Joint Estimation for Both AGC and DC Based on Distribution Function for OFDM Systems

In this paper, we derive the achievable rate of single-carrier frequency domain equalizer (SC-FDE) and orthogonal frequency division multiplexing (OFDM) systems, constrained on low resolution analog to digital converters (ADCs). We assume that there is a full knowledge of channels at both transmitter and receiver sides. We model low resolution ADCs with an additive uniform quantization noise. We consider an ultra-wide band millimeter wave (mm-Wave) system with a frequency selective channel. The channel is based on practical results at mm-Wave frequencies. Our numerical results show that SC-FDE systems with minimum mean square error (MMSE) equalizers outperform OFDM and SC-FDE systems with zero forcing (ZF) equalizers at high signal to noise ratio (SNR) regimes. Moreover, benefits of increasing the ADCs resolution are not significant due to exponentially increment in the power consumption of ADCs.

This paper presents an experimental investigation of the performance of an orthogonal frequency division multiplexing (OFDM) system in slow fading channels. The system has an 80 dB dynamic range and operates at 340 MHz with an 8 MHz 3 dB RF bandwidth. The peak user data rate achieved is 24 Mbits/sec. A novel automatic gain control (AGC) algorithm is proposed to address the high peak to average power ratio (PAPR) that occurs with OFDM. Experimental results show that the new AGC has superior performance compared to the standard analog AGC.

In this paper, we consider out-of-band radiations caused by high sidelobes in orthogonal frequency division multiplexing (OFDM) system. Suppression of sidelobes in OFDM system allows higher spectral efficiency and less interference with existing legacy systems in OFDM based overlay system. We combine different sidelobe suppression techniques to suppress the sidelobes in OFDM system. The multiple-choice sequence (MCS) is combined with cancellation carriers (CCs) and also MCS is combined with subcarrier weighting (SW) technique. MCS is applied prior to CCs or SW techniques. Simulation results show that combining MCS with CCs and MCS with SW techniques the sidelobes in OFDM system can be significantly reduced, which allow a successful co-existance between OFDM overlay system and an existing legacy system.

Orthogonal frequency division multiplexing (OFDM) signals have a generic problem of high peak to average power ratio (PAPR) which is defined as the ratio of the peak power to the average power of the OFDM signal. The drawback of the high PAPR is that the dynamic range of the power amplifiers (PA) & digital-to-analog (D/A) converters required during the transmission and reception of the signal is higher. As a result, the total cost of the transceiver increases, with reduced efficiency. This paper includes an idea of the PAPR constraint, along with the implementation and analysis of some specific algorithms for PAPR reduction of the OFDM signals. All the PAPR reduction algorithms are implemented and tested in the OFDM transceiver designed using Matlab. Analyzing some specific algorithms, we propose a novel algorithm by combining the subcarrier phase adjustment and linear combination of the time domain OFDM signals. Simulation results for a OFDM system employing Quadrature Amplitude Shift Keying (QASK) symbols and Quadrature Amplitude Modulation (QAM) can achieve a PAPR reduction of 5 dB.

Signal-to-noise ratio (SNR) is an important parameter for adaptive orthogonal frequency division multiplexing (OFDM) systems as it serves as a standard measure of signal quality. In this work, we present a novel SNR per subcarrier estimation scheme for wireless OFDM systems in frequency selective channels. The novel scheme can operate both in time and frequency, requiring only one previously known OFDM symbol. Firstly the noise variance per subcarrier estimate is obtained using the estimated channel coefficients and a novel analysis of its estimation error is provided. The noise variance per subcarrier estimate is used to obtain the SNR per subcarrier estimate. The estimation scheme takes under consideration the frequency selectivity of the channel and the variation of the noise statistics across the OFDM subcarrier index. The estimator's performance, in means of normalized mean square error (NMSE), is evaluated through simulations on a variety of channel models and is compared with another estimation scheme. The NMSE of the proposed scheme demonstrates its ability to identify the local statistics of the noise efficiently resulting into a reliable estimator in contrast with other similar approaches.

