Millimeter wave link configuration robust to radio frequency impairments
Millimeter wave (mmWave) bands offer several gigahertz of bandwidth that can support high data rate applications. To efficiently use the spectrum at mmWave frequencies, the wireless link between the transmitting and receiving radios must be configured properly. The link configuration problem at mmWave, however, is challenging due to the use of large antenna arrays and radio frequency (RF) impairments that are less severe in common lower frequency systems. Some of these impairments include the low resolution of RF phase shifters, carrier frequency offset, and the misfocus effect in near field systems with large arrays.
An important characteristic of mmWave multiple-input multiple-output (MIMO) channels is sparsity. The sparse nature of such channels has allowed the use of compressed sensing (CS) for fast link configuration through channel estimation or beam alignment. CS techniques usually involve random sensing matrices to acquire a compressed channel representation and an optimization algorithm to estimate the sparse channel. Unfortunately, common random CS matrices result in poor link configuration under RF impairments. In the first part of this dissertation, we construct a new class of CS matrices that achieve robustness to the low resolution of RF phase shifters and the carrier frequency offset. To aid our construction, we propose a framework called FALP and develop an algorithm called Swift-Link within this framework. Swift-Link achieves better beam alignment than standard CS-based solutions at a reduced computational complexity.
In the second part of this dissertation, we investigate the misfocus effect in near field beamforming. The beams in a near field communication scenario can focus RF signals in a spatial region called the focal point. The focal point, however, changes with the frequency of operation in a wideband phased array that uses a center frequency-based beamformer. This effect called misfocus, reduces the effective operating bandwidth of the near field system. To mitigate the misfocus effect, we propose a technique called InFocus that constructs beams which are well suited for massive wideband phased arrays. InFocus enables massive wideband phased array-based radios to achieve a higher data rate than comparable beamforming solutions.