Investigation on energy efficiency of OFDM-based two-stage cooperative multicast with CP combining

This paper focuses on the energy efficiency of two-stage cooperative multicast with CPC (CP combining). Aiming to provide a coverage of 95%, cooperative multicast with SC (selective combining) has been analytically investigated and a power saving of 43% can be achieved compared with conventional one-stage multicast [9]. However, CP combining which is more practical can provide a stronger received signal than SC and thus is expected to be able to provide further energy saving. Assuming high user density, analysis is carried out on the coverage performance of OFDM based two-stage cooperative multicast with CPC, which is decided by the BS (base station) transmission power at the 1^st stage and the number and locations of MRs (mobile relays) at the 2^nd stage. Given a fixed coverage, it is difficult to obtain the BS transmission power and MR scheme that provide the optimal total power consumption. Therefore, two sub-optimal schemes are proposed, i.e., BS coordination and MR coordination, which are based on the schemes for cooperative multicast with SC [9] and reduce the BS transmission power and the number of MRs, respectively. Numerical results show that with BS and MR coordination, cooperative multicast with CPC can save 9% and 17% compared to that with SC, respectively. Moreover, MR coordination provides more uniform coverage than BS coordination. Simulations are carried out to verify the numerical results. It is shown that MR coordination provides better coverage than BS coordination for all investigated user densities and the performance gap increases as the density decrease.