How does LoRa prevent collision when several nodes are transmitting simultaneously?
LoRa (Long Range) technology utilizes a spread spectrum modulation technique called Chirp Spread Spectrum (CSS) to enable long-range, low-power wireless communication. LoRaWAN, which is built on top of LoRa technology, is designed to allow multiple nodes to communicate efficiently in a shared medium without collisions. Here's how LoRa prevents collisions when several nodes are transmitting simultaneously:
Spread Spectrum Modulation: LoRa uses spread spectrum modulation, where each bit is modulated into a much wider frequency band, known as a chirp, instead of a single frequency. This chirp spreads the signal over a broader spectrum, making it less susceptible to interference and noise.
Coding Rate and Spreading Factor: LoRaWAN devices can adjust their transmission parameters, including the spreading factor and coding rate. The spreading factor determines the signal's chirp rate and bandwidth, and the coding rate controls error correction. Different nodes can use different spreading factors, which helps in distinguishing and decoding overlapping transmissions.
Time Division Multiple Access (TDMA): LoRaWAN employs a time-based access scheme known as TDMA. In LoRaWAN, gateways allocate specific time slots for individual end devices to transmit. Each device is assigned a unique uplink slot in which it can send data. This time-division approach ensures that nodes don't interfere with each other during transmission.
Random Backoff: To further reduce the chances of collisions, LoRaWAN devices employ a random backoff mechanism. After a device decides to transmit in its designated time slot, it may introduce a random delay before actually transmitting. This helps stagger the transmissions of different nodes, reducing the probability of simultaneous transmissions.
Listen Before Talk (LBT): In some regions where LoRa operates in unlicensed frequency bands, there may be requirements for LBT. LBT involves the device checking for channel activity before initiating a transmission. If the channel is busy, the device will wait for an idle period before attempting to transmit. This helps prevent collisions when multiple nodes want to access the channel.
Adaptive Data Rate (ADR): LoRaWAN supports adaptive data rate, where the network can adjust the data rate of individual devices based on signal quality and distance from the gateway. Devices farther from the gateway may use slower data rates, which reduces the chances of collision as they transmit less data in the same time slot.
These mechanisms, combined with LoRa's inherent resilience to interference, help ensure that LoRaWAN networks can accommodate a large number of devices while minimizing collisions and maximizing network efficiency. However, network planning and proper configuration of parameters like spreading factor, coding rate, and channel allocation are crucial for optimal performance in a crowded environment.