Multi-Beam Phased Array with Full Digital Beamforming for SATCOM and 5G Divaydeep Sikri and Rajanik Mark Jayasuriya Satixfy UK Ltd., Farnborough, U.K

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Multi-Beam Phased Array with Full Digital Beamforming for SATCOM and 5G Divaydeep Sikri and Rajanik Mark Jayasuriya SatixFy UK Ltd., Farnborough, U.K. As we usher in the age of large capacity wireless access systems demanding high spectral efficiencies, array antennas are playing an increasing role. MIMO antenna arrays have become integral to the standards for cellular and wireless local area networks. These active antenna arrays will play an equally important role in next-generation high throughput satellite (HTS) communications. Also, the large low Earth orbit (LEO) and medium Earth orbit (MEO) constellations planned by companies like OneWeb, Telesat, SES and SpaceX will need ground terminal antennas that track multiple satellites. This convergence of trends is driving a shift from passive antennas with static fixed beam patterns to fully steerable, active smart antennas. In this article, we discuss the advantages of digital beamforming (DBF) for capacity, control and flexibility. Until now, DBF was largely a concept because of the cost and complexity to implement a usable solution. We will describe a commercial ASIC implementing DBF with true time delay (TTD) that realizes its potential. DBF combined with an integrated RF front-end (RFFE) enables modular electronically-steerable multi-beam array (ESMA) antenna systems for a wide range of applications.

obile wireless communications systems re- in next-generation HTS communications. The develop- quire increasingly high data rates with vir- ment of large LEO and MEO constellations, planned by tually worldwide coverage. Because terres- companies like OneWeb, SES and SpaceX, will require trial networks do not cover the globe, high ground terminals able to track multiple satellites. Para- Mdata rate services are not available in remote areas or bolic dish antennas have been the defacto design for onboard ships and aircraft. SATCOM and SATCOM-on- SATCOM Earth antennas. They have advantages such the-move (SOTM) are essential capabilities to achieve as good performance, power consumption and cost, yet high capacity communications with global coverage. they are stationary and have lower efficiency. In compari- With large capacity wireless access requiring high spec- son, electronically-steerable antennas have many ben- tral efficiency, array antennas have emerged as a key efits: self-installation, multi-SATCOM, satellite tracking architecture for wireless communication systems, and and their payloads can be more flexible, enabling tech- MIMO antenna arrays are included in the standards niques such as multi-beam, beam hopping and flexible for cellular and wireless local area networks. These ac- beam shaping. All-electronic control eliminates mechan- tive antenna arrays will play an equally important role ical parts, which are slow and more likely to malfunction.

diation pattern defining the direc- power consumption, and the com- Analog tion of the energy radiated by the plexity scales with the size of the antenna. An antenna’s gain and antenna. directivity go hand in hand: the With analog baseband beam- Baseband RF Processing Chain greater the gain, the more directive forming, beamforming occurs in the NA the antenna. It is this feature of the baseband, after down-conversion antenna that has become the focus and before up-conversion, enabling for increasing capacity, particularly (a) use of higher precision phase shift- with the next-generation of wireless ers. However, the size of the phase Digital communications systems for both shifters and the complexity of the RFFE SATCOM and 5G. BFN—mixers in each RF chain and Beamformers comprise an array a network of baseband splitters and Baseband RFFE of antennas making the combined Processing combiners—are challenges. ND aperture directive. They control Digital Beamforming RFFE the radiation pattern through the constructive and destructive super- With digital beamforming (DBF), (b) position of signals from the differ- beamforming is performed digitally Analog ent antenna elements. In general, at baseband, requiring one beam- Digital beamforming can be classified as former and RFFE at each antenna passive and active. Passive beam- element. Offering a high degree of formers are fixed directive anten- control, DBF is considered the most NA Baseband nas made of passive components, flexible beamforming approach and Processing ND such as transmission lines, that superior to ABF for receiving and point the beam in a fixed direc- transmitting wideband signals and, tion. Active beamformer anten- more importantly, for multi-beam NA nas—commonly known as phased applications. The digital implemen- (c) arrays—have active phase shifters tation has greater reconfigurability at each antenna element to change and enables treatment of RF impair-  Fig. 1 Analog RF (a), digital (b) and ments at each antenna element. hybrid (c) beamformers. the relative phase among the elements; because they are active, the However, it requires data converters As Ku-Band capacity is widely beam can be dynamically steered. and RFFEs for each antenna ele- available from the existing geo- Electronically-steerable antennas ment, increasing the complexity and stationary (GEO) satellite networks can adopt one of three approaches power consumption. Fortunately, above the planet, market interest to beamforming: analog, digital recent advances in silicon processes has largely been for satellite ser- and hybrid (see Figure 1). have reduced the complexity, pow- vices at Ku-Band, namely digital TV er and cost of digital beamforming, broadcast, broadband internet ac- Analog Beamforming making it feasible for some phased cess and IoT networks. The growth Analog beamforming (ABF) can arrays. of these services will depend on the be implemented in three ways: development of new high perfor- RF, local oscillator (LO) and analog Hybrid Beamforming mance and low-cost user terminals baseband. Hybrid beamforming uses the with the ability to track satellite po- With RF beamforming, phase best of both alternatives: analog sition while in motion. The antenna shifting is implemented in both and digital. To reduce the com- at the terminal must be capable of the RF Rx and Tx paths prior to the plexity of digital beamforming, wide-angle scanning while keeping mixer. Reduced component cost is requiring control at each antenna fabrication costs as low as possible, one of the reasons for its popular- element, the hybrid approach uses since most applications are con- ity, particularly at mmWave, where “two stage” beamforming—the sumer markets. For low-cost appli- the small size of the phase shifter al- concatenation of analog and digital cations such as the IoT, the cost of lows better integration in the RFFE. beamforming—and provides a rea- the antenna can be reduced using However, phase shifter precision sonable compromise between per- energy efficient waveforms, such and noise figure degradation due to formance and complexity. Each ana- as half-duplex, which optimize link the phase shifters are performance log beamforming network serves as and resource utilization. The cost challenges for this technique. Also, a subarray for the next level of digi- using such waveforms can be re- the phase shifters and beamforming tal beamforming, forming a more duced with a single antenna that network (BFN) must be designed for directive “super element” whose can serve both receive (Rx) and the frequency of operation. signal is coherently combined in the transmit (Tx). LO beamforming uses the LO digital domain with the signals from distribution network for phase shift- the other super elements. Hybrid BEAMFORMING OPTIONS ing, addressing the noise figure beamformers provide limited multi- Antennas convert RF signals into challenge by shifting the phase beam capability, although the per- electromagnetic transmission and shifter from the signal path to the formance is sub-optimal compared vice versa. Each antenna has a ra- LO path. However, this increases to digital beamforming. TechnicalFeature

