Channel Allocation Schemes - Spread Spectrum

Spread Spectrum

Spread spectrum can be considered as an alternative to complex DCA algorithms. Spread spectrum avoids cochannel interference between adjacent cells, since the probability that users in nearby cells use the same spreading code is insignificant. Thus the frequency channel allocation problem is relaxed in cellular networks based on a combination of Spread spectrum and FDMA, for example IS95 and 3G systems. Spread spectrum also facilitate that centrally controlled base stations dynamically borrow resources from each other depending on the traffic load, simply by increasing the maximum allowed number of simultaneous users in one cell (the maximum allowed interference level from the users in the cell), and decreasing it in an adjacent cell. Users in the overlap between the base station coverage area can be transferred between the cells (called cell-breathing), or the traffic can be regulated by admission control and traffic-shaping.

However, spread spectrum gives lower spectral efficiency than non-spread spectrum techniques, if the channel allocation in the latter case is optimized by a good DCA scheme. Especially OFDM modulation is an interesting alternative to spread spectrum because of its ability to combat multipath propagation for wideband channels without complex equalization. OFDM can be extended with OFDMA for uplink multiple access among users in the same cell. For avoidance of inter-cell interference, FDMA with DCA or DFS is once again of interest. One example of this concept is the above mentioned IEEE 802.11h standard. OFDM and OFDMA with DCA is often studied as an alternative for 4G wireless systems.