Overview

Walkie-talkies provide a subscription-free method of communicating over short distances. Although their popularity has been diminished by the rise of cell phones, they are still useful when lack of reception or high per-minute charges hinders the use of cell phones.

Modern walkie-talkies operate on the FRS/GMRS standards. Both standards use frequency modulation (FM) at 462 or 467 MHz, which is in the UHF (Ultra High Frequency) band. The USRP® device in this example will transmit messages at either 462 or 467 MHz, in a manner that is compatible with FRS/GMRS devices.

Structure of the Example

This is the top-level block diagram of the model:

Source Signals

Below is the subsystem of sources for the model:

The source signal can be a pure tone sine wave, a chirp signal, or a multimedia file. To switch between these sources, double-click theModel Parametersblock to bring up a GUI. This GUI also allows you to set the pure tone frequency or the chirp signal target/sweep time (which controls the duration of the chirp signal). This example works properly with tones as low as 500 Hz and as high as 4 kHz.

When using a multimedia file, a resampler converts the sample rate from 22050 Hz to 8 kHz. This provides a convenient intermediate sample rate from which to resample to the final 200 kHz rate, consistent with the sample rate of the USRP® hardware in this example.

Continuous Tone-Coded Squelch System (CTCSS)

Walkie-talkies operate on a shared public channel, allowing multiple users to access the same channel simultaneously. The CTCSS[3]method filters out undesired communication or interference from these other users by generating a tone between 67 Hz and 250 Hz and transmitting it along with the source signal. The receiver contains logic to detect this tone, and acknowledges a message if the detected tone matches the code setting on the receiver. The receiver filters the tone out, so that the user will not hear the tone.

The CTCSS tone generator generates a continuous phase sine wave with frequency corresponding to the selected private code. The amplitude of the tone is usually 10%-15% of the maximum amplitude of the modulating signal. Note that since the maximum amplitude of all the source signals is 1, the default amplitude of 0.1 for the CTCSS tone corresponds to 10% of the modulating signal's maximum amplitude.

Resampler and FM Modulator

An FIR Interpolator block converts the sampling rate of the sum of the modulating signal and CTCSS tone to match the USRP® device's sampling rate of 8 kHz * 25 = 200 kHz. The resampling filter is designed using the FIRInterpolator System object (TM) from the DSP System Toolbox™.

Below is the block diagram of theFM Baseband Modulatorsubsystem:

This example uses the FM Modulator Baseband System object whose maximum frequency deviations and sample rate are set to 2.5 kHz and 200 kHz, respectively.

Running the Example

Turn on your walkie-talkie, set the channel to be one of the 14 channels (numbered 1 to 14) and the private code to be either one of the 38 private codes (numbered 1 to 38) or 0, in which case no squelch system is used and all received messages are accepted. Note that the private codes above 38 are digital codes and are not implemented in this example.

Set the channel and private code in theModel ParametersGUI so that they match the walkie-talkie. Run the model and see if you can hear the voice or tone output from the walkie-talkie. If you cannot, try adjusting the amplitude parameter slightly upward. You can change the channel and private code, as well as toggle between the pure tone, the chirp signal, and the multimedia file without stopping and restarting the model.

If you hear some dropouts or delay in the sound, run the model in Accelerator mode. From the model menu, select Simulation->Accelerator, then click the run button. If you still experience dropouts or delay in Accelerator mode, try running the model in Rapid Accelerator mode.

You can also run this model alongside an additional USRP® device running the FRS/GMRS receiver example, instead of with a commercial walkie-talkie.

Exploring the Example

Due to hardware variations among FRS radios and the USRP® boards, you may not hear continuous transmission on your FRS radio. In that case, you can vary the gain of the source signals in theSourcesblock and examine the resulting behavior.

Part 95.637 (Modulation standards) of the FCC wireless standards %[4]states that the maximum frequency deviation is 2.5 kHz for FRS and 5 kHz for GMRS. In practice, it is usually set to 2.5 kHz for both systems. If the maximum signal amplitude increases, the frequency deviation parameter should decrease. Otherwise, the receiving walkie-talkie will not decode the CTCSS code correctly. You can try to use a different signal with different values for frequency deviation to see if your walkie-talkie works properly. If the frequency deviation value is too large, you may not hear anything from your receiver when using a non-zero CTCSS private code. Set the CTCSS code to 0, which disables the squelch system. If you hear the transmitted signal, the CTCSS decoding of the non-zero code is incorrect.

You can reduce the amplitude of the CTCSS tone to determine the minimum amplitude required for your receiver to work correctly.

References

Copyright Notice

USRP® is a trademark of National Instruments Corp.