Pedro Bento

A multi-antenna technique for mm-wave communications with large constellations and strongly nonlinear amplifiers

It is well-known that mm-wave (millimeter wave) communications have high potential for future wireless broadband systems. However, there are important challenges that need to be overcome when implementing those systems, both at the hardware level and due to the hostile propagation conditions. This is especially important for spectrally efficient communications based on large constellations, since the power requirements are higher, as well as the amplification difficulties, specially for large, dense constellations. In this paper we propose a multi-antenna transmitter combined with a frequency-domain receiver for broadband mm-wave systems based on large and dense constellations. Our transmitter is compatible with highly-efficient, low-cost, strongly nonlinear amplifiers, while allowing spacial multiplexing gains.

Measuring the Magnitude of Envelope Fluctuations: Should We Use the PAPR?

The PAPR (Peak-to-Average Power Ratio) is widely employed to measure the magnitude of the envelope fluctuations of a given signal. It is particularly used to define the required amplifier back- off for multi-carrier modulations such as OFDM (Orthogonal Frequency Division Multiplexing) schemes. However, the PAPR increases with the block size (i.e., the number of subcarriers, in the OFDM case), but the signals distribution converges to a Gaussian distribution for a large number of subcarriers. Therefore, one might ask if the PAPR is indeed the best parameter to define the amplifiers back-off. In this paper we address this issue. It is shown that the required amplifiers back-off for a given non-linear distortion level, i.e. referred to the input amplifier saturation power, is almost independent of the number of subcarriers and, therefore, the PAPR is not the most suitable to define it. Therefore, we recommend the use of a parameter related to the instantaneous envelope power distribution to define the amplifier back-off.

Efficient LINC Amplification for 5G through Ring-Type Magnitude Modulation

The upcoming 5G generation is driven the expected huge growth in user bit rates and overall system throughput. This means a substantial spectral efficiency increase, which must be achieved while maintaining or even improving the power efficiency. To overcome the necessity of using inefficient linear HPA in typical communication systems, this paper addresses the use of the LINC technique for power amplification, since it employs grossly NL HPA that are simpler and much more energy efficient than the linear ones. However, this techniques efficiency is harshly damaged for signals with a large PAPR, as for systems employing high order spectral efficient modulations and/or close to Nyquist bandwidth limiting pulse-shaping, due mainly to the LINC signal separation and power combining procedures. However, given that any M-ary constellation can be split as a sum of several OQPSK signals, a new MM method is proposed for OQPSK signals with bandwidth close to Nyquist, that limit both maximum and minimum envelope excursions of the signal without spreading its spectrum. This method is also shown to improve the transmitters resilience to small gain and phase imbalances between the LINCs amplifiers. The proposed scheme can play a critical role on the upcoming 5G, since arranging several units of the proposed transmitter in an array/parallel configuration has the potential to simultaneously achieve the required high power and spectral efficiency.

Frequency-Domain Detection without Matrix Inversions for mmWave Communications with Correlated Massive MIMO Channels

Massive MIMO (m-MIMO) schemes operating at mmWave bands are being proposed to be used in next generation wireless systems, bringing new challenges both in terms of detection techniques and the definition of channel models. In this paper, frequency-domain iterative detection schemes which do not require matrix inversions and are suitable for m-MIMO systems employing Single- Carrier with Frequency-Domain Equalization (SC- FDE) modulations are considered. These techniques are evaluated using a general clustered channel model for multilayer m-MIMO systems. Performance results show that these techniques can approach the Matched Filter Bound (MFB) with just a few iterations, even with significant correlation between different antenna elements.

Beamforming Optimization for Multiuser Wireless Systems Using Meta-Heuristics

In the next generation wireless systems, where m-MIMO and mmWave should be combined with beamforming techniques, array optimization at receiver ensuring a good SINR level can become a very large problem. Meta-heuristics can be an adequate approach to solve this problem with a mild computational effort. In this work, different meta-heuristics are implemented and applied to this problem, with Differential Evolution presenting promising connection setup time results that expectedly would enable its use in m-MIMO systems.