Upper Bound on Side Lobe Levels for mMIMO Antennas to Evaluate the Beam Steering Capability

A promising concept to meet the growing requirements of future wireless systems is the use of massive multiple-input-multiple-output (mMIMO) systems. Specifically, in an attempt to mitigate the high cost due to the complexity of these systems, sub-array based antenna architectures are highly anticipated, where not every antenna element is independently controllable, but groups of elements are connected. Consequently, sharing the steering circuitry reduces degrees of freedom in the design of effective beam patterns. Due to this, sub-array based antenna systems suffer from severely increased side lobe levels (SLLs), which limit the steering capability of mMIMO systems. Therefore, it is difficult to establish a distinct general optimum tool for beam pattern design. In this letter, we explore theoretically a mechanism to describe SLLs due to beam steering in uniform sub-array based mMIMO systems, and describe an analytical upper bound on the SLL distance as a function of the steering angle. The derived expression can be used to evaluate the steering capabilities of sub-array based configuration.