OFDM-Based Radar Network Providing Phase Coherent DOA Estimation

Abstract

Next-generation radar sensors require imaging capabilities with high angular resolution. As for a single sensor, the aperture, and thus the achievable resolution, is limited due to the constraints of the front end, radar networks consisting of multiple sensors are a possible solution. However, their incoherency usually makes joint angle estimation impossible. This article presents a network concept consisting of an orthogonal frequency-division multiplexing (OFDM) radar and repeater elements, which receive the reflections from targets and retransmit them back to the radar. Thereby, any frequency conversion from radio frequency to baseband and vice versa is omitted such that the signal remains coherent to the initial transmit signal. To distinguish the bistatic signal transmitted by the repeater from the monostatic one of the OFDM radar, the orthogonal subcarrier structure of OFDM waveforms is exploited by combining a sparse radar transmit signal with a low-frequency modulation in the repeater. This allows to evaluate the bistatic signals at the radar with standard multiple-input-multiple-output (MIMO)-OFDM signal processing, leading to separate range-Doppler images for each virtual channel. Finally, it is shown that this method offers a coherent angular estimation based on the extended aperture of the network. For this purpose, a method to establish phase coherency by a reconstruction of the modulation phase is presented. The network concept is proved with measurements at 77 GHz.

Multiplexing of OFDM-Based Radar Networks

“…The repeaters consist of a double-sideband (DSB) mixer and amplifiers [10], [11]. Based on their receive (Rx) signal x Rp (t), their modulated Tx signal can be described as [1] y Rp (t) = x Rp (t) α Rp e +j(fmodt+φmod) +e −j(fmodt+φmod) . (1)…”

“…For example, measurements from different aspect angles of objects can be combined to provide an enhanced estimation of the targets properties. Moreover, by exploiting the enhanced aperture of the coherent network, a more precise direction of arrival (DoA) estimation is possible [1], [2].…”

“…A promising realization of a coherent sensor network is the radar-repeater network as presented in [1], [3], [4]. It combines one MIMO radar with several repeater nodes.…”

“…Through the frequency multiplexing capabilities of the repeater nodes, the application of orthogonal frequency-division multiplexing (OFDM) signals in the network is a proper choice due to its inherent subcarrierbased frequency-division multiplexing (FDM) signal structure. If the repeater nodes shift the signal by an integer multiple of the OFDM subcarrier spacing, the bistatic signal may be interleaved in the OFDM signal structure [1]. However, subcarrier interleaving reduces the unambiguous range and this problem multiplies with additional transmit (Tx) channels and repeaters.…”

Time-and-Frequency Hybrid Multiplexing for Flexible Ambiguity Controls of DFT-coded MIMO OFDM Radar

“…On the other hand, digital-oriented MIMO radar systems employing phase-modulated continuous-wave (PMCW) signals [2], [12] or orthogonal frequency-division multiplexing (OFDM) signals [23]- [44] have been studied for next-generation automotive radar systems. The MIMO OFDM radar, which has many advantages such as proliferation of radio-frequency (RF) front-ends from communication system [29]- [30], flexible digital design [31]- [32], and interference mitigation [34]- [35] is expected to be widely used in a radar sensor network [36]- [37], passive radars [38]- [40], and joint radar-communication (JRC) [31]- [34], [41]- [44] as the fields of future technology. Since OFDM-based JRC systems are seriously considered to be promising digital radars, the OFDM waveforms have been intensively studied to further increase the communication capacity [41]- [42] or improve the range and Doppler resolutions of radars [43]- [44].…”