The current systems that we are using now, like 3GPP LTE or LTE-A, are commonly called 4G mobile communication systems. Then, what are 5G mobile communication systems? To discuss 5G, we should first look into what the mega trends in mobile services are these days.
 
[M1. Traffic growth] The biggest trend would be soaring demands for multimedia and social network services witnessed recently. As a result of that, mobile traffic has been growing tremendously. The mobile traffic is expected to explode as an enormous number of things are to be capable of interacting with each other with the advent of the Internet of Things (IoT). So, the ever-expanding mobile broadband services and the growing number of communication-enabled things will continue to cause traffic increase. According to the Cisco VNI Global Mobile Data Traffic Forecast, mobile data traffic is expected to surge about 10 times - from 1.5EB (Exa Bytes, 1EB = 1,000,000TB) in 2013 to 15.9EB in 2018.[1]
 
[M2. Increased number of devices] No. 2 trend would be, as mentioned above, a sharp increase in the numbers of mobile devices and things that can be connected to the Internet (network) - from 7 billion and 12.5 billion in 2013 to 10.2 billion and 50 billion in 2018, respectively.[1]-[3]  Like more mobile devices are being introduced in the market every day, an increasing number of new things (e.g. wearable device, sensor, actuator, etc.) designed to realize this future 5G mobile service, IoT service, are being brought to market as well. The more these services get popular, the more mobile devices and connected things for the services will be available. These changes in the market and communication environment will give users more use cases to choose from, constantly causing new requirements to be added in the system.
 
[M3. Higher dependency on cloud]  Thanks to the growing user demands for cloud computing systems, various solutions aimed at mobile (personal) clouding computing market are actively being developed. As a result, the transition from the current PC era to the new mobile cloud computing era is likely to be accelerated.[4] Cisco shared the similar point of view, predicting the mobile cloud traffic would increase continuously and account for 70% (twice the current 35%) of the total mobile traffic by 2020.[5] In that respect, most 5G mobile services will most likely be provided through mobile cloud computing systems.
 
[M4. Various mobile convergence services] Last but not least, there will be also fast-growing demands for mobile-based convergence services in various fields such as augmented reality/virtual reality, ultra high-accuracy location-based service, hologram service, smart healthcare service, etc.[6] Accordingly, development of 5G mobile communication systems that can satisfy all the requirements of these various services should be followed to ensure seamless and reliable supports for the services.
 
5G mobile communication systems must be designed with the foregoing four mega trends (traffic growth, increased number of devices, higher dependency on clouding computing, and various 5G convergence services) in mind. To this end, some suggestions have been made by some interested countries and companies in relation to selecting key performance indicators to be used for 5G mobile communication systems. Then, based on such suggestions, ITU-R (International Telecommunication Union – Radiocommunication Sector) WP (Working Party) 5D selected the final 8 candidates as seen in Table 1. [7][8]  
 
<Table 1> 8 key candidate performance indicators by ITU-R WP 5D

     
Unlike the current 4G mobile communication systems, 5G systems have some key characteristics that make them different from their precedent. Below we will closely look into the two most important characteristics of the systems: latency and user experienced data rate.
  
[R1. End-to-End Latency] 
First of all, most previous and current mobile communication systems have been focused merely on improving peak data rates of a user device. But, for 5G systems, more attention and interest are being flown to improvement of end-to-end latency.

Soon, more real-time interactive multimedia services, such as augmented reality, virtual reality, real-time online games, etc., will become available in the market. What is essential for users to enjoy these interactive services seamlessly is low delay.[9,10] In general, when visual and auditory information is perceived through media, visual information must be delivered within a tolerable latency of about 10 ms, and auditory information within about 100 ms.[11] If not, that is, if the latency is longer, users feel disturbed, unable to enjoy the seamless service.

In addition, new mobile communication services will be released in various mobile communication-applicable areas like transportation, sports, education, medical field, manufacturing, etc. And obviously in these services, users' tolerable end-to-end latency will become shorter, even down to several ms.

For example, services like vehicular-to-vehicular (V2V) communications or vehicle-to-infrastructure (V2I) communications require extreme low latency to ensure traffic safety-related services are provided in time. Or in case remote robot-assisted surgery is needed on a moving ambulance, again a mobile communication technology that can guarantee extreme low latency would be the key factor for reliable and successful surgery.   

 
Tactile Internet, one of the most noted core features of 5G mobile communication, can support a low-latency mobile communication service capable of delivering tactile information in time. This kind of service requires extreme low latency to properly respond to users' requests. [9,10,12]  According to [13,14], if tactile (the most latency-sensitive sense of all the five senses of human) information is to be delivered through a mobile communication system, the tolerable latency must be less than 1 ms to prevent users from experiencing any delay or lag.

Otherwise, users would suffer from discomfort, feeling so-called "cyber sickness". To avoid this cyber sickness, user-oriented service scenarios and relevant technologies must be secured so that, based on them, a mobile communication system with extreme low latency can be developed.

It is known that it takes up to 120m/s for an electronic signal to be delivered through a human's nervous system. This means it takes less than 10 ms for tactile information from a user's hand to reach his brain. So, in order to make user experience in a mobile communication system as natural as an electronic signal traveling through the nervous system, end-to-end latency less than several ms should be guaranteed.  
 
<Figure 1> illustrates the effects of data rate (bandwidth) and round trip time (RTT) on HTTP's page load time (PLT). [15] As seen in the figure, data rate improvement does not have significant effects on reducing HTTP PLT once it exceeds certain level.

This means a system aimed just for improvement of data rate without considering RTT would not be effective in further reducing PLT. To address these issues in 5G, active R&D on low-latency mobile communication technologies and services in various aspects like physical, medium access control (MAC), network, and transport layers should be followed.
 

