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.
1.5 Chapter Summary
This chapter has dealt with the problem of sharing a single wireless communication channel among multiple communication links. We have used the simplest possible communication model that captures important features of the shared wireless medium, such as broadcast and interference, where the latter is modeled as a collision. We have adopted a packet to be the atomic unit of transmission, meaning that either the whole packet is received correctly or it is completely lost. The objective has been to introduce the main ideas for sharing the channel, such as TDD and TDMA and sketch the elements of a protocol that closely approximates the practical protocols. Where relevant, we have also discussed how the full-duplex capability of the wireless devices can contribute to the design of protocols that are more efficient compared to the case of half-duplex devices.
1.6 Further Reading
A classical book that introduces elements of data networking, along with rigorous models is the one by Bertsekas and Gallager [1992]. For rendezvous and link establishment procedures, the reader is referred to the operation of various standards, such as 4G LTE in Dahlman et al. [2013] (chapter 14, Access Procedures), or 802.11 Wi-Fi networks, both in ad hoc and infrastructure mode, see Standards [2016]. Besides Bertsekas and Gallager [1992], another book that offers insights into models for communication over a shared channel as well as stochastic modeling of communication traffic is Rom and Sidi [2012].
1.7 Problems and Reflections
1 State machine for a TDMA system. Describe a possible state machine through which the devices and the base station implement the protocol from Figure 1.7.
2 An even more practical state machine. Extend the state machine from the previous model in order to make the protocol practical, such as introduction of timeout mechanisms, dealing with device mobility, etc.
3 More than one rendezvous channel. Assume that there are different rendezvous channels. Two nodes can establish a link only if one of them sends and the other one receives on the same channel. At a given time, a device can use only one channel. Devise a strategy for establishing a link between two devices and try to compare its performance to the case when there is a single rendezvous channel. When and why would it be useful to have channels for rendezvous?
4 Unequal slots. Consider a generalization of a frame, which consists of a header, followed by communication resources. However, now assume that the communication resources are not organized into equal slots and instead the frame can contain slots with different lengths. Discuss how does this affect the type and amount of signaling bits used in the header.
5 Reservation with variable number of data slots. The analysis in Section 1.4.3 is done for the case in which the number of data slots following the reservation slots is fixed. Let us now consider the case in which is variable and adapted to the actual number of resources required in the reservation slots. Assume that each of the devices can request up to resources through the reservation packets.Find the number of bits that are required in the reservation packets and the allocation packet.Using the assumptions for bit duration from Section 1.4.3, find the maximal throughput that can be offered in a given frame. NB: the maximal throughput depends on the amount of resources requested by the devices.In practical systems, the allocation packet A may not be received by some of the devices due to errors caused by noise or interference. When is the impact of not receiving A worse, when is fixed or when is variable? How do you suggest to design the system to be more robust to this type of error?
Notes
1 1 The term broadcast outside of information theory is used to denote the message that is sent to all devices within the range. In that sense, the term multicast is used when the message is sent to a subset of two or more devices within the range. Hence, in this sense a broadcast is a special case of multicast. On the other hand, broadcast in an information-theoretic sense means that the transmission of a device is received by multiple devices, regardless of who the intended receiver of the message is. Thus, we can say that a transmission over a wireless medium is a broadcast, but the actual message sent may be intended as a unicast to a specific device or multicast to a group of devices.
2 2 Here we conservatively assume that a device sends a reservation packet even if it has no data to send.
2 Random Access: How to Talk in Crowded Dark Room
In the previous chapter, the dark room analogy was used to introduce the problem addressed by rendezvous protocols. Thinking about the same analogy, let us assume that Basil is in a dark room and some of the other people in the room want to talk to him. Basil cannot use visual cues, such as a raised hand, in order to schedule who should speak at him at a given time. Furthermore, the room is crowded, there are many other people in it, but only a few of them are active, in the sense that they want to say something to Basil at a given time instant. With this in mind, it is clearly not efficient to ask the people one by one if they have something to say, as most of them will be just silent and thus most of the time will be spent inefficiently. This observation paves the way for random access protocols, in which the reservation slots or data transmission slots are not exclusively pre-allocated to a device. The attribute “random” comes from the fact that the decision to transmit is randomized. The randomness can be caused by random packet arrival to the device. Alternatively, when the packet is already in the buffer of the device, the device can make a deliberate randomized choice to transmit or not. The dark room reflects the fact that we need to use the same wireless medium both to obtain the right to transmit data, which is a form of metadata, and to send the actual data.
Similar to the rendezvous protocol, random access is an indispensable solution when the devices need to perform an initial access. The objective of the initial access is to connect a device to the base station Basil, potentially going through a process of authentication, allocation of a temporary short address,