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Part 1 of this article provided an overview of WiMAX: its architecture; the standards on which it is based; and general operating principles.
Part 1 is available at The designer's basic guide to WiMAX technology, standards and implementation.
Part 2 takes a somewhat deeper dive into the technology, briefly describing some of the most important ones including, the protocol stack, randomizer, Reed-Solomon encoder, convolution encoder, interleaver, QPSK modulation, Orthogonal Frequency Division Multiple Access (OFDMA) and a few others.
Protocol stack
The IEEE 802.16 protocol reference model has three planes: User, control and management, as is shown in Figure 3.
Click here for Figure 3
Figure 3: Protocol stacks as per IEEE 802.16 MAC.
The IEEE standard 802.16TM-2004 deals with user and control planes. It defines two layers in these planes: Medium Access Control layer (MAC) and Physical layer (PHY). The MAC layer has three sub-layers: Service-Specific Convergence Sublayer (CS), Common Part Sub-layer (MAC CPS) and Security Sub-layer. CS provides the required adaptation for the up-layer incoming traffic while MAC CPS makes available key link layer functions for solving several broadband wireless communication issues.
Randomizer
Most telecommunication transmission systems are designed to give optimum performance when uncorrelated data is transmitted. But user data is highly correlated—such as long strings of 1s and 0s—and their transmission could result in saturation, that is, synchronization drift.
The randomizer combines user data with known synchronization frame data bits. Usually the two most significant bits of frame bits are combined using the XOR function. The result is efficient scrambler data.
The incoming data is first XOR-ed with the synchronized frame data generated from PN sequence generator (PN sequence register consists of a shift register and an XOR gate, and the shift register is initialized with the same know frame bits) after performing this operation. The resulting bit will be non-correlated.
At the receiver end, the scrambler data is XOR-ed with the same synchronized frame data used by the transmitter.
Reed Solomon encoder
Reed Solomon encoders and decoders are used in Galois Field arithmetic to map blocks of communication into larger blocks. Their computational complexity and low power requirement make Reed Solomon decoders ideal for communication systems with dedicated hardware blocks.
This functionality generally relates to polynomial-generated error correction and detection for both encoding and decoding methods. Each block corresponds to an over simplified polynomial based on the input block.
The method includes adding 'N' zeros to a data and performing successive homer reduction of the polynomial. It also includes calculating syndromes as the dot product of the codeword with syndrome vectors.
Convolution encoder
This is another type of channel coding usually used to compensate for random error introduced in the channel. A tree code is distinguished from a block code by its encoding process depending on past history/memory of the input symbols. Convolution code is a popular tree code.
The memory of the encoder is characterized by its state and represented by a v-bit binary number. For every 'm' input bits, the encoder outputs 'n' bits based on the 'm' input, and 'v' state bits, and then transitions to the next state. The code rare for convolution encoder is defined by R=m/n<1. In a real time application requiring high data rate, the state bit 'v' is limited to '<6.'
Using 'punctured' code obtained by periodically deleting some of the output bits, the decoding of high rate 'R' convolution codes can be simplified. The lower the bit rate form 1/n to m/k, the easier will be its decoding using simple modification.
The encoding of convolution is performed using convolution ally encoding digital data with rate 4/5 convolution codes. Using a puncture map, a non-optimal rate of 1/2, sixteen-state convolution code is punctured to a rate of 4/5. Incoming data steams can be arranged using the 4/5-code, and at this rate the convolution encoder is coupled to receive an input stream to be encoded.
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