Fundamental limits of cooperation

Abstract

Cooperation is viewed as a key ingredient for interference/nmanagement in wireless networks. This paper shows that/ncooperation has fundamental limitations. First, it is established/nthat in systems that rely on pilot-assisted channel estimation the/nspectral efficiency is upper-bounded by a quantity that does not/ndepend on the transmit powers; in this framework, cooperation/nis possible only within clusters of limited size, which are subject/nto out-of-cluster interference whose power scales with that of the/nin-cluster signals. Second, an upper bound is also shown to exist/nif the cooperation extends to an entire (large) system operating/nas a single cluster; here, pilot-assisted transmission is necessarily/ntranscended. Altogether, it is concluded that cooperation cannot/nin general change an interference-limited network to a noiselimited/none. Consequently, existing literature that routinely assumes/nthat the high-power spectral efficiency scales with the logscale/ntransmit power provides only a partial characterization. The/ncomplete characterization proposed in this paper subdivides the/nhigh-power regime into a degrees-of-freedom regime, where the/nscaling with the log-scale transmit power holds approximately,/nand a saturation regime, where the spectral efficiency hits a ceiling/nthat is independent of the power. Using a cellular system as an/nexample, it is demonstrated that the spectral efficiency saturates/nat power levels of operational relevance.