Aeronautical Telemetry Systems
(Under Construction)
Summary
Digital communication systems are ubiquitous in the modern world. These are used to move data between and within computers. For example, digital communication theory facilitates WiFi, GPS, cellular phone calls and texts, UART, JTAG, Bluetooth, USB, and many other systems. Such systems, however, require several subsystems to work in tandem to get information from one device to another.
When a wireless connection is required, radios allow transmitters and receivers to move information without a physical link between devices. They can send and receive massive amounts of information, but both the physical environment and manufacturing imperfections prevent them from reaching their theoretical limits. We develop new methods to improve performance to reach these theoretical limits, specifically for applications in aeronautical telemetry.
Background
Many digital communication schemes rely on the phase of the received waveforms to carry information. For example, in binary phase shift keying (BPSK), there are two options to communicate digital information: a positive cosine represents a binary 1, and a negative cosine represents a binary 0. In other words, the information can be communicated by transmitting with no phase offset (the positive cosine) or a phase offset of 180 degrees (the negative cosine).
The problem with such a system is how the receiver determines whether the received signal has the phase offset of 0 or 180 degrees. Imperfection in manufacturing might cause the receiver's local oscillator to run at a faster or slower frequency than the transmitter's. In this case, the receiver's estimated phase would drift over time. In the presence of a frequency offset, the receiver would make the wrong decision half the time! This would make communication impractical.
Even if we could ensure that the transmitter's and the receiver's local oscillators oscillate at exactly the same frequency, relative motion between the two radios would complicate matters. Every time the radios move either a half wavelength toward or away from each other, a phase offset of 180 degrees would be introduced from the physical distances of the radios. This is called the Doppler Effect. When the relative motion is great, as it is in the case of airplanes, this alone could render and otherwise effective system unusable.
Such complications are avoided through the use of phase error detectors that drive a control system called a phased locked loop (PLL) that can remove the effects of Doppler effects and manufacturing variations.
Our Contribution
We have developed novel methods for phase carrier acquisition that allow for both fast PLL lock times and improved noise bandwidth performance.
We simulate channels for 16-APSK communication systems that include effects like frequency and phase offsets. We then estimate channel characteristics like noise, signal attenuation, frequency offset, and phase offset. We compare various methods to determine which methods are the most reliable. We also do theoretical derivations for optimal channel estimation algorithms and implement those in our channel simulations.
In addition, we have tested our algorithms using real data and have confirmed that these algorithms can, in fact, ensure reliable digital communication systems.