Recently, a lot of efforts have been devoted to develop this network architecture (Porambage et al., 2018) with the aim of exploiting the full potential of current IoT deployments at the time of reducing their permanent dependency on the cloud (Ashouri et al., 2018). Among others, there are two main reasons for this paradigm shift: Limiting the amount of data sent to the cloud, hence strengthening data privacy and controlling the consumed bandwidth in the backhaul network, and reducing the latency of the transactions as they may be served at the edge of the network infrastructure (Lopez Pena and Munoz Fernandez, 2019). Besides, as investigated in Cui et al. (2019) and Mocnej et al. (2018), additional aspects related to end-device’s power consumption are also important when adopting an edge computing architecture in order to find a trade-off between local processing and communication tasks, which may permit to improve energy efficiency of end-devices.

The range of services and applications enabled by the integration of edge-nodes with certain processing and storage capabilities within IoT architectures is huge (Sanchez-Iborra et al., 2018). As end-devices usually present highly restricted computation power, task offloading has been extensively studied as it can notably enrich the capabilities of constrained IoT units (Xu et al., 2019), even considering Quality of Service (QoS) aspects (Song et al., 2017). Given the proximity of computation and storage resources to end-devices, many user-centric or contextual services have been developed (Breitbach et al., 2019). For example, quick indoor positioning (Santa et al., 2018), data caching, or digital twins (Santa et al., 2020) are widely-extended applications enabled by edge computing together with network virtualization techniques such as Software Defined Networking (SDN) and Network Function Virtualization (NFV) (Santa et al., 2020). In this line, edge computing may also help in networking tasks such as introducing novel data communication paradigms, e.g. Named Data Networking (NDN) (Wang et al., 2021), helping to select the most adequate settings for end-device transmissions (Sanchez-Iborra et al., 2018), or supporting IoT data security (Alaba et al., 2017).

Finally, edge computing has also enabled the integration of ML in IoT infrastructures. As discussed in Atitallah et al. (2020), the alliance between IoT and ML techniques paves the way for the development of innovative services, for example, in the frame of smart cities or e-health. Under this umbrella, an intelligent DSS framework on the edge for the automatic monitoring of diabetes patients through the analysis of sensed data was presented in Abdel-Basset et al. (2020). Work in Mrozek et al. (2020) also proposed an edge-based remote monitoring solution, in this case for fall-detection. Authors explored different ML models to evaluate their suitability in the proposed architecture, which is a crucial aspect, especially considering the processing and memory limitations of IoT entities.

The use of ML in edge-nodes has also been exploited for other purposes, such as intelligent network management. Work in Veeramanikandan et al. (2020) proposed a distributed deep learning solution in the IoT-edge environment focused on the integration of data flows with the aim of reducing the latency of the communications at the time of increasing accuracy since the data-generation stage. In line with this work, authors of Liu et al. (2019) presented an IoT network dynamic-clustering solution based on edge computing using deep reinforcement learning. The aim was to enable high-performance IoT data analytics, while fulfilling data communication requirements from IoT networks and load-balancing requirements from edge servers.

In this paper, we propose an IoT-edge computing multi-layer intelligent architecture to enable decision making at the highest level of the hierarchy, i.e. an edge-node, but considering the needs claimed by the individual elements deployed in a certain scenario. With this solution, computations are locally performed by the end-devices and the edge-node, which allows to have shorter response times as well as detaching the IoT system from the cloud. While most of previous research has focused on the task-offloading problem and the integration of ML at the edge, the coordinated operation of TinyML-enabled devices and edge-nodes has been scarcely addressed. Therefore, different from the related literature, we explore this synergy, which is crucial to build hierarchical distributed ML schemes as described in the following.

Ensemble ML has been employed during the last years in order to increase the accuracy of single models or to make complex system-wide decisions (Rokach, 2010). Concretely, the stacking technique, also known as stacked generalization, consists of using the output of several ML decisors as the input of another one, known as meta-model (Wolpert, 1992), (Chatzimparmpas et al., 2021).

