# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 4 Aeronautical Mobile Telemetry
### Channel Coding in the 20 th Century
The value of \(E_{b}/N_{0}\) was set by a variable attenuator (the thermal noise source was the noise generated by the receiver). The bit error rate performance, measured by the bit error rate tester (BERT) operating on a local copy of the PN sequence, was quantified experimentally for both soft decision decoding and hard decision decoding and for different IF filter bandwidths. Soft decision decoding showed little to no improvement over hard decision decoding. This was explained by the fact that the limiter/discriminator output is not conditionally normal, at low signal-to-noise ratios, and displays well-known \"pop noise\" that the soft decision metrics are not equipped to handle. The decoded bit error rate improved as the IF bandwidth increased. This is explained by the fact that inter-symbol interference--something the decoder is not designed for--diminishes as IF bandwidth increases. Coding gain was limited to 2.2 dB. Had a pre-modulation filter been used, the inter-symbol interference would have been more severe thus reducing the coding gain even further. In the end, it was not felt that coded PCM/FM, in the form examined in [129], was a practical solution for improving link reliability. Coded PCM/FM systems with good performance were still 40 years in the future. In the late 1980s, to address the undesirable bandwidth expansion accompanying channel codes, [PERSON], [PERSON], and [PERSON] [130; 131; 133; 134] investigated the application of trellis-coded modulation (TCM) [156; 157; 158; 159] to aeronautical mobile telemetry. Trellis-coded modulation is a form of coded modulation well suited to bandwidth-constrained applications. For a fixed information bit rate, the signal set is expanded to account for the code rate; the sequence of symbols in the expanded signal set is restricted by the code to maximize the Euclidean distance between allowed sequences. The encoder/modulator is illustrated in Figure 20. Two versions were studied: coded modulation for 8-PSK shown in Figure 20a and coded modulation for 16-PSK shown in Figure 20b. Both modulations were based on the same rate-1/2 4-state trellis code shown in the figures. The signal set was restricted to PSK to create a modulated carrier with a constant envelope--required because the RF power amplifiers operated in full saturation. The corresponding detector comprised the Viterbi algorithm operating on the matched filter outputs with the 4-state trellis. Quantized versions of the matched filter outputs were studied [130; 133; 134; 160; 161]. The simulation results with quantization showed that the 8-PSK system achieved a 1.5 dB improvement over the QPSK and that the 16-PSK system achieved a 0.5-1.0 dB improvement over QPSK. The limited improvements were due to quantization, the use of a 4-state code, and the PSK constraint. The TCM system was originally developed under a NASA grant for the space station flight telerobotic servicer [131]. It was certainly considered by the aeronautical mobile telemetry community but was probably not given serious consideration due to limited performance gains relative to the increase in complexity. To the author's knowledge, this system was never flown on a test range. The first \"close call\" using channel coding was the Bell Boeing V-22 Osprey [162] tests conducted at the Naval Air Station (NAS), the Patuxent River, MD, USA, in the late 1990s. The fundamentals of PCM telemetry are required to understand the rationale behind the coding strategy. PCM telemetry transmits _framed_ data continuously. _Frame_ refers to the periodic insertion of markers, such as major frame and minor frame markers, to divide the data into frames even though transmission is continuous. A very simple PCM frame structure is illustrated in Figure 21. One round through the commutator connected to \(N-1\) sensors produces \(N-1\) data words. [See Figure 2a] Each data word is 4-32 bits in length. The \(N-1\) data words define a minor frame. The start of a minor frame is marked by a minor frame synchronization word, 16-33 bits in length.