Microwave Journal
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Pivotal

Breaking Down mmWave Barriers with Holographic Beam Forming®

February 12, 2020

Editor’s Note: As 5G mmWave applications are being implemented, the issues of propagation distance and blocking have limited the success of initial deployments. However, new array beam forming technologies have been developed to help overcome these limitations by enabling higher power/gain, higher efficiency and improved scanning angles. Our cover feature introduces two array beam forming technologies that promise to improve mmWave systems: the first is Holographic Beam Forming from Pivotal Commware, and the other is dielectric resonator antenna technology from Antenna Company. So stay tuned as we address semiconductor technology improvements for 5G mmWave applications in our April issue.

mmWave wireless services have been traditionally used in short range wireless transport applications such as fronthaul/backhaul, point-to-point links and SATCOM. 5G in the U.S. will push the role of mmWave beyond simple transport to become a capacity layer for fixed wireless access (FWA) and enhanced mobile broadband (eMBB). Taking mmWave from its fixed beam roots to a dynamic, reconfigurable access layer will depend critically on beam forming techniques to assure link margins without requiring massive capital outlays.

5G mmWAVE BEAM FORMING TECHNOLOGIES

Figure 1

Figure 1 A canonical TDD phased array architecture.

Figure 2

Figure 2 A canonical MIMO array architecture.

Figure 3

Figure 3 A canonical HBF architecture.

Figure 4

Figure 4 39 GHz HBF antenna.

Beam forming uses planar antenna arrays with typical antenna gain ranging from 17 dB (4 square wavelength arrays) to 29 dB (64 square wavelengths). Phased arrays are a classic architecture used for beam forming. A canonical 4-element time division duplexed (TDD) phased array is shown schematically in Figure 1. In such an array, the antenna elements are spaced roughly one-half wavelength apart and every element is backed by a module consisting of a transmit power amplifier (PA), receive low noise amplifier (LNA), phase shifter and switches to swap between the transmit and receive modes. These elements are typically packaged as 4-element modules on a single chip. This distributed architecture is notoriously expensive and power hungry but is well understood by antenna engineers and industry and has been the standard for analog beam forming for decades. At mmWave, phased arrays have seen heavy usage in defense and satellite applications but have not seen widespread use in commercial terrestrial applications.

Phased array beam forming is both named for and enabled by a phase shifter behind every element. In transmission mode, the radio signal is distributed through a corporate feed network, providing the same phase shift to every element in the array. Phase shifters at each element then impart the appropriate delay in carrier phase to cause constructive interference of the radiated fields in desired direction while also causing destructive interference in others. The finite nature of the antenna aperture (radiating area of the antenna surface) gives rise to sidelobes or unavoidable radiation in undesired directions. Sidelobes are usually 13.4 dB or more below the beam peak. Receiving from the antenna array is the reverse of transmission, with the PA and LNA chains swapping for receive mode while the phase shifters align the received signals from the desired direction to constructively add while destructively combining the other directions. In both transmit and receive, the antenna radiation pattern is well defined and is typically measured in anechoic chambers.

Massive Multiuser MIMO (MU-MIMO) is a newer technique, shown in Figure 2, that replaces analog phase shifters with direct digital conversion of the analog waveform. At low frequencies (sub-3 GHz) high resolution digital to analog (DAC) and analog to digital converters (ADC) are used to directly create the radio waveform with the appropriate phase shift. More interestingly, different phase shifts can be imparted on a per-carrier basis and create beams carrying different information for the same sub-carrier. Receiving is similar with the received waveform being digitized at each antenna element. The digital bus-work connects each element to a coordinated radio/baseband system which post-processes the signals into their respective digital streams.

In practice, direct DAC in MIMO is rarely done at mmWave. RF up/down conversion is used to reduce sampling requirements to those of baseband signals. Even so, the DAC/ADC and RF conversion pairs are even more expensive and power hungry than a phased array. For traditional cellular this may prove acceptable as base station densities are modest. At mmWave, deployed densities are expected to be much higher due to the shorter link distances at mmWave rendering MU-MIMO unsuitable for real deployments.

