 a significant advantage there is a downside to the process. The first is that it reduces the system capacity. The reason for this is simply that it requires two channels or codes, one for each base station. It is estimated that this can reduce system capacity by as much as 40 per cent, but this is dependent upon many factors including the duration of the handover, the size of the cells, and the level of overlap between the cells. There are disadvantages for the mobile as well. It requires two receivers or demodulators to be able to monitor the two cells. While this adds complexity to the mobile, it can be overcome by integrating the additional functions into the main IC, and therefore the cost is relatively small. Despite these disadvantages, the advantages of a much smoother and more reliable handover are worth the additional costs. Although one of the main advantages claimed for CDMA is a vast improvement in the efficiency of the use of the spectrum, this parameter is very difficult to calculate in real terms. This is because it depends to a large extent on a variety of aspects including the level of interference from outside the cell, the cell density and such like. Even SO some useful comparisons can be made with systems such as GSM. It is accepted that 15 voice channels can be placed on each CDMA carrier, and as each carrier is 1.25 MHz wide (1.2288 MHz in exact terms), this represents a spectrum usage figure of 12 voice calls per megahertz. GSM can carry eight voice channels per carrier and there are five carriers per megahertz. This appears to provide a usage of 40 calls per megahertz. However, this is not the full story because neighbouring cells have to use different frequencies. With a cluster size of 12, i.e. 12 cells in a cluster, this figure reduces to 3.3 calls per megahertz, although when the frequency hopping facility of GSM is employed, cluster sizes of seven can be used and the frequency usage rises to six calls per megahertz. When comparing the two systems, other factors such as handover, whic