MAXIMAM LIKELIHOOD ESTIMATION FOR ALAMOUTI ENCODED OQPSK TRANSMISSION OVER AERONAUTICAL TELEMETRY DATA LINK

Abstract

For aircraft Fight testing, telemetry data plays a major role in evaluating the performance of the aircraft in real time while ensuring flight safety. Maintaining the data continuity during the entire course of the fight is mandatory considering the risk involved in fight testing. To achieve this, two antennas are generally installed (one at back and another one at belly of the aircraft), which provide approximately omni-directional data coverage for telemetry data transmission. However, the structure of the aircraft may obstruct transmission from any one of the antennas to the ground telemetry station. Problem arises when the signal received from both the on-board antennas are of same magnitude and opposite phase. In this situation, the signal may cancel each other at the receiver leading to self-interference. This phenomenon is termed as \two-antenna problem" in aeronautical telemetry. To combat the self-interference, transmission from both the antennas can be realized on two different frequencies. Since this method increases transmission bandwidth by two times, it is rarely used. As an alternative, continuous antenna beam steering (considering both antennas as antenna array) can be performed to direct the signal towards telemetry station. This method is mechanically complex, and requires continuous update on the precise telemetry location with reference to aircraft. Alternate solutions have been investigated to solve aforementioned self-interference issue. Space time code (STC)is one such solution which promises to deal with the \two-antenna problem". One of the variant of STC, speci c to the case of two transmit and one receive antennas, is called Alamouti code. In this code, two consecutive symbols are simultaneously transmitted from both the antennas in the rst time slot. In the next time slot, same two symbols are transmitted in such a way that they are orthogonal to the symbols transmitted in the previous time slot. Shaped O set Quadrature Phase Shift Keying (SOQPSK-TG) and Feher's Quadrature Phase Shift Keying (FQPSK-B and FQPSK-JR) are standard modulation schemes used in aeronautical telemetry. As these schemes are nonlinear in nature, application of Alamouti codes is not obvious. Alamouti encoded symbols are used to modulate the CPM phase states of the modulation scheme. As the STC encoded signals from the two antennas pass through the channel, they get exposed to channel impairments (frequency o set, channel attenuation and time delay). Detection of the signal at the receiver needs estimation of these three parameters. In this thesis, estimation of these three parameters is carried out by placing pilot bits in the transmitted signals. A joint maximum likelihood (ML) estimation of these unknown parameters is carried out and analyzed. Since the joint ML estimator needs a complex 3-Dimensional search, a sequential estimator requiring a less complex search is derived and analyzed. The performances of both joint as well as sequential ML estimators are found to be satisfactory and meeting the Cram er􀀀Rao bound (CRB) criteria. Due to partial response nature of SOQPSK-TG frequency pulse (spanning across 8 bit period), maximum likelihood sequence estimation (MLSE) has 512 states. Implementation of branch metric calculation for 512 states is highly complex. Pulse shaping has been carried out to generate the SOQPSK-TG approximation, which is subsequently been used in the sequential estimator. CPM approximation of FQPSK-JR has also been considered as another candidate for pulse shaping. The mean squared error (MSE) performances for both of these schemes are found to be satisfactory and meeting the CRB criteria. As both of these schemes use frequency pulse spanning across 2 bit periods, STC decoder based on MLSE has 8 states. Due to the lesser number of states, decoder complexity gets signi cantly reduced.