Petar Popovski - Wireless Connectivity

Circuit-switched operation is useful in minimizing signaling whenever it is known in advance which resources are required over a certain period. In practice, TDMA allocation can be more complex than what is described above. Let the demanded data rates be: for Zoya, for Yoshi and for Xia. An example of a TDMA allocation that can satisfy these data rate demands is depicted in Figure 1.5(c).

In real systems, even in the case of static, circuit-switched allocation, it is unrealistic to assume that the logical channels and frames will stay ideally allocated for an indefinite period. For example, there might be a period of time in which Basil has no data to send to Zoya. If Zoya does not receive anything within several consecutive frames, she might easily get out of synchronization with Basil, which would result in irrecoverable errors. If the internal clocks of Zoya and Yoshi have a large relative drift, then Zoya might start to receive the data for Yoshi, not knowing that it is not intended for her. This cannot be prevented in the described simple TDMA scheme, since no resources are spent in sending control information after the initial, circuit-switched allocation. This control information would be used to describe what kind of data is sent in a particular slot. Therefore the periodic frame structure with fixed allocations to the users is only an approximation, as there must be flexibility to change the allocation in the frame when new devices are coming in the system, as well as to release slots when some devices are leaving the system.

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 1.6can 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.

Using the ideas described so far, we can create a rudimentary, but fully functional scheme for medium access control (MAC) that allows using the shared channel between Basil and a group of terminals in his range. This scheme works provided that the communication channel behaves according to the collision model adopted in this chapter. Each frame header has a duration of . We define four frame types, each one associated with its respective frame header. A frame type can be represented by two bits, which is sufficient to encode four frame types. The frame header should include these two bits in order to identify the type of frame that follows the header. The frame headers are denoted , and the meaning of each header is specified as follows:

link establishment frame

start of a link termination

this frame contains slots for downlink transmission

this frame contains slots for uplink transmission.

It should be noted that the number of users is a predefined value, not conveyed through the header, such that we must assume it is known by Basil and the devices. Basil acts as a central controller and each header can be treated as a command transmitted from Basil to the devices. By default, each device is in a receive state (recall the hierarchy!) in order to detect the header and it subsequently takes action as instructed by the header.

We first describe how the headers and work. A frame that starts with either of these two headers has a total duration of s. For the example in Figure 1.7(a), it is assumed that . During the link establishment process, a terminal is allocated a number between 1 and 4. Zoya is allocated the slot number 2 and if Zoya detects a header , she expects a downlink packet (of bits) in slot 2 after that header. If Zoya detects , she is allowed to transmit an uplink packet in slot 2. It should be noted that, by system design, after or is sent, then no new header from Basil can arrive within the next s.