RENEWABLE ENERGY BASE STATION AND COVERAGE ADJUSTMENT METHOD THEREFOR, AND WIRELESS CELLULAR SYSTEM CLAIM OF PRIORITY The present application claims priority from Chinese patent application No. 20 13 10075703.6 filed on March 1 1, 20 13, the content of which is hereby incorporated by reference into this application. BACKGROUND OF THE INVENTION 10 This invention relates to a renewable energy base station and a coverage adjustment method therefor, and a wireless cellular system including the renewable energy base station. In a wireless cellular system including a plurality of base stations, as a way to reduce the energy consumption of the base station, there has been 15 proposed a technology to adjust the coverage area of the base station (also referred to as "cell zooming"), adjusting the base station coverage area in accordance with the traffic variation and the user quality of service (QoS) requirement. Adjustment to the coverage area of the base station is usually 20 achieved by adjusting the transmitting power of the base station. When it is assumed that the user is always connected to the base station having the strongest signal, adjusting the transmitting power can then change the user's connection condition. When a base station detects that the outage probability of the base station is relatively high, the base station reduces the 25 transmitting power, and requests its neighboring base station(s) to enhance the power in order to share the load pressure or the energy consumption pressure, until the outage probability is less than a given threshold value. In recent years, renewable energy base stations utilizing solar and wind energies have been increasingly used in wireless cellular systems. Compared with the previous ordinary base station, the renewable energy base station has a renewable energy power supply system. Take a renewable energy base station utilizing the solar energy as an example, the renewable energy power supply system in the base station collects the solar 5 energy through solar cell arrays and solar charge controllers, and stores the collected energy into a storage battery, and further supplies power to the main body of the base station through the storage battery. In addition, the renewable energy base station commonly used can be supplied with power by the power grid like the ordinary base station, but in order to save energy 10 consumption, the renewable energy base station preferably uses the electric power stored in the storage battery. Only when the electric power of the storage battery is insufficient to meet the user requirement, the power grid is enabled to supply the commercial power for the supplement. However, because power generation utilizing the solar and wind 15 energies is unsustainable, the energy condition of the renewable energy base station may not necessarily be stable. Take the solar energy as an example, when the sunlight beats down directly, the rate of collecting the energy is relatively high, and the electric power of the storage battery can be maintained at a relatively high level. When the sunlight is blocked due to a 20 weather condition, however, the rate of collecting the energy is low, and the electric power of the storage battery may be in a relatively low state. Based on the above-mentioned characteristics of the energy state of the renewable energy base station, it is not enough to only consider as usual the outside factors of the base station, such as the traffic variation and the 25 user QoS requirement, in adjusting the coverage area of the base station in the wireless cellular system, but the energy state of the renewable energy base station (i.e. the electric power of the storage battery) also needs to be taken into consideration. SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an object of this invention is to provide a renewable energy base station and a coverage adjustment method therefor, and a wireless cellular system including the 5 renewable energy base station, which are capable of adjusting a coverage area of the base station by taking into consideration an energy state of the renewable energy base station, improving a utilization rate of the renewable energy in a wireless cellular system, and reducing an energy consumption of the whole wireless cellular system. 10 In order to achieve the above-mentioned object, according to one embodiment of this invention, there is provided a renewable energy base station, which is used in a wireless cellular system, including: an adjustment request determining part for determining whether an adjustment to a coverage area of the renewable energy base station is required based on 15 at least one of network resource usage information of the renewable energy base station and storage battery electric power information of the renewable energy base station; an adjustment request transceiver in case where it is determined that the adjustment to the coverage area of the renewable energy base station is required, sending a coverage area adjustment request to 20 neighboring base stations in the wireless cellular system, and receiving an adjustment request response from the neighboring base stations; an adjustment request responding part for determining whether the coverage area adjustment request from the one of the neighboring base stations is allowed based on at least one of the network resource usage information and 25 the storage battery electric power information of the renewable energy base station in case where receiving the coverage area adjustment request from one of