# GAE and OBE Enhanced Interference Mitigation Techniques in LDACS
From the literature of L-band spectrum deployment [8, 9, 10, 11], some of the legacy users are distance measuring equipment (DME), the military tactical air navigation (TACAN) system, and the joint tactical information distribution system (JTIDS) used for navigation aids (Figure 1). Apart from these, universal access transceiver (UAT) at 978 MHz and secondary surveillance radar (SSR), airborne collision avoidance system (ACAS) at 1030 and 1090 MHz are allotted with fixed channels. With this knowledge, the LDACS system is deployed in the L-band either as an inlay system between the legacy users, such as DME, or as an overlay system in the unoccupied spectrum [12]. Though the overlay method is less complex and selected for GSM, such as LDACS2 (960-975 MHz), spectrum scarcity is a noticeable challenge [13, 14, 15]. LDACS1, as an inlay approach, takes the advantage of the 1 MHz spectral gap between DME signals. LDACS1 involves two-way communication: a forward link (FL) from the ground station (GS) to the air station (AS) and a reverse link (RL) from AS to GS. It provides frequency division duplexing (FDD) of 63 MHz spacing between FL (962-1213 MHz) and RL (1025-1150 MHz) with the opportunistic access of the paired spectrum. From Figure 2, the possible interference scenario for the LDACS1 inlay system can be recognized as follows:  
Figure 1: L-Band spectrum occupancy [12]. (a) LDACS1 FL is affected by DME GS (FL) not by DME airborne station (AS), as (RL) is not active in this part of spectrum (b) LDACS1 RL is affected by both the DME GS (FL) and DME AS (RL), (c) DME FL is affected by interference from both the LDACS1 GS (FL) and LDACS1 AS (RL), and (d) DME RL is affected with interference from LDACS1 AS (RL) not from LDACS1 GS (FL), as it is not active in this part of the spectrum [16]. Relative studies between LDACS1 and LDACS2 have shown that LDACS1 has significantly better performance than LDACS2, resulting in a conclusion that LDACS1 can be the final choice due to the capability of supporting a diversity of services and the cellular compatibility, and is referred to as LDACS [17; 18]. The utilization of the L-band for aeronautical applications is very challenging, due to the occupancy of legacy systems. From the detailed study of potential levels of interference from the various legacy systems [19; 20], it is found that DME is the prime legacy user occupying most of the 960 MHz to 1164 MHz spectrum. Thus, the deployment of LDACS as an inlay system with licensed users in the L-Band can cause interference to license users and vice versa. Any malfunctioning of a licensed system is a serious issue, as it is directly related to flight safety. Consequently, interference mitigation is an essential component in an LDACS receiver. The rest of the paper is organized as follows: Section 2 briefs the literature survey about the merits and demerits of the existing DME interference mitigation techniques in LDACS. Section 3 discusses the system and noise model used for this work. Section 4 includes the mathematical functioning of proposed nonlinear estimators on the received data, and Section 5 characterizes the difference in the performance of these proposed nonlinear estimators in terms of the accuracy of the received data.