WiMAX, meaning Worldwide Interoperability for Microwave Access, is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access. WiMAX ormed in June to promote conformity and interoperability of the standard. Post a Comment. Using multiple HARQ channels can compensate the propagation delay of the stop-and-wait scheme, that is, one channel transmits data while others are waiting for feedbacks. Therefore, using a small number of HARQ channels e.
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Receivers detecting a corrupted message will request a new message from the sender. In Hybrid ARQ, the original data is encoded with a forward error correction FEC code, and the parity bits are either immediately sent along with the message or only transmitted upon request when a receiver detects an erroneous message.
The ED code may be omitted when a code is used that can perform both forward error correction FEC in addition to error detection, such as a Reed—Solomon code. The FEC code is chosen to correct an expected subset of all errors that may occur, while the ARQ method is used as a fall-back to correct errors that are uncorrectable using only the redundancy sent in the initial transmission.
As a result, hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple hybrid ARQ is better, and above which basic ARQ is better. When the coded data block is received, the receiver first decodes the error-correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block.
If the channel quality is bad, and not all transmission errors can be corrected, the receiver will detect this situation using the error-detection code, then the received coded data block is rejected and a re-transmission is requested by the receiver, similar to ARQ.
When the first transmission is received error free, the FEC parity bits are never sent. Also, two consecutive transmissions can be combined for error correction if neither is error free. FEC, on the other hand, can often double or triple the message length with error correction parities.
In terms of throughput, standard ARQ typically expends a few percent of channel capacity for reliable protection against error, while FEC ordinarily expends half or more of all channel capacity for channel improvement.
In standard ARQ a transmission must be received error free on any given transmission for the error detection to pass. If received error free, it's done. If data is received in error, the second transmission will contain FEC parities and error detection. If received in error, error correction can be attempted by combining the information received from both transmissions.
In practice, incorrectly received coded data blocks are often stored at the receiver rather than discarded, and when the re-transmitted block is received, the two blocks are combined.
While it is possible that two given transmissions cannot be independently decoded without error, it may happen that the combination of the previously erroneously received transmissions gives us enough information to correctly decode.
There are two main soft combining methods in HARQ:. Several variants of the two main methods exist. For example, in partial Chase combining only a subset of the bits in the original transmission are re-transmitted. In partial incremental redundancy, the systematic bits are always included so that each re-transmission is self-decodable.
The puncturing pattern used during each re transmission is different, so different coded bits are sent at each time. Although the HSDPA standard supports both Chase combining and incremental redundancy, it has been shown that incremental redundancy almost always performs better than Chase combining, at the cost of increased complexity.
HARQ can be used in stop-and-wait mode or in selective repeat mode. Stop-and-wait is simpler, but waiting for the receiver's acknowledgment reduces efficiency. Thus multiple stop-and-wait HARQ processes are often done in parallel in practice: when one HARQ process is waiting for an acknowledgment, another process can use the channel to send some more data.
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