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easily quanti able usually expressed as a sum of all the mean data rates. However, although maximizing the overall system throughput is an easy target, in both theoretical analysis and real-life practice, it is usually subject to several other system or user constraints such as fairness and QoS as described below. 2. Channel Quality. The quality of the wireless channel for each device is inevitably different and is time-varying because each device exhibits different mobility and experiences different signal propagation conditions. A device under a better channel condition suffers fewer packet losses compared with one under a poor channel condition. Thus, from the system s perspective, it is wise to allocate more resources (e.g., more timeslots) to a device with a better channel quality because such a device delivers a higher data throughput. However, this is in con ict with the goal of providing fairness as described below. A related problem is how the scheduler can determine the channel conditions of individual users. A feasible method is to require the device to report the channel conditions to the scheduler from time to time using some feedback control channels. A critical issue is then about the accuracy and timeliness of the channel condition information. This is because outdated channel condition information can obviously lead to erroneous resource allocation. 3. Fairness. Strictly favoring devices with good channel conditions is obviously unacceptable to those devices suffering from poor channel conditions because, more often than not, all devices are of equal status in the system (e.g., the users pay the same service subscription fee). Thus, the scheduler needs to provide some sort of fairness. A critical question is then how fairness should be quanti ed. However, there is still no universally accepted notion of fairness. Different considerations lead to different de nitions of fairness. Another issue is that even if we have a well-de ned notion of fairness, it is not easy to map it to some metric that is quanti able from an application perspective. 4. Application QoS Requirements. The application running on the wireless device may demand some speci c QoS requirements. For example, a videophone application running on a multimedia cellular phone may specify a minimum video frame rate, a minimum response time, and a maximum delay jitter. Thus, given these requirements, the scheduler may need to adjust the strategy of maximizing system throughput or providing a high level of fairness. Indeed, even if a device is suffering from a poor channel condition, the scheduler may still need to allocate many resources to it because the device has very urgent packets to send. 5. Admission Control. Another responsibility of a scheduler is to determine whether a new device is to be admitted into the system given the current traf c demand conditions. Speci cally, since the requests of many devices are recurrent in nature (e.g., video or audio services that need regular allocation of resources across different transmission frames), a scheduler has to accurately judge whether a new user should be admitted into the system because such a user would inevitably aggravate the contention among the existing users.
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7.5 DESIGN EXAMPLE A JOINTLY ADAPTIVE TDMA MAC PROTOCOL In this section, we describe in detail a design example on jointly adaptive TDMA MAC protocols. We rst present an overview of the issues involved in an advanced TDMA protocol. We then discuss the various design considerations of our example, which is called synergistic channel-adaptive multiple access. 7.5.1 Overview of TDMA MAC Protocol Design
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Figure 7.8 depicts a partial taxonomy of TDMA-based MAC protocols. In general there are two types of TDMA-based protocols: pure TDMA and dynamic TDMA (DTDMA). In a pure TDMA protocol, the slot assignment, which is determined statically, is xed throughout the communication process without regard to the actual requirements of the users. Thus, even if a user does not have any information packet to send, the assigned timeslot is still occupied and therefore is wasted. In view of the poor utilization and in exibility, pure TDMA protocols are not used in practical wireless networks because of the dynamic user population involved. Dynamic (DTDMA) protocols can be classi ed as TDD (time-division duplex)- or FDD (frequency-division duplex)-based. A TDD-based protocol can exploit the asymmetry between the uplink (heavily loaded) and downlink (lightly loaded) communications in that the bandwidth for downlink communication can be converted for uplink communication when mobile traf c load is high. However, a TDD-based system suffers from the long and irregular delay in both the uplink and downlink communications.A FDD-based scheme, by contrast, can exploit the almost instantaneous acknowledgment (i.e., short propagation delay) in both uplink and downlink transmissions, and as such,
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