Abstract
Full-dimension MIMO (FD-MIMO) using planar active antenna systems (AAS) is considered a critical technology for fifth-generation (5G) cellular systems to improve network capacity. An AAS is typically subject to hardware impairments that negatively impact network capacity. Hence, this article focuses on impairments that cause phase and magnitude errors between radio frequency (RF) chains and shows why they are particularly difficult to avoid in practical AAS. Although previous investigations show these impairments to degrade performance, they are not useful in deriving measurable impairment margins for practical FD-MIMO deployments. Knowing impairment limits are critical for system designers to make hardware design tradeoffs such as AAS configuration, component selection, implementation complexity, and cost. Moreover, it also helps set conformance limits for critical lab verification. Therefore, the paper first investigates the impact of the impairments and derives their practical limits for FD-MIMO by explicitly considering the cumulative effects of the channel model, inter-cell interference, link adaptation, and channel aging due to feedback delays. It is shown that a lower number of digitized RF chains can be a better choice under lower impairments. Next, the sources of impairments are investigated by using measurements carried out in the lab and the field during live operation in a commercial LTE network. Phase drift from local oscillators (LO) and internal temperature variations are identified as two significant sources. The tradeoffs and shortcomings of some of the existing solutions in massive MIMO literature are discussed. Finally, in order to address the shortcomings, a novel and practical coherent LO distribution architecture and array calibration mechanism are proposed. This solution is shown to be applicable to both TDD and FDD FD-MIMO. Measurement results are provided to prove the high degree of coherency and stability achieved on a unique array architecture called high definition active antenna system (HDAAS).
Highlights
M ASSIVE multiple-input and multiple-output (MIMO) using large active antenna arrays are viewed as a critical solution to solve capacity demands in Fifth Generation New Radio (5G-NR) cellular networks operating in sub6 GHz bands
EFFECT OF PHASE AND MAGNITUDE ERRORS ON Full-dimension MIMO (FD-MIMO) PERFORMANCE we present an analysis of the impact of phase and magnitude errors on the FD-MIMO system’s performance with different hybrid beamforming-based antenna systems (AAS) configurations
In the case of the FD-MIMO system proposed in 3rd Generation Partnership Project (3GPP) standards, this requires overcoming non-ideal behavior due to radio frequency (RF) impairments
Summary
M ASSIVE multiple-input and multiple-output (MIMO) using large active antenna arrays are viewed as a critical solution to solve capacity demands in Fifth Generation New Radio (5G-NR) cellular networks operating in sub GHz bands. From a practical AAS design point of view [3], some of the key challenges include: routing a large number of high-speed digital I/O and corporate feeds; high-speed digital processing required to support digital front end (DFE) and lower-level physical layer processing [4]; power consumed by the analog front end (AFE), data converters, digital processors and high speed I/O; and thermal design to control heat dissipation Operating such a complex system under space and thermal constraints subjects it to non-deal behavior from radio frequency (RF) components that degrades performance. A careful analysis of existing methods is required to understand their drawbacks and devise a practical scheme Such a calibration scheme should ideally apply to both TDD and FDD based FD-MIMO in addition to solving transmission-related challenges and meeting error tolerance limits under the worst-case scenario.
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