Yong Huat Chew

A New Blind Joint Timing and Frequency Offset Estimator for OFDM Systems Over Multipath Fading Channels

A blind joint timing and frequency synchronization algorithm is derived for orthogonal frequency-division multiplexing (OFDM) signals transmitting over multipath fading channels. The proposed estimator requires neither the knowledge of the signal-to-noise ratio (SNR) nor the power delay profile of the fading channels, except that the maximum delay spread L should be known and should be shorter than the guard interval. In the proposed estimator, the transmitted time-domain OFDM samples are modeled as an independent Gaussian random sequence, and the received time-domain OFDM samples are partitioned into a few subsets in such a way that the neighboring entries in each subset are uncorrelated. This is achieved by picking out OFDM samples that are spaced L samples apart into each subset. By modeling the channel gains as unknown deterministic variables, a joint symbol timing and frequency offset (FO) estimation algorithm is derived based on these subsets. Simulation results show that, in terms of lock-in probability, the proposed blind ST estimator achieves better performance than the estimators studied by Van De Beek and Speth , as well as the approximate maximum-likelihood estimator described by Lee and Cheun when the SNR is sufficiently high. For FO estimation, in terms of mean square error (MSE), the proposed estimator outperforms the estimators studied by Van De Beek and Speth at medium and high SNR. In terms of bit error rate (BER), the proposed estimator is superior to the estimators studied by Van De Beek and Speth Furthermore, the simulation results show that the performance of the proposed estimator is tolerant to the variation in the number of delay paths. This means that the proposed estimator will still adequately perform, even when the number of delay paths is time varying and is not estimated at the receiver. Finally, simulation results show that the use of virtual subcarriers has negligible performance loss in the proposed estimator.

On the Modeling of a Non-Cooperative Multicell OFDMA Resource Allocation Game with Integer Bit-Loading

We address the adaptive allocation of subcarrier, bit and power resources among the base stations (BSs) of a downlink multi-cell OFDMA system using the non-cooperative game theoretic approach. Unlike the information theoretic approach where continuous bit-loading is used, our algorithm allocates integer number of bits to each subcarrier. Using a utility function with linear pricing on both the transmit bit rate and power, we first show that for a simple two-cell system with single user and fixed modulation, a Nash equilibrium (NE) solution always exists. However, for games formulated on more complicated systems such as with more than two cells, with more than one user in a cell, or when adaptive modulation is used, we observed that the existence of NE cannot be guaranteed. If NE points exist, the game can be played repeatedly with each BS (the player) selects its best strategy until the solution converges, or known as the myopic game. Based on the framework of potential games with coupled constraints, we propose an algorithm which can achieve a stable solution among the players despite that NE solution to the originally problem does not necessarily exist. The convergence of the proposed algorithm is verified by simulation.

A Linear Programming Solution to the Subcarrier-and-Bit Allocation of Multiclass Multiuser OFDM Systems

Adaptive subcarrier-and-bit allocation in orthogonal frequency division multiplexing (OFDM) systems can significantly improve spectral efficiency and reduce power consumption. The combinatorial optimization on the power, bit and subcarrier allocation in multiclass multiuser OFDM systems normally involved nonlinear objective functions and constraints, resulting in highly complex and computation intensive optimization problems. In this paper, we propose a method to linearize and simplify the optimization problem formulated on subcarrier-and-bit allocation in multiclass multiuser OFDM systems. With a drastically reduced complexity, the converted linear optimization problem can be solved in close to real time and the solutions found are optimal, as no relaxation is made during the linearization. Different approximate relationships between symbol error rate (SER) and bit error rate (BER) are also used to investigate the calculation on the required transmit power.

A Linear Programming Solution to Subcarrier, Bit and Power Allocation for Multicell OFDMA Systems

We address the adaptive allocation of subcarrier, bit and power in the downlink of multicell OFDMA systems which are operating at a frequency reuse factor of one. When a centralized scheduler is used, the presence of co-channel cell interference inter-relates the assignment of the subcarriers and powers in all the cells, making the optimal allocation of radio resources a challenging problem. The lack of optimal solution in the literature for such a centralized approach is attributed to the difficulties in solving the mixed integer nonlinear programming problem. We show that through some manipulations, the solution to the original problem can be decoupled into solving two sub- problems sequentially: first solve for power allocation which is a linear programming and then subcarrier-bit allocation which is a binary linear programming. The elimination of the nonlinear constraints in our approach results in the significant reduction in the computation complexity, making optimal solution becomes possible. Although optimal solution not always can be computed in real time when the number of subcarriers is large, it can still serve as a performance benchmark to future developed heuristic algorithms. Finally, results in our paper are more practical since discrete values are used for bit-loading. This is different to most of the reported work in the literature which adopted information theoretic approach.

