average data service rate is defined as the average number of bits the transmitter forwards to the receiver reliably. On the other hand, the effective capacity is the maximum constant data arrival rate at the transmitter buffer that the data service process can support under prescribed QoS constraints, i.e., data buffer overflow and buffering delay violation probability constraints. In the sequel, we provide expressions for the average data service rate and the effective capacity given a general class of energy arrival and demand processes, and then exemplify our results by constant energy demand rate and fixed data transmission rate for a clear presentation of our analysis.

Average Data Service Rate {#sec:reliable_throughput}
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We assume that the transmitter sets beforehand the energy demand process, $p(i)$, considering the energy overflow and outage probability constraints. Then, it adjusts the transmission strategy over the channel, $r(i)$, to maximize the average data service rate. As long as there is no energy outage, the transmitter does not change the transmission strategy. On the other hand, when there is an energy outage, the transmitter uses all the available energy in the battery and re-adjusts the transmission rate according to the available power, again to maximize the average 