## 3 Physical Layer Design
### High-Bandwidth Antennas  
The most important criteria for the antenna is that it should be embedded on a die while supporting high bandwidths. Therefore, it needs a small footprint, which in turn dictates the choice of the carrier frequency as the antenna size is proportional to the wavelength of the carrier. Mm-wave antennas will have dimensions of the order of a millimeter or less making their footprint acceptable for on-chip implementations. Moreover, traditional off-chip interconnects such as substrate traces or PCIe provide multi-gigabit bandwidths. In order to provide an energy-efficient alternative to the traditional inter-chip interconnects the wireless interconnects must also provide competitive data rates. Therefore, to enable multi-GHz bandwidths and a small antenna footprint, the minimum carrier frequencies need to be in the mm-wave range higher than few tens of GHz. Wireless interconnects using on-chip antennas tuned to mm-wave bands offer a mature interconnect technology as the antennas can be fabricated using the Back End of Line (BEOL) steps of CMOS fabrication processes. Many on-chip antennas such as meander dipole, zigzag dipole, slot, loop, inverted F, bow-tie, and Yagi have been designed and investigated [55]. To enable the inter-chip wireless interconnection, multiple cores in the chip needs to be equipped with antennas as discussed in Section 2.2. Therefore, the antennas should be compact enough to fit within the footprint of the core, which is typically a few square millimeters in current and future technology nodes. Relatively smaller antennas such as zigzag monopole antenna have been designed [56]. Zigzag monopole antenna provides a compact size and can be easily fabricated. The mm-wave zig-zag on-chip antennas and their co-planar waveguide feeds were designed to resonate in the 60 GHz frequency band [11]. Figure 4 shows this antenna with its major dimensions. This antenna has a footprint of about 0.385 mm\({}^{2}\). A co-planar feed structure is chosen for the antenna as it has low losses compared to other feed structures such as microstrips. A simple model of the chips was assumed without any heat sinks or complex structures that are typically found in such environments. All the antennas were tuned to 60 GHz with less than \(-25\) dB of return loss and the worst transmission coefficient between the pairs was found to be around \(-35\) dB [11]. In addition to the above antenna designs, novel techniques can be used to enhance various aspects of the antenna characteristics. Compact antennas are generally not very directional. In order to improve directional gains of on-chip antennas fixed beam log-periodic antennas are also investigated [57]. On the other hand, phased antenna arrays provide the advantage of high directive gain with beam steering although, with additional feeding circuitry. However, given the wavelengths of a few millimeters and die sizes of few millimeters across, it is challenging to design antennas arrays to enable beam-steering for intra- and inter-chip wireless communication. Leaky wave antennas (LWA) are potentially viable alternatives to phased antenna arrays [58]. The bandwidth of on-chip antennas can be increased by using a combination of multiple antennas operating at different center frequencies. This will effectively create multiple frequency bands. Meta-surfaces such as an Artificial Magnetic  
Figure 4: Fabricated metal zigzag antenna (microscopic picture) [56]. Conductor (AMC), which are periodic structures of elements can be designed in conjunction with the antennas to enhance the transmission of the on-chip antennas as well [59; 60; 61]. These structures can be used to reflect the antenna transmission outside the die reducing energy penetration into the substrate. However, the constraints on footprints in the on-chip environment need to be considered while designing meta-surfaces.