Patent ID: 7359733

Claim:
A method for downlink beamforming in a frequency-division-duplex wireless communications system comprising a base station with an antenna array defining a communication cell and terminals that are physically remote from said base station, the method comprising the steps of: dividing the communication cell into a plurality of sectors; identifying uplink nulls which would yield pseudo nulls in a sector; constraining use of the system to those terminals in a sector in which no pseudo nulls will be generated; receiving at said base station antenna array combinations of arriving signals from said plurality of remote terminals, each signal having a beam pattern incorporating a main beam and one or more nulls; identifying an uplink beamforming weight for a signal; generating a downlink beamforming weight based on the signal's uplink beamforming weight using the null constraint method, wherein the null constraint method comprises determining an uplink beam pattern's nulls: z u,k ( i ), i= 1 , . . . , M− 1, using a polynomial formed from the uplink beamforming weight: ∑ M i = 1 ⁢ w u , k ⁡ ( i ) ⁢ z - i + 1 = w u , k ⁡ ( 1 ) ⁢ ( 1 - z u , k ⁡ ( 1 ) ⁢ z - 1 ) ⁢ ⁢ … ⁢ ⁢ ( 1 - z u , k ⁡ ( M - 1 ) ⁢ z - 1 ) ; transforming phase components of the uplink beam pattern's nulls and obtaining phase components of a downlink beam pattern's nulls as: φ d , k ⁡ ( i ) = f d f u ⁢ φ u , k ⁡ ( i ) , where ⁢ ⁢ z u , k ⁡ ( i ) = A u , k ⁡ ( i ) ⁢ ⅇ jφ u , k ⁡ ( i ) , for i=1, . . . , M−1; and constructing the downlink beam pattern's nulls: z d,k ( i )= A d,k ( i ) e jψ d,k (i) , for i= 1 , . . . , M− 1; and constructing the downlink beamforming weight: ∑ M i = 1 ⁢ w d , k ⁡ ( i ) ⁢ z - i + 1 = w d , k ⁡ ( 1 ) ⁢ ( 1 - z d , k ⁡ ( 1 ) ⁢ z - 1 ) ⁢ ⁢ … ⁢ ⁢ ( 1 - z d , k ⁡ ( M - 1 ) ⁢ z - 1 ) ; and transmitting downlink signals using downlink beamforming weights.