UG-1828 Preliminary Technical Data
Rev. PrC | Page 214 of 338
in order to suppress the spectral regrowth down to the required ACPR. It is important not to include higher order power terms than
needed, which might cause the DPD unstable.
DPD Model 4 consists of four taps as shown in the example tap arrangement diagram in Figure 199. The four taps can be classified into
three categories:
The main tap – The main tap of the DPD adaptive filter that suppresses most of the spectral regrowth due to intermodulation; hence it
has the greatest number of polynomial terms. It is labelled as TAP
1
.
The side taps – There are two side taps on each side of the main taps. They are memory terms that compensate for frequency-dependent
distortion in the frequency domain, and time misalignment between the transmit and receive captured data. The side taps have the same
number of polynomial terms; and each side tap has about half of the number of polynomial terms of the main tap. They are labelled as
TAP
0
and TAP
2
.
The cross-term tap – The cross term is designed for further suppressing the residual spectral regrowth left over by the other three taps.
The number of polynomial terms is usually equal to or less than that of each side tap. It is labelled as TAP
3
.
Figure 199. Example Polynomial Constellation Configuration for Model 4
Note that in Figure 199, k represents the order. To handle 7th harmonics, the main tap must include the power terms up to k = 7.
To find the best model tap order for a specific PA design model, the user can take the following recommended procedures:
1. Set the amplifier output power to have a compression ratio of 1 dB or slightly less, as shown in Figure 182, i.e. the maximum peak of
the output signal is 1 dB below ideal linearity.
2. Determine the initial highest polynomial order of the main tap, TAP1, by measuring the spectral regrowth bandwidth to carrier
bandwidth ratio. All lower order polynomial terms must be included. For example, if the bandwidth ratio is 5, set the initial highest
polynomial to k = 5,that is, x(t – 1)|x(t – 1)|
4
3. Set the other taps to zeros, that is, turn off the other taps.
4. Use only TAP1 to execute DPD with order 5, then 6, and 7 (i.e. up to two orders above the initial estimate). Measure the ACPR for
each case. Select the one that yields a better ACPR. If the difference is small, select a lower order one, say 5.
5. While keeping the main tap determined above, set the side taps, TAP0 and TAP2, to about half the order of the main tap. In the
above example, the main tap has an order of 5, select the initial side tap order to be 2. Execute DPD with the main tap of order 5, and
the side tap order of 2, then 3, and 4. Select the side tap order that yields the lowest ACPR, say 3.
6. While keeping the main tap and side tap orders determined above, set the initial cross term tap order, TAP3, to be 2. Execute DPD
with the cross-term tap order of 2 and 3. Select none, 2 and 3, which yields the best ACPR. If the difference is small, select the lower
order one, including “none” taps. Some power amplifiers do not need a cross term.
7. Iterate the above procedure as necessary with different combinations until you are confident with the selections. Keep all tap order
selections to be minimal that satisfies your ACPR requirement with a 5 dB margin, which helps to keep DPD more stable. For
example: if your ACPR requirement is −60 dB, set your ACPR target to be −65 dB.
x(
t
–2)x|(t–2)|
k–1
k = 1:3
x(
t
)|x(
t
)|
k–1
k = 1:3
x(
t
–1)|x(
t
–1)|
k–1
k = 1:7
DECREASING
t
IN x(
t
)
DECREASING
t
IN |x(
t
)|
k
INCREASING
t
IN |x(
t
)|
k
INCREASING
t
IN x(
t
)
x(
t
–1)|x(
t
–2)|
k–1
NONE or k = 2:3
7
3
3
3
SIDE TERMS
MAIN TERMS
CROSS TERMS
TAP
1
TAP
3
TAP
2
TAP
0
24159-155
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