This work surveys the recent advances and key results and challenges toward the realization of the 5th generation mobile cellular communication systems. The surplus provided by The key enabling technologies for the 5th generation c ellular system such as massive MIMO, signal propagations and communications at the millimeter wave band, cellular network densification, network softwarization, and interference mitigation are explicitly highlighted and explained. Furthermore, the main op portunities and the challenges toward the system design and realization are also explained for further research and scientific investigations . © 2017 The Authors. Published by Oases Vox Ed. This is an open access article licensed under a Creative Commons Attribution -Non-Commercial 4.0 International License . 20 International Journal of Innovative Technical and Applied Sciences Vol 1. Issue 1 (2017 ) 012–018 1. Introduction Mobile cellular communications systems had seen remarkable development since its first deployment th ree decades ago. Developing the cellular concept, interference management and enabling handover represent the key driving ideas for the first generation syst em which used mostly for voice communications [1 -3]. The key enabling technologies for the second g eneration was the making use of digital interface for voice and SMS transmission. Driven by the need for providing data communication barrier with the voice communication making u se of wideband transmission and system architecture development were the main enabling technologies for the third system. In the third -generation cellular system, the problem of the wide band frequency selectivity is toggled by making use of Wideband Code Division Multip le Access (WCDMA) [4]. Furthermore, we have seen a remarkable system architecture evolution from second generation to the third generation by including additional barrier for data communication equipped with advanced communication protocols. In the 4th generation sys tem, the problem of frequency selectivity is toggle d by making use of Orthogonal Frequency Division Multiple Access (OFDMA) which provide more flexibility in resource assignment and resource scheduling [5 -7]. In addition to OFDMA, the 4th generation systems make use of Multiple Input Multiple Output (MIMO ) antenna technology which also adds remarkable increase in the overall system capacity [8]. Moreover, the System Architecture Evolution (SAE) developed in the 4th generation cellular systems enables the additional add up features s uch as reducing transmis sion latency and providing high data rate with multiple quality of services (QoS). On the way to the development of the 5th generation cellular system, the main seen as enabling technologies are: Softwarizati on of the network, utilizing massive MIMO system s, employing and incorporating more intelligent interference reduction, power control and resource allocation algorithms, densification of the system deployment and employing wider bandwidth by making use of the millimeter wave band,. The aim of the work i s to review the key enabling technologies and highlight the main challenges toward the realization of the 5th generation cell ular system. The rest of the paper is organized as follows: section 2, highlights the opportunities provided by the existing techn ologies toward the realization of the 5th generation system features. Section 3, summarize the key challenges and research problems. And section 4, conclude the work. 2. Opportunities In the Existing Technologies 2.1. Using Millimeter –wave band To address the growing needs in the bandwidth to connect the internet of people and the internet of things of the future, making use of the millimeter -wave at the band 30 GHz ~300 GHz is proposed[9 -11]. The available spectrum in this band is 200 times greater than the e ntire current cellular spectrum allocated to the systems of today. This large spectrum clearly contributes to the overall system da ta rate, which upper bounded by the relation [12]:     NIS nwmCR 1 log)(2 (1) Where: represents the cellular system upper bounded data rate, represents the system spatial multiplexing gain factor which increased by making use of multiple antennas, is the system load factor which reduced with the increase in the network densificat ion. represents the total bandwidth or the system allocated spectrum and , , represents the desired signal power, noise power and total interference p ower respectively. The total system interference of the next generation cellular system will be greatly reduced with the making use of the excellent proposed intelligent power control and scheduling strategies. Signal at the millimeter wave tend to have a reasonably small wave length, which enables deployment of larger numbers of ante nna (massive MIMO) in a small area [13 -15]. This feature also represents an additional surplus point in the system design according to the relation explained in equation (1). Recently, a number of specialized research groups have been working on channel characterization and deve loping channel models at the millimeter wave band. These groups include: METIS2020, COST IC1004, ETSI mmWave SIG, 5G mmWave Channel Model Alliance, MiWEBA, mmMagic, and NYU WIRELESS [9, 16 -20]. The developed empirical models (based on extensive measuremen t on the millimeter wave), suggest that perfect channel path loss and shadowing parameter can be perfectly predicted. Thus, these mode ls can be used for system design, prototyping and performance testing at this stage for the next generation system design. Also some promising results in this area shows that deployment of intelligent self-configuration algorithms can compensate the path loss and shadowing effect at high data rate[12]. The reported NYC channel models developed from the measurement data collec ted from the city center of New York at 28 GHz, 38 GHz, and 73 GHz show comparable pattern and characteristics [9]. 