CDMA for communications in the aeronautical environment

MITRE's Center for Advanced Aviation System Development (CAASD) is involved in planning a next-generation voice/data network for the FAA's Air-Ground/Ground-Air communication system. This system has some unique operational requirements, which make the comparison between TDMA and CDMA a difficult one (as it also is in current cellular radio systems). This work, in support of CAASD, attempts to outline the key attributes of CDMA communication systems, specifically direct-sequence spread spectrum systems. A brief comparison with more traditional multiple access schemes (TDMA and FDMA), is made and then direct-sequence CDMA modulation and demodulation for a simple transmitter and receiver, in a single "cell" environment, is described. The CDMA waveform description yields expressions which allow estimation of system capacity-the number of simultaneously active channels the system can support. Non-idealities such as additional (interfering) cells and imperfect power control are also considered. Finally, TDMA capacity expressions are reviewed and some comparative estimated capacity results for our VHF Air-Ground/Ground-Air application are shown, using waveforms based upon terrestrial cellular schemes.