5G is now a reality and the first stage of its infrastructure (sub-6 GHz) is already deployed in major cities around the world. The high data rate demand for 5G mobile users is shown to be fulfilled using the famous Multiple-Input Multiple-Output (MIMO) technology . The next deployment stage of 5G is expected to utilize the millimeter-wave (mmWave) frequency spectrum, and the forthcoming base station antennas will operate at frequency bands around at 28 GHz and 39 GHz. At these high frequencies, a steerable RF beam can reliably serve a communication device in a much better way compared to an isotropic RF radiator having inefficient directivity, and this is possible by performing beamforming at the base station end, illustrated in Fig. 1.
Beamforming is a technique by which a radiator is made to transmit radio signals in a particular direction. A communication device that performs this function is called a beamformer. The most common and simplest type of a beamformer is an array of half-wavelength spaced antennas connected to a single radio frequency (RF) source via a network of power dividers. Such a beamformer is referred to as a corporate-feed array. More sophisticated beamformers involve a bank of phase shifters connected to each antenna element to add beam steering capability to a simple corporate-feed array. Advanced beamformers involve digitally controlled phase shifters, lens structures, intelligent and meta-surfaces, etc., which enhances the beamformer performance.
Designing 5G-ready beamformer hardware at mmWave is challenging due to three major reasons:
1. Huge losses faced by the electromagnetic waves while propagating through the free space, hence highly directive radiation is desirable.
2. The required network of phase-shifters and power dividers to add beam steering capabilities is lossy and expensive.
3. The theoretical array theory supporting MIMO require each antenna to be connected separately to the baseband processing unit, making the overall system prohibitively expensive, especially when it comes to implementing a 64 or 128 element mmWave massive MIMO system.