Stacked ensemble ML has been widely studied in the literature. In his work, Pavlyshenko (2018) explored different stacking techniques employed for time series forecasting and logistic regression with highly imbalanced data. From the attained results, authors demonstrated that stacking models are able to achieve more precise predictions than single models. The fields of application of this technique are multiple, e.g. sentiment analysis (Emre Isik et al., 2018), children disability detection (Mounica et al., 2019), star and galaxy classification (Chao et al., 2020), or early illness detection (Ksia̧żek et al., 2020; Wang et al., 2019), among many others. In the following, we provide some detailed examples.

Work in Silva and Ribeiro (2006) presented a two-level hierarchical hybrid model combining Support Vector Machine (SVM)–Relevance Vector Machine (RVM) to exploit the best of both techniques. Authors demonstrated the validity of their solution on a text classification task, in which the first hierarchical level made use of an RVM to determine the most confident classified examples and the second level employed an SVM to learn and classify the tougher ones. Authors of Kowsari et al. (2017) also tackled the text classification problem but employing a hierarchical Deep Learning (DL)system in order to provide specialized understanding at each level of the document hierarchy. In this case, different DL models were combined to increase the accuracy of conventional approaches using Naive Bayes or SVM. In Elwerghemmi et al. (2019), SVM was employed in a stacked fashion for Quality of Experience (QoE) prediction. Authors confirmed its applicability for the manipulation of non-stationary data in real-time applications, outperforming a range of well-known QoE predictors in terms of accuracy and processing complexity. Finally, work in Cheng et al. (2020) proposed a stacking-based ensemble ML scheme for predicting the complex dynamics of unmanned surface vehicles. In this case, authors made use of tree-based ensemble models, as well as DL techniques for producing different combinations of stacked classifiers, resulting in a notably improved accuracy in comparison with other state-of-the-art techniques.

As can be seen, stacking-based ensemble learning can be employed for a plethora of applications. In our case, as mentioned above, we present a distributed multi-layer stacked DSS that permits to make decisions affecting several end-devices by aggregating individual decisions made by these elements. Therefore, the isolated interests of end-devices, which compose the lowest layer of the intelligent architecture, are gathered and considered by a higher-level intelligent instance, which looks for common benefits for the whole deployment. This approach poses a step further compared to previous proposals, which just leverage multi-layer ML schemes aiming to improve the performance of conventional models, usually in terms of accuracy. Besides, with the proposed system we achieve a reduction in end-device’s communication activities, as the raw data are processed on the same device instead of sending them to the infrastructure.

TinyML is a concept proposed in Warden and Situnayake (2019), which is attracting great attention from both academia and industry (Sanchez-Iborra and Skarmeta, 2020). It permits to integrate ML-based intelligence in resource-limited devices by optimizing and porting ML models built in non-constrained platforms. This paves the way for obtaining truly-intelligent IoT units able to make decisions without the support of additional devices or servers. This approach reduces end-device communication activities as the sensed data is locally processed, which permits to increase battery lifetimes given that wireless transmission are highly power demanding as mentioned above. Besides, reducing raw data exchanges between IoT devices and the infrastructure limits privacy and security risks (Sanchez-Iborra and Skarmeta, 2020).

Given the novelty of this paradigm, not many works can be found in the literature exploiting the full range of opened possibilities. A clear field of TinyML application is vehicular scenarios, for example, to improve the performance of autonomous driving in mini-vehicles (de Prado et al., 2020) or to increase driver safety (Lahade et al., 2020). Thanks to TinyML, heavy computation tasks that are usually performed by powerful processors such as image or audio processing (Wong et al., 2020; Pontoppidan, 2020) are being investigated in order to be executed by end-devices. From a wider perspective, other works have proposed TinyML-based DSSs for environmental parameter prediction (Alongi et al., 2020) and smart-farming applications (Vuppalapati et al., 2020), obtaining highly promising results although additional efforts should be performed to increase the efficiency of TinyML models.