<Figure 1> Changes of PLT by bandwidth and round trip time changes (source: Vodafone)[15] 
 

[R2. User Experienced Data Rate]
In addition to its No. 1 goal, achievement of lower end-to-end latency, 5G mobile communication systems have another goal, improvement of user experienced data rates. Then what is the difference between peak data rates and experienced user data rates? ITU-R WP 5D's 5D/TEMP/390-E defines them as follows:[8] 

• Peak data rate: peak data rate refers to the maximum achievable data rate per user. Future IMT systems should provide very high peak data rate capability that leads to high network capacity enabling new differentiated services and enriching the end user experience.
• User experienced data rate: user experienced data rate is defined as the minimum data rate per user that should be achievable anytime anywhere. Future IMT systems should have the capability to provide anytime, anywhere [gigabit] data rate experience to mobile users. Also, Future IMT systems should provide an [“edgeless”] experience to the mobile users unlike the existing systems where the user experience is limited by the cell edge performance.

Here, Future IMT systems refer to 5G mobile communication systems. As seen in the definition above, for consistent and guaranteed user experienced data rates, 5G mobile communication systems should be able to provide users with [gigabit] data rates anytime anywhere. None of the previous systems up to 4G has been able to present a solution for their biggest drawback, degraded performance at cell edges.

So, improving user experienced data rates through upgrading cell edge performance has been one of the most important goals that 5G mobile communication systems must achieve. To this end, many companies including Samsung Electronics, being well aware of that, have included this guaranteed 1 Gbps anywhere (inner/outer/edge) as one of the key system requirements of 5G mobile communication systems.[16]

To ensure that 5G systems offer better user experienced data rates than 4G, researches on many different 5G candidate technologies, such as interference management technology, including interference alignment/neutralization, small cell network/heterogeneous network technology, and advanced MIMO/beamforming technology are being conducted actively. 
 

<Figure 2> Example of uniform experience regardless of user-location (source: Samsung Electronics)[16] 
 
So far we have discussed the various mega trends in mobile services (traffic growth, increased number of devices, higher dependency on cloud, various mobile convergence services). We also learned 8 key system requirements that ITU-R WP 5D derived from the trends (user experienced data rates, peak data rates, mobility, latency, connection density, energy efficiency, spectrum efficiency, and traffic volume density).

As such, compared to 4G and earlier systems, 5G has additional and different requirements to satisfy, including latency and user experienced data rates, the two most important requirements discussed in detail here.
 
Other important issues to be considered when developing 5G mobile communication systems include:

• Connection density related to the increase of devices/access points (AP)/base stations, and their density 
• Energy efficiency related to addition of new features to devices, and use of more complicated/efficient protocol/algorithm, etc.

5G mobile communications systems are to be developed by considering and meeting these new requirements, and are expected to provide users with user experience different and better than that of 4G.
 
 
References
 
[1] VNI Global Mobile Data Traffic Forecast 2013-2018, Cisco, 2014
[2] Internet of Things, Cisco, 2013
[3] J. Gubbi et al., Internet of Things (IoT): A vision, architectural elements, and future directions, Elsevier Future Generation Computer Systems, pp.1645-1660, Feb. 2013
[4] Forbes, 6 Big Internet Trends to Watch for in 2012, Dec, 2011
[5] The Mobile Economy, GSMA, 2014
[6] ICT R&D Mid- and Long-term Strategies (2013-2017), Ministry of Science, ICT and Future Planning, 2013
[7] 20th ITU-R WP 5D meeting, TTA, 2014
[8] IMT Vision – Framework and overall objectives of the future development of IMT for 2020 and beyond, ITU, Feb. 2014
[9] G. Fettweis and S. Alamouti, “5G: Personal Mobile Internet beyond What Cellular Did to Telephony,” IEEE Communications Magazine, vol. 52, no. 2, pp. 140-145, Feb. 2014.
[10] G. Fettweis, “The Tactile Internet – Applications and Challenges,” IEEE Vehicular Technology Magazine, vol. 9, no. 1, pp. 64-70, Mar. 2014.
[11] M. T. G. Pain and A. Hibbs, “Sprint Starts and the Minimum Auditory Reaction Time,” J. Sports Sciences, vol. 25, no. 1, pp. 79–86, Jan. 2007.
[12] Korea Communications Agency (KCA), “New Issues regarding 5G Mobile Communication, Tactile Internet Overview,” Broadcasting Communication Technology: Issues and Perspectives, Korea Communications Agency, no. 49, Feb. 2014.
[13] E. Steinbach et al., “Haptic Communications,” Proc. IEEE, vol. 100, no. 4, pp. 937–56, Apr. 2012.
[14] T. DeFanti and R. Stevens, “Teleimmersion,” Ch. 6, The Grid: Blueprint for a New Computing Infrastructure, Elsevier Series in Grid Computing, pp 131–55.
[15] Walter Haeffner, “Networks at the Speed of Light,” Symposium Das Taktile Internet, Oct. 2013.
[16] Wonil Roh, “5G Mobile Communications for 2020 and Beyond - Vision and Key Enabling Technologies,” EUCNC, Jun. 2014.
 

About author

Professor Howon Lee (hwlee@hknu.ac.kr, http://wsl.hknu.ac.kr)

• Assistant professor at Department of Electrical, Electronic and Control Engineering, Hankyong National University, and adjunct professor at KAIST institute for IT Convergence
• GLOBECOM/VTC/WCNC/PIMRC Technical Program Committee (TPC) Member
• Division of 5G Forum Service Committee Member 
• Research interests: 5G Wireless Communications, Ultra-Dense Distributed Network, Internet of Things