In this letter, we propose a simple adaptive switching scheme to enhance the performance of space-time/frequency block coded OFDM systems (STBC/SFBC-OFDM). Since STBC-OFDM and SFBC-OFDM undergo severe performance degradation in time- and frequency-selective fading channels, respectively, performance enhancement can be achieved by switching between STBC-OFDM and SFBC-OFDM over a continuously varying channel environments. Thus, a new switching scheme based on the characteristics of the actual channel is proposed. The effectiveness of the proposed scheme is demonstrated by computer simulations.

An adaptive subcarrier allocation (SA) algorithm is proposed for both the enhancement of system capacity and the practical implementation in a clustered OFDM system. The proposed algorithm is based on the two dimensional comparison of the channel gain in both rows and columns of the channel matrix to achieve higher system capacity. Simulation results demonstrate that the proposed algorithm outperforms the SA algorithm based on only one dimensional comparison in terms of system capacity, and furthermore, it performs as well as the optimal SA algorithm at relatively low computational cost.

This letter proposes a high precision ranging scheme based on the time of arrival estimation technique for the IEEE 802.15.4a chirp spread spectrum system. The proposed scheme consists of a linear channel impulse response estimation process with the zero forcing or minimum mean square error technique and the multipath delay estimation process with matrix pencil algorithm. The performance of the proposed scheme is compared with that of a well known MUSIC algorithm in terms of computational complexity and ranging precision. Simulation results demonstrate that the proposed scheme outperforms the MUSIC algorithm even though it has comparatively lower computational complexity.

This letter considers frame synchronization in non-synchronized sampling discrete multi-tone (DMT) based asymmetric digital subscriber line (ADSL)/very high speed DSL (VDSL) systems in the presence of timing error. We propose a frame synchronization method which is based on the observation that the normalized correlation between two sequences separated by the FFT length is Cauchy random variable. The proposed approach uses less number of correlators, reducing computational complexity as well as demodulation delay than a previous approach. Therefore, ADSL/VDSL modems can be more power efficient and computationally less complex via the proposed frame synchronization method. Simulation results demonstrate the effectiveness of the proposed approach, comparing with the previous approach.

Instead of pilot tones, the impulse sample is exploited for channel estimation in Impulse Postfix OFDM systems [1]. As the magnitude of impulse sample is increased, the accuracy of channel estimation can be enhanced, but it may significantly increase the PAPR of generated OFDM symbols. In this letter, based on the statistical analysis of the generated OFDM symbol, we propose a decision scheme for determining the magnitude of impulse sample. By using the proposed scheme, we can determine the magnitude of impulse sample that provides the enhancement of BER performance as well as the avoidance of PAPR increase. The validation of the proposed scheme is demonstrated by computer simulations.

In OFDMA systems, various subcarrier allocation (SA) algorithms have been developed and adopted to realize the low-cost implementation or the optimized usage of resources, such as bandwidth and total transmit power, at the cost of increased complexity. Regardless of SA algorithms, however, it is computationally inefficient for a user who uses only a small number of subcarriers to use a full fast Fourier transform (FFT) for multicarrier demodulation at a conventional receiver in a downlink environment. In response, this letter proposes a novel low complexity FFT scheme that demodulates a set of desired multicarriers with smaller-size FFTs is proposed for computationally efficient and/or low hardware-cost receiver in OFDMA systems. Furthermore, a decision rule for the optimum size of FFT in the known transform decomposition methods is provided.

In this study, a computationally efficient ranging scheme exploiting a minimum mean square error (MMSE) and a matrix-pencil (MP) technique is proposed for the IEEE 802.15.4a chirp spread spectrum (CSS) system. Based on the characteristics of the CSS signal, a practical methodology for the MMSE process is devised and the appropriate values of parameters, which are cutoff bandwidth, number of samples and sampling duration in frequency domain, are investigated and numerically determined to enhance the performance. The performance of proposed scheme is analyzed in terms of the computational complexity and the ranging estimation errors. Simulation results demonstrate that the proposed scheme performs as well as the conventional scheme at remarkably reduced computational costs.