Fundamental Limits of Cooperation

• Published in: IEEE transactions on information theory 2013-09, Vol.59 (9), p.5213-5226
• Main Authors: Lozano, Angel, Heath, Robert W., Andrews, Jeffrey G.
• Format: Article
• Language: English
• Subjects: Applied sciences; Channel estimation; Coherence; Cooperation; Cooperative systems; Detection, estimation, filtering, equalization, prediction; Electric noise; Equipments and installations; Exact sciences and technology; Fading; Information theory; Information, signal and communications theory; Interference; Mobile radiocommunication systems; Noise; Power; Radiocommunications; Receivers; Signal and communications theory; Signal, noise; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Transmitters; Wireless communications; Wireless networks
• ISSN: 0018-9448; 1557-9654
• DOI: 10.1109/TIT.2013.2253153

Cooperation is viewed as a key ingredient for interference management in wireless networks. This paper shows that cooperation has fundamental limitations. First, it is established that in systems that rely on pilot-assisted channel estimation, the spectral efficiency is upper-bounded by a quantity that does not depend on the transmit powers; in this framework, cooperation is possible only within clusters of limited size, which are subject to out-of-cluster interference whose power scales with that of the in-cluster signals. Second, an upper bound is also shown to exist if the cooperation extends to an entire (large) system operating as a single cluster; here, pilot-assisted transmission is necessarily transcended. Altogether, it is concluded that cooperation cannot in general change an interference-limited network to a noise-limited one. Consequently, the existing literature that routinely assumes that the high-power spectral efficiency scales with the log-scale transmit power provides only a partial characterization. The complete characterization proposed in this paper subdivides the high-power regime into a degree-of-freedom regime, where the scaling with the log-scale transmit power holds approximately, and a saturation regime, where the spectral efficiency hits a ceiling that is independent of the power. Using a cellular system as an example, it is demonstrated that the spectral efficiency saturates at power levels of operational relevance.