Millimeter Wave

1. Data Demand and Additional Spectrum

The development of wireless communication has always been closely tied to spectrum. The radio spectrum carries calls, texts, videos, and data. It is a finite resource, so rising demand for wireless data increases congestion in some bands.

For decades, cellular networks from 1G through 4G used frequency bands below $6$ GHz. These bands, often called the sub-6 GHz spectrum, can propagate over relatively long distances and penetrate buildings better than higher bands, which supports wide-area mobile coverage. Their availability is constrained by national allocations and growing traffic. Smartphones, video streaming, and many applications have increased load, limiting the speeds available in congested areas.

To increase 5G speed and capacity, engineers also turned to higher spectrum ranges, including millimeter wave bands. Availability depends on regulation and deployment, while wide channels can complement the coverage provided by lower bands. Deployment also depends on available site locations, backhaul, and interference coordination.

What Are Millimeter Waves (mmWave)?

Millimeter wave (often abbreviated as mmWave) refers to a specific range of very high radio frequencies. It is formally defined as the band of spectrum between $30$ GHz and $300$ GHz. In 5G, the term is sometimes used more broadly for bands around $24$ GHz and above. The FR2 ranges depend on the specification release: current profiles include FR2-1 from 24.25 to 52.6 GHz and FR2-2 from 52.6 to 71 GHz. Not every band between 24 and $100$ GHz therefore has the same status in 5G terminology.

The name "millimeter wave" comes from the physical length of the radio waves at these high frequencies. Wavelength is inversely proportional to frequency; the higher the frequency, the shorter the wavelength. At frequencies in the tens of GHz, the wavelength shrinks to just a few millimeters, hence the name. This differs from the meter-long waves of FM radio and the tens-of-centimeters-long waves of traditional cellular bands. The short wavelength affects propagation and antenna design, while the bands can provide wide bandwidth.

2. Available Bandwidth in mmWave

The main practical advantage of moving to the millimeter wave spectrum is the amount of available bandwidth.

Bandwidth: Channel Width

In wireless communication, a wider channel can carry more data at the same time, provided that the signal-to-noise ratio, modulation, coding, and other radio conditions support it.

• 4G LTE Channels: A typical 4G LTE channel has a bandwidth of $10$ or $20$ MHz. Even with advanced techniques like Carrier Aggregation, combining a few of these channels might result in a total bandwidth of $100$ MHz.
• 5G mmWave Channels: In selected 5G bands, a single operator can use a continuous channel with a bandwidth of $100$ MHz, $200$ MHz, or $400$ MHz. The $800$ MHz value can arise in carrier aggregation and deployment profiles rather than as a universal single-channel bandwidth.

This represents a large increase in available channel width. Such bandwidth can support the multi-gigabit eMBB rates associated with 5G, a 4K movie download in seconds, high-resolution VR streaming, and wireless home internet. Protocol overhead and scheduling reduce application throughput relative to the channel rate. The final rate also depends on modulation, SNR, user count, MIMO, and carrier aggregation, so gigabit links are possible in selected lower-band configurations as well. mmWave adds capacity where radio and regulatory conditions allow it.

3. Propagation Limits in mmWave

While the bandwidth available in the mmWave spectrum is a major advantage, deploying these ranges requires careful planning. High-frequency signals have different physical properties from lower ranges, and those properties present challenges for mobile communication.

4. Technologies Supporting mmWave

mmWave deployments commonly combine two related 5G technologies: large Massive MIMO arrays and beamforming. Together they help use wide channels and compensate for part of the link loss.

Massive MIMO Arrays

The short wavelength of mmWave signals helps with antenna design. An antenna element's size is related to the wavelength, and at 28 GHz the wavelength is about a centimeter. Engineers can therefore pack many small elements, such as 64, 128, 256, or more, into a compact array. This is an example of Massive MIMO in the mmWave context; the actual element count depends on the equipment. The array supplies the degrees of freedom needed for beamforming.

Beamforming

Beamforming uses the antenna array to direct energy into a narrow, steerable beam, much like a spotlight. The resulting antenna gain can compensate for part of the path loss and increase signal strength toward the user. The resulting range still depends on power, blockage, SNR margin, and the required service level.

Beamforming also helps address some blockage. A direct line-of-sight path often provides the best link margin, but the system can form beams that use reflections from surfaces such as buildings or other NLOS paths. Their quality depends on the geometry and materials of the environment.

Advanced Beam Management

Because the connection can rely on narrow beams that are sensitive to blockage, 5G NR includes beam managementprocedures. The base station performs "beam sweeping" to identify usable beams, and after connection the device reports measurements that support tracking and adjustment. These procedures use reference signals and reported measurements, with timing affected by mobility and numerology. If the primary beam is blocked, beam-failure and recovery procedures can switch to another beam or path when one is available.

5. Practical Deployments and Use Cases for mmWave

Given its unique combination of high speed, limited range, and limited penetration, mmWave 5G usually operates as a capacity layer. Operators select locations where wide channels provide the greatest benefit and use lower bands to complement coverage.

• Dense Urban Hotspots: An important use for mmWave is providing data capacity in dense environments. This includes downtown centers, airports, train stations, sports stadiums, and concert venues. Thousands of people use devices in a small area, while wide channels and beamforming-enabled spatial reuse support quality service together with the other network layers.
• Fixed Wireless Access (FWA): mmWave can deliver "fiber-like" internet speeds to homes and businesses in suitable line-of-sight deployments. An operator can place a mmWave small cell on a utility pole or lamppost and provide gigabit service to homes within clear line of sight, typically a few hundred meters. A clear path does not guarantee gigabit service because load, backhaul, and rain can also affect the rate. Deployment time and cost depend on site density, backhaul, and civil works; avoiding a fiber run to every address can reduce them in some deployments.
• Indoor Enterprise and Public Venues: For large indoor spaces like shopping malls, convention centers, or corporate campuses, a network of indoor mmWave small cells can provide high wireless capacity and speed for employees, visitors, and operational systems.
• V2X and Smart City Infrastructure: In the future, mmWave can be used for ultra-high-bandwidth vehicle-to-infrastructure communication, such as cars downloading high-definition maps from lampposts or uploading large amounts of sensor data to the cloud.