A cognitive radio, CR, communication apparatus, comprising: a signal receiving unit which is configured to receive signals from a primary user (111,112) of a primary system and from a secondary transmitter of a secondary system through a plurality of M antennas, the received signals of the primary system and secondary system including elements associated with a plurality Np of signals known to the CR communication apparatus; and a determination unit which is configured to: - determine the number M of antennas; - determine the number Np of known signals to the CR communication apparatus; - determine a number Ns of OFDM symbols to recognize a signal of the primary system wherein the signal receiving unit and the determination unit are configured such that the following steps are repeatedly performed with respect to Ns number of times: - the signal receiving unit receiving the signals; - the determination unit determining correlation matrices based on a difference between the received signals and the elements associated with the number Np of signals known to the CR communication apparatus, wherein the number of the correlation matrices is the same as the number Np of known signals to the CR communications apparatus and wherein the size of the correlation matrices is MxM; - the determination unit determining a sum of the correlation matrices; and - the determination unit determining a metric based on the sum of the correlation matrices; wherein the determination unit is further configured to: - determine whether the signal of the primary user exists from among the received signals based on the metric. a signal receiving unit which is configured to receive signals from a primary user (111,112) of a primary system and from a secondary transmitter of a secondary system through a plurality of M antennas, the received signals of the primary system and secondary system including elements associated with a plurality Np of signals known to the CR communication apparatus; and a determination unit which is configured to: - determine the number M of antennas; - determine the number Np of known signals to the CR communication apparatus; - determine a number Ns of OFDM symbols to recognize a signal of the primary system wherein the signal receiving unit and the determination unit are configured such that the following steps are repeatedly performed with respect to Ns number of times: - the signal receiving unit receiving the signals; - the determination unit determining correlation matrices based on a difference between the received signals and the elements associated with the number Np of signals known - determine the number M of antennas; - determine the number Np of known signals to the CR communication apparatus; - determine a number Ns of OFDM symbols to recognize a signal of the primary system - the signal receiving unit receiving the signals; - the determination unit determining correlation matrices based on a difference between the received signals and the elements associated with the number Np of signals known to the CR communication apparatus, wherein the number of the correlation matrices is the same as the number Np of known signals to the CR communications apparatus and wherein the size of the correlation matrices is MxM; - the determination unit determining a sum of the correlation matrices; and - the determination unit determining a metric based on the sum of the correlation matrices; wherein the determination unit is further configured to: - determine whether the signal of the primary user exists from among the received signals based on the metric. - the determination unit determining a sum of the correlation matrices; and - the determination unit determining a metric based on the sum of the correlation matrices; - determine whether the signal of the primary user exists from among the received signals based on the metric.