5G Uniform Linear Arrays With Beamforming and Spatial Multiplexing at 28, 37, 64, and 71 GHz for Outdoor Urban Communication: A Two-Level Approach
Summary (2 min read)
Introduction
- Abstract—Multiple-input multiple-output (MIMO) spatial multiplexing and beamforming are regarded as key technology enablers for the fifth-generation (5G) millimeter wave mobile radio services.
- Hybrid beamforming can be employed by having a group of elements connected to one RF chain or an array of sub-arrays, where each sub-array has several interconnected antenna elements but its own RF chain.
- Phase shifts are digitally controlled by realizing discrete phase shifts which causes quantization phase error leading to formation of quantization lobes, which occur at the grating lobe angles during beam steering.
A. Current Limitations in SU-MIMO
- In the case of SU-MIMO, more than one spatial stream is exchanged between two arrays.
- The antenna spacing and array orientation can significantly affect the system performance [19], [20].
- Employing a specific hybrid precoding technique or comparison of the existing hybrid precoding techniques such as for optimizing the hardware resources in mmWave MIMO is not the focus of this paper.
- Rather, their paper provides an alternative approach to the Rayleigh distance criterion that enables SM for SU-MIMO in LOS mmWave channels.
- The existing 3GPP channel models [1][45] do not include the effects of directional local scattering at the Tx and Rx in an outdoor urban environment for mmWave propagation, yet real-world measurements in New York City show the existence of directional propagation in urban environments, leading to the formation of SLs [2], [46], [47].
B. System-Level Architecture
- In order to meet the contradictory requirements of beamforming which requires co-polarized closely spaced antenna elements typically at 𝜆𝜆 2 with high coherence, and SM which requires no coherence between antenna elements to ensure simultaneous separate parallel data streams [19], a twolevel (2L) hybrid beamforming architecture is proposed as illustrated in Fig.
- Under low SNR conditions when employing beamforming, the same architecture can be configured with the sub-arrays resulting in formation of a larger array, which the authors call “super-array” in this paper.
- MMWAVE CHANNEL MODEL A. 3D Statistical Spatial Channel Model.
- With an increase in the number of sub-array elements, the azimuth HPBW becomes less than 6.3° (±3.3°) and is taken as 7° which is the lower limit in NYUSIM v1.5.
- The number of independent non-zero rows and columns depends on the amount of scattering, reflection and the length 𝐿𝐿𝑦𝑦 of the Tx and Rx arrays.
C. MIMO Channel Matrix Condition Number
- The channel condition number and its statistical properties are important for characterization of the MIMO channel.
- The correlation between the channel paths increases if angular separation of the channel paths decreases.
- A low channel condition number usually corresponds to a high rank and vice versa; the matrix has full rank (the highest rank) when the channel condition number is equal or close to 0 dB (the lowest theoretical condition number).
- The channel condition number is an important design parameter in MIMO systems as it has been shown to drastically affect the detection, error and performance of linear Rxs in MIMO systems.
- Performance of linear detectors such as zero-forcing (ZF), maximum-likelihood (ML) and minimum mean square error (MMSE) detectors has been investigated indicating strong dependence of the detector performance on the channel condition number [64]-[70].
A. Sub-Array Antenna Element
- The azimuth and elevation sub-array radiation pattern would depend on 𝑀𝑀 in each sub-array and their individual element radiation pattern (𝐸𝐸(𝜃𝜃)) .
- Both circular and rectangular patches have similar gain, beam position and efficiency.
- A circular pin-fed antenna patch is employed in this paper and its design parameters are given in Fig. 5 [80], where 𝐷𝐷, 𝑆𝑆𝜋𝜋 , 𝑅𝑅, 𝐻𝐻, 𝜖𝜖𝑟𝑟 and 𝑡𝑡𝑡𝑡𝑚𝑚𝑡𝑡 are the patch diameter, feed offset, feed pin radius, substrate height, relative permittivity and loss tangent due to the substrate medium respectively.
- The dominant mode is 𝑇𝑇𝑀𝑀11 and the radiation pattern is a single lobe with maximum in the direction normal to the plane of the antenna.
B. Super-Array Element Spacing and Grating Lobes
- 𝐵𝐵𝑚𝑚𝑡𝑡𝑒𝑒 𝑗𝑗2𝜋𝜋𝜆𝜆 𝑑𝑑 𝑚𝑚𝑚𝑚𝑚𝑚𝛿𝛿𝑁𝑁𝑇𝑇𝑚𝑚𝑡𝑡=1 (7) where ∑ 𝐵𝐵𝑚𝑚𝑡𝑡𝑒𝑒 𝑗𝑗2𝜋𝜋𝜆𝜆 𝑑𝑑 𝑚𝑚𝑚𝑚𝑚𝑚𝛿𝛿𝑁𝑁𝑇𝑇𝑚𝑚𝑡𝑡=1 is the array factor 𝐴𝐴𝐴𝐴𝑆𝑆𝐴𝐴for the super-array.
- 𝐴𝐴𝐴𝐴𝑆𝑆𝑆𝑆𝑆𝑆𝐴𝐴)𝐴𝐴𝐴𝐴𝑆𝑆𝐴𝐴 (8) The array pattern 𝐴𝐴(𝜃𝜃) of 𝐶𝐶𝑀𝑀4 super-array with two subarrays and elements with radiation pattern 𝐸𝐸(𝜃𝜃) obtained is plotted in Fig. 6 with MATLAB R2016a® where the subarray separation is 𝑑𝑑 = 2𝜆𝜆 .
- The elevation HPBW remains constant and is taken as 45° that is the upper limit in NYUSIM v1.5.
- As the number of MIMO channels increases for the same lateral separation of the sub-arrays, the channel condition number increases indicating unfavourable propagation conditions for MIMO SM.
A. Channel Condition Number
- With a view to improve the channel condition number for 𝐶𝐶𝑀𝑀4 in Fig. 7, 𝑑𝑑 is increased from 2𝜆𝜆 to 4𝜆𝜆 for the 5×5 MIMO 28 GHz channel.
- The channel condition number decreases for super-array with a higher number of elements as this increases the sub-array spacing indicating favourable MIMO propagation even for further increases in number of 𝑁𝑁𝑅𝑅 × 𝑁𝑁𝑇𝑇 elements at L2.
- A. Poon and M. Taghivand, “Supporting and enabling circuits for antenna arrays in wireless communications,” Proc. IEEE, vol.
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Cites background from "5G Uniform Linear Arrays With Beamf..."
...At the cell edge, the boresight array gain is the key factor for coverage, as sidelobes off of the main beam will be far below the boresight power levels [49]....
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