WiMAX (Worldwide Interoperability for Microwave Access) is a telecommunication protocol that provides fixed and fully mobile internet access. The current WiMAX revision provides up to 40 Mbit/s with the IEEE 802.16m update expected offer up to 1 Gbit/s fixed speeds. The name "WiMAX" was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard.
2.1. WiMAX Network Topology
Figure 2-1 shows the WiMAX topology. The transmission supports LOS (Line-of-Sight) and NLOS (Non-Line-of-Sight). The topology is like GSM network. There are BSs and MSs and BSs can communicate with each other. But the different is that WiMAX backbone network is IP based core network.
Figure 2-1: WiMAX Network Topology from www.xarxaneta.org
2.2. MAC Layer Technical Overview
A characteristic of WiMAX MAC layer is that it is connection-oriented. Each connection is distinguished with a 16-bit connection identifier (CID). When performing network entry, an MS sets up multiple connections with the BS. The connections are created based on the services mapped to the MS, including broadcast, management and data transmission services.
Each data connection is associated to a QoS level. Connections are dynamically added or dropped if services are initiated or terminated with the MS.
The WiMAX MAC also uses a scheduling algorithm for which the subscriber station needs to compete only once for initial entry into the network. After network entry is allowed, the subscriber station is allocated an access slot by the base station. The time slot can enlarge and contract, but remains assigned to the subscriber station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription, the scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control quality of service (QoS) parameters by balancing the time-slot assignments among the application needs of the subscriber stations.
2.2.1. MAC PDU
MAC protocol data unit (PDU) is a data unit for protocol communication between the MAC layers of BS and MS. Basically, data traffic comes in the form of service data units (SDUs) from upper layers. The MAC layer tunnels upper layer traffics without knowledge of the payload content (shown in Figure 2-2).
Figure 2-2: SDU and PDU Diagram
According to Reference [1], two MAC header types are specified:
– Generic MAC Header: Used in MAC PDUs containing payload data. The generic MAC header indicates length, destination CID, encryption key and included subheader type of the PDU.
– Bandwidth Request Header: Used for requesting uplink bandwidth by MS.
As Figure 2-2 shown above, the payload length is variable. Also, the SDU length from upper layer isn’t the fixed value. Because of those unpredictable reasons, there is a packing/fragmentation mechanism for the PDU payload. Each data fragment under packing/fragmentation should be attached to a packing/fragmentation sub-header.
Figure 2-3 shows examples of the ordering of data fragments and sub-headers.
GMH FSH SDU Fragment CRC
GMH PSH SDU or
Fragment PSH SDU or
Fragment PSH SDU or
Fragment CRC
MAC PDU with Fragmentation
MAC PDU with Packing
Figure 2-3: Examples of the Ordering of Payload and Sub-headers in a MAC PDU
2.2.2. MAC Frame Structure
WiMAX supports both time division duplex (TDD) and frequency division duplex (FDD) modes, but the OFDMA mode supports only TDD. For TDD mode, uplink and downlink use the same spectrum, and a PHY frame is separated into downlink and uplink subframe in time domain. A Typical frame structure layout is shown in Figure 2-4.
Figure 2-4: Typical TDD Frame Structure under OFDMA Mode [1]
1. Preamble:
Each downlink frame starts with a preamble, which lasts for one OFDMA symbol and occupies the entire spectrum. The preamble is robustly modulated in BPSK across subcarriers
2. Frame Control Header (FCH):
FCH contains Downlink Frame Prefix, which indicates the coding scheme and length of DL-MAP. MSs decode the DL-MAP according to information provided in FCH. The FCH uses the four logical subchannels following the preamble.
3. Downlink Map (DL-MAP):
The DL-MAP describes the DL subframe. By specifying subchannel and OFDMA symbol allocation to each user, the DL-MAP enables MSs to decode the DL subframe.
Modulation of DL-MAP is fixed to QPSK, and the coding scheme is specified in the FCH.
4. Uplink Map (UL-MAP):
UL-MAP is similar to the DL-MAP. The UL-MAP describes the UL subframe, and namely bandwidth allocation among the served users. The UL-MAP is embedded in the first DL burst.
5. Downlink/Uplink Bursts:
DL/UL bursts contain data and messages to be transmitted by BS/MS. Each DL burst is mapped with a DL Interval Usage Code (DIUC), and the burst profile is provided in the DL-MAP. Similarly, each UL burst has a UL Interval Usage Code (UIUC), and its PHY characteristics are described in the UL-MAP.
6. Ranging Subchannels:
MSs use ranging subchannels to perform initial ranging, periodical ranging , handover ranging and bandwidth requests. Ranging is a process in which an MS adjusts its PHY parameters according to indicated by BS.
7. Transition Gaps:
Receiver mode and transmitter mode are separated by transition gaps to ensure proper operation. The gap from DL to UL subframe is Transmit Transition Gap (TTG), while the one from UL to DL is Receive Transition Gap (RTG).