Yuxi Chen, Muhammad Iqbal Rochman, Kyler McCormack, Jakob Kotz, Monisha Ghosh
原文摘要
Direct-to-cell (DtC) satellites serve unmodified LTE phones today from low Earth orbit, where the satellite's motion creates Doppler shifts of tens of kHz. Since large offsets such as these cannot be compensated adequately by the user equipment (UE), the satellite pre-compensates the Doppler shift of each beam, but it can do so for only one reference point in the beam. A UE elsewhere in the beam still has to compensate a residual offset; previous field studies report residuals of up to 10 kHz, beyond the $\pm$7.5 kHz that the 15 kHz subcarrier spacing can resolve. In this paper we corroborate this effect with our own detailed measurements of Starlink transmissions and further correlate ineffective handover performance to inadequate residual offset compensation. We analyze the Doppler compensation problem with detailed standard-specific simulations and propose a compensation method at the UE, based on the location of the UE with respect to the beam reference point used for the Doppler compensation and the known satellite location, that improves the efficiency of handover. With UE cell-specific pre-compensation, the neighbors are identified, read correctly and handed over to in time rather than lost to radio-link failure.
Downlink (DL) precoding in mid-band MIMO relies on uplink (UL) sounding reference signals (SRS) with reciprocity-based singular value decomposition (SVD) or channel state information reference signals (CSI-RS) with codebook-based precoding matrix indicator (PMI) feedback. Although SVD precoding offers unquantized beamforming, it depends on a channel estimate from SRS transmitted at the limited user equipment (UE) power, whereas Type-I PMI is quantized but derived from a high-power CSI-RS based channel estimate. In this paper, we compare SRS-SVD and CSIRS-PMI for single-user MIMO through an end-to-end, 3GPP-compliant link-level simulation at 3.75 GHz with 100 MHz bandwidth, using a 192 antenna element gNB mapped into 32 ports, the 3GPP Release 19 UE antenna model, and ray-traced (RT) channels in Manhattan and Chicago. We show that neither scheme is uniformly superior: CSIRS-PMI serves weak and shadowed users better and reduces outage, whereas SRS-SVD achieves higher spectral efficiency (SE) and enables more multi-layer transmission for users with strong channels. The results on the Clustered Delay Line (CDL)-C statistical channel also confirm this crossover. We further show that by utilizing a simple logistic regression based on the received power of the standard CSI reference signal (CSI-RSRP) and the channel quality indicator (CQI), the gNB can select the optimal transmission scheme with around 90% accuracy, achieving an SE that closely approaches an ideal per-user selection.