Wireless Sensor Network Project LoRa Multi-Node Monitoring

Design and implement a multi-node wireless sensor network using LoRa technology that collects environmental data from distributed sensor nodes and aggregates it at a central gateway for monitoring and analysis.

Project Architecture

Network Topology

Understanding LoRa Technology

What Makes LoRa Special

LoRa stands for Long Range and uses a patented Chirp Spread Spectrum (CSS) modulation technique developed by Semtech. Unlike traditional modulation where a signal stays at one frequency, LoRa continuously sweeps the signal frequency across the bandwidth. This chirp signal can be detected even when it is 20 dB below the noise floor — meaning LoRa receivers can decode signals that are 100 times weaker than the background noise.

LoRa Physical Layer Parameters

The link budget of LoRa is extraordinary. With SF12 and 125 kHz bandwidth, the receiver sensitivity reaches -137 dBm. Combined with a typical transmit power of +14 dBm, the total link budget is 151 dB. Compare this to WiFi's typical link budget of 110-120 dB, and you understand why LoRa can reach 10+ kilometers.

Spreading Factor Deep Dive

Think of Spreading Factor like speaking speed. SF7 is like speaking quickly — you convey information fast but the listener needs to be close to understand. SF12 is like speaking very slowly and clearly — even someone far away can understand, but it takes much longer to say the same thing.

Hardware Design

Sensor Node Components

Power Budget Calculation

Power management is critical for WSN nodes. Let us calculate battery life:

• Active mode (sensing + LoRa transmit): 120mA for 2 seconds
• Sleep mode: 5μA (Arduino power-down + LoRa sleep)
• Duty cycle: Transmit every 5 minutes = 2s active / 300s total

Average current = (120mA × 2s + 0.005mA × 298s) / 300s = 0.805mA

With a 2600mAh battery: Life = 2600 / 0.805 = 3230 hours ≈ 134 days ≈ 4.5 months

Adding a 1W solar panel that provides 3-4 hours of charging per day extends this to indefinite operation.

Communication Protocol Design

Packet Structure

Each sensor node transmits a compact data packet to minimize time on air (which saves power and reduces collision probability):

Time Division Approach

With multiple nodes transmitting on the same frequency, collisions can occur. We implement a simple TDMA-like scheme where each node is assigned a time slot. Node 1 transmits at second 0, Node 2 at second 10, Node 3 at second 20, and so on. A random jitter of ±2 seconds prevents permanent collisions if clocks drift.

Acknowledgment and Retry

The gateway sends a brief acknowledgment after receiving each packet. If a node does not receive an ACK within 1 second, it waits a random backoff time (1-5 seconds) and retransmits. Maximum retry count is 3 before the node gives up and waits for the next scheduled transmission.

Gateway Implementation

The gateway uses an ESP32 which provides both LoRa reception (via SPI-connected SX1276) and WiFi connectivity for cloud upload. It operates continuously (mains powered) and performs these functions:

1. Listens on the LoRa frequency for incoming packets
2. Decodes the packet structure and validates CRC
3. Sends ACK back to the transmitting node
4. Stores locally in case WiFi is temporarily unavailable
5. Uploads to cloud platform via WiFi using MQTT or HTTP
6. Monitors node health — flags nodes that miss expected transmissions

WSN Concepts Demonstrated

Performance Results

Key Takeaways

1. LoRa achieves extraordinary range (10+ km) by trading off data rate — ideal for sensor networks that send small packets infrequently
2. Chirp Spread Spectrum modulation allows signal detection below the noise floor, providing robustness against interference
3. Spreading Factor is the primary trade-off control: higher SF means longer range but slower data rate and more power consumption
4. Proper duty cycling (sleeping 99%+ of the time) extends battery life from hours to months
5. Star topology is simplest for WSN; mesh topology adds redundancy but increases complexity and power consumption
6. Time-slotted transmission prevents packet collisions in multi-node networks
7. The complete IoT stack spans physical sensing → wireless communication → internet protocols → cloud storage → visualization