Green Base Station Solutions and Technology

Zhiping Chen and Licun Wang

(ZTE Corporation)

Abstract:Base station power consumption is the biggest power issue concerning wireless networks.Saving power in base stations is therefore the primary focus in green wireless network development.This paper discusses green base stations in terms of system architecture,base station form,power saving technologies,and green technology applications.It explores effective ways of reducing power consumption in base stations.

Keyw ords:software defined radio;baseband unit;remote radio unit;high efficiency power amplifier;green power

E nvironmental protection is a globalconcern,and for telecom operators and equipment vendors worldwide,developing green,energy-saving technologies for wireless communications is a priority.A base station is an important element of a wireless communications network and often the main focus of power saving in the whole network.

In a wireless communications network,the base station should maintain high-quality coverage.It should also have the potential for upgrade or evolution.As network traffic increases,power consumption increases proportionally to the number of base stations.However,reducing the number of base stations may degrade network quality.So green base stations are proposed.Akey issue is how to save energy and reduce power consumption while guaranteeing service and coverage for users and ensuring the base station is capable of evolution.

This paper discusses green base stations in terms of system architecture,base station form,key power-saving technologies,and green technology applications.It aims to find an effective approach to power saving.

1 Multimode Wireless Base Station System

A wireless mobile network is a sophisticated network often with several generations of a system and different frequency bands in coexistence.It contains a 2G system—represented by GSM—and a 3G system—represented by Universal Mobile Telecommunications System(UMTS).Long Term Evolution(LTE)technology has begun to be commercially used.Each generation has its own communications equipment,including a complete set of network elements from base station to core network.For multiple systems to coexist,the traditionalnetwork construction approach has been to add new generation devices into the existing network.With more new generation technologies being employed,the number of network devices and auxiliary devices has increased significantly,and this has led to a dramatic increase in network power consumption.Therefore,to reduce power consumption and guarantee QoSin the context of multi-networks,the current network construction approach must be changed.

Based on Software Defined Radio(SDR)technology,the SDRsoft base station platform is designed to improve the base station system architecture[1],[2].The biggest difference between a traditional base station and an SDRsoft base station is that the Radio Frequency Unit(RU)of the soft base station is capable of software programming and redefining.So an SDRsoft base station can intelligently allocate spectrum and support several standards.By employing broadband multicarrier digital signal processing technology and software configuration,the Radio Frequency(RF)module of the soft base station can support GSM,UMTS,and LTE,as well as transmit and receive multimode RFsignals.The baseband module of the soft base station adopts Micro Telecommunications Computing Architecture(MicroTCA)and is characterized by small size,low power consumption,modularization,and good scalability.It can also process baseband signals of several modes,including GSM,UMTS,and LTE.

The SDRsoft base station platform enables a telecom operator to combine networks of different modes and different bands into one network.It simplifies network structure and greatly decreases the number of Network Elements(NEs)and auxiliary facilities,thus reducing power consumption base station power consumption.

The 2G/3G swapping project of a leading telecom operator in Asia-Pacific is a good example of how power consumption can be reduced using the SDRsoft base station platform.In the old network,one base station used three cabinets for GSM900,GSM1800,and UMTS2100 devices.Its overall power consumption was 4280 W.After the old base station was swapped with SDR,UMTS900 system was included and power consumption decreased by 57%.In addition,power used by the air conditioner was also reduced because there was less equipment in the room.The SDRbase station was effective in saving power.

2 Distributed Base Station and Super Baseband Pool

The modular design of an SDRsoft base station allows innovation on the base station's form.Two innovative forms are distributed base station and super baseband pool.In distributed base station,the Base Band Unit(BBU)is separated from the Remote Radio Unit(RRU),making network deployment more flexible.Asuper baseband pool allows baseband resources to be reused and shared and network resources to be used more efficiently.

2.1 Distributed Base Stations

The SDRsoft base station platform retains the rack structure form of traditionalindoor and outdoor base stations.But it can also launch distributed base stations with BBU and RRU deployed at different places and connected with optical fiber[2],[3].

