Radio access technologies for cellular mobile communications are typically characterized by multiple access schemes, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and OFDMA. In the 4th generation (4G) mobile communication systems such as Long-Term Evolution (LTE) (Au et al., Uplink contention based SCMA for 5G radio access. Globecom Workshops (GC Wkshps), 2014. doi:10.1109/GLOCOMW.2014.7063547) and LTE-Advanced (Baracca et al., IEEE Trans. Commun., 2011. doi:10.1109/TCOMM.2011.121410.090252; Barry et al., Digital Communication, Kluwer, Dordrecht, 2004), standardized by the 3rd Generation Partnership Project (3GPP), orthogonal multiple access based on OFDMA or single carrier (SC)-FDMA is adopted. Orthogonal multiple access was a reasonable choice for achieving good system-level throughput performance with simple single-user detection. However, considering the trend in 5G, achieving significant gains in capacity and system throughput performance is a high priority requirement in view of the recent exponential increase in the volume of mobile traffic. In addition the proposed system should be able to support enhanced delay-sensitive high-volume services such as video streaming and cloud computing. Another high-level target of 5G is reduced cost, higher energy efficiency and robustness against emergencies.

Non-Orthogonal Multiple Access (NOMA) for cellular future radio access

To address the vast variety of user requirements, applications, and channel conditions, flexibility support is strongly highlighted for 5G radio access technologies (RATs). For this purpose, usage of multiple orthogonal frequency division multiplexing (OFDM) numerologies, i.e., different parameterization of OFDM-based subframes, within the same frame has been proposed in the third-generation partnership project discussions for 5G new radio. This concept will likely meet the current expectations in multiple service requirements to some extent. However, since the quantity of wireless devices, applications, and heterogeneity of user requirements will keep increasing toward the next decade, the sufficiency of the aforementioned flexibility consideration remains quite disputable for future services. Therefore, novel RATs facilitating much more flexibility are needed to address various technical challenges, e.g., power efficiency, massive connectivity, latency, spectral efficiency, robustness against channel dispersions, and so on. In this paper, we discuss the potential directions to achieve further flexibility in RATs beyond 5G, such as future releases of 5G and 6G. In this context, a framework for developing flexible waveform, numerology, and frame design strategies is proposed along with sample methods. We also discuss their potential role to handle various upper-level system issues, including the ones in orthogonal and nonorthogonal multiple accessing schemes and cellular networks. By doing so, we aim to contribute to the future vision of designing flexible RATs and to point out the possible research gaps in the related fields.

Flexible Radio Access Beyond 5G: A Future Projection on Waveform, Numerology, and Frame Design Principles

One of the effective ways to address the exponentially increasing traffic demand in mobile communication systems is to use more spectrum. Although licensed spectrum is always preferable for providing better user experience, unlicensed spectrum can be considered as an effective complement. Before moving into unlicensed spectrum, it is essential to carry out proper coexistence performance evaluations. In this paper, we analyze WiFi 802.11n and Long Term Evolution (LTE) coexistence performance considering multi-layer cell layouts through system level simulations. We consider a time division duplexing (TDD)-LTE system with an FTP traffic model for performance evaluation. Simulation results show that WiFi performance is more vulnerable to LTE interference, while LTE performance is degraded only slightly. However, WiFi throughput degradation is lower for TDD configurations with larger number of LTE uplink sub-frames and smaller path loss compensation factors.

Licensed-assisted access for WiFi-LTE coexistence in the unlicensed spectrum

A maximum likelihood (ML) estimator for digital sequences disturbed by Gaussian noise, intersymbol interference (ISI) and interchannel interference (ICI) is derived. It is shown that the sampled outputs of the multiple matched filter (MMF) form a set of sufficient statistics for estimating the input vector sequence. Two ML vector sequence estimation algorithms are presented. One makes use of the sampled output data of the multiple whitened matched filter and is called the vector Viterbi algorithm. The other one is a modification of the vector Viterbi algorithm and uses directly the sampled output of the MMF. It appears that, under a certain condition, the error performance is asymptotically as good as if both ISI and ICI were absent.