most effective way to increase chan- as hemi-spheroidal 3D antennas or nel capacity. With SATCOM, it en- other conformal shapes can be im- ϴ ables simultaneous communication plemented using DBF. d sin ϴ with multiple satellites. DBF sup- d ports large numbers of beams using TTD BEAMFORMING the entire antenna aperture, which As shown in Figure 2, with a provides the same antenna gain uniform linear array, the incident and directivity for each beam. wavefront at an angle θ results in x(t) x(t – τ) x(t – Nτ) Fast beam steering: DBF sup- a delay (τ…Nτ) for the signals ar-  Fig. 2 Uniform linear array geometry. ports fast beam switching and steer- riving at different elements. This ing, i.e., within microseconds. This delay causes the antenna array to DIGITAL BEAMFORMING WINS enables fast acquisition and track- have a pattern depending on the Given the ongoing improvement ing in high dynamic channel envi- frequency. To have a flat pattern in silicon technology, DBF is the pre- ronments. over the desired frequency range, ferred approach for phased array Flexibility: Active beamforming the antenna’s coherent bandwidth antennas. It offers: with flexible reconfiguration -en should be greater than the band- Wideband signal reception ables the array to adapt for multiple width of the signal. This implies that and transmission: Wider signal applications, such as online calibra- Nτ<

20 20 19 GHz 19 GHz 20 GHz 20 GHz 21 GHz 21 GHz 15 15 Antenna Defocused (Beam Squint) 10 10

5 Gain (dB) 5 Gain (dB)

0 0

–5 –5 –10 0 5 10 15 20 25 30 35 40 45 –5 0 5 10 15 20 25 30 35 40 45 (a) Scan Angle ϴ (deg) (b) Scan Angle ϴ (deg)

 Fig. 3 Phased array radiation patterns showing beam squint vs. frequency (a) and no beam squint with true time delay (b). TechnicalFeature

ity to build large antennas. Prime beam (see Figure 6). 600 digitizes the signal at each antenna • Equalization/pre-equalization 500 element with high speed analog- and digital predistortion for each 400 Operational SNR > 20 dB to-digital converters (ADC) and beamformer chain. True Time Delay 300 digital-to-analog converters (DAC), • 2 GHz analog baseband inter- processing more than 2 Tbps data face. 200 rates. Prime connects to RFFEs con- • Tight integration with SatixFy’s 100 taining the RF transceivers via a high Sx3000 modem via SERDES in-

