1. Analog to Digital Conversion OFDM Signal follows a two dimensional or complex Gaussian distribution.

2. Effect of Receiver ADC on BER performance of an OFDM ADC transforms the continuous analog input signal to discrete valued digital signal. Original signal is not preserved perfectly. Dynamic range of ADC is limited. Amplitude clipping occurs if input signal exceeds ADC’s maximum input amplitude.

3. Effect of Receiver ADC on BER performance of an OFDM Linear ADC quantizes input signal into set of n equidistant output values with a span of [-a a]. Two errors occurs…… 1. Quantization Error due to finite step size 2a/n 2. Amplitude clipping for input values from outside this range.

4. Effect of Receiver ADC on BER performance of an OFDM Both I and Q components are Gaussian distributed. Mean Square ADC Error normalized to average input signal powe vs clipping amplitude for different quantization resolution values Input signal is modelled as a gaussian function with standard deviation Mean square quantization error is relative to mean input power Amplitude limit a is in unit of

5. Effect of Receiver ADC on BER performance of an OFDM

6. MSE..mean power of an additional random noise superimposed on quantization signal. ADC noise is not gaussian..Hence BER estimation with gaussian noise is not adequate. Due to predominant clipping High ADC noise occurs eventhough small dynamic range of 2a For a=1, normalized ADC noise power is -8 dB and is independent of resolution of ADC For higher values of a, ADC noise is dominated by effect of increasing step size. Based on resolution, an optimal value of ‘a’ for which ADC noise is minimal.

7. Effect of Receiver ADC on BER performance of an OFDM Both Amplitude limitation and step size are seperately analyzed on BER. ADC resolution is b Amplitude limit is a

8. Effect of Receiver ADC on BER performance of an OFDM Average ADC noise power for a=3 and a=10 is shown… a=3 results -34 dB due to amp clipping a=10 results …below -54 dB For step size above 0.2 (5 bit resolution or less) the gap betweeen two curves closed. Noise power is dominated by quantization noise.

9. BER performance of 512 subcarrier OFDM modem coherent QPSk and DQPSK The expts are conducted for a=3 with step size fro 0.1 to 0.5 (reso..6 and 3 bits). Curves with out markers represent BER performance in the absence of ADC quantization and clipping noise. SNR penalty at a BER of 10^-4 with step size of 0.1 is 0.15 dB and 0.2 dB for QPSK and DQPSk. Resolution corresponds to 60 steps in the range from -3 to 3..requires 6 bit resolution. For higher step sizes SNR penalty is more severe. For 0.5 step size …SNR penalty is 2.8 dB and 3.2 dB for QPSK and DQPSK