Nonlinearities are usually something to be avoided in orthogonal frequency division multiplexing (OFDM) transceivers. However, it has been demonstrated that nonlinear (NL) OFDM can outperform linear OFDM, since NL distortion has useful information on the transmitted signals that can be exploited by the optimum maximum likelihood receiver. The optimum ML receiver for NL OFDM is too complex and it is not easy to obtain its performance, although some theoretical performance bounds are available for scenarios with a large number of subcarriers and signal-to-noise ratio (SNR). However, the optimum performance of NL OFDM in small-to-moderate SNR values is still not well studied. In this paper, we consider the optimum detection of NL OFDM schemes, where we take advantage of the NL distortion to improve performance. We present a detailed study of the contributions to the signal associated with a given subcarrier from the distortion received at the other subcarriers. This allowed us to obtain a bound on the maximum likelihood (ML) performance of NL OFDM that is valid over a wide range of SNR values subcarriers and demonstrates that the performance gains observed in the asymptotic SNR region also take place for lower SNRs. Additionally, we propose a practical, iterative decision-directed receiver that exhibits large performance gains over linear OFDM in both uncoded and coded scenarios.

In satellite communications power consumption and reliability are key concerns. The unreliable nature of the wireless channel can cause erroneous transmissions, which must be compensated using retransmissions, at the expense of more power. This issue is aggravated by the low amplification efficiency of high data rate schemes such as Orthogonal Frequency Division Multiplexing (OFDM). Ideally, retransmissions could be avoided entirely by increasing transmission power, however, this would drastically lower the overall energy efficiency of the system. Sophisticated Hybrid Automatic Repeat Request (HARQ) techniques can simultaneously reduce the need for retransmissions and increase the success likelihood of each retransmission attempt (a critical aspect for delay-constraint services over satellite links), while minimizing average consumed power. In this work, we propose a novel HARQ technique where retransmitted signals are submitted to a soft clipping operation that lowers the Peak-to-Average Power Ratio (PAPR) of the OFDM signal, thereby increasing the amplification efficiency. A Generalized Approximate Message Passing (GAMP) based receiver makes use of the additional distortion in the retransmission to increase the likelihood of a successful decoding. It is shown that this approach can outperform Chase Combining (CC) schemes even for Super Quadrature Amplitude Modulation (Super-QAM), while decreasing the power expended during retransmissions.

Non-linear distortion imposed to orthogonal frequency division multiplexing (OFDM) signals from non-linear power amplifiers (PAs), has always been a challenging problem in wireless communications. This is due to the high peak-to-average power ratio (PAPR) of OFDM signals. To overcome this issue, receiver side compensation techniques based on distortion cancellation and iterative detection have been proposed. However, these techniques require knowledge of the non-linearity (NL) parameters. Therefore, in this paper we focus on NL parameter estimation and propose a least squares (LS) based estimation technique by transmitting a training signal. We study the performance of the proposed technique for the solid state power amplifier (SSPA) model and the Bussgang model. The studies are done considering both perfect channel knowledge and imperfect channel knowledge at the receiver. The results show that the proposed technique has good accuracy and performance for distortion cancellation and iterative detection based receivers such as the Bussgang receiver. The results also indicate that the estimation technique is more sensitive to channel estimation error than the additive Gaussian noise.

In wireless communication systems, the achievable data rates are always dependent on the characteristics of the wireless channel. Although a system is designed for an expected channel link quality, the channel may deteriorate for certain periods of time due to the changing nature of the propagation environment. A weak channel may lead to transmission errors, which can be corrected by employing Hybrid Automatic Repeat Requests (HARQ). In particular, type II HARQ protocols have become an appealing option since they only involve the transmission of additional parity bits, instead of repeating the original transmission. These additional parity bits can be obtained by puncturing a low rate channel code. However, finding an optimum pattern of puncturing for a specific code is non-trivial. In this work, we propose a retransmission scheme that considers various Low-Density Parity-Check (LDPC) codes of different rates that can be concatenated to perform the encoding and decoding operations, resulting in a lower encoding overhead for successful transmissions, while maintaining efficient decoding. We show that this technique can maintain high throughput and goodput, and a low outage probability, even when the received signal power is suddenly reduced due to the loss of the Line-of-sight (LOS) component.

The design of a wireless catheterization laboratory (CathLab) presents considerable challenges, especially for intravascular ultrasound (IVUS) systems. In fact, the huge bit rates, together with the highly demanding quality of service (QoS) requirements such as high reliability, very low latency and secure communications, make the design of a wireless CathLab particularly difficult, especially due to the limited signal processing capabilities at the transmitter side. In this paper, we consider the design of a secure wireless CathLab system using a physical layer security scheme that takes advantage of the Multi-Input Multi-Output (MIMO) channel characteristics. It is shown that the physical layer security scheme can have excellent secrecy characteristics, even for an eavesdropper close to the transmitter or receiver. Moreover, the signal processing requirements are very small, especially for the data transmission phase, making this technique particularly interesting for a wireless IVUS scenario.

Simultaneous wireless information and power transmission (SWIPT) is one of the popular technique used in radio frequency energy harvesting (EH). Frequency splitting based SWIPT (FS-SWIPT) is a novel SWIPT technique that can be adopted with frequency domain multiple access signal transmission protocol for the near field EH application in biomedical sensors. In this paper, we implement FS-SWIPT in orthogonal frequency division multiplexed (OFDM) signal, the OFDM signal has separate sub carriers for the data symbols and the energy carrying symbols. At the receiver, the energy and information symbols are based on their sub carriers by using FS-SWIPT technique. The amplification of high energy OFDM signal at transmitter by using a high power amplifier are susceptible to non linear distortion (NLD), which degrades the bit error rate (BER) performance. We analyze the impact of NLD on the BER performance of FS-SWIPT and validate the analytical results by using simulations.

This paper considers massive multiple-input, multiple output (MIMO) schemes for single-carrier with frequency-domain equalization (SC-FDE) modulations. We present low-complexity turbo multi-user receivers based on the maximum ratio combining (MRC) and equal gain combining (EGC) techniques. Our receivers are implemented in the frequency-domain and allow excellent performances, even for moderate numbers of antennas.