In a wireless communication system such as a cellular system which expands its service over wide area, it is necessary to ensure that every base station (Base Station: BS) communicates independently of its adjacent base stations. Thus, conventionally applied is a method such as using different frequencies, different spreading codes, or different time domains. In recent years, an OFDM (Orthogonal Frequency Division Multiplexing) method has been standardized as IEEE802.16e (Mobile WiMAX), in which a usable frequency band is divided into narrow and orthogonal subcarrier frequency bands for communication. In the Mobile WiMAX system, provided that independently separated subcarriers are allocated, the same frequency band is used in contigubus sectors (refer to Non patent document 1).
An FDMA (Frequency-division multiple access) method using different frequencies is obviously disadvantageous in a view point of frequency utilization efficiency. In a CDMA (Code-division multiple access) method using the same frequencies with spreading codes, communication capacity is reduced because of the interference by signals from contiguous sectors. In a Mobile WiMAX system, a heavier communication load increases collision probability, causing the interference effect over its negligible level.
In a Mobile WiMAX system, an adaptive array system (AAS) using a multi-antenna is standardized (refer to Non patent document 2). Depending on a flexibility of the antenna, the interference to mobile subscriber stations (Mobile Subscriber Station: MSS) in its own sector can be prevented. However, no attention is paid to avoid interference to mobile subscriber stations in sectors contiguous to the own sector.                Non-Patent Document 1:IEEE 802.16-2004 (8.4.4.4 Allocation of subchannels for FCH, and logical subchannel numbering)        Non-Patent Document 2:IEEE 802.16-2004/Cor 1-2005 (8.4.6.3.3 AMC support for SDMA)        Patent Document 1:Japanese Patent Laid-Open No. 2002-319894        Patent Document 2:U.S. Pat. No. 6,067,290        