Effects of multiple users´ CFOs in OFDM-SDMA up-link - an interference model

OFDM-SDMA systems are intended to allow multiple users to communicate simultaneously with a base station in the same frequency band. In these systems, each user signal may be distorted by an independent carrier frequency offset (CFO), which will result in a severe degradation of the system performance if no CFO compensation is carried out. The standard compensation algorithms known from single-user SISO systems do not apply in this context, hence novel compensation techniques have to be introduced. We analyse the influence of multi-users´ CFOs on the OFDM-SDMA signal transmission and develop a scheme for their compensation with low complexity. Due to a disturbance of the inherent structure of the OFDM system, the scheme is not capable to compensate for all the effects generated by the CFOs. The remaining interferences may be treated as additional noise, thus setting tight limits for the size of the CFOs which are supported by the system.