OFDM (orthogonal frequency division multiplexing) systems are very sensitive to carrier frequency offset. Although frequency offset is generally compensated for by an automatic frequency control (AFC), residual frequency offset remains after the AFC. Moreover, OFDM signals are also affected by phase noise. Thus, a kind of phase tracking is needed to compensate for these effects. In an OFDM system such as IEEE 802.11a, a phase tracking scheme that uses pilot subcarriers to detect the phase rotation caused by residual frequency offset and phase noise is employed. However, the spectrum of the AFC output signal is shifted by residual frequency offset. That is, the detected phase is the composite phase of the pilot subcarrier and the adjacent subcarrier because of inter-carrier interference (ICI). The compensation based on the composite phase sometimes overcorrects or undercorrects the phase rotation. In this paper, we propose a phase tracking scheme using iterative estimation of residual frequency offset in OFDM systems. The proposed phase tracking scheme estimates residual frequency offset in the frequency domain, and compensates for it in the time domain. The compensation in the time domain mitigates the spectrum shift of the AFC output. Therefore, the proposed scheme can reduce ICI, and track phases more accurately than the conventional phase tracking scheme.

Orthogonal frequency division multiplexing (OFDM) systems suffer from high sidelobes level. In some applications, as cognitive radios, this phenomenon can be a serious matter. This paper proposes Minimum shift keying (MSK)-OFDM to address this problem. The proposed method is an OFDM system using MSK modulation in each subcarrier. The corresponding transmitter and receiver are introduced. The method doesn't entail great changes in the main parts of OFDM. Like OFDM systems, the transceiver can be implemented using DFT/IDFT. Simulation results show great reduction in out of band radiation in comparison with conventional OFDM.

ABSTRACTWhile inter-symbol interference is not of concern, inter-carrier interference that is caused by carrier frequency offset (CFO) is more concerned in orthogonal frequency division multiplexing (OFDM) systems. The estimation of channel impulse response (CIR) is another important parameter of interest in the system. Recently, a number of approaches for the joint estimation of CFO and CIR in OFDM systems have been proposed in the literature, and most of them are traditional maximum-likelihood-based approaches. In this paper, we propose a number of mixture cost-reference particle filtering (CRPF) approaches for joint estimation of CIR and CFO, and a couple of them can be applied without the knowledge of both state and measurement noise statistics. Conventional least mean squares algorithm or recursive least squares algorithm is jointly employed with recently developed CRPF for nonlinear and multiple parameters estimation in OFDM systems. The computer simulation results show that mixture CRPFs are more robust against particle impoverishment (PI) problem than standard mixture particle filtering (PF) and mixture regularized PF (RPF) approaches although RPF was particularly developed to tackle the PI problem. In this paper, we investigate a number of mixture PF approaches including ones that do not need the information of noise statistics for the joint estimation of CFO and channel in OFDM systems.

Orthogonal frequency division multiplexing (OFDM) system has been widely used for the high speed digital communication area and it is very promising for the next generation communication standard because of high data rate capability. However, OFDM system has inherent serious problem of high peak-to-average power ratio (PAPR) that makes the OFDM signal be distorted in the nonlinear HPA (high power amplifier). In this paper, CI (carrier interferometry)-OFDM system is considered to overcome this problem and is newly analyzed in the selective fading and narrow-band interference channel. CI-OFDM system distributes each parallel data to all sub-carriers in differently phase-rotated form. So, PAPR becomes lowered and it shows the frequency diversity effects. BER (bit error rate) performance is analyzed in the selective fading and narrow-band interference environment. From the simulation results, it is confirmed that CI-OFDM system calms down the high PAPR problem and CI-OFDM system is more robust than the ordinary OFDM system in the selective fading and narrow-band interference channel.

This paper considers an identification method of the relay transmission characteristics including multipaths and coupling effects at the relay stations of a single-frequency network in an orthogonal frequency division multiplexing (OFDM) system. Estimating the transmission characteristics over the entire frequency range of OFDM transmission paths is extremely difficult because the transmitted signals in an OFDM system are limited to some bandwidth. Therefore, an objective of this paper is to propose a new approach that uses the guard interval feature of the OFDM signal to identify the relay transmission characteristics, multipath characteristics, coupling wave characteristics, and sensitivity characteristic of a closed-loop system including the coupling wave canceler, and evaluate the identification errors. The effectiveness of the proposed method is demonstrated through numerical examples. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(12): 10– 19, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20230

