Have you ever encountered these scenarios?
The Wi-Fi at home has full signal after passing through the wall, but the network speed is slow?
The Bluetooth headset suddenly disconnected in the mall?
The image transmission of the drone freezes at 500 meters away?
Behind all this, there are four "invisible commanders" in the field of wireless transmission technology, namely: frequency band, power, sensitivity, and air rate.
In the design and optimization of wireless communication systems, the working frequency band, transmission power, receiving sensitivity, and air rate are the four core parameters that determine performance. They restrict and relate to each other, and directly affect the transmission distance, data throughput, anti-interference ability and system energy consumption. This article will explore the mechanism of action of these four in depth, and analyze their engineering challenges and solutions in combination with typical frequency band scenarios such as SUB-1G, 2.4GHz, and 5GHz.
Ⅰ. Working frequency band: trade-off between penetration, coverage and rate
1. Frequency band division and characteristics
The wireless frequency band ranges from SUB-1G (<1GHz) to millimeter wave (30-300GHz), and the physical characteristics vary significantly:
SUB-1G (such as 433MHz, 868MHz, 915MHz): long wavelength (0.3-0.7m), strong diffraction ability, low penetration loss, can "go around" obstacles (such as through walls and across trees), suitable for wide area coverage (such as smart meters, agricultural Internet of Things). However, the bandwidth is limited, and the theoretical rate is usually <100kbps.
2.4GHz/5GHz (Wi-Fi/BT/ZigBee): mainstream frequency bands that balance coverage and speed, with bandwidth up to 160MHz (Wi-Fi 6), supporting Mbps-level speeds, but with attenuation of up to 15-30dB when penetrating concrete walls. 2.4GHz has wide coverage but is prone to "traffic jams" (Bluetooth and microwave ovens are all competing for the road), 5GHz has many lanes (large bandwidth), fast speeds but weak penetration through walls (concrete walls attenuate by 30%). So why do Bluetooth headsets disconnect in shopping malls? -- The 2.4GHz band is "crowded" by dozens of Wi-Fi signals
Millimeter wave (28/60GHz): ultra-large bandwidth (GHz level), supports 10Gbps+ speeds (such as 5G NR), but has a short transmission distance (hundreds of meters), is easily absorbed by rain and fog, and requires beamforming compensation.
2. Band selection
Coverage vs. rate: SUB-1G sacrifices rate for kilometer-level coverage, while mmWave does the opposite.
Interference avoidance: The 2.4GHz band is crowded (Wi-Fi/BT/ZigBee coexistence), requiring dynamic channel selection (DFS) or frequency hopping (FHSS).
Regulatory restrictions: Countries have strict restrictions on the transmit power and duty cycle of the ISM band (such as FCC Part 15, ETSI EN 300 220).
Ⅱ. Transmit power: the driver of distance and the enemy of energy consumption
1. Power and link budget
Transmit power (unit: dBm) directly determines the effective isotropic radiated power (EIRP),
Where, is the receiving sensitivity. Improving can extend the transmission distance, but it is limited by regulations and power consumption.
Transmit power is like the volume of a voice - the louder the voice, the farther it travels, but too loud will disturb people and waste electricity.
2. Typical scenarios
Low power wide area (LPWA): NB-IoT/LoRa uses 17dBm (50mW) and spread spectrum technology to achieve 10km+ coverage.
High-speed short-range: Wi-Fi 6 APs are often configured with 23-27dBm (200-500mW) and use MIMO and OFDMA to increase capacity.
Compliance challenges: For example, ETSI stipulates that the EIRP in the 2.4GHz band is ≤20dBm, and a compromise needs to be made between antenna gain and power.
Ⅲ. Receiver sensitivity: the "last mile" of weak signal capture
1. Definition of sensitivity and influencing factors
Receiver sensitivity (unit: dBm) is the minimum effective signal power that the receiver can decode, which is determined by the noise figure (NF), bandwidth (BW), and signal-to-noise ratio (SNR).
Bandwidth: Double the bandwidth, sensitivity deteriorates by 3dB (such as LoRa 125kHz vs. 500kHz).
Modulation: High-order modulation (such as 256QAM) requires higher SNR, and sensitivity deteriorates.
Sensitivity is the ability of a receiver to capture weak signals in noise - just like hearing someone's whisper in the vegetable market.
2. Improve sensitivity
LoRa's "time for sensitivity":
Using a longer spreading factor (SF12) for transmission, the rate drops to 0.3kbps, but the sensitivity is as high as -148dBm (20dB stronger than 4G!).
5G mobile phone's "anti-interference headset":
Millimeter wave mobile phones have built-in AI noise reduction algorithms to extract effective signals from noise.
Ⅳ. Air rate: the trade-off between speed and distance
1. The relationship between rate and physical layer parameters
The air rate (PHY Rate, unit: Mbps) is determined by the formula:
NSC: number of subcarriers (OFDM system)
M: modulation order (such as QPSK=2, 256QAM=8)
CR: coding rate (such as 1/2, 3/4)
TSYM: symbol period
2. The contradictory trade-offs of rate optimization
Increase the rate:
oAdd subcarriers (Wi-Fi 6 has 4 times more subcarriers than Wi-Fi 5
oHigher-order modulation (upgrade from QPSK to 1024QAM)
oIncrease the coding rate (from 1/2 to 5/6)
Reduce the speed to maintain the distance: LoRa uses SF12 (rate 0.3kbps) in exchange for -148dBm sensitivity, which increases the link budget by 20dB compared with SF7 (5kbps).
Conclusion: The "impossible triangle" of wireless performance and future breakthroughs
Under the mutual constraints of frequency band, power, sensitivity, and rate, wireless design often faces the **impossible triangle of "distance-rate-energy consumption"**. In the future, through AI dynamic parameter tuning (such as adaptive modulation and coding), metamaterial antennas (improving gain and directivity), joint communication perception (environmental intelligent perception) and other technologies, it may break the traditional boundaries and open up new ways of wireless transmission.