  This letter proposes a new user cooperative offloading protocol called user
reciprocity in backscatter communication (BackCom)-aided mobile edge computing
systems with efficient computation, whose quintessence is that each user can
switch alternately between the active or the BackCom mode in different slots,
and one user works in the active mode and the other user works in the BackCom
mode in each time slot. In particular, the user in the BackCom mode can always
use the signal transmitted by the user in the active mode for more data
transmission in a spectrum-sharing manner. To evaluate the proposed protocol, a
computation efficiency (CE) maximization-based optimization problem is
formulated by jointly power control, time scheduling, reflection coefficient
adjustment, and computing frequency allocation, while satisfying various
physical constraints on the maximum energy budget, the computing frequency
threshold, the minimum computed bits, and harvested energy threshold. To solve
this non-convex problem, Dinkelbach's method and quadratic transform are first
employed to transform the complex fractional forms into linear ones. Then, an
iterative algorithm is designed by decomposing the resulting problem to obtain
the suboptimal solution. The closed-form solutions for the transmit power, the
RC, and the local computing frequency are provided for more insights. Besides,
the analytical performance gain with the reciprocal mode is also derived.
Simulation results demonstrate that the proposed scheme outperforms benchmark
schemes regarding the CE.
