5G will have to cope with a high degree of heterogeneity in terms of services and requirements. Flexible and efficient use of all available spectrums for different network deployment scenarios is one challenge for the future 5G. To maximize spectrum efficiency, the 5G air interface technology will also need to be flexible and capable of mapping various services to the best suitable combinations of frequency and time resources. In this work, we study and fairly compare the performance of different waveforms (UFMC, FBMC, and legacy LTE waveforms) in cellular networks with relaxed synchronization. First, a unified frame structure is discussed to introduce the context. Then we compare the robustness of the waveform contenders in the envisaged scenario. Based on these results, a bit loading algorithm is proposed to cope with the non-uniform distribution of the interference across the carriers in order to increase the spectral efficiency.