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LOW-COMPLEXITY MILLIMETER WAVE COMMUNICATION SYSTEM DESIGNSAuthor: Jagyasi Deepa Date: 2021-06-23 Report no: IIIT/TH/2021/76 Advisor:Ubaidulla P AbstractMillimeter Wave (mmWave) communication is considered as one of the important frontiers in the suite of technologies for next generation wireless communication, as it offers a huge unlicensed spectrum and can accommodate ever-increasing data traffic and user demands. However, these potential benefits are accompanied by many challenges that include path loss, loss due to blockages, limited coverage, and absorption and penetration losses. Highly directional beamforming and advanced beamforming techniques are considered as potential ways to overcome these losses. The small wavelength of mmWaves allows the packing of a large number of antenna elements in small volumes, thus enabling MIMO implementation and resulting in large beamforming gains from MIMO. Here, we focus on the design of mmWave communication systems to overcome some of the key challenges encountered while exploiting the benefits of MIMO. The high cost and power consumption of mixed signal components at mmWave frequencies make the practical implementation of MIMO beamformers difficult. Hybrid beamforming, which offers a trade-off between analog and fully-digital beamforming, is a potential technique to overcome this challenge. In this thesis, low-complexity hybrid beamforming architecture is adopted in all the proposed mmWave communication system designs in order to achieve practical feasibility. In particular, to address the underlying challenges, we propose the design of hybrid transceivers for MIMO equipped mmWave systems in different communication scenarios: (i) multi-user MIMO downlink communication, (ii) multi-user MIMO interference channel-based communication, and (iii) MIMO amplify-forward (AF) relay-assisted cooperative communication. For all the considered communication scenarios, we propose low-complexity hybrid transceiver designs. To obtain these, first, the fully-digital filters are derived by solving a specifically designed optimization problem, which is then decomposed into RF/baseband hybrid filters using sparse approximation techniques. The distinguishing factor among these scenarios is the difference in the optimization problem considered to address related communication challenges. Specifically, the first scenario considers that the base-station (BS) is simultaneously communicating with multiple user-equipments (UEs) over a MIMO downlink channel. A sum-meansquare-error (SMSE) minimization problem is formulated under a constraint on the total transmit power. This is a non-convex problem and a closed-form solution is hard to achieve. We propose a joint iterative algorithm for a reliable solution towards this problem. The second scenario, which is a multi-user MIMO interference channel-based scenario, considers a complex network, wherein several mmWave transmitters are simultaneously communicating with their intended receivers. The signal from all remaining transmitters is co-channel interference at each intended receiver. Three different designs are obtained for this scenario. First, an overall SMSE is minimized under total transmit power constraint for all the transceiver pairs to obtain the low-complexity mmWave system design. Later, its dual problem, i.e., minimization of transmit power at the transmitting units while achieving the desired quality-ofservice (QoS) criterion (in terms of SMSE), is addressed. The solution to both the SMSE minimization and its dual problem is derived by an iterative algorithm. At last, the maximization of signal-to-leakageplus-noise-ratio (SLNR) under a constraint on the transmit power is considered. The proposed leakagebased problem results in a set of decoupled sub-problems and hence admits a closed-form solution. This leads to reduction in computation complexity as compared to the first two iterative solutions obtained for this scenario. The third scenario considers a MIMO AF relay-assisted communication system design in which two UEs are communicating with each other via an AF relay. This case addresses the short-range and non-line-of-sight (NLOS) communication challenges of mmWaves. Again, an SMSE minimization problem is considered with a constraint on the total relay transmit power. Two design solutions are obtained based on the operating mode viz., half-duplex (HD) mode and in-band full-duplex (FD) mode, of all the communicating nodes. First, a transceiver and relay-filter design are proposed considering the HD AF relay and HD UEs. Further, this design is extended to in-band FD mode where all the nodes operate in FD mode and utilize the same frequency resources. In in-band FD communication, due to simultaneous transmission and reception, the problem of loopback-self-interference (LSI) prevails. The LSI can be mitigated using the existing cancellation techniques. However, these do not provide perfect cancellation and hence some residual LSI still remains. This residual LSI accumulates over time and hugely affects the overall system performance. In the proposed in-band FD mmWave design, the low-complexity filters are obtained while mitigating the effect of the residual LSI at all the nodes. Channel state information (CSI) is a critical factor in the overall system performance. The availability of perfect CSI is not guaranteed due to the presence of various errors such as pilot contamination, estimation errors, quantization errors, etc., that corrupt the CSI. Hence, in this thesis, the perfect CSIbased proposed designs are also extended to robust designs for almost all the communication scenarios while making them resilient to the CSI errors. The efficacy of the proposed designs is validated through various performance metrics such as SMSE, sum-rate, bit-error-rate (BER), and hardware and computational complexities. The asymptotic analysis is also performed in order to study the upper bounds on the performance of the system. The numerical results demonstrate that the proposed solutions with reduced hardware complexity achieve comparable performance to the full-complexity digital solutions. Furthermore, the results also demonstrate the resilience of the robust design in the presence of CSI errors as compared to design assuming perfect CSI knowledge. Full thesis: pdf Centre for Others |
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