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Impact of an RF Receiver on Communication System Performance

This example shows how to use the SimRF™ Circuit Envelope library to measure the effect of noise figure on bit error rate (BER) of a SimRF RF receiver model and to verify the result by comparing to a Communications System Toolbox™ reference model.

RF Receiver System Architecture

The Modulator and Channel subsystems consist of Communications System Toolbox blocks that model:

  • An QPSK-modulated waveform of random bits.

  • A raised-cosine pulse-shaping filter for bandwidth reduction.

  • Free-space path loss.

The RF receiver sub-system, shown in light blue, consists of:

  • An Inport block, which assigns the input waveform to the specified RF carrier. To ensure that the SimRF model has the same available power as the reference model, the Source type parameter is set to Power.

  • A cascaded RF amplifier and RF mixer with specified noise figure and gain.

  • An Outport block, with the Source type parameter set to Power.

  • Note the presence of an S-parameter block preceding the Mixer. This block ensures that the only frequency entering the mixer will be the carrier at 2.1 GHz.

The reference system, shown in red, consists of:

  • A Communications System Toolbox Receiver Thermal Noise block that raises the noise level of the signal according to the value calculated by the Friis Equation.

  • A Gain block that models the combined gain of the RF receiver.

Communications System Toolbox filters and demodulators process the received signal.

The example model defines variables for block parameters using a callback function. To access model callbacks, select File > Model Properties > Model Properties and click the Callbacks tab in the Model Properties window.

Circuit Envelope Simulation of the RF Receiver

  1. Type open_system('simrfV2_friis') at the Command Window prompt.

  2. Select Simulation > Run.

Error Rate Calculation blocks compute the BER for the system and reference. To observe the BER as it approaches steady state, increase the total simulation time. For this example, the steady-state bit error rate is .12%.

Computing RF Receiver Noise Figure

To model noise in the SimRF circuit envelope environment:

  • In the Configuration block dialog, select Simulate noise.

  • Specify the Noise figure (dB) parameter of RF Amplifier and RF Mixer blocks in your system. The following specifications for the RF receiver in this example produce a combined noise figure of 6.3218 dB (as per the Friis Equation): LNA gain of 20 dB, LNA noise figure of 6 dB, and RF mixer noise figure of 15 dB.

$$G_1 = 100 \mbox{  (20 dB)}$$

$$F_1 = 3.9811 \mbox{  (6 dB)}$$

$$F_2 = 31.6228 \mbox{  (15 dB)}$$

$$F_{sys} = F_1 + \frac{F_2 - 1}{G_1} = 4.2873$$

$$NF_{sys} = 10\log_{10}{F_{sys}} = 6.3218 \mbox{ dB}$$

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