A method of receiving a downlink signal in a narrow frequency band of a legacy broadband wireless communication system by a user equipment, the method comprising: acquiring a first cell identifier from a first carrier; receiving, through the first carrier, carrier information for a second carrier; and receiving downlink data on the second carrier based on the carrier information and a narrowband reference signal, NB-RS, wherein each of the first and second carriers occupies a frequency band of one resource block, RB, wherein the first carrier is a carrier on which a narrowband synchronization signal, nSS, and a narrowband physical broadcast channel, nPBCH, are present and the second carrier is a carrier on which the nSS and the nPBCH are not present, characterized in that: the carrier information includes cell identifier information indicating whether a second cell identifier used for a legacy cell specific reference signal, CRS, on the second carrier is the same as the first cell identifier, if the cell identifier information indicates that the second cell identifier used for the legacy CRS is the same as the first cell identifier, the number of antenna ports for the legacy CRS is the same as the number of antenna ports for the NB-RS, and if the cell identifier information indicates that the second cell identifier used for the legacy CRS is different from the first cell identifier, the carrier information further includes number-of-antenna ports information and the number of antenna ports for the legacy CRS is the same as the number of antenna ports corresponding to the number-of-antenna ports information. acquiring a first cell identifier from a first carrier; receiving, through the first carrier, carrier information for a second carrier; and receiving downlink data on the second carrier based on the carrier information and a narrowband reference signal, NB-RS, wherein each of the first and second carriers occupies a frequency band of one resource block, RB, wherein the first carrier is a carrier on which a narrowband synchronization signal, nSS, and a narrowband physical broadcast channel, nPBCH, are present and the second carrier is a carrier on which the nSS and the nPBCH are not present, characterized in that: the carrier information includes cell identifier information indicating whether a second cell identifier used for a legacy cell specific reference signal, CRS, on the second carrier is the same as the first cell identifier, if the cell identifier information indicates that the second cell identifier used for the legacy CRS is the same as the first cell identifier, the number of antenna ports for the legacy CRS is the same as the number of antenna ports for the NB-RS, and if the cell identifier information indicates that the second cell identifier used for the legacy CRS is different from the first cell identifier, the carrier information further includes number-of-antenna ports information and the number of antenna ports for the legacy CRS is the same as the number of antenna ports corresponding to the number-of-antenna ports information. the carrier information includes cell identifier information indicating whether a second cell identifier used for a legacy cell specific reference signal, CRS, on the second carrier is the same as the first cell identifier, if the cell identifier information indicates that the second cell identifier used for the legacy CRS is the same as the first cell identifier, the number of antenna ports for the legacy CRS is the same as the number of antenna ports for the NB-RS, and if the cell identifier information indicates that the second cell identifier used for the legacy CRS is different from the first cell identifier, the carrier information further includes number-of-antenna ports information and the number of antenna ports for the legacy CRS is the same as the number of antenna ports corresponding to the number-of-antenna ports information.