The RRU can be installed on a roof or in an iron tower and connected to the antenna with a jumper several meters long instead of a feeder cable(which is often tens of meters long).Consequently,the cost of excessive feeder cable is reduced.The Power Amplifier(PA)is also required to output less power,and power of the RRU is saved.With decreased power consumption,the RRU can be ventilated in a natural way,saving on air conditioners or fans.As a result,power consumption and noise of auxiliary facilities is also reduced.The BBU can be inserted into a traditional rack or installed on the wall or on a support to save space.This cuts the cost of land acquisition,equipment room construction,and air conditioning.

▲Figure 1.Wireless access network based on super baseband pool.

2.2 Super Baseband Pool

With the BBU and the RRU separate,multiple BBUs can be placed together and connected to their RRUs with optical fibers.These BBUs form a baseband poolso that baseband resources can be shared,scheduled,and controlled in a centralized way.This solution is called super baseband pooling.

Super baseband pooling changes the form of base station greatly,allowing the baseband's processing capability to be centralized,shared,and virtualized[4].Processing devices in the baseband pool can be dynamically scheduled to process baseband signals of different RRUs.This enables the base station to adapt to the tidal effect of mobile communications systems and maximize utilization of baseband resources.The RRUcan be deployed near the terminal user.This reduces the transmit power on the network and user sides without affecting coverage and reduces power consumption of the whole wireless access network.By centralizing scheduling and control,this solution(Fig.1)considerably reduces the need for base station equipment rooms,and allows facilities,power supply,and transmission to be shared to the greatest possible extent.

3 Power SavingTechnologies for Green Base Stations

Innovation in SDRbase station architecture and form brings about greater efficiency in mobile network construction,more reasonable configuration of devices,and maximum sharing of infrastructure and auxiliary devices.Hence,resources are saved.High efficiency PA and intelligent power saving technologies can reduce overall power consumption of the base station even further[5]-[9].

3.1 High Efficient Power Amplifier

Among all components in a base station,the RFunit consumes the most power.And within the RFunit,the PA consumes the most power,accounting for about 80%of the RF's total power consumption.Decreasing power consumption in a base station reduces the heat generated by equipment and the amount of air conditioning needed for cooling.Therefore,improving the PA's efficiency reduces power consumption of the main devices in a base station.

?Figure 2.Technicalprinciple of D-PT.

When designing high efficiency PAs,circuit application,component selection,and process innovation must be taken into account.PAs have developed from the expensive feed-forward linear type and class AB high powered type to Doherty Pas,which come with Digital Pre-Distortion(DPD)technology.The PA chip has also developed,from Laterally Diffused Metal Oxide Semiconductor(LDMOS)and GaN to High Voltage Heterojunction Bipolar Transistor(HVHBT).PA efficiency has increased from less than 10%to 45%and is now targeted for over 50%.Currently,amplifiers combining DPD and Doherty dominate in base stations in wireless communications systems.

The demand for continuous improvement in efficiency drives the development of PA technologies.New PA technologies include Envelope Tracking(ET)PA and digital switch PA.

3.2 Intelligent Power Saving Technology

The mobility of mobile users means base station traffic varies dramatically from hour to hour.Intelligent power saving technologies are designed to assess traffic in real-time and take power saving measures based on assessment results.Idle resources can be made dormant by intelligent carrier adjustment or intelligent channel power-off.The PA's power supply can be configured by intelligent PA control or dynamic voltage scaling.Here,dynamic voltage scaling and intelligent carrier adjustment are analyzed.

(1)Dynamic Voltage Scaling Dynamic voltage scaling is also called Dynamic Power Tracking(D-PT).D-PTinvolves intelligently controlling the PA's power supply by tracing load changes and using variable voltage.Fig.2 shows the technical principle of dynamic voltage scaling.When the power output of the PA is high,voltage provided to the PA is large.When the output power falls below a certain threshold,the voltage is lowered too.This ensures the PA works at its best under different power loads and saves power in different configurations.An intelligent and highly efficient power supply system can adjust the power supply as the load changes.When the load is at maximum,power efficiency is as high as 92%.Generally speaking,a base station's power consumption can be reduced by 12%using this technology.

