Link adaptation

Lore & Background

Link adaptation comprises adaptive modulation and coding (AMC) and other techniques. It requires channel state information at the transmitter, which can be acquired in time-division duplex systems by assuming channel reciprocity, or measured at the receiver and fed back. Adaptive modulation systems exhibit great performance enhancements over fading channels compared to systems without transmitter channel knowledge.

In HSDPA, link adaptation is performed by choosing the modulation type—QPSK for noisy channels and 16QAM for clearer channels—and by varying the forward error correction (FEC) code rate. The FEC code has a base rate of 1/3, but can be effectively varied by bit puncturing and hybrid automatic repeat request (HARQ) with incremental redundancy. Good link conditions allow more puncturing and higher information bit rate; poor conditions require all redundant bits, reducing bit rate; very bad conditions trigger HARQ retransmissions, ensuring correct reception but further decreasing bit rate.

WiMAX uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to radio channel quality, affecting bit rate and robustness. The process is dynamic, with signal and protocol parameters changing as conditions change.

Reader's Guide

Link adaptation is significant because it directly improves the rate of transmission and bit error rates by exploiting channel state information at the transmitter. Over fading channels, which model wireless propagation environments, adaptive modulation systems show great performance enhancements compared to systems that do not use such channel knowledge. The practical impact is illustrated by HSDPA, which adapts up to 500 times per second to achieve very high bit rates—up to 14 megabit/sec on clear channels using 16QAM and near 1/1 coding rate, while on noisy channels it provides reliable communications using QPSK and 1/3 coding rate at about 2.4 megabit/sec. This dynamic adaptation allows efficient use of the radio spectrum and robust data transmission under varying conditions. The concept extends beyond modulation and coding to include other signal and protocol parameters, and is a core component of modern wireless systems like WiMAX and UMTS. Its legacy lies in enabling high-speed, reliable wireless data services by continuously matching transmission parameters to the instantaneous link quality.

Did You Know?

• Link adaptation can occur every 2 milliseconds in HSDPA.
• Adaptive modulation systems require channel state information at the transmitter, which can be obtained via channel reciprocity in time-division duplex systems.