1. A reception device comprising:
a reception unit to receive a plurality of transmission signals via propagation paths orthogonal to each other, the transmission signals being obtained by multiplexing a plurality of first signals by using a precoding matrix, the fist signals being L (L is an integer of 2 or larger) complex baseband signals whose real component and imaginary component are modulated independently of each other;
a splitting unit to split each of a plurality of received signals received by the reception unit into a real component and an imaginary component;
a narrowing unit to narrow down possible signal point candidates of a real component of the first signal to signal point candidates on a basis of the real component of the split received signal and narrow down possible signal point candidates of an imaginary component of the first signal to signal point candidates on a basis of the imaginary component of the split received signal;
a signal point candidate hypothesizing unit to hypothesize one signal point candidate of a real component from the signal point candidates of a real component obtained by the narrowing down and hypothesize one signal point candidate of an imaginary component from the signal point candidates of an imaginary component obtained by the narrowing down; and
a signal estimation value calculating unit to estimate a real component of the first signal on a basis of the one hypothesized real component signal point candidate and estimate an imaginary component of the first signal on a basis of the one hypothesized imaginary component signal point candidate.

2. The reception device according to claim 1, wherein
the first signals are modulated by a 22N-QAM
(Quadrature Amplitude Modulation) technique (N is an integer of 2 or larger), and
the narrowing unit selects, from among possible signal point candidates of the first signal, a signal point candidate that is closest to a signal point of the received signal and a signal point candidate that is closest to the received signal in each inverted bit among signal point candidates including an inverted bit with respect to each bit of the closest signal point candidate, and outputs the selected signal point candidates.
3. The reception device according to claim 1 or 2,
wherein
the first signals are modulated by a 22N-QAM technique (N is an integer of 2 or larger), and
when ym and yn represent input signals, §mn and §nm represent elements of a real number precoding matrix, and Am and An represent transfer functions of the propagation paths,
in a case where the input signals ym and yn satisfy formula (2), the narrowing unit narrows down signal point candidates to a total of (2N_1 + 1) signal point candidates including a signal point candidate present in a region satisfying formula (2) and a signal point candidate that is closest to formula (1) in a region satisfying formula (3), and
in a case where the input signals ym and yn satisfy formula (3), the narrowing unit narrows down signal point candidates to a total of (2N_1 + 1) signal point candidates including a signal point candidate present in the region

satisfying formula (3) and a signal point candidate that is closest to formula (1) in the region satisfying formula (2).
rr^rm V = V ... (1)
A cb
A y ...(3)
4. The reception device according to any one of claims 1
to 3, further comprising:
(L-2) stages of signal point candidate hypothesizing units (L is an integer of 3 or larger) to hypothesize a signal point candidate for one component of a received multiplexed signal, the (L-2) stages of signal point candidate hypothesizing units being located before the narrowing unit, wherein
the narrowing unit performs narrowing down on the complex baseband signals on a basis of the one component of the received multiplexed signal.
5. The reception device according to any one of claims 1
to 4, wherein
the estimated signal calculating unit calculates the real component of the first signal from a value of the one hypothesized real component signal point candidate, a real component of a received signal, and an equivalent channel vector associated with the one hypothesized real component signal point candidate, and
the estimated signal calculating unit calculates the real component of the first signal from a value of the one hypothesized imaginary component signal point candidate, an imaginary component of a received signal, and an equivalent

channel vector associated with the one hypothesized imaginary component signal point candidate.
6. The reception device according to any one of claims 1
to 5, further comprising:
a likelihood calculating unit to calculate, on a basis of a real component and an imaginary component of the first signals estimated by the signal estimation calculating unit, likelihood of each of the real component and the imaginary component of the first signals, and output the likelihood.
7. A reception method comprising:
receiving a plurality of transmission signals via propagation paths orthogonal to each other, the transmission signals being obtained by multiplexing a plurality of first signals by using a precoding matrix, the fist signals being L (L is an integer of 2 or larger) complex baseband signals whose real component and imaginary component are modulated independently of each other;
splitting each of a plurality of received signals into a real component and an imaginary component;
narrowing down possible signal point candidates of a real component of the first signal to signal point candidates on a basis of the real component of the split received signal and narrowing down possible signal point candidates of an imaginary component of the first signal to signal point candidates on a basis of the imaginary component of the split received signal;
hypothesizing one signal point candidate of a real component from the signal point candidates of a real component obtained by the narrowing down and hypothesizing one signal point candidate of an imaginary component from the signal point candidates of an imaginary component

obtained by the narrowing down; and
estimating a real component of the first signal on a basis of the one hypothesized real component signal point candidate and estimating an imaginary component of the first signal on a basis of the one hypothesized imaginary component signal point candidate.
8. A communication system comprising:
a transmission device including a transmission unit to transmit a plurality of transmission signals via propagation paths orthogonal to each other, the transmission signals being obtained by multiplexing a plurality of first signals by using a precoding matrix, the fist signals being L (L is an integer of 2 or larger) complex baseband signals whose real component and imaginary component are modulated independently of each other; and a reception device including:
a reception unit to receive the transmission signals;
a splitting unit to split each of a plurality of received signals received by the reception unit into a real component and an imaginary component;
a narrowing unit to narrow down possible signal point candidates of a real component of the first signal to signal point candidates on a basis of the real component of the split received signal and narrow down possible signal point candidates of an imaginary component of the first signal to signal point candidates on a basis of the imaginary component of the split received signal;
a signal point candidate hypothesizing unit to hypothesize one signal point candidate of a real component from the signal point candidates of a real component obtained by the narrowing down and hypothesize one signal

point candidate of an imaginary component from the signal point candidates of an imaginary component obtained by the narrowing down; and
a signal estimation value calculating unit to estimate a real component of the first signal on a basis of the one hypothesized real component signal point candidate and estimate an imaginary component of the first signal on a basis of the one hypothesized imaginary component signal point candidate.