Signal Bandwidth (MHz) 0 Phased Array bandwidth I/Q interface. Within terface. each DBF, the ADCs and DACs are • Support for an external modem 0 40 80 120 160 200 Number of Elements connected to high-resolution digital with an L-Band interface. phase shifters and digital delay cir- • Very high speed beam tracking  Fig. 4 Maximum signal bandwidth vs. cuits which implement TTD to avoid and beam steering. number of elements in a uniform linear beam squints, enabling wideband • Linear and circular polarization array. signal transmission and reception. control. The DBF chips are connected to • Self-calibration with internal syn- each other via a high speed digital chronization engines. serial bus (SERDES), which enables • Antenna control integrated with a highly integrated, controllable the Sx3000 modem. and scalable antenna system. The • Power saving modes and con- key features of the Prime DBF are: figurations tailored to the appli- • Over 1 GHz instantaneous signal cation. bandwidth. • Multi-beam capability: up to 32 RF TRANSCEIVER beams with independent phase, A companion to the Prime DBF, gain and delay control for each Satixfy’s first-generation RFFE is

 Fig. 5 SatixFy Prime DBF ASIC. Quadrature Mixer Ip BB BB Input Gain RF 1 Beam, 2 Beams, Im Out V >1 GHz ≤ 880 MHz PA LPF Driver PA +/–90˚ Phase Shift TX LO LO*2 Power Distribution ∑ Splitter Circuit Quadrature LO Multiplier RF Out H PA 4 Beams, 32 Beams, Driver PA ≤ 440 MHz 55 MHz Qp BB BB Input Gain Qm Quadrature Mixer LPF

(a)  Fig. 6 Prime DBF capability: number of beams vs. bandwidth. Quadrature Mixer Ip BB BB Output Gain RF Im In V LNA2 LPF LNA3 LNA 1 +/–90˚ Phase Shift RX LO LO*2 Power Distribution Splitter ∑ Circuit Quadrature LO Multiplier RF In H LNA2 LNA 1 LNA3

Qp BB BB Output Gain Qm Quadrature Mixer LPF (b)

 Fig. 7 SatixFy Beat Ku-Band front-end.  Fig. 8 Block diagram of a single element, circularly polarized Tx (a) and Rx (b). TechnicalFeature

ESMA Beat 1 Sx3000 Prime I/Q Interface RF Interface I/Q PA Transceiver AFE1 LNA

Beat 2

I/Q PA Antenna SERDES SERDES AFE2 Modem and and DBF Transceiver Panel Control Control LNA

Beat 8

I/Q PA AFE8 Transceiver LNA

 Fig. 9 System architecture. a Ku-Band RFIC which links the is connected to the Beat RFFE via with an LNA and PA without up- or Prime’s I/Q signals with the Ku-Band an analog I/Q interface and to the down-converters. antenna elements (see Figure 7). Sx3000 modem via high speed The modular architecture of the Called Beat, the RFFE integrates the SERDES. This level of integration ESMA enables it to be scaled to transmit driver and power ampli- enables a highly configurable an- larger arrays by tiling. An example is fier, transmit up-converter, receive tenna supporting different applica- shown in Figure 10, where a single low noise amplifier, receive down- tions. Within this architecture, the tile of 32 antenna elements requires converter and antenna polarization DBF is band-agnostic, meaning to eight Beats and one Prime. The tiles control, either linear or circular (see build phased array antennas for dif- are daisy-chained via high speed Figure 8). A single Beat supports ferent satellite (Ku-, Ka- or X-Band) SERDES, which provides both data four Ku-Band antenna elements op- or 5G (sub-6 or 28 GHz) bands, only and the control plane to and from erating in half-duplex mode. the RFFE and antenna panel need the antenna controller. Figure 9 shows the block dia- to be modified. The backbone of SatixFy recently introduced the gram of a fully integrated ESMA the BFN remains the same, greatly world’s first fully digital 256-ele- system composed of the Prime DBF, simplifying antenna designs for dif- ment ESMA for Ku-Band SATCOM Beat RFFE and the antenna panel. ferent applications and frequency (see Figure 11 and Table 1). The The Prime DBF at the heart of the bands. For phased array antennas ESMA antenna can serve both as a electronically-steerable antenna at VHF and UHF, Prime can be used standalone IoT terminal or a build-

