In this thesis, we explore radio resource management algorithms for OFDMA based cellular networks. Firstly, we combine the OFDMA technology with multiple antennas technology and handle the resource allocation problem for the MISO-OFDMA systems. We take care of the fairness issue among users to prevent the users, which have good channel conditions, to obtain most of the system resources. Thus, we propose a fairness aware resource allocation algorithm and compare it with the existing schemes. Next, we enhance the conventional cellular network structure with mobile relays and examine relay selection and resource allocation algorithms for the OFDMA-based mobile relay enhanced cellular networks. We propose a novel relaying frame structure with efficient resource management algorithms in order to reveal the opportunities of the mobile relays. Then, we consider the queue-lengths of the users and propose a queue and channel aware joint relay selection and resource allocation algorithm to use the system resources efficiently. Finally, we combine mobile relaying and offloading technologies in order to overcome the capacity and coverage problems of the conventional cellular networks. We focus on the radio resource management problem for the OFDMA-based mobile relay-enhanced heterogenous cellular networks that contains multiple radio access technologies. We propose a network interface selection algorithm that consider the bandwidth availability information of each network in order to prevent sending the users to overloaded networks.