the neighboring base stations,; and a response transmitter for sending the adjustment request response to each of the neighboring base stations based on a result of the determination made by the adjustment request responding part. Further, according to one embodiment of this invention, there is provided a coverage adjustment method for a renewable energy base station, 5 which is performed in a wireless cellular system including a renewable energy base station, the coverage adjustment method including: an adjustment request determining step of determining, by the renewable energy base station, whether an adjustment to a coverage area of the renewable energy base station is required based on at least one of network 10 resource usage information of the renewable energy base station and storage battery electric power information of the renewable energy base station; an adjustment request receiving and sending step of in case where it is determined that the adjustment to the coverage area of the renewable energy base station, the renewable energy base station sending a coverage area 15 adjustment request to neighboring base stations in the wireless cellular system and receiving an adjustment request response from the neighboring base stations; an adjustment processing determining step of determining, by the renewable energy base station, whether the coverage area is able to adjust based on the adjustment request response from the each of the 20 neighboring base stations; and an adjustment processing step of determining, by the renewable energy base station, in case where it is determined that the coverage area is able to adjust, at least one of an adjustment amount to the coverage area of the renewable energy base station and an adjustment amount to the coverage area of the neighboring 25 base stations, and collaborating with the neighboring base stations to perform adjustment processing to the coverage areas. Further, according to one embodiment of this invention, there is provided a wireless cellular system, including a renewable energy base station and neighboring base stations adjacent to the renewable energy base station, which use renewable energy, the renewable energy base station including: an adjustment request determining part for determining whether an adjustment to a coverage area of the renewable energy base station is required based on at least one of network resource usage information of the 5 renewable energy base station and storage battery electric power information of the renewable energy base station; and an adjustment request transceiver for in case where it is determined that the adjustment to the coverage area of the renewable energy base station is required, sending a coverage area adjustment request to each of neighboring base stations in the wireless 10 cellular system, and receiving an adjustment request response from the neighboring base stations, the each of the neighboring base stations including: an adjustment request responding part for determining whether the coverage area adjustment request from the renewable energy base station is allowed based on at least one of the network resource usage 15 information of the neighboring base stations and the storage battery electric power information of the each of the neighboring base stations; and a response transmitter for sending the adjustment request response to the renewable energy base station based on a result of the determination made by the adjustment request responding part. 20 According to the renewable energy base station, the coverage adjustment method for a renewable energy base station, and the wireless cellular system including the renewable energy base station, because the adjustment to the coverage area of the base station takes into consideration the energy state of the renewable energy base station, the utilization rate of 25 the renewable energy in the wireless cellular system is improved, thereby the energy consumption of the whole wireless cellular system is reduced. BRIEF DESCRIPTION OF THE DRAWINGS The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: FIG. 1 is an explanatory diagram illustrating a principle of adjustment to the coverage area of a base station according to a first embodiment of this invention; FIG. 2 is an explanatory diagram illustrating a principle of adjustment to the coverage area of the base station according to a second embodiment of this invention; FIG. 3 is an explanatory diagram illustrating a principle of adjustment to the base station according to a third embodiment of this 10 invention; FIG. 4 is an explanatory diagram illustrating a principle of adjustment to the base station according to a fourth embodiment of this invention; FIG. 5 is a diagram illustrating the functional configuration of a 15 renewable energy base station; FIG. 6 is a flowchart illustrating the adjustment to the coverage area of a renewable energy base station; FIG. 7 is a flowchart illustrating processing of a renewable energy base station after receiving a coverage area adjustment request; 20 FIG. 8 is a flowchart illustrating processing of an ordinary base station after receiving a coverage area adjustment request; and FIG. 9 is a sequence diagram illustrating a timing of an adjustment processing to the coverage area of the base station of a wireless cellular system. 