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From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband by Martin Sauter

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5.4 Physical Layer Framing

The structure of a PHY frame in point-to-multipoint operation depends on the duplex mode used in the network.

5.4.1 Frame Structure in FDD Mode for Point-to-Multipoint Networks

In licensed bands, operators usually deploy FDD base stations, where data in the uplink and downlink directions are transmitted on different frequencies as shown on the right side of Figure 5.2. While the base station can always send and receive data on the two frequency bands simultaneously, SSs can only be full- or half-duplex. While full-duplex devices are slightly more expensive owing to the independent transmission and reception chains, they are able to support the highest possible transmission rate in both directions simultaneously. Half-duplex devices, on the other hand, cannot benefit directly from FDD, as they have to stop sending data to be able to receive new data from the network. This is problematic if a device has a lot of data to send and receive. In this case, the theoretical bandwidth of a cell that only serves a single subscriber is cut in half, as 50% of the downlink time and 50% of the uplink time cannot be used by the SS. Most applications, like web browsing, are asymmetric and subscribers usually receive more data than they transmit. In these cases, half-duplex devices are not at such a big disadvantage as they mostly receive data and only rarely switch into transfer mode.

In most scenarios, more than one subscriber is served by a cell. Thus, a network still ...

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