Code frequency division multiple access: A downlink multiplexing in support of effective multi-user equalization

The downlink of a mobile communication system consists of multiplexed channels that are broadcast to all mobile stations. Due to the common physical source, all signals are synchronized and, for each specific mobile receiver, are subject to the same channel distortion. Judicious choices of signals can correspondingly be made in order to limit inter-user interference on such links. The paper presents a new multiplexing scheme that takes advantage of this possibility. Basically, the scheme separates the users by frequency division and spreads the resulting signal with a spreading code that is common to all users within the same cell. Users from different cells are distinguished by the use of different codes. On non-frequency selective channels, the scheme leads to perfect orthogonality. In the frequency selective case, a good user separation can be maintained by inserting minimum mean square error equalizers. The strength of the scheme is that the training of this equalizer is dominated by the properties of the transmission channel rather than by the interferers. This holds at least in the absence of strong inter-cellular interference. This fact, the great flexibility in the allocation of bandwidths to the various users and the possibility to implement simple suboptimum receivers make the scheme very attractive for mobile communications