25 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Firstly, a meaning of an outage probability of a renewable energy base station and a calculation method therefor are described. The outage probability (outage) of a base station is defined as a statistical probability that a user quality of service (QoS) is not met in the coverage area of the base station. The outage occurs generally because, under the condition of a given frequency band of a base station, the user quality of service requirement exceeds the service capacity of the base 5 station. In case where a renewable energy base station has insufficient energy, the renewable energy base station closes a part of the frequency band, resulting in a decrease of the service capacity of the renewable energy base station. The energy of the renewable energy base station therefore also becomes a factor contributing to the occurrence of the outage. That is to 10 say, for a renewable energy base station, the outage may occur due to overloading or insufficient energy, and hence the outage probability is a function based on load and energy. For example, in the case of a given service rate (bandwidth) requirement, the outage probability of the renewable energy base station can 15 be calculated by the following equation ( 1). Pout=Pr(rlo/o), it is determined that an adjustment to the coverage area of the local base station is required. When the adjustment request determining part 2 determines that an 25 adjustment to the coverage area of the local base station is not required (Step S 12, "NO"), the processing ends. Normally, each of base stations has a neighboring base station information table 10, and updates the information of each of neighboring base stations through periodic information exchange. When the adjustment request determining part 2 determines that an adjustment to the coverage area of the local base station is required (Step S12, "YES"), the adjustment request transceiver 3 sends a coverage area adjustment request to each of neighboring base stations stored in the neighboring base station information 5 table 10 and receives an adjustment request response from each of the neighboring base stations (Step S13). The processing after each of the neighboring base stations receives the coverage area adjustment request is described later with reference to FIG. 7 and FIG. 8. Then, the adjustment processing determining part 4 determines 10 whether the coverage area can be adjusted (Step S14). The adjustment request responses can be divided into two kinds of responses, specifically, a positive response allowing the adjustment to the coverage area and a negative response denying the adjustment to the coverage area, and the positive response includes at least the network resource usage information 15 that can be provided by the neighboring base station. Under the condition that the number of neighboring base stations allowing the adjustment request is a predetermined number or more (e.g., two or more) (Step S14, "YES"), the adjustment processing part 5 determines an adjustment amount of the coverage area of the local base station and an 20 adjustment amount of the coverage area of each of the neighboring base stations, and collaborates with each of the neighboring base stations to carry out the adjustment processing to the coverage area of the base stations (Step S15). Detailed operations of the adjustment processing part 5 of the local base station and each of the neighboring base stations are described later 25 with reference to FIG. 9. Next, after the coverage area adjustment processing is completed, the user requirement checking part 6 determines whether the coverage area (transmitting power) after the adjustment processing can meet the user requirement (Step S16). Under the condition that it is determined that the user requirement can be met (Step S16, "YES"), the processing ends. Under the condition that it is determined that the user requirement is not met (Step S16, "NO"), the power grid power supplying part 7 calculates the electricity demand except the power supplied by the storage battery, and 5 supplies power from the power grid to the renewable energy base station based on the electricity demand (Step S18), and then the whole processing ends. On the other hand, in Step S14, when the number of neighboring base stations allowing the adjustment request is less than the predetermined 10 number (Step S 14, "NO"), it is once again determined whether the electric power of the storage battery of the renewable energy base station is less than the above-mentioned second threshold value (Step S17). When the electric power of the storage battery of the renewable energy base station is less than the second threshold value, e.g., less than 20% of the total electric power of 15 the storage battery (Step S17, "YES"), the power grid power supplying part 9 enables the power grid to supply power to the renewable energy base station (Step S18), and then the whole processing ends. On the other hand, when the electric power of the storage battery of the renewable energy base station is the second threshold value or more (Step S 17, "NO"), then after a certain 20 period, the processing following Step S 12 is carried out again. FIG. 7 is a flowchart illustrating processing of a renewable energy base station after receiving a coverage area adjustment request. After receiving the coverage area adjustment req~~esftro m a neighboring renewable energy base station (Step S21), the adjustment 25 request responding part 11 of the renewable energy base station determines whether the coverage area adjustment request from the neighboring base station is allowed based on the network resource usage information and the storage battery electric power information of the local