Pilot symbol assisted modulation (PSAM) is widely used to obtain the channel state information (CSI) needed for coherent demodulation. It allows the density of pilot symbols to be dynamically chosen depending on the channel conditions. However, the insertion of pilots reduces the spectral efficiency, more severely when the channel is highly time-variant and/or frequency-selective. In these cases a significant amount of pilots is required to properly track the channel variations in both time and frequency dimensions. Alternatively, non-coherent demodulation does not require any CSI for the demodulation independently of the channel conditions. For the particular case of up-link (UL) based on massive single input-multiple output (SIMO) combined with orthogonal frequency division multiplexing (OFDM), we propose to replace the traditional reference signals of PSAM by a new differentially-encoded data stream that can be non-coherently detected. The latter can be demodulated without the knowledge of the CSI and subsequently used for the channel estimation. We denote our proposal as hybrid demodulation scheme (HDS) because it exploits both the benefits of a coherent demodulation scheme (CDS) and a non-coherent demodulation scheme (NCDS) to increase the spectral efficiency. The mean squared error (MSE) of the channel estimation, bit error rate (BER), achieved throughput and complexity are analyzed to highlight the benefits of this differential data-aided channel estimation as compared to other approaches. We show that the channel estimation is almost as good as PSAM, while the BER performance and throughput are improved for different channel conditions with a very small complexity increase.

Massive multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) are wireless technologies adopted by the Fifth Generation (5G) of mobile communications. The channel estimation and pre/postequalization processes in coherent detection schemes for massive MIMO-OFDM are a challenging task, where several issues are faced, such as pilot contamination, channel calibration, matrix inversions, among others. Moreover, they increase the energy consumption and latency of the system. A non-coherent technique relying on DPSK constellation has been proposed for a singlecarrier scheme, assuming flat-fading. In our paper, we extend this technique to be combined with OFDM, where the channel is doubly dispersive (time and frequency). We will show that the differential modulation can be performed either in the time or frequency domain, where the latter suffers from an additional phase rotation, which should be estimated and compensated. We provide the analytical expression of the signal-to-interferenceand-noise ratio (SINR) for both cases, and we show numerical results in order to verify our analysis.

Orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) are wireless radio technologies adopted by the new Fifth Generation (5G) of mobile communications. A very large number of antennas (massive MIMO) is used to perform the beam-forming of the transmitted signal, either to reduce the multi-user interference (MUI), when spatially multiplexing several users, or to compensate the path-loss when higher frequencies than microwave are used, such as the millimeter-waves (mm-Waves). Usually, a coherent demodulation scheme (CDS) is used in order to exploit MIMO-OFDM, where the channel estimation and the pre/post-equalization processes are complex and time consuming operations, which require a considerable pilot overhead and also increase the latency of the system. As an alternative, non-coherent techniques based on a differential modulation scheme have been proposed for the up-link (UL). However, it is not straightforward to extend these proposals to the down-link (DL) due to the (usually) reduced number of antennas at the receiver side. In this paper we overcome this problem, and assuming that each user equipment (UE) is only equipped with one single antenna, we propose the combination of beam-forming with a differential modulation scheme for the DL, enhanced by the frequency diversity. The new transmission and reception schemes are described, and the signal-to-interference-plus-noise ratio (SINR) and the complexity are analysed. The numerical results verify the accuracy of the analysis and show that our proposal outperforms the existing CDS with a significant lower complexity.

Superimposed training (ST) is a semiblind channel estimation technique, proposed for orthogonal frequency division multiplexing (OFDM), where training sequences are added to data symbols, avoiding the use of dedicated pilot-subcarriers, and increasing the available bandwidth compared with pilot symbol assisted modulation (PSAM). Filter bank multicarrier offset quadrature amplitude modulation (FBMC-OQAM) is a promising waveform technique considered to replace the OFDM, which takes advantage of well-designed filters to avoid the use of cyclic prefix and reduce the out-band-emissions. In this paper, we provide the expressions of the average channel capacity of the FBMC-OQAM combined with either PSAM or ST schemes, considering imperfect channel estimation and the presence of the pilot sequences. In order to compute the capacity expression of our proposal, ST-FBMC-OQAM, we analyze the channel estimation error and its variance. The average channel capacity is deduced considering the noise, data interference from ST, and the intrinsic self-interference of the FBMC-OQAM. Additionally, to maximize the average channel capacity, the optimal value of data power allocation is also obtained. The simulation results confirm the validity of the capacity analysis and demonstrate the superiority of the ST-FBMC-OQAM over existing proposals.

Under scenarios of high mobility, the traditional coherent demodulation schemes (CDS) have a limited performance, due to the fact that reference signals cannot effectively track the variations of the channel with an affordable overhead. As an alternative solution, non coherent demodulation schemes (NCDS) based on differential modulation have been proposed. Even in the absence of reference signals, they are capable of outperforming the CDS with a reduced complexity. The literature on NCDS laid the theoretical foundations for simplified channel and signal models, often single carrier and spatially uncorrelated flat fading channels. In this work, the most recent results assuming orthogonal frequency division multiplexing (OFDM) signaling and realistic channel models are explained, and the impact of some hardware impairments such as the phase noise (PN) and the non linear high power amplifier (HPA) are also considered. Moreover, new potential research lines are also highlighted.