Orthogonal Frequency Division Multiplexing Time Research Articles

Physical Layer Security in an OFDM Time Reversal SISO Communication With Imperfect Channel State Information

A frequency domain time-reversal (TR) precoder is proposed to perform physical layer security in single-input single-output (SISO) systems using orthogonal frequency-division multiplexing (OFDM) and artificial noise (AN) injection. This scheme guarantees the secrecy of a communication towards a legitimate user, Bob, by exploiting the frequency diversity selective behaviour in multipath channels. The transmitter, Alice, has imperfect channel state information (CSI) of the legitimate link thanks to the channel reciprocity in time division duplex systems and does not know the instantaneous CSI of a potential eavesdropper, Eve. Three optimal decoding structures at Eve are considered in a block fading environment depending on the handshake procedure between Alice and Bob. Closed-form approximations of the signal-to-noise ratio required at Bob and the maximal CSI error that can be made at Alice, in order to guarantee a communication ergodic secrecy rate (ESR), are derived. Furthermore, the optimal amount of AN energy to inject, considering imperfect CSI, is also given as a closed-form expression. A trade-off on the choice of the spreading factor of the TR precoder is established between maximizing the ESR and decreasing the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\epsilon }-$ </tex-math></inline-formula> achievable secrecy rate. Finally, thanks to these results, Alice can be a priori aware of the ESR over which she can establish a secure communication.

Extended Spatial-Index LED Modulation for Optical MIMO-OFDM Wireless Communication

An efficient optical modulation technique for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) visible light communication system is proposed in this paper. The proposed modulation technique is termed as extended spatial-index light-emitting diode (LED) modulation. In the proposed technique, the indices (the spatial domain) of the LEDs are exploited in a dynamic style to not only get rid of the optical OFDM time-domain ( OFDM t d ) shaping problem but also to expand the LED indices spatial modulation domain. The indices of the active LEDs in the proposed technique are changed from the two LEDs active situation to the situation where all or several LEDs are active. Moreover, within the selected active LED indices, the power weight distribution and the positions of the OFDM components are varied to expand the resultant spatial domain. Therefore, the proposed technique offers a considerable spectral efficiency improvement over the up-to-date LED index OFDM modulation schemes even with a lower number of LEDs. The key idea of the proposed technique is to maximize the LEDs’ indices spatial position (spatial domain) utilization, where both the power weight allocation and the positions of the complex OFDM time domain components are varying several times over the same active LED indices combination, which improve the optical system spectral efficiency. The simulation results asserted the superiority of the proposed technique, as it improves both the average bit error rate (ABER) and the achievable data rate (R) compared with existing up-to-date OFDM-LED index modulations with even lower computational complexity.

Compressed Sensing OFDM Time Delay Estimation Based on Atomic Evolution and Elimination

Aiming at the problem of time delay estimation for orthogonal frequency division multiplexing (OFDM) signals with small samples, a new compressed sensing time delay estimation method based on atomic evolution and elimination is proposed. This method introduces the idea of evolution and elimination from Darwin's evolutionism, abandons the fixed and invariable way of grid points in the traditional algorithm, takes grid points as adjustable parameters, and updates grid points in each iteration process to promote the evolution of atoms. In order to deal with the off-grid effect, the exponential function is directly modified instead of linear approximation, which effectively eliminates the modeling error caused by the original off-grid algorithm. In the process of calculation, the atomic elimination mechanism is adopted, which reduces the amount of calculation and improves the speed of it. The simulation results show that the proposed algorithm can significantly improve the performance of time delay estimation, especially when using coarse grids. Based on the universal software radio peripheral (USRP), the validity of the proposed algorithm is verified by the actual signal.

Removal of the OFDM Segment without Affecting Signalling BER to Provide Higher Data Rate as well as a Cost Efficient Time Processing or a MIMO-OFDM Relay Network

ABSTRACTOne of the most serious problems in relay systems is power constraint. Due to this limitation, complex algorithms are replaced by cost efficient techniques. In conventional orthogonal frequency division multiplexing (OFDM) systems at the relays, signal processing techniques, such as equalization is performed considering signalling in the frequency domain. Employing such techniques, the received OFDM time signals in the first hop have to be converted into frequency domain at the relays, equalized and converted back to time domain again via the next hop for retransmission. In this paper, by employing the Toeplitz matrix structure that converts frequency selective channels into an equivalent channel matrix, the convolution of input signal and the channel is transferred into multiplication of input signal with the equivalent channel matrix causing the removal of the OFDM procedure at the relay. Since the span of the channel is calculated by a channel matrix, the signal length is not increased causing the transmission rate to be increased without the loss of full system BER.