Holographic Beam Forming (HBF)

HBF is a new technique derived from metamaterial concepts. Functionally, HBF is an analog beamformer with performance equivalent to analog phased arrays. As with other planar arrays, the antenna area dictates the achievable beamwidth and maximum realized gain. The HBF technique uses one RF chain that feeds all the elements in the array through a single RF port. Rather than a corporate feed as in phased arrays or digital network in MIMO, HBF uses a series feed that distributes the RF signal along multiple individual antenna elements. Slots, patches, dipoles and other small antenna types have all been shown to work with the HBF technique as radiating elements. A varactor tuned feed coupler transfers energy from the distribution network into the antenna element and this is shown schematically in Figure 3. As shown, each element sees a different progressive phase shift from the feed point. Beam forming is accomplished by using the capacitance shift in the varactor to vary the coupling to selected elements with the proper phase alignment needed to point the beam in the desired direction.

HBF is named for its similarity to conventional optical holography. In optical holography, an external source excites the hologram surface which converts the excitation into an output wave our eyes perceive as an image. In HBF, the radio is the source excitation and the beam is the output wave. The pattern of element coupling converting the guided excitation into a free space wave is exactly a hologram. As a passive system, the technique is fully reciprocal with reception and transmission using the same hologram. No switching at the antenna level is required. All the amplifiers and baseband components can remain within the radio unit.

Figure 4 shows a typical HBF implemented in a multilayer printed circuit board at 39 GHz. The holographic technique uses a denser lattice than half wavelength along one dimension. As suggested from the schematic view of Figure 3, many of the elements will have the wrong phase for the desired beamshape and are turned off. In order to avoid the efficiency penalty for having significant portions of the array in the “off” state, the elements are packed at more than double the density of a traditional array to allow uniform aperture excitation and efficient beam forming. The element miniaturization along one axis has an additional benefit in that it widens the scanning volume to nearly ±80 degrees with less than 10 dB loss from antenna broadside.

5G mmWAVE SYSTEM DESIGN

Current 4G networks suffer from limited available spectrum to serve ever-increasing data demands from users. As mmWave spectrum is being used to close this gap, more systems showcase the need for cost-effective, low-power beam forming techniques.

Figure 5

Figure 5 A high-level 5G cellular network architecture.

A high-level cellular network architecture is shown in Figure 5. Perhaps the most challenging link in the above architecture is the radio access network (RAN) link. Front-haul, backhaul and other transport is generally expected to use fiber as the physical medium. RAN, by its nature, requires wireless connectivity. In the context of 5G, the RAN link can be decomposed into the following node types:

  • The Base Station (gNB in 5G): The gNB connects the core network to end user equipment.
  • User Equipment (UE): The UE connects to the gNB for access and provides the user with service.
  • Repeater: Repeaters are optional components that connect the gNB and UE.

These node types all have mmWave capability. The mmWave requirements differ among node types which drives different architecture decisions for practical implementation of 5G equipment.

A mmWave gNB must comply with requirements standardized by 3GPP, the primary cellular RAN standards development organization. In practical terms, mmWave gNBs are driven to meet the following high-level requirements:

  • Hybrid beam forming architecture with 2 or 4 subapertures each capable of an independently steered beam
  • Moderate-to-high output power, with effective isotropic radiated power (EIRP) from 50 to 70 dBm
  • Beamwidths on the order of 5 to 10 degrees
  • Fast switching capable of complying with timing requirements at the slot and symbol level, translating to beam-to-beam switching times of ~4 μs

HBF is well-suited to mmWave gNB designs. Hybrid beam forming with HBF results in low RF and thermal complexity. Phased-array-based hybrid beamformers use monolithic modules to realize the required antenna gain, with each module generating heat due to the phase shifter and amplifier losses. This leads to a thermal design challenge with heat generated throughout the PCB. With HBF beamformers, there is a single RF PA per HBF from which to extract heat. This also allows for digital pre-distortion (DPD), which enhances efficiency and reduces power consumption. HBF also lends itself to rapid beam-switching implementations, without any RF gain or phase adjustment required at fast time scales.



