The present invention provides a method of generating mutually orthogonal reference signals for different user terminals in and OFDM system that span different but overlapping subcarriers. The subcarriers allocated to the user terminals are divided into a plurality of non-overlapping subcarrier blocks. Each user terminal is then allocated one or more subcarrier blocks. For each subcarrier block, a user terminal is assigned a reference signal comprising a base reference sequence and a linear phase rotation. To ensure mutual orthogonality among all user terminals, user terminals allocated the same subcarrier block use the same base reference sequence with different linear phase rotations.