Feature: Digital design
DPD performance DPD performance can be enhanced with more resources. For example, more GMP coefficients help model the PA’s behaviour more accurately. Tus, as bandwidths widen, this becomes one element of a strategy to maintain or improve performance. But, there are limits to this method: a point of little returns is reached where additional resources provide almost no benefits. DPD algorithm developers need to take more creative approaches to eke out further enhancements. ADI’s approach is to augment the base
algorithm generalised memory polynomial with more general basis functions and higher order Volterra products. In the effort to build a model that will accurately predict PA behaviour, data accumulation and manipulation become essential. Developers can draw on the data captured at successive time and power levels when shaping model behaviour; see Figure 5. Note the more extensive data capturing/ observation nodes coupled with digital power monitoring, which helps with dynamics. Prior, stored models can be called on to mitigate dynamic transients. GaN process technology brings with
it many distinct advantages in terms of efficiency, bandwidth and operating frequency, but, it suffers from a long-term memory effect called “charge trapping”. GMP-based DPD corrects some of these
Figure 6: Long-term gain errors introduced by GaN PA charge trapping
errors, but any residual error will impact signal quality. Tis distortion induces a corresponding rise in EVM; see Figure 6. Note the PA gain fluctuations and their
temporal nature, the trap and de-trap states and that de-trapping occurs on the lower- power symbols. Since the temporal effect is long-term,
traditional approaches would suggest the acquisition of a very large number of sample points and, hence, a large amount of data to be stored and processed. Memory costs, silicon-area and processing costs make this approach commercially not feasible. DPD
developers must negate the effects of charge trapping, but in a way that lends itself to efficient implementation and operation. Charge-trap correction is supported at low cost in terms of power and compute time in ADI’s ADRV9029 transceiver. It has been shown to recover the EVM to a level within the 3GPP specifications. Te next-generation transceiver,
ADRV9040, will be a more elaborate solution, delivering better performance in dynamic scenarios and better coverage against an increasing number of GaN PAs with unique charge-trap personalities.
Figure 7: Balancing the elements of DPD performance against the challenges
www.electronicsworld.co.uk December 2021/January 2022 27
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