re 10.12 Interference to a neighbouring cell by mobiles reusing the same carrier frequency telecommunications services to its subscribers. This allocated spectrum is a precious resource, for which an operator may need to spend billions of dollars. Frequency usage efficiency is very important. For example assume a 10+10 MHz of spectrum (downlink and uplink) is allocated to a typ- ical operator to operate a 1G mobile telecommunications system. Further assume each voice channel requires 25~30 kHz to provide an acceptable quality, and therefore only 350~400 channels are possible in a 10 MHz spectrum. These channels have to be reusable if a large portion of the population of a region is to be served. If two mobile phones were simultaneously to use the same uplink channels (i.e. the same car- rier frequencies) for the purpose of conducting a voice call to a base station on a mobile network, the radio signals would interfere with each other. Therefore the same frequency carrier may not be used in the same cell. Similarly the same carrier frequency may not be used in a neighbouring cell as the level of interference may still be too large. An example is shown in Figure 10.12 where two users at cell borders communicate with one base station while interfering with the other. This problem was especially acute in the 1G standards which used analogue frequency modulation radio technology for communications between mobiles and base stations. Interference in these systems manifested itself as crosstalk: a conversation could be overheard by anyone using the same carrier frequency. This meant that 1G analogue mobile telecommunications systems had to be designed to ensure that interference between users was below a designed level. It follows that a certain distance needs to be kept between two base stations which use the same carrier frequency. In the 1G, some 2G and to some extent 4G standards each cell is allo- cated a subset of all available carriers. A neighbouring cell is provided with a different (and mutually exclusive) set 