Low complexity energy efficiency analysis in millimeter wave communication systems

Millimeter wave (mm-wave) system performance may be degraded if the operation mechanism is not properly designed, because mm-wave systems suffer severe path loss and very short coherence time. Thanks to the sparse channel model and directional transmission property, it is usually sufficient to use analog beam codebooks in beam training to estimate dominant channel components instead of complete instantaneous channel matrices. With this viewpoint, we first characterize the achievable beam gain by the number of antennas and beamwidth, and then propose a low complexity mechanism that employs the offline designed analog beam codebooks for both beam training and data transmission. This mechanism not only avoids high overhead and delay caused by the online beamforming design based on instantaneous channels but also enables a much longer quasi coherence time, which is the new concept proposed in this work. In addition, it can realize a theoretical analysis of the role of system parameters in energy efficiency. We consider a phase-controlled point-to-point (P2P) mm-wave system example to illustrate the proposed concepts and mechanism. Numerical simulations also verify the effectiveness of the theoretical analysis result and also provide a suggestion for system design.