Читать книгу Wireless Connectivity - Petar Popovski - Страница 23

The conclusion from the previous subsection is that a robust system operation requires some of the physical communication resources to be invested into transmission of metadata or control information. In this way, the base station can regularly inform the mobile devices about the actual allocation of the data in the logical channels.

Figure 1.6 Introduction of a header in the TDMA frame. (a) Periodic TDMA system with full occupancy of the channel. (b) The header brings flexibility: the central controller (Basil) can decide to start the frame at an arbitrary time after the previous frame is finished.

A step towards achieving such flexibility is the introduction of a frame header of duration , as illustrated in Figure 1.6(a). In the simplest form of circuit-switched operation, the channel allocation in each TDMA frame remains fixed, such that the frame header is only required to mark the start of the frame, and not carry information about the allocation. Even so, the introduction of a frame header that carries only information “This is the frame start”, introduces additional flexibility, as depicted in Figure 1.6(b). In the example, there are served in a frame, each of them getting an equal share of the resources, such that the total frame duration is . The allocation is still circuit-switched, but now a given terminal does not locate its communication resource (slot) in terms of absolute time, but rather the relative time, measured with respect to the frame header: for example, Yoshi receives the information sent in the second slot after the frame header. Basil can now decide when to start a frame and actually leave some blank inter-frame space. This is very important, since shared communication channel in the inter-frame space can be used for other purposes, such as link establishment, as it will be readily seen.

The frame header can further be enhanced in order to support time division between downlink and uplink traffic. For example, the frame headers in Figure 1.6 can contain information “This is a downlink frame”, such that Zoya knows that she should receive her data during the slot allocated to her. The reasoning for the uplink is analogous. Besides marking the frame start, now the header contains an additional, single bit of information to announce whether the frame is intended for downlink or uplink, respectively. Based on that, Zoya knows whether to receive or transmit during the slot that is allocated to her. Now the system can flexibly allocate resource for communication in both directions (uplink/downlink), such that the system operates with a flexible time-division duplex (TDD) mode.