A transmission apparatus includes (N+1) or more transmission antennas. A first receiving apparatus detects N or fewer transmission antennas from a received signal. A second receiving apparatus detects more than N transmission antennas from a received signal. The transmission apparatus uses a first radio resource for outputting a known signal from each of transmission antennas and uses a second radio resource for outputting a known signal from at least one of the transmission antennas. A timing difference between the first and second radio resources is smaller than or equal to a first determined difference and a frequency difference therebetween is smaller than or equal to a second determined difference. The first receiving apparatus estimates a channel state based on the known signal outputted on the first radio resource. The second receiving apparatus estimates a channel state based on the known signals outputted on the first and second radio resources.