Radio Link Control (RLC) | WiMAX Operation

Radio link control protocol is a layer 2 (link layer) that is used to coordinate the overall flow of data packets across the radio link. RLC uses error detection and data retransmission to increase the reliability of the radio link while reducing the error rate. WiMAX radio link control functions include power level control, periodic ranging, burst profile changes and bandwidth requests.

Power control is the process of adjusting the power level in a wireless system where the base station receiver monitors the received signal strength of mobile radios. Control messages are transmitted from the base station to the mobile telephone commanding it to raise and lower its transmitter power level as necessary to maintain a good radio communications link.

Ranging may need to be performed after the subscriber station has been inactive for a while. A timer (the T4 timer) that is continuously reset as the subscriber station communicates with the system helps determine this. If the subscriber station (SS) has not communicated with the system in awhile, the timer will not be reset and it will expire. If the timer expires, the SS must again perform ranging with the system.

The base station is responsible for assigning burst profiles. However, the subscriber station may request changes to the burst profile. This may occur as a result of an increase in the bit error rate of the received signal due to fading or interference. The subscriber station may request a change in burst profile that is more robust or offers a higher data transmission rate. The base station may grant the request, negotiate parameters or reject the request.

During a WiMAX communication session, changes in bandwidth may be requested. The subscriber station may send bandwidth request messages to the base station to increase or decrease its allocated bandwidth. Bandwidth request messages may be sent as independent messages or they may be piggybacked with other messages.

Medium Access Control | WiMAX Operation

Medium access control is the process used by communication devices to gain access to a shared communications medium or channel. The methods for controlling access to WiMAX systems may be assigned (“non-contention based”) or random (“contention based”).

When the WiMAX system uses contention free access control the subscriber station must wait for polling messages before responding. If contention based access control is used (e.g. best effort service), the subscriber device must compete for access to send its packets. The WiMAX system can mix contention free and contention based access on the same radio channel.

Contention free access is provided by defining time periods that specific devices will use when communicating with the system. Because all the devices listening to the WiMAX radio channel can hear these messages, devices will not transmit during the assigned time periods.

Contention based access is provided through the use of contention slots and the collision sense multiple access (CSMA) process. The WiMAX channel descriptors define specific time periods (“contention slots”) that contention based WiMAX devices must use when accessing the WiMAX system. Contention slots are dedicated time intervals (time slots) on a communication channel that can be used to allow devices to randomly request service from a system.

When contention based WiMAX subscriber stations access the system, they first obtain the contention time slot interval and the system access parameters (e.g. initial access transmit power level). After the contention slot time period has started, the subscriber station begins to transmit an access message at a low RF power level. If the subscriber station hears a positive response to its access request message, it can transmit its package. If the subscriber device does not hear a response (e.g. another device has transmitted at the same time), it must stop transmitting and wait a random amount of time before attempting to access the system again. Each time the device attempts to access the system and fails, it must wait a longer amount of time before attempting to access the system again. This prevents the possibility of many collisions between devices that are attempting to access the system at approximately the same time.

Figure 1 illustrates how the WiMAX system can mix contention free and contention based access control on a WiMAX radio channel. This diagram shows that the downlink channel contains downlink and uplink descriptor messages that define when subscriber stations are allowed to transmit. For unicast polled devices (contention free), they are assigned specific time periods to transmit from a polling message. For multicast polled, broadcast polled or best effort devices (contention based), they compete during the contention time slot periods.

Figure 1: WiMax Access Control

Initial Ranging | WiMAX Operation

Initial ranging is the process of estimating the distance or propagation time between a transmitter and receiver. Ranging information may be used to assist in the establishment of operating parameters for the transmitter and receiver. The transmitter power level and packet transmission delay time ensure packets do not overlap with transmission from other devices.

During the initial ranging process, the base station is assigned the basic CID that will be used to control the radio operations of the subscriber device. After the basic CID is assigned, a primary management CID may be assigned to allow for authentication and the establishment of other CID channels. A secondary CID may be assigned to allow the downloading of configuration files and the assignment of an IP address using dynamic host configuration protocol (DHCP).

Figure 1 depicts the basic channel acquisition processes that may be used in the WiMAX system. The subscriber station begins by scanning a set of potential WiMAX frequencies. If it finds a WiMAX radio channel, it synchronizes with the RF channel and acquires the downlink channel descriptor (DCD) and uplink channel descriptor (UCD) messages to determine how to access the system. The subscriber station then sends initial ranging request messages to get the attention of the system and to receive timing adjustment information. This process starts by transmitting at a lower RF power level and gradually increasing until the system responds with an assignment of basic and primary control identifiers (CID). The subscriber station then sends its transmission capabilities to the base station and the WiMAX system responds with an authorization or denial of service for these transmission capabilities.

Figure 1: 802.16 Channel Acquisition and Initial Ranging

Channel Acquisition | WiMAX Operation

Channel acquisition is the process of finding and acquiring access to a communication channel. When WiMAX devices initialize (e.g. when they are turned on), they begin a channel scanning process. Channel scanning is the process of searching through multiple radio channels to find signals that indicate a channel is available on which to communicate. The WiMAX device will typically have a stored list of frequency channels for it to scan in order to reduce the amount of scanning time. These frequency channels may be preprogrammed by or for a WiMAX system operator so the WiMAX device will initially try to connect to a specific WiMAX system.

When the WiMAX device has found one or more WiMAX radio channels, the device will decode the channel and look for packets of data that have a frame control header that contains a downlink channel description (DCD) message and an uplink channel description (UCD) message. The DCD message contains parameters that are necessary or that will assist it to access the device in receiving information from the downlink channel on the communication system. The UCD message provides the device with the parameters that are necessary to access the communication system.