Presence Of Frequency Offsets Research Articles

Selection Combiner in Time-Varying Amplify Forward Cooperative Communication

This research presents the diversity combining schemes for Multiple Symbol Double Differential Sphere Detection (MSDDSD) in a time-varying amplify-and-forward wireless cooperative communication network. Four diversity combiners, including direct combiner, Maximal Ratio Combiner (MRC), semi MRC and Selection Combiner (SC) are demonstrated and explained in details. A comprehensive error probability and outage probability performance analysis are carried through the flat fading Rayleigh environment for semi MRC and SC. Specifically, error performance analysis is obtained using the PDF for SC detectors. Finally, power allocation expression based on error performance minimization approach is presented for the proposed SC performance optimization. It is observed that the performance analysis matches well with the simulation results. Furthermore, the proposed SC scheme offers better performance among the conventional MRC and direct combiner schemes in the presence of frequency offsets.

Joint low‐complexity equalization and CFO estimation and compensation for UWA‐OFDM communication systems

SummaryFrequency synchronization has a great importance in preserving the performance of the underwater acoustic (UWA) orthogonal frequency division multiplexing (OFDM) systems. The carrier frequency offset (CFO) estimation can be blind or data‐aided. In this paper, the Zadoff‐Chu (ZC) sequences are used for OFDM synchronization in UWA communications, and they are compared with different data‐aided algorithms. We propose a low‐complexity algorithm for CFO estimation based on ZC sequences. Also, a joint equalization and CFO compensation scheme for UWA‐OFDM communication systems is presented. Simulation results demonstrate that the proposed CFO estimation algorithm allows estimation of the CFO accurately with a simple implementation in comparison with the traditional schemes. Also, the performance of the UWA‐OFDM system can be preserved in the presence of frequency offsets.

Timing Synchronization Performance of Preamble Signal with Orthogonal Frequency Multiplexed Data Signal in the Presence of Frequency Offset

Wireless machine-to-machine (M2M) has been applied to a very wide area of industry and has been used for today’s communication systems. Automatic control of industrial machinery is one of the applications of wireless M2M and requires high reliability and efficiency of data transmission. In order to shorten frame length, a frame format that multiplexes a data signal to the IEEE 802.11a short preamble sequence has been proposed. This paper evaluates the performance of an iterative frequency offset compensation scheme in terms of a timing synchronization probability and a bit error rate (BER). Three iterations of frequency offset compensation achieve about 0.06 improvement on the timing synchronization probability and the BER degradation is suppressed within 3 dB from the theoretical BER performance.

Does amplify-and-forward cooperative relay improve OFDM/OFDMA ergodic capacity?

The ergodic information rate for Orthogonal Frequency-Division Multiplexing / Orthogonal Frequency-Division Multiple Access with amplify-and-forward (AF) relaying systems in the presence of frequency offsets is evaluated. Unlike previous work, per-subcarrier adaptive power allocation is performed on each relay to optimize the system ergodic information rate. For a given frequency offset and total number of relays M, the AF ergodic information rate is proven to be a monotonically increasing function of a (the ratio of the power allocated to the source node and the total transmit power), implying that the maximum ergodic information rate can be obtained at α=1 (i.e., there is no cooperative relay). Furthermore, the proof of "cooperative relays cannot improve the AF ergodic information rate in a quasi-static wireless channel" is also provided in this letter.

