Chase Combining HARQ versus Incremental Redundancy HARQ
This page on Chase Combining HARQ vs. Incremental Redundancy HARQ describes Chase Combining HARQ and Incremental redundancy HARQ techniques and provides difference between Chase Combining HARQ and Incremental Redundancy HARQ techniques.
Chase Combining HARQ
In this chase combining type of HARQ, same information and parity bits are retransmitted each time. The retransmission of the same packet helps increase Eb/No at the receiver due to MRC technique. This technique is also called as repetition coding as same coded information is transmitted all the time.
Moreover at receiver previous packets are stored in a buffer; so that retransmitted packets are summed up with previously received erroneous packets before they are passed to decoder. The technique used at receiver to sum the packets is the simple MRC (maximum Ratio Combining) technique.
Buffer Size = No. of coded symbols per coded packet
Application- WiMAX as per IEEE 802.16e-2005 OFDMA Physical layer.
Fig.1,Chase Combining HARQ
Incremental redundancy HARQ
In this Incremental redundancy type of HARQ, multiple different set of code bits are generated for the same information bits used in a packet unlike chase combining type. These different sets are transmitted under different channel conditions. These sets consist of different redundancy flavors obtained by different puncturing configurations.
This scheme in which some additional redundant information bits are transmitted in each re-transmission and receiver needs to decode on each re-transmission is the IR type II HARQ.
In type III HARQ, each re-transmitted packet is self decodable.
Buffer Size = No. of coded bits of the total transmitted coded packets
Application- HSDPA as per UMTS R5 standard
Fig.2,Incremental redundancy HARQ
Chase combining and incremental redundancy techniques are very useful for engineers working in physical layer development of advanced wireless technologies as per Mobile WiMAX and LTE standards.
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