Maximum Likelihood Receiver for Multiple Channel Transmission Systems

This paper proposes nonorthogonal sharing of available resources between latency-critical and latency-tolerant communication for fulfilling tight requirements of ultrareliable low-latency communication (URLLC) as well as avoiding inefficient spectrum utilization of grant-based (GB) access for sporadic URLLC traffic. In the proposed system, grant-free (GF) access is adopted for URLLC to reduce transmission latency, whereas GB access is used for latency-tolerant communication. Due to GF access, collision emerges between the communications, and use of OFDM technology for both communications leads to wideband interference (WB-I) on URLLC. Therefore, a novel nonorthogonal multiple accessing (NOMA) scheme based on orthogonal frequency division multiplexing (OFDM) and OFDM with index modulation (OFDM-IM) is proposed in order to reduce the impact of the collision on URLLC, that requires 99.999 ${\%}$ success probability within 1 ms. OFDM-IM technology is used for latency-tolerant communication since WB-I is converted to either narrowband dominant interference or narrowband interference by fractional subcarrier activation in OFDM-IM. In this way, URLLC is partially affected by latency-tolerant communication. It is shown that the proposed NOMA scheme significantly reduces the latency in comparison to classical NOMA scheme based on pure OFDM while guaranteeing $10^{-5}$ reliability for URLLC, via both computer-based simulations and theoretical analysis.

NOMA With Index Modulation for Uplink URLLC Through Grant-Free Access

The increasing number of mobile devices and the demand for large throughput requiring applications has hampered access to the limited frequency spectrum. The goal of this work is to introduce a new degree of freedom to the reuse of occupied resources by intentionally creating co-existence between different types of signals. First, the co-existence of orthogonal frequency division multiple access and single carrier-frequency division multiple access signals is introduced and analyzed for a conventional successive interference cancellation (c-SIC) processing. Then, the average bit-error-rate is derived for the proposed co-existence approach as a baseline. The results of the analysis lead to the design of an improved adaptive multi-user detection (MUD) approach, which outperforms the c-SIC receiver. The proposed MUD approach performs iterative likelihood testing and a signal-to-interference plus noise ratio based processing to improve the decoding performance. Additionally, three different power control schemes are proposed for heterogeneous networks to improve the gain further and observe the performance in the system-level. Our results show that the proposed combination of methods works well in dense mobile communication environments.

Theoretical Analysis of the Co-Existence of LTE-A Signals and Design of an ML-SIC Receiver

Interference suppression for the LTE uplink

In this paper, we proposed a cognitive radio (CR) implementation by using standard wireless communication laboratory equipments such as signal generator and spectrum analyzer. Equipments are controlled through MATLAB instrument control toolbox to carry out CR capabilities specified by IEEE 802.22 WRAN standard. Energy detection and maximum minimum eigenvalue detection algorithms are employed to sense spectrum for opportunistic access. The aim of this work is to provide a CR environment for spectrum sensing algorithms to perform a comparative study considering wireless microphone signals for research and educational purposes.

Spectrum sensing testbed design for cognitive radio applications

Interference cancellation is expected to have significant importance for next-generation wireless communication systems due to various co-channel deployment scenarios and dense frequency reuse. In this study, an interference cancellation receiver that exploits the unique characteristics of single-carrier frequency-division multiple access based systems is proposed. The proposed receiver suppresses the co-channel dominant interference by employing a soft window to the frequency-domain samples where the desired and interfering signals overlap. In order to improve the performance, demodulation and regeneration stages are repeated multiple times by initially accommodating a group of reliable symbols before the iterations. The simulation results indicate that proposed methods work particularly well for low overlap ratios compared to interference coordination and no cancellation methods.

Interference suppression based on soft blanking and iterative likelihood test for LTE uplink

Interference cancellation will become more important in future wireless communication systems due to various co-channel deployment scenarios and denser frequency reuse. In this study, an interference cancellation receiver is proposed which can be used for single-carrier frequency-division multiple access based systems. In the proposed receiver, impact of dominant interference (DI) is mitigated by an iterative blanking algorithm. If the DI is detected on the desired band, overlapping DI spectrum is blanked, followed by demodulation and regeneration of the desired signal. Afterward, the blanked overlapping samples are replaced by the corresponding samples of the regenerated signal. In order to improve the performance, demodulation and regeneration stages can be repeated multiple times. Performance of the proposed approach is compared with those of interference coordination and no cancellation through link-level and system-level simulations.

Mehmet Bahadir Celebi

Papers

Theoretical Analysis of the Co-Existence of LTE-A Signals and Design of an ML-SIC Receiver

Interference suppression for the LTE uplink

Spectrum sensing testbed design for cognitive radio applications

Interference suppression based on soft blanking and iterative likelihood test for LTE uplink

Interference Mitigation for LTE through Iterative Blanking