Method for polar decoding with dynamic successive cancellation list size and polar decoder

1. A method performed by a polar decoder for polar decoding the method comprising:

receiving, from a demodulator, a demodulated radio signal; and

decoding the demodulated radio signal into a number, N, of bits with Successive Cancellation List by:

determining, by a processor of the polar decoder, at the i-th level of a binary tree for decoding the i-th bit of the N bits, where 1≤i≤N, and when the i-th bit is an information bit by:

calculating, by the processor, a path metric for each of 2*L i-1 candidate paths at the i-th level, where L i-1 is an SCL size at the (i−1)-th level and L 0 =1, the calculating the path metric for each of 2*L i-1 candidate paths comprises, for each of the 2*L i-1 candidate paths: calculating a Logarithmic Likelihood Ratio, LLR, and calculating, by the processor, the path metric based on the LLR;

setting, by the processor, an SCL size at the i-th level, L i , based on and a statistical distribution of the path metrics calculated for the 2*L i-1 candidate paths; and

selecting, by the processor, L i surviving paths from the 2*L i-1 candidate paths based on their respective path metrics;

determining a difference between the largest and the smallest of the path metrics calculated for the 2*L i-1 candidate paths, and dynamically setting L i based on the difference; and

when the i-th bit is a frozen bit:

setting, by the processor, L i to be equal to L i-1 and determining Li surviving paths for the frozen bit.

2. The method of claim 1 , wherein said setting L i comprises:

setting L i to be larger than L i-1 when the difference between the largest and the smallest of the path metrics calculated for the 2*L i-1 candidate paths is smaller than a first predetermined threshold,

setting L i to be equal to M when a difference between the largest and the smallest of the M+1 largest path metrics among the path metrics calculated for the 2*L i-1 candidate paths is larger than a second predetermined threshold, where M<L i-1 , and

setting L i to be equal to L i-1 otherwise.

3. The method of claim 2 , wherein

setting L i to be larger than L i-1 comprises setting L i =2*L i-1 ; and/or

setting L i to be equal to M comprises setting L i =M=L i-1 /2.

4. The method of claim 2 , wherein L i is set to be larger than L i-1 only when L i-1 is smaller than a maximum allowable value of SCL size, and/or L i is set to be equal to M only when L i-1 >1.

5. The method of claim 2 , wherein the first predetermined threshold is smaller than the second predetermined threshold.

6. The method of claim 1 , further comprising:

applying a Cyclic Redundancy Check, CRC, to each of the LN surviving paths at the N-th level; and

when only one of the L N surviving paths passes the CRC:

determining N decoded bits from the one surviving path;

when two or more of the L N surviving paths pass the CRC:

determining N decoded bits from one of the two or more surviving paths that has the largest path metric among the two or more surviving paths; or

when none of the L N surviving paths passes the CRC:

determining N decoded bits from one of the L N surviving paths that has the largest path metric among the L N surviving paths.

7. The method of claim 1 , further comprising:

determining N decoded bits from one of the L N surviving paths that has the largest path metric.

8. A polar decoder comprising:

a processor; and

memory, the memory including instructions executable by the processor whereby the polar decoder is configured to perform a method for polar decoding a demodulated radio signal received from a demodulator into a number, N, of bits with Successive Cancellation List, SCL by:

determining at the i-th level of a binary tree for decoding the i-th bit of the N bits, where 1≤i≤N, and when the i-th bit is an information bit by:

calculating a path metric for each of 2*L i-1 candidate paths at the i-th level, where L i-1 is an SCL size at the (i−1)-th level and L 0 =1, the calculating the path metric for each of 2*L i-1 candidate paths comprises, for each of the 2*L i-1 candidate paths: calculating a Logarithmic Likelihood Ratio, LLR, and calculating the path metric based on the LLR;

setting an SCL size at the i-th level, L i , based on L i-1 and a statistical distribution of the path metrics calculated for the 2*L i-1 candidate paths; and

selecting L i surviving paths from the 2*L i-1 candidate paths based on their respective path metrics;

determining a difference between the largest and the smallest of the path metrics calculated for the 2*L i-1 candidate paths, and dynamically setting L i based on the difference; and

when the i-th bit is a frozen bit:

setting L i to be equal to L i-1 and determining Li surviving paths for the frozen bit.

9. A computer program product comprising a non-transitory computer readable storage medium having computer program instructions stored thereon, the computer program instructions comprising:

when executed by a processor in a polar decoder, causing the polar decoder to perform a method for polar decoding a demodulated radio signal received from a demodulator into a number, N, of bits with Successive Cancellation List, SCL, the method including:

determining at the i-th level of a binary tree for decoding the i-th bit of the N bits, where 1≤i≤N, and when the i-th bit is an information bit by:

calculating a path metric for each of 2*L i-1 candidate paths at the i-th level, where L i-1 is an SCL size at the (i−1)-th level and L 0 =1, the calculating the path metric for each of 2*L i-1 candidate paths comprises, for each of the 2*L i-1 candidate paths: calculating a Logarithmic Likelihood Ratio, LLR, and calculating the path metric based on the LLR;

setting an SCL size at the i-th level, L i , based on L i-1 and a statistical distribution of the path metrics calculated for the 2*L i-1 candidate paths; and

selecting L i surviving paths from the 2*L i-1 candidate paths based on their respective path metrics;

determining a difference between the largest and the smallest of the path metrics calculated for the 2*L i-1 candidate paths, and dynamically setting L i based on the difference; and

when the i-th bit is a frozen bit:

setting L i to be equal to L i-1 and determining Li surviving paths for the frozen bit.