OFDM-Based Spatial Data Focusing for Wireless Physical Layer Geocasting in Multipath Channels

Abstract

OFDM-based spatial data focusing (OFDM-SDF) is proposed as a novel means of performing wireless physical layer geocasting, i.e. spatially confined broadcasting. It is shown that this approach overcomes beamforming and directional modulation (DM) limitations by exhibiting higher spatial precision with a reduced number of antennas and offering uncoupled range-angledependent focusing. This paper describes the OFDM-SDF system model for multipath channels, including multipath robust equalization, design rules for steering phases and sidelobe mitigation, analytical geocast delivery zone derivation, and optimized symbol mapping. Using density-based clustering of the spatial bit error rate distribution, a procedure for identifying a practical geocast delivery zone and evaluating its precision and connectivity is proposed. OFDM-SDF's performance and multipath robustness are evaluated through Rice channel simulations as a function of the Rice factor. In particular, it is shown that a 2-antenna OFDM-SDF array matches the radial and angular precision of, respectively, a 6 and 12-antenna DM array in recent literature, while robustness is ensured for 5G small cell channels.

Frequency Diverse Array Spatial Data Focusing: Free Space and Multipath Experimental Validation

“…3) Receiver-side Signal Processing: From (10) it is clear that, as anticipated, multipath distortion of the LOS phase prevents conventional SDF ZF equalization of the received OFDM blocks (11) by a reference channel estimation Ĥ0 [q] from yielding the desired residual phase shift of the free space FDA-SDF equalized symbols (7). Therefore, just like OFDM-SDF in [14], a 2-stage equalization is required to emulate this desired behavior and ensure that the free space geographical FDA-SDF properties from [16] remain valid. Specifically, the n-th antenna's equalized OFDM blocks are computed as…”

“…This was first demonstrated in the angular domain by time-based SDF (T-SDF) [12] that uses time-orthogonal distributed transmission. It was extended to the range domain by exploiting OFDM subcarrier orthogonality and frequency diversity in OFDM-based SDF (OFDM-SDF) [13], [14]. Moreover, a hybrid FDA-based SDF (FDA-SDF) approach has been proposed in [15], [16].…”

Frequency Diverse Array Spatial Data Focusing for High Precision Range-angle-based Geocasting

“…Compliance to the steered residual phase condition (13) should be ensured at the target position. Thus, the final expression for the steering phase to be added to the symbols transmitted from the n-th antenna is found by evaluating (10a) and (10b) at the target coordinates (d steer , θ steer ) and substituting them for τ n and ∆τ n in (14).…”

“…Note that by exploiting time-orthogonality in the transmitted signals xn(t), no restrictions apply to the choice of frequency offsets ∆fn. In contrast to previous SDF[12],[13] and SAMC FDA[15] schemes, that employ frequency-orthogonality.…”

“…However, it is limited, just as PA itself, to focusing in the angular domain only. Range-angle-dependent SDF has been achieved by exploiting OFDM frequency resources in OFDM-based spatial data focusing (OFDM-SDF), both in free space [12] and multipath scenarios [13]. However, as frequencies are restricted to OFDM subcarriers, flexibility is limited and significant manipulation of the geocast delivery zone is challenging.…”