Link adaptation

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Link adaptation, or adaptive modulation and coding (AMC), is a term used in wireless communications to denote the matching of the modulation, coding and other signal and protocol parameters to the conditions on the radio link (e.g. the pathloss, the interference due signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). For example, EDGE uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is a dynamic one and the signal and protocol parameters change as the radio link conditions change -- for example in HSDPA in UMTS this can take place every 2 ms.

[edit] An Example of Link Adaptation

In HSDPA link adaptation is performed by:

choice of modulation type -- the link can employ QPSK for noisy channels and 16QAM for clearer channels. The former is more robust and can tolerate higher levels of interference but has lower transmission bit rate. The latter has twice higher bit rate but is more prone to errors due to interference and noise hence it requires stronger FEC (forward error correction) coding which in turn means more redundant bits and lower information bit rate;
choice of FEC coding rate -- the FEC code used has a rate of 1/3, but it can be varied effectively by bit puncturing and HARQ with incremental redundancy. When the radio link conditions are good more bits are punctured and the information bit rate is increased. In poor link conditions all redundant bits are transmitted and the information bit rate drops. In very bad link conditions retransmissions occur due to HARQ which ensure correct reception of the sent information but further slow down the bit rate.

Thus HSDPA adapts to achieve very high bit rates, of the order of 14 megabit/s, on clear channels using 16-QAM and close to 1/1 coding rate. On noisy channels HSDPA adapts to provide reliable communications using QPSK and 1/3 coding rate but the information bit rate drops to about 2.4 megabit/s. This adaptation is performed up to 500 times per second.