This letter rigorously derives explicit closed-form expressions for the joint/marginal probability density functions of the uplink/downlink multipaths' time-of-arrival (TOA) and azimuth angle-of-arrival (AOA) in a wireless-communication fading channel. This derivation is based on a new "geometrical model" of omnidirectional scatterers as spatially distributed uniformly on a two-dimensional hollow-disc (i.e., a thick ring) centered upon the mobile. By varying the hollow-disc's thickness, this spatial density degenerates to the well-known uniform-ring or uniform-disc densities.Index Terms-Dispersive channels, fading channels, land mobile radio cellular systems, land mobile radio diversity systems, land mobile radio equipment, mobile communication, radio communication equipment.
This paper proposes a new digital predistortion (DPD) scheme for linearizing millimeterwave hybrid beamforming transmitters using observation receivers with low-bit resolution analog-to-digital converters (ADCs). To train the DPD function required to compensate for the distortions exhibited by a given sub-array (also called main sub-array), an error signal is produced by out-of-phase combining the main subarray transmitted signal and the one generated by another sub-array (also called auxiliary sub-array) using anti-beamforming modules. The error signal is then frequency down-converted and digitized using a lowbit resolution ADC. Proof-of-concept validation experiments are conducted by applying the proposed DPD system to linearize an off-the-shelf hybrid-beamforming array comprised of four 64-element sub-arrays, operating at 28 GHz and driven with up to 800 MHz orthogonal frequency-division multiplexing modulated signals. Using the proposed DPD scheme, an observation receiver with a 4-bit ADC was sufficient to improve the adjacent channel power ratio by 10 dB and the error vector magnitude was reduced from 5.8% to 1.6%. These results are similar to those obtained using an observation receiver with 16-bit ADC.
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