167 research outputs found
Preamble-Based Channel Estimation for CP-OFDM and OFDM/OQAM Systems: A Comparative Study
In this paper, preamble-based least squares (LS) channel estimation in OFDM
systems of the QAM and offset QAM (OQAM) types is considered, in both the
frequency and the time domains. The construction of optimal (in the mean
squared error (MSE) sense) preambles is investigated, for both the cases of
full (all tones carrying pilot symbols) and sparse (a subset of pilot tones,
surrounded by nulls or data) preambles. The two OFDM systems are compared for
the same transmit power, which, for cyclic prefix (CP) based OFDM/QAM, also
includes the power spent for CP transmission. OFDM/OQAM, with a sparse preamble
consisting of equipowered and equispaced pilots embedded in zeros, turns out to
perform at least as well as CP-OFDM. Simulations results are presented that
verify the analysis
Antenna Array Design for LOS-MIMO and Gigabit Ethernet Switch-Based Gbps Radio System
The high spectrum efficiency of multiple-input multiple-output (MIMO) transmission traditionally
depends on the high multiplexing gain in rich scattering environments, which will
not always hold in the line-of-sight (LOS) environments, especially at higher microwave frequency
band. In this paper, a novel antenna array design rule is proposed to guarantee full
multiplexing gain for LOS-MIMO systems with one- or two-dimensional antenna arrays in
LOS scenarios, and the strict perpendicular constraint is released in the two-dimensional case.
The minimum antenna array area and the performance sensitivity to the area error are also
obtained to guide the practical system design. Then, a demo MIMO-OFDM system with the
designed square antenna array at 15 GHz carrier is implemented on a novel Gigabit Ethernet
(GE) switch-based software defined radio (SDR) platform, which combines the hardware
accelerating units (HAUs) with the general-purpose processors (GPPs). The field evaluation
results show that the system throughput and spectrum efficiency are greater than 1 Gbps and
15 bps/Hz, respectively. To the best of our knowledge, it is the first time to demonstrate
the Gbps LOS-MIMO-OFDM system at such microwave bands in the world, which can be a
successful design example for the next generation wireless backhaul or fixed wireless access
Phase-domain Injected Training for Channel Estimation in Constant Envelope OFDM
Constant envelope orthogonal frequency division
multiplexing (CE-OFDM) is a multi-carrier waveform with 0 dB
peak-to-average power ratio (PAPR). This property enables the
exploitation of multi-carrier waveforms with non-linear power
amplifiers, avoiding the undesirable clipping effects. However,
the existing channel estimation techniques designed for OFDM
cannot be reused, since the use of a phase modulator makes
CE-OFDM a non-linear waveform. Previous works assumed that
the channel estimation process relies on the transmission of
preambles, and the data symbols are equalized using a frequency
domain equalizer (FDE). To avoid the overhead induced by
preambles, a phase-domain injected training (PIT) is proposed,
where the pilot sequence is embedded in the phase dimension of
the data symbols. This novel approach does not waste time and/or
frequency resources as in preamble-based schemes. Moreover, it
does not require additional power for the training. The received
symbols are averaged with a dual procedure, and owing to
the particular structure of CE-OFDM, the channel estimates
are recovered. Also, the analytical expression of the channel
estimation mean squared error (MSE) is derived. Finally, several
numerical results illustrate the performance of the proposal,
showing that the MSE, bit error rate (BER) and achievable rate
are improved, as compared to the existing works.This work was supported by the Spanish National Project IRENE-EARTH under Grant PID2020-115323RBC33/AEI/10.13039/501100011033. The work of Andrea M. Tonello was supported in part by the Chair of Excellence Program of the Universidad Carlos III de Madrid.Publicad
On bounds and algorithms for frequency synchronization for collaborative communication systems
Cooperative diversity systems are wireless communication systems designed to
exploit cooperation among users to mitigate the effects of multipath fading. In
fairly general conditions, it has been shown that these systems can achieve the
diversity order of an equivalent MISO channel and, if the node geometry
permits, virtually the same outage probability can be achieved as that of the
equivalent MISO channel for a wide range of applicable SNR. However, much of
the prior analysis has been performed under the assumption of perfect timing
and frequency offset synchronization. In this paper, we derive the estimation
bounds and associated maximum likelihood estimators for frequency offset
estimation in a cooperative communication system. We show the benefit of
adaptively tuning the frequency of the relay node in order to reduce estimation
error at the destination. We also derive an efficient estimation algorithm,
based on the correlation sequence of the data, which has mean squared error
close to the Cramer-Rao Bound.Comment: Submitted to IEEE Transaction on Signal Processin
High Dimensional Modulation and MIMO Techniques for Access Networks
Exploration of advanced modulation formats and multiplexing techniques
for next generation optical access networks are of interest as promising
solutions for delivering multiple services to end-users. This thesis addresses
this from two different angles: high dimensionality carrierless amplitudephase
(CAP) and multiple-input multiple-output (MIMO) radio-over-fiber
(RoF) systems.
High dimensionality CAP modulation has been investigated in optical
fiber systems. In this project we conducted the first experimental demonstration
of 3 and 4 dimensional CAP with bit rates up to 10 Gb/s. These
results indicate the potentiality of supporting multiple users with converged
services. At the same time, orthogonal division multiple access
(ODMA) systems for multiple possible dimensions of CAP modulation has
been demonstrated for user and service allocation in wavelength division
multiplexing (WDM) optical access network.
2 x 2 MIMO RoF employing orthogonal frequency division multiplexing
(OFDM) with 5.6 GHz RoF signaling over all-vertical cavity surface
emitting lasers (VCSEL) WDM passive optical networks (PONs). We have
employed polarization division multiplexing (PDM) to further increase the
capacity per wavelength of the femto-cell network. Bit rate up to 1.59 Gbps
with fiber-wireless transmission over 1 m air distance is demonstrated.
The results presented in this thesis demonstrate the feasibility of high
dimensionality CAP in increasing the number of dimensions and their potentially
to be utilized for multiple service allocation to different users.
MIMO multiplexing techniques with OFDM provides the scalability in increasing
spectral effciency and bit rates for RoF systems.
High dimensional CAP and MIMO multiplexing techniques are two
promising solutions for supporting wired and hybrid wired-wireless access
networks
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