5G mMTC: Challenges and Solutions

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5G would cater to a diverse set of use cases and would solve many inherent shortcomings of MTC technologies.

Curated by Vinay Prabhakar Minj

5G is a big buzzword in the industry. It’s the next generation of mobile technology. It is not just about high data rates but is an ecosystem of technologies which is going to cater to a wide range of use cases and requirements. This will impact a lot of industries, businesses and lives of people. mMTC or massive machine-type communication (mMTC) is a very important aspect of 5G.

5G features and services

It is known that 5G will take care of a lot of use cases and requirements. And all of it can be grouped into one of these three services:

  1. EMBB (Enhanced Mobile Broadband) – It has a high data throughput of the order of more than 10 Gbps, high system capacity of the order of more than 1000 times that of LTE and a much better spectral efficiency than LTE (3 – 4 times that of LTE). Its use cases are high-speed mobile broadband, virtual reality, augmented reality, gaming, etc.
  2. URLLS (Ultra-Reliable Low Latency Services) – It focusses on low latency, high reliability and high availability aspects. The expectation is of the order of less than 1 millisecond of latency and availability of the order of 99.9999 percent. This is basically for mission-critical use cases and applications.
  3. mMTC (Massive Machine-type Communication) – It aims to provide connectivity to a huge number of devices whose traffic profile is typically a small amount of data (spread) sporadically. So, latency and throughput are not a big concern. The main concern is the optimal power utilisation of those devices because they are battery powered and the expectation of battery life is around 10 years or so.

MTC: Machine Type Communication

It is also referred to as IoT and is basically about connecting devices without human intervention. These devices typically generate some data that is sent to a cloud server to analyse and take action on that. These devices can be fitness bands, smart watches, connected home appliances, smart lights and so on. Such devices are getting connected through MTC.

Speaking of MTC, already lots of technologies are out there which are competing with each other to provide services. These technologies can be divided into two groups: non-3GPP MTC and 3GPP MTC technologies.

Non-3GPP or proprietary – These are also known as LPWANs. Popular non -3GPP MTC technologies include ZigBee, LoRa, Ingenu, Sigfox and Weightless. These technologies use unlicensed spectrum that makes them cheaper (cost-wise) for the user. Their infrastructure is not present throughout the world, it’s present at some locations only and is not 3GPP standardised. The use open or proprietary  standards, but since they have been around for a while, they have some sort of early mover advantage.

3GPP MTC technologies (2G & 4G)  – Within the 3GPP group, we have three main technologies:

  • EC-GSM (Extended Coverage GSM): This is an upgrade of GSM suited for the requirements of IoT-based use cases.
  • LTE-M: It is also called as Emtc (enhanced MTC) and Cat M (Category M) is low bandwidth and low throughput variant of LTE. It operates in 1.4MHz spectrum. It gives good throughput and mobility.
  • NB-IoT (Narrow Band IoT): It has also evolved from LTE and is a new technology altogether. It has a completely different physical layer. It is much more optimised for low throughput, sporadic sort of data communications needed by MTC devices. Its USP provides a lot of flexibility in terms of deployment. It has three modes:
  • in-band: In this mode, NB-IoT can be deployed in one of the physical resource blocks of an LTE cell.
  • guard-band: In this mode, it can be deployed in the guard-band between two LTE carriers.
  • standalone: In this mode, it can be deployed in one of the carriers of GSM frequency. It has a lot of flexibility from the deployment point of view.

The above technologies use licensed spectrum, so they are slightly costlier than the non-3GPP ones. But the advantage is that the cellular networking infrastructure is already present throughout the world. Apart from that, they are controlled by 3GPP standards that makes them quite lucrative interoperability-wise.

Need for mMTC technology

The existing technologies have some shortcomings which can be improved in a new generation of technology.

  • The present technologies focus only on some specific use cases, not all of them.
  • They are unable to meet the latency and reliability requirements of new use cases such as connected cars, industrial automation and so on. So far for NB-IoT and Cat M, the number of devices per cell was 40000 -50000. In 5G, that number can reach upto 1 million devices per cell and 50 billion devices worldwide.
  • These technologies have evolved from LTE and originally LTE was not designed for IoT. So, they are not optimised to handle IoT specific characteristics like small data packets, sporadic transmission, uplink centric transmission, power optimisation and so on.

5G MTC

Since a lot of evolution of these technologies has happened themselves, it is being said that the initial phases of 5G would include only an evolution of NB-IoT and Cat M. Proper 5G MTC technology might come a bit later.

The standardisation of 5G MTC is still in progress and would be a part of the second phase. It would be included in Release 16 of 3GPP specification.

It has two aspects:

  1. mMTC caters to scalable connectivity for a huge number of devices and latency agnostic applications. And it should be optimised for small packets, sporadic transmissions and uplink centric activity. Typical use cases are energy meters, connected home appliances, connected fitness bands, smart watches and so on.
  2. uMTC or Ultra-reliable Machine Type Communication focusses on reliability and low latency. Typical use cases are automated industries, connected cars, remote surgeries and so on.

