A network access node and a client device for generating and using cubic phase polynomial sequences (si, sj) of length L with a third order coefficient value a3 belonging to the subset of sequences () are described. The network access node transmits a control message to the client device, wherein the control message indicates the cyclical shift value NCS and the third order coefficient value a3. The client device receives the control message and determines a cubic polynomial phase sequence (si) belonging to the subset of sequences () based on the cyclical shift value NCS and the third order coefficient value a3. The client device thereafter transmits the determined cubic polynomial phase sequence (si) as a random access preamble to the network access node.