Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 17

When a dynamic spectrum access decision is made locally, it can be made quickly. If we are considering a point‐to‐point link that has the flexibility to choose between different frequencies and its spectrum sensing has a simple SNIR threshold to compare to when using one frequency, it can simply decide that SNIR on a used frequency is low and switch to a different available frequency. In a distributed MANET, where we have to make cooperative distributed DSA decisions, propagation time of spectrum sensing information, processing time, and the need for making synchronized decisions will produce a relatively slower decision‐making process. In a centralized decision‐making construct, the decision time will even be longer as the spectrum allocation arbitrator has to obtain information from a large‐scale set of heterogeneous networks and make slower, more lasting decisions. Centralized decision making is global, has to coordinate between all networks, has to consider geographical separation between networks, and cannot be performed every short period of time.

Figure 1.3 shows a conceptual view of this decision‐making process timing where T₁ represents local decision‐making time, T₂ represents distributed decision‐making time, and T₃ represents centralized decision‐making time. In Figure 1.3, T₁ T₂ and T₃ are expressive of both the speed of making DSA decisions and the time periods between making DSA decisions. Figure 1.3, by no means uses an accurate time scale but is meant to illustrate that in a hybrid DSA decisions system many local decisions can occur before a distributed cooperative decision can occur, and many distributed cooperative decisions can occur in different networks before a centralized arbitrator decision can occur.

Figure 1.3 Conceptual view of DSA decision‐making process timing.

It is important to consider this hierarchy of decision making when designing a hybrid DSA system. Local DSA decisions have to be quick, distributed DSA decisions have to be more insightful, and centralized DSA decisions have to be long lasting in order for the design to achieve the aspects described below and avoid the pitfalls described in Section 1.5.

Taking into consideration the hierarchy of DSA decision making, a comprehensive DSA design will consider these important aspects:

1 Avoid rippling effects. Rippling effects here means that the machine‐based decision‐making processes switches a user or a network from frequency f1 to frequency f2 only to decide quickly to switch back from f2 to f1. This rippling effect can reduce the throughput efficiency of the network and diminish the optimized use of the spectrum resources pool of frequency.

2 Consider traffic demand. In a large‐scale set of heterogeneous networks, centralized DSA decisions can be more successful in allocating more frequency bands to networks where there is high traffic demand. This will increase the overall throughput of the managed networks. With DSA, spectrum is a commodity that can be distributed to overcome interference and resources should be increased where demand increases.

3 Consider secondary user rules. If the system being designed is for an opportunistic use of spectrum as a secondary user, the design has to adhere to the secondary user rules. Secondary user rules give the primary user the first right to spectrum use and spectrum sensing has to always be on the lookout for primary user activities. Secondary user rules are discussed in more details in Chapter 2.

4 Consider hidden node effects. The challenges associated with hidden nodes are discussed in the next chapter. Being able to sense spectrum use from different locations helps avoid this challenge. In a hybrid system with a centralized spectrum arbitrator and using a large set of spectrum sensors, the hidden node challenge can be mitigated effectively.

5 Consider the hierarchy of response times mentioned above. Local decisions are fast, distributed decisions are slower and more insightful, centralized decisions are intended to be long lasting. A hybrid DSA decision system design has to consider making the different DSA hierarchal levels work in harmony.

6 Avoid conflicts with other decision‐making processes. This challenge can surface as SDR and SDN allow the incremental addition of software modules that can be prone to making contradicting decisions. Consider this example: a software module makes adaptive power control decisions. This module can obtain SNIR information from the physical layer and decides to increase power to overcome low SNIR assuming that the peer node, from which it received a signal with a low SNIR, is suffering from the same degradation reciprocally. In the meantime, a DSA agent obtaining the same SNIR metric attempts to switch frequency to overcome the low SNIR. There are pros and cons for each of these decisions. Increasing spectrum emission power can quickly fix the low SNIR challenge but may reduce frequency reusability in a large‐scale set of networks. Switching to a new frequency can also address the low SNIR challenge radically but can be a greedy decision. The new frequency may have been more needed by a neighboring network. The main point here is for the designer to consider how SDN and SDR can have other software modules that address spectrum conflicts from different angles and make sure DSA decisions are made in harmony between all spectrum access techniques. Chapter 5 explains the value of using a generic cognitive engine skeleton at all entities of DSA decision making with a single information repository and a single decision maker. This skeleton design ensures that a single process makes all the DSA‐related decisions (i.e., offers all DSA services) at any given DSA service entity.

Clearly, DSA decisions are not “one size fits all”. Depending on so many factors and other design decisions, one has to articulately consider all the design aspects of DSA. A comprehensive DSA system may have a mix of local, distributed, and centralized decision making and a good design must consider the order of time⁵ these decisions ought to take, which decision to make at which area, and how to coordinate between all DSA‐related decision‐making capabilities.