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Modulation Design

In digital communications, modulation is the process in which a bitstream is converted to a waveform compatible with a transmission channel.

Some design choices, such as modulation, were indirectly affected by the need for interoperability. Modulation is the process by which a signal is encoded for transfer over a data link, and demodulation is the process by which it is decoded upon receipt for further processing. Different modulation schemes have varying abilities to deal with noise and interference. In the case of Mode S, the choice of using interoperable frequencies affected modulation because interference became a large concern. However, the easiest way to ensure backwards compatibility was to make as few equipment changes as possible. This approach was also endorsed by the contractors that would eventually manufacture the equipment since it required fewer changes to their production and testing facilities. In the end, a compromise was reached that utilized a combination of old and new modulation techniques.

To minimize transponder cost, the designers of Mode S experimented with only binary modulation schemes, such as pulse amplitude modulation (PAM), frequency shift keying (FSK), and phase shift keying (PSK). More sophisticated techniques typically found in today's wireless communication systems, thus, could not be used.

The researchers initially believed that both the uplink and downlink transmission should use pulse modulation. Pulse modulation had been used for both the uplink and downlink transmissions in the ATCRBS system. In addition, the circuitry was both inexpensive and easy to implement. Therefore, if the Lincoln Labs researchers wanted to use a different scheme, they had to convince the FAA and transponder manufacturers first.

Interrogation:

Frequency 1030 MHz
Modulation Differential Phase-Shift Keying (DPSK)
Data Rate 4 Mbps

To investigate the sensititivy of candidate Mode S modulation systems to interference, either from ATCRBS transmissions or multipath reflections, mathematical models were done. The LL researchers examined three types of environments: additive white gaussian noise, ATCRBS interfering waveforms with noise, and ATCRBS interference with noise. The results indicated that DPSK had a clear performance advantage over both PAM and FSK.

The researchers faced an uphill battle to convince the FAA and manufacturers of transponder companies to use DPSK modulation. ``It was a hard sell,'' reflected Goblick. ``[They] were not used to this technology and its production problems.'' There had also been some question among the researchers whether DPSK demodulation could be implemented at an affordable cost. The problem with DPSK was convincing the FAA and transponder manufacturers that inexpensive demodulators could be built. They also had to show that these demodulators had a performance equal to the theoretical optimum. Eventually, the LL researchers was able to prove that the cost of implementing DPSK for uplink modulation would only be slightly higher than using PAM. Since DPSK had a clear performance advantage and was less susceptible to interference than PAM, "the cost-effective choice is DPSK for the [Mode S] uplink" (Thomas Goblick, ATC27). It had been a difficult sell, but eventually, the LL researchers prevailed.

Bit error probability versus signal-to-noise ratio comparison (PAM vs. DPSK)
("DABS Modulation and Coding Design" (ATC-52), T. J. Goblick, March 12, 1976)
Reply

Frequency 1090 MHz
Modulation Pulse Position (PPM)
Data Rate 1 Mbps

However, the recommendations for the downlink was not to use DPSK. The additional transmitter that would be needed on a transponder had been found to increase the cost by more than 20%.``One of the FAA officials was adamant,'' recalls Goblick. ``I want things absolutely low-cost.'' As a result, pulse-position modulation (PPM), a form of PAM modulation with a slightly better resistance to ATCRBS interference, was chosen.

The selection of the modulation scheme was a direct product of the desire to maintain interoperability, through the choice of frequency. In the old ATCRBS system, ground stations would often receive replies even when they had not recently sent an interrogation. The resulting situation was known as a FRUIT (False Replies Uncorrelated in Time) environment. The Lincoln Lab researchers had been inclined to select DPSK because it provided the most resistance to the interference caused by the ATCRBS FRUIT environment. When cost constraints discouraged the use of this modulation for the downlink transmission, the researchers opted to use PPM, which provided slightly better resistance than PAM.



The Story of Mode S: An Air Traffic Control Data Link Technology last modified: 12.06.2000