Cellular technologies have evolved over time starting from the first generation (1G) to the current fourth generation (4G) with the objective of improving several factors such as spectral efficiency, capacity, coverage, power consumption, and user experience. This has been possible with the continuous advances in electronics and signal processing technologies in different segments of the cellular system architecture. Currently, we are in the stage of conceptualizing the next generation of cellular communications, i.e., 5G.
In all previous cellular generations, there was clearly a single dominating technology, i.e., Frequency Division Multiple Access (FDMA) for 1G, Time Division Multiple Access (TDMA) for 2G, Code Division Multiple Access (CDMA) for 3G, and Orthogonal Frequency Division Multiple Access (OFDMA) for 4G. However, for the upcoming 5G wireless, no clear dominating technology has been foreseen yet. Based on the current activities in industries and academia, it seems to be a mixture of technologies which are supposed to address the main emerging requirements such as high data rate, low energy consumption, low latency and the support/integration of heterogeneous devices/networks. In this regard, this blog will shed a light on some of the key technologies along with their potential advantages and challenges.
The potential techniques to meet the aforementioned requirements are ultra-densification, millimetre wave (mmWave) communications, massive Multiple Input Multiple Output (MIMO), full duplex technology, adaptive three dimensional (3D) beamforming, dynamic spectrum access and advanced multiple access schemes. Besides these techniques, several aspects such as software defined radio/networking, Internet of Things (IoT), intelligent caching, cloud computing and big data are being considered as important enablers for 5G wireless. In addition, advanced Wireless Fidelity (WiFi) networks, infrastructure sharing, integration of heterogeneous networks such as cellular networks, public switched telephone network, power line communication, electricity distribution network and satellite networks in a single platform, machine type communication, body area networks, and vehicle to vehicle communications are also emerging in the wireless community.
One potential technique of meeting the complicated requirements of 5G communications is by maximizing network densification via massive deployment of small cells of different types including licensed small cells and unlicensed WiFi access points. This densification approach has been already adopted in existing wireless cellular networks, which essentially results in a multi-tier Heterogeneous Network (HetNet). The investigation of suitable resource allocation algorithms that efficiently utilize radio resources such as bandwidth, transmission power and antenna while mitigating inter-cell and inter-user interference and guarantee acceptable Quality of Service (QoS) for active users is one of the most critical issues. In addition, design and deployment of reliable backhaul networks that enable efficient resource management and coordination with practical energy efficiency constraints are also important aspects to be studied.
Due to huge amount of network data traffic caused by the popularity of video, internet gaming and social media across a range of new devices such as tablets and smartphones, it is almost certain that this explosive traffic growth problem of cellular networks cannot be addressed by just upgrading the existing networks. Besides, several studies have shown that more than 70 % of the current traffic originates from an indoor environment. In most metropolitan indoor environments where traffic congestion is more critical, Wireless Fidelity (WiFi) Access Points (APs) are already available. Also, it has been reported in the past studies that WiFi system consumes significantly less energy than the existing 2G and 3G systems and deploying more WiFi hotspots is significantly cheaper than that of upgrading 3G or Long Term Evolution (LTE) networks. In this regard, advanced WiFi networks can be promising candidates to meet the data rate requirements of the next generation 5G wireless systems. However, existing WiFi APs are mostly equipped with a single antenna whose radiation pattern is omnidirectional. Recently, the deployment of multiple antennas on WiFi APs has received an important attention. This will enable APs to control the radiation pattern of transmitted and received radio signals adaptively which will consequently help to improve the QoS experience of the users, and to meet the capacity requirements of the future 5G wireless networks.
Dynamic spectrum access has been considered as one of the enablers to address the spectrum scarcity problem in future wireless networks. In this context, investigation of suitable techniques in order to foster the implementation of cognitive radio systems in practical scenarios is crucial. In this direction, future works are needed in order to understand the performance of cognitive radio systems in the presence of imperfect channel knowledge, asynchronous primary user traffic, and various practical inevitable imperfections such as noise uncertainty, channel uncertainty, noise/channel correlation, hardware impairments such as phase noise, frequency offset, amplifier nonlinearity, analog to digital converter inaccuracies, calibration issues, etc. Another important issue is how to tackle their harmful effects such as interference to the licensed (primary) system and the performance degradation of the unlicensed (secondary) system.
