The next generation wireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA) need to operate in widely different deployment and usage scenarios. Thus, support for flexible resource allocation is important. In this paper we investigate the performance of different memory- less and memory-based channel estimators for different OFDMA subcarrier allocation schemes and different pilot patterns. We evaluate the performance in various fading environments and for different user terminal velocities. The results show that channel estimation can perform well enough for time-frequency localized resources as small as 22 channel symbols with two pilots in many important scenarios. The results provided can be used to identify appropriate subcarrier allocations for the next generation OFDMA based wireless systems.