A Comparitive Study of LTE Based M M Communication Technologies for Internet of Things
With the technological advancement at its peak, the world has seen as massive increase in the number applications in the field of Internet of Things. With a rapid increase in the number of connected devices, there comes a big need for new communication protocols that are lightweight and efficient in terms of power consumption, speed and coverage. M2M communication protocols devised by 3GPP aims to achieve the same with the introduction of NB-IoT and eMTC in their release 13. These two LTE based cellular IoT technologies have diverse applications and will be able to network with millions of devices in the near future while also coexisting with the current cellular infrastructure used for mobile broadband communication. The paper briefly describes the technological aspects behind these two protocols and compares them to determine their applications.
Keywords: Long Term Evolution(LTE), Machine to Machine communication (M2M), Low Power Wide Area (LPWA), Enhanced Machine Type Communication (eMTC), Narrowband Internet of Things (NB-IoT), Power savings management (PSM), Extended discontinuous reception (eDRX), Global System for Mobile (GSM)
Internet of Things (IoT) is undoubtedly one of the major revolutions that the wireless industry has seen and it is likely to be the key element in driving the future of cellular technology as well. With widespread adoption of the internet, it is evident that every device and every human is being connected to each other today. IoT is the key enabler for this by delivering machine-to-machine (M2M) communication on a massive scale. Ericsson predicts there will be around 28 Billion connected devices by 2021, of which more than 15 billion will be connected M2M and consumer-electronics devices.
Connectivity is the foundation for IoT, and the type of access required will depend on the nature of the application. Many IoT devices are served by radio technologies such as Bluetooth that operate on unlicensed spectrum and are ideal for short-range connectivity for a home or indoor environment. In order to achieve wide area connections of IoT devices, two alternatives are available. On one hand, there are the current proprietary LPWA technologies, such as SigFox and LoRa, which typically operate on unlicensed spectrum. On the other hand, there are 3GPP standardized cellular IoT technologies such as NB-IoT and eMTC which typically operate on licensed spectrum. We are going to talk about the latter set of IoT technologies in this paper.
Cellular IoT (CIoT) is defined as a set of technologies under the 3GPP standardization that enables IoT connectivity using the licensed frequencies while co-existing with the legacy cellular broadband technologies, such as LTE. Owing to its capabilities, numerous services are envisioned for cellular IoT, including utility meters, vending machines, automotive (fleet management, smart traffic, real time traffic information to the vehicle, security monitoring and reporting), medical metering and alerting. But, LTE was primarily designed for mobile communication and there are numerous requirements of the M2M communication systems and applications that LTE needs to deliver. Some of the major requirements are as follows:
Cost Reduction: The cost of LTE supporting cellular modules embedded on phones are not considered very expensive with respect to the other components on a typical smartphone. For M2M, the cost of the communication unit has to be drastically reduced to be integrated with IoT devices because devices such as wearable devices and smart meters lie in a lower price range as compared to phones. Various cost reduction techniques have been considered by the Third Generation Partnership Project (3GPP), including reduced computational complexity, reduced data rate, single antenna support, and half duplex operations.
Reduced Power Consumption: This is another important aspect of the battery powered IoT devices because millions of them are deployed in various locations across the world. Regular changing of batteries in these devices is not at all feasible.
Enhanced Coverage: There exists IoT applications where devices are sometimes located in remote or not so easily reachable locations such as basements and deep indoors, where there are high chances of path loss between the transmitter and the receiver. Thus, M2M communication services may require a 15–20 dB coverage enhancement with respect to regular cellular services.
Scalability: In order to enable a Massive IoT market, networks need to scale efficiently and be able to support millions of devices.
Diversity: Every IoT application will have a different set of requirements in terms of cost, latency, data rate and performance. LTE based M2M communication should be able to handle such diverse set of requirements from different applications.
In order to meet the above requirements, 3GPP release two new technologies namely, NB-IoT and eMTC that share the spectrum with the already existing LTE. Section II and Section III of the paper gives a brief introduction to these two technologies in terms of its functionalities and architecture while section IV thoroughly analyzes the difference between them in terms of performance and other parameters. Finally, in the Section IV, we conclude the paper with a brief note on our understanding about their differences followed by the references.
