Mitigating interference via spatial and spectral nulling

Phase‐only transmit nulling may help the next generation of radar systems operate in an over‐crowded radio frequency spectrum. The reiterative uniform weight optimisation (RUWO) algorithm generates constant modulus radar waveforms with spatial, frequency or space–frequency nulls for multiple‐input multiple‐output (MIMO) transmit arrays by utilising the maximum signal‐to‐interference plus noise ratio framework in a reiterative fashion. This study presents loop‐back and open air experimental results, obtained using an eight‐channel X‐band coherent MIMO radar test bed, that demonstrate the use of RUWO to generate constant modulus waveforms that possess spatial and/or frequency nulls. The RUWO algorithm is first used to deterministically generate phase‐only weights that produce spatial, frequency or space–frequency nulls at baseband in a loop‐back configuration. The algorithm is then utilised to generate weights to produce spatial nulls, both deterministically and adaptively, for the full eight‐channel test bed in an open air configuration. The deterministic and adaptive strategies are compared; results demonstrate that RUWO can be used to create spatial nulls and highlight the need for careful calibration of both the transmitter and the receiver.