In TDMA, the time slots are available to the ground stations with full bandwidth. In FDMA, the frequency bands are available to the ground stations with continuous time availability. By combining these two multiple access techniques, a hybrid technique called Code Division Multiple Access (CDMA) can be formed. Frequency hopping can be used so that the frequency bands assigned to the users are reordered in an essentially random manner. Fig shows the frequency time structure of CDMA technique.
|Technique||Sharing of overall bandwidth of satellite transponder.||Sharing of time of the satellite transponder.||Sharing of bandwidth and time both.|
|Interference effects||Adjacent frequency bands interference. Generated because of nonlinearity of satellite transponder amplifier.||Interference between the user of adjacent time slots. Generated because of incorrect synchronization.||Performance is affected because of adjacent time and frequency slots. But due to frequency hopping the effect is less.|
|Synchronization||No synchronization is required.||Time synchronization is essential.||No synchronization is required.|
|Code word||No code word is required.||No code is required||Code words are required by the ground station.|
|Power efficiency||Power efficiency is reduced.||Full power efficiency is possible.||Full power efficiency is possible.|
|Guard times and bands||Guard bands are required.||Guard times are required.||Guard times and bands both are required.|
Thus during time slot 1, user 1 occupies, frequency band 1, user 2 occupies frequency band 2 and user 3 occupies frequency band 3. During second time slot, user 1 hops to frequency band 1 and user 3 hops to band 2.
Even though CDMA looks complicated, it has following advantages :
- CDMA doesnot require any external synchronization.
- As number of users are increased, the performance of Code Division Multiple Access CDMA declines gradually.
- The noise rejection capability of Code Division Multiple Access CDMA is better.
- See More : Space division multiple access
- See More : Direct sequence spread spectrum
- See More : Multiple access techniques