Power-indexed look-up table design of digital pre-distortion for power amplifiers with dynamic nonlinearities
This invention is a method of power amplifier digital pre-distortion which measures a current power level of the power amplifier, stores in a look up table entries consisting of a power level and a corresponding set of digital pre-distortion coefficients, selects a set of digital pre-distortion coefficients corresponding to the measured power level. If the measured current power level is near a power level index, the digital pre-distortion coefficients correspond to the power level index. If the measured current power level is greater than the maximum power level entry, the digital pre-distortion coefficients is of the maximum power level entry. If the measured current power level is less than the minimum power level entry, the digital pre-distortion is of the minimum power level entry. If the measured current power level is not near a power level index, the digital pre-distortion coefficients are an interpolation.
1. A method of power amplifier digital pre-distortion comprising the steps of:
measuring a current power level of the power amplifier;
storing in a look up table a set of entries, each entry consisting of a power level and a corresponding set of digital pre-distortion coefficients;
selecting an set of digital pre-distortion coefficients by
if the measured current power level falls within a range associated with a power level index in the look up table, recalling from the look up table the set of digital pre-distortion coefficients corresponding to the power level index,
if the measured current power level is greater than the maximum power level entry in the look up table, recalling from the look up table the set of digital pre-distortion coefficients corresponding to the maximum power level entry in the look up table,
if the measured current power level is less than the minimum power level entry in the look up table, recalling from the look up table the set of digital pre-distortion coefficients corresponding to the minimum power level entry in the look up table, and
if the measured current power level does not fall within a range associated with a power level index in the look up table
recalling a first set of digital pre-distortion coefficients corresponding to a next power level index below the measured power level, recalling a second set of digital pre-distortion coefficients corresponding to a next power level index above the measured power level, and
interpolating a third set of digital
pre-distortion coefficients from the first set of digital pre-distortion coefficients and the second set of digital pre-distortion coefficients;
generating a digital pre-distortion from the selected set of digital pre-distortion coefficients; and
applying the digital pre-distortion to the power amplifier.
2. The method of claim 1 , wherein:
said range comprises P′−ε1 P′+ε2, where: P′ is the nearest power table index to the measured power level and ε1 and ε2 are small empirical constants.
3. The method of claim 1 , wherein:
said step of selecting an set of digital pre-distortion coefficients further includes comparing the measured power level with multiple power levels, each of the multiple power levels corresponding to one index power level of the look up table.
4. The method of claim 1 , wherein:
said step of selecting an set of digital pre-distortion coefficients further includes
determining a power level change between the measured power level and a prior power level, and
selecting a next power level in the look up table if the determined power level change is beyond a preset range.
5. The method of claim 1 , wherein:
said step of interpolating a third set of digital
pre-distortion coefficients includes
w
current
=
w
high
*
f
(
α
)
+
w
low
*
(
1
-
f
(
α
)
)
f
(
α
)
=
1
-
(
1
-
α
)
n
n
=
1
,
2
,
…
5
α
=
10
log
10
(
P
current
/
P
low
)
10
log
10
(
P
high
/
P
low
)
P
current
∈
(
P
low
,
P
high
)
where: w current is a DPD coefficient to be calculated
corresponding to DPD coefficients w high of P high and w low of P low ; P current is the current measured power level; P high is the index power level in the look up table next higher than P current ; P low is the index power level in the look up table next lower than P current ; and P current , P low , P high and α are in the dB domain.
6. The method of claim 1 , further comprising:
measuring distortion of a signal output of the power amplifier;
calculating a digital pre-distortion coefficients to compensate for the measured distortion; and
updating data stored in the look up table based upon the calculated digital pre-distortion coefficients and a corresponding measured power level.
7. The method of claim 6 , wherein:
the look up table has fixed power level indices; and
said step of updating includes replacing the digital pre-distortion coefficients stored at one of the fixed power level indices with the calculate digital pre-distortion coefficients when a measured power level matches the one the fixed power level indices.
8. The method of claim 6 , wherein:
said step of updating includes replacing both power level indices and the corresponding digital pre-distortion coefficients.
9. The method of claim 8 , wherein:
said step of updating includes replacing both power level indices and the corresponding digital pre-distortion coefficients includes
if the measured current power level is near a power level index in the look up table, replacing the look up table the set of digital pre-distortion coefficients corresponding to the power level index with the calculated digital pre-distortion coefficients,
if the look up table does not store a maximum number of entries, storing a new entry at an index power level corresponding to the measured power level having the calculated digital pre-distortion coefficients, and
if the measured current power level is not near a power level index in the look up table and the look up table stores a maximum number of entries
determining the gaps between the measured power level, the next greater power level index stored in the look up table and the next lower power level index stored in the look up table, and
replacing a current entry in the look up table which is the lower power level of the smallest determined gap with a new entry having a power level entry corresponding to the measured power level and the calculated digital pre-distortion coefficients.
10. The method of claim 8 , further comprising:
segmenting the look up table into at least two portions, each portion storing only entries having a power level index within a corresponding range of power; and
wherein said step of updating includes updating only entries of the look up table with the portion of the segmented look up table having a power range including the measured power.
11. The method of claim 10 , wherein:
said step of segmenting the look up table permits entries with a higher power density for higher power.
12. The method of claim 8 , further comprising:
determining when the distortion over time of the power amplifier is unstable; and
said step of updating includes setting an expiration time of an updated entry in the look up table upon determination the distortion over time is unstable and deleting a look up table entry upon elapse of the corresponding expiration time.
13. A method of power amplifier digital pre-distortion comprising the steps of:
measuring a current power level of the power amplifier;
if the measured current power level falls within a range associated with a power level index in a look up table, recalling from the look up table a set of digital pre-distortion coefficients corresponding to the power level index;
if the measured current power level is greater than a maximum power level entry in the look up table, recalling from the look up table a set of digital pre-distortion coefficients corresponding to the maximum power level entry in the look up table;
if the measured current power level is less than a minimum power level entry in the look up table, recalling from the look up table a set of digital pre-distortion coefficients corresponding to the minimum power level entry in the look up table; and
if the measured current power level does not fall within a range associated with a power level index in the look up table:
recalling a first set of digital pre-distortion coefficients corresponding to a next power level index below the measured power level, recalling a second set of digital pre-distortion coefficients corresponding to a next power level index above the measured power level, and
interpolating a third set of digital pre-distortion coefficients from the first set of digital pre-distortion coefficients and the second set of digital pre-distortion coefficients;
generating a digital pre-distortion from the selected set of digital pre-distortion coefficients; and
applying the digital pre-distortion to the power amplifier.
14. The method of claim 13 , said range comprises P′−ε1-P′+ε2, where: P′ is the nearest power table index to the measured power level and ε1 and ε2 are small empirical constants.
15. The method of claim 13 , further including comparing the measured power level with multiple power levels, each of the multiple power levels corresponding to one index power level of the look up table.
16. The method of claim 13 , further comprising:
measuring distortion of a signal output of the power amplifier;
calculating digital pre-distortion coefficients to compensate for the measured distortion; and
updating data stored in the look up table based upon the calculated digital pre-distortion coefficients and a corresponding measured power level.