A joint blind timing and frequency offset estimator for OFDM systems over frequency selective fading channels

In this paper, a joint symbol timing and frequency-offset estimator based on the maximum likelihood (ML) criterion, is derived for OFDM systems over frequency selective fading channels. .We use the linear minimum mean square estimation technique (LMMSE) to estimate the nuisance parameters such as transmitted signals and fading gains, and arrive at an algorithm to jointly estimate symbol timing and frequency offset. Simulation results show that our proposed symbol timing estimator performs better than the estimator reported by van de Beek et al (1997), while our proposed frequency offset estimator achieves a lower MSE at higher SNR (ges 15 dB). Simulation results also show that our proposed symbol timing and frequency offset estimator has better MSE performance than the cyclostationarity-based estimator studied in T. Pollet and M. Moeneclaey (1995) at higher SNR (for example 15 dB). However, in terms of probability of correct timing synchronization and MSE, our symbol timing estimator achieves roughly the same performance as the estimator by Lee et al (2002), although a different approach is used in our estimator. Our proposed estimator is next modified to two practical ones independent of SNR, furthermore, one of which needs only the knowledge of the maximum delay spread to perform estimation. We also extend our proposed estimator to applications where multiple OFDM symbols can be observed to perform synchronization

Sensitivity study of location management area partitioning in cellular communication systems

The problem of finding the appropriate location area (LA) partitioning that minimizes the total signaling cost in cellular radio network location management is important since it maximizes the bandwidth available for revenue-generating services. All of the existing work involves finding the solution for a given fixed set of parameters. However, we feel that the result obtained is generally insufficient to the problem, since values taken by the parameters of a mathematical programming model at the time of its formulation are only estimates of real-life conditions. The optimal solution should instead be treated as a starting point for further sensitivity study of the problem. This is particularly important in helping to identify any vast deviation of the parameters that affect the system performance. Such a study is still lacking for in cellular communication systems, which normally have their design parameters varying or fluctuating due to unpredictable human movements. In this paper, we present the sensitivity study on the static location management design of a GSM cellular communication system which is modeled as a non-linear programming problem. We investigate the sensitiveness of the total signaling cost with one of these parameters: cell crossing rates, call arrival rates, paging and update costs, deviates from the design value. The bounds for these parameters within which the parameters can change but the LA partitioning remains still optimal are presented.

A new approach for finding optimal base stations configuration for CDMA systems jointly with uplink and downlink constraints

The problem of finding an optimal base station (BS) configuration for CDMA networks given a set of candidate BS sites is investigated. The design is first formulated as an optimization problem. Features which are unique to CDMA, such as power control, intra-cell and other cell interference, etc., are included in our consideration. In particular, the signal-to-interference ratio (SIR) in both uplink and downlink of each MS is taken as the quality of measure. The solution to the problem is simplified by using a heuristic algorithm. The heuristic begins from all candidate BS sites being activated and assignment of each mobile station (MS) to BS is based on smallest path loss. It then proceeds to deactivate the sites one by one until the minimum number of sites needed to serve the traffic is obtained

An EM-Based Semiblind Joint Channel and Frequency Offset Estimator for OFDM Systems Over Frequency-Selective Fading Channels

We propose an estimator that will jointly estimate the channel and the carrier frequency offset (CFO) for orthogonal frequency-division multiplexing (OFDM) systems based on the maximum-likelihood criterion. The proposed estimator uses only one received OFDM symbol to perform estimation and does not require the knowledge of power delay profile, except that the number of delay paths should be known. The search range for the CFO is first partitioned into a number of subranges. The expectation-maximization (EM) and Newtons methods are then used within each subrange in an alternative manner to jointly obtain the channel and the CFO estimates. By assuming that the transmitted time-domain OFDM signals can be modeled as Gaussian random variables, we show that the EM method has closed-form expressions and, thus, can be efficiently implemented. We then study the mean-square-error (MSE) performance of the estimator under various signal-to-noise (SNRs) ratios and various numbers of cyclic prefix samples through simulations. Finally, we compare the performance of our proposed estimator with that of some existing estimators in terms of bit error rate (BER). Our simulation results show that the proposed estimator performs better than the reported estimator, which sequentially estimates the channel and the CFO. By suitably choosing the number of subranges, our estimator also performs better than a reported estimator, which makes use of pilot symbols to obtain the initial CFO and channel estimates and then uses a decision-directed technique for overall eventual estimation.

Performance Study on the Effects of Cell-Breathing in WCDMA

The performance of CDMA cellular systems over the air is generally interference-limited. A phenomena arising from this is the effect of cell-breathing in 3G cellular systems. Cell-breathing is the expansion or contraction of the effective coverage of a cell in response to the number of active mobiles (MSs) in a network. If it is not well controlled, communication failure may result. With 3G offering different classes of services, planning and dimensioning of such high-quality radio networks thus requires extensive planning tools. Despite the progressive rolling out of 3G systems worldwide, the effects of cell-breathing are not well understood. In this work, a simulator is set up to study the effect of cell breathing. Simulation results show how the area where reliable coverage can be provided by a base station (BS) may varies as the density of MSs changes within it. Current work emphasizes on the downlink simulation with a single BS followed by a similar simulation involving 2 BS. The results demonstrate how cell-breathing may cause communications failure

An optical filter of adjustable finesse using an amplified fiber ring resonator

We consider an optical filter constructed from a simple ring resonator with two electro-optical directional couplers and a fiber amplifier in the ring. By adjusting the coupling coefficient of the electro-optical directional couplers, the finesse of this optical filter can be controlled over a specified range. The variation in the insertion loss of the optical filter can be overcome by adjusting the gain of the fiber amplifier simultaneously. This paper analyzes theoretically the performance of the optical filter, as a function of the coupling coefficient of the electro-optical directional coupler and the gain of the fiber amplifier. We also show that it is possible to design an optical filter of wide adjustable finesse by changing only the coupling coefficient of the electro-optical directional couplers, leaving the gain of the fiber amplifier unchanged. This simpler method results only in a slight change in the insertion loss if an appropriate condition is chosen. The filter will be useful in coherent optical systems, where the bandwidth of the optical filter needs to be appropriately adjusted under varying biasing and signaling conditions, or as an optical discriminator with an adjustable Q-factor.