Channel Coding | WiMAX Radio

Channel coding is a process where one or more control and user data signals are combined with error protected or error correction information. The WiMAX system channel coding processes include error correction coding, interleaving and randomization.

Error Correction Coding

Error correcting codes are additional information elements that are sent along with a data signal that can be used to detect and possibly correct errors that occur during transmission and storage of the media. Error correction codes conform to specific rules or formulas to create the code from the data that is being sent. Error correction codes require an increase in the number of signal elements that are transmitted which increases the required data transmission rate. The WiMAX system can use a variety of error coding methods including Reed Soloman coding, convolutional coding (optional) and block turbo coding (optional).

Interleaving

Interleaving is the reordering of data that is to be transmitted so that consecutive bytes of data are distributed over a larger sequence of data to reduce the effect of burst errors. The use of interleaving greatly increases the ability of error protection codes to correct for burst errors. Many of the error protection coding processes can correct for small numbers of errors, but cannot correct for errors that occur in groups. The WiMAX system uses interleaving to map data onto non-adjacent sub-carriers to help overcome the effects of frequency selective (e.g. multi-path) distortion.

Randomization

Randomization is a process that rearranges data components in a serial bit sequence to statistically approximate a random sequence. For communication systems, randomization involves using a known randomization code or process in the transmitter and using the same code to decode the randomized sequence at the receiver.

The WiMAX system uses a pseudo-random binary sequence (PRBS) randomization process that ensures that there are no long sequences of bits that would cause the modulator to produce a high peak to average power ratio (PAPR) signal. PAPR is a comparison of the peak power detected over a period of sample time to the average power level that occurs over the same time period. A high PAPR would require the use of a more linear RF amplifier assembly increasing cost and decreasing power conversion efficiency (e.g. shorter battery life).

Channel Descriptors | WiMAX Radio

Channel descriptor is a set of information parameters that describe the characteristics associated with a communication channel. The use of a channel descriptor can permit more accurate and successful reception and decoding of information that is sent on a communication channel.

The WiMAX system periodically sends (broadcasts) channel descriptors on the downlink channel to allow the subscriber stations to understand how to decode and transmit messages. Channel descriptors provide information about the uplink and downlink channels.

The downlink channel descriptor contains a downlink frame prefix that provides the information to the receiver about the frame structure of the downlink channel and a downlink map (DL-MAP) that defines what information will be transmitted. The DL-MAP contains a downlink interval usage code (DIUC) that defines when information will be transmitted on the downlink and what formats it is supposed to use (burst profile).

The uplink channel descriptors contain an uplink map (UL-MAP) message that defines when a subscriber station is allowed to transmit on the uplink and what formats it is supposed to use (burst profile). The UL-MAP contains an uplink interval usage code (UIUC) that defines when a subscriber station is allowed to transmit on the uplink and what formats it is supposed to use (burst profile).

Figure 1 demonstrates how the WiMAX system uses channel descriptors to define the allocated times and burst types that are used on WiMAX radio channels.

Figure 1: WiMax Channel Descriptors

Radio Packets (Bursts) | WiMAX Radio

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.

Figure 1: WiMax RF Packets