Signal Processing Methods for Multicarrier Passive Radar and Communication Systems

Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission technique that facilitates efficient information transfer through a wireless channel by dividing the large frequency bandwidth into multiple smaller subbands. Besides being a favorable technology for the communications systems, OFDM waveforms can be utilized in radar systems as well. In particular, there has been a rising interest on the passive radar systems that operate without transmitters of their own and opportunistically use the illumination offered by existing com-munication or broadcast systems that often use multicarrier modulation. This thesis develops signal processing techniques for both the OFDM-based communications systems and passive radars utilizing OFDM broadcast systems as their illuminators. The thesis proposes novel target detection algorithms for passive radar systems that are based on the sec-ond generation digital television (DVB-T2) signals. It is shown that these methods, which are based on the control symbol structure of the DVB-T2 transmission, are able to detect targets even when the information of the transmitted data is not available at the radar receiver. Analyt-ical derivations of the probability distributions of the test statistics are provided in the thesis and the results are verified through simulations using different target fluctuation models. DVB-T2 transmitters can be arranged in a single-frequency network (SFN) configuration. This means that multiple transmitters are using the same frequency at the same time. Because of the identical transmitted signal waveforms, a passive radar receiver is not able to easily associate the measurements of the target reflections with correct transmitters. A solution to this association problem is formulated in the thesis. It is shown that the proposed algorithm can produce a reliable association result already with a single measurement per each available transmitter. Simulations also verify that the performance is significantly improved when the number of measurements increases. The thesis presents a design of a space-time block code (STBC) that can be used in multiple-input multiple-output (MIMO) SFN broadcasting systems where four transmit antennas are divided into two separate groups. This kind of arrangement can be found from the handheld digital television systems (DVB-NGH). The proposed STBC is shown to provide a performance that is very close to the earlier codes designed for similar systems while at the same time de-creasing the maximum likelihood decoding complexity. Another contribution of the thesis to the OFDM communications technology is a proposal of a low-complexity channel estimation method for Multiband-OFDM (MB-OFDM) based ultra-wideband (UWB) systems. This algo-rithm improves the performance of the least squares channel estimation without a significant increase in computational complexity by introducing a novel iterative interpolation technique.