his gives two advant- Microcells 48/11 ages: the exposure to interference on any one frequency is reduced, properly. This is achieved by transmitting a fixed bit pattern (the and the possibility of being in a fading null is eliminated. Training Sequence) in the middle of each time slot. Knowing this The benefits of frequency hopping are mainly noticed on the sequence, the receiver can estimate the transfer function in the time downlink, as the uplink will normally have space diversity which domain between the transmitter and receiver by means of a correla- already provides a similar effect. Frequency hopping on its own tion process implemented in a Viterbi equaliser. may improve the path by 3dB, but if space diversity is already in use the nett improvement may be as low as 1dB. There are two different methods of implementing frequency hop- ping at the base station. The most straightforward is to provide a 48.9.3.7 Handover separate transceiver for each frequency in the hopping sequence (6 is about the minimum to obtain any worthwhile benefit). Then each Handovers are used to maintain the link to the MS as it moves from user's voice frames will be sent successively on different trans- cell to cell. The algorithms used in PCN networks are similar to ceivers. The disadvantage of this method is that in the PCN environ- GSM, but are tailored to the small cell environment. The two basic ment only 2 or 3 transceivers are typically necessary to support a methods used are: cell's traffic, and to provide 6 represents a major investment when applied across the whole network. 1. The 'Most Suitable Connection' method, where the mobile is PCN networks will tend to use a more difficult approach, which always connected to the BS which provides the best path is to actually switch the frequency of the transceiver between budget. successive timeslots. This fast switching speed (less than 30us) is 2. The 'Minimum Acceptable Performance' method where the difficult to achieve, but allows hopping to be implemented with the MS is only h