A meta-analysis of the comparative distance education (DE) literature between 1985 and 2002 was conducted. In total, 232 studies containing 688 independent achievement, attitude, and retention outcomes were analyzed. Overall results indicated effect sizes of essentially zero on all three measures and wide variability. This suggests that many applications of DE outperform their classroom counterparts and that many perform more poorly. Dividing achievement outcomes into synchronous and asynchronous forms of DE produced a somewhat different impression. In general, mean achievement effect sizes for synchronous applications favored classroom instruction, while effect sizes for asynchronous applications favored DE. However, significant heterogeneity remained in each subset.

In this paper, the issues upon user scheduling in the downlink packet transmission for multiple services are addressed for coordinated multi-point (CoMP) OFDMA networks. We consider mixed traffic with voice over IP (VOIP) and best effort (BE) services. In order to improve cell-edge performance and guarantee diverse quality of service (QoS), a utility-based joint scheduling algorithm is proposed, which consists of two steps: ant colony optimization (ACO) based joint user selection and greedy subchannel assignment. We compare the proposed algorithm with the opimal algorithm and the greedy user selection (GUS) based scheme. Via simulation results, we show that the performance of our proposed algorithm well approaches to that of the optimal solution. It is also observed that 95% of BE users are satsified with average cell-edge data rate greater than 200kbps by using either of the two algorithms. Whereas, our proposed algorithm ensures that more than 95% of VoIP users are satisfied with packet drop ratio less than 2%, compared to 78% by the GUS based algorithm.

Coordinated multi-point (CoMP) joint transmission is considered as a key technique to mitigate inter-cell interference and improve the cell-edge performance in 3GPP LTE-Advanced. In this paper, a utility-based joint resource allocation approach is proposed to support CoMP joint transmission in orthogonal frequency division multiple access systems. Two different utility functions are considered with diverse quality of service provisioning. The objective is to maximize the sum utility of a clustered CoMP network, considering mixed real-time voice over IP (VoIP) and best-effort (BE) traffic patterns. A centralized joint resource allocation algorithm is proposed by decoupling the optimization problem into per-subchannel subproblems. Further more, a greedy user selection based low-complexity algorithm is proposed for practical implementation. For the sake of performance comparison, the proportional-fair based joint scheduling, as well as two non-coordinated scheduling schemes are also evaluated. Via system-level simulation we show that the proposed algorithms not only substantially improve the cell-edge spectrum efficiency of BE users, but also significantly decrease the packet drop ratio and call outage ratio of VoIP users.