Spread spectrum communications is presently being used for a number of commercial applications and is expected to proliferate as the demand for untethered communications increases.
A number of consortiums have been formed to develop satellite based Personal Communications Systems (PCS) with global coverage. Some examples of these systems include Globalstar (Globalstar System Application before the FCC by Loral Cellular Systems, Corp., Jun. 3, 1991) and Odyssey (Application of TRW Inc. before the FCC to Construct a New Communications Satellite System "Odyssey," May 31, 1991), among others. The intent of these systems is that a subscriber can place telephone calls directly through the satellite network from almost anywhere on the Earth, using a portable handset much like the present cellular telephones. Both of the systems mentioned intend to use spread spectrum CDMA techniques for a number of reasons.
The return link signal, as proposed in the above filings, is direct sequence (DS) CDMA spread spectrum. This type of signaling, while having some desirable characteristics, suffers from a number of disadvantages for satellite PCS application. Among these are the difficulty of rapid acquisition, the sensitivity of system capacity to power control error, and the Eb/No degradation due to access noise (these systems typically require Eb/No&gt;8 dB at BER=0.001 in order to achieve reasonable user capacities).