In this paper, we present a novel advance in comparison with previous works by presenting a hierarchical TinyML scheme which is validated in a real use-case. As mentioned above, adopting a vertically and horizontally distributed TinyML scheme is a proposal not addressed yet in the related literature. Besides, the selected case of study (smart-agriculture) is attracting a lot of attention from the IoT research community (Raj et al., 2021). Concretely, our solution permits to decide whether to separately irrigate or fertilize different subsets of plantations within the same green house attending to environmental parameters sensed along the scenario. The insights of this use-case and its implementation are provided in the following sections.

The empirical methodology consisting of the design and development of a distributed DSS with two decision levels, aiming at considering the particular needs identified by individual IoT end-devices, while dealing with the severe processing and communication limitations of these elements. The first decision level (end-device) consists of a set of p zones, and each zone contains r slave devices, so the number of elements at this level is r × p. Each IoT device contains a TinyML model that outputs its individual decision using a set of inputs { x 1 , x 2 , … , x n } collected from the environment. In turn, the second level consists of a single master device (edge-node) which provides a final-decision TinyML model using as input a categorical vector of r dimensions { s 1 , s 2 , … , s r }, which represents the outputs of the individual devices within a zone, and provides as output a final decision for that zone.

The training and evaluation of the different ML models have been carried out in a Python non-constrained environment and used the criterion of accuracy (for balanced data) and the balanced accuracy (for unbalanced data) as performance metrics. The conversion of these ML models into TinyML ones was done by using compatible TinyML libraries (specified below) and their evaluations were performed using the following criteria: Flash memory, SRAM, and latency as figures of merit. As further explained in following sections, diverse ML algorithms have been investigated under these conditions to select the most adequate regarding the defined criteria. Finally, as communication technology to connect end-devices with the edge-node, we have considered the use of an LPWAN-based solution, due to its low power consumption and long coverage range, which are highly valued characteristics for IoT deployments.

Therefore, the followed empirical methodology consists of the next steps: 1.

Produce a dataset of m samples and n features ( m × n ).

We consider the case of a green house equipped with (i) a range of ground sensors that monitor the status of the plantation and (ii) a set of fixed sprinklers that cover certain plantation zones. Each of these elements is equipped with a communication module that connects it with an edge-node (Fig. 3). Each ground sensor is able to detect the needs of its surrounding plants in terms of moisture, nutrients, etc. However, as each sprinkler is placed in the green house ceiling and covers an area monitored by a number of sensors, the irrigation decision (and its composition) should be made considering the individual needs of the affected plants. Therefore, firstly each ground sensor decides the needs of its sensed plants by using its embedded TinyML model and, then, this decision is submitted to the edge-node (instead of transmitting all the raw sensed data). Once the edge-node gathers every decision made by the sensors under a common sprinkler, it finally decides the action of this sprinkler by using a top-level meta-TinyML model. As can be seen, the decision made for each sprinkler takes into consideration the individual needs of the irrigated plants thanks to the hierarchical TinyML scheme, hence obtaining a greater irrigation precision and adapting it to the actual needs of the individual plants. This may permit the exploitation of the green house for different types of plantations as well as increasing the efficiency of the irrigation and fertigation systems.

For the sake of clarity, in the following we explore the proposed hierarchical TinyML scheme by individually describing the two levels of decision explained previously (Fig. 1).

