We present an analytical model of throughput-delay tradeoff of the proportional fair scheduling of OFDMA Systems in context of user multiplexing. For this purpose, we propose simple and efficient resource allocation schemes which are based on 2-dimensional resource partitioning in frequency and time domain. Basically, frequency-domain partitioning is determined by degree of frequency diversity and time-domain partitioning is determined by degree of multiplexing which is number of users to be served in a sub-channel of a frame. Our approach is aimed at maximizing the delay constrained throughput by choosing the appropriate degree of multiplexing for a given degree of frequency diversity. We investigate the effect of degree-of-multiplexing and frequency diversity on delay QoS performance of OFDMA system by analyzing the effective capacity, and then develop an approach to finding the optimal degree of multiplexing by the asymptotic approximation.