LDACS1 Frame Structure: Components and Functionality

This page describes the LDACS1 frame structure used for resource access and communication between Aircraft Stations (ASs) and a ground station for air traffic control.

LDACS1 is the L-band Digital Aeronautical Communication System operating in FDD configurations at the airport and along the flight path from origin to destination. As we know, OFDM frames are constructed using OFDM symbols. Different types of frames are generated depending on the MAC data frame to be transmitted. These include data segments, dedicated control segments, and random access frames.

LDACS1 uses OFDMA-TDMA for transmissions from Aircraft Stations (ASs) to the Ground Station (GS).

OFDMA in LDACS1

In OFDM, all subcarriers of the entire symbol are assigned to one signal user (i.e., subscriber). In OFDMA, the total subscribers are divided into subchannels or groups of subcarriers (either contiguous or distributed). These are assigned to multiple users or subscribers, or in this case, Aircraft Stations.

In LDACS1, two users have been assigned subcarriers in one single FFT across one time symbol. A minimum of one tile structure is assigned to each Aircraft Station. One tile is composed of 6 symbols across the time axis and 25 subcarriers across the frequency axis.

LDACS1 Frame Structure Details

Figure 2 depicts the LDACS1 frame structure used by Aircraft Stations (ASs) for communication with the Ground Station (GS) in LDACS1.

As shown, the LDACS1 frame consists of superframes, multiframes, and tiles. Each tile has a duration of 0.72ms. One superframe has a duration of 240ms.

This superframe consists of a Random Access subframe of duration 6.72ms and four multiframes, each with a duration of 58.32ms. The data carried on the reverse link is in the form of a PHY-PDU provided by the MAC sublayer. The size of the RL PHY-PDU varies based on different frames and tiles. One RA PHY-PDU is mapped to one RA subframe, as shown.

Each multiframe is composed of a Dedicated Control Segment and a Data Segment.