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In linear-minimum-mean-square-error (LMMSE) channel estimation for multicarrier systems, one needs to know the channel correlation function. This poses a problem for systems with a small number of pilots operating in time-varying channels. We propose to approximate the channel power-delay profile (PDP) with a shape that can completely be described in two parameters, namely, the mean delay and the root-mean-square (RMS) delay spread. Furthermore, we develop a simple technique to estimate these delay parameters. The approximate PDP is then used to generate the LMMSE filter coefficients for data-subcarrier channel estimation. Mathematical expressions are derived that can be used to predict the accuracy of the various estimates, and they are verified by simulation. The proposed technique is applicable to both point-to-point communication and multiaccess communication where different users may experience different channel conditions. As a practical application, we also specialize the proposed technique to Mobile WiMAX signals and investigate the resulting performance. [ABSTRACT FROM AUTHOR]

This correspondence presents an algorithm for compensation of I/Q imbalance for a direct-conversion packet-switched orthogonal frequency-division-multiplexing (OFDM) communications system, which accounts for transmitter I/Q imbalance, receiver I/Q imbalance, phase/frequency error, and dispersive multipath fading. The proposed estimation algorithm is then presented, which operates within the framework of existing multiuser OFDM radio standards (802.11a). It is shown that this algorithm accurately estimates and corrects transceiver I/Q imbalance on a packet-by-packet basis, all within the receiver's digital baseband. [ABSTRACT FROM AUTHOR]

This paper evaluates the performance of a new estimator for Fine Frequency Offset in OFDM packed-based applications. The estimator, which operates in the frequency domain, is based on the Inter-Carrier Interference (ICI) in the received preamble. A new theoretical expression for the estimator as a function of the input frequency offset is proposed. The estimator performance is evaluated for WIMAX receivers, showing an estimation error variance similar to that obtained with other well-known methods based on the processing in the time domain. [ABSTRACT FROM AUTHOR]

Although the voice over Internet protocol (VoIP) service is expected to be widely supported in wireless mobile networks, the feasibility and capacity of the protocol have not previously been studied in relation to the emerging IEEE 802.16e orthogonal frequency-division multiple access (OFDMA) system. Informing subscriber stations about new downlink or uplink resource assignments creates a signaling overhead that influences the system throughput. In particular, the performance of VoIP services is seriously affected by the signaling overhead because the symmetry between the downlink and uplink causes immense downlink overheads. However, previous research on the IEEE 802.16e OFDMA system has not taken the signaling overhead into consideration. In the analytical model presented here, the signaling overhead is considered in the evaluation of the uplink and downlink performance of VoIP services in the IEEE 802.16e OFDMA system. [ABSTRACT FROM AUTHOR]

The recent Orthogonal Frequency Division Multiple Access (OFDMA) transmission technique is gaining popularity as a preferred technology in the Broadband Wireless Access (BWA) emerging standards. In standards 802.16-2004 and 802.16e, the basic allocation units are comprised of sub-channels and OFDMA time symbols; each sub-channel is a group of sub-carriers, so that all the sub-channels are considered equally adequate to all users. We study the naturally arising new approach of two-dimensional mapping of incoming requests into the matrix that represents the system resources, where each allocation is of an arbitrary multi-rectangular shape (to the best of our knowledge, this approach has not been discussed elsewhere). We define a cost model and constraints related to practical OFDMA systems, which depend on the spatial shape of the two-dimensional allocation; the main objective function is the spatial efficiency. We show that the arising problem, even in its simplest form, is NP-hard [1}. We present run-time efficient heuristic solutions for various mapping problems, taking into account the above Q0S and OFDMA related constraints. In particular, a novel solution for two-dimensional mapping under priority constraints is suggested. Extensive simulations with parameters of real systems were used to investigate the performance of the proposed solutions in terms of throughput, delay and system load. The results show that high throughput can be achieved with relatively simple mapping algorithms. We believe that the proposed two-dimensional mapping approach is prospective, due to its fitness to modern standards. [ABSTRACT FROM AUTHOR]