Luciano Leonel Mendes

5GNOW: non-orthogonal, asynchronous waveforms for future mobile applications

This article provides some fundamental indications about wireless communications beyond LTE/LTE-A (5G), representing the key findings of the European research project 5GNOW. We start with identifying the drivers for making the transition to 5G networks. Just to name one, the advent of the Internet of Things and its integration with conventional human-initiated transmissions creates a need for a fundamental system redesign. Then we make clear that the strict paradigm of synchronism and orthogonality as applied in LTE prevents efficiency and scalability. We challenge this paradigm and propose new key PHY layer technology components such as a unified frame structure, multicarrier waveform design including a filtering functionality, sparse signal processing mechanisms, a robustness framework, and transmissions with very short latency. These components enable indeed an efficient and scalable air interface supporting the highly varying set of requirements originating from the 5G drivers.

Generalized Frequency Division Multiplexing for 5th Generation Cellular Networks

Cellular systems of the fourth generation (4G) have been optimized to provide high data rates and reliable coverage to mobile users. Cellular systems of the next generation will face more diverse application requirements: the demand for higher data rates exceeds 4G capabilities; battery-driven communication sensors need ultra-low power consumption; and control applications require very short response times. We envision a unified physical layer waveform, referred to as generalized frequency division multiplexing (GFDM), to address these requirements. In this paper, we analyze the main characteristics of the proposed waveform and highlight relevant features. After introducing the principles of GFDM, this paper contributes to the following areas: 1) the means for engineering the waveforms spectral properties; 2) analytical analysis of symbol error performance over different channel models; 3) concepts for MIMO-GFDM to achieve diversity; 4) preamble-based synchronization that preserves the excellent spectral properties of the waveform; 5) bit error rate performance for channel coded GFDM transmission using iterative receivers; 6) relevant application scenarios and suitable GFDM parameterizations; and 7) GFDM proof-of-concept and implementation aspects of the prototype using hardware platforms available today. In summary, the flexible nature of GFDM makes this waveform a suitable candidate for future 5G networks.

Generalized Frequency Division Multiplexing in a Gabor Transform Setting

This letter shows the equivalence of the recently proposed generalized frequency division multiplexing (GFDM) communications scheme with a finite discrete critically sampled Gabor expansion and transform. GFDM is described with the terminology of Gabor analysis and the Balian-Low theorem is applied to prove the non-existence of zero-forcing receivers for certain configurations, having strong impact on the system performance. An efficient algorithm for calculation of specific GFDM receiver filters is derived and numerical examples confirm the theoretical results.

A synchronization technique for generalized frequency division multiplexing

Generalized frequency division multiplexing (GFDM) is a block filtered multicarrier modulation scheme recently proposed for future wireless communication systems. It generalizes the concept of orthogonal frequency division multiplexing (OFDM), featuring multiple circularly pulse-shaped subsymbols per subcarrier. This paper presents an algorithm for GFDM synchronization and investigates the use of a preamble that consists of two identical parts combined with a windowing process in order to satisfy low out of band radiation requirements. The performance of time and frequency estimation, with and without windowing, is evaluated in terms of the statistical properties of residual offsets and the impact on symbol error rate over frequency-selective channels. A flexible metric that quantifies the penalty of misalignments is derived. The results show that this approach performs practically as state-of-the-art OFDM schemes known in the literature, while it additionally can reduce the sidelobes of the spectrum emission.

Widely Linear Estimation for Space-Time-Coded GFDM in Low-Latency Applications

This paper presents a solution for achieving transmit diversity with generalized frequency division multiplexing (GFDM). Compared to previous works, the proposed solution significantly improves symbol error rate (SER) performance and latency, where both aspects are crucial for future 5G cellular networks. It is shown that widely linear estimation at the receiver side can jointly equalize and demodulate the space-time encoded GFDM signal. Moreover, maximum ratio combining can further increase the SER performance with multiple receive antennas. SER performance is evaluated in Rayleigh fading multipath channels.

Expectation Propagation for Near-Optimum Detection of MIMO-GFDM Signals

Generalized frequency division multiplexing (GFDM) as a nonorthogonal waveform aims at diverse applications in future mobile networks. To evaluate its performance, its capacity limits are of particular importance. Therefore, this paper analyzes its constellation-constrained capacities for cases where the channel state information (CSI) is unknown at the transmitter and perfectly known at the receiver. In frequency selective channels, GFDM may provide advantage over the conventional orthogonal frequency division multiplexing (OFDM) scheme. In order to achieve near-capacity performance, the interaction of data symbols in time and frequency combined with multiple antennas (MIMO) challenges the design of GFDM receivers. This paper, therefore, applies expectation propagation (EP) for systematic receiver design. It is shown that the resulting iterative MIMO-GFDM receiver with affordable complexity can approach optimum decoding performance and outperform MIMO-OFDM in a rich multipath environment. Simulations are also used to illustrate the impact of channel delay spread on the constellation-constrained capacities and on the performance of the novel receiver algorithm.

Robust WHT-GFDM for the Next Generation of Wireless Networks

This paper presents the combination of generalized frequency division multiplexing (GFDM) with the Walsh-Hadamard transform (WHT) to achieve a scheme that is robust against frequency-selective channels (FSC). The proposed scheme is suitable for low-latency scenarios foreseen for 5G networks, specially for Tactile Internet. The paper also presents analytical approximations that can be used to estimate the bit error rate of GFDM and WHT-GFDM over frequency-selective channels in single shot transmission. Simulation results for encoded GFDM are included for further comparison.

Frequency-Shift Offset-QAM for GFDM

This paper presents a novel perspective to apply the offset quadrature amplitude modulation (OQAM) scheme on top of the multicarrier waveform termed Generalized Frequency Division Multiplexing (GFDM). The conventional time-shift OQAM is described for GFDM and, with the introducing of the general use of unitary transform, an interesting counterpart, i.e., frequency-shift OQAM, is proposed. The conventional long prototype pulse with time-shift of one half subsymbol becomes a short prototype pulse with frequency-shift of one half subcarrier. The frequency-shift OQAM scheme offers advantages such as low out-of-band emission and low implementation complexity. The concept can be applied to the broader scope of filtered OFDM without penalties in terms of performance in time variant frequency-selective channels.

Asynchronous multi-user uplink transmission with generalized frequency division multiplexing

This paper investigates the applicability of generalized frequency division multiplexing (GFDM) for an uplink scenario where several users are not perfectly synchronized, as it appears in wireless sensor networks (WSN). We compare the performance in terms of inter-user interference (IUI) caused by time and frequency misalignments between users, where the physical layer is realized with GFDM and OFDM. It is shown that IUI can be significantly reduced when using GFDM. Furthermore, we propose a data-aided phase error estimation and compensation algorithm which is capable of correcting residual phase errors at the receiver.

LTE-compatible 5G PHY based on generalized frequency division multiplexing

The soft transition between generations of mobile communication systems is a desirable feature for telecommunication operators and device manufacturers. Looking to the past, clock compatibility between WCDMA and LTE allowed manufacturers to build inexpensive multi-standard devices. In this paper it is shown that GFDM, a candidate waveform for the 5G PHY layer, is able to use the LTE master clock and the same time-frequency structure as employed in todays generation of cellular systems. Two approaches for coexistence of 4G/5G waveforms are presented in the paper. The first GFDM setting is aligned with the LTE grid; in the other one GFDM acts as a secondary system to the primary LTE. The second approach introduces a new way of positioning subcarriers that further enhances the flexibility of GFDM. In addition, the paper also considers low latency aspects for autonomous and human controlled device communication in future application scenarios.