WE CLAIM:
1. A transmission device comprising:
a control unit to allocate one or more layers per terminal for transmission to one or more terminals, and configure positions in a frequency domain and a time domain of reference signals of each of the layers on the basis of at least one of information indicating a position of each of the terminals and information indicating condition of each of channel between the transmission device and the one or more terminals;
an arranging unit to arrange the reference signals of each of the layers in the frequency domain and the time domain on the basis of the positions in the frequency domain and the time domain configured by the control unit; and
a transmission processing unit to perform space-division multiplexing on signals of one or more layers arranged by the arranging unit, and transmit the multiplexed signals.
2. The transmission device according to claim 1, wherein
the transmission device transmits control information
including the positions of the reference signals in the frequency domain and the time domain and information indicating sequences used as the reference signals to each of the one or more terminals.
3. The transmission device according to claim 2, wherein
the transmission device changes density of reference
signals within resources in a schedule unit on the basis of
a moving speed or a channel condition of each of the one or
more terminals, the schedule unit being a unit of

allocation of resources in the frequency domain and the time domain.
4. The transmission device according to claim 3, wherein the transmission device changes the density of the reference signals by determining whether or not to arrange the reference signals in units of reference signal groups, the reference signal group being a group of reference signals whose positions are determined in advance.
5. The transmission device according to claim 4, wherein positions of different reference signal groups within the resources in the schedule unit are different in the time domain.
6. The transmission device according to claim 4, wherein positions of different reference signal groups within the resources in the schedule unit are different in the frequency domain.
7. The transmission device according to any one of claims 4 to 6, wherein the transmission device defines a position of one of the reference signal groups as a reference position within the resources in the schedule unit, defines positions of the other reference signal groups by offset values in at least one of the frequency domain and the time domain, and includes, in the control information, the offset value of each of the reference signal groups and a flag indicating whether or not to arrange reference signals for each of the reference signal groups.
8. The transmission device according to claim 2 or 3, wherein the transmission device includes one or more

reference signal groups within resources in a schedule unit, the schedule unit being a unit of allocation of resources in the frequency domain and the time domain, the reference signal group being a group of reference signals whose positions are determined in advance, defines a position of one reference signal group as a reference position, defines positions of the other reference signal groups by offset values in at least one of the frequency domain and the time domain, and includes, in the control information, the offset value of each of the reference signal groups.
9. The transmission device according to any one of claims 4 to 8, wherein the transmission device selects whether to perform a first arrangement process of determining arrangement of the reference signals in the frequency domain and the time domain within resources of one schedule unit or a second arrangement process of determining arrangement of the reference signals in the frequency domain and the time domain within resources of a plurality of schedule units.
10. The transmission device according to claim 9, wherein an amount of data used for the reference signals within the resources of one of the plurality of schedule units when the second arrangement process is performed is smaller than an amount of data used for the reference signals within the resources of the one schedule unit when the first arrangement process is performed.
11. The transmission device according to any one of claims 1 to 10, wherein the transmission device multiplies reference signals, arranged at identical positions in the

frequency domain and the time domain in different layers, by different orthogonal codes for different layers.
12. The transmission device according to any one of claims 1 to 11, wherein the arranging unit performs scrambling on the reference signals, and arranges the scrambled reference signals in the frequency domain and the time domain.
13. The transmission device according to any one of claims 1 to 12, wherein the transmission device selects and performs either of first allocation in which identical arrangement in the frequency domain and the time domain is used in different layers and second allocation in which at least one of arrangement in the frequency domain, arrangement in the time domain, and orthogonal codes used in multiplication is different in different layers.
14. The transmission device according to claim 13, wherein the transmission device selects the first allocation or the second allocation depending on a distance between the terminals.
15. The transmission device according to claim 13, wherein the transmission device acquires, from each of the terminals, information indicating a direction in which a reception strength is maximum when signals emitted in different directions transmitted from the transmission device are received by the terminal, and selects the first allocation or the second allocation on the basis of differences in the acquired direction between the terminals.
16. The transmission device according to claim 13, wherein

the transmission device acquires channel information from each of the terminals, calculates spatial correlation between the terminals on the basis of the channel information, and selects the first allocation or the second allocation depending on the spatial correlation.
17. The transmission device according to claim 1, wherein
a schedule unit being a unit for allocation of resources in
the frequency domain and the time domain is constituted by
a matrix of 12 subcarriers of zeroth to eleventh
subcarriers and 14 symbols of zeroth to thirteenth symbols,
reference signals arranged at the second symbols on the
first and second subcarriers and the third symbols on the
first and second subcarriers are defined as a first
reference signal group, reference signals arranged at the
second symbols on the third and fourth subcarriers and the
third symbols on the third and fifth subcarriers are
defined as a second reference signal group, the first and
second reference signal groups are multiplied by orthogonal
codes of two bits that are different for different layers,
the first and second reference signal groups are defined as
a basic pattern, the basic pattern is duplicated, and the
duplicated basic pattern is arranged at positions shifted
by six subcarriers in a frequency direction.
18. A communication system comprising:
one or more terminals; and
a transmission device, wherein
the transmission device includes:
a control unit to allocate one or more layers per terminal for transmission to the terminals, and configure positions in a frequency domain and a time domain of

reference signals of each of the layers on the basis of at least one of information indicating a position of each of the terminals and information indicating condition of each of channel between the transmission device and the one or more terminals;
an arranging unit to arrange the reference signals of each of the layers in the frequency domain and the time domain on the basis of the positions in the frequency domain and the time domain configured by the control unit; and
a transmission processing unit to perform space-division multiplexing on signals of one or more layers arranged by the arranging unit, and transmit the multiplexed signals.
19. A transmission method performed by a transmission device, the transmission method comprising:
a first step of allocating one or more layers per terminal for transmission to one or more terminals, and configuring positions in a frequency domain and a time domain of reference signals of each of the layers on the basis of at least one of information indicating a position of each of the terminals and information indicating condition of each of channel between the transmission device and the one or more terminals;
a second step of arranging the reference signals of each of the layers in the frequency domain and the time domain on the basis of the positions in the frequency domain and the time domain configured in the first step; and
a third step of performing space-division multiplexing on signals of one or more layers arranged in the second step, and transmitting the multiplexed signals.