How to use sub in indoor and outdoor environments through simple link budget analysis

In the home, industrial, and building automation sectors, wireless applications operating in the sub-GHz (less than 1 GHz) frequency band are becoming increasingly popular. This trend highlights the need for system designers to thoroughly understand various aspects such as design methods, estimation techniques, cost implications, and trade-offs involved. Alongside distance estimation formulas, a solid grasp of sub-GHz wireless channels and propagation environments is essential. Typically, RF and wireless engineers start by creating a link budget before beginning any RF design. A link budget accounts for multiple factors like distance, transmit power, receiver sensitivity, antenna gain, frequency, reliability, and environmental conditions. It calculates how well an RF link will perform in the sub-GHz range, considering electromagnetic wave behavior such as reflection, diffraction, and scattering through different media. Sub-GHz wireless networks are particularly cost-effective in low-data-rate systems, whether it's a simple point-to-point connection or a more complex mesh network. These systems offer long-range, reliable communication and extended battery life—key advantages for many applications. Features like higher output power, reduced energy absorption, and narrowband operation enhance transmission distances, while improved signal propagation and efficient circuit design help reduce power consumption and prolong battery life. The sub-GHz spectrum is part of the unlicensed ISM (Industrial, Scientific, and Medical) band, making it ideal for low-cost systems. Sub-GHz nodes can reduce costs by up to 30–40% compared to more advanced wireless solutions, and their protocol stacks require less memory. Various protocols operate in this band, including ZigBee, automation protocols, cordless phones, and proprietary solutions like MiWi. However, due to the shared nature of the ISM band, interference from other devices such as mobile phones and cordless systems can occur. A link budget is essentially a calculation of all gains and losses across the entire wireless communication system—from the transmitter through the medium to the receiver. It helps determine the signal strength at the receiving end. To analyze and estimate the link budget, key parameters like antenna gain, transmit power, and receiver sensitivity must be considered. Other secondary factors, such as required distance, available bandwidth, data rate, and protocol compatibility, also play a role. Although receiver sensitivity isn't always included in the link budget, it’s still crucial for determining the system's ability to receive a signal. The basic link budget formula is: Receive Power (dBm) = Transmit Power (dBm) + Gain (dB) – Loss (dB). Using this method, designers can build a system that meets performance requirements within a defined cost range. Some losses may vary over time, such as increased bit error rates in digital systems or decreased signal-to-noise ratios in analog systems. For testing, the PICtail™ board with the Microchip MRF89XA module and the MRF49XA sub-GHz transceiver is commonly used. These modules are FCC, ETSI, and IC certified, offering modular PCB antennas. PICtail boards typically use wire-type antennas, which are mounted on development or daughter boards. The hardware interface between the PIC® microcontroller and the transceiver forms a wireless node, as shown in Figures 1 and 2. To conduct distance and performance experiments, at least two wireless nodes are needed. Testing can be done using identical sub-GHz modules or combined based on application needs. The environment significantly affects signal propagation, so tests should be conducted in both indoor and outdoor settings. Factors like obstacles, terrain, and antenna orientation (vertical or horizontal) influence results. Indoor measurements may be affected by office equipment, Wi-Fi, Bluetooth, or microwave signals, as well as materials like concrete, glass, wood, and metal. For accurate distance testing, it's important to consider module mounting, antenna orientation, and consistent power supply. Antenna placement—whether vertical or horizontal—depends on desired output power, space constraints, and frequency characteristics. The length of a wire antenna can be calculated by dividing the frequency by 7500 (in cm). For example, a 433 MHz antenna would be about 17.3 cm, and a 915 MHz one around 8.2 cm, provided the wire doesn’t exceed a quarter of the wavelength. To measure distance, the MiWi P2P demo code is programmed into two sub-GHz transceivers. One node is placed on a pole and set to receive mode, while the other is moved at intervals of 1.5 to 3 meters. At the critical point, the actual and radial distances are measured, and communication is checked again after moving back. This method, shown in Figure 4, demonstrates how distance affects signal quality. Packet Error Rate (PER) tests assess the effective coverage between two wireless nodes. According to IEEE 802.15.4 standards, a PER below 1% is considered reliable. If needed, packet delays can be adjusted to improve accuracy. Bit Error Rate (BER) tests use pseudo-random sequences to evaluate signal integrity, as true randomness is hard to achieve. Modulation modes can improve BER performance at low signal-to-noise ratios. Sensitivity tests involve reducing the receiver input power until PER drops below 1%. This process helps determine the minimum signal level the receiver can handle. A setup with two sub-GHz modules, as shown in Figure 5, allows users to find the maximum communication distance where PER remains under 1%. In conclusion, sub-GHz RF technology enables the creation of wireless products that can run for up to 20 years on a single battery. These systems excel in low-data-rate environments where long-range communication and extended battery life are critical. Their narrowband operation allows for greater transmission distances, and lower signal attenuation helps overcome obstacles. It's recommended that proprietary low-duty-cycle links use the sub-GHz ISM band to avoid interference. The low noise spectrum improves efficiency and reduces power consumption. Both power efficiency and system range depend on receiver sensitivity and transmit frequency. Narrower bandwidths increase sensitivity, ensuring better performance at lower data rates. In urban areas, increasing the link margin by 12 dB can help double the transmission distance. Optimizing receiver sensitivity is the first step in extending range, as other variables may require multiple adjustments to achieve similar results. Due to multipath fading, signal loss can exceed 30–40 dB, so designing with sufficient link margin is essential to maintain reliable communication.

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