The energy efficiency of OFDM systems is known to be low due to the high peak-to-average power ratio problem. The linear-amplification-with-nonlinear-component (LINC) technique can solve the problem by decomposing the input signal into two constant-envelop component signals and applying high-efficient nonlinear amplifiers. However, the power combiner, a key component used to combine the amplified signals, is difficult to implement. Combinerless LINC systems employ two transmit antennas such that two component signals can be naturally combined at the receiver. Unfortunately, the performance of combinerless LINC-OFDM systems is seriously degraded if difference exists, even small, between the two channels that the two component signals propagate. In this paper, we propose an enhanced zero-forcing equalizer to solve this problem. The main idea is to reduce a self-interference induced by the channel difference. A closed-form expression for the optimum parameter of the proposed equalizer is also derived. Simulations show that the proposed equalizer can effectively enhance the performance of combinerless LINC-OFDM systems.

In this paper, the authors propose a watermarking scheme that embeds both image-dependent and fixed-part marks for dual protection (content authentication and copyright claim) of JPEG images. To achieve the goals of efficiency, imperceptibility, and robustness, a compressed-domain informed embedding algorithm, which incorporates the Lagrangian multiplier optimization approach and an adjustment procedure, is developed. A two-stage watermark extraction procedure is devised to achieve the functionality of dual protection. In the first stage, the semifragile watermark in each local channel is extracted for content authentication. Then, in the second stage, a weighted soft-decision decoder, which weights the signal detected in each channel according to the estimated channel condition, is used to improve the recovery rate of the fixed-part watermark for copyright protection. The experiment results manifest that the proposed scheme not only achieve dual protection of the image content, but also maintain higher visual quality (an average of 6.69 dB better than a comparable approach) for a specified level of watermark robustness. In addition, the overall computing load is low enough to be practical in real-time applications.