I wanted to ask how can we calculate SNR on the basis of channel estimation using Pilot Symbols and make CQI table for Adaptive modulation coding according to the estiamted SNR values.
1 Answer
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Assume that it's a SISO Rayleigh channel and can be modeled as $y=\sqrt{E_S}hc+n$. Firstly, the transmitter transmits the Pilot Symbols $c=c_0$, the receiver receives the signal as $y=\sqrt{E_S}hc_0+n$. Estimate $h$ by computing the expected value of $y$ and divided by $\sqrt{E_S}c_0$ $$h=\frac{\varepsilon\{y\}}{\sqrt{E_S}c_0}$$ the SNR can be expressed as, $$ \text{SNR} = \frac{E_s||h||^2}{\sigma^2} $$ Then, make the CQI table according to the Bit Error Rate (BER) requirement of the communication system. Each modulation method has a minimum SNR requirement and only can be used when the SNR is higher than that. It can be found from the $P(\rho)$ curve between the error rate and SNR.
For example, the SNR requirement of BPSK and QPSK are 10dB and 20dB. If the estimated SNR is 15dB, we will use BPSK modulation to make sure its reliability and sacrifice some capacity.
