The duplex spacing keeps locally generated transmit power out of the receive path in the user's device. In a TDD system the function of duplex spacing is performed in the time domain by ensuring that transmit time slots do not overlap receive time slots. The function of the duplex gap can only be achieved in the time domain if all networks are synchronised together and if all base stations are CO sited. Specifically, the frame structure configuration needs to be coordinated between adjacent cells to avoid simultaneous TX and RX on the same frequency at the same time. Table 4.4 Traditional FDD duplex band plan Other Guard Operational Duplex gap Operational Guard Other users bandwidth bandwidth users lower duplex upper duplex Duplex spacing Telecommunications Economies of Scale An argument can be made that a time division duplexed (TDD) radio layer is more efficient at handling mobile broadband data traffic than present FDD networks. In terms of traffic handling a TDD radio layer can be made to be asymmetric as required, SO it is potentially equivalent to an ADSL rather than VDSL connection. As the radio channel is reciprocal (the same frequency is used on the uplink and downlink) it is easier to characterise the channel. This in turn makes it easier to extract some peak data rate gain from multiple antenna (MIMO) systems (covered in more detail in Chapter 5). It is also easier to adaptively noise match the receive chain and power match the transmit chain. At the radio planning level there is no need for a duplex gap. This releases additional spectrum. However, there are some practical problems with TDD. Even if internetwork syn- chronisation is achieved, users in different networks may have varying asymmetry, which means that one set of users TX slots overlap another set of users receive slots. The assumption here is that there is generally enough physical distance between users to mitigate any interfer- ence effects. If interference does occur it is managed by handing over to other time slots or channels. Both Wi