of Blurring the Line Between Signals and Noise

The key to CDMA is code assignment: A spreading code is used for signals on the transmitting end, enabling their transmission over a wide frequency band. This makes them difficult to differentiate from random noise, thus improving privacy of the information contained therein. (Going back to our squirrel example, this is similar to when the little critter hides its nuts under fallen leaves, in tree hollows and other such widely dispersed locations.)

The wireless signals are received by a receiver using the same spreading code and decoded (despread) to separate the information-containing signal from the noise. With our squirrel, this represents memory of where it hid its nuts, as remembering this location reliably will enable it to unearth the food cache later regardless of where it is hidden or how many leaves pile up atop that hiding place. However, the comical truth is that squirrels often just forget where they hid their food.

When utilizing CDMA to receive a signal on one channel, it is not possible to distinguish between noise and the signals on other channels which use the same frequency band. In short, by utilizing different codes, CDMA offers the advantage of enabling numerous channels (users) to share the same frequency band. Going back to squirrels again, this is much like multiple squirrels successfully remembered their respective food hiding places while coexisting in the same forest to survive the winter.

GPS Harnesses CDMA's Advantages to the Fullest

All GPS satellites use the same frequency, yet it is possible to tell them apart without confusion. This is due to the unique spreading code assigned to each satellite—in other words, each positioning signal. In the GPS field, this is referred to as the pseudorandom noise (PRN) code, which is a mathematically generated bit string for purposes of preventing signal interference while making the signal appear as nothing more than random noise. Because the code resembles noise, the encoded signal also looks that way, which enables numerous satellites to share the same frequency band with no inter-satellite interference.

CDMA is also used as a means of maintaining time accuracy in GPS recievers. A PRN code utilized by the receiver for signal decoding is known as a "replica." Decoding is carried out by precisely correlating the received signal with the replica's position (time) to check the difference between the transmitting satellite's timestamp in the decoded signal and the receiver's timestamp. This is multiplied by the speed of light to determine the distance between the satellite and the receiver. It goes without saying that this operation is an important first step in positioning calculations and time synchronization.

To review, the CDMA communication method has the following salient characteristics: (1) resistance to interference caused by noise, (2) enables the coexistence of multiple signals on the same frequency band, and (3) can be used for precise time measurements. Looking back on it now, it almost seems as if CDMA were designed specifically with GPS in mind, which makes one really admire the GPS technology developers who wisely integrated these CDMA advantages into the core workings of massive-scale GPS systems. Moreover, CDMA has proven to be an invaluable asset in contemporary society and the various developments in wireless communication which came thereafter. In fact, GPS was the first large-scale application of CDMA—something I wish more people knew!

Similarities Between a Car's Tires and an Antenna

Let's change the subject and talk about FURUNO's AU-500, a dual-band, multi-GNSS-compatible antenna, as well as its AU-300 single-band counterpart. Both of these products are capable of receiving information-containing signals buried within noise from a GNSS. These new antenna models were released alongside the dual-band-compatible GT-100 GNSS receiver module for time synchronization applications.

A GNSS receiver antenna has a comparable function to the tires which make contact with the road in the system known as an automobile. No matter how powerful a car may be, its performance cannot exceed the performance limits of the tires, meaning the tires' limits define the limits of the system as a whole.
In a GNSS receiver system, the antenna serves to bridge the gap between the analog signals which it receives and the digital signal processing within the system. As follows, only analog information captured by the antenna can be acquired and processed by the receiver system as a whole, so the antenna's ability to capture signals reliably is what defines its performance in practice as a system component.

With the AU-500 and AU-300, the band filter designed specifically for LTE base station transmissions, and the integrated ground plane (a metal plate installed on the back of the element to improve receiving sensitivity) serve to bolster the system's performance. But what else determines the actual performance of the system? That comes down to the unit's core durability—its ability to withstand adverse environmental factors and realize reliable performance over the long term without malfunctions or breakdowns.

Antennae That Survived the Most Tortuous Tests

During the AU-500 and AU-300 development processes, they were exposed to a wide range of sustained tests of the most torturous variety. These included heavy, side-pelting rain tests and inundation to simulate disaster-level rainstorms, saltwater exposure tests to simulate wave strikes, and even boiling tests—although I'm not sure what they were going for with this last one, as it's hard to picture a situation in which antennas would be boiled in saucepan....

There were also tests involving exposure to corrosive gas and other chemical substances, application of ±8,000-volt static electricity, surge tests intended to simulate lightning strikes, ultraviolet irradiation, and also tests involving vibrations, impacts and dropping in anticipation of possible problems experienced during transport and installation.

And don't forget thermal shock testing, wherein the unit underwent a repeated cycle of rapid cooling from 85°C to –40°C (185°F to –40°F) in less than five minutes' time followed by heating up again. This, of course, was conducted to eliminate the possibility of thermal stress issues which might lead to breakage or fracture, since the product combines disparate materials with differing coefficients of thermal expansion. In some cases, numerous thermal stress cycles can lead to the gradual formation of small cracks. With these two antenna models, a microscope was used to check for cracks after each cycle during tests, and no problem was observed even after the completion of 1,000 cycles.

One member involved in testing explained, "Under certain test conditions, some of the test-pieces cracked. While 200 test cycles is typically sufficient, we increased the number of cycles to 1,000. Having passed these rigorous tests, our products are exceptionally durable, allowing for a 'set and forget' approach in real-world applications."

These antennas adopt a mushroom-like form that is designed to minimize snow pileup, as well as an integrated ground plane.

Actual mushrooms that one finds in the forest serve to break down cellulose and help revitalize plant life. This "mushroom" by FURUNO ELECTRIC, on the other hand, serves to cultivate the "forest" of information transmitted daily in the form of data. It plays the pivotal role of providing ultra-precise timing information—something I hope you will keep in mind the next time you spot an antenna in your own town.