Link adaptation for fixed relaying with untrusted relays: Transmission strategy design and performance analysis

Wireless cooperative communication through link adaptation with untrusted relay assignment is considered. Using sharp channel codes in different transmission modes, reliability for destination and security in the presence of untrusted relays are provided through rate and power allocation. Scenarios with single available relay and opportunistic relaying in the presence of multiple relays are investigated. These scenarios are analyzed separately in terms of performance over Rayleigh fading channel for constant power and adaptive power transmission. In constant power case, closed form expressions for average spectral efficiency is derived through an effective approximation. Retaining the sum transmission power of source and relay unchanged, the performance of system is enhanced through power allocation. Numerical results reflect the effectiveness of the approximations for analytic performance evaluation when comparing to the simulation results.