Aerospace and Electronic Systems Magazine November 2017 - 27

Bilzhause et al.

Figure 3.

LDACS inlay concept (left) and recorded LDACS and DME signals from flight experiments [39], [40] (middle, right; courtesy of the authors).
C

Overlapping cell coverage: Interfering with the signal of multiple
ground stations is more difficult
than attacking the signal of one.
Use of location data and directed
antennas: By using surveillance
data to acquire location information, the expected angle of arrival
can be calculated. Directional antennas can then be focused on the
direction from which the signal is
estimated to be received. This is
particularly useful for aircraft.
Multiple datalinks: By continuously monitoring a set of features, such as signal-to-noise
ratio, packet error rate, location,
speed, and Doppler shift, unauthorized signal sources can
be detected. If an unauthorized
signal is detected, air traffic
management communication is
switched to a secondary datalink
using a different technology, e.g.,
a satellite datalink.

Figure 4.

LDACS frame structure: LDACS uses two radio channels for the forward link (FL; ground to air) and
reverse link (RL; air to ground) directions separated by frequency division duplex. Both channels are
structured into multiframes of a constant duration of 58.32 ms. Each multiframe is internally structured
into a control slot (dedicated control slot (DC) or common control slot (CC) in the figure), and a data
slot. The slots are created from frames on the FL and tiles on the RL. The random access (RA) and
broadcast control (BC) slots are inserted every four multiframes to create a superframe with a 240 ms
duration. FL and RL multiframes are interleaved in time for optimized control slot spacing.

All of these measures come at significant cost. The robustness
of the LDACS signal may be strengthened by increasing the transmission power. However, this measure might cause interference toward the DME system, which is not acceptable. The other measure
discussed above requires changes in the infrastructure: multiple
ground stations, directed antennas, secondary datalinks, or a combination thereof. The SESAR program is pursuing the multilink
concept, i.e., the use of secondary datalinks.

SECURITY IN THE MEDIUM ACCESS CONTROL SUBLAYER

Multiframe Level
LDACS structures the OFDM symbols of the physical layer into multiframes with a 58.32 ms duration, as illustrated in Figure 4. Every four
NOVEMBER 2017

multiframes, an additional random access or broadcast control slot with
a 6.72 ms duration is inserted to support the initial cell entry of aircraft.
Together this constitutes a superframe with a 240 ms duration.
Introducing security on the multiframe level would require covering the complete multiframe with additional security data. Assuming s-bit security information, the overhead would be s/19,656
in the worst case.6 Although this approach adds little overhead, it is
problematic when it comes to latency requirements, error recovery,
and trust relationships.
Multiframe-based security allows security verification on the
receiving side only to be performed after the complete multiframe
6

Each forward link multiframe comprises 9 OFDM frames. In the
most robust coding and modulation scheme, this corresponds to
9 × 2,184 = 19,656 bits for the forward link multiframe. Using
less robust coding and modulation schemes, the forward link
multiframe can grow to 88,992 bits.

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine November 2017