Introduction
The deployment of 5G networks requires new spectrum bands to support increased data rates and connectivity. One of the key frequency bands being utilized for 5G is the 28GHz millimeter wave (mmW) band. This high frequency range allows for multi-gigabit data speeds, but also presents design challenges particularly related to radio components like filters and antennas. This article provides an overview of 28GHz mmW filters and antennas for 5G networks.
28GHz mmW Band Overview
The 28GHz band, from 27.5-28.35GHz, is being used for 5G deployments worldwide. Some key advantages of 28GHz:
- Large amount of spectrum available – up to 850MHz depending on the region
- High bandwidth channels to support multi-Gbps data rates
- High frequency allows antenna arrays for beamforming and spatial multiplexing
However, the higher frequency also results in increased path loss and sensitivity to blockages. Omnidirectional coverage is difficult, so highly directional beamforming antennas are required. The small wavelength also leads to more challenging filter and antenna designs.
Spectrum Allocations
The 28GHz band plan varies regionally:
- North America: 27.5-28.35GHz (850 MHz)
- Europe: 24.25-27.5GHz (3.25 GHz)
- Asia: 26.5-29.5GHz (3 GHz)
The amount of spectrum directly impacts the maximum data rate per user or cell, making the North American allocation most attractive for operators.
Propagation Characteristics
Due to the high frequency, 28GHz signals experience higher free space path loss and atmospheric absorption compared to sub-6GHz 5G bands. The Small wavelength also leads to diffraction loss around obstructions.
Typical path loss exponents range from 2.5 to over 4 depending on the environment. Signals can be blocked by buildings, foliage, human bodies, etc.
This leads to shorter communication range, on the order of a few hundred meters cell radius in urban areas. More cell sites are required to maintain coverage compared to lower frequencies.
Beamforming and MIMO
To compensate for the reduced range, 28GHz systems utilize beamforming and MIMO antenna arrays. Highly directional beams between the user and base station maintain link budget. Phased array antennas allow rapid beam steering and tracking.
MIMO techniques like spatial multiplexing are employed to increase data capacity using multiple streams. The small wavelength allows dozens of antenna elements to be integrated into a compact array. 5G specifications target up to 256-element arrays for mmW systems.
28GHz Filter Requirements
Filters play a critical role in the 28GHz radio front end to reject out of band interference and noise. The small wavelength places strict demands on filter performance and technology.
Insertion Loss
Minimum insertion loss is critical to maintain link budget. Each 1dB of loss cuts the effective communication range. Target specifications are 2dB or less over the passband.
Bandwidth
The filter must have sufficient bandwidth to pass the full 28GHz spectrum allocation, up to 850MHz for North America 5G bands. Minimum fractional bandwidth is >3%.
Rejection
Strong rejection of adjacent frequency bands is needed to avoid interference and blockers. >30dB rejection should be maintained within 100MHz of band edges. >50dB rejection further away.
Power Handling
Transmitted power is limited for 28GHz, but filters must handle at least 30dBm transmit power levels without distortion. Higher power handling reduces insertion loss.
Size
Extremely compact size is required to integrate filters into the RF front end. Surface mount packaging with <5mm footprint is typical. Size is driven by manufacturability.
Cost
Low cost is needed for wide adoption in mmW products and infrastructure. Simple architectures with easy manufacturing are preferred. Tuning and adjustment must be minimal.
28GHz Filter Technologies
Many filter technologies have been researched and developed for 28GHz applications:
LC Resonator Filters
- Advancements in MEMS and lithography enable miniaturized LC filters up to 30GHz
- Low loss, moderate rejection, compact size
- Parallel plate/overlay capacitors and spiral inductors are commonly used
- Bandwidth control can be challenging
Cavity Filters
- Waveguide or dielectric resonator cavities for high Q, low loss
- Excellent rejection and power handling
- Bulky size and higher cost
- Limited tuning flexibility
Surface Acoustic Wave (SAW) Filters
- Very compact footprint suitable for mmW
- Low cost, simple manufacturing
- Moderate insertion loss and bandwidth
- Power handling limited to ~25dBm
Bulk Acoustic Wave (BAW) Filters
- Low loss, good power handling
- Narrow bandwidth 2-4% typical
- Requires matching to 50 ohms
- High Q resonators limit frequency tuning range
Acoustic Waveguide Filters
- Low loss, wide bandwidth 3-5%
- Compact planar or thin film designs
- Moderate rejection and power handling
- Narrow passband requires precise manufacturing
Summary:
- LC filters provide the best combination of low loss and wide bandwidth but require advanced MEMS or semiconductor fabrication.
- SAW and BAW suitable for low cost, moderate performance filters up to 30GHz. Limitations on loss and bandwidth.
- Cavity and acoustic waveguide filters for high performance, but higher cost and larger sizes.
