# A History of Channel Coding in Aeronautical Mobile Telemetry and Deep-Space Telemetry
## 2 Early History and Definitions
Even so, the \"digital\" words could be multiplexed with the A/D outputs. To help address spectral congestion on busy test ranges, the bandwidth of the PCM/FM signal was reduced by inserting a low pass filter between the PCM data stream and the FM modulator. The low pass filter was called a pre-modulation filter. A block diagram of the corresponding ground station is shown in Figure 2b. Here, the received signal is demodulated using a limiter/discriminator. The output of the limiter/discriminator is a noisy version of the low-pass filtered PCM signal. The limiter/discriminator output was viewed as a binary baseband PAM signal and applied to a binary baseband PAM detector comprising a detection filter (usually an integrate-and-dump operation) and a timing PLL to recover the PCM bits [3]. The words representing the quantized values of the sensor outputs were recovered using a de-commutator. In deep-space telemetry, PCM/PSK/PM was quickly adopted as the preferred multiplexing/transmission method in the 1960s. This is because the performance advantage of coherent detection over non-coherent detection was required to close the space-to-Earth link. An illustration of a typical configuration for space telemetry is shown in Figure 3a. The earliest space probes using PCM typically had two or more telemetry streams in operation. A common configuration dedicated one data stream to \"engineering telemetry data\" and another to \"scientific telemetry data\". The data rate for the engineering telemetry data was usually much lower than that of the scientific telemetry data. The data stream was itself composed of time-multiplexed data words. The telemetry data streams were frequency multiplexed by applying each one to a different subcarrier as shown. The subcarrier modulation was PSK. The PCM/PSK subcarriers were combined and used to phase-modulate  
Figure 1: An illustration of pulse code modulation (PCM): (**a**) the continuous-time sensor output and the corresponding 4-bit PCM representation using the NRZ pulse; (**b**) the two most common pulses used to represent bits: the non-return-to-zero (NRZ) pulse and the bi-phase (bi-\(\phi\)) pulse. (PM) an RF carrier. The PM modulation index was usually less than \(\pi/2\) radians to produce a residual carrier component at the PM demodulator output [6]. The residual carrier component was used by the phase synchronizer to achieve lock when the signal-to-noise ratio was very low [7]. A block diagram of the corresponding ground station is shown in Figure 3b. The first step was to apply a coherent PM demodulator to the received signal. A coherent phase reference was extracted from the received signal as explained in the previous paragraph. Coherent PSK demodulators--comprising a matched filter (usually an integrate-and-dump  
Figure 3: A block diagram illustrating PCM/PSK/PM used in aerospace telemetry: (**a**) the space based multiplexer/transmitter; (**b**) the ground-based demodulator/demultiplexer. Figure 2: A block diagram illustrating PCM/FM used in aeronautical mobile telemetry: (**a**) the airborne multiplexer/transmitter; (**b**) the ground-based demodulator/demultiplexer. operation), a carrier phase PLL, and a timing PLL--were applied to each subcarrier signal. The output bits were then de-multiplexed to access the data words for each sensor. As a truly \"digital\" multiplexing technique, PCM was extremely flexible; it accommodated a wide variety of multiplexing requirements with relative ease. In addition, PCM offered the \"digital advantage\" of noise immunity. For example, in 1965 Stampfl, in reference to the Nimbus I weather satellite, observed [8]  
_[O]nly an ultimate data accuracy of 4-5% on the 5-channel IR data could be achieved primarily due to tape recorder properties. Conversion to binary form prior to recording offers the only convenient means for accuracy improvement._  
Each additional bit added to the binary code representing the sample amplitude increases the signal-to-noise ratio of the sample by 6 dB [9].