5G UEs pose a major challenge for device designers. The inherently lower power added efficiency at mmWave, compared to traditional cellular bands, leads to devices operating at higher temperatures while simultaneously reducing battery life. The simplified architecture for DPD with HBF is particularly important in handset UEs for this reason, as early 5G handsets have seen thermal issues with multiple mmWave arrays in a confined physical envelope. Mass-market UEs also have even more stringent cost requirements than gNBs, with a constant “race-to-the-bottom” in price pushing designers to embrace novel technologies if they have a corresponding cost decrease.

As a contrast to 5G gNBs and UEs, 5G repeaters are less defined in standards. Generally, repeaters reproduce an incoming signal with minimal processing, typically limited to L1 (physical layer) or L0 (RF layer) only. Regardless of whether a repeater processes a signal at L0 or L1, beam forming is needed to support mobile network operator (MNO) requirements. The cost, complexity and power draw are expected to be less than a full gNB, allowing repeaters to serve as network extension nodes without the effort of deploying a complete gNB. Cost is a major driver for repeater deployment by MNOs: the lower the cost, the more easily repeaters can be deployed to cover gaps in the network. HBF is thus a natural fit for the beam forming subsystem of a 5G mmWave repeater.

When compared to a full gNB solution, repeaters do not need full-stack processing to operate. The lower the layer of processing, the lower cost of the signal processing chain, as FPGAs and ASICs are not required. A L0 repeater consists primarily of a system controller, power supply and management unit, RF chain(s) and beamformer(s). These modules are required in a L1 repeater with the addition of frequency conversion (to IF or I/Q), ADC/DAC chains and baseband processing. L0 repeaters provide the lowest cost architecture, while L1 repeaters can provide additional signal conditioning in the presence of interference.

Repeaters extend a signal to locations unreachable by the original source. As an example, the free space path loss at 28 GHz is approximately 71 dB 10 ft. from the source. A repeater which extends coverage to an additional 10 ft. (say from a window, inside a home) should have at least 71 dB of gain to compensate. With typical window or wall penetration loss adding at least 5 dB, this example repeater needs 50 dB of electronic gain with an additional 26 dB of gain contributed by the Tx and Rx antennas.

With a restriction to L0-only processing, the repeater must be able to remain unconditionally stable across all deployment environments: this presents the fundamental design issue for repeaters given the large electronic gain required within a single enclosure. Careful RF design is needed to prevent oscillation under any conditions, as stability is affected by temperature, scattering environment and even window material. Pivotal’s L0 repeater using HBF has been proven in real-world deployments to effectively extend the coverage of mmWave in both indoor and outdoor settings. The use of low-cost mmWave repeaters is a crucial piece in the 5G deployment puzzle to avoid over-deployment of gNBs, which leads to a node density and cost that is unacceptable for MNOs.

5G mmWAVE HBF REPEATERS IN FIELD TRIALS

Figure 6

Figure 6 Pivotal Echo 5G™.

Penetration through building material presents a challenge for mmWave deployments. It is not uncommon to encounter over 40 dB of loss when transitioning from outdoor to indoors. Since the channel is reciprocal, the same degradation occurs in the uplink direction, from UE to gNB. To overcome mmWave propagation and outdoor to indoor penetration loss, Pivotal Commware created two mmWave beam forming repeater products: the Echo 5G (see Figure 6), a self-installable, tablet-sized, on-the-window, beam forming repeater for facilitating outdoor to indoor propagation, and the Pivot 5G, a professionally installed network element/repeater for supplementing and, in some cases, replacing gNB in commercial network deployments. This setup has been tested in numerous environments with a U.S.-based carrier and commercial gNBs and has completed interoperability testing with network equipment from three gNB vendors.

Apartment Buildings

Pivotal conducted one trial in a suburban apartment building shown in Figure 7. The gNB was located 700 ft. away and a commercial UE was inside the home. The Echo 5G was placed on the patio window (in yellow) with line-of-sight (LOS) to the gNB at 45 degrees from window broadside (parallel arrows). The unobstructed gNB-to-UE link supported peak throughput of 1.5 Gbps. The UE was then moved around the interior and tests run in various locations within the apartment.

