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Abstract : |
This paper explores a spectrally ecient, variable rate transmission scheme under a service outage constraint in a block fading channel. The service outage constraint is motivated by real time variable rate applications as well as the coverage versus capacity tradeos in cellular systems. In a service outage, the transmission code rate is below a given basic rate and may in fact be zero. Unlike an information outage in the capacity versus outage problem, asymptotically error free communication is maintained during a service outage by employing a transmission code rate equal to the instantaneous mutual information. We solve the problem of maximizing the time average information rate subject to the probability of a service outage being suciently small. When the service outage constraint is feasible, the optimum power policy is shown to be a combination of water lling and channel inversion allocations. The optimum power allocations for delay limited capacity, capacity versus outage, and ergodic capacity are special cases of this optimum scheme. Numerical results for a Rayleigh fading channel are given. In addition, coverage versus capacity tradeos are examined for geographic attenuation due to path loss. Key Words: Service outage, adaptive transmission, block-fading channel, real time applications, spectral eciency, |
