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Energy-Efficient Two-Stage Cooperative Multicast: Effect of User Density

Exploiting the spatial diversity and path-loss gain provided by the cooperative transmission with the mobile relay (MR), two-stage cooperative multicast (CM) can provide the same performance as conventional multicast does with reduced power consumption. However, this target cannot always be achieved, such as when the user density is low. To investigate the impact of user density on the energy efficiency of CM, selective combining based on average received signal strength (SCA) is assumed for the second-stage signal processing, and maximum ratio combining (MRC) is employed to collect useful signals from the first and second stages. Then, a try-best MR selection scheme is proposed, choosing the successful mobile station closest to the unsuccessful MS as its MR. Based on this MR selection scheme, the coverage analysis is carried out. A lower bound of user density can be numerically obtained, below which the two-stage CM is energy inefficient. The analysis is verified by simulations. It is shown that to guarantee a given coverage performance such as 95%, for a cell with a radius of 1500 m and a multicast data transmission efficiency of 0.45 b/s/Hz, the lower bound of user density for CM to be energy efficient is found to be 13. The total power consumption of two-stage CM is significantly reduced when the number of MSs (N) increases. For N = 100, a reduction of 50% could be achieved compared with that of conventional multicast. Moreover, for the signal processing scheme, the SCA assumption is verified to be reasonable for low user density, and MRC could effectively reduce the lower bound of user density from 24 to 13. Finally, the energy efficiency of the proposed CM scheme with try-best MR selection is shown to be much higher than that of existing schemes.