Given that the proposed scheme presents two different decisors, two different datasets for training each of the respective models are needed. Regarding the end-device level, we have produced a large dataset of 10,000 samples in which we have assigned random values to the input parameters following an uniform distribution within certain ranges, namely, T ∈ [ 17 ∘ C , 33 ∘ C ], M ∈ [ 0 % , 100 % ], PH ∈ [ 0 , 14 ], and EC ∈ [ 1.1 S / m , 6.3 S / m ]. Then, making use of the thresholds shown in Table 1, an automatic labelling algorithm has been developed to assign the proper action { a 0 , a 1 , a 2 } to each input vector. After a first round of automatic data labeling, a subsequent manual tuning has been conducted, especially in those samples with values close to the established thresholds. This process has been conducted with the support of expert agriculture engineers from the Mohammed VI Polytechnic University. Finally, the dataset has been z-score normalized and partitioned by using the k-fold cross validation method, with k = 10, for training and validating the different TinyML models that have been evaluated in real IoT devices.

Regarding the top-level decisor, a different dataset has been generated. Considering that the input vector consists of 4 elements with 3 possible values ( { a 0 , a 1 , a 2 }), there is a limited range of possible inputs, namely, 3 4 = 81. We have applied the mode statistical operator to obtain the final decision ( O = { A 0 , A 1 , A 2 }) for each input combination and we have manually revised each of the 814-elements input vector, aiming at selecting the most adequate output in the case of conflict. Given the limited number of possible input samples, all of them have been employed for the training and testing phases of the different TinyML models evaluated in the edge-node.

We consider that the followed dataset generation procedures are sufficient for the purpose of validating our proposal at this point of our research. Thus, the generation of datasets from field-sampled data has been left for future work in order to better adjust our models. Besides, for replicability purposes, we have made the end-device-level dataset publicly available.3³

https://github.com/ramonjsi/Smart-Agriculture

As described above, the first step to produce a TinyML model is to obtain a regular ML model in a non-constrained platform. To this end, we have used the well-known Python’s Scikit-learn library to produce a series of ML models with different configurations that will be described in Section 5. Concretely, we have considered the following ML algorithms: Multi-Layer Perceptron (MLP), Decision Tree (DT), Random Forest (RF), and Support Vector Machine (SVM), given that all of them are supported by the TinyML toolkits employed in our experiments.

Once the non-constrained ML model is produced and adjusted, it should be ported to be runnable in constrained units. For this task, we have employed a series of TinyML toolkits, depending on the involved ML algorithm, given that each toolkit is compatible with a limited set of algorithms. We have employed the emlearn4⁴

https://github.com/emlearn/emlearn

We have selected the Arduino Uno board as the target device for evaluating the performance of the developed models for both of the decisors described above (end-device and edge levels). We have chosen this unit given its popularity, low-cost, and notable processing and memory constraints, which make it a good benchmarking tool for efficient IoT developments. It is equipped with a 16 MHz 8-bit processor (ATmega 328p) with flash and Static RAM (SRAM) memories of 32 KB and 2 kB, respectively. Considering these resources, the Arduino Uno belongs to the most constrained type of Micontroller Units (MCUs) (Class 0) according to the classification in Bormann et al. (2014).

As communication technology to connect the deployed sensors and the edge-node, we have selected LoRA, an LPWAN-based solution that permits long-range transmissions with a great energy-efficiency (Sanchez-Iborra and Cano, 2016), which are highly valued characteristics for the scenario under study. However, these attractive characteristics are achieved at the expense of notably reducing the transmission data-rate, leading to very long transmission times, even more than a second under certain configurations. Besides, given that LoRa makes use of unlicensed frequency bands, a strict duty cycle-based regulation that restricts LoRa transmissions to 1% of the available time, i.e. a maximum of 3.6 seconds per hour per device, is established. For these reasons, it is highly desirable to limit the number of communications using this type of communication technology, which is achieved in the proposed solution. For our experiments, we have employed the Semtech SX1272 LoRa modem (Semtech, 2019).

As explained previously, we have evaluated the performance of different types of ML algorithms, namely, Multi-Layer Perceptron (MLP), Decision Tree (DT), Random Forest (RF), and Support Vector Machine (SVM), with different model configurations for each of them. Aiming at selecting the best setup among the plethora of evaluated alternatives for each model, the accuracy when making a decision with respect to the labels assigned in the generated dataset (see Section 4.4) has been adopted as a principal figure of merit. To this end, we have employed the grid search technique that permits to obtain the optimum configuration for each model.

