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LDACS1 Physical Layer

LDACS is under development as data link communication between Aircraft Stations and Ground station for Air traffic control and maintenance. LDACS stands for L-Band Digital Aeronautical Communication System. There are two systems proposed by EUROCONTROL, viz. LDACS1 and LDACS2. LDACS1 is based on OFDM modulation scheme and supports FDD topology. LDACS2 is based on GSM and supports TDD configuration. Transmission link from Aircraft Station (AS) to the Ground Station(GS) is referred as reverse link and transmission link from Ground Station(GS) to Aircraft Station is referred as forward link. We will see LDACS1 physical layer used in Aircraft Transmitter and Ground Station receiver. LDACS1 physical layer for the reverse link is divided into transmitter part used in AS and receiver part used in GS.

OFDM symbol in LDACS Reverse Link(Aircraft Transmitter)

Specifications Value
FFT Size 64
Number of Guard Carriers 7(Left side), 6(Right side)
Number of pilot subcarriers 12
Number of DC carrier 1
Number of Used Data carriers Total 50 (25 per user)
Subcarrier spacing 9.765625KHz
Useful Symbol Time 102.4 µS
Cyclic Prefix Time(11/64) 17.6µS
Total OFDM symbol time 120µS
Guard Time (Tg) 4.8µS
Total FFT Bandwidth 625KHz

Three types of MAC sub-frames pass through the LDACS1 physical layer. They are Data segment, Dedicated control segment and Random Access Frame. Based on these different types there will be variation in frame formation module as mentioned below. Pilot subcarrier values are also different for each case.

LDACS1 Transmitter Part

LDACS1 physical layer

On this page we will analyze the processing of RL Data PHY PDU through LDACS1 physical layer. Let us assume following modulation-coding parameters as defined in specifications.
•  Modulation Scheme: QPSK
•  Coding Rate: 1/2
•  RS parameter: (16,14,1)
•  Total Coding rate: 0.44
•  Number of uncoded bits: 112
•  Number of coded bits: 268

Following are the steps involved through LDACS1 physical layer transmitter chain.

Step 1: MAC sub layer data i.e. PHY PDU of size 14 bytes are fed as input to the RS encoder which produces 16 bytes as output.
Step 2: Six zero tail bits are added to the 16 bytes, makes it about 134 bits.
Step 3: This 134 bits pass through zero tail biting convolution encoder of rate 1/2. Which produces encoded bits of double the input size.
Step 4: The data is interleaved as per permutation equation provided which produces 268 interleaved data bits.
Step 5: These 268 data bits get modulated as per QPSK symbol table which produces 134 qpsk data symbols.
Step 6: These data symbols are processed differently for different MAC sublayer frames. Frame formation for the Data segment is explained below in the figure. Total 12 pilots are added to each symbol. 4 PAPR symbols are added for PAPR reduction purpose. After removing DC, 12 pilots and 4 PAPR we are left with 50 useful data carriers per FFT of size 64. For detailed LDACS1 frame structure refer LDACS1 frame structure. As mentioned one tile is made of 25 carriers and 6 symbols. The tile is minimum resource allocated for each user. Across one FFT there are two users allocated as shown as USER#1 and USER#2. Rest of the symbols are allocated for other users or to the same user#1 and user#2 based on data bandwidth requirement.
Step 7: 64 point IFFT is applied on these frame of data.
Step 8: Cyclic prefix and guard interval is added to the samples of OFDM symbol obtained in the step7.

LDACS1 Physical Layer Receiver

LDACS1 physical layer receiver

As shown in the figure-2 following steps are performed using LDACS1 physical layer receiver; on received impaired IQ data after it is passed from the RF subsystems.
Step 1: Time and frequency offset estimation and correction is done first.
Step 2: Cyclic prefix and guard intervals are removed.
Step 3: 64 point FFT is performed.
Step 4: Respective tile structure is segregated for the particular user. Based on the left or right pilot structure, channel estimation and correction is done.
Step 5: Frame deformation is carried out, i.e. pilots, guard and DC subcariers are removed from the received OFDM symbols.
Step 6: Data demapping (i.e. QPSK demapping) is performed, which produces data bits.
Step 7: De-interleaving is applied to the demapped data.
Step 8: Viterbi decoding is applied to the de-interleaved data which corrects any errors in the received impaired data packet.
Step 9: Reed Solomon decoding(RS decoder) is applied to the data obtained in step 8.

LDACS(L Band Digital Aeronautical Communication System) RELATED LINKS

Following links covers LDACS1 and LDACS2 Physical layers, LDACS1 and LDACS2 frame structures and provide difference between LDACS1 and LDACS2.
Difference between LDACS1 versus LDACS2
LDACS1 frame structure
LDACS2 frame structure
LDACS1 physical layer
LDACS2 physical layer
Air Traffic Management

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RF and Wireless Terminologies

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