Chapter 7
Link Adaptation
So far we have studied the modulation, coding, scrambling, channel-modeling, multicarrier, and multi-antenna transmission schemes used in the LTE (Long Term Evolution) standard. We have examined in detail multiple MIMO (Multiple Input Multiple Output) transmission modes, the best operating condition for each mode, and the peak data rates achievable for the system. We have not yet examined the mechanisms involved in the transition between various transmission modes or the criteria for these changes. In this chapter, we will overview the dynamic nature of the LTE standard and the way in which it chooses various parameters in order to optimize the spectral efficiency in time-varying channel conditions.
Spectral efficiency is an important measure used to evaluate the performance of mobile communications systems. The LTE standard, for example, has specific requirements in terms of average, cell-edge, and overall spectral efficiency relative to 3G (third-generation) standards 1. Spectral efficiency is defined as the average data rate per bandwidth unit (Hz) per cell. This definition by itself reveals the tradeoffs involved in designing mobile systems. For a given bandwidth allocation, you can increase the spectral efficiency by augmenting the data rate through the use of higher-order modulation or higher-dimension MIMO techniques; in noisy channel conditions, however, such a selection may increase the probability of error and thus have a detrimental effect on ...
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