21 International Journal of Innovative Technical and Applied Sciences Vol 1. Issue 1 (2017 ) 012–018 Figure 1 : Channel Path loss at 28 GHz, 38 GHz and 73 GHz[21] As shown in figure 1, the measurements also indicate that the smaller wavelengths introduce an increased sensitivity of the p ropagation models to the scale of the environment and show some frequency reliance of the path loss as well as increased occurrence of signal blockage. Furthermore, the penetration loss is highly dependent on the material and tends to increase with frequency. The shadowing fad ing and angular spread parameters are larger and the boundary between LOS transmission and NLOS transmission depends not only on antenna heights but also on the local communication environment (urbanization elements) [21, 22]. The small -scale fading characteristics of the channel such as delay spread and angular spread and the multipath richness is somewhat simil ar over the frequency range, which is encouraging result for enlarging the available on hand 3GPP models to the wider frequency range at the millimeter wave band[13, 19]. 2.2. Making use of Massive MIMO The primary and pioneering works reported in[23 -25] s hows that there is a lot of gain that can be extracted from the deployment of multi - element antennas at both communication link ends. This high predicted spectral efficiency can be extracted in terms of divers ity/or spatial multiplexing gains, which are bo th directly related to the overall system spectral efficiency[8, 26]. The predicted additional spectral efficiency gain associated with Multiple Input Multiple Output (MIMO) channel is due to the premise that the rich scattering communicati on environment affords parallel independent Single Input Single Output (SISO) channel paths from the transmitter to the receiver. As a result , for any simple point -to-point MIMO channel with N_T transmit antennas and M_R receive antennas, the overall rate scales linearly with the increase in the number of antennas at the transmitter and receiver (min(N_T,M_R)) [27]. Unlike the point to point MIMO link, another advanced configuration of multi -element antennas is the multi -user multiple input multiple output (MU -MIMO) arrang ement. This configuration is slightly more complex than the simple MIMO channel on the way that the MU -MIMO system makes use of the additional multi -user spatial dimension to further improve the overall system spectral efficiency. In MU -MIMO channel config uration scenario, the Base station / or Access point is equipped with multiple antennas and concurrently communicates with a group of users. Each individual user is also equipped with multiple antennas. To maximize the MIMO channe l gain utilization for sin gle user MIMO and MU -MIMO channels, baseband signal processing techniques known as precoding is applied at the transmitter[28, 29]. With this precoding signal processing at the transmitter, the non -causal known interference is pre -subtracted or reduced, which also come with more spatial efficiency gain. Massive Multiple Input Multiple Output (MIMO)on the other hand is an emergi ng MIMO technology aims to scale up MIMO deployment by order of magnitude compared to what is used in the 4th generation system[14, 15]. Figure 2, shows the evaluation of channel spectral efficiency in term of (bit//sec/Hertz) vs. the SNR in atypical massive MIMO confi gurations. It shows clearly that massive MIMO technology will have great contribution on the 5th generation systems. 22 International Journal of Innovative Technical and Applied Sciences Vol 1. Issue 1 (2017 ) 012–018 Figure 2: channel capacity increase as the number of antenna increase 2.3. Network Densification Network densification is an increase in the number of base station/ spectrum deployed in the given geographical area. The net work densification gain is an increase of the data rate relative to the increase of network density. As depicted in figure 3, netw ork densification can be practically achieved by two ways: spatial aggregations which explicitly mean using large amount of Electromagnetic spectru m band or spatial densification which requires an increase of the factor (m/n) in equation (1)[12]. This can be achieved by an increase in the number of antennas or reduce the user served by the specific base station. In general, network densification and network throughput are related by the following relation: 𝑛𝑒𝑡𝑤𝑜𝑟𝑘 𝑡ℎ𝑟𝑜𝑢𝑔 ℎ𝑝𝑢𝑡 =𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙 𝑒 𝑠𝑝𝑒𝑐𝑡𝑟𝑢𝑚 ×𝑐𝑒𝑙𝑙 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ×𝑠𝑝𝑒𝑐𝑡𝑟𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 (2) Where network throughput is defined in (bit/sec/km2), available spectrum is given in (Hz), cell density is defined in (cell/k m2) and the spectral efficiency is given in ( bits/sec/Hz/cell). Figure 3: Network Densification Classification 0 2 4 6 8 10 12 14 16 18 20100200300400500600700800900 (Eb/[N0+I])[dB] bit/sec/Hertz m=50,(w/n)=1 m=70,(w/n)=1 m=100,(w/n)=1 m=120,(w/n)=1 m=150,(w/n)=1 23 International Journal of Innovative Technical and Applied Sciences Vol 1. Issue 1 (2017 ) 012–018 The main challenges toward the realization of network densifications are the extensive need for backhaul densification, tight control of inter-cell interference and the realization of the cloud radio access network with coordinated multi­point CoMP. Given all these challenges, there are also a lot of opportunities. 