Interference mitigation in downlink multi-beam LEO satellite systems using DS-CDMA/CI

The interference from adjacent beams of a low earth orbiting (LEO) satellite degrades the performance of users in area illuminated by desired beam. This paper investigates the advantages of employing Carrier Interferometry (CI) codes in downlink of a DS-CDMA based LEO satellite link to mitigate such interference. Qualitatively, use of these codes in a downlink LEO satellite channel offers following advantages: (1) Improved BER performance in interference dominant satellite channel resulting in improved capacity; (2) Significant reduction in bit error rate (BER) floor; (3) Lesser variation in performance with changes in number of active users; (4) Uniform cross-correlation characteristics over code space for uniform quality of service; and (5) Orthogonal CI codes with arbitrary integer spreading length can be generated. Quantitatively, these codes provide improvement in capacity by at least 50 percent at 15 dB and 20 degrees elevation, which further improves with increase in elevation angle. Moreover, the achievable BER floor is at least as low as that provided by 10 degree elevation gain in traditional codes. To obtain realistic results, Loo's statistical model for land mobile satellite (LMS) channel is assumed and the channel parameters for this model were taken from linear regression fits of experimental measurement results of real-world LMS channel.