II. Enhanced machine type communication (EMTC)
In the Release 13 standardization, 3GPP released a favorable cellular LPWA technology eMTC which is also called LTE Cat-M1. This was done with an intention to minimize the cost, complexity and power consumption over the legacy UE’s proposed in the previous releases for machine type communication (category 0). eMTC achieves this by extending most of the existing LTE physical layer procedures and introducing a set of new features to it. An eMTC UE adopts a narrowband operation for transmitting and receiving signals over the physical channels where the maximum channel bandwidth is limited to 1.08 MHz (or 6 LTE RB’s) out of the available 1.4MHz. The two remaining PRB’S of 180 KHz each are used as guard bands to mitigate the interference levels. The value of 6 RB’S is used to make sure that eMTC can use the same signals and channels used by a regular LTE UE to perform all the cell search and random access procedures. The physical channel and signals are always constrained within the 1.08 MHz irrespective of the cell bandwidth by using a new frequency unit called a narrowband. This also ensures that eMTC coexists with other UE’s and can be controlled by the same eNB with just minor software updates and no other infrastructure changes. In terms of the downlink control channel, eMTC has introduced a few new mechanisms in the Release 13. Instead of the legacy control channel (PDCCH), a new control channel called MTC Physical Downlink Control Channel MPDCCH is introduced. This new control channel spans up to six RBs in the frequency domain and one sub frame in the time domain. The mechanism is handled in a way that it removes the necessity to decode PCFICH and have no Physical Hybrid Automatic Retransmission Request (ARC) Indicator Channel (PHICH) as well. With the deployment of eMTC network, series of multiple narrowband regions can be configured. That is, it is possible to configure 6 PRBs each within the LTE carrier for narrowband Physical Downlink Shared Channel (PDSCH) and MPDCCH for data scheduling purposes.
Cat-M1 devices can achieve a maximum throughput of up to 1 Mbps in both uplink and downlink operations for massive IoT. Also, eMTC targets 15 dB coverage enhancement with respect to legacy LTE, which results in 155.7 dB maximum coupling loss between transmitter and receiver. This ensures coverage for IoT devices deployed in remote regions or locations. eMTC is standardized to ensure that for Massive IoT deployment and coverage, it supports long battery life of about 10 years with a 5 Watt-Hour battery system for effective utilization. This technology uses power savings management (PSM) and extended discontinuous reception (eDRX) as its power savings mechanisms to achieve long battery life for Cat-M1 devices. Thus, we see that eMTC enhances the legacy MTC mechanisms to reduce cost and extend coverage while also seamlessly coexisting with the existing LTE.
Figure 1: Scheduling in Legacy LTE PDSCH and eMTC
III. Narrowband – internet of things (NB-IOT)
LTE Cat-NB1 also known as the NB-IoT was another cellular low power wide area technology for IoT, introduced in the Release-13 of 3GPP as an evolution to eMTC. NB-IoT further decreases the operation bandwidth as compared to eMTC from 1.08MHz to 180 KHz for both uplink and downlink operations. Although this ensures a major reduction in device complexity, it can cause a massive reduction in the peak data rate from 1Mbps to up to 30Kbps as compared to eMTC. Thus, this makes NB-IoT ideal for ultra-low end IoT applications such as remote sensors, smart buildings and smart meters where low data rates are tolerable. Like eMTC, even NB-IoT uses power saving mode and eDRX to attain a long battery life of up to 12 years (for a 5 WH battery) and making it power efficient.
Let us now look into the different ways in which NB-IOT can be deployed in LTE.
In-Band Operation: In this case, NB-IOT is deployed within the LTE wideband system and comprises of 1 resource block of 180 KHz. The transmit power is shared between LTE and NB-IOT at the eNB. Sharing of PRBs between NB-IoT and LTE allows for more efficient use of the spectrum and seamless increase in NB-IoT capacity as more devices are added to the network.
Standalone Operation: In this case, NB-IoT is deployed in a standalone 200 KHz spectrum and all the transmit power at the eNB can be utilized entirely. This provides a better coverage as compared to the In-Band operation with a covering power of up to 164 dB.
Guard-Band Operation: In this case, the NB-IoT channel is placed in a guard band of an LTE channel. In guard-band operation, the NB-IoT downlink can share the same power amplifier (PA) as the LTE channel, thus effectively also sharing the transmitted power.
