Low-complexity spectral shaping method for OFDM signals with dynamically adaptive emission mask

Orthogonal frequency division multiplexing (OFDM) signals with rectangular pulses exhibit low spectral confinement. Shaping their power spectral density (PSD) is imperative in the increasingly overcrowded spectrum to benefit from the cognitive radio (CR) paradigm. However, since the available spectrum is non-contiguous and its occupancy changes with time, the spectral shaping solution has to be dynamically adapted. This work proposes a framework that allows using a reduced set of preoptimized pulses to shape the spectrum of OFDM signals, irrespective of its spectral width and location, by means of simple transformations. The employed pulses combine active interference cancellation (AIC) and adaptive symbol transition (AST) terms in a transparent way to the receiver. They can be easily adapted online by the communication device to changes in the location or width of the transmission band, which contrasts with existing methods of the same type that require solving NP-hard optimization problems.

A Modified Pulse and Design Framework to Halve the Complexity of OFDM Spectral Shaping Techniques

Orthogonal frequency division multiplexing (OFDM) is a widespread modulation but suffers from high out-of-band emissions (OOBE). Spectral shaping strategies such as precoding, active interference cancellation (AIC) and time-domain methods are effective at reducing the OOBE but entail optimization procedures and real-time implementation costs which might be considerable. This letter proposes a modification of the conventional OFDM waveform aimed at reducing the cost associated to many of the state-of-theart spectral shaping techniques and sets a framework for future works that want to benefit from the same reduction. This approach may reduce both the number of coefficients involved in the optimization and the number of products of its implementation by up to 50%.

Wideband Ultrasonic Acoustic Underwater Channels: Measurements and Characterization

In this work we present the results of a measurement campaign carried out in the Mediterranean sea aimed at characterizing the underwater acoustic channel in a wideband at ultrasonic frequencies centered at 80 kHz with a width of 96 kHz, covering two octaves from 32 to 128 kHz. So far, these type of wideband measurements are not found in the literature. Periodic orthogonal frequency division multiplexing (OFMD) sounding signals using Zadoff-Chu sequences have been specially designed for this purpose. The collected data has been post-processed to estimate the time-variant impulse and frequency responses and relevant parameters for system design like the time coherence, bandwidth coherence, delay spread and Doppler bandwidth. The statistical behavior of the channel gain random fluctuation has also been analyzed. This information has been extracted for both the global channel and each path separately. The wide bandwidth of the measurements have allowed the characterization of the channel in a scarcely explored ultrasonic band with an accuracy that is far beyond what is reported in previous works.