Efficient multiuser spectral precoding for reducing out-of-band emission for OFDM-based cognitive radios

When multiple users in an OFDM system can be synchronized, a novel efficient multiuser precoding approach for reducing out-of-band emission (OOBE) is developed. The main idea is to view all users as a virtual single user and design a common virtual precoder for all users. The precoder of a particular user is then constructed using the corresponding columns of the virtual precoder according to the allocated spectrum of that user. Without suffering from any degradation in BER performance, the proposed approach achieves higher spectral efficiency, larger OOBE reduction and better peak to average power ratio (PAPR) performance than the existing precoding approaches at the cost of higher complexity. Moreover, unlike the existing approaches, the spectral efficiency loss of the proposed approach does not increase as the number of users increases. It is also remarkable that the proposed approach allows each user to use noncontiguous spectra without losing additional spectral efficiency; this property is essential for enabling granular resource allocations.