Orthogonal frequency division multiplexing (OFDM) has been incorporated in standards/draft standards such as IEEE 80.15.3c, IEEE 802.11ad for building multi-Gigabit systems operating in a few GHz of bandwidth. The digital implementation of the receivers for such a system is challenging because high precision (6+ bits/sample) analog-to-digital conversion (ADC) at such high speeds is power hungry and expensive. In this paper, we show that by suitable subcarrier power allocation we can get good performance even with low precision ADC (1-4 bits/sample without oversampling). We derive an analytical expression for the uncoded SER of an M-QAM OFDM system with finite precision ADC. By accounting for automatic gain control (AGC), we show that equal received subcarrier power (ERSP) leads to less quantization noise power than equal transmit subcarrier power (ETSP). Furthermore, for high SNR, ERSP has a lower symbol error rate (SER) than ETSP. But for lower SNR, ETSP is better, and hence we also use convex combinations of ETSP and ERSP power allocations. We illustrate the accuracy of our analytical results with simulations for the Saleh-Valenzuela channel model with log-normal fading. Our results show that for 16-QAM, at SER of 0.01, with a 3-bit ADC and a combination of ERSP and ETSP, we can come within 1 dB of ETSP of the full precision case (while ETSP with 3-bit ADC has a SER floor above 0.04).

It is well known that Orthogonal Frequency Division Multiplexing (OFDM) systems suffer from intercarrier interference (ICI) in mobile environment due to loss of orthogonality among subcarriers caused by Doppler shifts. There exist many ICI mitigation techniques in the literature to improve the performance of OFDM systems. However, most of the existing ICI mitigation techniques assume the OFDM transmission bandwidth is narrow enough that the frequency offsets on all subcarriers are identical. In a wideband OFDM transmission or a non-contiguous OFDM spanning over large bandwidth, the Doppler shifts on different subcarriers are different, especially in high speed aerial vehicle communication systems. In this paper, we analyze the wideband OFDM system in high mobility environment where the frequency offsets vary from subcarrier to subcarrier. We then propose a novel ICI cancellation scheme to eliminate the ICI effect and offer the wideband OFDM system significantly improved BER performance. Simulation results in AWGN channel and multipath fading channel confirm the effectiveness of the proposed scheme in the presence of frequency offset and time variations in the channel, offering the best BER performance available which matches the BER performance of wideband OFDM system without ICI. To our knowledge, this paper is the first to address the ICI problem of varying frequency offsets across subcarriers in wideband OFDM system.

A three-dimensional (3D) orthogonal frequency division multiplexing (OFDM) system with peak-to-average power ratio (PAPR) reduction method for the wireless sensor networks (WSNs) is presented. The transmit power of wireless nodes in an ad hoc network is strictly limited. Thus, high PAPR of the 3D OFDM system is a possible drawback when it is considered as a physical layer transmission scheme for the WSNs. Here, we propose an improved partial transmit sequence (PTS) technique to reduce PAPR of the OFDM system with 3D modulation formats. In the proposed algorithm, the components of 3D signals are assigned to different subblocks using a diagonal rule (DR), increasing randomness of the signals in the disjoint subblocks forcibly. As a result, the proposed method reduces PAPR of the 3D OFDM system significantly without increase in computational complexity. Hence, the proposed algorithm makes the 3D OFDM system be a possible candidate for a physical layer transmission scheme in future WSNs.

Based on superposition of signals in the time domain, we propose a new low-complexity scheme for the reduction of the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme first generates signal candidates by combining an OFDM signal and its cyclically delayed versions with different delays and phases, and then selects the candidate signal with minimum PAPR. It is shown that the complexity of the proposed scheme is significantly lower than that of the selective mapping (SLM) at the cost of some degradation in the bit error rate (BER) performance and, at similar PAPR and coded BER performance, is lower than that of the low-complexity SLM proposed recently.

Signa1 modulation recognition (SMR) is an essential and challenging topic in orthogonal frequency-division multiplexing (OFDM) systems, and also it is the fundamental technique for signal detection and recovery. However, traditional feature extraction based SMR methods cannot effectively acquire the characteristics of the OFDM signals. Hence, the modulated OFD-M signal cannot be reliably identified. In this paper, we propose a deep learning (DL) based SMR method for recognizing OFDM signals, which is combined with a convolutional neural network (CNN) trained on in-phase and quadrature (IQ) samples. In the network model, the batch normalization (BN) layer and dropout layer are used to speed up model training and prevent overfitting, respectively. Three convolution layers with different convolution kernels perform well than traditional feature extraction methods in obtaining intrinsic properties of OFDM signals. The same number of multiple modulated signals are mixed and sent to the trained model for identification. Experiments are conducted to show that the method we proposed performs better than the traditional methods, mainly reflected in a higher probability of correct classification (PCC) and better consistency.