MIMO Communication Using Single Feed Antenna Arrays

Multi-input-multi-output (MIMO) communication has emerged as a promis-
ing technology for meeting the increasing demand on higher data rates. The
technology exploits the spatial resource dimension by sending the datas-
treams to different locations in the multi element array (MEA) domain while
decoding the signals at the receive end based on the signalsŠ unique spatial
signatures. To this end, the MEA is conventionally assumed to be attached
to a number of radios for independently modulating and up-converting (de-
modulating and down-converting) the set of signals at the transmit (receive)
end.
While the implementation of a MIMO system is affordable at basestations
(BS) and access points (AP), this is not true when considering simple, low-
cost, battery-based mobile terminals with limited physical area. It is the
subject of this thesis to bring a new philosophy regarding the design of
reduced complexity MIMO systems by revisiting the MIMO wireless prop-
agation from a signal space point of view. The main objective is to enable
MIMO transmit functionalities with a sole radio and a single RF chain.
The general approach for achieving the aforementioned objective is to mod-
ulate one datastream conventionally to a single antenna element while mod-
ulating the other datastreams in the analogue RF domain, using simple
switched antenna systems (SAS) or sophisticated reactance-assisted antenna
systems.
The use of a SAS is found simple to implement, but can hardly handle high
order signal formats, and is best suited for binary phase shift keying (BPSK)
signal formats. The idea there is to encode the remaining datastreams to
the active antenna index by moving the excitation point using an RF switch.
In the second case, reactance-assisted antenna systems are found important
for scaling the single-radio MIMO objective to high order signaling dimen-
sions. The idea is to exploit a number of passive radiators surrounding the
central driving one for creating the desired multiplexing relations of a set
of predefined basis beams. By this, information is encoded over the an-
gular variations of the beampattern, which finally conveys the datastreams
over the propagation channel to the intended receiver. However the second
technique requires a complex baseband control circuitry when compared to
the SAS approach. The proposed approach is finally validated in an indoor
office environment using a 2.6 GHz prototype. The experiments show that
the proposed beamspace MIMO approach provides performance compara-
ble to a conventional MIMO system, but at a reduced size and hardware
complexity.