Multiband digital pre-distorter with reduced dimension requirements
There is provided mechanisms for operating a DPD for an access node. A method comprises receiving a plurality of input signals, each from a respective frequency band, wherein each signal is of memory order M. The method comprises, for each frequency band l=1, . . . , B of the frequency bands, generating a plurality of pre-distorted signals for the frequency band/based on the plurality of input signals coming from all the B frequency bands and using LUTs, according to which there is one LUT structure with LUT-based transforms per each combination of frequency band l=1, . . . , B and memory order m=0, . . . , M, wherein each of the LUT structures comprises combinations of LUTs of at most dimension B−q, where 1≤q≤B. The method comprises, for each frequency band l=1, . . . , B of the frequency bands, combining the plurality of pre-distorted signals for the frequency band to provide a combined pre-distorted signal for the frequency band.
1. A method for operating a digital pre-distorter, (DPD) for an access node, the method comprising:
receiving a plurality of input signals, each from a respective frequency band, wherein each signal is of memory order M;
for each frequency band l=1, . . . , B of the frequency bands:
generating a plurality of pre-distorted signals for the frequency band l based on the plurality of input signals coming from all the B frequency bands and using Look-Up Tables (LUTs) according to which there is one LUT structure with LUT-based transforms per each combination of frequency band l=1, . . . , B and memory order m=0, . . . , M, wherein each of the LUT structures comprises combinations of LUTs of at most dimension B−q, where 1≤q<B; and
combining the plurality of pre-distorted signals for the frequency band to provide a combined pre-distorted signal for the frequency band.
2. The method of claim 1 , wherein address entries of the LUTs are determined using a basis function {tilde over (S)} operating on B−q-dimensional inputs of the plurality of input signals, and wherein there is one LUT for all terms in {tilde over (S)} with same address entry and different LUTs for all terms in {tilde over (S)} with different address entries.
3. The method of claim 1 , wherein the LUT-based transforms have adapted DPD coefficients β.
4. The method of claim 3 , wherein adapted DPD coefficients β are determined through training of the DPD with reference signals.
5. The method of claim 1 , wherein each dimension represents a respective address entry to the LUTs.
6. The method of claim 1 , wherein generating the plurality of pre-distorted signals comprises:
transforming the plurality of input signals coming from all the B frequency bands using the LUT-based transforms, wherein which of the LUT-based transforms to use for transforming the plurality of input signals are determined by using the plurality of input signals as input to the LUTs.
7. The method of claim 1 , wherein the LUT structures for frequency band l and memory order m comprises a first set of LUTs adapted to use as input a one-dimensional input formed by the plurality of input signals from a respective individual frequency band of the B frequency bands.
8. The method of claim 7 , wherein the LUT-based transforms of the LUTs in the first set of LUTs for frequency band l at time index n for transforming input signal x are denoted H l (i) (x,n) and defined as:
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9. The method of claim 1 , wherein the LUT structures for frequency band l and memory order m comprises a second set of LUTs adapted to use as input B−q down to 2-dimensional input formed by combinations of B−q of the frequency bands down to 2 of the frequency bands, where q>B−1.
10. The method of claim 9 , wherein the LUT-based transforms of the LUTs in the second set of LUTs for frequency band B−r, for r={q, q+1, . . . , B−2}, at time index n for transforming input signal x are denoted H l (B−r) (x,n) and defined as:
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11. The method of claim 1 , wherein the LUT structures for frequency band l and memory order m comprises a third set of LUTs adapted to use as input a one-dimensional input formed by weighted linear combinations of the plurality of input signals from all B frequency bands.
12. The method of claim 11 , wherein the LUT-based transforms of the t:th LUT in the third set of LUTs when q>B−1 at time index n for transforming input signal x are denoted H l (wsm,t) (x,n) and defined as:
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13. The method of claim 11 , wherein the LUT-based transforms of the LUTs in the third set of LUTs for frequency band B−r at time index n for transforming input signal x when q=B−1 are denoted H l (B−r,wsm,t B−r ) (x,n) and defined as:
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14. The method of claim 11 , wherein the weighted linear combinations of the plurality of input signals are weighted using weights w b (t) , w b (t B−r ) .
15. The method of claim 14 , wherein
the weights w b (t) are iteratively determined.
16. The method of claim 1 , wherein the LUTs have adapted grid points, the grid points being identified using parameter estimation.
17. The method of claim 1 , wherein only a subset of all available LUTs are used when generating the plurality of pre-distorted signals, and wherein which LUTs to be included in the subset of LUTs is determined by empirical analysis.
18. The method of claim 1 , where q=B−1.
19. A digital pre-distorter, (DPD) for an access node, the DPD comprising processing circuitry, the processing circuitry being configured to cause the DPD to:
receive a plurality of input signals, each from a respective frequency band, wherein each signal is of memory order M;
for each frequency band l=1, . . . , B of the frequency bands:
generate a plurality of pre-distorted signals for the frequency band l based on the plurality of input signals coming from all the B frequency bands and using Look-Up Tables, LUTs, according to which there is one LUT structure with LUT-based transforms per each combination of frequency band l=1, . . . , B and memory order m=0, . . . , M, wherein each of the LUT structures comprises combinations of LUTs of at most dimension B−q, where 1≤q<B; and
combine the plurality of pre-distorted signals for the frequency band to provide a combined pre-distorted signal for the frequency band.
20. A non-transitory computer readable storage medium storing a computer program for operating a digital pre-distorter (DPD) for an access node, the computer program comprising computer code which, when run on processing circuitry of the DPD, causes the DPD to:
receive a plurality of input signals, each from a respective frequency band, wherein each signal is of memory order M;
for each frequency band l=1, . . . , B of the frequency bands:
generate a plurality of pre-distorted signals for the frequency band l based on the plurality of input signals coming from all the B frequency bands and using Look-Up Tables, LUTs, according to which there is one LUT structure with LUT-based transforms per each combination of frequency band l=1, . . . , B and memory order m=0, . . . , M, wherein each of the LUT structures comprises combinations of LUTs of at most dimension B−q, where 1≤q<B; and
combine the plurality of pre-distorted signals for the frequency band to provide a combined pre-distorted signal for the frequency band.