## Optimized Two-Tier Caching with Hybrid Millimeter-Wave and Microwave Communications for 6G Networks  
[PERSON] 1, [PERSON] 1,*2, [PERSON] 1, [PERSON] 2, [PERSON] 3 and [PERSON] 3\({}^{\text{\text{\textregistered}}}\)4  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 3: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 4: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 5: [[https://www.mdpi](https://www.mdpi)]([https://www.mdpi](https://www.mdpi)). is a de facto technology that is envisaged for future cellular networks [2]. For mmWave communications, high-gain directive antennas are employed to achieve data rates of up to 7 Gbit/s, and this is anticipated to grow up to 40 Gbit/s. Given the abundance of spectra in the mmWave band, which are capable of providing multi-gigabit data services, optimizing communication over mmWave bands has become a prominent research area [3]. To handle an enormous amount of data traffic, the dense deployment of SBSs, exploitation of huge mmWave bandwidth, and edge data caching at SBSs' level and the UE's level are foresighted in 6G networks. The mmWave communications operating at high carrier frequencies experience more path loss than at low carrier frequencies. For instance, at the 60 GHz band, mmWave experiences 28 dB more free space path loss than at 2.4 GHz [4]. To overcome the higher level of channel attenuation, antenna gain is improved by employing beamforming through directive antennas at both the transmitter and receiver ends [5; 6; 7]. The beam steering feature of the mmWave communication can greatly reduce path loss and mitigate the interference footprint. mmWave also helps to reduce the antenna size due to its shorter wavelength. Despite mmWave communication providing interference elimination, it is more susceptible to non-line-of-sight (NLoS), penetration loss, and blockages than \(\mu\)Wave frequencies [8; 9]. One of the noteworthy architectural boosts in future cellular networks will be device-to-device (D2D) communications, which enable UE to establish a direct connection with other UE in proximity without traversing through the SBS [10; 11]. As a significant portion of data traffic is generated and consumed locally, D2D can improve spectral efficiency tremendously. D2D communications are a promising approach to lower signaling overhead, improve energy efficiency, enhance the QoS, and reduce backhaul traffic. The UE's data demands trend toward a few popular pieces of content that are requested repeatedly [12]. Therefore, edge caching is a promising technique where SBS and UE storage can function as data cache resources. Thus, when content is requested by a user, the request can be fulfilled through its own cache or a user in proximity via a D2D link [13; 14]. Nevertheless, if the desired content is not available in the cache resources of the neighboring UE within the D2D range, then the desired content is obtained from the serving SBS without imposing a data traffic burden on the backhaul link. The cache resources are limited, which poses a significant challenge to optimizing cache decisions. The caching process consists of two phases. In the first phase, data is cached at the network edge. In the second phase, the UE's data demands can be fulfilled through the cache resources if the requested content is available in the cache. The ability to access data via cache resources can provide robustness and flexibility in data access [15; 16]. Moreover, caching content close to the users offers dual benefits.