base station (Step S22). Specifically, a fourth threshold value for determining whether the network resource is sufficient and a fifth threshold value for determining whether the electric power of the storage battery of the base station is sufficient can be predetermined, and under the condition that the amount of 5 the network resource of the local base station used is smaller than the fourth threshold value (e.g., the network resource occupied is smaller than 60%) and the electric power of the storage battery of the local base station is larger than the fifth threshold value (e.g., the remaining electric power of the storage battery is larger than 50%), it is determined that the coverage area 10 adjustment request from the neighboring base station is allowed. The fourth threshold value and the fifth threshold value can be specific numerical values, or ratios in the form of percentage, etc. Under the condition that it is determined that the coverage area adjustment request is allowed (Step S22, "YES"), the response transmitter 12 15 sends a positive response allowing the coverage area adjustment (Step S23). The positive response includes at least the network resource usage information that can be provided by the neighboring base station. Under the condition that it is determined that the coverage area adjustment request is not allowed (Step S22, "NO"), the response transmitter 12 sends a negative 20 response denying the coverage area adjustment (Step S24). The processing ends. FIG. 8 is a flowchart illustrating processing of an ordinary base station (base station that does not utilize renewable energy) after receiving a coverage area adjustment request. 25 Different from Step S22 of FIG. 7, in Step S25 of FIG. 8, the ordinary base station determines whether the coverage area adjustment request from the neighboring base station is allowed based on only the network resource usage information of the local base station. Other operations are the same as the operations of FIG. 7, and thus the repeated description is omitted herein. Further, in order to improve the utilization rate of the renewable energy in the whole wireless cellular system, it is preferred that the renewable energy base station requesting the adjustment to the coverage 5 area preferentially select a renewable energy base station(s) with sufficient electric power to carry out the subsequent operation of increasing the coverage area. FIG. 9 is a diagram illustrating the timing sequence of adjustment processing to the coverage area of the base station of a wireless cellular 10 system. The operations of the adjustment processing part 5 of the local base station and each of the neighboring base stations are below described with reference to FIG. 9. As illustrated in FIG, 9, the adjustment processing part 5 of the local base station determines an adjustment amount to the coverage area of the 15 local base station and an adjustment amount to the coverage area of each of the neighboring base stations based on the network resource usage information each of the neighboring base stations is able to provide, and then sends the adjustment amount to the coverage area of each of the neighboring base stations to each of the neighboring base stations. For 20 example, under the condition that four neighboring base stations each can provide a bandwidth of 10 M or more, the adjustment processing part 5 of the local base station determines to decrease the coverage area (transmitting power) of the local base station by an amount corresponding to the bandwidth of 40 M, and to increase the coverage area of each of four 25 neighboring base stations by an amount corresponding to the bandwidth of 10 M, and notifies each of the neighboring base stations of the adjustment amount. After receiving the respective adjustment amount, the adjustment processing part 5 of each of the neighboring base stations adjusts the respective coverage area based on the corresponding adjustment amount. After the adjustment to the coverage area is completed, each of the neighboring base stations notifies the local base station that the adjustment has been completed. 5 Then, after receiving the notifications that the adjustment has been completed from all of the neighboring base stations, the adjustment processing part 5 of the local base station adjusts the coverage area of the local base station, based on the adjustment amount to the coverage area of the local station as determined previously. The coverage area adjustment 10 processing of the base stations including the local base station is thus completed. Specifically, the adjustment processing part 5 determines the adjustment amount to the coverage area of the local base station and the adjustment amount to the coverage area of each of the neighboring base 15 stations based on the obtained network resource usage information each of the neighboring base stations is able to provide, and then collaborates with each of the neighboring base stations to carry out the adjustment to the coverage area. For example, under the condition that four neighboring base stations each can provide a bandwidth of 10 M or more, it is determined to 20 decrease the coverage area (transmitting power) of the local base station by the amount corresponding to the bandwidth of 40 M, and to increase the coverage area of each of four neighboring base stations by the amount corresponding to the bandwidth of 10 M. This invention is not limited to the above-described embodiments but 25 includes various modifications. The above-described embodiments