Energy Saving in BTS : The impact of SON !!

Presentation1
Many MNO’s are notorious ( among many other failings ) for their high dependency on diesel to fuel their base stations. One would think that faced with falling voice ARPU and hypothetical additional data revenue , energy expenditures would top the list to reduce OPEX since oil prices will remain stubbornly high at + $ 100 pb. Reduction in fuel OPEX requires CAPEX because it implies purchasing more energy efficient equipment or switching to renewable energy power solutions. While solar and wind remain the most prominent green technologies used to power off grid base stations , SON is another technical innovation within the 3GPP standards to save on BTS energy consumption.

So what is SON and how does it save on energy consumption ? A self-organizing Network (SON) is an automation technology designed to make the planning, configuration, management, optimization and healing of mobile radio access networks simpler and faster. SON functionality and behavior has been defined and specified in generally accepted mobile industry recommendations produced by organizations such as 3GPP and the NGMN . The first technology making use of SON features is LTE, but the technology has also been retro-fitted to older radio access technologies such as UMTS since Telcos began to understand that to meet the rising demand for data, it could be more cost-effective for them to expand HSPA and HSPA+ high-speed data capacity on the existing 3G infrastructure in many locations. SON promises enhancements in network efficiency, reductions in CAPEX and OPEX, improvements in customer experience (with potential reductions in churn).

So what are some of the benefits of SON in your BTS topology ?? A SON delivers an intelligent network where base stations self-optimize their operational algorithms and parameters in response to changes in network, traffic and environmental conditions. With operational intelligence at the access point, a SON can collect live network and call data, process it in real time, and either preview the changes or automatically deploy them live. SON offers offline planning capabilities for rapidly modeling the optimization of several parameters, including cell list additions, handover, interference control, and QoS enforcement. 3GPP Rel-11 has defined two energy saving states for a cell with respect to energy saving namely: not Energy Saving state and energy Saving state. When a cell is in an energy saving state it may need neighboring cells to pick up the load. However, a cell in energy Saving state cannot cause coverage holes or create undue load on the surrounding cells. All traffic on that cell is expected to be drained to other overlaid/umbrella cells before any cell moves to energy Saving state.

It is an indisputable fact that the traffic load in mobile networks is very unevenly distributed both over time and over cells. Excessive waste of energy occurs in low traffic situations since the radio system is optimized for maximum load. The NGMN Alliance has measured the daily average traffic distribution for an urban scenario .There is no communication activity in the cell in the two hours between approximately 04:00 and 06:00, and traffic is lower than 20 percent in the seven hours between approximately 0:00 and 07:00. So subscribers use more communication services during the day and very few in the wee hours of the morning. Knowing such usage habits is useful to determine how resources should be allocated so that the maximum amount of power can be saved. With SON base stations the network’s coverage and capacity can be optimized when SON base stations can dynamically alter parameters such as antenna tilt and reference power offsets to compensate for lapses in coverage and ensure adequate capacity where it’s needed.

Drastic improvements can be achieved by adapting to the actual traffic demand in a mobile network. The solutions include automatically switching off unnecessary cells, modifying the radio topology, and reducing the radiated power with methods such as bandwidth shrinking and cell micro-sleep. The challenge is to maintain reliable service coverage and quality of service (QoS) in the related area, while simultaneously consuming the lowest energy. The self organizing network (SON) supports proper selection of the appropriate energy saving mechanism and automatic collaborative reconfiguration of cell parameters with the neighbour cells.Mobility features like handoffs from one cell to the next can be optimized in a SON when base stations can balance load traffic among contiguous cells

Most engineers know that the amplifier power supply uses 60 percent to 80 percent of the energy consumed by base stations .If the RF power amplifier (PA) works at full power when there is no traffic or the load is very low, then power is wasted. Fortunately PA voltage can be dynamically adjusted according to the traffic load and required output power. When the output power is relatively low, the voltage required by a power amplifier is set lower than its maximum output voltage. In this way, power amplification is improved when the traffic load is light, and power consumption is reduced.In conventional amplifiers this power is independent of the amplifier input signal, i.e., of the current traffic load.

The key approach to saving energy is to make the power consumption proportional to the traffic load, either by implementing a partial shut down of amplifiers or by employing enhanced power amplifiers. During power-off of the amplifiers, further power savings can be achieved by also switching off the baseband signal processing, and indirectly, in the AC/DC power conversion and in the cooling fans.From the energy consumption point of view, low loads should be avoided. Instead, two types of mechanisms can be applied to reduce idle and unused capacities. As a first step, all energy-consuming equipment should implement power-reduction mechanisms while in operational mode, adapting to the actual load (short -term strategies). Second, the traffic should be reshuffled to a smaller number of highly loaded sectors or processing entities, and the others should be switched-off (long-term strategies). However, the coverage and the quality of service must not be degraded.

When there is no traffic, power can be saved by adjusting the PA voltage. Power can also be saved in transmitting LTE OFDM subcarriers. OFDM symbols can be automatically turned off when there is no baseband data transmission. This reduces power in the PA. Turning off PA OFDM symbols is more efficient than keeping them turned on all the time, especially when there is light or no traffic. PAs, cells, and power supply can also be intelligently turned off in the same way as OFDM symbols. A basestation’s handling of RACH ( random-access channel ) offers another optimization metric. Automatically setting up a SON base station’s RACH config parameters such as the number of preambles on a packet and ramp-up power can reduce synchronization times, call setup times, and handover delays while improving other aspects of RACH performance.

SON base stations are able to automatically configure themselves from the moment they are first powered up and before they join a wireless network. Once power is supplied, the base station would configure its physical cell identity, including its Internet Protocol (IP) address, and it would authenticate its software and configuration data. Following the completion of these baseline tasks, the SON base station would initialize the configuration of its radio by setting up its relationships with its neighboring cells and compiling its neighbor list. Based on a number of predetermined operational criteria such as energy savings, range requirements, and interference conditions, a SON base station will begin the self-optimization process once its initial configuration has been completed and it has joined the network. One of the first optimization tasks it will undertake will be to dynamically prune and select the base stations that are on its neighbors list.

And this is where it really counts !!! A SON base station can save up to 40% of power usually consumed.

The combination of dynamic PA voltage adjustment and intelligent turning off of OFDM symbols is unique in the industry and can save about 32% of power consumption. Suppose the average power consumed by each base station is 1500 W (configured with three sectors). A single station can save up to 5200 kWh each year. This means more than 5.2 million kWh can be saved for a network with 1000 base stations each year, which is a saving of 1730 tons of standard coal and a reduction of 4500 tons of carbon dioxide a year. If base station power consumption is reduced, then less auxiliary power and heat-dissipation devices are required and less network OAM is necessary. The power needed for these devices is also reduced. New energy sources such as solar, wind, and bioenergy can be used in conjunction with these innovative energy-saving technologies. In this way, network energy consumption can be cut by + 50%.

The recent deployment of LTE to address the growing data capacity crunch, has highlighted the need and value of self-organizing capabilities within the network that permits reductions in operational expenses (OPEX) during deployment as well as during continuing operations. Self-optimizing capabilities in the network will lead to higher end user Quality of Experience (QoE) and reduced churn, thus allowing for overall improved network performance. Self-Organizing Networks (SON) improve network performance, but in no way replace the wireless industry’s important need for more spectrum to meet the rising mobile data demands from subscribers !!

Sadiq Malik ( Telco Strategist )

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