Another way of enhancing the utilization of the available spectrum resources is to enable full duplex operation on a radio node so that it can transmit and receive on the same radio channel. In a wireless system, full duplex operation can provide several benefits such as increased link capacity, wireless virtualization, improved physical layer security, reduced end-end and feedback delays, and improved spectrum utilization efficiency by allowing simultaneous sensing and transmission, and simultaneous transmission and reception. However, there exist several research problems in realizing the full duplex operation in heterogeneous wireless networks such as strong Self-Interference (SI), imperfect cancellation of SI due to residual hardware impairments, increased aggregate interference, high power consumption. In this regard, it is crucial to investigate advanced multi-antenna based signal processing techniques such as adaptive beamforming and antenna selection/switching, self-interference estimation/detection techniques and innovative power control strategies in order to handle the issues of the residual SI.
Furthermore, another key enabling technique is three dimensional (3D) beamforming which has recently received important attention in order to enhance the capacity of future wireless networks. In contrast to the conventional 2D beamforming, the 3D beamforming controls the radiation pattern in both elevation and azimuth planes, thus providing additional degrees of freedom while planning a cellular network. The main research challenge here is the investigation of low-complexity hybrid beamforming solutions which can control the radiation pattern in both elevation and azimuthal planes.
Massive MIMO and mmWave technologies provide vital means to resolve many technical challenges of the future wireless 5G Networks and they can seamlessly be integrated with the current networks and access technologies. In a rich scattering environment, massive Multiple Input Multiple Output (MIMO) technique can enable significant performance gains with simple beamforming strategies such as maximum ratio transmission or zero forcing. This technology uses a very large number of service antennas at the base station which helps to eliminate the multiuser interference with the help of very sharp beams. Despite its several other benefits such as system throughput improvement, higher energy efficiency, reduced latency, and the simplification of medium access layer, several challenges such as pilot contamination, the effect of hardware impairments, correlation and synchronization issues need to be addressed with the help of future research works.
Besides, another promising way of solving spectrum scarcity problem and meeting capacity demand of future wireless systems is to enable mobile communications using millimetre wave (mmWave) frequencies. The capacity requirement of the next-generation wireless network would inevitably demand us to exploit the mmWave frequencies ranging 30GHz-300GHz which is still under-utilized but can offer huge spectrum. Most importantly, as the mmWaves have extremely short wavelength, it becomes possible to pack a large number of antenna elements in a small form factor which consequently helps to realize massive MIMO at the base stations and user terminals. Furthermore, mmWave frequencies can be used for outdoor point-to-point backhaul links or for supporting indoor high-speed wireless applications (e.g., high-resolution multimedia streaming). However, there are several challenges to be solved including propagation issues, mobility aspects, hardware imperfections such as power amplifier non-linearity and low efficiency of radio frequency components at these frequencies.
In addition to the existing multiple access schemes such as TDMA, FDMA, CDMA, OFDMA and Space Division Multiple Access (SDMA), several multiple access schemes such as Polarization Division Multiple Access (PDMA), Interweave Division Multiple Access (IDMA), Universal Filtered Multi-Carrier (UFMC), Sparse Code Multiple Access (SCMA), Generalized Frequency Division Multiple Access (GFDMA) and Non-Orthogonal Multiple Access (NOMA) schemes are being investigated by several researchers as promising multiple access techniques for 5G wireless.
Although several aforementioned techniques are being considered as promising technologies for 5G, it’s not yet clear which combination of these technologies will define the so called fifth generation (5G) of cellular communications since 5G standardization is still in its infancy. However, it is clear that all the modified techniques and network architectures being investigated in the community will not be mature enough for 5G by the time 5G will be deployed and many of them will eventually propagate to the next generations beyond 5G.