are explained in details for better understanding of this invention and are not limited to those including all the configurations described above. A part of the configuration of one embodiment may be replaced with that of another embodiment; the configuration of one embodiment may be incorporated to the configuration of another embodiment. A part of the configuration of each embodiment may be added, deleted, or replaced by that of a different configuration. For example, in addition to the network resource usage information 5 each of the neighboring base stations is able to provide, the positive response in Step S23 described above may further include the storage battery electric power information of each of the neighboring base stations. When there are relatively more neighboring base stations sending the positive responses to the local base station, the neighboring base station(s) 10 with more electric power can be selected to carry out the subsequent operation of increasing the coverage area based on the storage battery electric power information of each of the neighboring base stations received. In this manner, the electric power of the renewable energy can be utilized more reasonably. 15 The above-described configurations; functions, processing modules, and processing means, for all or a part of them, may be implemented by hardware: for example, by designing an integrated circuit. The above-described configurations and functions may be implemented by software, which means that a processor interprets and executes programs 20 providing the functions. The information of programs, tables, and files to implement the functions may be stored in a storage device such as a memory, a hard disk drive, or an SSD (a Solid State Drive), or a storage medium such as an IC card, or an SD card. The drawings shows control lines and information lines as considered necessary for explanation but do not show all control lines or information lines in the products. It can be considered that almost of all components are actually interconnected. WHAT IS CLAIMED IS: 1. A renewable energy base station, which is used in a wireless cellular system, comprising: an adjustment request determining part for determining whether an 5 adjustment to a coverage area of the renewable energy base station is required based on at least one of network resource usage information of the renewable energy base station and storage battery electric power information of the renewable energy base station; an adjustment request transceiver in case where it is determined that 10 the adjustment to the coverage area of the renewable energy base station is required, sending a coverage area adjustment request to neighboring base stations in the wireless cellular system, and receiving an adjustment request response from the neighboring base stations; an adjustment request responding part for determining whether the 15 coverage area adjustment request from the one of the neighboring base stations is allowed based on at least one of the network resource usage information and the storage battery electric power information of the renewable energy base station in case where receiving the coverage area adjustment request from one of the neighboring base stations; and 20 a response transmitter for sending the adjustment request response to the neighboring base stations based on a result of the determination made by the adjustment request responding part. 2. The renewable energy base station according to claim 1, wherein at 25 least one of in case where a network resource of the renewable energy base station is insufficient and in case where the electric power of the storage battery of the renewable energy base station is insufficient, the adjustment request determining part determines that the adjustment to the coverage area of the renewable energy base station is required. 3. The renewable energy base station according to claim 1, wherein: the network resource usage information includes information indicating a used amount of a network resource of the renewable energy 5 base station, and the storage battery electric power information includes information indicating a remaining electric power of the storage battery of the renewable energy base station; and at least one of in case where the used amount of the network resource of the renewable energy base station is higher than a 10 predetermined first threshold and in case where the remaining electric power of the storage battery of the renewable energy base station is lower than a predetermined second threshold, the adjustment request determining part determines that the adjustment to the coverage area of the renewable energy base station is required. 15 4. The renewable energy base station according to claim 1, wherein the adjustment request determining part calculates an outage probability of the renewable energy base station based on the network resource usage information and the storage battery electric power information of the 20 renewable energy base station, and determines that the adjustment to the coverage area of the renewable energy base station is required when the outage probability is higher than a predetermined third threshold value. 5. The renewable energy base station according to claim 1, wherein in 25 case where a network resource of the renewable energy base station is sufficient and the electric power of the storage battery of the renewable energy base station is sufficient, the adjustment request responding part determines that the coverage area adjustment request from the one of neighboring base stations is allowed. 