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Chapter 7: Detection Strategies and Cancellation Schemes In A MC-CDMA System

7.3 Multiuser Interface In CDMA Systems

In contrast to FDMA and TDMA techniques which are frequency bandwidth limited, CDMA systems are interference limited. In CDMA systems, each user's data is spread by a unique pseudorandom code. All users then transmit in the same frequency band and are distinguished at the receiver by the user specific spreading code. All other signals are notdespread because they use different codes. These signals appear as interference to the desired user because of non-zero cross-correlation values between the spreading codes. As the number of users increases, the signal to interference ratio (SIR) decreases until the resulting performance is no longer acceptable: Thus, this multi-user interference must be reduced to achieve higher capacities and that is strictly connected to the cross-correlation factor between different users. We can reduce cross-correlation in spread spectrum systems by:

• Spreading the signal by orthogonal codes which have zero cross-correlation. This technique is very efficient in downlink transmission, because a base station can transmit to all users simultaneously and the signals are spread synchronously at chip level. Transmitting asynchronously in the uplink, to restore the orthogonality of the codes, the mobile users can be time-aligned by a synchronisation method.
• Cancellation schemes that usually work subtracting the interference caused by other users and require a significant processing power; they are very useful especially to solve near-far problems.

More in general other important techniques like power control schemes, voice activity detection or antenna sectorization are used to minimise the performance degradation caused by the total co-channel interference (intra and inter-cell) from other users.

7.4 Synchronisation In a System

Synchronisation is one of the most determinant parameters to reduce multi-user interference. The main issue of synchronous MC-CDMA is timing. For the downlink, timing is not a problem because there is only one station that generates the codes [7]. The uplink is much more complicated, because different transmitters have different clock offsets and propagation delays, so uplink synchronisation seems to be difficult to establish when we face a fading multipath mobile radio channel. In an ideal radio channel the perfectly orthogonal spreading codes maintain orthogonality only if the codes remain strictly synchronised. Due to multipath propagation the orthogonality is partly lost; the delayed multipath signals are clearly not synchronised to the first arriving path and besides the dominant components are time-shifted for the different propagation delays. Because some synchronicity error exists, special orthogonal codes with small cross-correlation must be used. A great advantage of MC-CDMA is its relatively long symbol duration that could allow to establish a quasi-synchronous channel state.

The synchronisation offsets should be a small fraction of the chip period if we want to minimise multi-user interference by orthogonal signals. Increasing the chip period would make it easier to synchronise the signals. However, for a given processing gain, an increase in the chip period results in an increase in the symbol period, hence a decrease in the data rate; the solution for such a problem is the adoption of a multicarrier signalling scheme able to maintain the original data rate. Increasing the chip period by a factor M, the number of subcarriers, reduces the bandwidth of each of the subband signals by a factor M relative to the bandwidth of the original spread spectrum signal so that the overall bandwidth is approximately the same as that of the single-carrier signal...