Low-Power Correlation for IEEE 802.16 OFDM Synchronization on FPGA

This brief compares the use of multiplierless and DSP slice-based cross-correlation for IEEE 802.16d orthogonal frequency division multiplexing (OFDM) timing synchronization on Xilinx Virtex-6 and Spartan-6 field programmable gate arrays (FPGAs). The natural approach, given the availability of embedded DSP blocks on these FPGAs, would be to implement standard multiplier-based cross-correlation. However, this can consume a significant number of DSP blocks, which may not fit on low-power devices. Hence, we compare a DSP48E1 slice-based design to four different quantizations of multiplierless correlation in terms of resource utilization and power consumption. OFDM timing synchronization accuracy is evaluated for each system at different signal-to-noise ratios. Results show that even relatively coarse multiplierless coefficient quantization can yield accurate timing synchronization, and does so at high clock speeds. Multiplierless designs enjoy reduced power consumption over the DSP48E1 Slice-based design, and can be used where DSP Slice resources are insufficient, such as on low-power FPGA devices.

OFDM-Based STBC with Low End-to-End Delay for Full-Duplex Asynchronous Cooperative Systems

We propose a new space-time block coding (STBC) for asynchronous cooperative systems in full-duplex mode. The orthogonal frequency division multiplexing (OFDM) transmission technique is used to combat the timing errors from the relay nodes. At the relay nodes, only one OFDM time slot is required to delay for a pair-wise symbol swap operation. The decoding complexity is lower for this new STBC than for the traditional quasi-orthogonal STBC. Simulation results show that the proposed scheme achieves excellent performances.

Time and Frequency Synchronization for MIMO-OFDM Systems

Any carrier frequency offset will cause a loss of subcarrier orthogonality which results in ICI and hence performance degrades severely in MIMO-OFDM systems. In this paper, a time and frequency synchronization solution for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is proposed. The synchronization is achieved using one Constant Amplitude Zero Auto Correlation (CAZAC) sequence-based preamble which is simultaneously transmitted from all transmit antennas in the same OFDM time instant. The synchronization is accomplished sequentially by coarse time synchronization, fractional frequency offset estimation, integral frequency offset estimation and fine time synchronization. The simulation shows that the proposed algorithm can estimate the timing and frequency offsets efficiently in MIMO-OFDM systems, especially in low signal-to-noise ratio condition.

OFDM channel estimation and data detection with superimposed pilots

AbstractWe propose three iterative superimposed‐pilot based channel estimators for Orthogonal Frequency Division Multiplexing (OFDM) systems. Two are approximate maximum‐likelihood, derived by using a Taylor expansion of the conditional probability density function of the received signal or by approximating the OFDM time signal as Gaussian, and one is minimum‐mean square error. The complexity per iteration of these estimators is given by approximately O(NL2), O(N3) and O(NL), where N is the number of OFDM subcarriers and L is the channel length (time). Two direct (non‐iterative) data detectors are also derived by averaging the log likelihood function over the channel statistics. These detectors require minimising the cost metric in an integer space, and we suggest the use of the sphere decoder for them. The Cramér–Rao bound for superimposed pilot based channel estimation is derived, and this bound is achieved by the proposed estimators. The optimal pilot placement is shown to be the equally spaced distribution of pilots. The bit error rate of the proposed estimators is simulated for N = 32 OFDM system. Our estimators perform fairly close to a separated training scheme, but without any loss of spectral efficiency. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Combination of selected mapping and statistical redistribution method for reducing peak to average power ratio in OFDM systems

Orthogonal frequency-division multiplexing (OFDM) is an attractive transmission technique for high-bit-rate communication systems. One major drawback of OFDM is the high peak-to-average power ratio (PAPR) of the transmitter’s output signal. The selected mapping (SLM) approach provides good performance for PAPR reduction, but it requires a bank of inverse fast Fourier transforms (IFFTs) to generate a set of candidate transmission signals, and this requirement usually results in high computational complexity. In selected mapping (SLM) method a whole set of candidate signals is generated representing the same information, and then the most favourable signal as regards to PAPR is chosen and transmitted. SLM scheme is one of the initial probabilistic approaches for reducing the PAPR problem, with a goal of making occurrence of the peaks less frequent, not to eliminate the peaks. The scheme can handle any number of subcarriers and drawback associated with the scheme is the overhead of side information that needs to be transmitted to the receiver. This paper studies the problem of peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) system. It introduces a new PAPR reduction method that performs signal processing in the OFDM time domain together with the SLM method. The simple non-linear amplitude pre-distortion process of the proposed method changes the statistical characteristics of the OFDM symbol from Rayleigh distribution into Gaussian like distribution. This change in statistical distribution is accompanied by changes of the peak and average power values and consequently reduces in the PAPR value. For the QPSK OFDM system with 128 sub-carriers, 3dB reduction in PAPR value is achieved. The proposed method shows that SLM with square rooting have higher PAPR reduction compare to the original SLM method. The approach is very flexible and can work with arbitrary number of sub-carries without restriction on the type of modulation applied on them.

