Capacity Considerations on the Uplink of a Multi-User DMT OFDMA System Impaired by Time Misalignments and Frequency Offsets

The uplink of an asynchronous DFT/OFDM based communication system is studied and capacity implications are derived. Mulitple users share a Gaussian channel through DMT OFDMA modulation and multiplexing. We consider a bank of single user detectors at the output of which multiple access interference arises whenever time misalignments and frequency offsets exist among users. Consequently, we determine an inner bound to both the capacity of a given user link and the region of achievable information rates for joint reliable communications (i.e. capacity region). Such capacity inner bounds are random variables and function of several system parameters. The associated complementary cumulative distribution functions are defined, and are evaluated for several system scenarios characterized by different tone assignment schemes and strategies for power allotment to subcarriers. As a result, a pragmatic approach for enlarging the capacity region is devised. The approach is based on the appropriate insertion of a guard time, the partition of tones to users and the choice of the power profile to be assigned to the sets of tones.