Challenges for 5G

The fifth generation (5G) network is projected to support a huge amount of data traffic. It is meant to serve millions of wireless connections. With the utilization of certain advanced technologies like Small Cells, Massive MIMO, etc, 5G can support a wide range of devices and applications which will further give rise to an enhanced future of Internet of Things (IOT). However, these technologies have their own challenges that render the establishment of 5G difficult.

Technologies and Challenges :
These are explained as following below.

1. Milimeter Waves :
   5G uses millimeter waves called so because they have wavelengths lying between 1 mm and 10 mm.

   
   

   Figure – Range of Milimeter Waves for 5G

   These are high frequency waves ranging between 3 GHz and 300 GHz out of which the range from 24 GHz to 100 GHz has been proposed for 5G. This will enable a huge amount of devices and technologies to be able to use the bandwidth as well as result in better streaming of higher quality videos and other multimedia content.

   However, this further exposes certain hindrances to 5G.
   Since millimeter waves are high frequency waves, they are more susceptible to blockage from buildings, trees and other structures. Additionally, they can be attenuated by clouds and rains. For that reason, traditional cellular towers being used on a large scale today may render futile. It is so because the relationship between the wave frequency and antenna size is inversely proportional. This is where the next technology comes in.

2. Small Cell :
   ‘Small cells’ is a term for low powered radio access nodes that help in providing services to both indoor and outdoor areas. Small cell antennas have a range between 10 m to 2 km.
   To extend the coverage of a macrocell, distributive antenna systems (DASs) are used in conjunction with the cell tower. DASs take a signal from the base station and boost it to increase the area the signal can reach. Small cells will be a crucial component for 5G networks, because they increase network capacity, density, speed and coverage.

   Apart from all the convenience provided by small cells, there are a handful of challenges or drawbacks while their implementation, them being:

   1. Small cells must be low cost because they are set up for fewer subscribers within a lesser range.
   2. The number of small cells required to build a 5G network may make it hard to set up in rural areas.
   3. It should be easy for the mobile network operator to diagnose potential problems and maintain small cells.
   4. They should be physically small and lightweight in order to be deployed on streetlight poles, sides of building walls, etc.
   5. These should have high weather reliability.
3. Massive MIMO :
   MIMO is a radio communication technology and stands for Multiple Input Multiple Output. As the basic framework of MIMO is to have multiple antennas at the transmitter and receiver. MIMO ensures reliable communication at high data rates as it takes advantage of the multiple paths that exist between the various transmitters and the receivers.

   

   For older technologies, one cell can have only up to ten antennas but for 5G the very cell can have up to a hundred antennas this means one single cell can serve a lot more users at the same time and that with greater efficiency and speed. But everything comes with a cost, this, meaning that massive MIMO possesses its own complications; Antennas broadcast information in all the directions at once, this could cause immense amount of interference. This issue can be solved by using another 5G technique called beamforming.
4. Beamforming :
   Beamforming is a MIMO technique in which the transmitter or antenna focuses a narrow signal beam in the direction of the receiver. It requires that the transmitter knows the wireless channel.
   Multiple antennas placed in proximity, broadcasting a signal at slightly varied timings are deployed in the working of beamforming. The overlapping waves will produce constructive or destructive interference that will make the signal strong or weak respectively.If this is executed properly, beamforming focuses the signal to its path.

   The access point forms a narrow beam which has high gain in a specific direction, rather than across a wide angle. This beam points to the subscriber from which it has to receive data, intersecting with the subscriber’s beam and receiving its data. The limitations include the computing resources needed as they require more time and power. Beamforming computationally is the linear combination of the outputs of the elements with which a beam can be computed.

5. Non-Orthogonal Multiple Access (NOMA) :
   NOMA is used to address the challenges such as high spectral efficiency and massive connectivity. The typical approach to NOMA is to group users and superpose their data signals using different transmission powers before transmitting the group’s signal in the same way, using the same beamforming. NOMA superposes multiple users in the power domain although its basic signal waveform could be based on (Orthogonal frequency division multiple access) OFDMA.

   There are certain limitations and challenges attached to this technique. These are:

   1. Code-domain multiplexing has a potential to enhance spectral efficiency but requires a high transmission bandwidth and is not easily applicable to the current systems
   2. In NOMA, since each user requires to decode the signals of some users before decoding its own signal, the receiver computational complexity will be increased when compared to OMA, leading to a longer delay.
   3. Information of channel gains of all users should be fed back to the base station (BS), but this results in a significant channel state information (CSI) feedback overhead
   4. Furthermore, if any errors occur during SIC processes at any user, then the error probability of successive decoding will be increased.
6. Software Defined Network (SDN) :
   In SDN, the control plane is segregated from its respective data plane physically i.e. the application and OS layer are separated from the hardware centralizing its intelligence and abstracting its architecture. A single control plane is made up of all the individual control planes and it does exactly the same job as earlier, just for a larger number of devices as a whole which means that there is a defined control logic in a centralized manner. Communication between the two planes is done through APIs. For making controller to switch communication, protocols like OpenFlow can be used.

   There are two main challenges in the implementation of SDN:

   (i). Rule Placement Problem –

   • Forwarding in SDN is done using flow tables defined by the centralized controller. The size of the memory called ternary content addressable memory (TCAM) is limited.
   • In addition to this TCAM is very expensive.

   (ii). Controller Placement Problem –

   • Controllers define flow rule according to the requirements and must be able to handle all the requests which becomes prone to causing delays.
   • If we have very less controllers for a large network, it might get congested.

Regardless of the most advanced technologies ready to be brought into service by the fifth generation network, there are significant numbers of other challenges that thwart the successful establishment of 5G worldwide. These are listed below:

• The lack of coordination of frequencies worldwide. Different countries are observed to have different frequencies.
• Making calibrated measurements of frequencies and bandwidth is very expensive, time-consuming and requires a lot of expertise.
• Inter Modulation Distortion (IMD) could be caused due to the proximity of the bands of 5G NR and legacy LTE systems.
• The ability to cover wide ranges of geographic spectrum and fitting into the footprints of today’s cellular network with high frequency ranges is a big challenge within itself.
• Rural and suburban areas are less likely to enjoy 5G investment, and this will potentially widen the digital divide.

However, 5G networks and services, are forecast by independent economic studies to deliver very significant economic gains by this decade.