5G 28GHz Antenna Requirements
Like filters, antennas operating at 28GHz mmW face stringent demands for 5G performance. Key parameters include:
Gain
High gain is essential to counter path loss and close the link budget. Required EIRP reaches up to 55dBm with base station antenna gains over 30dBi.
Beam Steering
Electrically steered directional beams for capacity and range. Wide azimuth and elevation scanning range supports beamforming and spatial multiplexing.
Bandwidth
Antenna bandwidth must cover the full 28GHz band up to 850MHz. Impedance matching required over the band. Gain variation < 3dB.
Efficiency
Minimize loss mechanisms like conductor and dielectric loss. 70%+ radiation efficiency needed to support high EIRP levels.
Size and Weight
Compact size and low weight desired to enable dense deployments on poles, rooftops, etc. Size under 8″ diameter x 4″ depth typically required.
Reliability
Robustness for outdoor operation in harsh environments. Stable performance over temperature and humidity extremes.
Cost
Making 5G mmW deployments commercially viable requires low cost antenna arrays and components, without sacrificing performance.
28GHz Antenna Technologies
Similar to filters, meeting these specs requires advanced antenna technologies and architectures:
Substrate Integrated Waveguide (SIW) Arrays
- Low loss propagation in integrated waveguide form
- Beam scanning via frequency tuning or phased array
- Moderate bandwidth, gain up to ~25dBi
- Integration with PCB and semiconductor manufacturing
Microstrip Patch Arrays
- Low profile, lightweight, low cost
- Gain up to 30dBi with 1000+ elements
- Limited scan range and bandwidth
- Dielectric and conductor losses increase with frequency
Reflectarrays/Transmitarrays
- Parabolic reflector performance made planar
- Extremely high gain and efficiency
- Steered beams with tunable phase shifters
- Narrow bandwidth and limited scan range
- Complex feed array required
Dielectric Resonator Arrays
- Very low loss, high radiation efficiency
- Moderate bandwidth and gain up to 28dBi
- Complex feeds and power distribution
- High Q resonance limits steering agility
Summary
- Microstrip patches optimal for low cost phased arrays with moderate performance
- SIW arrays combine high performance with easier manufacturing
- Advanced architectures like reflect/transmit-arrays provide highest gain and beam control
MIMO and Multi-Beam Arrays
MIMO spatial multiplexing at 28GHz uses multi-beam antennas or arrays mounted in various orientations to provide diverse spatial channels for multiple data streams.
Typical configurations utilize:
- 4 to 16 antenna arrays per base station
- Each array may have up to 256 dual-polarized antenna elements
- Arrays distributed to provide 360 degree azimuth coverage
- Antenna mounting directions optimized to maximize channel separation
Multi-beam arrays allow simultaneous transmission/reception with multiple UEs to increase capacity. Each array generates multiple fixed or steerable beams using sub-arrays with phase shifters or tuning elements.
Conclusion
The shift to 5G in mmW bands like 28GHz brings formidable challenges in designing radio components like filters and antennas. High performance, small size, and low cost need to be simultaneously achieved. A variety of filter and antenna architectures show promise in targeting the demanding requirements for 28GHz operation. Ongoing research and product development continue to optimize mmW components and arrays to make high frequency 5G commercially viable worldwide. Careful selection of filter and antenna technologies allows balancing performance, size, and cost.
Frequently Asked Questions
What is the main driver for using the 28GHz band in 5G?
The large amount of spectrum available in the 28GHz range, up to 850MHz in some regions, enables very high data rates up to multi-Gbps speeds per user. The wide bandwidths support high capacity 5G networks.
Why are highly directional antennas needed at 28GHz?
Due to the high free space path loss at such high frequencies, directional antennas with high gain are essential for closing the link budget and achieving reasonable range. Omnidirectional coverage is very difficult. Directional beamforming maintains signal strength.
How does beam steering work for 28GHz antennas?
Phased array antennas are commonly used for beam steering at 28GHz. By adjusting the phase of the signal at each antenna element, the beam direction can be electronically pointed without mechanically moving the antennas. This allows fast adaptation of the beams for capacity and coverage optimization.
What is a typical data rate achievable with 28GHz 5G?
Using advanced modulation up to 256QAM and large channel bandwidths allocated at 28GHz, data rates up to 2Gbps may be achievable with 28GHz NR. This supports applications like 4K/8K video streaming, mobile broadband, and fiber-like wireless connectivity.
Why is filter rejection important for 28GHz?
Strong out-of-band rejection is critical for 28GHz filters to avoid interference from adjacent frequency bands and blockers that could desensitize the receiver. The filter must provide high isolation from nearby spectrum to maintain sensitivity. 30dB rejection within 100MHz of the band edge is typical.
