Project "NetMIMO"
Marie Curie IRG 224755

The initial cellular systems deployed in the 1980s and 1990s featured conservative frequency reuse patterns in order to ensure a high signal-to-interference-plus-noise ratio (SINR). This allowed operating the links comfortably with limited signal processing at the expense of having a small number of concurrent links. Altogether, the system spectral efficiency (bits/s/Hz/cell) was low and soon became insufficient to satisfy the explosive growth in demand for wireless communication. With the soaring cost of bandwidth and of real estate, the need for higher efficiencies became dire. Since then, we have witnessed a sustained improvement in system spectral efficiency driven, mostly, by advances in communication theory and by Moore’s law. Specifically, the successive introduction of advanced techniques (forward error correction, power control, link adaptation, incremental redundancy, etc) and the massive increases in processing power have enabled:

A progressive rise in link spectral efficiency, which in emerging systems (3GPP Long-Term Evolution, IEEE 802.16 WiMAX) is already approaching capacity.
Operation at diminishing SINRs, enabling ever more aggressive frequency reuse patterns. In fact, the aforementioned systems are reaching the point of universal frequency reuse and are thereby limited first and foremost by their own interference.
With link efficiencies approaching capacity and with frequency reuse reaching universality, this marks the end of the road for the approach followed thus far to improve the system spectral efficiency.

In recent years, the introduction of multiple-input multiple-output (MIMO) techniques has provided powerful new means for enhancing the performance of wireless systems. MIMO techniques enable spatial frequency reuse within each cell, but still subject to the high levels of interference from other cells. It is becoming increasingly clear that major new improvements in spectral efficiency will have to entail addressing such intercell interference. Such is, precisely, the premise of NetMIMO.

In the traditional modus operandi of cellular systems, a user is assigned to a cell site on the basis of certain criteria (e.g., signal strength) and it then communicates with that cell site while causing interference to all other sites in the system. The key tenet of NetMIMO is that, in the uplink specifically, intercell interference is merely a superposition of signals that were intended for other cell sites, i.e., signals that happen to have been collected at the wrong place. If these signals could be properly classified and routed, they would in fact cease to be interference and become useful in the detection of the data they bear. (A dual observation can be made about the downlink.) This insight naturally leads to the conclusion that, ultimately, the goal should be to serve all users through all the sites within their range of influence. While challenging, this is theoretically possible by virtue of the fact that the cell sites are connected by a powerful backbone network. This ambitious idea leverages the almost unlimited bandwidth available in optical-fiber wireline networks to transcend the burden of wireless intercell interference. Operationally, this can be interpreted as a form of MIMO that, through the backbone network, spans the entire system as opposed to only each cell separately. Hence the term NetMIMO. With NetMIMO, in fact, the notion of a cell gets blurred once users are no longer assigned to specific sites. Ultimately, there is a network of sites serving a population of users. While this is a conceptually simple proposition, it poses numerous hurdles and challenges that this project aims to resolve.