 determining the required link-level results, we build upon the mutual infor- mation (MI) method [14]. This works by applying a formula from SINR to MI, and then from MI to BER (bit error ratio) and PER (packet error ratio) to get the packet error probability. For the first step, the performance data of modulation schemes typically come from link-level simulations. MI has the meaning of the number of effective bits that can be transported at a certain SINR level. It is always below the Shannon bound: In reality, each modulation grade comes with its own MI level, depending on SINR. For further analysis, we use an analytic expression developed by fitting the link-level result data with a suitable function [22]. Figure 8.5 shows the resulting graph. In low-SINR regions, MI is limited by the Shannon bound. In high-SINR regions, it is saturated and limited by the number of bits that the modulation scheme © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology QAM16 QAM32 QAM84 QAM128 QAM256 Shannon SINR (dB) Figure 8.5 Mutual information as a function of the SINR. The modulation schemes perform close to the Shannon bound. This graph does not yet contain the error correction statistics. supports (m). The region in between is influenced by both effects and handled by this formula: MI(SINR,m) = 1 : ([s MIshannon(SINR)]-w+m-w)1/w shannon using the following abbreviations =s(m) = 0.95 - 0.08 X (m modulo 2) w=w(m) = 2 m + 1 where m is the modulation index-that is, the number of bits per symbol (1 = QPSK (quadrature phase shift keying).. 8 = QAM (quadrature amplitude modu- lation) 256). The scale factor s(m) reveals the remarkable fact that square-shaped modulation constellations (m = 2, 4, 6, 8) perform slightly better than the other I/Q-asymmetric constellations. The MI value has the unit of bits per second per hertz, SO we can derive the data rate by multiplying by the bandwidth of the subchannel. The net bit rate of the PHY layer is less than this value because of channel coding.