Aspects of the disclosure provide an apparatus that includes a baseband processing circuit and a transmitting circuit. The baseband processing circuit is configured to encode a reference signal based on a specific sequence to generate a digital stream. The specific sequence has non-zero values at selected positions, and the number of coincidences of non-zero value positions between the specific sequence and a shifted copy of the specific sequence is smaller than a threshold. The transmitting circuit is configured to transmit wireless signals in response to the digital stream.