# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 2 Early History and Definitions
The deep-space channel is characterized by its power constraint--extremely low power at the receiver input. This is caused by the enormous distances between the transmitter and receiver, often measured in multiples of the astronomical unit (1 AU = \(149.6\times 10^{6}\) km). Consequently, deep-space links operate at low bit rates and must rely on low-rate channel codes to close the link. The corresponding increase in bandwidth due to the use of a low-rate channel code is an accepted cost. In contrast, the aeronautical mobile telemetry channel is characterized by its bandwidth constraint. Because aeronautical mobile telemetry originates near the Earth's surface, interference and channelization constraints are placed on aeronautical mobile telemetry signals in a way they are not for space telemetry. The increasing complexity of test articles increased the bit rate requirement for the telemetry downlink. This is the same trend that overwhelmed the capacity of FM/FM systems in the 1950s mentioned above. By 1969, the primary constraint in telemetry system design was bandwidth [15]. The bandwidth problem was exacerbated by a series of auctions that reassigned spectrum_from_ aeronautical mobile telemetry _to_ commercial services [16]. For a fixed information bit rate, channel coding increases bandwidth. Thus, for a channel code to be viable in aeronautical mobile telemetry, the performance improvement due to the channel code must be seen to be worth the cost of compounding the bandwidth problem. Consequently, a good channel code for aeronautical mobile telemetry is a high-rate code coupled with a bandwidth-efficient modulation. High-rate channel codes are not as powerful as the low-rate channel codes that prevail in deep-space telemetry. The corresponding increase in required bit energy at the receiver is the accepted cost. An additional characteristic of aeronautical mobile telemetry is link loss. The loss comprises a small static component defined by losses of waveguides, cables, connectors, radomes, etc., and a time-varying component due to a variety of factors such as disadvantageous propagation from the test article and multipath interference for low-elevation angle propagation. The time variations of the loss cause link outages that produce error bursts that can be quite long. The hope has been that channel coding would correct the long error bursts. Unfortunately, the extraordinarily long interleavers required to break up the long error bursts introduce an unacceptably long transmission delay. **Encryption. ** When encryption was introduced to aeronautical mobile telemetry, the requirement was for encryption to be the \"last thing\" applied to the data before modulation. The arrangement is shown in Figure 4a. The reason this arrangement was required is that PCM framing and channel encoding both insert markers, in the form of known synchronization patterns, and in the data. The codeword markers are indicated by the black fields in the figure. Encryption worked best when there was no structure (i.e., markers) present in the encrypted data stream. The impact of this arrangement on channel coding was profound. First, because decryption had to be performed before decoding could be performed, the demodulator had to make hard decisions (the decryptor requires binary inputs). This removed the possibility for soft decision decoding. Second, errors in the demodulator output (the decryptor input) tend to cause long bursts of errors in the decryptor output. It is usually the case that the error bursts exceed the error-correcting capability of the hard decision decoding algorithm. Later, the no-structure requirement was relaxed to allow markers to be inserted in the encrypted data stream. This opened the door for the arrangement shown in Figure 4b to be used. Here, encryption precedes encoding, thus placing the encoder as the last thing before modulation. This allowed the decoder to be the first thing after demodulation which is required for soft-decision decoding.