(2)Intelligent Carrier Adjustment The load of a base station dynamically changes—traffic in peak hours differs greatly from that at other times.In a base station,the number of carriers is usually configured according to peak hour traffic.As a result,in idle hours,the power of some carriers is used in control channels rather than in traffic channels,leading to very low power utilization.

Intelligent carrier adjustment enables a base station to dynamically adjust the number of active carriers based on traffic,and inactive carriers are shut down to reduce power overhead.In an S222 configuration,this technology can reduce base station power consumption by about 40%in idle hours.

4 Application of Green Base Station Technologies

Temperature control devices such as air conditioners are accountable for a considerable amount of power consumption in a base station.Before green base stations are put into use,one of the key issues is how to effectively save power in temperature control system.Solar and wind is now being widely applied in the telecommunication industry to power base stations.

4.1 Intelligent Temperature Control System in the Equipment Room

Fans in wireless devices and air conditioners in equipment rooms consume a lot of energy in order to create a favorable working environment for system devices.An effective way to save energy is to reduce power consumption of fans and air conditioners.

Automatic Control Systems(ACS)can be used for this purpose.Temperature sensors are installed indoors and outdoors to measure temperatures,and then the indoor temperature is controlled with natural wind.Only when the difference between indoor and outdoor temperatures is small and the indoor temperature exceeds a predefined threshold does the ACSrun the air conditioner.The structure of the ACSis illustrated in Fig.3.

ACScan be used independently or with the air conditioner.By taking advantage of natural conditions,the temperature of a base station can be adjusted in all weather.This intelligent temperature control system uses fans instead of air conditioners for ventilation for about 80%of the year.As a result,the annual running time of an air conditioner is greatly reduced.Compared with a traditional equipment room,an ACS-cooled room can save up to 70%energy.

?Figure 3.Automatic control system for equipmentroomtemperature.

?Figure 4.Hybrid power supply solution with wind and solar energy.

4.2 Power Supply Solution with Green Energy

Asharp decrease in power consumption in a base station makes it possible to replace the traditional electricalpower supply with solar or wind energy.Among other solutions,solar and hybrid solar-wind power has gradually been applied in base stations.Solar and wind generated power is clean,inexhaustible,and cheap.Long-term benefit can be gained from an initial investment,and so it conforms to the global trend towards power saving and emissions reduction.However,these solutions depend on favourable weather conditions.For solar energy,the average daily solar irradiance must reach 4 KWh/m and for wind power,wind speed must be at least 3.5 m/s to make the turbine work.

At present,hybrid wind and solar energy is the most feasible green power solution(Fig.4).During the day,a solar panel and wind turbine provide power to base station equipment;while at night,equipment is powered by wind and batteries.When there is neither wind nor sunshine,the batteries are used.Wind and solar devices can be flexibly configured based on site situations.In principle,the ratio of wind energy to solar energy can vary from 2:8 to 5:5.But in any case,power supplied using wind cannot exceed 50%of the total power supply.

5 Conclusions

The green base station solution involves base station system architecture,base station form,power saving technologies,and application of green technologies.Using SDR-based architecture and distributed base stations is a different approach to traditional multiband multimode network construction.Using this approach,power consumption in the network can be reduced greatly.Advancement of PAtechnologies and the introduction of intelligent power saving also improve resource utilization and reduces emissions.Intelligent temperature control and new energy sources make wireless base stations greener.Although reducing power consumption and emissions in a wireless network requires various power saving means and technologies,technical updates and innovations in the base station itself are key for green base stations.

SDR-based BBU+RRU base stations have been deployed globally,and with their environmental protection features,have won the trust of global users.However,innovation in green base stations has not come to an end.Continuous improvement in wireless communications network will bring a better future.