Beamwidth-adjustable low sidelobe beamforming for space-time diversity

Extending space-time coding (STC) from element to beam domain has been considered in the past without taking into account the condition that diversity branches must be independent and identically distributed (i.i.d.). Although in element-space such a simple statistical channel model can be used under the assumption that antenna elements are placed sufficiently apart one from another, in beam-space a more realist multipath channel model must be dealt with. The required inter-element spacing to guarantee i.i.d. conditions can be theoretically determined given the nominal angular spread of the channel, and for relatively narrow angular spreads such inter-element spacing can be prohibitive. A major issue is, therefore, whether beamforming can extract i.i.d. diversity branches from a multipath scenario, thus enabling beam-space when element-space is not practical. In this paper we propose a beamforming method that allows the design of beamwidth-adjustable low sidelobe beampatterns and investigate its application to extracting i.i.d. channel conditions out of a multipath channel with Gaussian path density spatial distribution.