An Overview of Link 16

It is common practice now for tactical data links to use frequency-hopping spread spectrum (FHSS) in order to improve immunity to jamming. Link 16, a tactical data link standard used by NATO, is a jam-resistant high-speed digital data link which operates in the radio frequency band 960–1,215 MHz.  Reserved on a worldwide basis by the ITU Radio Regulations, Link 16 is used for the operation and development of airborne electronic aids to air navigation.

• 960–1164 MHz:  Aeronautical Mobile and Aeronautical Radio Navigation
• 1164–1215 MHz: Aeronautical Radio Navigation and Radio Navigation Satellite

It is interesting to note that these frequency ranges limit the exchange of information to users within line-of-sight of one another. Link 16 has the following data rates:

Information Data Rates (kbps)

Channel Data Rates (kbps)

1-Standard: 26.88

1-Standard: 59.520

2-Packed-2: 53.76

2-Packed: 2 119.040

3-Packed-4: 107.52

3-Packed: 4 238.080

Link 16 is a TDMA-based secure data link; it works with assigned time slots. The Link 16 time intervals are:

• 24 hours = 112.5 epochs
• 1 epoch = 64 Frames
• 1 frame = 1,536 time slots

Each time slot lasts 7.8125 ms and uses the following format:

1. Jitter period
2. Synchronization symbols
3. Message Transmission symbols
4. Guard period

The symbols can be packed as follows:

1. Standard (3 words)
2. Packed-2 (6 words)
3. Packed-4 (12 words)

There are four options to transmit Link-16 symbols:

1. Standard symbols are always transmitted with a double-pulse structure (STD-DP).
2. Packed-2 symbols can be transmitted with a single-pulse (P2SP) or double-pulse structure (P2DP).
3. Packed-2 symbols can be transmitted with a double-pulse structure (P2DP).
4. Packed-4 symbols are always transmitted with a single-pulse structure (P4SP).

A Link 16 communication terminal is called a Joint Tactical Information Distribution System (JTIDS). The terminal interface standards (hardware/software) are presented in the JTIDS System Segment Specification (SSS) DCB79S4000C. In Link-16, there are no nodes required to maintain network operation. This means that the loss of any unit does not cause a system failure.

Link 16 implements a hybrid direct-sequence/frequency-hopping spread spectrum (FHSS) system, which means the transmit frequency is not held constant. The frequency hopping occurs over 51 frequencies (also called bins).

• Link 16 hops at a rate of 1/13 ms (76,923 hops per second)
• Link 16 has 2 IFF notches centered at 1030 and 1090 MHz

Developing Link 16 applications on the Radio420X

Link 16 operates in frequency bands supported by Nutaq's Radio420X platform. As we have seen in a previous blog post,"Cognitive OFDM on PicoSDR", the Nutaq PicoSDR has frequency hopping capabilities which make it suitable for developing frequency-hopping spread spectrum (FHSS) applications like Link 16.

In this example, the FPGA logic is used to change the frequency of the VCO (in fact it is the frequency of the PLL is changed). This clarification is important because our designs also support changing the frequency from an external computer via Gigabit Ethernet, which is slower. The setup time, when changing the frequency from the FPGA, is comprised of 12 SPI register writes that configure the Tx and Rx frequency. According to be SPI_busy signal, it is 270 clock cycles (on average) per SPI register writes. With a clock of 80 MHz that means:

1/80 MHz * 270 * 12 = 40.5 µs (Rx and Tx)

The Radio420X is designed around the LMS6002D chip. An additional period of 20 µs is necessary for the PLL inside the LMS6002D to stabilize (see page 5 of the Radio420X data sheet). This must be added to the 40.5 µs, resulting in a total of 60.5 µs (16.5 hops per ms).

The LMS6002D remains functioning at the current frequency as long as you do not send the last command (12th write to SPI register). If we need to change the Tx and Rx as quickly as possible in response to an external factor, we must consider the time required to perform 12 register writes, as is the case in the above example.

In a case of "hopping" from a known sequence (such as for Link 16), we only need to consider the last command and the stabilization time of 20 s of the LMS6002D’s internal PLL, which leads to lower than 60.5 µs results (over 16.5 hops per ms).

The Link-16/JTIDS standard will remain in use for many years to come, so research on how to defend against hostile interference will continue to generate interest. Nutaq’s PicoSDR system, equipped with the Radio420X, is the perfect platform for prototyping improvements to Link-16 or other tactical data links that use frequency-hopping spread spectrum (FHSS).