Читать книгу Wireless Connectivity - Petar Popovski - Страница 29

Our half-duplex model assumes ideal switching between transmit and receive state, without causing any additional inefficiency in the system. As mentioned before, achieving two-way communication by switching in time between a transmit and a receive state is known as TDD. In practical TDD wireless systems, switching between transmission and reception is not without cost, as it takes a certain turnaround time. Hence, one should avoid having protocols that require from a half-duplex device to switch frequently between transmission and reception. For our discussion, this implies that it is not desirable to have the situation from Figure 1.8(a), where a device needs to switch multiple times from receiving the header, transmitting the reservation packet, receiving the allocation and, potentially, be the first transmitter in the data slots. A possible solution is shown in Figure 1.8(b), where the reservation slot is used to reserve data slot in the next frame or another frame in the future, instead of the current one, thus achieving some form of pipelining.

If the wireless devices have the capability for full-duplex transmission/reception, then there is no need to use TDD and separate the uplink and the downlink frame in time. Figure 1.9 shows a possible configuration of a TDMA frame in which the downlink and the uplink transmission take place simultaneously. The gain, usually claimed from full-duplex, is the doubling of the overall data rate or the throughput, as the same communication resource can be simultaneously used twice. This figure illustrates the gain that full duplex brings in terms of latency, which is the time since the packet in a terminal is ready to be transmitted until the time the terminal gets the opportunity to actually transmit it over the air. The header H-U announces the allocation of the uplink slots that follow it, such that the user allocated to slot 1 can start to send immediately after receiving H-U. By contrast, in the previous example of TDD with half-duplex transceivers, the first slot available for transmission is slot 0 in the next frame.