Enhanced Illumination Sensing Using Multiple Harmonics for LED Lighting Systems

This paper considers frequency division multiplexing (FDM) based illumination sensing in light emitting diode (LED) lighting systems. The purpose of illumination sensing is to identify the illumination contributions of spatially distributed LEDs at a sensor location, within a limited response time. In the FDM scheme, LEDs render periodical illumination pulse trains at different frequencies with prescribed duty cycles. The problem of interest is to estimate the amplitudes of the individual illumination pulse trains. In our previous work, an estimation approach was proposed using the fundamental frequency component of the sensor signal. The number of LEDs that can be supported by this estimation approach is limited to around 100 LEDs at a response time of 0.1 s. For future LED lighting systems, however, it is desirable to support many more LEDs. To this end, in this paper, we seek to exploit multiple harmonics in the sensor signal. We first derive upper limits on the number of LEDs that can be supported in the presence of frequency offsets and noise. Thereafter, we propose a low complexity successive estimation approach that effectively exploits the multiple harmonics. It is shown that the number of the LEDs can be increased by a factor of at least five, compared to the estimation approach using only the fundamental frequency component, at the same estimation error.

Secondary Transceiver Design in the Presence of Frequency Offset between Primary and Secondary Systems

When both primary and secondary systems are orthogonal frequency division multiplexing modulated and are non-cooperative, carrier frequency offset between the systems is inevitable to cause harmful interference. In this paper, we jointly optimize secondary transceivers assuming that the frequency offset between the secondary transmitter (ST) and the primary receiver (PR) and different channel information from the ST to the PR are known at the ST. We first derive unified interference constraints and obtain the secondary transceivers minimizing the mean square error through convex optimization techniques. We then derive closed-form transceivers for several special cases to reveal the impact of the frequency offset on the secondary transceivers. We show that when there is no frequency offset between the ST and the PR, the optimal processing at the ST is power allocation. Otherwise, both power allocation and precoding are necessary. The impact of the frequency offset on the performance of both systems increases as the interference constraints become tighter and the bandwidth of the primary system becomes smaller. When the proposed transceivers are used, the performance of the secondary system is robust to the frequency offset and the performance of the primary system degrades little due to the remanent frequency offset.

Iterative detection of turbo-coded offset QPSK in the presence of frequency and clock offsets and AWGN

The key contribution of this work is to develop transmitter and receiver algorithms in discrete-time for turbo- coded offset QPSK signals. The proposed synchronization and detection techniques perform effectively at an SNR per bit close to 1.5 dB, in the presence of a frequency offset as large as 30 % of the symbol-rate and a clock offset of 25 ppm (parts per million). Due to the use of up-sampling and matched filtering and a feedforward approach, the acquisition time for clock recovery is just equal to the length of the preamble. The carrier recovery algorithm does not exhibit any phase ambiguity, alleviating the need for differentially encoding the data at the transmitter. The proposed techniques are well suited for discrete-time implementation.

Improved OFDMA uplink transmission via cooperative relaying in the presence of frequency offsets—Part II: Outage information rate analysis

AbstractIn Part I of this paper, the ergodic information rate is derived for an orthogonal frequency‐division multiplexing access (OFDMA) uplink with cooperative relaying in the presence of frequency offsets. In this part, the outage information rate of the system is analysed. Both amplify‐and‐forward (AF) and decode‐and‐forward (DF) relays are considered. Each node can play the roles of the source node and the relay, simultaneously, at different subcarriers. Both the outage information rate and the diversity gain are derived for the interference‐limited environment. Numerical results illustrate the superior performance of the proposed cooperative scheme over conventional transmission (without relaying) with regard to outage information rate. Copyright © 2009 John Wiley & Sons, Ltd.

Cooperative OFDM Channel Estimation in the Presence of Frequency Offsets

Channel estimation in the presence of frequency offsets is developed for cooperative orthogonal frequency-division multiplexing (OFDM) systems. A two-time-slot cooperative channel estimation protocol is proposed. The source broadcasts the training sequence to the relays and the destination (first time slot), and the relays retransmit the training sequence (second time slot). Pilot designs for amplify-and-forward (AF) and decode-and-forward (DF) relays are derived. These designs eliminate interrelay interference (IRI), which occurs due to the simultaneous relay retransmissions, and minimize the mean square error (MSE). Consequently, the number of AF and DF relays is constrained to be less than lfloorN/(2L - 1)rfloor and lfloorN/Lrfloor, respectively, where N is the total number of subcarriers, L is the channel order, and lfloorarfloor is the maximum integer part of alpha. The pairwise error probability (PEP) of orthogonal space-time coding in cooperative OFDM due to both frequency offset and channel-estimation errors is also evaluated. The optimal power allocation ratio between the source and the relays to minimize the PEP is derived for AF and DF relays. When L < 16, DF relays outperform AF relays in terms of PEP. With L = 4 and 16 active relays, the gap is 9 dB for a frequency offset error variance of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , and this gap increases to about 11.3 dB when the variance increases to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> .

