In a wireless cellular service, a service area is typically divided into a multiplicity of cells. A base station is employed in each cell to serve mobile terminals, e.g., cellular radiotelephones, in the cell to realize wireless communications. In a well known manner, the base station performs call administration, and establishes and maintains telephone connections between mobile terminals in the corresponding cell and other communication terminals, which may or may not be mobile terminals, via, e.g., a public switched telephone network (PSTN) connected to the base station. After a telephone connection is established, the base station receives in a wireless manner communication information from a mobile terminal at one end of the connection, and transmits same to a communication terminal at the other end thereof, and vice versa.
It is common to use a multi-sector antenna arrangement in the base station for transmission and reception of communication information to and from mobile terminals in the cell. The cell is divided into N typically, but not necessarily, equal sectors, where N is an integer greater than one. If the sectors are equal, each sector covers an angular span of 2.pi./N radians of the cell. The multi-sector antenna arrangement includes multiple antennas for transmitting and receiving N sector beams containing the communication information to and from the N sectors, respectively. It is generally believed that the number of mobile terminals which can be effectively served in a cell increases linearly with the number of the sector beams used, i.e., N.
When considering the optimization of the cellular wireless service performance, the focus of the prior art is invariably on the design of a radiation pattern of a sector beam. The radiation pattern typically includes a main lobe flanked by sidelobes. The main lobe represents the bulk of power of the sector beam transmitted to the corresponding sector. The sidelobes represent the remaining power of the sector beam radiated outside the sector, which causes undesirable interference to the transmissions to other sectors. Such interference is known as "inter-sector interference." The prior art design of the radiation pattern typically involves pre-selecting a set of constraints on the radiation pattern to attempt to, for example, shape the sidelobes into a desired pattern to minimize the inter-sector interference. These constraints include, for example, requirements of the power levels of the maxima of the sidelobes, locations of the sidelobe maxima with respect to the main lobe, etc. A solution satisfying the pre-selected constraints is then obtained if such a solution exists at all. However, the solution, if any, generally does not account for all important characteristics of the design, which can be defined only after the design is realized.
Moreover, in practice, a base station normally implements multiple sector beams in a cell, and each sector in the cell is afflicted by inter-sector interference aggregately caused by those sector beams transmitted to other sectors in the same cell. However, the pattern and effect of such inter-sector interference contributed by more than one sector beam are hardly predictable based on the design of the radiation pattern of an isolated sector beam, on which the prior art technique focuses. The unpredictability of the inter-sector interference is exacerbated if the sectors are unequal. As a result, use of the prior art technique to achieve the optimal service performance is, at best, precarious, and whether such performance is achievable thereby is also in question.
Accordingly, there exists a need for a dependable methodology to improve the wireless cellular service performance by, for example, effectively reducing the inter-sector interference.