As millimeter wave (mmWave) systems experience severe propagation and penetration losses, establishing a reliable communication link is quite challenging. To address this problem, directional beamforming (BF) utilizing electrically large antenna arrays is re- quired both at the base station (BS) and at the user equipment (UE) to overcome the high losses. The process of determining these BF directions is referred to as beam alignment (BA). Owing to the sparsity of mmWave channels, BA can usually be performed if the BS and the UE know their respective angle-of-arrival (AoA) that corresponds to the most dominant scatterer in the channel. However, due to the very low initial signal-to-noise ratio (SNR) before any BF, this AoA information is difficult to obtain. In this thesis, to enhance the UE’s sensing and processing capabilities, the UE is assumed to be equipped with a hybrid intelligent reflecting surface (IRS), which enables the UE to simultaneously sense and reflect a portion of an incoming signal. By exploiting this capability, a complete BA scheme is proposed for a communications scenario in which only the BS is transmitting pilot symbols. In the resulting scheme, the BS and the UE both probe a random subset of the angular range in every slot of BA with high gain and vary the probed direction such that the entire angular range is explored after as few slots as possible. Then, the UE first estimates its AoA in each slot based on all so far received signals using a multi-slot maximum likelihood (ML) estimation method. Once this AoA estimate converges to a stable value, the UE activates the IRS and configures it for reflection towards its estimated AoA. Afterwards, the BS finds its AoA by employing a similar estimation method as the UE. The BA scheme is terminated after a predefined maximum number of slots is exceeded. Extensive simulations demonstrate that the pro- posed scheme is suitable for BA if the SNR at the UE is higher than −15 dB. Otherwise, AoA estimation at the BS becomes infeasible due to the very high path loss of the two- way channel. For SNRs higher than −15 dB, the performance of the proposed scheme is found to improve significantly with increasing SNR as well as with an increased number of slots. Compared to other approaches established in the literature, the proposed scheme is inferior for very low SNRs of less than −10 dB and superior for higher SNRs. Moreover, by increasing the number of antennas at both the BS and the UE, the performance of the proposed scheme improves significantly compared to other approaches.      «

As millimeter wave (mmWave) systems experience severe propagation and penetration losses, establishing a reliable communication link is quite challenging. To address this problem, directional beamforming (BF) utilizing electrically large antenna arrays is re- quired both at the base station (BS) and at the user equipment (UE) to overcome the high losses. The process of determining these BF directions is referred to as beam alignment (BA). Owing to the sparsity of mmWave channels, BA can usu...     »

As millimeter wave (mmWave) systems experience severe propagation and penetration losses, establishing a reliable communication link is quite challenging. To address this problem, directional beamforming (BF) utilizing electrically large antenna arrays is re- quired both at the base station (BS) and at the user equipment (UE) to overcome the high losses. The process of determining these BF directions is referred to as beam alignment (BA). Owing to the sparsity of mmWave channels, BA can usually be performed if the BS and the UE know their respective angle-of-arrival (AoA) that corresponds to the most dominant scatterer in the channel. However, due to the very low initial signal-to-noise ratio (SNR) before any BF, this AoA information is difficult to obtain. In this thesis, to enhance the UE’s sensing and processing capabilities, the UE is assumed to be equipped with a hybrid intelligent reflecting surface (IRS), which enables the UE to simultaneously sense and reflect a portion of an incoming signal. By exploiting this capability, a complete BA scheme is proposed for a communications scenario in which only the BS is transmitting pilot symbols. In the resulting scheme, the BS and the UE both probe a random subset of the angular range in every slot of BA with high gain and vary the probed direction such that the entire angular range is explored after as few slots as possible. Then, the UE first estimates its AoA in each slot based on all so far received signals using a multi-slot maximum likelihood (ML) estimation method. Once this AoA estimate converges to a stable value, the UE activates the IRS and configures it for reflection towards its estimated AoA. Afterwards, the BS finds its AoA by employing a similar estimation method as the UE. The BA scheme is terminated after a predefined maximum number of slots is exceeded. Extensive simulations demonstrate that the pro- posed scheme is suitable for BA if the SNR at the UE is higher than −15 dB. Otherwise, AoA estimation at the BS becomes infeasible due to the very high path loss of the two- way channel. For SNRs higher than −15 dB, the performance of the proposed scheme is found to improve significantly with increasing SNR as well as with an increased number of slots. Compared to other approaches established in the literature, the proposed scheme is inferior for very low SNRs of less than −10 dB and superior for higher SNRs. Moreover, by increasing the number of antennas at both the BS and the UE, the performance of the proposed scheme improves significantly compared to other approaches.      «

As millimeter wave (mmWave) systems experience severe propagation and penetration losses, establishing a reliable communication link is quite challenging. To address this problem, directional beamforming (BF) utilizing electrically large antenna arrays is re- quired both at the base station (BS) and at the user equipment (UE) to overcome the high losses. The process of determining these BF directions is referred to as beam alignment (BA). Owing to the sparsity of mmWave channels, BA can usu...     »