Design of distributed unique words for enhanced frame synchronization

A novel approach is introduced for the design of a unique word (UW) for frame synchronization in time division multiplexing/time division multiple access (TDM/TDMA) systems. In particular, the UW symbols are distributed in each frame in a specific pattern in order to optimize the UW autocorrelation properties. The distribution patterns are scalable, since they preserve their autocorrelation properties under truncation. The resulting frame structure enables the use of several types of UWs, including the silent-UW (SUW) and the unit-UW (UUW), that are novel approaches for which the optimal Bayes detector is derived. An analytical performance characterization is presented along with comparisons with state-of-the-art alternatives. We show that SUW can significantly outperform the classic UW in the presence of frequency offsets or whenever data interference is the main impairment to cope with, i.e. at moderate to high signal to noise ratios. Interestingly, this performance improvement comes with the further advantage of a considerable complexity reduction. Hence UUW is shown to provide both performance improvement and complexity reduction.

Matched nonlinearities for frame synchronisation in presence of frequency offsets

In communication systems data are transmitted in frames. The frame boundaries are indicated by transmitting a number of a priori known symbols. The process of detecting the frame boundaries is essential to correct data reception at the receiver. Frame synchronisation also frequently needs to be achieved prior to carrier frequency synchronisation. A number of techniques that achieve frame synchronisation prior to carrier synchronisation have been described in the technical literature. It is shown that matched nonlinearities can be used to improve the performance of the best performing, maximum-likelihood based frame synchroniser known in the literature, by about 1 dB in required signal-to-noise ratio, for the same accuracy in frame synchronisation, with essentially the same implementation complexity.

A Reduced-PAR Opportunistic STBC Scheme for Frequency-Selective Channels in the Presence of Frequency Offset

Opportunistic space-time block codes (STBC) have been investigated for flat-fading channels in the literature. In this paper, we present a novel scheme for integrating them with orthogonal frequency division multiplexing (OFDM) for frequency-selective channels with an unequal error protection capability making it attractive for multimedia and multicasting applications. In addition, our new scheme optimizes the tradeoff between coding gain and peak-to-average ratio (PAR) while minimizing inter-carrier interference in the presence of carrier frequency offset. We show that our scheme achieves a 2 dB reduction in PAR over the conventional scheme.

PAR-Constrained Training Signal Designs for MIMO OFDM Channel Estimation in the Presence of Frequency Offsets

Training signals for OFDM channel estimation should possess low PAR to avoid nonlinear distortions at the transmit amplifier and at the same time they should be robust against frequency offsets. In this letter, we show that the above two requirements of the training signals are conflicting. We propose two new training signal designs for MIMO OFDM frequency-selective channel estimation. Our proposed training signals achieve more robust channel estimation performance against frequency offsets while satisfying the PAR constraints compared to training signals designed to achieve a fixed low PAR but without any consideration for robustness to frequency offsets.

