# GAE and OBE Enhanced Interference Mitigation Techniques in LDACS
## 5 Results and Discussions  
This section discusses the variation in performance of proposed nonlinear estimators GAEPPA, GAEPPL, JGAEPPA, JGAEPPL, OBEPPA, OBEPPL, JOBEPPA and JOBEPPL in reducing DME interference when employed in OFDM-based LDACS communication. The discussion is based on the results obtained from the Matlab simulation of the LDACS forward link communication prototype. The performance of proposed methods is comparedwith the existing pulse blanking method in terms of variation in BER at the LDACS receiver for different SNRs. The mathematical model of LDACS FL GS transmitter (Figure 3) and LDACS FL AS receiver (Figure 4) are developed in accordance with standards of the LDACS system for all the inner building blocks. At the transmitter side, random data of 91 bytes are generated by the data source and given as the input of RS code (91,101) for external coding. Once external encoding is performed by the RS encoder, 6-bit zero padding is performed before passing through internal encoding by the convolutional encoder (171,133). The encoded bits from the output of the convolutional code with a native coding rate of half are further interleaved (using the permutation interleaver), mapped to symbols (using symbol mapper). The mapped symbols form complex values when it passes through the QPSK modulation block. The frame composer block forms the LDACS FL Data/CC frame with proper insertion of pilot values (158), null values (728) and complex data values (2442) over a total of 3328 subcarriers. Further, the time domain composite waveform of this OFDM frame is generated by passing the frame through the IFFT block of length 64. The effect of the introduction of IFFT (windowing) is canceled by adding 16 cyclic prefix bits. Table 1 holds the OFDM system parameters used in this simulation study. The block diagram of the proposed LDACS receiver is shown in Figure 4. Each block of the receiver is designed in such a way that, it is capable of retrieving the data transmitted from the transmitter if there is no noise in the channel. From the received data, the cyclic prefix bits are removed initially. The resulting data are further converted into the frequency domain so that pilot removal and complex data segregation from corresponding subcarriers can be done at the frame decomposer block. The complex data values further undergo QPSK demodulation and symbol de-mapping to obtain the bitstreams. The bitstreams are de-interleaved and decoded using the de-interleaver and vitterbi decoder, respectively. From the output of the vitterbi decoder, redundant bits are removed and decoded, using the RS decoder to get the original data. In order to analyze the performance degradation of the LDACS FL AS receiver due to DME interference, the AWGN channel is considered. The BER variation of the received signal when passed through the AWGN channel without the influence of the DME interference is obtained as shown in Figure 7. For the study of interference on LDACS, DME signals are generated in accordance with (3) for a duration \(\Delta t\) of 12 us. The baseband DME pulse pairs are modulated to the relative carrier frequency of the channel to 0.5 MHz left and to the 0.5 MHz right of the LDACS1 system bandwidth. A reduction in performance of the LDACS FL AS receiver can be observed when DME noise is allowed to affect the transmitted data. Figure 7 also shows how the existing simple noise reduction method