A mapping apparatus for generating a frozen vector associated with a polar code codeword of length N on the basis of the frozen set F r of each row component code of a product code and of the frozen set F c of each column component code of the product code, the product code comprising row component codes having N r bits and column component codes having N c bits, wherein the frozen set F r is the same or is different for the N r rows of the product code and the frozen set F c is the same or is different for the N c columns of the product code, and wherein the frozen sets F r and F c are represented as vectors with a 0 if the corresponding bit is an information bit and a 1 if it is a frozen bit, the mapping apparatus comprising a processing unit configured to: replicate a frozen set F r N c times to generate an expanded matrix row which is a matrix consisting of the replicated frozen set F r ; replicate a frozen set F c N r times to generate an expanded matrix column which is a matrix consisting of the replicated frozen set Fc; generate the frozen vector on the basis of the expanded matrix row and the expanded matrix column, wherein a respective bit value of the frozen vector equals 1 if a respective corresponding bit of the expanded matrix row or a further respective corresponding bit of the expanded matrix column equals 1 and, otherwise, the respective bit value of the frozen vector equals 0; wherein the processing unit is further configured to change at least one bit of the frozen vector that is not frozen into a frozen bit, wherein the at least one bit of the frozen vector is at a position at which a reliability of a corresponding bit of the polar code codeword is lower than a predetermined threshold value; and/or the processing unit is further configured to change at least one further bit of the frozen vector that is frozen into an information bit, wherein the at least one further bit of the frozen vector is at a position at which a reliability of a corresponding bit of the polar code codeword is higher than a predetermined threshold value. replicate a frozen set F r N c times to generate an expanded matrix row which is a matrix consisting of the replicated frozen set F r ; replicate a frozen set F c N r times to generate an expanded matrix column which is a matrix consisting of the replicated frozen set Fc; generate the frozen vector on the basis of the expanded matrix row and the expanded matrix column, wherein a respective bit value of the frozen vector equals 1 if a respective corresponding bit of the expanded matrix row or a further respective corresponding bit of the expanded matrix column equals 1 and, otherwise, the respective bit value of the frozen vector equals 0; wherein the processing unit is further configured to change at least one bit of the frozen vector that is not frozen into a frozen bit, wherein the at least one bit of the frozen vector is at a position at which a reliability of a corresponding bit of the polar code codeword is lower than a predetermined threshold value; and/or the processing unit is further configured to change at least one further bit of the frozen vector that is frozen into an information bit, wherein the at least one further bit of the frozen vector is at a position at which a reliability of a corresponding bit of the polar code codeword is higher than a predetermined threshold value. <EOS>