Adaptive Modulation and Coding (AMC) in WIMAX
- Set of modulation/coding schemes
- QPSK, 16QAM, 64QAM
- distributed over one sector:
- SINR distribution
- automatic selection mechanisms
- Maximum throughput per sector
- Highest modulation scheme
- Lowest coding protection
- Mean throughput offered per sector
- Contribution of different modulations
- Average over the area
AMC is a link adoption (LA) method to adapt the transmission parameters to take advantage of prevailing channel conditions. The principle of LA aims to exploit the variations of the wireless channel (over time, frequency, and/or space) by dynamically adjusting certain key transmission parameters to the changing environmental and interference conditions observed between the base station and the subscriber. In practical implementations, the values for the transmission parameters are quantized and grouped together in a set of modes. AMR modes are modulation and coding scheme.
The channel quality is identified by a channel quality indicator, or channel state information (CSI), that provides knowledge about the channel. The task is to adjust a number of signal transmission parameters to the variations of that indicator. There are various metrics that may be used as Channel State Information. Typically, SNR (Signal-to-noise-ratio) or SiNR (signal-to-noise-plus-interference ratio) may be available from the physical layer (e.g., by exploiting power measurements in slots without intended transmit data (e.g. pilot channels)).
The figure shows the dependence between modulation schemes (spectral efficiency) and Signal-to-Noise Ratio (SNR) requirements at the air interface. The figure also represents the link-level spectral efficiency (SE) performance (b/s/Hz) vs. the short-term average SNR in dB, for four different uncoded modulation levels referred to as binary phase shift keying (BPSK), quaternary PSK (QPSK), 16-quadrature amplitude modulation (QAM), and 64-QAM. It is seen that each modulation is optimal for use in different quality regions, and LA selects the modulation with the highest SE for each link.