A radio packet is a short transmission (a burst) of information that occurs on a radio channel. Radio bursts contain reference sequences (preamble and possible a midamble), control information and payload of data.
The radio packet burst may have different types of radio characteristics such as modulation type, error coding, preamble length and transmission guard time periods. The combination of these characteristics is called the burst profile.
A burst set is a single transmission (RF packet) that contains a preamble along with one or more bursts of information. The bursts of information contained within the RF packet may have different modulation and coding types. A burst frame is the complete set of information that is contained in a transmission burst.
The bursts within a burst set are sequenced according to their modulation complexity. Bursts with lower complexity modulation types are located at the beginning of the radio packet. Bursts that follow can use modulation types with higher complexity (e.g. QPSK, QAM). This allows subscriber stations to receive and decode all the bursts up to the burst with the highest modulation type it can receive.
RF bursts start with a sequence of bits (a preamble) that the receiving device can recognize and lock onto. Once the receiving device locks onto the preamble, it knows where to find the rest of the packets.
For longer RF bursts, midamble sequences may be periodically inserted to assist receivers in the decoding of bursts. A midamble is a sequence of bits that the receiving device can recognize and lock onto to help decode the bits surrounding the midamble. Increased mobility (speed) can be tolerated when preambles and midambles are sent more frequently.
Burst profiles may continually change on a WiMAX system. Subscriber stations can send burst profile change request messages that define a desired new burst profile characteristic (such as modulation type or error coding). The request may be a result of an increase in the error rate using an existing burst profile.
Data packets may be inserted (embedded) within the payload of a single RF burst, they may be divided (fragmented) so they can be distributed over several radio packet bursts or multiple small packets may be combined (packed) into the payload.
Figure 1 depicts an example of a WiMAX radio packet which is made of a preamble and a set of bursts. The figure shows that the modulation type of the burst starts simple (BPSK) and gets more complex as additional bursts follow.