Table 2 presents the performance of the best configurations for each of the ML algorithms under consideration and the values assigned to their basic configuration parameters. Regarding accuracy, observe the notable performance of all the algorithms although DT and RF stand out with an almost perfect accuracy of 99.9%. Considering the memory footprints of the models on the Arduino device, the MLP model is the heaviest one in terms of flash memory and SRAM. In turn, RF and, especially, DT are lighter models, which is a highly valued aspect considering the severe storage and memory constraints of the target device. It is remarkable that the optimized SVM model exceeded the flash memory available on the device, hence it could not be deployed on it. In order to obtain an SVM model that could be embedded on the selected unit, we had to simplify it very much, thus dramatically reducing the model accuracy. This behaviour was already detected in Sanchez-Iborra and Skarmeta (2020).

Decision-making latency is another key aspect when evaluating a TinyML model, given the computation restrictions of the target MCU. Again, the DT algorithm presents the best performance in comparison with RF and MLP, which is the slowest one. This behaviour is justified by the simplicity of the former, as the MLP and RF produce more complex models as evidenced by their memory and RAM footprints discussed above. Finally, comparing the performance of the TinyML toolkits under consideration, observe that in all cases, the models generated by MicroMLGen are lighter and faster than those produced by emlearn.

In the light of these results, the DT model generated by MicroMLGen would be the most adequate one to implement the end-device-level decisor, which permits each sensor to decide the most proper action for its monitored plants.

Given that this decisor should not be subject to any uncertainty, we have selected the simplest model for each of the considered algorithms that provides a perfect accuracy of 100%. Therefore, after evaluating a large set of different configurations, for the sake of simplicity, we only show the performance results of the finally chosen models (Table 3). Please note that as the SVM algorithm was not able to provide a perfect accuracy for this decision model (best result of 81.5% with linear kernel), we have not included it in the following discussion and in the table.

Regarding the MLP algorithm, we have obtained that the simplest configuration obtaining a perfect decision accuracy is using 8 hidden layers with 6 neurons in each layer with the ReLu activation function and a limit of 1000 training iterations. The rest of omitted parameters have been set to the default values of the Scikit-learn library (also applicable for the following algorithms). For the DT algorithm, we employed the entropy function for the split criterion and a tree depth of 7 levels for obtaining a perfect accuracy. Finally, the RF algorithm was configured with the same split-criterion function as the DT model and 3 estimators. With these configurations, the memory requirements of each model and their computation delays are shown in Table 3.

Comparing the different families of algorithms, again the MLP presents more demanding requirements in terms of memory than the DT and RF algorithms. It is also noticeably slower than the others. As expected, DT models demand less memory and are faster than the RF ones. Finally, comparing the models generated by both emlearn and MicroMLGen libraries, it can be seen how the latter produces slightly more optimized (in terms of memory) and faster models than the former.

In the light of the attained results, as in the previous case, the DT model produced by MicroMLGen is the most convenient one for implementing this decisor, given its perfect accuracy with low impact on the device (13% and 17% of total flash and SRAM memories in the device, respectively) and its quick decision-making time.

In the following, we explore the performance of the whole system considering the communication activities between end-devices and edge-node, too. As mentioned above, using an LPWAN-based solution such as LoRa, permits reliable long communications at the expense of severely reducing the data-rate, hence, increasing the time needed for completing a transmission. Although one of the key characteristics of LPWAN technologies is their reduced energy consumption, the communication activities are still the most power-demanding task for an end-device. For those reasons, we explore the performance of the systems from both latency and energy consumption perspectives.

