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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

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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 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. Reprinted with permission of MICROWAVE JOURNAL® from the March 2019 issue. ©2019 Horizon House Publications, Inc. TechnicalFeature 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 ele- ments; 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<<Ts, where TS is the duration
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