Читать книгу Wireless RF Energy Transfer in the Massive IoT Era - Hirley Alves - Страница 15

LOng RAnge (LoRa). LoRa is a proprietary long-range, low-power communication technology that operates at sub-GHz unlicensed frequency bands, namely industrial, scientific, and medical (ISM) frequency bands. LoRa employs chirp spread spectrum and uses six practically orthogonal operational bands, denoted as spreading factors, which allow rate and range adaptation [11].

LoRa wide area network (LoRaWAN), regulated by LoRa Alliance [12], uses LoRa as its physical layer. LoRaWAN builds upon a star of stars topology where devices communicate in a single-hop with a gateway, which connects to a network server via standard IP protocol. The medium access control (MAC) is ALOHA-like. LoRa characteristics enable multiple devices to communicate simultaneously using different spreading factors, thus supporting 65536 connections per gateway [11].

LoRaWAN provides several configurations based on six distinct spreading factors and bandwidths, usually 125 kHz or 250 kHz for uplink, depending on regional regulation. The distinct spreading factor configuration allows symbol duration and transmission rate flexibility. For instance, higher spreading factors extend the symbol duration, thus increasing robustness with a lower rate. Such flexibility comes at the cost of increased time-on-air, yielding channel usage increase, thus leading to higher collision probability as extensively discussed in [13]. However, code replication [14], superposition [15], adaptive data rate [16, 17] are techniques that help mitigate these effects. Since the technology operates in the ISM band, it is vulnerable to interference from neighboring deployments and other technologies impacting the performance [18].

Unlike other proprietary solutions, LoRa is open, with a relatively vast amount of information about its operation, thus being one reason for its popularity in many communities, particularly the academic community. Many research works in a broad range of topics (propagation measurements, network performance, and characterization, simulators, among others) in recent years. See, for instance, [13-20] for a comprehensive survey of the recent advances.

SigFox. This technology operates in a star topology, and devices connect to any base station (BS) in range using ultra narrowband signals of 100 Hz in ISM frequency bands. The signal carries a 12-byte payload and travels two seconds over the air, with a fixed data rate of 100 or 600 bits/s using D-BPSK modulation. These characteristics enable reduced energy consumption and broad coverage [21]. Unlike LoRa, much less information is available about SigFox. Only a few works are analyzing the technology, for instance [22-24].

Long Term Evolution for Machines (LTE-M). The mobile communications industry has an ever-growing interest in the IoT and has coined the term MTC leading to new services.

MTC operation mode was introduced in Release 12 (namely LTE category 0 (Cat 0)) to support communication via carrier network in a cost-effective and power efficient way compared to the legacy network. Thus, LTE Cat 0 constitutes a preliminary attempt to somewhat support IoT connectivity, and comprises a low complexity machine-oriented specification but with standard control information under full carrier bandwidth. Enhanced MTC, introduced in Release 13 (Cat M1), mitigates this issue, reducing spectral and energy wastage.

Later releases continued the enhancements and that led to improvements in coverage, latency, battery life, and capacity² with the changes in bandwidths (from 1.4 MHz in Cat 0 to 5 MHz in the evolution), and half-duplex operation, and features such as power saving mode (PSM) and extended discontinuous reception (eDRX) [9, 25].

Besides global coverage, LTE-M supports high data rates, real-time traffic, mobility and voice, which in turn makes this technology versatile, unlike other technologies discussed so far, although at cost of complex architecture and costly transceiver.

Narrowband-IoT (NB-IoT). NB-IoT inherits many of its features from LTE, and as its name suggests, it focuses on narrowband signals that occupy 180 kHz bandwidth, which corresponds to one resource block in the legacy LTE system. NB-IoT design is leaner than its counterpart, LTE-M, to match the requirements of many battery-constrained IoT applications. Therefore, it offers good indoor coverage, support to massive connectivity, low power consumption and optimized network architectures (also a characteristic in LTE-M), while operating under licensed spectrum [9, 28]. These two cellular technologies differ vastly by their target.

