# A Photonically-Excited Leaky-Wave Antenna Array at E-Band for 1-D Beam Steering
## 5 Featured Application  
In a potential scenario, we discuss the application of the LWA for beam alignment in wireless data center interconnections. Wireless interconnections in data centers were proposed in [28] to provide scalability and reconfigurability, which are difficult to achieve by fiber interconnected nodes as the number of servers increases. Mm-wave links at 60 GHz or sub-THz frequencies, better suited for tens of Gbps data rates, could provide a flexible alternative to an all-fiber scenario.  
Figure 11a depicts a simplified data center node structure with server racks distributed in a hexagonal lattice. The servers form sub-groups that are interconnected by fiber, whereas the wireless links provide flexible interconnection between them, thus forming a hybrid architecture. More details on hybrid architectures can be found in [28].  
Electronic beam steering with phased arrays is frequently proposed in the literature as a means to align the different line-of-sight (LoS) links [16; 28]. However, mechanical alignment is more affordable, given the cost and energy consumption of many active elements required in a high gain phased array. As discussed next, these high-frequency data links feature high directivity and narrow beamwidth, which require dedicated hardware, as well as the corresponding protocol to control the stringent alignment.  
In [18] and in the present manuscript, we reported on the LWA that produces a beam squint with frequency, which is one of the simplest forms to steer the beam. It shows a medium directivity of about 18-20 dBi, points within the 22\({}^{\circ}\) scan range, and works at the E-band (from 75 to 85 GHz). Owing to the antenna scalability, these characteristics can be easily adapted for a lower HPBW and enhanced pointing resolution. Indeed, the row composed of 19 periods presents an HPBW of 4\({}^{\circ}\) (18.5 dBi gain for one row), while maintaining good input impedance matching. Besides, the same concept can likewise be employed at other frequencies [29]. In this use case, the LWA would be co-located with the transceivers on top of the racks in a structure that would offer in-plane mechanical rotation, as shown in Figure 11b. After, the LWA could be used to establish optimum alignment between transceivers in the link. During alignment, a signal with reduced bandwidth or lock-in detection could be employed, lowering the demand for highly directive antennas and eluding a dedicated QO system. For instance,  
Figure 10: Detected power after the QO system.  
for a detector NEP of \(2\,pW/\sqrt{Hz}\)[30], and a signal bandwidth of 1 MHz, it suffices with 21 dBi directive antennas to obtain an SNR = 10 (0 dBm noiseless radiated power, 10 m distance, \(f\) = 75 GHz, and \(D\) = \(G\) for the Rx antenna). For 10 GHz bandwidth, the required directivity would increase to 31 dBi.  
This solution would offer a fast and simple option for data center reconfigurability. Additionally, the transceiver hardware, represented in blue in Figure 11b, could be partially reused. For instance, it would suffice with an optical switch for a photonically generated signal to select either of the antennas. Therefore, this solution would play little role in the overall system cost or size. The PCB in which the antenna has been fabricated is 30 mm by 30 mm, with an approximate packaging footprint of around 5-10 cm\({}^{3}\).