The two pictures in Figure 7 depict “before and after” the unit was utilized. Figure 7 summarizes throughput results. The results printed in green are all LOS locations from the unit. The results in amber are non-LOS locations from Echo that Echo still served very well.

Without this product, full rate was only observed by placing the UE at the balcony door, as most of the interior was mmWave shadowed by the building wall (in red). Only the kitchen area indoors was not in the mmWave shadow, as it had LOS connectivity to gNB through the window. For other test locations, a mostly full rate experience was observed and locations that previously had no connectivity showed substantial throughput. Signal level measurements showed 20 to 30 dB improvement.

f7.jpg

Figure 7 Suburban apartment trial results, before and after Echo 5G deployment.

The number of gNBs needed to cover an area is the dominant factor in calculations of CAPEX and OPEX associated with a network deployment. This type of unit reduces the number of initially needed base stations dramatically, with 1 km distant LOS links becoming possible even with outdoor to indoor penetration challenges.

Figure 8

Figure 8 Outdoor vs. indoor coverage, urban, without Echo (red) and with Echo (blue).

Modeling suggests the served addressable market (SAM) can be doubled, even quadrupled, if this product is utilized to bring the mmWave signals indoors without adding additional gNBs. Figures 8 and 9 show projections for urban and suburban environments, respectively. In both figures, the baseline is achieved outdoor coverage (x-axis). The benefit Echo 5G brings is the additional indoor coverage (y-axis) depicted with the blue line in each of the plots.

Figure 9

Figure 9 Outdoor vs. indoor coverage, suburban, without Echo (red) and with Echo (blue).

Repeater Applications

Figure 10

Figure 10 Pivot 5G prototype.

Figure 11

Figure 11 Live demo setup for MWC Los Angeles 2019.

f12.jpg

Figure 12 The layout of the indoor space and photo of the boardroom.

Pivot 5G is an outdoor network repeater shown in Figure 10. As a base station proxy, the unit redirects mmWave signals from the gNB around obstacles and extends the range of 5G base stations.

A bring-your-own-device demonstration was set up for MWC Los Angeles 2019. This demonstrated Echo 5G and Pivot 5G in a real-world environment, with commercial gNB and 5G UEs. The demo location was a meeting boardroom near the MWC venue, and the room lacked LOS to the gNB. Echo 5G was deployed on the window facing South Figueroa Street. From this position, the Echo 5G was located 300 ft. away from the gNB but lacked LOS to the gNB. Pivot 5G was deployed in the hotel parking lot to redirect coverage to Echo 5G. The topology is depicted in Figure 11.

Testing used a standard Samsung S10 5G phone. Each of the demos was benchmarked from inside the boardroom showing that 5G throughout is impossible without the units. In each test, the phone either could not connect to 5G or it connected with low throughput (< 100 Mbps). With Echo 5G and Pivot 5G on, throughput of 1000 Mbps on the 5G network, with the phone positioned 15 to 20 ft. in LOS of the Echo 5G, was consistently observed.

Moving into a hallway behind the boardroom, the phone did not initially connect to 5G. With the Echo 5G and Pivot 5G turned on, throughput of 800 Mbps was shown even with the phone positioned 20 to 30 ft. away and not in LOS of the Echo 5G.

Finally, 150 Mbps was achieved in a second hallway - two boundaries and over 30 ft. away from the window - by the Echo 5G and Pivot 5G. Without them, there was no 5G reception. The layout and the results are depicted in Figure 12.

SUMMARY

This article demonstrated the importance of beam forming in 5G mmWave networks and described three beam forming technologies: phased arrays, MU-MIMO and HBF. It claimed that HBF is particularly well-suited to 5G mmWave system designs and explained why, from a systems perspective, the use of low-cost mmWave repeaters is a crucial piece in the 5G deployment puzzle to avoid over-deployment of gNBs. Finally, the article elaborated on field trials and early success stories associated with 5G mmWave repeaters that use HBF.n