Low-Complexity All-Digital Sample Clock Dither for OFDM Timing Recovery

Based on phase adjustment, this work investigates a low-complexity all-digital sample clock dither (ADSCD) to perform coherent sampling for orthogonal frequency-division multiplexing (OFDM) timing recovery. To reduce complexity, only tri-state buffers are acquired to build a multiphase all-digital clock management (ADCM), which can generate more than 32 phases over gigahertz without phase-locked or delay-locked loops. Following divide-and-conquer search and triangulated approximation, the phase adjustment is simple but efficient, such that four preambles are adequate to make analog-to-digital (A/D) sampling coherent. Performance evaluation indicates that the proposed ADSCD can tolerate ±400-ppm clock offsets with 0.8 ~ 1.3-dB signal-to-noise ratio (SNR) losses at 8% PER in frequency-selective fading. Hence, this scheme involves a little overhead to ensure fast recovery and wide offset tolerance for OFDM packet transmissions.

Orthogonal Frequency Division Multiplexing Timing Synchronization in Multi-Band Orthogonal Frequency Division Multiplexing Ultra-Wideband Systems

Problem statement: MB-OFDM UWB systems have attractive features and received a great attention. It has the capability of sending/receiving data at rates in access of 100 Mbits sec-1. Owing to the sensitivity of OFDM systems to synchronization errors, a fast synchronization algorithm is required to discern for the start of OFDM symbol. Existing approaches to OFDM time synchronization rely on their proposed preamble. Approach: In this study, a comprehensive algorithm for Orthogonal Frequency Division Multiplexing (OFDM) timing synchronization suitable for Multi-Band OFDM Ultra-Wideband (MB-OFDM UWB) systems was proposed. Firstly, we proposed a technique for packet detection in the multi-band scenario. Secondly, a new timing metric for timing synchronization is proposed. Then, we propose a technique for frame synchronization. Finally, time frequency code identification and low-complexity timing refinement technique was proposed. Results: The performance of the timing synchronization (coarse synchronization and timing refinement) is evaluated in terms of Mean Squared Error (MSE) in MB-OFDM UWB Channel Models (CM1, CM2, CM3 and CM4). It was found that the proposed method performs better than existing approaches. Conclusion: A suitable timing synchronization steps for MB-OFDM UWB systems was proposed. The proposed method can be applied on any preamble signal.

Effect of imperfect noise estimation in timing synchronization

This paper calculates the performance of an orthogonal frequency division multiplexing (OFDM) system confronted with Rayleigh fading channels in terms of sync detection probability. We adopted a timing synchronization scheme employing simple repeated constant amplitude zero auto-correlation (CAZAC) training symbol with relevant correlation techniques, and analyzed the effect of imperfect noise estimation. Some probability expressions of OFDM timing synchronization for system measurement are evaluated.

Peak-to-average power ratio reduction in OFDM using cyclically shifted phase sequences

Two approaches for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) are proposed that are relied on a set of cyclically shifted phase sequences (CSPS) and implemented using the time domain circular convolution. After multiplying CSPS with the frequency domain data, the signal candidates can be expressed as weighted sum of the circularly shifted OFDM time domain data in the first method, which is called CSPS method. In the second method, weighted coefficients for generating the signal candidates in CSPS method are optimally selected to improve its performance; thus, the second method is referred to as optimised CSPS (OCSPS) method. The performances of the CSPS and OCSPS methods are evaluated using simulated data and compared with those of selective mapping (SLM) and partial transmit sequences (PTS). The simulation results show that both the CSPS and OCSPS methods can reduce the PAPR effectively, and that the OCSPS performs even better than the CSPS. The OCSPS can achieve the same performance as compared to the PTS. A distinct feature of the proposed methods is that only one inverse discrete Fourier transform is needed, and thus, the candidates can be calculated in time domain directly.

OFDM Timing Estimation Method without Training Symbol

In this letter, we present timing synchronization method using two consecutive orthogonal frequency division multiplexing (OFDM) symbols which are designed to be cyclic-shifted against each other. Our approach can be viewed as an expansion of Minn's correlation methods. Using the proposed OFDM signal, however, we can estimate the timing offset without any training symbol.

Analysis of OFDM Timing Synchronization Using Multipath Exploitation

This letter derives and computes the detection probability for timing synchronization in an orthogonal frequency division multiplexing (OFDM) system encountered with a multipath Rayleigh fading channel. OFDM timing synchronization using constant amplitude zero auto-correlation (CAZAC) training symbols and correlation techniques is adopted. With this provision, we focus on the numerical analysis for OFDM timing synchronization scheme employing a preadvancement technique to reduce the inter-symbol interference (ISI). For measuring system performance, the sync detection performance derived in the considered system is presented in a multipath Rayleigh fading channel.

Blind OFDM symbol synchronization in ISI channels

We present a new algorithm for blind symbol synchronization in orthogonal frequency division multiplexing (OFDM) systems. The new algorithm declares symbol synchronization when a certain autocorrelation matrix, constructed from the received signal, achieves minimum rank. Unlike previously proposed blind algorithms, the new rank method guarantees correct symbol synchronization, even in the presence of intersymbol interference. Also, it does not assume that the OFDM time samples are i.i.d. In particular, the rank method works even with OFDM systems that employ pulse shaping. The increased complexity of the algorithm would be acceptable for systems, such as fixed-receiver broadcast systems, that require guaranteed synchronization under all conditions.