transmission, HARQ schemes are included in the present wireless standards like. LTE, LTE-Advanced and WiMAX. Conventional HARQ systems use the same. Hybrid automatic repeat request (hybrid ARQ or HARQ) is a combination of high- rate forward . phone networks such as UMTS, and in the IEEE standard for mobile broadband wireless access, also known as “mobile WiMAX”. Hybrid Automatic Repeat Query (HARQ) is an error correction technique that has Keywords— channel estimation, HARQ, incremental redundancy, WiMAX.

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The CRC bits are generated based on the contents of the information bits. It is used widely in Wimax and WiFi communication systems. From Wikipedia, the free encyclopedia.

If the channel quality is good enough, all transmission errors should be correctable, and the aimax can obtain the correct data block. Piscataway Township, New Jersey: If the information have to be retransmitted several times, it will take long for the receiver to obtain the complete – final – information.

As mentioned above, these HARQ channels are independent of each other; retransmissions of a data burst can only haqr done by its initial sending HARQ channel.

What is Retransmission, ARQ and HARQ? – telecomHall

But if the combination of these two packages still does not give us the complete information, the process must continue – and another ‘NACK’ is sent. The main one is HARQ. What does Orthogonal means in Wireless Networks? In practice, in real World, this is very difficult to happen, because the links can face the most different adversities.

Now it is stored in a ‘buffer’.

We send a package, and it arrives with errors: Also, two consecutive transmissions can be combined for error correction if neither is error free. In fact, we can even ‘expect’ errors when it comes to Wireless Data Transmission. The puncturing pattern used during each re transmission is different, so different coded on are sent at each time. HARQ can be used in stop-and-wait mode or in selective repeat mode.


All telecomHall articles are originally written in Portuguese. In the following example, we have a packet being sent from the transmitter to the receiver.


First because from wrong packets 1 and 2 we can get a correct one, since we do not discard erroneous packets anymore. CRC protection is iwmax practically in all existing Voice and Data applications. This is the maximum number supported by ‘buffer’.

The data packets or retransmissions are separately decoded. If the information arrived properly completethe receiver is ready to receive and process new data.

But when we use HARQ, and retransmit packet [2. The received signal is stored in a ‘buffer’, and will be combined with next retransmission.

And what is the limit of these retransmissions?

Hybrid automatic repeat request

Wimad Read Edit View history. Taking the above example, when we send the package [2], and it arrives with errors, it garq discarded. Similarly the second copy of data burst 2 is sent at time frame 6 via channel 2. There are various ways of Foward Error Correction FECbut the main idea is, given a level of quality in the link, try to get the lowest number of required retransmissions. We transmit an information, which arrived wrong, and we need to do a retransmission.

Performance Analysis of HARQ in WiMAX Networks Considering Imperfect Channel Estimation

The last row shows the data bursts storing in the SS buffer at the corresponding time frame waiting to be forwarded to the upper layer. If a piece of information is lost, and is retransmitted, the conversation becomes intelligible.


That is, sends another ‘NACK’ message. In standard ARQ a transmission must be received error free on any given transmission for the error detection to pass. And then, if we retransmit less information only redundancywe spend less energy, and that will run much faster.

Data bursts 1 and 2 have been erroneously received and are waiting for correct copies while data burst 3, even though has been correctly received, is also waiting so that it may be forwarded further in the correct order. Below, you can see how some retransmission schemes work. The package [1] arrives, and garq ‘OK’. For example, in partial Chase combining only a subset of wiimax bits in the original transmission are re-transmitted.

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. Above a certain error level, the process is eliminated. Thus, a negative acknowledged NAKed data burst can only be resent via the initial sending garq until it is successfully received.

The first row shows the consecutive time frames. If received error free, wimmax done. The following diagram shows a simplified demonstration of how the CRC is used. The transmitter sends a package [1]. Or retransmit that same information again; Or retransmit only the redundancy. Similarly, at time frame 5, data bursts 1 through 4 are in the buffer. In practice, we work with a number of ‘processes’, which may vary eimax example from 4, 6 or 8.