6. The renewable energy base station according to claim 1, wherein: the network resource usage information includes information indicating a used amount of a network resource of the renewable energy 5 base station, and the storage battery electric power information includes information indicating a remaining electric power of the storage battery of the renewable energy base station; and in case where the used amount of the network resource of the renewable energy base station is lower than a predetermined fourth 10 threshold value and the remaining electric power of the storage battery of the renewable energy base station is higher than a predetermined fifth threshold value, the adjustment request responding part determines that the coverage area adjustment request from the one of neighboring base stations is allowed. 15 7. The renewable energy base station according to claim 1, further comprising: an adjustment processing determining part for determining whether the coverage area is able to adjust based on the adjustment request 20 response from the neighboring base stations; and an adjustment processing part for determining, in case where it is determined that the coverage area is able to adjust, at least one of an adjustment amount to the coverage area of the renewable energy base station and an adjustment amount to the coverage area of each of the 25 neighboring base stations, and collaborating with the adjustment processing part of the neighboring base stations to perform adjustment processing to the coverage areas. 8. The renewable energy base station according to claim 7, wherein the adjustment processing part notifies the neighboring base stations of the determined adjustrnent amount to the coverage area of the each of the neighboring base stations, and decreases the coverage area of the renewable 5 energy base station based on the determined adjustment amount to the coverage area of the renewable energy base station after the coverage area of the each of the neighboring base stations is increased based on the received adjustment amount to the coverage area of the each of the neighboring base stations. 9. The renewable energy base station according to claim 7, wherein the adjustment processing part increases the coverage area of the renewable energy base station based on the adjustment amount to the coverage area of the renewable energy base station received from the neighboring base 15 stations. 10. A coverage adjustment method for a renewable energy base station, which is performed in a wireless cellular system including a renewable energy base station, the coverage adjustment method including: an adjustment request determining step of determining, by the renewable energy base station, whether an adjustrnent to a coverage area of the renewable energy base station is required based on at least one of network resource usage information of the renewable energy base station and storage battery electric power information of the renewable energy base station; an adjustment request receiving and sending step of in case where it is determined that the adjustment to the coverage area of the renewable energy base station, the renewable energy base station sending a coverage area adjustment request to neighboring base stations in the wireless cellular system and receiving an adjustment request response from the neighboring base stations; an adjustment processing determining step of determining, by the renewable energy base station, whether the coverage area is able to adjust 5 based on the adjustment request response from the neighboring base stations; and an adjustment processing step of determining, by the renewable energy base station, in case where it is determined that the coverage area is able to adjust, at least one of an adjustment amount to the coverage area of 10 the renewable energy base station and an adjustment amount to the coverage area of the neighboring base stations, and collaborating with the neighboring base stations to perform adjustment processing to the coverage areas. 15 11. The coverage adjustment method for a renewable energy base station according to claim 10, wherein the adjustment request determining step includes step of determining, by the renewable energy base station, that the adjustment to the coverage area of the renewable energy base station is required at least one of in case where a network resource of the renewable 20 energy base station is insufficient and in case where the electric power of the storage battery of the renewable energy base station is insufficient. 12. The coverage adjustment method for a renewable energy base station according to claim 10, wherein: 25 the network resource usage information includes information indicating a used amount of a network resource of the renewable energy base station, and the storage battery electric power information includes information indicating a remaining electric power of the storage battery of the renewable energy base station; and the adjustment request determining step includes a step of determining, by the renewable energy base station, that the adjustment to the coverage area of the renewable energy base station is required at least one of in case where the used amount of the network resource of the 5 renewable energy base station is higher than a predetermined first threshold and in case where the remaining electric power of the storage battery of the renewable energy base station is lower than a predetermined second threshold. 