Comtech EF Data CDM-760, Installation And Operation Manual

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Appendix L Revision 4
CDM-760 Advanced High-Speed Trunking Modem MN-CDM760
L–5
The predistorter alters the uplink signal particularly in terms of the signal’s magnitude
distribution. Due to this, the new ‘best’ inverse system identified on the next iteration will differ
slightly. After several iterations (typically 5-10), the complex coefficients converge for the
current conditions. The trained coefficients are relayed to the predistorter on a regular interval
of every 0.5 seconds. In addition to providing initial convergence, the dynamic nature of the
predistorter continues the optimization process in the midst of varying link conditions or user-
applied transmit configuration changes. Furthermore, the constantly updating approach
automatically optimizes for physical changes over the life cycle of the satellite transponder.
When adjusting for the best transponder output backoff power, as detailed later in this
appendix, the predistorter adaptively determines the best predistorter for the particular
backoff. Additionally, the best predistorter is identified and stored for the 15 most recent
ModCods employed. This is particularly valuable with ACM, where switching between ModCods
can occur frame by frame. Many of these tradeoffs are challenging or impossible to fully
optimize with a stagnant predistorter.
L.1.3 Crest Factor Reduction (CFR)
The objective of CFR is to preemptively act upon the uplink waveform so that the downlink
received signal has less distortion. Though the objective is similar, how CFR achieves this is
conceptually quite different from predistortion.
CFR reduces the magnitude peaks of the uplink waveform by applying a scaled, opposing
compensation pulse to the signal’s largest magnitude peaks. Since this process includes the
possibility of creating new magnitude peaks when cancelling others, the technique is re-applied
in successive stages. Additionally, the compensation pulses are shaped so the frequency
spectrum of the uplink signal is unchanged from that of the normal square-root raised-cosine
shape.
Figure L-7. Peak Cancellation Block Diagram
Similar to predistortion, this process intentionally introduces distortion to the uplink signal. Also
comparable to predistortion, the level of CFR applied is determined dynamically depending on
the amount of non-linear compression in the satellite link. But, different from predistortion, the
added distortion from CFR is not intended to invert the effects of the transponder. Instead, the
aim is essentially to avoid the most distorting effects of the transponder that would have been
applied to the largest magnitude peaks. Though limited distortion is intentionally applied, on
balance there is less overall less distortion on the downlink signal than if the technique were not
applied. Compounding the benefit, with CFR the predistorter concentrates resources on
reducing distortion over a narrower range of signal magnitudes, further improving performance.