Link adaptation, comprising adaptive coding and modulation (ACM) and others (such as Power Control), is a term used in
wireless communication
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
s to denote the matching of the
modulation,
coding and other
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
and
protocol parameters to the conditions on the
radio link
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
(e.g. the
pathloss
Path loss, or path attenuation, is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system ...
, the
interference due to signals coming from other transmitters, the sensitivity of the receiver, the available transmitter power margin, etc.). For example,
WiMAX
Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options.
The WiMAX ...
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
High-Speed Downlink Packet Access
High Speed Packet Access (HSPA) is an amalgamation of two mobile protocols—High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA)—that extends and improves the performance of existing 3G mobile telecommunic ...
(HSDPA) in
Universal Mobile Telecommunications System
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the In ...
(UMTS) this can take place every 2 ms.
Adaptive modulation systems invariably require some
channel state information at the transmitter. This could be acquired in
time-division duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
systems by assuming the channel from the transmitter to the
receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel knowledge can also be directly measured at the receiver, and fed back to the transmitter. Adaptive modulation systems improve
rate of transmission, and/or
bit error rate
In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
The bit error rate (BER) ...
s, by exploiting the
channel state information that is present at the transmitter. Especially over
fading channels which model
wireless
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
propagation environments, adaptive modulation systems exhibit great performance enhancements compared to systems that do not exploit channel knowledge at the transmitter.
Example
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
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 (FEC) coding which in turn means more redundant bits and lower information bit rate;
* Choice of FEC
code rate—the FEC code used has a rate of 1/3, but it can be varied effectively by ''bit
puncturing'' and ''
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 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.
See also
*
Cliff effect
In telecommunications, the (digital) cliff effect or brickwall effect is a sudden loss of digital signal reception. Unlike analog signals, which gradually fade when signal strength decreases or electromagnetic interference or multipath increases, ...
*
*
*
*
Hierarchical modulation
Hierarchical modulation, also called layered modulation, is one of the signal processing techniques for multiplexing and modulating multiple data streams into one single symbol stream, where base-layer symbols and enhancement-layer symbols are sy ...
*
Radio resource management
References
{{Reflist
Quantized radio modulation modes
Radio resource management