Tile 1 Tile 2 Tile N

Tx SERDES Tx SERDES Tx SERDES Prime Prime Prime Rx SERDES Rx SERDES Rx SERDES

Rx SERDES

Antenna Controller/ Modem Tx SERDES

 Fig. 10 Tiled ESMA to increase aperture size. TechnicalFeature

ing block for a larger array. The an- using appropriate waveforms,1-2 TABLE 1 tenna is a single board design with making it possible to communicate Ku-BAND ESMA a shared aperture antenna (Rx and at very low SNR. Tx), operating from 11 to 12 GHz Topology Tx/Rx Half Duplex Broadband Communications for TDD for Rx and 13.75 to 14.5 GHz for Land, Maritime and Aeronautical Tx. The 256-element ESMA com- # Beams Up to 32 Applications Simultaneous Beams prises eight Primes daisy-chained and 64 Ku-Band Beats. The anten- High capacity GEO networks Frequency Rx: 11 to 12 GHz and new constellations of LEO and Coverage Tx: 13.75 to14.5 GHz na can simultaneously point, track and manage multiple beams with MEO satellites will help serve the # Elements 256 multiple polarizations. Figure 12 demand for broadband access, # DBFs 8 Primes shows the antenna radiation pat- both for fixed terminals in remote # RFICs 64 Beats terns, measured in an anechoic areas and SOTM applications. Dur- chamber. ing the past decade, the demand RF Bandwidth 1 GHz for broadband connectivity and in- Channel 880 MHz APPLICATIONS flight entertainment on commercial Bandwidth The flexible ESMA architecture airlines has demonstrated the need Tx Antenna Gain 28 dBi enables low-cost, adaptive and for low drag and highly reliable an- Rx Antenna Gain 26.5 dBi steerable antenna system with low tenna systems, making a conformal antenna based on ESMA a good Modem Sx 3000-Based weight and power consumption. Modem This makes this system viable and solution. The simultaneous multi- attractive for various applications: beam capability enables simulta- Digital neous connectivity with multiple Interconnectivity 4 SerDes Lanes at 9.4 Gbps/Lane IoT satellites and make-before-break In rural areas, satellites can pro- connections to ensure seamless Terminal Self-Sufficient vide the missing coverage to con- connectivity—particularly when Functionality System, Single nect sensors and other entities to switching beams with LEO satel- Board Design, the cloud, such as sensors for ag- lites at high speed. These same Minimal External riculture, water metering, weath- benefits extend to land mobile Interfaces er, petrol and gas metering. The and maritime applications, where ESMA enables compact, low-cost ESMA based SATCOM links can and low-power IoT antennas that co-exist with terrestrial wide area can automatically search, acquire communications. The ESMA can and track satellites. Advantages be scaled according to the re- of the ESMA include eliminating quired link budget, physical size, bulky mechanical structures and weight and power consumption self-installation and tracking, which constraints of the platform. significantly reduce installation cost 5G Fixed Wireless Access and enables mobile applications on The jump in 5G data rates,  Fig. 11 Ku-Band 256-element ESMA. vehicles, ships, aircraft and drones. The small antenna size is feasible compared to 4G, relies on smart

0 0

–5 –5

–10 –10

–15 –15

–20 –20 Scan Angle = –60˚ Scan Angle = –30˚ –25 Scan Angle = –60˚ –25 Scan Angle = 0˚ Scan Angle = –30˚ Scan Angle = 30˚ Scan Angle = 60˚ Normalized Gain (dB) Scan Angle = 0˚ Normalized Gain (dB) –30 –30 cos1.3() Pattern Scan Angle = 30˚ Scan Angle = 60˚ –35 –35 cos1.4() Pattern

–40 –40 –90 –60 –30 0 30 60 90 –100 –80 –60 –40 –20 0 20 40 60 80 100 (a) Polar Angle (°) (b) Polar Angle (°)

 Fig. 12 Measured 256-element ESMA H-plane radiation patterns vs. scan angle: Tx at 13.75 GHz (a) and Rx at 11.7 GHz (b). TechnicalFeature

antennas with multiple, wide- up- and down-conversion, which is band, directive beams. ESMA’s the interface between the DBF and beamforming capability can in- the antenna element. The chipset crease spectrum utilization by up enables a flexible and scalable ar- to two orders of magnitude. The chitecture, with the resulting ESMA high precision phase shifters and achieving extremely small size, low TTD in the DBF makes it suitable power consumption and low-cost, for the both mmWave and sub-6 compared to other approaches. GHz arrays. The ESMA’s flexibility Products based on ESMA will sup- enables dynamically reconfiguring port a wide range of applications, the beams, combined with 1D and including SATCOM (GEO, LEO and 2D dual-polarized scanning for MEO) and 5G. both line-of-sight and non-line- of-sight channel conditions. With References TTD beamforming, high gain and 1. D. Rainish and A. Freedman, “VL- SNR Implementation for Mobile squint free antenna patterns can st be achieved across the entire cel- Broadband Application,” 21 Ka and Broadband Communications lular band. Conference, Bologna, Italy, Octo- ber 2015. SUMMARY 2. A. Freedman and D. Rainish, “Air This article introduced a scalable Interface for Low Power Operation ESMA with two building blocks: of a Satellite Terminal,” 21st Ka and a digital ASIC (Prime) with TTD, Broadband Communications Con- which performs the signal process- ference, Bologna, Italy, October ing and beamforming, and an RFFE 2015. containing the RF amplification and