Link adaptation
Example
Example
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In HSDPA link adaptation is performed by:
* Choice of modulation type—the link can employ [QPSK](/wiki/QPSK "QPSK") for noisy channels and [16QAM](/wiki/Quadrature_amplitude_modulation "Quadrature amplitude modulation") for clearer channels. The former is more robust and can tolerate higher levels of interference but has lower [spectral efficiency](/wiki/Spectral_efficiency "Spectral efficiency"), which means it may transmit at a lower bit rate for a given bandwidth. The latter has double the spectral efficiency, but is more prone to errors due to interference and noise hence it requires stronger [forward error correction](/wiki/Forward_error_correction "Forward error correction") (FEC) coding which in turn means more redundant bits and lower information bit rate;
* Choice of FEC [code rate](/wiki/Code_rate "Code rate")—the FEC code used has a rate of 1/3, but it can be varied effectively by *bit [puncturing](/wiki/Punctured_code "Punctured code")* and *[hybrid automatic repeat request](/wiki/Hybrid_automatic_repeat_request "Hybrid automatic repeat request") (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](/wiki/Retransmission_%28data_networks%29 "Retransmission (data networks)") occur due to HARQ which ensures correct reception of the sent information but further decreases the bit rate.

Thus HSDPA adapts to achieve very high bit rates, of the order of 14 megabit/sec, 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/sec. This adaptation is performed up to 500 times per second.


What are the implications of HSDPA's adaptation frequency on network performance?