Dynamic Spectrum Access Decisions. George F. ElmasryЧитать онлайн книгу.
6.3 Stages of 5G SI Cancellation
The previous sections focused on spatial modeling and evaluation metrics without considering some DSA techniques that can decrease the impact of SIR on the transmission capacity of 5G infrastructure. This section focuses on one important aspect of DSM in 5G, the maximization of SI cancellation. In order to increase the infrastructure capacity, 5G uses different stages of SI cancellation, as shown in Figure 6.9.
Figure 6.9 5G FD communications with different stages of noise cancellation.
Let us consider the following aspects of SI cancellation with FD communications that makes DSM more efficient and possible with the higher frequency bands 5G is utilizing:
1 Directionality. 5G beam forming relying on MIMO antenna technology means the signal is as narrow as possible where the spectrum is concentrated to the receiving node, with minimal spectrum leaks to other transmitting and receiving node pairs using the same frequency.
2 5G MIMO antennas implement SI cancellation using multipath fading analysis stages that reach up to 20 dB gain at both the transmitting and receiving antenna. This is shown in Figure 6.9 as the MIMO antenna cancellation.
3 After using a low noise amplifier, the receiver implements further analog noise cancellation of the RF signal.
4 After the analog to digital converter, the 5G receiver further implements other digital signal noise cancellation techniques.
The 5G protocol stack and the SDR concepts of 5G allow the service provider more flexibility in implementation. Figure 6.10 shows the 5G protocol stack at a higher level. The use of 5G in specific industrial applications has the flexibility of adding software modules that can allow for adapting 5G to the industrial application needs. For example, a service provider can explore using a form of network coding in the open wireless architecture (OWA) layer for further dB gain that can be achieved when using 5G mm‐wave links for long‐range reachability. This technique will trade some bandwidth for additional error control coding redundancy.
Figure 6.10 5G protocol stack.
The reader is encouraged to explore antenna design literature for more details about how 5G MIMO antenna interference cancellation is achieved in the mm‐wave range, which is beyond the scope of this book. However, the next chapter introduces some MIMO techniques that can be considered for adapting 5G for military communications systems.
6.4 5G and Cooperative Spectrum Sensing
Cooperative spectrum sensing was originally explored in the context of CR networks where the secondary user (SU) has the ability to detect spectrum utilization and collect spectrum sensing information either as a standalone radio terminal, cooperatively with peer nodes, or relying on an external spectrum sensing information. This was covered in Part 1 of this book, where energy detection and other sensing techniques that can be used by the SU are explained. Part 1 also covered DSA for heterogeneous MANET networks with some focus on military communications challenges and approaches to solving the challenges of DSA with heterogeneous MANETs using hybrid design that include cooperative distributed decision fusion. The 5Gs take on cooperative spectrum sensing has some different and unique angles such as:
energy conservation of end‐user devices
leveraging the fact that 5G networks can be dense
avoiding information redundancy
optimizing resource utilization with heterogeneous networks overlay
the consideration that end‐use devices can connect to each other with device‐to‐device (DTD) communications protocols.
One of the core concepts in 5G is the use of a spectrum agent (SA) in order to offload some of the burdens of spectrum sensing and fusion away from the 5G end‐user devices. Notice that 5G can use both licensed and unlicensed spectrum, making it important to detect what frequency bands other communication systems operating in the same geographical location are using.
The dense deployment of 5G small cells is leveraged to turn each cell into a SA and turn inactive15 small cells into SAs. Figure 6.11 illustrates the compound overlay of active and inactive entities in a 5G deployment in an urban area where we can have a macrocell with the largest area of coverage shown in the figure with the large tower at its center, multiple picocells as illustrated by the small towers, and many femtocells as illustrated with the femtocell icons. This illustration uses inactive femtocells as SAs where an end‐user device can obtain spectrum awareness information from an SA even though the device has not made the decision to select this SA as an access point or to use other access points.
Figure 6.11 The compound overlay of 5G entities in a 5G deployment.
Notice in Figure 6.11 that a macrocell user can be utilizing a licensed spectrum band while an end user obtaining spectrum awareness information from a SA can end up using unlicensed spectrum band using the femtocell as the access point. With this approach, the end user does not have to perform the spectrum sensing functionality or the computationally extensive spectrum sensing information fusion. Instead, the end‐user device obtains processed information with decision fusion results from the SA. One can see from this compound overlay that if and when an SA acts as a femtocell, it may have to resort to unlicensed spectrum because the macrocell is using the licensed spectrum bands. As illustrated be the dotted lines in Figure 6.11, an end user would send the SA a spectrum service request and the SA can then do any of the following:
1 Periodically detect spectrum utilization in the area and decide if an available licensed band can be used or if an unlicensed band must be used. This decision is based on the SA periodically detecting and analyzing spectrum utilization in the area. Here, the SA is acting as a spectrum fusion center, deciding spectrum availability and broadcasting it to the end users in the area.
2 Not perform fusion and rely on the macrocell as the fusion center where all the SAs, femtocells and picocells send the macrocell their own spectrum sensing information and the macrocell performs the fusion and send the fusion results to the SAs, femtocells, and picocells.16
In any case, the SA would respond to the spectrum access service request from the end user with a frequency band it can use regardless of whether that band is licensed or unlicensed, and regardless of whether the decision is made by the SA or the macrocell.
Note that 5G standards give the service provider flexibility in how to implement spectrum sensing, how to fuse sensing data, and how to create decision. A different hybrid form of local, cooperative distributed, and centralized17 spectrum sensing technique can