Regarding end-to-end latency, we should consider both processing and transmission times. In LoRa, the transmission time for a given message is markedly determined by its low-level configuration, especially the Spreading Factor (SF) parameter. As the SF increases the data-rate is reduced, therefor,e the robustness of the transmission is enhanced but the transmission time notably grows. In order to explore the system performance, we have made use of the two extreme SF configurations, i.e. SF7 (high data-rate, low transmission robustness) and SF12 (low data-rate, high transmission robustness). The rest of LoRa configuration parameters have been fixed as follows: Bandwidth (BW): 125 kHz, Coding Rate (CR): 4/5, and CRC check: On. Given that there are only 3 possible messages to be exchanged between end-devices, the edge-node, and the sprinklers, i.e. “no action”, “irrigation”, and “fertigation”, just 2 bits are needed for their codification, which will be the data payload transported in each transmission.

Thereby, the end-to-end latency for making a decision with the previously selected models at each level (DTs) and assuming that the environmental data are already collected, is presented in Table 4. As can be seen, processing times are negligible in comparison with transmission times. This happens for the finally selected models (DTs), but observe that other algorithms such as MLP would introduce a non-negligible delay (see Table 2 and Table 3), hence this fact should be taken into consideration when designing systems as the one presented. Other delays related to the transmission coordination among end-devices could be also considered, although these aspects are out of the scope of this work. Given that the greatest contribution to the end-to-end latency comes from communication activities, it is necessary to reduce them, for example, by avoiding the transmission of big volumes of raw data from the end-devices to the edge-node, as we propose in our solution.

Regarding energy consumption, we have considered the processing and transmission times shown in Table 4, as well as the consumption charts that can be found in the employed equipment’s datasheets (Arduino Uno’s microcontroller (ATmega 328p) and Semtech SX1272 LoRa modem). We have calculated each device’s energy consumption by assuming LoRa’s SF7 and 20 dB gain, with the microcontroller working at 16 MHz and 5 V of operating voltage (configuration by default). Please, note that this is a theoretical estimation of the involved processing and communication activities’ consumption without considering other devices’ tasks, e.g. environmental sensing. The attained results are shown in Table 5. As discussed in previous sections, communication activities are notably much more consuming than computation tasks. Nevertheless, the reduced consumption of LoRa technology and the limited number of transmissions per day permit devices to have long battery lifetimes employing usual power-banks used in IoT deployments. This is a crucial feature for ensuring the system scalability and manageability.

In the light of the attained results, we consider that the proposed solution may be of high interest for introducing an intelligent, low-cost, and efficient automation system into current non-digitalized green-houses or other cultivation facilities, hence enabling the transition of the traditional agriculture towards the smart-agriculture paradigm.

This threat concerns the evaluation process which can be inaccurate, hence leading to biased conclusions. To mitigate this possible issue, all the models were trained and tested using a 10-fold cross validation to overcome the internal limitations and to prevent the overfitting of our classifiers that may appear using the random train-test split. Moreover, we used the grid search strategy to tune the hyper-parameters of the different employed classifiers in order to search the optimum configuration for each of them.

External validity is related to the extent to which the results obtained in this study can be generalized outside the context of this study. In our case, the used datasets were generated by using a set of empirical knowledge provided by agriculture engineers. It concerned the models’ classification characteristics (temperature, soil moisture, soil PH, and soil electrical conductivity), as well as the defined classes (irrigation and fertigation). This approach helps to the generalization of the findings of this study, especially in real-life case-studies. Besides, we have comprehensively detailed the followed empirical methodology to ease the reproducibility of this study in other fields of application.

Construct validity addresses the reliability of the predictive model performance obtained through this study. To reduce this potential limitation, four criteria were used: three are related to the constraints of end-devices in an IoT context (flash memory, SRAM, and latency), and one is related to the ML domain (accuracy). Other ML criteria could be added to assess the reliability of our classifiers such as Area Under Curve (AUC), F1-score, precision and recall. Moreover, statistical tests or ranking methods have been left for future study to assess the significance of performances and to obtain an overall rank of our classifiers in a real deployment.