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Design Notes

802.11a Receiver Sensitivity Test Bench

This project demonstrates an 802.11a transmitter (TX) and receiver (RX) system along with several related measurements. The transmitter consists of blocks that implement the scrambling, convolutional encoding, interleaving, mapping and modulation defined in the 802.11a specifications. The receiver performs the reverse operations for extracting the transmitted bits. The receiver can employ either Hard or Soft Decisions Viterbi Decoding. Currently, it is set to use Soft Decisions.

This project uses 802.11a TX and RX for testing transmitter power amplifiers (PA) based on a number of measurements. The transmitter PA is defined through its AM/AM and AM/PM characteristics, which were extracted from a harmonic balance (HB) simulation of a transistor level PA circuit.

The output of the OFDM modulator is attenuated before being fed into the PA. The level of attenuation is determined by variable Atten_dBm in Global Definitions, which yields different levels of backoff from the 1 dB compression point. Atten_dBm=20 is used for generating results with backoff of approximately 10 dB, while Atten_dBm=13.5 is used for results with a backoff of approximately 5 dB.

Spectral regrowth as a function of the level of backoff from the 1 dB compression point is investigated using two identical PA's driven with different signal levels. They are loaded at approximately 10 dB and 5 dB backoff, respectively. The outputs of these PA's are shown in the "TX Spectrum" graph. The 5dB backoff spectrum shows larger spectral regrowth than that of the 10dB backoff.

A VSA block is connected to the transmitter PA and is used to plot its AM to AM and AM to PM characteristics. Results are shown in AMtoAM and AMtoPM graph, along with reference curves.

Bit error rate (BER) results are calculated as a function of the signal level at the input of the receiver through Monte Carlo simulations. Signal attenuation at the input of the transmitter PA is used to set the appropriate backoff level. The other reference curves shown in the "BER" graph include BER results when a transmitter PA is not used and BER results for PA loading at approximately 5 and 10 dB backoff. The receiver sensitivity is evaluated based on the signal level needed to achieve the target error rate. At 1e-5 BER, the sensitivity is about -76.5 dB and -70.6 dB for 10 and 5 dB backoff respectively. The BER results can be monitored in the System Simulator Output window.

System Diagram - 80211a

Graph - IQ

Graph - TX Spectrum

Graph - AMtoAM and AMtoPM

Graph - BER