781 research outputs found
Adaptive OFDM Index Modulation for Two-Hop Relay-Assisted Networks
In this paper, we propose an adaptive orthogonal frequency-division
multiplexing (OFDM) index modulation (IM) scheme for two-hop relay networks. In
contrast to the traditional OFDM IM scheme with a deterministic and fixed
mapping scheme, in this proposed adaptive OFDM IM scheme, the mapping schemes
between a bit stream and indices of active subcarriers for the first and second
hops are adaptively selected by a certain criterion. As a result, the active
subcarriers for the same bit stream in the first and second hops can be varied
in order to combat slow frequency-selective fading. In this way, the system
reliability can be enhanced. Additionally, considering the fact that a relay
device is normally a simple node, which may not always be able to perform
mapping scheme selection due to limited processing capability, we also propose
an alternative adaptive methodology in which the mapping scheme selection is
only performed at the source and the relay will simply utilize the selected
mapping scheme without changing it. The analyses of average outage probability,
network capacity and symbol error rate (SER) are given in closed form for
decode-and-forward (DF) relaying networks and are substantiated by numerical
results generated by Monte Carlo simulations.Comment: 30 page
Adaptive Turbo-Coded Hybrid-ARQ in OFDM Systems over Gaussian and Fading Channels
In this paper, an analytical approach for spectral efficiency maximization of coded wideband transmissions is presented based on OFDM. The approach exploits Type-III Hybrid-ARQ, enabling all sub-carriers to be employed in codeword transmission regardless of the sub-carrier conditions. The effects of imperfect sub-channel estimation are characterized and compensated for during code rate and signal constellation optimization. The results of the paper highlight that by independently adapting the code rate and signal constellation to individual OFDM sub-carriers based on an estimated sub-carrier CSI, the overall spectral efficiency of the system is maximized
COMPARATIVE ANALYSIS OF MODULATION SCHEMES USING ADAPTIVE EQUALIZATION AS FADING MITIGATION TECHNIQUE
Inter-symbol interference (ISI) due to multipath fading is a vital problem in high-speed wireless communication which restricts communication quality and capacity. Therefore, in addition to choosing a fading mitigation technique, it is also important to strategically select a modulation scheme for effective data transmission. Recent literature review on wireless standards, such as 3G and 4G indicates that QAM and QPSK are suitable choices for data transmission. In this paper, a comparative analysis on selected modulation schemes is performed in a fading environment. The mitigation of fading is done using adaptive equalization technique. Also, we show that the signal to noise ratio (SNR) is an important parameter to choose. It is observed that, even when an adaptive equalizer is used at the receiver, a very low SNR gives very high symbol error rate (SER). We derive some important conclusions from our simulation result: QPSK shows minimum SER, whereas 256-PSK and 256-PAM perform worse. Given its spectral efficiency and a low SER, the best choice is 256- QAM
Adaptive Modulation and Coding Using Signal to Noise Ratio Switching Threshold
Orthogonal frequency division multiplexing (OFDM) is one of the key enabling
technologies for fourth generation (4G) wireless system. It offers high data rate
transmission with high spectral efficiency, immunity to multipath fading and simple
implementation using fast Fourier transform (FFT). However inefficient utilization of
the channel will result when OFDM system is designed for worst-case channel
conditions. Thus adaptive transmission scheme that can be adjusted to channel
conditions is one of the techniques to improve the performance of OFDM systems.
In this thesis, three types of subband adaptive transmission scheme namely adaptive
modulation (AM), adaptive coding (AC) and adaptive modulation and coding (AMC)
based on SNR switching threshold are investigated. The performances of these
systems are evaluated using an efficient adaptation algorithm. The efficient adaptation
algorithm is based on the average value of the SNR of the subcarriers in the subband. First the performance of adaptive modulation using quadrature amplitude modulation
(QAM) and phase shift keying (PSK) system are evaluated. The results obtained
showed that a significant improvements in terms of bit error rate (BER), spectral
efficiency and throughput can be achieved. To further enhance the system,
convolutional coding is employed. However convolutional coding causes the
maximum throughput to be limited. To solve this problem adaptive coding schemes
which provides another area of flexibility is investigated. Finally the combination of
adaptive modulation and adaptive coding is examined.
Simulations results have shown that the performance of adaptive transmission
schemes are superior compared to fixed (nonadaptive) transmission schemes. A high
throughput performance can be achieved without sacrificing the BER. The
performance comparisons of the proposed system with the conservative system
showed that the proposed scheme is able to meet the BER target of 10-3 with a slightly
better throughput performance around 0.3 Bps/Hz
Adaptive 2×2 MIMO employed Wavelet-OFDM-Radio over Fibre Transmission
Due to high peak-to-average-power ratio (PAPR) and low spectral-efficiency, the conventional Fast Fourier Transform based orthogonal frequency division multiplexing (OFDM) is losing its place to the other multicarrier modulation schemes. Alternatively, the wavelet treated multiple-input multiple-output (MIMO)-OFDM is gaining its popularity in realization of futuristic 5G networks due to proffering high spectral-efficiency, low-cost and low phase-noise. Subsequently, this work demonstrates a RF transmission system in S-band employing \times 2 Alamouti's space-time block code (STBC) is implemented in this work to overcome this non-linearity and to realize a less-complex detection. The work is further extended to realize an adaptive MIMO-RoF system employing W-OFDM scheme to adjust itself to a suitable available phase shift keying strategy as per the link-situation to retain an optimal balance of link-quality and spectral-efficiency
A Survey of Blind Modulation Classification Techniques for OFDM Signals
Blind modulation classification (MC) is an integral part of designing an adaptive or intelligent transceiver for future wireless communications. Blind MC has several applications in the adaptive and automated systems of sixth generation (6G) communications to improve spectral efficiency and power efficiency, and reduce latency. It will become a integral part of intelligent software-defined radios (SDR) for future communication. In this paper, we provide various MC techniques for orthogonal frequency division multiplexing (OFDM) signals in a systematic way. We focus on the most widely used statistical and machine learning (ML) models and emphasize their advantages and limitations. The statistical-based blind MC includes likelihood-based (LB), maximum a posteriori (MAP) and feature-based methods (FB). The ML-based automated MC includes k-nearest neighbors (KNN), support vector machine (SVM), decision trees (DTs), convolutional neural networks (CNNs), recurrent neural networks (RNNs), and long short-term memory (LSTM) based MC methods. This survey will help the reader to understand the main characteristics of each technique, their advantages and disadvantages. We have also simulated some primary methods, i.e., statistical- and ML-based algorithms, under various constraints, which allows a fair comparison among different methodologies. The overall system performance in terms bit error rate (BER) in the presence of MC is also provided. We also provide a survey of some practical experiment works carried out through National Instrument hardware over an indoor propagation environment. In the end, open problems and possible directions for blind MC research are briefly discussed
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