Millimeter waves are electromagnetic waves with wavelengths ranging from 10 mm to 1 mm and frequencies between 30 GHz and 300 GHz. Communication using these waves is known as millimeter wave communication, which can be divided into two main types: millimeter wave waveguide communication and millimeter wave radio communication. The millimeter wave band typically refers to the frequency range of 30 GHz to 300 GHz, corresponding to wavelengths from 1 mm to 10 mm. This type of communication uses millimeter waves as a carrier for transmitting information. Current research often focuses on specific "atmospheric window" frequencies and "attenuation peak" frequencies.
One key characteristic of millimeter waves is their line-of-sight propagation. As part of the very high frequency band, they travel through space as direct waves, with narrow beams and strong directionality. While this makes them susceptible to atmospheric absorption and rainfall fading, resulting in shorter single-hop distances, it also means less interference due to the high frequency band, leading to stable and reliable transmission. This makes millimeter wave communication ideal for high-quality wireless transmission with consistent channel parameters.
Another important feature is the presence of "atmospheric windows" and "attenuation peaks." These refer to specific frequency bands where millimeter waves experience minimal or maximum signal loss, respectively. For instance, 35 GHz, 45 GHz, 94 GHz, 140 GHz, and 220 GHz are considered atmospheric windows, while 60 GHz, 120 GHz, and 180 GHz are known for significant attenuation. These characteristics make certain bands suitable for point-to-point communication, while others are used in secure multi-channel systems.
Rainfall significantly affects millimeter wave propagation, causing more severe signal attenuation compared to microwaves. Factors such as rainfall intensity, distance, and raindrop size influence the level of attenuation. To mitigate this, designers must account for sufficient margin when setting up millimeter wave communication systems.
In addition, millimeter waves have strong penetration capabilities against dust and smoke. Unlike lasers or infrared light, which struggle to pass through such obstacles, millimeter waves can traverse sand and smoke almost without loss. Even under conditions of intense scattering from explosions or metal foil strips, any fading caused is usually temporary and recoverable.
Current millimeter wave communication systems include both terrestrial and satellite-based applications. Terrestrial systems are widely used in relay communications where privacy is essential. Due to the high frequency and narrow beam, interception and interference are challenging. Satellites benefit from the abundant frequency resources of millimeter waves, making them ideal for long-distance and high-capacity communication. The U.S. tactical satellite system, for example, uses 60 GHz for interstellar communication, taking advantage of the high atmospheric loss at that frequency.
Military applications of millimeter wave technology are extensive, including radar, guidance, communication, and electronic countermeasures. These systems offer advantages like high data rates, good confidentiality, and strong anti-interference capabilities. In electronic warfare, millimeter waves are used for secure communication and jamming resistance, making them a valuable tool in modern conflicts.
Looking ahead, millimeter wave communication has promising potential in both military and civilian fields. It can support broadband multimedia services, vehicle collision avoidance, remote sensing, and satellite links, among other applications. With ongoing research and technological advancements, its role in future communication systems is expected to grow significantly.
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