SOFT DECISION APPARATUS, SOFT DECISION METHOD AND PROGRAM
Provided is a soft decision device that performs a soft decision on an (n+1)th symbol of a transmission signal with a symbol multilevel degree of m, the soft decision device including: a control unit that estimates a branch metric, which is a distance that is obtained by a Viterbi algorithm, is in a distance function, and indicates a likelihood of transition from an nth symbol of the transmission signal to each candidate for the (n+1)th symbol, based on a received signal and an estimated transfer function that is an estimation result of a transfer function of a transmission line, estimates a path metric, which is a sum of a distance that is obtained by the Viterbi algorithm, is in a distance function, and indicates a likelihood that each candidate for the (n+1)th symbol of the transmission signal is a symbol of the transmission signal and a distance of a predetermined remaining path of the transmission signal, based on a result of the branch metric estimation process, and estimates a likelihood that a kth bit in the (n+1)th symbol of the transmission signal is a predetermined bit by using the path metric of each candidate for the (n+1)th symbol of the transmission signal.
1 . A soft decision device that performs a soft decision on a main symbol that is an (n+1)th symbol (n is an integer from 1 to N, N is an integer of 1 or more) of a transmission signal with a symbol multilevel degree of m (m is an integer of 2 or more), the soft decision device comprising:
a controller that executes
a branch metric estimation process of estimating a branch metric, which is a distance that is obtained by a Viterbi algorithm, is indicated by a predetermined distance function, and indicates a likelihood of transition from an nth symbol of the transmission signal to each candidate for the (n+1)th symbol, based on an estimated transfer function that is a previously obtained estimation result of a transfer function of a transmission line through which the transmission signal propagates and a received signal that is a result of reception of the transmission signal,
a path metric estimation process of estimating a path metric, which is a sum of a distance that is obtained by the Viterbi algorithm, is indicated by a predetermined distance function, and indicates a likelihood that each candidate for the (n+1)th symbol of the transmission signal is a symbol of the transmission signal and a distance of first to nth remaining paths of the transmission signal, based on a result of the branch metric estimation process, and
a bit likelihood estimation process of estimating a likelihood that a kth bit (k is an integer from 1 to m) in the (n+1)th symbol of the transmission signal is a predetermined bit by using the path metric of each candidate for the (n+1)th symbol of the transmission signal obtained by the path metric estimation process, and without using path metrics other than the path metric of each candidate for the (n+1)th symbol of the transmission signal,
wherein the controller acquires the likelihood estimated by the bit likelihood estimation process as a result of the soft decision.
2 . The soft decision device according to claim 1 , wherein the controller estimates the likelihood by also using a standard deviation of a distribution of noise added to the transmission signal on the transmission line.
3 . The soft decision device according to claim 2 , wherein the standard deviation is obtained based on a difference between the received signal and a result of applying the estimated transfer function to correct data indicating a correct answer of the transmission signal.
4 . The soft decision device according to claim 2 , wherein the controller changes the likelihood based on a sign opposite to a sign of the estimated likelihood, a predetermined constant, and the standard deviation when a determination result of the nth symbol obtained by calculation in order from the (n+1)th of the transmission signal to the nth is different from a determination result of the nth symbol obtained by calculation in order from the nth of the transmission signal to the (n+1)th.
5 . The soft decision device according to claim 4 , wherein the predetermined constant is determined based on the likelihood indicating a positive peak of the distribution and the standard deviation.
6 . A soft decision method for performing a soft decision on a main symbol that is an (n+1)th symbol (n is an integer from 1 to N, N is an integer of 1 or more) of a transmission signal with a symbol multilevel degree of m (m is an integer of 2 or more), the soft decision method comprising:
a control step of executing
a branch metric estimation process of estimating a branch metric, which is a distance that is obtained by a Viterbi algorithm, is in a predetermined distance function, and indicates a likelihood of transition from an nth symbol of the transmission signal to each candidate for the (n+1)th symbol, based on an estimated transfer function that is a previously obtained estimation result of a transfer function of a transmission line through which the transmission signal propagates and a received signal that is a result of reception of the transmission signal,
a path metric estimation process of estimating a path metric, which is a sum of a distance that is obtained by the Viterbi algorithm, is in a predetermined distance function, and indicates a likelihood that each candidate for the (n+1)th symbol of the transmission signal is a symbol of the transmission signal and a distance of first to nth remaining paths of the transmission signal, based on a result of the branch metric estimation process, and
a bit likelihood estimation process of estimating a likelihood that a kth bit (k is an integer from 1 to m) in the (n+1)th symbol of the transmission signal is a predetermined bit by using the path metric of each candidate for the (n+1)th symbol of the transmission signal obtained by the path metric estimation process, and without using path metrics other than the path metric of each candidate for the (n+1)th symbol of the transmission signal,
wherein the control step includes acquiring the likelihood estimated by the bit likelihood estimation process as a result of the soft decision.
7 . A non-transitory computer readable storing a program for causing a computer to function as the soft decision device according to claim 1 .