In the home, building, and industrial applications markets, short-range wireless technologies—especially those operating in the sub-GHz (less than 1 GHz) band—are becoming increasingly popular. As a result, system designers must understand the various methods, cost considerations, and trade-offs involved in such implementations. Beyond just distance estimation formulas, it's crucial to grasp how sub-GHz wireless channels behave and how environmental factors influence signal propagation.
Typically, RF and wireless engineers begin with a link budget before starting any RF design. A link budget accounts for factors like distance, transmit power, receiver sensitivity, antenna gain, frequency, reliability, and the physical characteristics of the propagation medium—such as reflection, diffraction, and scattering. These elements collectively determine the performance of sub-GHz RF links.
Sub-GHz wireless networks are particularly cost-effective in low-data-rate systems, whether they're simple point-to-point connections or more complex mesh networks. They offer advantages such as long-distance communication, reliable RF links, and extended battery life. Higher output power, reduced energy absorption, less spectral interference, and narrowband operation all contribute to longer transmission ranges. Improved signal propagation, efficient circuit design, and lower memory usage help reduce power consumption, allowing devices to run for years on a single battery.
The sub-GHz band is part of the license-free ISM (Industrial, Scientific, and Medical) spectrum. Sub-GHz nodes are typically used in cost-sensitive applications, where each node can be up to 30–40% cheaper than advanced wireless solutions. Their protocol stacks also require less storage space. Many protocols operate in this band, including ZigBee (which supports both 2.4 GHz and sub-GHz versions), automation protocols, cordless phones, wireless Modbus, remote keyless entry (RKE), tire pressure monitoring systems, and proprietary protocols like MiWi. However, due to shared spectrum use, there's potential for RF interference from other devices such as mobile phones or cordless systems.
A link budget is essentially an accounting of all gains and losses in a wireless communication system, from the transmitter through the channel to the receiver. It helps estimate the signal strength at the receiving end. To calculate it accurately, you need to consider parameters like antenna gain, transmit power, and receiver sensitivity. Other secondary factors, such as required distance, available bandwidth, data rate, protocol, and interference, should also be taken into account. Even though receiver sensitivity might not directly factor into the link budget, it’s essential for determining the minimum signal level that the receiver can still process effectively.
The basic link budget formula is: Received Power (dBm) = Transmitted Power (dBm) + Antenna Gain (dB) – Path Loss (dB). Using this calculation, engineers can design systems that meet performance requirements within a specific 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 PICtailâ„¢ board based on the Microchip MRF89XA module and MRF49XA sub-GHz transceiver is commonly used. The MRF89XA module is FCC, ETSI, and IC certified, offering modular PCB antennas (often called "snakes"). PICtail boards usually come with wire-type antennas for different frequencies, which are mounted on development or daughter boards.
The hardware interface between the PIC® microcontroller and the transceiver module is referred to as a wireless node. As shown in Figures 1 and 2, wireless nodes can be built by combining a PIC MCU development board with a PICtail daughter board.
Distance and performance experiments require at least two wireless nodes. The test setup usually involves two development boards with the same sub-GHz module, though they can also be combined depending on the application.
The working environment significantly affects wave propagation. Distance tests should be conducted in various indoor and outdoor settings to understand module performance. Considerations include open vs. obstructed paths, terrain type, and antenna orientation (vertical or horizontal).
Factors affecting indoor measurements include office equipment, proximity to Wi-Fi®, Bluetooth, or microwave signals, as well as materials like concrete, walls, glass, wood, and metal. For accurate distance testing, it's important to maintain consistent battery power and consider module mounting and antenna placement.
Antenna orientation plays a key role in performance. Vertical mounting creates elevation lobes, while horizontal mounting results in azimuth lobes. The length of a wire antenna can be calculated using the formula: Length (cm) = Frequency (MHz) / 7500. This applies when the antenna is less than a quarter of the wavelength.
To perform distance measurements, the MiWi P2P demo code is programmed into two sub-GHz transceiver nodes. After setting a working channel, one node is placed on a pole in receive mode, while the other acts as the active node. The active node is moved in increments of 1.5 to 3 meters to measure communication stability.
Once the critical distance is identified, the actual and radial distances are measured. Communication is tested intermittently at the critical point, and then again after moving back 1.5 meters to ensure stability.
Packet Error Rate (PER) tests evaluate the effective coverage between two wireless nodes. PER is calculated by sending a set number of packets, typically 1000, and measuring the percentage of errors. A PER of 1% or lower is considered reliable. If needed, packet delays can be adjusted to improve accuracy.
Bit Error Rate (BER) tests involve transmitting pseudo-random data sequences and comparing the received data to the original. Since true randomness is hard to achieve, pseudo-random sequences are used for accurate BER measurements. The modulation method impacts BER performance, especially at low signal-to-noise ratios.
Sensitivity tests determine the lowest input power level at which a receiver can still function reliably. An attenuator gradually reduces the input power until the PER exceeds 1%, indicating the sensitivity limit.
In conclusion, sub-GHz RF technology enables the creation of simple, long-lasting wireless products that can operate for up to 20 years on a single battery. Sub-GHz networks are ideal for low-data-rate systems where long-range communication and extended battery life are key benefits. Lower signal attenuation and diffraction effects allow sub-GHz signals to travel over 1 km without requiring multiple hops.
Using the sub-GHz ISM band for proprietary low-duty-cycle links minimizes interference and improves efficiency. Narrowband operation enhances transmission range and reduces power consumption. Receiver sensitivity and channel bandwidth are inversely related, with narrower bandwidths providing higher sensitivity and better performance at lower data rates.
In urban environments, increasing the link margin by 12 dB can double the transmission distance. Optimizing receiver sensitivity is the most effective way to extend range, as other system variables require more effort to achieve similar improvements. Finally, accounting for multipath fading losses (up to 30–40 dB) is crucial when designing a robust wireless system.
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