A Tutorial on Wideband XL-MIMO: Challenges, Opportunities, and Future Trends

As identified by international telecommunication union (ITU), extremely large-scale multipleinput multiple-output (XL-MIMO) is one of the pivotal enablers for sixth generation (6G) mobile communication networks across mmWave, sub-THz, and emerging mid-band (FR3) frequencies. These systems employ very large antenna arrays to achieve high spatial resolution and throughput. However, the conventional phase-shift-based beamforming which is designed under the narrowband assumption faces limitations due to large bandwidth and array sizes, which subsequently lead to beam squint. This phenomenon exists in both near-field and far-field regions of the antenna. This frequency-dependent misalignment of the beam degrades array gain, increases bit error rate, and limits data rates, especially in wideband scenarios. This paper presents both a comprehensive survey and a unifying framework for beam squint mitigation in XL-MIMO systems. Motivated by the challenges of maintaining consistent beamforming across frequency, we categorize existing solutions into two main classes: subband divisionbased methods, which are based on filtering the wideband signal to narrowband signals and applying a subband-dependent set of phase shifts to each subband to reduce the beam squint, and subarray divisionbased techniques, which involve partitioning the entire array into smaller subarrays or utilizing a subarray of true-time-delays (TTDs). We compare these solutions and outline their advantages and disadvantages. Our approach includes theoretical derivation of the beam squint channel for both the far-field and nearfield channels, comparative analysis of the beam squint mitigation approaches, and numerical simulations. Finally, we outline open research challen