Hybrid Spread Spectrum Techniques
In addition to the frequency hopped and direct sequence, spread spectrum multiple access techniques, there are certain other hybrid combinations that provide certain advantages. These hybrid techniques are described below
Hybrid FDMA/CDMA (FCDMA)
- This technique can be used as an alternative to the DS-CDMA techniques
- Figure 7.12 shows the spectrum of this hybrid scheme.
- The available wideband spectrum is divided into a number of subspectras with smaller bandwidths
Figure 7.12 Spectrum of wideband CDMA compared to the spectrum of a hybrid, frequency division, direct sequence multiple access technique
- Each of these smaller sub channels becomes a narrowband CDMA system having processing gain lower than the original CDMA system
- This hybrid system has an advantage in that the required bandwidth need not be contiguous and different users can be allotted different sub spectrum bandwidths depending on their requirements
- The capacity of this FDMA/CDMA technique is calculated as the sum of the capacities of a system operating in the subspectra
Hybrid Direct Sequence/Frequency Hopped Multiple Access (DS/FHMA)
- This technique consists of a direct sequence modulated signal whose center frequency is made to hop periodically in a pseudorandom fashion.
- Figure 7.13 shows the frequency spectrum of such a signal
- Direct sequence, frequency hopped systems have an advantage in that they avoid the near-far effect.
- Frequency hopped CDMA systems are not adaptable to the soft handoff process since it is difficult to synchronize the frequency hopped base station receiver to the multiple hopped signals.
Figure 7.13 Frequency spectrum of a hybrid FH/DS system.
Time Division CDMA (TCDMA):
- In a TCDMA (also called TDMA/ CDMA) system, different spreading codes are assigned to different cells.
- Within each cell, only one user per cell is allotted a particular time slot
- Thus at any time, only one CDMA user is transmitting in each cell
- When a handoff takes place, the spreading code of the user is changed to that of the new cell
- TCDMA has an advantage in that it avoids the near-far effect since only one usertransmits at a time within a cell.\
Time Division Frequency Hopping (TDFH):
- This multiple access technique has an advantage in severe multipath or when severe co-channel interference occurs.
- The subscriber can hop to a new frequency at the start of a new TDMA frame, thus avoiding a severe fade or erasure event on a particular channel.
- This technique has been adopted for the GSM standard, where the hopping sequence is predefined and the subscriber is allowed to hop only on certain frequencies which are assigned to a cell.
- This scheme also avoids co-channel interference problems between neighboring cells if two interfering base station transmitters are made to transmit on different frequencies at different times.
- The use of TDFH can increase the capacity of GSM by several folds