Recently, multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) combined with time division multiplexing (OFDM/TDM) based on frequency domain equalization (FDE) has been proposed to improve the performance of conventional OFDM-based relaying in terms of high peak-to-average power ratio (PAPR) and the bit error rate (BER). The PAPR problem, however, is not completely eliminated and thus, the effect of peak-limited channel is not negligible. In this paper, we present the BER performance analysis of MIMO-OFDM/TDM in a peak-limited and frequency-selective channel. The theoretical equalization weights in a nonlinear channel for MIMO-OFDM/TDM are derived to capture the negative effect of peak-limitation. Performance analysis presented in this paper was confirmed by the computer simulation and it was shown that the achievable BER performance of MIMO-OFDM/TDM in a peak-limited and frequency-selective channel is a function of the OFDM/TDM design parameter.

Future wireless Internet services require a broadband frequency spectrum with high data rates. Cognitive radio (CR) concept is a novel approach to improve the spectrum efficiency. The CR is based on the opportunistic usage of frequency spectrum, which is not occupied by the primary users. Conventional multi-user access in bi-directional CR network may be done by using either time division multiple access (TDMA), frequency division multiple access (FDMA) or code division multiple access (CDMA). Without adaptive or dynamic frequency reuse, TDMA and FDMA have lower spectrum efficiency in comparison with CDMA. However, the problem of CDMA in a multipath channel is a multi-user interference (MUI). In this paper, we present a bi-directional CR network with wireless network coding (WNC) in a multipath channel. Unlike the conventional multi-user bi-directional CR network, where the users access the spectrum holes in different time-slot or frequency, the proposed method allows secondary users (SUs) to access the spectrum holes simultaneously. The performance of bi-directional CR network with WNC is theoretically analyzed in terms of spectrum efficiency and the maximum number of SUs. The numerical results show that the spectrum efficiency and the maximum number of SUs of the proposed method increases in comparison with conventional CR network.

Orthogonal frequency division multiplexing (OFDM) is an effective technique for high-speed digital transmission over time-dispersive channels. However, for coherent detection, a reliable channel estimation (CE) is required. OFDM is characterized by its high peak-to-average power ratio (PAPR), which makes it very sensitive to nonlinear distortions that may affect the channel estimation accuracy leading to a bit error rate (BER) performance degradation. In this paper, we present closed-form BER expression for OFDM with a pilot-assisted CE in a nonlinear and frequency-selective fading channel. We discuss how, and to what extent, the nonlinear degradation affects the BER performance with the CE based on a time/frequency division-multiplexed (TDM/FDM) pilot. The analysis is based on a Gaussian approximation of the nonlinear noise due to both HPA amplitude limitation and quantization. We also evaluate the estimator's mean square error (MSE) with both TDM and FDM pilots. Our results show that pilot-assisted CE using FDM pilot is more sensitive to nonlinear distortions than the CE using a TDM pilot, since its pilot subcarriers are affected by nonlinear noise due to both the HPA and the quantization.

1 IPSA Institute, 71000 Sarajevo, Bosnia and Herzegovina 2Alcatel-Lucent Bell N.V., 2018 Antwerpen, Belgium 3Communication Group at Energy and Communication Department, IPSA Institute, 71000 Sarajevo, Bosnia and Herzegovina 4Department of Electrical and Communication Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 5 Jozef Stefan Institute, 1000 Ljubljana, Slovenia

Orthogonal frequency division multiplexing (OFDM) system with frequency domain equalization (FDE) requires reliable channel estimation (CE). OFDM has a problem of high peak-to-average power ratio (PAPR), which makes it very sensitive to nonlinear distortions, affecting the channel estimation accuracy. In this paper, we investigate the effect of the nonlinearity to the the OFDM system with pilot-assisted CE based on time or frequency division multiplexed (TDM/FDM) pilot. A closed-form bit error rate (BER) expressions for OFDM system are derived in a nonlinear and frequency-selective fading channel. The analysis is based on the Gaussian approximation of the nonlinear noise, which is also confirmed by computer simulation. Our results in terms of BER and mean square error (MSE) show, that FDM-pilot based CE is more sensitive to nonlinear distortions as compared to CE based on TDM-pilot.

Analog network coding (ANC) as a simple implementation of physical layer network coding based on orthogonal frequency division multiplexing (OFDM) has been proposed to increase the network capacity and reliability of bi-directional link between a pair of users. In ANC protocol, an information between a pair of users is exchanged through two orthogonal time phases (i.e., multiple-access and broadcast phases). On the other hand, the phase noise (PN) introduces phase offset and inter carrier interference (ICI) to the useful signal. Thus, in ANC scheme PN will affect the useful signal during both multiple-access and broadcast phases. In this article, we present a performance analysis of ANC scheme using OFDM in the presence of PN in frequency-selective fading channel. We derive the total composite variance of ANC scheme in the presence of PN to obtain the signal-to-interference-plus-noise ratio (SINR) expression. Then, we evaluate the system's performance in terms of bit error rate (BER), SINR degradation, and ergodic capacity through both numerical and computer simulations. Computer simulated average BER has been consistent with the numerical results, validating the presented analysis. Our results have shown that the ANC scheme is more sensitive to the PN introduced during the broadcast phase (i.e., at destination) than during the multiple-access phase (i.e., at relay). This is because of the higher ICI to the useful signal and enhanced noise due to the imperfect self-information removal at the destination. In addition, the performance degradation of ANC scheme based on OFDM in the presence of PN is highly expressed for the PN linewidth values up to 20 Hz.