Characteristics of mMTC

  • Small packets transmitted from devices typically of a few bytes (10 bytes, 20 bytes)
  • Huge number of devices (300,000 – 1 billion devices per cell)
  • Mostly uplink transmissions
  • Low user data rates, around 10 kb/s per user
  • Sporadic user activity (the device will send data seldom and randomly)
  • Low device complexity and cost
  • Optimal power usage and long battery life

Challenges in front of mMTC

  • A common framework is required which can take care of all MTC use cases (latency agnostic, latency-sensitive and so on).
  • Current packet sizes, channel estimation pilots, link adaptation mechanisms are unsuitable for MTC. The present ones are more suited for longer sessions and bigger data packets.
  • Small packets have their own challenges. Radio resource allocation has to be done at a finer granularity as the amount of data is small. And present channel coding schemes are inefficient with small data packets.
  • Inefficient control signalling: a lot of control signalling happens before data can be sent.
  • It will require handling a huge number of devices which are accessing the networks in an uncoordinated manner sporadically.
  • Coverage enhancement. It is the ability of devices to be able to work in bad signal conditions.

Control signaling optimisation

It involves integrating various protocol procedures into leser procedures. LTE access and data transfer involves the following steps:

Random Access Resource Allocation à RRC Connection Setup à Authentication à NAS Level Security à Access Level Security à Data Bearer Setup à Data Transmission.

So, a lot of signaling take place before the final data transmission occurs, which might be a very small amount (10 – 20 bytes). We might be sending 100 bytes of control information just for 10 bytes of data. So this is an area where optimisation can be done.

One potential optimisation solution for this is combining various steps of signaling flow into fewer steps like combining authentication and security with initial access and RRC connection setup. One important aspect of it is that it is signature-based access. In this, every UE sends a signature to the network side which uniquely identifies that UE. So, from the signature itself the UE can be authenticated.

The signature is basically a sequence of preambles which is unique for each device because it is created using the unique identity of that device. Thus, reducing the signaling flow leads to better spectral efficiency, lower power utilisation of the device and would prevent any type of “storms ” of signaling on the networking side.

Efficient initial access

Initial access is one area which can be optimised for mMTC. Some proposed techniques are:

  • Non-orthogonal multiple access (NOMA) at device side
  • Multi-user detection at the network side
  • Grant free one shot transmission

In orthogonal multiple access, for every radio resource only one device can be asked to transmit. From reception point of view, there are no inter-user collisions/interferences. But when we have a huge number of devices and limited radio resources, this might not be optimal from a spectral efficiency point of view.

In non-orthogonal multiple access, we deliberately give the same resource to multiple users and they are multiplexed in the power domain. Their transmission is separated in the power domain. If multiple users transmit on the same resource, they are bound to create collisions. The collisions are resolved on the receiver side by successive interference cancellation techniques which leads to multi-user detection at the receiver side. So, this mechanism definitely leads to more complexity on the receiver but gives a very good resource utilisation, which is needed for mMTC.

NOMA at device and SIC at base station 

What happens in non-orthogonal multiple access is two devices transmitting to the base station with a significant difference in their power, because they are at different propagation delays. At the base station, we receive both the signals – one with a higher power and the other with lower power. The base station then extracts the signal of both the devices one-by-one by using successive interference cancellation.

LTE: Multi Stage Access Protocol

This is the initial access to LTE. There is an Access Notification àResource Grant/Allocation à Contention Resolution à Data Transmission.

There is significant overhead for small amount of data leading to latency and device power wastage.

Here there is a scope of optimization and a new mechanism has being proposed for it, known as one stage access protocol.

One Stage Access Protocol: It has grant free access (common grant is given to all the devices). Collision resolution happens at the base station through successive interference cancellation. In all, this is much shorter signaling and gives us low latency and better spectral efficiency and power utilisation from device point of view.

Coverage Extension

It is basically for devices to be able to work in places where the signal is not good like basements. For this, every transmission in uplink or downlink is repeated several times to improve the chances of reception.

CE levels or Coverage Extension levels are present in Cat M and NB-IoT and can be extended to 5G as well. It is based on the levels of reception of the signals at the device. The number of repetitions of the transmission of the device would depend on the CE level. If it is normal, then there might be 8 repetitions. If it is robust (i.e less than normal), then there might be 16 repetitions and so on.

This mechanism is very good if we have grant-based access, where the network communicates to the device about  how many repetitions is required. So, both the network and device know how many repetitions would be done and at the end of that, the network would acknowledge whether it received correctly or not.

But in grant free access, the device is randomly selecting a resource and sending it. The device calculates its own CE level and the network does not know how many repetitions the device is going to make (because network hasn’t communicated with the device). This can lead to confusion between the network and the device.

The solution  

There is one solution to the above problem. The CE level which decides the number of repetitions of the device is scrambled (i.e sequence corresponding to the CE level is put) on the data when the data is sent from the device.

At the de-scrambling step, the network is able to know the CE level of the device and knows how many repetitions the device is going to make. Hence, there is no confusion and everything goes fine.

About the author

The article is an extract from a speech presented by Ajay Uppal, Senior Principal Engineer – Hughes Systique Corporation (HSC), at IOTSHOW.IN2019.

 

 

 

Longjam Dineshwori

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