Mohammad Javad Omidi

Parallel structures for joint channel estimation and data detection over fading channels

Joint data and channel estimation for mobile communication receivers can be realized by employing a Viterbi detector along with channel estimators which estimate the channel impulse response. The behavior of the channel estimator has a strong impact on the overall error rate performance of the receiver. Kalman filtering is an optimum channel estimation technique which can lead to significant improvement in the receiver bit error rate (BER) performance. However, a Kalman filter is a complex algorithm and is sensitive to roundoff errors. Square-root implementation methods are required for robustness against numerical errors. Real-time computation of the Kalman estimator in a mobile communication receiver calls for parallel and pipelined structures to take advantage of the inherent parallelism in the algorithm. In this paper different implementation methods are considered for measurement update and time update equations of the Kalman filter. The unit-lower-triangular-diagonal (LD) correction algorithm is used for the time update equations, and systolic array structures are proposed for its implementation. For the overall implementation of joint data and channel estimation, parallel structures are proposed to perform both the Viterbi algorithm and channel estimation. Simulation results show the numerical stability of different implementation techniques and the number of bits required in the digital computations with different estimators.

Joint Data and Kalman Estimation for Rayleigh Fading Channels

Channel estimation is an essential part of many detection techniques proposed for data transmission over fading channels. For the frequency selective Rayleigh fading channel an autoregressive moving average representation is proposed based on the fading model parameters. The parameters of this representation are determined based on the fading channel characteristics, making it possible to employ the Kalman filter as the best estimator for the channel impulse response. For IS-136 formatted data transmission the Kalman filter is employed with the Viterbi algorithm in a Per-Survivor Processing (PSP) fashion and the ove rall bit error rate performance is shown to be superior to that of detection techniques using the RLS and LMS estimators. To allow more than one channel estimation per symbol interval, Per-Branch Processing (PBP) method is introduced as a general case of PSP and its effect on performance is evaluated. The sensitivity of performance to parameters such as fading model order and vehicle speed is also studied.

Improved channel estimation using noise reduction for OFDM systems

A new channel estimation algorithm for OFDM systems is proposed in this paper based on a noise reduction method. The performance of a least squares (LS) channel estimator is improved by using the colored noise characteristics in the LS estimation of the channel impulse response (CIR). The noise, which is in the CIR (signal) subspace, is estimated and suppressed based on the correlation between two parts of noise, which are within and outside the signal subspace. Knowledge of the CIR statistical parameters is not needed in the proposed estimation algorithm as long as the CIR length is not longer than the cyclic prefix interval.

Equalisation of SIMO-OFDM systems with insufficient cyclic prefix in doubly selective channels

Time variations of a doubly selective wireless channel and insufficient cyclic prefix (CP) length of an orthogonal frequency division multiplexing (OFDM) transmission system cause intercarrier interference (ICI) and interblock interference (IBI) as significant limitations. This paper investigates the problem of joint ICI and IBI mitigation in single-input multiple-output OFDM (SIMO-OFDM) systems. It is assumed, unlike most existing literature, that the channel delay spread is larger than the CP, and also the channel varies on each OFDM block. First, doubly selective channel is modelled using basis expansion model (BEM) and a closed-form expression for signal-to-interference-plus-noise ratio (SINR) is derived. Then, a time-domain equaliser is developed, which maximises the SINR for all subcarriers. Moreover, a frequency-domain equalisation approach is proposed which is based on the MSE minimisation per tone. A low-complexity implementation of the per-tone equaliser is also derived. An important feature of the proposed equalisers is that no bandwidth expansion or redundancy insertion is required except for the CP. Finally, complexity comparison and simulation results over Rayleigh fading channel are provided to illustrate the effectiveness of the proposed approaches. Since both equalisers are designed in the frequency domain, they provide significant interference cancellation.

Joint data and channel estimation using the per-branch processing method

Joint data and channel estimation in a wireless communication system and overfrequency selective Rayleigh fading channels can be performed by implementing maximum likelihood sequence estimation (MLSE) using the per-survivor processing (PSP) method. However PSP can be used only if there is one channel estimation per symbol interval. In this paper we introduce the per-branch processing (PBP) method as a general case of PSP, which has the advantages of PSP and allows more than one estimation per symbol to improve the receiver error performance in fast fading. The Kalman filter is considered for channel estimation and the overall bit error rate (BER) performance is shown to be superior to that of detection techniques using the RLS and LMS estimators. Three different square-root methods for implementation of the Kalman filter are analyzed and compared with the RLS and LMS algorithms based on different number of bits required for implementation.

