1. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment performs radio communication with a beam, and when the user equipment detects a beam disappearance state being a state incapable of maintaining communication quality with the base station, the user equipment transmits a notification of the beam disappearance state with a beam having a wider half width than a half width before detection of the beam disappearance state.
2. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment performs radio communication with a beam,
when the user equipment detects a beam disappearance state being a state incapable of maintaining communication quality with a first base station, the user equipment transmits a notification of the beam disappearance state to a second base station configuring dual connectivity with the first base station, and
when the second base station receives the notification of the beam disappearance state, the second base station gives a command to the first base station to perform beam re-detection processing with the user equipment.
3. A communication system comprising:

a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment performs radio communication with a two-stage beamforming method by using a multi-element antenna, and
the user equipment transmits information for identifying an attribute of each first-stage beam to the base station.
4. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
when the user equipment communicates with a first base station while not communicating with a second base station, the user equipment measures a degree that the second base station interferes with a transmission signal from the first base station, and transmits a measurement result to the first base station, and
the first base station changes transmission power of a signal to be transmitted to the user equipment, based on a received measurement result.
5. A communication system comprising:
a user equipment; and
a base station connected to the user equipment to perform radio communication with the user equipment, wherein
when the user equipment communicates with a first base station while not communicating with a second base station, the first base station adjusts a communication

condition for interference suppression between data transmission from the first base station to the user equipment and data transmission performed by the second base station, and requests the second base station to perform data transmission on the adjusted communication condition.
6. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment performs radio communication with a two-stage beamforming method by using a multi-element antenna, and
when the user equipment communicates with a first base station while not communicating with a second base station, the user equipment forms, as first-stage beams, a first beam having a main beam directed to the first base station and a null directed to the second base station, and at least one second beam having a null directed to the second base station and having directivity different from directivity of the first beam.
7. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment performs radio communication with a two-stage beamforming method by using a multi-element antenna, and
when the user equipment communicates with a first base station while not communicating with a second base station, the user equipment designs at least one beam

having a main beam directed to a direction of a multipath of the first base station and having a null directed to the second base station by adjusting a configuration number of the multipath, and forms the designed beam as a first-stage beam.
8. A communication system comprising:
a user equipment; and
a base station configured to be connected to the user equipment to perform radio communication with the user equipment, wherein
the user equipment is configured to perform reciprocity-using channel estimation being channel estimation using reciprocity of a channel, for each frequency band,
the user equipment transmits reciprocity capability information indicating whether or not the reciprocity-using channel estimation can be performed for each frequency band to the base station, and
the base station performs communication with the user equipment by using the reciprocity-using channel estimation in a frequency band in which both of the user equipment and the base station are allowed to perform the reciprocity-using channel estimation, based on the reciprocity capability information of the user equipment.