# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 4 Aeronautical Mobile Telemetry
### Channel Coding in the 20 th Century
(Major frames are usually identified by a subframe counter known as a subframe ID in Chapter 4 of IRIG 106 [163]. The subframe ID counts 1 to \(Z\), then starts over at 1; the \"1\" indicates the start of a major frame. When used the subframe ID occupies the position of word 1 in Figure 21). Counting the minor frame synchronization word, each minor frame comprises \(N\) words and is represented by a row in Figure 21. \(Z\) minor frames define a major frame. In this simple example, the \(W\)-th word in each minor frame is from the same sensor; the \(W\)-th column in Figure 21 comprises consecutive samples from the same sensor. The PCM frame structure is more complex when sub-commutation or super-commutation is used. See Chapter 4 of [163]. Figure 19: The experimental configuration for the coded PCM/FM experiment described in [129]: (**a**) the encoder and transmitter; (**b**) the demodulator and decoder. Figure 20: Trellis-coded modulation encoder/modulator studied by [PERSON], [PERSON], and [PERSON] [130; 131; 133; 134]: (**a**) the 4-state trellis code mapping to 8-PSK; (**b**) the 4-state trellis code mapping to 16-PSK. In the V-22 tests, the test article instrumentation included hardware to compute CRC parity bits (eight bits) for error detection on each PCM minor frame. The check bits occupied the position of Word 1 in Figure 21. The ground station was equipped with a bypassable hardware parity check unit called the \"frame thrower\" [164]. Minor frames that did not pass the parity check were discarded and not passed on for recording or display. The assessment of error detection was visual: strip chart plots of measurements were used, where errored minor frames were discarded to show smooth curves. This is because most sensor outputs were oversampled so the elimination of occasional samples did not negatively impact the reconstructed continuous-time measurements [164]. In contrast, the retention of errored frames produced strip chart plots with spikes due to the errors. Despite the advantages of error detection, the project managers did not approve the use of error detection during actual flight tests. The fear was that discarding minor frames with a failed parity check would lead to missing data. The customer preferred to see unusable data due to dropouts instead of clean data [164]. As of this writing, parity check bits on minor frames is an option in the IRIG 106 standards [163]. The standard recognizes two 16-bit options and one 32-bit option. The two 16-bit options are defined by the polynomials  
\[ \begin{split}\text{CRC-16-ANSI:}\quad X^{16}+X^{15}+X^{2}+1\\ \text{CRC-16-CCITT:}\quad X^{16}+X^{12}+X^{5}+1.\end{split}  \tag{14}\]  
The 32-bit option is defined by the polynomial CRC-32 given by  
\[X^{32}+X^{26}+X^{23}+X^{22}+X^{16}+X^{12}+X^{11}+X^{10}+X^{8}+X^{7}+X^{5}+X^{4 }+X^{2}+X+1. \tag{15}\]  
The length-32 CRC is defined in the IEEE 802.3 standard [165]. When used, the 16 or 32 parity check bits are positioned at the end of the minor frame. By the late 1990s, the bandwidth issue became an acute problem. The causes were mentioned in Section 2: increasing test article complexity increased bit rate requirements and federal spectrum auctions reduced the spectrum available for aeronautical mobile telemetry [16]. Combined, these trends reduced the spectrum available to aeronautical mobile telemetry at the same time the demand for spectrum was increasing. At the same time, the increasing complexity of test articles also exacerbated a problem that previously had been more of a secondary problem: data reliability. More complex test articles tend to have a wider operating range. The wider operating range increases the distance over which the telemetry link must be maintained. This, coupled with increased bit rates, reduced the received value of \(E_{b}/N_{0}\). The response was a series of initiatives funded by the advanced-range telemetry (ARTM) program [144; 145]. Of the many ARTM goals, two are relevant to the focus of this article: bandwidth-efficient modulation and channel coding.