The large amount of unlicensed and semi-unlicensed bandwidth available for millimeter (mm) wave communication enable multi-Gigabit wireless networking that can potentially transform the telecommunications landscape.
Intellectual Merit: This research investigates the use of the unlicensed 60 GHz ``oxygen absorption'' band for providing a quickly deployable broadband infrastructure based on multi-Gigabit outdoor mesh networking. Millimeter wave links are inherently directional: the directionality is required to overcome the increased path loss at higher frequencies, and is feasible for nodes with compact form factors using antenna arrays realized as patterns of metal on circuit board. This project addresses the cross-layer design of mesh networks with such highly directional links, in which implicit coordination using carrier sense mechanisms cannot be relied on, and there is no omni-directional mode for explicit coordination. In addition, the research will investigate new design principles for directional medium access control, with the challenge being to coordinate nodes despite the deafness induced by directionality, while taking advantage of the drastically reduced spatial interference. The project will also study methods for network discovery and topology updates, the interactions between scheduling and routing; and the impact of oxygen absorption on network capacity and protocol design/performance.
Broader Impact: The principal investigators will develop publicly available mm wave network simulation tool, intended to engage a larger research community in this emerging field. The investigators will also explore other mechanisms for broader impact including technology transfer, undergraduate research, and curriculum updates featuring mm wave communication.
The proliferation of smart phones and tablets has led to explosive growth in demand for mobile broadband. Industry studies project that a 1000-fold increase in the capacity of cellular networks is required in order to keep up with the growth of video-centric mobile applications (e.g., videoconferencing and entertainment on the go). The goal of this project was to explore some of the radically new design concepts required to deliver such orders of magnitude increases in cellular data capacity. We first sketch out the logic behind our research plan, and then summarize some key outcomes. Given the scarcity of wireless spectrum, orders of magnitude increases in cellular capacity can only be delivered by scaling down cell sizes so as to reuse spectrum. Thus, cell radii must shrink from kilometers down to 100s of meters, for example, with compact ``picocellular’’ base stations deployed on lampposts and rooftops in densely populated urban environments. One of the key bottlenecks to this vision, however, is backhaul: given the high data rates envisioned for fourth generation cells, the small cell base stations must be connected to the wired Internet by links with speeds of the order of Gigabits/second, but optical fiber connectivity to each lamppost would require an immense infrastructure investment. A core goal of this project was to investigate a wireless solution to this backhaul problem, with a mesh network that connects each base station to an Internet gateway with a small number of hops. In order to provide the high data rates needed, however, we avoid the overcrowded spectrum in current use, and propose use of the 60 GHz band, where there are 7 GHz of unlicensed spectrum. The characteristics of such a ``millimeter (mm) wave’’ band (so called because the wavelength is a few mm) are very different from those of today’s cellular and WiFi systems because of the order of magnitude smaller wavelength (the wavelength at 60 GHz is 5 mm, compared to the 6 cm wavelength at 5 GHz). Thus, while we can draw upon decades of wireless research, it is necessary to rethink many of the standard design prescriptions used in today’s wireless networks. We now summarize some of the key outcomes from the research carried out in this project. 1) Millimeter wave links must be highly directional in order to overcome the higher propagation loss at these frequencies. Fortunately, the small carrier wavelength also allows the realization of electronically steerable antenna arrays that can synthesize directional beams in a flexible fashion. However, the use of directional links completely changes the requirements on wireless network protocol design. We provide quantitative analysis that shows that the focus of wireless network protocols must shift from interference mitigation (interference is greatly reduced by directionality) to coordination (made difficult due to the ``deafness’’ induced by directionality). For example, the ``listen before talk’’ protocols used in today’s WiFi networks cannot be used. Based on the design guidelines from our analysis, we came up with a new network protocol that enables decentralized operation of mesh networks with highly directional links. The key ideas is to implicitly coordinate using learning and memory, and to be reactive rather than proactive in interference mitigation. 2) We also explored, using a mix of modeling, analysis and simulations, the capacity of the picocellular networks enabled by our backhaul design. While shrinking cell sizes enhances spectral reuse, a key insight from our work was that mutual interference between neighboring base stations increases, essentially because there are fewer obstacles blocking interfering signals from neighboring cells than in conventional macrocells. The key conclusion from our work was that, in order to deliver the potential capacity increases from shrinking cell sizes while using current cellular spectrum, sophisticated coordination mechanisms between neighboring base stations are required, which requires significant further design effort. 3) The analysis for picocellular networks using today’s cellular frequencies led us to contemplate a bolder approach to delivering the required 1000X capacity increases, by using mm wave transmissions directly from base station to mobile. This would make it possible to synthesize highly directional beams from base stations to mobiles, thus cutting down immensely on inter-cell interference and truly delivering on the promise of enhanced spatial reuse with small cells. A key challenge is the potential disruption of connectivity due to blockage: mm waves are easily blocked by obstacles such as human bodies, buildings, trees, and cars (``obstacles look bigger at smaller wavelengths’’). Another is the difficulty of adapting highly directional beams in the face of mobility. However, we obtained promising preliminary results towards the end of this project that indicate that it might be possible to overcome such hurdles. These results laid the groundwork for a new NSF proposal aimed at realizing the vision of 60 GHz to the mobile. This proposal has been funded, and work in this area is ongoing.
