Load-Modulated Single-RF MIMO Transmitters
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Multiple-Input Multiple-Output (MIMO) systems are utilized to enhance throughput in modern wireless networks. In a MIMO system, both transmitter and receiver sides are equipped with multi-antenna arrays. It has been shown that MIMO systems can boost the data rate of wireless systems without requiring any increase in power or bandwidth. Fundamental limits from information theory show that in a rich scattering environment with perfect channel knowledge, the higher the number of antennas, the higher the benefit of MIMO systems. Due to demands for high data rate in modern life and being limited in terms of power and bandwidth, MIMO systems with large number of antennas are promising for next generation of wireless networks. However, when it comes to hardware implementation, MIMO systems with large number of antennas face some challenges. The first issue is related to the cost of MIMO systems. Each antenna needs a separate RF-chain; therefore, the RF-cost increases with the number of antennas. The second issue is about the physical size of MIMO systems. To avoid the destructive effect of mutual coupling in antenna arrays, antennas are usually spaced at least half a wavelength apart from each other. This leads to the size issue in MIMO systems, e.g., the size issue in multi-antenna arrays in smart phones. Low-cost MIMO systems are being attractive due to the large RF-cost in large scale MIMO systems. In the current state of the art of MIMO transmitters, RF-cost grows with the number of antennas. Each RF-chain in MIMO transmitters includes a Power Amplifier (PA), a mixer and a Digital to Analog Converter (DAC). These elements are expensive and power consuming. In this dissertation, we introduce a new low-cost MIMO transmitter which uses a single-RF chain. The proposed transmitter called Load- Modulated Single-RF (LMSRF), has a central PA with very high power efficiency and does not need any DAC and mixer. LMSRF converts digital signals from baseband directly to the RF domain. Some passive load modulator blocks control the signal on the antennas using some high speed switches. LMSRF is completely compatible with the current MIMO receivers and does not need any change in receivers. LMSRF is a new implementation way of MIMO transmitters. We analyze the power efficiency of LMSRF in various cases. Next, we propose a precoding technique to be used together with LMSRF in massive MIMO base stations. The proposed precoding scheme enables a very efficient system in terms of power efficiency with slightly loss in the performance. Furthermore, we propose a new modulation scheme called Phase Modulation on the Hypersphere (PMH) which has the best power efficiency in LMSRF MIMO transmitters. We derive the performance of PMH in different channels. Some practical mapping schemes are presented and compared. Finally, a new pulse shaping method called spherical pulse shaping is introduced to be used in LMSRF with PMH modulation.