362 research outputs found
Sub-sampled OFDM based sub-band ultra-wideband system
In sub-band ultra-wideband (SUWB) systems, the use of spreading codes in conjunction with sub-banding enables energy efficient reduced sampling rate receiver designs. In this work, the orthogonal frequency division multiplexing (OFDM) technique is proposed for SUWB systems as a means to mitigate the multipath fading effects of the channel. The OFDM demodulation performed at the sub-sampled rate with reduced number of discrete Fourier transform (DFT) points provides scope for low power receiver implementations. Moreover, OFDM improves the flexibility as bandwidth resources can be allocated with improved granularity at integral multiples of the OFDM sub-channel bandwidth. The requisite correlation properties of the spreading codes is relaxed in the proposed OFDM-SUWB system and more number of spreading codes can be used when compared to the existing SUWB system. Also, a simple channel estimation method exploiting the low complexity advantage of the inherent spreading code based receiver is proposed. Simulation results in terms of the bit error rate (BER) performance are presented over the IEEE 802.15.4a channel models and also comparisons with the multi-band OFDM (MB-OFDM) system are made demonstrating the usefulness of the proposed scheme
Low Complexity AOFDM System for Time-varying Wireless Channels
Signal transmitted through a wireless channel undergoes distortion due to the pres ence of reflectors in the environment between a transmitter and a receiver as well as due to the Doppler shift caused by the relative movement of the receiver with respect to the transmitter. Distorted signal is recovered at the receiver side by means of predicting the channel responses and performing inverse operation that the channel introduced on the transmitted signal. Prediction of channel responses becomes more complex when the receiver moves with a varying speed since it directly affects the auto-correlation of the channel responses.
First part of this thesis provides a solution for recovering the transmitted data when the receiver is moving with varying speed. The system first tracks the receiver speed variations using the number of deep fadings (nulls) in the received signal enve lope of one sub-carrier during a fixed time period. If there is a significant change in receiver speed then the Kalman filter parameters are calculated and updated. Future channel responses are predicted using the updated Kalman filter parameters and used in equalizer to recover the distorted signal. The performance and computational effi ciency of the proposed system outperforms the conventional system which calculates predictor parameters at a fixed interval.
Second part of the thesis presents an adaptive modulation technique based on the signal-to-noise ratio and the receiver speed. Modulation schemes for different combinations of signal-to-noise ratio and receiver speeds are obtained by selecting the higher modulation scheme with the bit error rate less than target bit error rate. Boundaries of the selected modulation schemes are found using support vector ma chine classifiers. The receiver uses the designed system to select appropriate modula tion scheme by mapping the current modulation scheme and the channel conditions. The proposed system outperforms conventional adaptive modulation technique that uses instantaneous signa-to-noise ratio by a margin of 5 dB
Delay Alignment Modulation: Manipulating Channel Delay Spread for Efficient Single- and Multi-Carrier Communication
The evolution of mobile communication networks has always been accompanied by
the advancement of ISI mitigation techniques, from equalization in 2G, spread
spectrum and RAKE receiver in 3G, to OFDM in 4G and 5G. Looking forward towards
6G, by exploiting the high spatial resolution brought by large antenna arrays
and the multi-path sparsity of mmWave and Terahertz channels, a novel ISI
mitigation technique termed delay alignment modulation (DAM) was recently
proposed. However, existing works only consider the single-carrier perfect DAM,
which is feasible only when the number of BS antennas is no smaller than that
of channel paths, so that all multi-path signal components arrive at the
receiver simultaneously and constructively. This imposes stringent requirements
on the number of BS antennas and multi-path sparsity. In this paper, we propose
a generic DAM technique to manipulate the channel delay spread via
spatial-delay processing, thus providing a flexible framework to combat channel
time dispersion for efficient single- or multi-carrier transmissions. We first
show that when the number of BS antennas is much larger than that of channel
paths, perfect delay alignment can be achieved to transform the time-dispersive
channel to time non-dispersive channel with the simple delay pre-compensation
and path-based MRT beamforming. When perfect DAM is infeasible or undesirable,
the proposed generic DAM technique can be applied to significantly reduce the
channel delay spread. We further propose the novel DAM-OFDM technique, which is
able to save the CP overhead or mitigate the PAPR issue suffered by
conventional OFDM. We show that the proposed DAM-OFDM involves joint frequency-
and time-domain beamforming optimization, for which a closed-form solution is
derived. Simulation results show that the proposed DAM-OFDM outperforms the
conventional OFDM in terms of spectral efficiency, BER and PAPR.Comment: 16 Pages, 15 figure
Compensation of fibre impairments in coherent optical systems
Tese de mestrado integrado. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 201
New Concept of PLC Modems: Multi-Carrier System for Frequency Selective Slow-Fading Channels Based on Layered SCCC Turbocodes
The article introduces a novel concept of a PLC modem as a complement to the existing G3 and PRIME standards for communications using medium- or high-voltage overhead or cable lines. The proposed concept is based on the fact that the levels of impulse noise and frequency selectivity are lower on high-voltage lines than on low-voltage ones. Also, the demands for “cost-effective” circuitry design are not so crucial as in the case of modems for low-voltage level. In contract to these positive conditions, however, there is the need to overcome much longer distances and to take into account low SNR on the receiving side. With respect to the listed reasons, our concept makes use of MCM, instead of OFDM. The assumption of low SNR is compensated through the use of an efficient channel coding based on a serially concatenated turbo code. In addition, MCM offers lower latency and PAPR compared to OFDM. Therefore, when using MCM, it is possible to excite the line with higher power. The proposed concept has been verified during experimental transmission of testing data over a real, 5 km long, 22kV overhead line
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.
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