AMPS

The Advanced Mobile Phone System (AMPS) was developed by Bell Labs in 1983 and gained significant popularity throughout the 90s. This standard marked the advent of first-generation cellular technology, where voice transmission relied on frequency modulation (FM), akin to walkie-talkies. However, it utilized a segmented radio spectrum, integrating a digital control channel.

AMPS operated within the frequency bands of 825–845 MHz and 870–890 MHz, splitting the spectrum into 21 control channels and 395 traffic channels. Each user was assigned a dedicated channel for their calls. Interestingly, features such as roaming were also available, and dialing was relatively straightforward, paralleling modern phone systems. However, a major drawback was the analog nature of voice transmission—calls could be intercepted with a radio receiver tuned to the relevant frequencies. This vulnerability became a concern for journalists seeking to uncover secrets. In the 80s, the processing power of microcontrollers was insufficient for secure digital channels, leading to the use of digital streams solely for connection establishment while voice data was transmitted analogically.

Despite the discontinuation of AMPS services, enthusiasts can still create a short-range AMPS network using open-source software and Software-Defined Radio (SDR) technology. This venture can serve as a valuable educational tool or an engaging demonstration in exhibitions, making historical technology come to life rather than gathering dust in storage.

Important Note: Before engaging in any transmissions, it’s crucial to consult local radio regulations. This guide focuses on technical aspects rather than legalities. Generally, transmitting through a 50 Ohm dummy load instead of an antenna is permissible, allowing for short-range signal transmission. Higher power transmissions may necessitate a license.

Acquiring the Phone

Finding an AMPS phone can be straightforward—searching eBay with terms like “AMPS brick phone” yields numerous options starting around $20. Shipping and taxes might exceed the phone's price, however. A common issue with older phones is battery failure due to long inactivity. I opted to purchase two AMPS models—the Motorola StarTAC and the Motorola TeleTAC for testing calls between devices. The TeleTAC was affordable, and the StarTAC, the focus of this exploration, has a nostalgic appeal.

Be cautious, as some sellers may misrepresent their items. For instance, I purchased a phone labeled as “tested” but received it without a charger, rendering it useless for AMPS testing. While the seller technically didn't lie, it was frustrating. I ended up ordering another unit with a charger from a different seller as a backup. Always confirm details before purchasing.

Upon powering the device, the “NS” (no service) LED will blink, indicating it cannot connect to any network—understandable since AMPS signals were last transmitted in 2007.

Setting Up the Network

To establish our micro-network, we’ll require several components:

• Software-Defined Radio Transmitter: This device should transmit and receive wide-band signals in full-duplex mode. Options like USRP or LimeSDR are suitable for this task.
• Linux Computer: I utilized a Raspberry Pi 4 for convenience, avoiding the need to install Linux on my main system. A reliable power supply is essential for powering the transmitter through the Raspberry Pi.
• Transmitter Control Software: For AMPS, two software options are available: osmocom-analog and gr-amps.

#### Software Installation

The software was designed for Linux and, while it can be compiled on Windows, many libraries may be lacking. In Linux, everything should work seamlessly. Here’s the installation process:

First, update your system and install the SoapySDR library, which is necessary for hardware access. At the time of writing, version 0.7 is compatible with osmocom-analog:

sudo apt-get update sudo apt-get install build-essential autoconf gcc make cmake libasound2-dev pkg-config wget https://github.com/pothosware/SoapySDR/archive/refs/tags/soapy-sdr-0.7.2.tar.gz tar -xf soapy-sdr-0.7.2.tar.gz cd SoapySDR-soapy-sdr-0.7.2 && mkdir build && cd build cmake .. make -j4 && sudo make install && sudo ldconfig cd ../..

If using a USRP transceiver, install the USRP library:

sudo apt-get install libuhd-dev uhd-host wget https://github.com/pothosware/SoapyUHD/archive/refs/tags/soapy-uhd-0.4.1.tar.gz tar -xf soapy-uhd-0.4.1.tar.gz cd SoapyUHD-soapy-uhd-0.4.1 mkdir build && cd build cmake .. make -j4 && sudo make install && sudo ldconfig cd ../..

For LimeSDR users, install the LimeSuite library:

wget https://github.com/myriadrf/LimeSuite/archive/refs/tags/v20.10.0.tar.gz tar -xf v20.10.0.tar.gz cd LimeSuite-20.10.0 mkdir build && cd build cmake .. make -j4 && sudo make install && sudo ldconfig cd ../..

Finally, download and install the osmocom-analog software:

git clone git://git.osmocom.org/osmocom-analog cd osmocom-analog autoreconf -if ./configure make -j4 sudo make install cd ..

#### Testing the Setup

Once the software is ready, connect the transmitter to the Raspberry Pi and execute the following command:

sudo amps --sdr-soapy --channel 334 --channel 332 --samplerate 256000

This command opens two channels: control channel 334 and voice channel 332. The total channels required equal the number of phones plus one. You can also adjust transmission power and sensitivity as needed.

If configured correctly, the software will initiate, displaying ASCII graphics in the console:

Simultaneously, the phone will receive data from the control channel, changing its LED status to “online”:

You may notice the “Rm” (roaming) LED illuminated, as the “fake” operator ID doesn’t match the preprogrammed ID in the phone. This is not an issue, and you won't incur extra charges for roaming. At this stage, you can make and receive calls between the computer and the phone:

Extended call data will also be visible in the logs:

During a call, it’s fascinating to observe the AMPS signals in the radio spectrum, revealing a digital control channel alongside a voice channel:

Conclusion

Testing the operation of the 1G cellular network was an enlightening experience, yielding several surprising insights:

• The system functions effectively despite being developed in 1983. Dialing and answering calls bear a remarkable resemblance to current smartphones.
• The sound quality is unexpectedly high, shedding light on why users lamented the transition from AMPS to the initial GSM generation—FM modulation at 12 KHz performs well.
• Privacy concerns are significant in such a system, especially for public figures. AMPS allows anyone to intercept calls due to its standard FM modulation. However, this simplicity offers advantages, such as extensive coverage without requiring dense base station infrastructure—vital for rural areas, which contributed to AMPS's longevity in the USA.

While AMPS is now a relic of the past, it can still be explored for educational and demonstration purposes, which is a rewarding endeavor. A special thanks to the osmocom-analog contributors for making this revival possible.

Thank you for reading! If you're interested in exploring other “abandoned” standards, check out our article on testing the POCSAG paging protocol and two-way paging.