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Modern cellular networks commonly deploy rapid channel rate adaptation to vary the wireless capacity in response to channel conditions while maintaining a fixed target error rate (typically 1%). Although desirable in terms of throughput for inelastic applications that do not adapt to network conditions, a low fixed target error rate incurs the expense of significant power consumption, especially at high transmission rates. In this work, we show that elastic traffic, in particular TCP, benefits greatly from the perspective of power efficiency when we also incorporate target error rate adaptation. More specifically, TCP behavior, although sensitive to packet errors, is not uniformly so. When TCP has a small window, it requires extremely low packet error rates. However, for large windows, especially with a buffer, TCP can tolerate larger loss rates. The contribution of this work is in conducting a detailed and realistic investigation into how beneficial target error rate adaptation is for TCP in terms of reducing power and impact on throughput. Our work differs from past contributions in that we explicitly take into account the impact of the buffer and a variable channel. We devise simple local power-adaptation policies based on TCP behavior and study them with the help of a numerical model. Finally, we present a detailed investigation of our policies using actual modulation schemes and real channel traces collected on a commercial 1xEV-DO network. The results show that compared to the existing scheme, our policies save typically about 20% to 30% power with marginal or no reduction in throughput.
Date of Conference: 13-16 Oct. 2009