2.4. Network Softwarization: Network softwarization is an overall transformation trend of designing, implementing, deploying, managing and maintaining network by implementing of network function in software. The main objectives of network softwarization is to reduce both the capital expenditure (CAPEX) and operation expenditure (OPEX) by speeding up service creation, network deployment, service management and reducing power consumption. These obje ctives can be realized building following features[30]: • System Programmability • Network flexibility which includes reconfigurability, reusability and infrastructure sharing functions. • Network adaptability which include self configuration, self protection, self healing and self optimization[31]. • Adding various capabilities to the network such as automatic network management and computing power. Incorporating all these features in the 5 G network requires adding detailed system function to all 5 G network layers (Trans port network, Core network, Access Network, and Mobile edge Network) as illustrated in figure 3. Figure 4: 5G Net work Softwarization Functions 2.5. Incorporating Intelligent Interference Reduction, Power Control and Resource Allocation algorithms Cellular communication system interference includes adjacent channel interference (ACI), coexistence interference (CEI), C o-channel interference (CCI), and Multiple access interference (MAI). ACI refers to the component of interference between links that c ommunicate geographically close to each other using neighboring frequency bands. CEI arise in systems using the same freq uency band or between adjacent frequency bands: some examples include interferences between 2nd generation and 3rd generation networks and interferences bet ween different systems operating in the non -licensed bands. CCI is interference between links that r euse the same frequency band (channel), in cellular systems this is also known as inter -cell interference. MAI is the interference among the transmissions from multiple radios utilizing the same frequency resources to a single receiver. In addition to the classical interference mitigation / management techniques which include various methods based on frequency reuse, there is a lot of opportunities to reduce the level of interference targeting the following objectives: • Enhancing the cell edge user's thro ughput • Enhancing the energy efficiency by switching on/off some of the base units. • Reducing system overhead by for example clustering small cell. These features can be brought into practical systems by developing excellent algorithms based on power co ntrols, context awareness and optimum resource allocation. Furthermore, by making use of the self optimized network (SON), we can automate all these functi ons to be active at the system opera ration time. 3. Opportunities and Challenges From all abov e, we can summarize the technological opportunities for the 5th generation systems in the following points 24 International Journal of Innovative Technical and Applied Sciences Vol 1. Issue 1 (2017 ) 012–018 • The availability of spectrum at the conventional cellular systems band and the millimeter wave bands. • The possibility of making use of massive MI MO technology with millimeter wave band (overcome the restriction of the size of the antenna with a wavelength which means a massive BS antenna array does not have to be physically large). • Possibility of multi -band operation (deploying the system to oper ate at the conventional cellular band and millimeter wave band). • Possibility of building advanced self optimized network software functions to automate system configurations for optimum operating conditions. • Overall network throughput improvement. We c an summarize the technology challenges for the 5th generation systems in the following points: • Additional channel measurement and modeling at millimeter wave band. • Multi -band transceiver design complication issues (Amplification and Signal Processing). • Design for large path loss range from conventional band to millimeter wave band. • Deployment of massive MIMO in the millimeter wave band. • Pilot contamination issue in Massive MIMO system. • MAC layer design, power control and resource scheduling. • Cost for network densification. • Massive backhaul connectivity requirement. • Signal detection and parameter estimation for massive MIMO. 4. Conclusions This paper highlights the key benefits provided by the enabling technologies for the 5th generation c ellular communications system and summarizes the main challenges and opportunities of the current researched technology. The surplus of massive MIMO systems, t he possibility of making use of the millimeter wave band, network densifications, network softwar ization and interference mitigation are intuitively explained in the direction to meet the requirement of the next generation cellular systems. Furthermore, the ongoing research problem i n these areas was also explained to show out the way for further inv estigation and new research innovations.