Rack-Mount RDMS
TM
Telemetry Receiver
75
Quasonix, Inc.
4.1.5.21 Convolutional Decode
The Convolutional Decoder (also commonly referred to as the Viterbi Decoder) is available in PCM/FM and legacy
PSK modes. Its purpose is to decode data that has been encoded per “Consultative Committee for Space Data
Systems, Recommendation for Space Data System Standards, TM Synchronization and Channel Coding, CCSDS
131.0-B-1, Blue Book,” September 2003, Section 3.
Convolutional encoding adds redundant information to the transmitted data stream. The Convolutional Decoder uses
this information to detect and correct bit errors that may occur, particularly due to predominantly Gaussian noise.
The encoded data rate is twice the specified data rate, and the occupied bandwidth is also doubled.
For legacy PSK modes that use quadrature modulation (e.g., OQPSK), there are actually two independent decoders,
one for in-phase (“I”) data and one for quadrature (“Q”) data. This approach assumes a similar arrangement for the
encoders in the transmitter, which is true of all Quasonix transmitters.
The user may toggle the Convolutional Decoding value On or Off by pressing the Enter key on the front panel
keypad until the desired value displays.
Figure 134: Advanced Menu, Convolutional Decode Selection
4.1.5.22 SNR Estimator
The SNR Estimator menu is available for all waveform modes. SNR estimation is used primarily to achieve proper
channel weighting by the optimal ratio pre-detection diversity combiner. There are two algorithms available to
estimate SNR: DSP and AGC. Under normal conditions, with proper AGC zeroing, these two algorithms give
substantially similar results.
The user may toggle the SNR Estimator value to AGC or DSP by pressing the Enter key on the front panel keypad
until the desired value displays.
The DSP-based SNR estimator numerically measures received power relative to noise power at the demodulator
input, assuming the signal power is attributable to constant-envelope modulation and the noise power is attributable
to additive white Gaussian noise. This measurement is very accurate when the SNR is below 30 dB, but it will rarely
report a value above this (no matter how strong the signal). This selection is recommended for ARTM modulation
types (Tier 0, I, and II) or when an accurate AGC zeroing process is not feasible.
The AGC-based SNR estimator measures received power relative to the AGC zero point (i.e., noise floor). This
estimation is insensitive to modulation and does not distinguish between intended signal and interfering signal. This
selection is recommended for unknown signals or for signals with modulation that is not constant-envelope (for
example, unshaped PSK). This method is also recommended when the received signals may be more than 30 dB
above the noise floor. However, an accurate AGC zeroing process is required for accurate results.