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Type of Document Dissertation Author Al-Naser, Mahmud J, URN etd-08162006-190358 Title Polarimetric Vector Receivers for CDMA Multipath Signals Degree PhD Graduate Program Electrical Engineering Advisory Committee
Advisor Name Title Dr. Brian L. Hughes Committee Chair Keywords
- multipath
- DS-CDMA
- vector antennas
Date of Defense 2006-12-18 Availability unrestricted Abstract ABSTRACT
ALNASER, MAHMUD J. Polarimetric Vector Receivers for CDMA Multipath Sig-
nals. (Under the direction of Prof. Brian L. Hughes.)
In this dissertation, we investigate the design and performance of receivers for vector antenna systems on multipath fading channels. Vector antennas are co-located
antennas that can detect or excite more than one component of the received electro-
magnetic field. We study the design and implementation of vector antenna receivers
for code-division multiple-access (CDMA) signals in the presence of multipath signal
propagation. The use of antenna arrays in CDMA systems has been widely inves-
tigated in wireless communications, due to the significant gain of capacity relative
to a single antenna systems. Multiuser space-time detection schemes which utilize
antenna arrays (e.g., the beamformer-RAKE) have also been proposed for CDMA
systems to exploit space and time and to combat multiuser interference. Most of
this previous work has focused on the use of linear arrays of single- or dual-polarized
antennas, which typically measure one or two components of the received electromagnetic (EM) field in several locations. The use of vector antennas (arrays of antennas
that measure multiple components of the EM field in one location) for CDMA systems
has not been addressed in the literature.
We first consider space-time receivers for a single-user, direct-sequence CDMA
(DS-CDMA) system. Using electromagnetic theory, we derive a channel model that
describes joint multipath propagation of the electric and magnetic Żelds in space
and time. Using this model, we derive vector antenna receivers which are optimal
for various performance criteria, such as maximum-likelihood (ML), minimum mean-
square error (MMSE), matched filter (MF), and zero-forcing or decorrelator (DD).
The performance of these receivers is measured in terms of the bit-error-rate (BER) as
a function of the signal-to-noise ratio (SNR). Comparing the BER for various number
of antennas, we find that the diversity improvement obtained, for a fixed number of
multipaths L, increases with the number of elements in the vector antenna. For
example, three mutually orthogonal electric dipoles (known as a tripole antenna)
o®ers more diversity gain than a dual-polarized antenna. Similarly, a six-element
vector antenna provides more diversity gain than a tripole antenna.
Next we compare the performance of multiuser detection schemes for vector an-
tennas and a uniform linear array (ULA). A space-time MMSE (ST-MMSE) receiver
is derived for n users and L discrete multipath fading channels, assuming that the
multipath coe±cients and spreading codes of all users are known at receiver. When
channel information is not available, an adaptive implementation of the MMSE receiver is considered, in which training data bits are used to estimate the channel
coe±cients. Simulations of BER suggest that the vector antenna achieves significantly higher diversity gain than the ULA for a fixed number of multipaths. We next
consider vector antennas employed at the base station for a DS-CDMA systems with
M-ary orthogonal modulation. Here M-ary orthogonal modulation is used at mobile
station for data transmission and a beamformer-RAKE receiver is implemented at
base station. The capacity in terms of the number of users as a function of number
of antennas employed at the base station is evaluated. Simulations suggest that a
vector antenna can achieve a higher capacity than a uniform linear array (ULA) at
given SNR.
For multi-input multi-output (MIMO) channels, spatial multiplexing schemes such
as V-BLAST and adaptive transmission are considered. Using V-BLAST, the BER
of the normalized empirical data measurement for a single tripole antenna (i.e. three
mutually orthogonal dipoles) at the transmitter and the receiver is simulated. An
adaptive transmission scheme for the same antenna system is also simulated.
Finally, we investigate and analyze the signal-to-interference plus noise ratio (SINR)
of minimum variance (MV) beamforming at the receiver. An expression for the SINR
is derived as a function of the direction-of-arrival (DOA) and polarization of both the
desired signal and one interference signal in a multipath fading channel. Furthermore,
the beampattern of the vector antenna steered toward the DOA is compared with a
ULA. The vector antenna beampattern shows maximum gain in direction of desired
signal while ULA have many side loops in other directions.
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