Maximin Algorithm With a Step-Length Estimation Technique

We propose a technique that enhances the performance of Torrieri and Bakhru's maximin algorithm in the presence of frequency offsets and modulated interferences, a common scenario in multiple-access environments, where we observed that the Maximin algorithm suffers performance degradation in the aforesaid scenario. To combat modulated interferences and fading effects, we propose to update the step length during every smart antenna weight vector update. We found that our scheme improves the robustness of the maximin algorithm to a greater extent. Simulation results for a frequency-hopping system confirm the validity of the approach and demonstrate improvements in the performance of the maximin algorithm in the presence of frequency offsets and data modulated interferences in a fading channel environment

An Integrated Framework for MC-CDMA Reception in the Presence of Frequency Offsets, Phase Noise, and Fast Fading

The combination of code-division multiple-access and multicarrier (MC) modulation has been proposed to develop high data-rate wireless communication systems. Due to the longer symbol duration in comparison with single-carrier systems, MC systems are more sensitive to various imperfections, including phase noise and frequency offsets due to local oscillators and Doppler spreading due to motion that results in temporal channel variations. The performance of current systems is significantly limited by these imperfections because they disperse the transmitted power in a particular subcarrier into adjacent subcarriers, thereby causing interference between the subcarriers at the receiver. We consider a single-user communication system and use a canonical model for the received signal that efficiently captures the effects of all impairments. The model uses Fourier basis functions that are fixed for all imperfections while the expansion coefficients depend on imperfections. Using the model, we introduce a receiver structure that implements the matched filter (MF), and hence, optimal. The MF is implemented through a Rake receiver in the frequency domain. The receiver fully compensates for frequency offsets as well as phase noise, thereby eliminating the performance loss due to these factors. Furthermore, in contrast to existing designs, it delivers improved performance under fast fading by exploiting Doppler diversity. Finally, the same integrated receiver structure works for all imperfections eliminating the need for devising a separate correction technique for each.

Full-duplex fast estimation of echo and channel responses in the presence of frequency offsets in both far echo and far signal

In a previous publication, we proposed a full-duplex fast training procedure for simultaneously estimating echo and channel responses. It reduces the tap-setting time to half of that required by the traditional half-duplex fast training schemes. However, its estimation accuracy may be degraded by the frequency offsets in both far echo and far signal that are caused by the analog carrier network. We extend the previous work and develop a new algorithm that can compensate for the phase rotation components introduced by these frequency offsets. The performance of the new method is analyzed in terms of mean-square error. Simulation results are presented to confirm the analysis.

Carrier frequency acquisition and tracking for OFDM systems

We present and analyze a technique for fast acquisition and accurate tracking of the carrier frequency in orthogonal frequency division multiplexing (OFDM) receivers. The scheme is based on a data-aided frequency estimation algorithm. The presence of known symbol sequences periodically inserted in the OFDM frame allows the data demodulator to rapidly lock onto the carrier frequency during the acquisition phase, even in the presence of frequency offsets up to a few tenths of the overall signaling rate. Once acquisition is over, the circuit switches to a decision-directed mode to perform fine frequency tracking for reliable data demodulation. The algorithm performance is analyzed in terms of width of the lock-in frequency range and of lock-in probability in the acquisition mode, and of mean-square frequency estimation error in the tracking mode. Since OFDM is known to be extremely sensitive to carrier frequency errors, the impact of the carrier frequency synchronizer on the receiver error rate is also investigated.

Acquisition and Tracking Performance of a PLL Using a Class of Nonlinear Filters

A class of nonlinear filters has been proposed as an alternative loop filter of a PLL. This paper examines further aspects of the performance of a PLL incorporating such a nonlinear filter. The acquisition performance due to frequency offsets and the steadystate tracking performance in the presence of noise and frequency offsets are analyzed and compared with experimental and simulation results. The noise analysis is based on techniques using the FokkerPlanck equation. The effect of the nonlinearity in the loop filter is approximated using a quasi-linearization approach. A modification to the original nonlinear filter structure is proposed in order to obtain the best noise performance in the presence of frequency offsets. The overall improvement in performance of a PLL with a nonlinear filter is evaluated and compared with a conventional PLL and other methods of improved acquisition performance such as the sweep method. The necessary graphs and formulas are presented for the design of a PLL with a nonlinear filter. The extra available design parameter enables the compromise between acquisition and noise performance to be overcome to a significant degree.