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A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called orthogonal frequency-division multiple access (OFDMA) is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This paper presents our investigation into reducing the multiple access interference (MAI) introduced by symbol timing misalignment in an OFDMA uplink system. To combat the MAI, we provide a new OFDMA symbol frame employing a simple symbol repetition coupled with a cyclic time shift for typical OFDMA uplink scenarios. Theoretical analysis and computer simulations are used to assess the performance of the proposed OFDMA uplink system. It is found that this scheme can decrease the probability of destroying the orthogonality among the users and provide the MAI-free reception. Index Terms--Broadcasting, cyclic time shift, frequency division multiaccess, interference suppression, MAI, OFDMA uplink, symbol timing misalignment, synchronization, uplink broadcasting.

In this paper, we propose a differential space-frequency block code--orthogonal frequency division multiplexing (DSFBC-OFDM) scheme as a multiple-input multiple-output (MIMO) transmission technique for next generation broadcasting system. A linear decoding method for DSFBC, which performs comparably to the ML decoding method, is derived for the cases of two or four transmit antennas. A simple table lookup method is proposed to improve the efficiency of the encoding/decoding process of DSFBC for the case of non-constant modulus constellations. This not only reduces the computational load, but also removes the necessity of channel estimation. Also, synchronization techniques with a DSFBC-encoded phase reference symbol (PRS) are discussed. Finally, an MIMO channel model for the next generation broadcasting system is developed by extending the 3GPP MIMO model to fit broadcasting environments. The MIMO channel model is then used to compare BER performances of differential space block code schemes for various channel environments. Simulation results show that the DSFBC-16QAM scheme using either four transmit antennas with one receive antenna or two transmit antennas with two receive antennas achieves a performance gain of 12 dB, with a data rate twice faster than that of the conventional DQPSK scheme. Index Terms--DSFBC, linear decoding, MIMO channel model for broadcasting system, non-constant modulus constellation, OFDM, table lookup method.

In this paper, an orthogonal frequency-division multiplexing (OFDM)-code-division multiple-access (CDMA) system employing various diversity schemes is considered as a possible candidate for broadband wireless access network applications. With an emphasis on a preamble design for multichannel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in the space-time block-coded (STBC) OFDM-CDMA systems, respectively. Also, for a simple construction of the OFDM-CDMA system with the low peak-to-average power ratio (PAR) and reduced transmitter complexity, we investigate a simple PAR reduction scheme and its combination with both decimated selective mapping (SLM) and partial transmit sequence (PTS) methods. Taking into account the false side information introduced by the PAR reduction scheme, the effects of diversity techniques and PAR on the performance of the STBC OFDM-CDMA system are investigated and the maximum achievable diversity gain for a two-path Rayleigh-fading environment is evaluated. From the presented results, we find that the OFDM-CDMA signal with the low PAR is easily constructed by a simple phase transform and its combination version with the SLM and PTS approaches can be implemented to be less complex by decimating the OFDM samples. At the same time, the OFDM-CDMA system applying a space-time-frequency diversity with two and four transmit antennas can give the diversity of D = 4 and D = 8 for the multiuser case with half user capacity, respectively, even if encountered with the false side information. Index Terms--Broadband wireless access network, channel estimation, diversity, multichannel separation, orthogonal frequency-division multiplexing (OFDM)-code-division multiple access (CDMA), peak-to-average power ratio (PAR), preamble, space-time block code (STBC).

This paper investigates the peak-to-average power ratio (PAR) reduction scheme employing a simple symbol transform in OFDM-CDMA systems. This approach is very simple because of no additional complexity and works without restriction on the allocation of spreading code, maintaining an original transmission efficiency. Simulation results show that the investigated scheme gives the PAR reduction gain of 2 ~ 3 dB compared to the original OFDM and OFDM-CDMA signals, and can provide the further PAR reduction by combining the selective mapping (SLM) and partial transmit sequence (PTS) schemes, which are less complex compared to the ordinary SLM and PTS approaches, respectively. Index Terms--OFDM-CDMA, partial transmit sequence (PTS), peak-to-average power ratio (PAR), selective mapping (SLM), symbol transform.

In this paper, an OFDM-CDMA system employing various diversity schemes is considered for a possible candidate of broadband wireless access networks and broadcasting applications. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in OFDM-based multiple-antenna transmission systems. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be applied to the HIPERLAN/2 and IEEE 802.11a standards in the case of more than two transmit antennas. Also, an effect of diversity techniques on the performance of OFDM-CDMA based broadband wireless access networks is investigated and the maximum achievable diversity gain for a two-path Rayleigh fading environment is evaluated. Simulation results show that the OFDM-CDMA system applying a space-time-frequency diversity with a full-rate full diversity code can give the diversity of D = 4 and D = 8 for both multi-user cases of maximum user and half user capacities, respectively. Index Terms--Broadband wireless access network, channel estimation, diversity, multi-channel separation, OFDM-CDMA.

In this paper, we present a low-complexity sampling frequency-offset (SFO) estimation scheme suitable for the 3GPP long-term evolution (LTE) downlink system. First, a novel SFO estimation method for a pilot-aided orthogonal frequency division multiplexing system is proposed providing a good performance with reduced complexity. To achieve low complexity, the proposed estimation scheme uses a central conjugate-symmetric primary synchronisation signal specified in the LTE standard, which eases the SFO estimation at the receiver. For performance evaluation, we derive the mean square error expression of the proposed frequency estimation scheme. Simulations show that the proposed frequency estimation scheme can detect the SFO with higher accuracy and much lower complexity, compared with the conventional estimation schemes. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]