Robust or nonrobust: On MC-DS-CDMA acquisition in LEO satellite communications

Low Earth Orbit(LEO)satellite communications can provide global coverage in the sixth generation communication(6G)networks.To combat the strong Partial Band Interferences(PBIs)and multipath fading in LEO satellite communication systems,the Multicarrier Direct Sequence Code Division Multiple Access(MC-DS-CDMA)technique is a promising alternative to the traditional Single-carrier Direct Sequence Code Division Multiple Access(SC-DS-CDMA)system for its advantages of high bandwidth efficiency,superior interference rejection capability,and low complexity of parallel signal processing.However,limited studies have been conducted on the performance analysis of MC-DS-CDMA acquisition systems in the presence of a large Doppler shift,a unique characteristic of LEO satellite communications.To bridge this gap,we investigate the performance of MC-DS-CDMA systems with two-dimensional acquisition and noncoherent equal gain combining over Rician fading channel in the presence of the Doppler shift and PBIs.The performance metrics are detection probability and Mean Square Error(MSE)of the Doppler factor and delay.Specifically,we derive the closed-form expressions for the MSE and the Probability Density Function(PDF)of the acquisition decision variable and obtain the detection probability.We conduct extensive numerical experiments to verify the theoretical analysis and performance gain of MC-DSCDMA over the SC-DS-CDMA.The results show that MC-DS-CDMA with two-dimensional acquisition is more robust to multipath Rician fading than SC-DS-CDMA.Moreover,MC-DS-CDMA outperforms SC-DS-CDMA regarding the detection probability and MSE when combating the strong PBIs.

Multipair two-way massive MIMO AF relaying with ZFR/ZFT and hardware impairments over high-altitude platforms

HAPs(High-Altitude Platforms)are one of the most promising alternative infrastructures for providing wireless communications to overcome the shortcomings of terrestrial-tower-based and satellite systems.This paper investigates a multipair two-way MM-AF(Massive MIMO Amplify-and-Forward)relay system over HAPs,where multiple user pairs exchange information within a pair through a relay with a very large number of antennas,and a HAP channel is modeled to follow a Rician fading distribution because of the presence of a line-of-sight path.First,the effect of hardware impairments on a multipair two-way MM-AF system is taken into consideration and is modeled using transmit and receive distortion noises.Then,ZFR/ZFT(Zero-Forcing Reception/Zero-Forcing Transmission)processing matrices of HDR(Half-Duplex Relaying)and FDR(Full-Duplex Relaying)with imperfect channel state information are presented.Finally,the asymptotic expressions of an end-to-end SINR are derived.Theoretical analyses and simulation results show that when the number of relay antennas grows very large,the SE(Spectral Efficiency)of an MM-AF relay system is limited by hardware impairments at the users instead of those at the relay,or by other types of interference.Thus,the quality of the hardware at a relay can be decreased without significantly degrading performance.