A Unified Perspective of Different Multicarrier CDMA Schemes

In the conventional multicarrier CDMA schemes, different users utilize different spreading codes to realize the multiple accessing (which is the CDMA technique) and the bits from the same user are transmitted on different frequency subcarriers to realize the multiplexing (which is the OFDM technique). However, the function of the set of subcarriers and the function of the set of spreading codes can be switched. That is, the bits from the same user are spread with different accessing codes but they are transmitted on the same subcarrier. Based on this idea, we introduce a new multicarrier scheme which is named as CDM-OFDMA. We put the MC-DS-CDMA and CDM-OFDMA schemes into one unified formula format. Their performances are evaluated and compared. Under equal deterministic channel characteristic condition, it is found that the CDM-OFDMA scheme has a better performance than the MC-DS-CDMA scheme. The main reason is that the MAI from the same subcarrier is much bigger than the MAI from other subcarriers and in the CDM-OFDMA scheme the MAI from the same subcarrier is avoided by employing the orthogonal spreading codes