Mobile communication networks alone today consume 0.5 percent of the global energy supply. Meeting the rapidly increasing demand for more capacity in wireless broadband access will further increase the energy consumption. Operators are now facing both investing in denser and denser networks as well as increased energy cost. Traditional design paradigms, based on assumptions of spectrum shortage and high cost base station sites, have produced current cellular systems based on 3G and 4G (LTE) standards. The latter ones are characterized by very high spectrum efficiency, but low energy efficiency. Deployment has favored strategies with few high-power bases stations with complex antenna systems. The key method for indoor coverage has so far been to literally "blast signals through walls" - a solution that is neither energy-efficient nor very sound from a radiation perspective. As environmental aspects may be perceived as important from a societal perspective, the cost remains the short-to medium-term concern for operators of future mobile broadband systems. What becomes evident now is that the so far mostly neglected energy cost will be a major concern. Future system deployment has to balance infrastructure deployment, spectrum, and energy cost components. Ongoing incremental improvements in electronics and signal processing are bringing down the power consumption in the base station. However, these improvements are not enough to match the orders-of-magnitude increase in energy consumption cause by demands for more capacity. It is clear that solutions to this problem have to be found at the architectural level, not just by increasing the efficiency of individual components. In this article we propose a framework for a total cost analysis and survey some recent, more radical, "clean slate" approaches exploiting combinations of new spectrum opportunities, energy-efficient PHY layers, and novel deployment and backhauling strategies that target minimizing overall system cost. The latter involve network deployment tightly tailored to traffic requirements, using low-power micro base stations tailored specifically to decrease the power consumption compared to today's high-power macro base station schemes. To illustrate our findings, a power consumption model for mobile broadband access networks taking backhaul into account is presented, and the main trade-offs between infrastructure, energy, and spectrum costs are analyzed. We demonstrate optimal deployment strategies in some simple scenarios where a certain capacity has to be provided in a dense interference-limited scenario.
2011. Vol. 18, no 5, 18-24 p.