MIMO & Multi-Antenna: From 5G to Wi-Fi Engineering Insights

I. Why Is One Antenna Not Enough?

In wireless communication systems, multi-antenna technology has become a standard feature of 5G and Wi-Fi 6/7. However, a fundamental question remains: why is one antenna insufficient?

Wireless signals encounter reflection and scattering from objects like glass, metal, and walls during propagation, creating multiple paths. Because signals on these different paths arrive with varying times and phases, they can cancel each other out at certain locations, causing a sudden drop in signal strength—this is known as multipath fading.

If a device has only one antenna, communication quality degrades significantly when that position is in a fading spot. Adding multiple antennas allows for spatial diversity: antennas at different locations experience different fading patterns, so when one antenna receives a poor signal, another may still receive a good one.

II. The Two Core Functions of MIMO

MIMO (Multiple-Input Multiple-Output) technology uses multiple transmit and receive antennas to achieve two key objectives:

For example, a 2x2 MIMO system transmits two data streams simultaneously, theoretically doubling the transmission rate without requiring additional spectrum bandwidth.

III. Why Does 5G Typically Use 4 Cellular Antennas?

5G pursues high data rates, which depend not only on bandwidth but also on the number of spatial streams.

In the 4G era, the primary configuration was 2x2 MIMO (two spatial streams). With 5G's 100MHz bandwidth, using only two antennas would underutilize the available spectrum. 4x4 MIMO supports four independent data streams in parallel, making full use of large bandwidth resources.

The value of 4x4 MIMO is particularly significant in TDD bands with large bandwidths, such as N77 (3.3-4.2GHz) and N79 (4.4-5.0GHz). The high speed of 5G is the result of "large bandwidth + multi-stream MIMO" working together, not solely a function of frequency.

IV. A Key Engineering Challenge of Multi-Antenna Design: Isolation

As the number of antennas increases, the greatest engineering challenge is not simply "fitting them in," but ensuring each antenna operates independently.

Coupling and Isolation

When multiple antennas are placed close together, the signal transmitted by one antenna can be partially received by another, creating mutual interference known as coupling. The metric for measuring this interference is isolation, expressed in dB.

Higher isolation is better. Good isolation ensures each antenna can transmit and receive independently without mutual interference, allowing MIMO performance to be fully realized.

Factors Affecting Isolation

• Antenna Spacing: Greater spacing reduces coupling and improves isolation. However, internal device space is limited, requiring trade-offs between layout and performance.

• Antenna Type and Polarization: Adjacent antennas with different polarizations generally achieve better isolation, benefiting MIMO performance in compact layouts.

• Ground Plane and Surrounding Metal: The PCB ground, metal brackets, shields, and LCD metal frames near the antenna all affect current distribution and coupling paths.

V. Antenna Deployment Challenges in Vehicular Environments

Compared to smartphones, vehicle environments present more complex antenna deployment challenges:

These metal structures can alter the antenna's resonant frequency, radiation efficiency, and radiation pattern, causing performance to deviate from design expectations.

VI. Engineering Considerations for FPC Antennas in Metallic Environments

In vehicle dashboard scenarios, FPC antennas are commonly used. A frequent question arises: if there is metal behind the antenna but no direct contact with the FPC, does it still affect performance?

Conclusion: Lack of direct contact is beneficial, but it does not mean there is no effect.

Reason: Antennas have a near-field region around them. Even if the FPC does not physically touch the metal, electromagnetic fields still extend into the space. If metal enters the near-field, coupling occurs. When the antenna is adjacent to metal through a "floating ground" approach (e.g., via a supporting bracket), a loading effect can shift the resonant frequency.

Specific effects include:

• Resonant Frequency Shift: An antenna designed for 2.45GHz might shift to 2.35GHz after installation.

• Reduced Efficiency: Some electromagnetic energy couples into the metal, dissipating as heat.

• Pattern Distortion: Omnidirectional characteristics may be compromised, with signal strength enhanced in some directions and attenuated in others.

VII. Similar Considerations for Wi-Fi Multi-Antenna Systems

Wi-Fi 6/7 also use 2x2 or 4x4 MIMO. Taking 2x2 Wi-Fi as an example, both antennas require good isolation. If the antennas are too close or isolation is insufficient, MIMO performance degrades, and actual throughput may fall far below theoretical limits (e.g., theoretical 866Mbps but only 300Mbps in practice).

Additionally, the 2.4GHz band is more sensitive to surrounding metal structures due to its longer wavelength (approximately 125mm), whereas the 5GHz band (wavelength ~60mm) is relatively less affected. Therefore, in industrial device deployment, attention must also be paid to antenna spacing, clearance, and nearby metal structures to optimize Wi-Fi MIMO performance.

VIII. Engineering Practice Recommendations

The following practical recommendations can guide the deployment and optimization of multi-antenna systems:

1. Prioritize Isolation Over Antenna Count: An increased number of antennas requires good isolation to realize MIMO gains. Deployment decisions should be supported by electromagnetic simulation or measured verification.

2. Assess the Near-Field Environment: Metal structures near the installation location (enclosures, brackets, shields) will affect antenna performance, even if antennas are not in direct contact. Adequate clearance should be considered during the design phase, and antenna performance in critical positions should be verified through over-the-air (OTA) testing of the finished device.

3. Consider On-Site Environmental Factors: For 2.4 GHz devices, due to the longer wavelength, antenna performance is more sensitive to surrounding metal structures, making clearance planning even more critical than for 5 GHz devices. Multiple placement and orientation options should be compared during prototyping to evaluate signal strength and throughput.

4. Adopt a Systematic Optimization Approach: Antenna issues are often not solved simply by swapping the antenna. A holistic approach—considering overall layout, clearance, matching networks, isolation, and OTA performance—is required.

IX. Conclusion

From cellular MIMO to Wi-Fi multi-antenna, increasing the number of antennas fundamentally aims to achieve spatial multiplexing and diversity gains. However, the number of antennas alone does not determine performance; isolation, antenna layout, and the surrounding metal environment are equally, if not more, critical.

For the deployment of industrial IoT devices such as industrial routers, vehicle-mounted gateways, and CPEs, understanding these underlying principles helps in practical installation planning—avoiding performance degradation due to metal obstruction, insufficient spacing, or near-field interference, and ensuring that equipment delivers the expected communication performance in real-world environments.