A pulse detonation combustor including a plurality of nozzles engaged with one another via mating surfaces to support a gas discharge annulus in a circumferential direction. In one embodiment, the pre-chamber is part of a two-stroke combustion engine having a cylinder head with a sparkplug receptacle that has a generally frustoconical shape, and the pre-chamber is coupled to the sparkplug receptacle. The outer sleeve surrounds the inner sleeve. When the rotatable hood is stopped at any desired position between a closed position and an open position without holding by hand, the opening having a desired size can be obtained, while both hands of a user are free. The control channel element assignment unit is configured to assign a plurality of continuous CCE to the PDCCH addressed to the mobile station based on an aggregation level changed by the aggregation level determination unit, when failing to assign CCE to the PDCCH addressed to the mobile station.