10 13. The coverage adjustment method for a renewable energy base station according to claim 10, wherein the adjustment request determining step includes a step of calculating, by the renewable energy base station, an outage probability of the renewable energy base station based on the network resource usage information and the storage battery electric power 15 information of the renewable energy base station, and determining that the adjustment to the coverage area of the renewable energy base station is required when the outage probability is higher than a predetermined third threshold value. 14. The coverage adjustment method for a renewable energy base station according to claim 10, further including: an adjustment request responding step of determining, by the each of the neighboring base stations whether the coverage area adjlxstment request from the renewable energy base station is allowed based on the network 25 resource usage information and the storage battery electric power information of the each of the neighboring base stations in case where receiving the coverage area adjustment request from the renewable energy base station; and a response sending step of sending, by each of the neighboring base stations, the adjustment request response to the renewable energy base station based on a result of the determination made in the adjustment request responding step. 5 15. The coverage adjustment method for a renewable energy base station according to claim 14, wherein the adjustment request responding step includes a step of determining, by the each of the neighboring base stations, that the coverage area adjustment request from the renewable 10 energy base station is allowed when a network resource of the each of the neighboring base stations is sufficient and the electric power of the storage battery of the each of the neighboring base stations is sufficient. 16. The coverage adjustment method for a renewable energy base 15 station according to claim 14; wherein: the network resource usage information includes information indicating a used amount of a network resource of the base station, and the storage battery electric power information includes information indicating a remaining electric power of the storage battery of the base station; and 20 the adjustment request responding step includes a step of determining, by the each of the neighboring base stations, that the coverage area adjustment request from the one of neighboring base station is allowed in case where the used amount of the network resource of the each of the neighboring base stations is lower than a predetermined fourth threshold 25 value and the remaining electric power of the storage battery of the each of the neighboring base stations is higher than a predetermined fifth threshold value. 17. The coverage adjustment method for a renewable energy base station according to claim 10, wherein the adjustment processing step includes steps of: notifying, by the renewable energy base station, the neighboring base 5 stations of the determined adjustment amount to the coverage area of the each of the neighboring base stations; increasing, by the each of the neighboring base stations, the coverage area of the each of the neighboring base stations based on the received adjustment amount to the coverage area of the each of the neighboring base 10 stations; and decreasing, by the renewable energy base station, the coverage area of the renewable energy base station based on the determined adjustment amount to the coverage area of the renewable energy base station after the coverage area of the each of the neighboring base stations is increased. 1.5 18. A wireless cellular system, comprising a renewable energy base station and neighboring base stations adjacent to the renewable energy base station, which use renewable energy, 20 the renewable energy base station including: an adjustment request determining part for determining whether an adjustment to a coverage area of the renewable energy base station is required based on at least one of network resource usage information of the renewable energy base station and storage battery electric power information 25 of the renewable energy base station; and an adjustment request transceiver for in case where it is determined that the adjustment to the coverage area of the renewable energy base station is required, sending a coverage area adjustment request to each of neighboring base stations in the wireless cellular system, and receiving an adjustment request response from the neighboring base stations, the each of the neighboring base stations including: an adjustment request responding part for determining whether the coverage area adjustment request from the renewable energy base station is allowed based on at least one of the network resource usage information of the neighboring base station and the storage battery electric power information of the each of the neighboring base stations; and a response transmitter for sending the adjustment request response 10 to the renewable energy base station based on a result of the determination made by the adjustment request responding part. 19. A renewable energy base station, substantially as herein described, with reference to accompanying drawings and examples. 15 20. A coverage adjustment method for a renewable energy base station, substantially as herein described with reference to accompanying drawings and examples. 2 1. A wireless cellular system, substantially as herein described with reference to accompanying drawings and examples.