| dc.description.abstract | 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. | nb_NO |
