Cyclic code phase multiple access for inbound satellite communications

Abstract

A satellite communication system comprising of a central hub/gateway station and remote very small aperture terminal (VSAT) stations. VSAT inbound transmissions are spread in bandwidth using an assigned PN code sequence. This sequence is derived as a cyclic function of a base sequence for use by the group of VSATs. The gateway transmits a continuous outbound signal that functions as the network frequency and timing reference. In addition, the gateway periodically forms timing and frequency error estimates on each VSAT transmission and broadcasts these results over the outbound channel to the VSATs. Use of the outbound reference coupled with these time and frequency measurements enable all VSAT inbound transmissions within a group to be bit synchronized. Bit level synchronization enable the system to gain the full advantage of the correlation properties of the cyclic codes to improve multiple access performance as compared to asynchronous or chip synchronous code division multiple access (CDMA) techniques. N active users can be supported with an N length code in this system even for small values of N (N.ltoreq.15). The synchronized, cyclic relationship of the inbound transmissions enables the gateway to receive and demodulate a group of channels using a single receiver. The gateway station includes a correlator receiver which is synchronized with the gateways outbound network reference and which contains all the cyclic PN code sequences to detect correlations between the pseudo-randomly modulated data bits corresponding to the cyclic PN sequence bits of the VSAT stations.