Papr Reduction in OFDM Systems Using Polynomial-Based Compressing and Iterative Expanding

In this paper we propose a novel algorithm for PAPR reduction of an OFDM system, based on a companding scheme. In this method a compressing polynomial is appended to the IFFT block at the transmitter and at the receiver the FFT block is combined with a reverse expanding function where the iterative Jacobis method is used for solving equations. The proposed method entails less complexity at the transmitter in comparison with other PAPR reduction algorithms. It also requires less increase in SNR for the same BER compared to other companding methods. A trade off between complexity and performance can set the order of compressing polynomial and the number of iterations for the proposed algorithm at the receiver

Optimum Sensor Selection Based on Energy Constraints in Cooperative Spectrum Sensing for Cognitive Radio Sensor Networks

Cooperative spectrum sensing (CSS) has been proposed to improve the reliability of decisions made about the presence of primary users in cognitive radio networks. However, new solutions are required to face various challenges in the implementation of CSS. Efficiency in energy consumption for CSS is a challenging issue, which should be solved through the effective management of sensors for CSS. In this paper, all the subsets of sensors, which cooperatively satisfy the desired sensing accuracy, are formed. Our aim is to propose a heuristic algorithm, which chooses the subset with minimum average energy consumption for CSS. In addition, a sub-optimal algorithm is proposed to reduce the computational complexity of the heuristic algorithm. The simulation results confirm the advantages of the proposed algorithms in terms of energy-efficient sensor selection for CSS and low complexity compared with the other state-of-the-art methods.

Reduction of out of band radiation using carrier-by-carrier partial response signalling in orthogonal frequency division multiplexing

Orthogonal frequency division multiplexing (OFDM) is a mature and one of the most popular multicarrier modulation (MCM) techniques. Also, it is the main candidate for physical layer of cognitive radio (CR) networks. CR is a new method to satisfy ubiquitous demand for wireless services while there is no enough unlicensed spectrum. However, the most important shortcoming of OFDM-based CR systems is the high level of out of band (OOB) components that originate from simple fast Fourier transform (FFT)-based implementation. In this study, the authors propose a novel method to reduce side-lobes of OFDM spectrum. In this method proper carrier-by-carrier partial response signaling is used on the modulated symbols across the time. This method will allow for using other techniques for OOB radiation or peak-to-average power ratio (PAPR) reduction, while does not require high complexity at the transmitter, i.e. just the receiver should have either maximum likelihood sequence detector (MLSD) in case of no pre-coding or a simple slicer in case of pre-coding. Moreover, it will not affect the PAPR but will increase word error rate (WER). Simulation results show that about 7 dB reduction in OOB components can be expected by this method.

Analysis of a multi-user cognitive radio network considering primary users return

A complete distributed single-channel multi-user cognitive radio network with a common control channel is analyzed. To derive the necessary metrics, we exploit renewal reward theory by which we are able to carry out a continuous time analysis of the whole network. This approach can be used as a framework for some other scenarios with a few changes in the modeling of the renewal process.The imperfect behavior of secondary sensor as well as changes in the occupancy state of PU traffic during SU transmission is considered in deriving the probability of collision and it is introduced as a constraint for the problem of maximizing the network throughput. Consequently, the interference due to both primary user re-occupancy and sensing error in the detection of primary user are considered.We derive mathematical closed form expressions for the expected amount of interference and transmission time in a renewal cycle and hence for the collision and transmission rates as well as other performance measures, those which are important for network design and network planning.The analysis used in this paper can be easily extended to some other scenarios such as slotted transmission protocols. Furthermore, additional constraints such as energy can be easily included in the analysis since the complete continuous-time behavior of secondary and primary users in the spectrum access can be modeled. Multi-user cognitive radio networks have been considered in the literature recently. However, there is no analytical framework which can provide a model in order to derive the system performance metrics. This paper presents an analytical continuous time framework for a multi-user cognitive radio in which secondary users (SUs) communicate on a single common control channel, the most common protocol discussed in the multi-user category. The proposed analysis method is based on the renewal theory. We prove that the spectrum access of SUs with respect to the primary user (PU) traffic behavior forms a renewal process. Furthermore, the corresponding renewal cycle is derived and metrics such as collision probability and interference time due to both sensing error and PU re-occupancy are formulated in the renewal cycle. Thanks to the proposed continuous time analyses, the transmission efficiency for the secondary network is formulated. Finally, some numerical analyses are provided on the derived equations to discuss the optimum transmission time for SUs in order to have the maximum efficiency under the prescribed collision constraint. As an example, for the sensor, probabilities of false alarm and miss-detection are taken to be equal to 0.1 and 0.01, respectively, with a primary network in which the idle and busy cycle rates are 0.1 and 0.3; also, the transmission time must be bounded to 500 ms in order for the collision probability to remain under 0.01. At the end, we show that simulation results are consistent with the numerical analyses. Display Omitted

Sensitivity Analysis of OFDMA and SC-FDMA Uplink Systems to Carrier Frequency Offset

Orthogonal frequency division multiple access (OFDMA) and single carrier frequency division multiple access (SC-FDMA) are two technologies for the uplink transmission of present and next generation of broadband wireless systems. This paper studies and compares OFDMA and SC-FDMA in terms of sensitivity to carrier frequency offset (CFO) in the uplink. In order to calculate signal-to-interference ratio (SIR), we use a simple superposition principle approach where the contributions of different users are studied separately. We derive closed-form mathematical expressions for the desired signal, interference terms, and consequently SIR for both OFDMA and SC-FDMA systems in the uplink. It is pointed out that there is a strong relationship between sensitivity analysis to CFO and subcarrier allocation schemes. Also, we prove that the derived expressions for both systems are reduced to very simple forms in interleaved subcarrier allocation. Finally, the theoretical analysis are verified using Monte Carlo simulations in block, interleaved, and block-interleaved subcarrier allocations, and the two systems are compared upon these set of results.