Reception device, communication system, and method for calculating likelihood of modulation signal

A method for calculating a likelihood of a modulation signal in a reception device, the method comprising:

• receiving a plurality of multiplexed signals respectively through paths different from each other as a plurality of complex baseband signals, and dividing the complex baseband signals respectively into a real component and an imaginary component, the multiplexed signals being obtained by multiplexing a plurality of modulation signals by a real-number precoding matrix, each of the modulation signals having a real component and an imaginary component modulated independently from each other;
• narrowing down candidate signal points, which are obtainable by a real component of the multiplexed signal corresponding to a first complex baseband signal that is one of the complex baseband signals, to a first candidate signal point by using only a real component of the first complex baseband signal;
• narrowing down candidate signal points, which are obtainable by an imaginary component of the multiplexed signal corresponding to the first complex baseband signal, to a second candidate signal point by using only an imaginary component of the first complex baseband signal;
• calculating a first replica vector by using the first candidate signal point corresponding to the first complex baseband signal;
• calculating a second replica vector by using the second candidate signal point corresponding to the first complex baseband signal;
• and
• calculating a likelihood of the modulation signal by using the first replica vector and the second replica vector.

A control circuitry configured to control a reception device, the reception device comprising:

• a signal division circuitry configured to receive a plurality of multiplexed signals through propagation paths different from each other as a plurality of complex baseband signals, and to divide the plurality of complex baseband signals respectively into a real component and an imaginary component, the multiplexed signals being obtained by multiplexing a plurality of modulation signals by a real-number precoding matrix, each of the modulation signals having a real component and an imaginary component modulated independently from each other;
• a first maximum-likelihood point search circuitry configured to narrow down candidate signal points, which are obtainable by a real component of the multiplexed signal corresponding to a first complex baseband signal that is one of the complex baseband signals, to a first candidate signal point by using only a real component of the first complex baseband signal;
• a second maximum-likelihood point search circuitry configured to narrow down candidate signal points, which are obtainable by an imaginary component of the multiplexed signal corresponding to the first complex baseband signal, to a second candidate signal point by using only an imaginary component of the first complex baseband signal;
• a first replica-vector calculation circuitry configured to calculate a first replica vector by using the first candidate signal point corresponding to the first complex baseband signal;
• a second replica-vector calculation circuitry configured to calculate a second replica vector by using the second candidate signal point corresponding to the first complex baseband signal;
• and
• a likelihood calculation circuitry configured to calculate a likelihood of the modulation signal by using the first replica vector and the second replica vector.

A computer program product comprising a non-transitory computer usable medium having a computer readable program that causes a control circuitry configured to control a reception device, the reception device comprising:

• a signal division circuitry configured to receive a plurality of multiplexed signals through propagation paths different from each other as a plurality of complex baseband signals, and to divide the plurality of complex baseband signals respectively into a real component and an imaginary component, the multiplexed signals being obtained by multiplexing a plurality of modulation signals by a real-number precoding matrix, each of the modulation signals having a real component and an imaginary component modulated independently from each other;
• a first maximum-likelihood point search circuitry configured to narrow down candidate signal points, which are obtainable by a real component of the multiplexed signal corresponding to a first complex baseband signal that is one of the complex baseband signals, to a first candidate signal point by using only a real component of the first complex baseband signal;
• a second maximum-likelihood point search circuitry configured to narrow down candidate signal points, which are obtainable by an imaginary component of the multiplexed signal corresponding to the first complex baseband signal, to a second candidate signal point by using only an imaginary component of the first complex baseband signal;
• a first replica-vector calculation circuitry configured to calculate a first replica vector by using the first candidate signal point corresponding to the first complex baseband signal;
• a second replica-vector calculation circuitry configured to calculate a second replica vector by using the second candidate signal point corresponding to the first complex baseband signal;
• and
• a likelihood calculation circuitry configured to calculate a likelihood of the modulation signal by using the first replica vector and the second replica vector.