A base station (101, 102, 103) comprising: a downlink transmit path comprising circuitry configured to transmit a plurality of reference signals in two or more subframes, each subframe comprising one or more resource blocks, each resource block comprising a number S of orthogonal frequency division multiplexing, OFDM, symbols, each of the S OFDM symbols comprising a number N of subcarriers, and each subcarrier of each OFDM symbol comprises a resource element; and a reference signal allocator configured to: allocate a first group of the plurality of reference signals to selected resource elements of a first subframe according to a reference signal pattern, wherein the first group of the plurality of reference signals is for a first group of antenna ports, and allocate a second group of the plurality of reference signals to selected resource elements of a second subframe according to the same reference signal pattern, wherein the second group of the plurality of reference signals is for a second group of antenna ports consisting of different antenna ports from the antenna ports in the first group of antenna ports, wherein a period of the first group of reference signals and an offset for the first subframe are set by one of a broadcast signaling and a function of a cell-ID and a subframe number, and a different offset for the second subframe can be derived from the offset for the first subframe, wherein the period of the first group of reference signals is a smallest number of subframes starting from one subframe allocated with the first group of reference signals to a next subframe also allocated with the first group of reference signals, and wherein the offset for the first subframe is a number of subframes starting from a beginning of a radio frame to a subframe allocated with the first group of reference signals, wherein the radio frame comprises ten consecutive subframes. a downlink transmit path comprising circuitry configured to transmit a plurality of reference signals in two or more subframes, each subframe comprising one or more resource blocks, each resource block comprising a number S of orthogonal frequency division multiplexing, OFDM, symbols, each of the S OFDM symbols comprising a number N of subcarriers, and each subcarrier of each OFDM symbol comprises a resource element; and a reference signal allocator configured to: allocate a first group of the plurality of reference signals to selected resource elements of a first subframe according to a reference signal pattern, wherein the first group of the plurality of reference signals is for a first group of antenna ports, and allocate a second group of the plurality of reference signals to selected resource elements of a second subframe according to the same reference signal pattern, wherein the second group of the plurality of reference signals is for a second group of antenna ports consisting of different antenna ports from the antenna ports in the first group of antenna ports, allocate a first group of the plurality of reference signals to selected resource elements of a first subframe according to a reference signal pattern, wherein the first group of the plurality of reference signals is for a first group of antenna ports, and allocate a second group of the plurality of reference signals to selected resource elements of a second subframe according to the same reference signal pattern, wherein the second group of the plurality of reference signals is for a second group of antenna ports consisting of different antenna ports from the antenna ports in the first group of antenna ports, wherein a period of the first group of reference signals and an offset for the first subframe are set by one of a broadcast signaling and a function of a cell-ID and a subframe number, and a different offset for the second subframe can be derived from the offset for the first subframe, wherein the period of the first group of reference signals is a smallest number of subframes starting from one subframe allocated with the first group of reference signals to a next subframe also allocated with the first group of reference signals, and wherein the offset for the first subframe is a number of subframes starting from a beginning of a radio frame to a subframe allocated with the first group of reference signals, wherein the radio frame comprises ten consecutive subframes.