Both, NB-IoT and LTE-M, are optimized to reduce power consumption, extending devices battery life, though NB-IoT has even lower device power consumption and lower chipset cost. One advantage of these technologies is that both support in-band deployment with the legacy LTE (including LTE guard-bands in NB-IoT), or as standalone with dedicated spectrum.

A recent comparison of these LPWA technologies is available in [9, 27, 29]³. Figure 1.4 compares the LPWA technologies with respect to cell coverage and data rates. As discussed above, unlicensed technologies have limited data rates and cell coverage, usually ranging from few kilometers in urban environments up to tens of kilometers in rural areas, when compared to licensed technologies. On the other hand, device cost and network deployment are often advantageous. However, cellular technologies provide quality-of-service (QoS) guarantees via long-term service level agreements. At the same time, operation under licensed spectrum comes with the advantage of predictable and controllable interference through efficient mechanisms that enable a massive number of connections.

Figure 1.4 Comparison of LPWA technologies in terms of cell coverage and data rate.

Source: Illustrative numbers based on [29].

Massive MTC (mMTC). Massive IoT flourishing markets impose increasingly challenging connectivity demands. Third generation partnership project (3GPP) has taken revolutionary steps in Release 12 when introducing MTC, and evolutionary ones in posterior releases by enhancing the performance of LTE-M and introducing new services (NB-IoT and EC-GSM-IoT introduced in Release 13).

MTC revolutionized the mobile communications industry by shifting the focus from broadband services towards the IoT. Clear evidence comes with the fifth generation (5G) that inherently dedicates two-thirds of the service modes to the IoT. In 5G, MTC branched out to massive MTC (mMTC) and critical MTC (cMTC), also known as ultra-reliable low-latency communications (URLLC)⁴.

mMTC stands for massive referring to large number of users connected to the network, widely expected in many IoT applications. Therefore, in the communications community, mMTC and massive IoT are used as synonyms and interchangeably.

Even though mMTC is encrusted 5G jargon, it is an umbrella term that specifies the ensemble of solutions toward massive IoT. Thus, it comprises the cellular IoT technologies discussed so far, namely LTE-M and NB-IoT, and solutions under development toward current and next generations, as well as non-cellular technologies, such as LoRa and SigFox. This is because 1. LTE-M and NB-IoT are compliant with the evolution of mobile communications, thus both solutions operate in-band with 5G, and will evolve (so-called future proof) complying with 5G mMTC requirements [9, 25]; 2. mMTC encloses LPWANs; therefore, non-cellular LPWA technologies fall within this definition. Another point is that even with the advantages of cellular IoT, it is unlikely that a single technology will be ubiquitous in a fragmented market. Therefore, future generations beyond 5G are likely to coexist and complement unlicensed solutions as foreseen in [33].

mMTC key challenges are:

 Energy efficiency In many applications, devices rely solely on batteries, and very often, replacement is costly, dangerous, or simply not possible (see Chapter 2). Though battery lifetime may be extended with smart sleeping mode techniques, it may not be sufficient.

 Scalability Support to a massive number of connections. The network capacity should also scale to accommodate the demands of such large number of devices. Figure 1.1 illustrates the projected growth for cellular IoT solution.

 Coverage Deep indoor coverage is a crucial requirement for many applications, requiring regional, national, or even global coverage.

 Heterogeneity Different applications impose different requirements, e.g., in terms of data rates, latency, reliability, energy efficiency, coverage. Flexible connectivity is imperative to handle heterogeneous requirements.

 Device costCost is a critical factor in meeting the economies of scale and many use-cases. For instance, cellular IoT solutions have reduced peak rate and device complexity, and half-duplex operation and narrow bandwidths help address this challenge.

These are the most representative challenges of mMTC, although other capabilities exist, as initially identified by the ITU [34], e.g., peak and experienced data rates, spectrum efficiency, mobility, latency, reliability, and security. In this context, cellular IoT becomes advantageous by coping with the LPWAN requirements and key performance indicators (KPIs) while pushing some of these to their limits, whereas handling security and QoS requirements [4].