OFDM is a combination of multiplexing and modulation. The signal to be transmitted is first split into a number of separate channels modulated by the data and then re-multiplexed to create an OFDM carrier. HF modem research is ongoing, to investigate whether this type of modulation is well-suited to combat selective fading, which is a major problem in HF data communication. OFDM is used for Wireless LANs and ADSL. When OFDM is combined with coding it is known as Coded OFDM (COFDM), which is used in DAB-broadcasting to combat multi-path.