The RF bandwidth of NB-IoT physical layer is 200 kHz. In downlink, NB-IoT adopts QPSK modem and OFDMA technology with sub-carrier spacing of 15 KHz. In uplink, single-tone transmissions (3.75 KHz and 15 KHz channels) and multi-tone transmissions based on SC-FDMA with 15 KHz sub carrier spacing is adopted. The narrower bandwidth of the uplink signals also enables the multiplexing of a larger amount of UE in the same bandwidth.
Figure 2: In-Band, Standalone and Guard Band deployements in NB-IoT
In the previous two sections, we see that both eMTC and NB-IoT use various techniques to ensure fair coexistence with LTE while also being ideal in terms of reduced complexity, reduced power consumption and extended coverage. We shall now look into the differences between these two technologies and determine the applications for which each of these are apt.
IV. Comparison between emtc and NB-IOT
NB-IoT was devised to achieve a coverage enhancement of 20 dB as compared to GSM. Thus, if the maximal coupling path loss of GSM is 144 dB, the maximal coupling path loss of NB-IoT would be 164 dB although the real gain depends on the deployment methodology and configuration. In case of eMTC, the design objective is to achieve coverage enhancement of 15 dB compared to LTE whose maximal coupling path loss is 140 dB. Thus the maximal coupling path loss of eMTC would be 155 dB which is 30% smaller than that of NB-IoT.
Figure 3: Coverage comparion between NB-IoT and eMTC
Increased coverage enables operators to deploy cellular IoT technologies in the existing LTE base station grid. The target is to provide sufficient coverage for smart meters and other IoT appliances that are typically located in basements and similar deep in-building locations.
B. Power Consumption:
The power cosumption factor for eMTC and NB-IoT can be categorized into standby power and active power. Standby power can be defined as the service life of the battery which is dependent on the technology incorporated in these two protocols. Since both eMTC and NB-IoT incorporate PSM and eDRX for power consumption optimization, their standby power can be assumed to be equal to around 10 years. The active power consumption on the other hand depends on the product of transmitted power density and the lemgth of transmission. In downlink, since eMTC has a high throughput rate, its power consumption is seen to be much lower than that of NB-IoT. In uplink, owing to its support for higher modulation schemes, eMTC is seen to have a lower active power consumption when channel conditions are good. When the channel conditions are not good, NB-Iot performs better owing to its support for single tone transmissions.
The difference in costs of modules between NB-IoT and eMTC is meager and is dependent only of the modulation,encoding and decoding methodologies adopted. The below figure represents a typical 3GPP standard cellular module:
The main block that is different between the two technologiues is the baseband physical layer (represented in blue above) which is incharge of the digital signal processing of the modem. Since the procesing for 1.4MHz is done in eMTC as compared to 200 KHz processing in NB-IoT, the cost of NB-IoT is slightly lower. Typically, the eMTC modules cost less than $10 while the NB-IoT modules cost about $5.
Mobility management becomes as important aspect when your device is not fixed onto a single location over time. eMTC being based on LTE, supports both cell handover and redirection in real time. On the other hand NB-IoT cannot perform any of the mobility management tasks when in connected state and can only do it in idle state.
E. Latency and Speed:
Latency and speed are two factors which majorly determine for which applications, eMTC and NB-IoT ideal for. Owing to the high data rates and support for mobility that eMTC offers, it is ideal for more mission critical or real time applications. Since NB-IoT supports much lower data speeds, it cannot be used for real time applications. Instead, it is ideal for applications where the data communication needs to be done in intervals. The below image shows the wide range of applications that eMTC and NB-IoT support.
Figure 4: Applications supported by eMTC and NB-IoT
There is no clear winner between eMTC and NB-IoT because they both have their advantages and disadvantages making them suitable for very diverse set of applications. Iot applications can be categorized into massive IoT and critical IoT applications. Massive IoT includes smart metering, home security, etc. The requirements for massive IoT include years of battery life, scalability to very large number of devices, robust coverage, and deep indoor facilities. The NB-IoT technology is optimized for this use case. Critical IoT includes applications, such as health care and connected car, where very low latency levels on ultra-reliable networks, often combined with very high throughput is required. LTE Cat-M1 is optimized to meet these requirements.