162 research outputs found
Intersymbol and Intercarrier Interference in OFDM Transmissions through Highly Dispersive Channels
This work quantifies, for the first time, intersymbol and intercarrier
interferences induced by very dispersive channels in OFDM systems. The
resulting achievable data rate for \wam{suboptimal} OFDM transmissions is
derived based on the computation of signal-to-interference-plus-noise ratio for
arbitrary length finite duration channel impulse responses. Simulation results
point to significant differences between data rates obtained via conventional
formulations, for which interferences are supposed to be limited to two or
three blocks, versus the data rates considering the actual channel dispersion
Estimation and detection techniques for doubly-selective channels in wireless communications
A fundamental problem in communications is the estimation of the channel.
The signal transmitted through a communications channel undergoes distortions
so that it is often received in an unrecognizable form at the receiver.
The receiver must expend significant signal processing effort in order to be
able to decode the transmit signal from this received signal. This signal processing
requires knowledge of how the channel distorts the transmit signal,
i.e. channel knowledge. To maintain a reliable link, the channel must be
estimated and tracked by the receiver.
The estimation of the channel at the receiver often proceeds by transmission
of a signal called the 'pilot' which is known a priori to the receiver.
The receiver forms its estimate of the transmitted signal based on how this
known signal is distorted by the channel, i.e. it estimates the channel from
the received signal and the pilot. This design of the pilot is a function of the
modulation, the type of training and the channel. [Continues.
Performances of Weighted Cyclic Prefix OFDM with Low-Complexity Equalization
International audience—In this paper, we justify low-complexity equalization techniques for weighted cyclic prefix (WCP)-OFDM. This modulation technique refers to filter bank based multicarrier (FBMC) transmission system provided with short filters. It allows the use of non-rectangular waveforms in order to mitigate interference caused by time-frequency selective channels while preserving an efficient implementation. Index Terms—Time-varying multipath channels, filter bank based multicarrier modulations, equalization, efficient realization
Doctor of Philosophy
dissertationThe demand for high speed communication has been increasing in the past two decades. Multicarrier communication technology has been suggested to address this demand. Orthogonal frequency-division multiplexing (OFDM) is the most widely used multicarrier technique. However, OFDM has a number of disadvantages in time-varying channels, multiple access, and cognitive radios. On the other hand, filterbank multicarrier (FBMC) communication has been suggested as an alternative to OFDM that can overcome the disadvantages of OFDM. In this dissertation, we investigate the application of filtered multitone (FMT), a subset of FBMC modulation methods, to slow fading and fast fading channels. We investigate the FMT transmitter and receiver in continuous and discrete time domains. An efficient implementation of FMT systems is derived and the conditions for perfect reconstruction in an FBMC communication system are presented. We derive equations for FMT in slow fading channels that allow evaluation of FMT when applied to mobile wireless communication systems. We consider using fractionally spaced per tone channel equalizers with different number of taps. The numerical results are presented to investigate the performance of these equalizers. The numerical results show that single-tap equalizers suffice for typical wireless channels. The equalizer design study is advanced by introducing adaptive equalizers which use channel estimation. We derive equations for a minimum mean square error (MMSE) channel estimator and improve the channel estimation by considering the finite duration of channel impulse response. The results of optimum equalizers (when channel is known perfectly) are compared with those of the adaptive equalizers, and it is found that a loss of 1 dB or less incurs. We also introduce a new form of FMT which is specially designed to handle doubly dispersive channels. This method is called FMT-dd (FMT for doubly dispersive channels). The proposed FMT-dd is applied to two common methods of data symbol orientation in the time-frequency space grid; namely, rectangular and hexagonal lattices. The performance of these methods along with OFDM and the conventional FMT are compared and a significant improvement in performance is observed. The FMT-dd design is applied to real-world underwater acoustic (UWA) communication channels. The experimental results from an at-sea experiment (ACOMM10) show that this new design provides a significant gain over OFDM. The feasibility of implementing a MIMO system for multicarrier UWA communication channels is studied through computer simulations. Our study emphasizes the bandwidth efficiency of multicarrier MIMO communications .We show that the value of MIMO to UWA communication is very limited
Single- versus Multi-Carrier Terahertz-Band Communications: A Comparative Study
The prospects of utilizing single-carrier (SC) and multi-carrier (MC)
waveforms in future terahertz (THz)-band communication systems remain
unresolved. On the one hand, the limited multi-path components at high
frequencies result in frequency-flat channels that favor low-complexity
wideband SC systems. On the other hand, frequency-dependent molecular
absorption and transceiver characteristics and the existence of multi-path
components in indoor sub-THz systems can still result in frequency-selective
channels, favoring off-the-shelf MC schemes such as orthogonal
frequency-division multiplexing (OFDM). Variations of SC/MC designs result in
different THz spectrum utilization, but spectral efficiency is not the primary
concern with substantial available bandwidths; baseband complexity, power
efficiency, and hardware impairment constraints are predominant. This paper
presents a comprehensive study of SC/MC modulations for THz communications,
utilizing an accurate wideband THz channel model and highlighting the various
performance and complexity trade-offs of the candidate schemes. Simulations
demonstrate that discrete-Fourier-transform spread orthogonal time-frequency
space (DFT-s-OTFS) achieves a lower peak-to-average power ratio (PAPR) than
OFDM and OTFS and enhances immunity to THz impairments and Doppler spreads, but
at an increased complexity cost. Moreover, DFT-s-OFDM is a promising candidate
that increases robustness to THz impairments and phase noise (PHN) at a low
PAPR and overall complexity.Comment: 18 pages, 12 figures, journa
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