## 8 Comparison with Other Emerging Interconnect Technologies
Surface Plasmon Polariton resonance in the specific Graphene-based structures can emit photons in the THz bands as a result of electronic pulse excitation [106]. Single walled metallic CNT structures have been shown to operate as dipole antennas when excited by a light source such as a laser source [103]. By controlling the length of the CNT elements, they can be tuned to specific frequencies making it suitable to employ Frequency Division Multiple Access (FDMA) as the medium access mechanism. This can create multiple non-overlapping THz channels greatly improving the interconnect bandwidth. Wireless NoCs utilizing CNTs for intra-chip communication have been proposed in [31]. Arrays of THz antennas based on Graphene have been proposed in [104]. A survey of THz based on Graphene has been presented in [105]. Graphene THz antennas have been used for broadcast enabled wireless NoCs [10]. While THz wavelengths are known to suffer exponential path loss due to molecular absorption for specific frequencies, the inter-chip distances being below 1 m will have negligible absorption [104; 107]. Moreover, the multi-chip system can be packaged inside a controlled environment to reduce or eliminate specific molecules to mitigate the effect of molecular absorption if necessary. Table 4 presents comparisons between various alternative inter-chip interconnection technologies in terms of qualitative advantage, disadvantage as well as energy/bit for a single point-to-point link and physical bandwidth per channel. The energy and bandwidth numbers vary depending upon design and implementation technologies. However, the numbers in Table 4 represent specific designs and implementations which are used for system-level comparative evaluation. Figure 19 shows the average packet energy and average bandwidth per core at network saturation for various interconnection technologies discussed here considering 65 nm technology node characteristics. We have considered a system with 4 multicore chips each with 16 cores in them and 4 memory stacks with 4 layers in each. In each multicore chip, the 16 cores interconnected with a mesh-based wireline NoC. For all the interconnect technologies, the memory stacks are connected to the I/O modules of the processing chips through 4 channels each consisting of 128 bit (assuming \(\upmu\)-bump pitch of 50 \(\upmu\)m and 10 mm die edge) [96] wide channel operating at 1 GHz. Hence, this wide I/O provides a total bandwidth of 128 Gbps per channel with its neighboring processing chip with an  
energy consumption of 6.5 pJ/bit [96]. The 4 channels of the HBM memory are connected with this wide I/O to the adjacent 4 cores along the boundary of the adjacent chips. For the conventional I/O-based system we have considered an architecture where a middle core along an edge is connected to its counterpart on the nearest edge of the adjacent chip. The high speed serial I/O channels are adopted from 65 nm designs and are shown to have a bandwidth of 15 Gbps with an energy consumption of 5 pJ/bit [108]. As analog I/O circuitry do not scale well with technology node these parameters are representative of current technology trends [73]. For the mm-wave wireless multi-chip system we have considered the inter-chip communication to utilize the mm-wave transceivers that are located at one of the central cores of each chip achieving a WI density of 1/16 WI/core. This design principle is adopted from the discussion on topology selection in Section 2.2. In the photonic multi-chip system, the inter-chip communication happens through high bandwidth photonic interfaces. To make all the systems comparable, we consider one photonic interface at each chip at the same location as the WIs. We connect these interface switches through a single waveguide. For our experiment, we consider one waveguide with 8-way Wavelength Division Multiplexing (WDM) channels to each enable concurrent Single Write Multiple Read (SWMR) communication between the 4 photonic interfaces using two WDM channels per link.