DESIGN EXAMPLE A JOINTLY ADAPTIVE TDMA MAC PROTOCOL in .NET

Draw QR-Code in .NET DESIGN EXAMPLE A JOINTLY ADAPTIVE TDMA MAC PROTOCOL
DESIGN EXAMPLE A JOINTLY ADAPTIVE TDMA MAC PROTOCOL
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Requests acknowledgment (Nr ack slots) CSI Polling
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(b) Figure 7.12. Uplink (a) and downlink (b) frame structures of the SCAMA protocol for CBR, VBR, and ABR users.
7.5.2.4 Protocol Operations. The operation of the SCAMA protocol is divided into two phases: the request phase and transmission phase. In the request phase, mobile terminals that have packets to transmit will send a request packet in one of the Nr request minislots governed by the respective permission probability. The request packet is short (24 bits4) and occupies only a minislot as illustrated in Figure 7.12a. It contains the mobile terminal ID, request type (CBR, VBR, or ABR), data deadline, number of information data packets desired to transmit, and pilot symbols for CSI estimation. If more than one mobile terminal sends request packets in the same request minislot, collision occurs and all the request packets are lost if capture effect
The 24-bit packet size includes the payload only. Other common header and trailer bits, such as guard bits and CRC bits, are not shown here for brevity.
REVIEW OF MAC LAYER DESIGN FOR WIRELESS SYSTEMS
is not considered.5 After each request minislot, an acknowledgment packet will be broadcast from the base station through the acknowledgment minislot in the downlink frame as illustrated in Figure 7.13a. The acknowledgment packet contains only the successful request packet ID. Mobile terminals that fail to receive an acknowledgment will retransmit the request packet in the next request minislot, which is again governed by the permission probability. On the other hand, successfully acknowledged users will wait for announcement on the allocation schedule of the traf c slots from the base station. Unlike the traditional MAC protocols, the base station will collect all requests in the current request phase as well as the backlog requests from the previous frames before allocation of traf c slots. All the requests will be assigned priorities according to the deadline, the CSI, and the service type (CBR, VBR, or ABR), as well as the waiting time of the request (i.e., the number of elapsed frames since the request is acknowledged). The time slot allocation algorithm is conceptually depicted in Figure 7.13b. Since the physical layer offers a variable throughput that is dependent on the CSI, the rationale behind the SCAMA MAC protocol is to give higher priority to the mobile terminals that are in better channel conditions in the bandwidth allocation process. The motivation of this strategy is that a user with better channel condition, with the support of the variable-rate channel encoder, can enjoy a larger throughput and therefore, can use the system bandwidth more effectively. Nevertheless, for fairness, information slots should also be allocated to mobile terminals that are approaching their deadlines, despite their possibly worse channel states; otherwise, the queued information packets will be dropped.
7.5.2.5 Priority Function for Slot Allocation. In general, a priority function for ef cient slot allocation should satisfy the following goals:
Give priority to requests with high CSI value Maintain priority (i.e., prevent priority inversion) between different classes (CBR, VBR, or ABR) Maintain fairness (delay jitter) within each class
The SCAMA protocol employs a general priority function that provides a exible balance of these con icting goals. Furthermore, the slot allocation mechanism is also very exible for incorporating other types of allocation algorithms such as de cit round robin (DRR), weighted fair queueing (WFQ), and class-based queueing (CBQ). Speci cally, the priority metric of the ith request (which may be a new request or a backlog request), mi, is given by the following equation: