On SFBC Schemes for Enabling Virtual Array Concept in Monostatic ISAC Scenarios

Abstract

The integrated sensing and communication (ISAC) paradigm is being proposed for 6G as a new feature of the physical layer (PHY), for tackling dual-functional applications, i.e., demanding radio-sensing and communication functions, such as the Internet of Things (IoT) and autonomous driving systems. This work considers the integration of sensing and communications functionalities in a unique platform. To achieve this goal, the use of orthogonal space frequency block codes (SFBC) is proposed. SFBC code orthogonality enables both the separation of communications data streams at a user terminal and the estimation of target parameters. The SFBC enhances the communications link diversity without requiring channel state information knowledge at the transmitter and enable the virtual antenna array concept for enhancing the direction-finding resolution. The use of different SFBCs provides a tradeoff between achieved diversity and sensing resolution. For example, an Alamouti code, applicable for the case with two transmitting antennas, duplicates sensing resolution and achieves a diversity order of two while the use of a Tarokh code, applicable for a scenario with four transmitting antennas, provides a fourfold better resolution and diversity order of four. However, the code rate achieved with the Tarokh code is half of the one achieved with the Alamouti code. Furthermore, the unambiguous range is reduced since the bandwidth is divided to multiplex the different antenna signals. For its simplicity, good performance and reduced integration requirements, the method is promising for future ISAC systems.

Quasi-Orthogonal SFBC for Monostatic MIMO ISAC Scenarios

“…The P transmitting elements are spaced by a distance of /2 Q , where  denotes the wavelength. The specific spacing of the transmitting antennas obeys the virtual array concept [11], which enhance the attained angular resolution [14,15].…”

“…First, the CP is removed and the received signal deframed, then the symbols are decoded by the SFBC Tirkkonen decoding block, which follows the data estimate. After OFDM deframing, CP removal, and considering that the coherence bandwidth is equal or greater than the bandwidth of a sub-carrier block (i.e., four consecutive subcarriers for Tirkkonen SFBC), the received signal at the UE for the k th index frequency block and l th OFDM symbol, can be expressed as [15],…”

“…More recently, the contribution in [14] discussed the exploitation of Alamouti SFBC for the ISAC monostatic paradigm, where the results showed the suitability of this code for radio-sensing purposes. This result was extended to a general framework regarding orthogonal SFBC codes in [15], where Alamouti was compared to Tarokh in terms of radio-sensing and communication. The latter contribution showed that although Tarokh presents better resolution than Alamouti for radio-sensing, it also reports reduced unambiguous range, which can be catastrophic when ranging objects on the edge of the desired exploration zone.…”

“…The latter contribution showed that although Tarokh presents better resolution than Alamouti for radio-sensing, it also reports reduced unambiguous range, which can be catastrophic when ranging objects on the edge of the desired exploration zone. This is because the unambiguous range is inverse to the number of resources used by the SFBC in the frequency domain, i.e., the Tarokh code employs a frequency block with eight sub-carriers, while Alamouti uses only two sub-carriers [15]. This work evaluates the performance of Tirkkonen SFBC [16] in a joint radio-sensing and communication scenario.…”

“…4. Following OFDM deframing and CP removal, the received signal for the k th sub-carrier block and l th OFDM symbol at the q th received antenna can be described by [15], , , , ,…”