Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 116

1 1 5G literature often uses the term dynamic spectrum management (DSM) instead of dynamic spectrum access.

2 2 Technically mm‐wave starts at 30 GHz, but it is common in 5G literature to consider the band above 24 GHz to be the mm‐wave band.

3 3 Many military communications waveforms use a mix of TDMA and FDMA, creating time and frequency slots for the allocated spectrum that can be shared between network nodes in omni‐directional transmission. Part 1 of this book showed how military communications MANETs evolved to use directional sectored antennas to be able to transmit and receive on the same frequency simultaneously based on directionality. 5G introduces many antenna technologies that enhance this FD capability for commercial wireless systems.

4 4 FD and concurrent sensing can mitigate the hidden node problem explained in Chapter 2.

5 5 Figure 5.4 is an oversimplification of spatial separation. Base stations before 5G used sectored antennas allowing for frequency reusability, which is not illustrated in this figure.

6 6 There are different terminologies in the literature that expresses the signal to noise ratio. SNR is one term that stands for signal to noise ratio; SNIR is another term that expresses signal to noise interference ratio. SINR is a third term that stands for signal to interference plus noise ratio. SIR is a fourth term for signal to interference ratio where interference can be additive noise and/or from another user using the same frequency. While in the previous chapters we used the term SNIR because military communications seek to distinguish noise from interference that can be malicious, in this chapter we will use the term SIR to emphasize the role of SI in 5G.

7 7 Notice that not all transmitting nodes cause interference. Φ is the subset of all transmitting nodes that can introduce interference at location o.

8 8 Metrics such as the probability of granting a connection when requested, the probability of keeping a connection during the duration of the session, and the probability of meeting the data rate defined in the service agreement can be used to quantify the reliability of the 5G network.

9 9 Notice that β can be a tunable parameter. Some literature uses the expression “link closure” to indicate that the condition SIR > β is met.

10 10 Theoretically, one can always trade data rate for FEC to achieve link closure under low SIR. Practically, there is a computational power limitation for the use of FEC. A 5G node has limited modes of turbo code and can't keep trading data rate for lower SIR to achieve link closure. Also, a link is requested with a specific minimum data rate. The result of Equation 6.1 defines Γ in Equation 6.2.

11 11 Notice how β is related to the data rate. A link can always trade off bandwidth for higher dB gain using error control coding.

12 12 Note that different flows may be requesting different data rates.

13 13 Notice that the use of a Rayleigh model means that the number of transmitters is a multiplier in the exponential part of Equation (6.3).

14 14 Note that such decisions to increase infrastructure resources can use the metrics runtime evaluation to get insight into this need but should use the more comprehensive post‐processing evaluation explained in Chapter 5 to make the ultimate decision of adding infrastructure resources.

15 15 Inactive here means that there are no end‐user devices utilizing the cell. The cell is always active as a SA.

16 16 One can see the parallels and differences when comparing this 5G approach to the military communications approach of cooperative distributed and centralized fusion decisions. One can also see that regardless of the problem domain, good DSA design requires considering some form of local, distributed, and centralized spectrum fusion. System designers should be striving to create the appropriate hybrid approach for optimizing spectrum resources use.

17 17 Local can mean the SA or femtocell has fusion capability, distributed cooperative can mean the different cell types and SAs share spectrum sensing information and make distributed cooperative decisions, and centralized can mean the macrocell is the main performer of spectrum sensing information decision fusion.

18 18 From the software‐defined network perspective, a centralized spectrum fusion and arbitration in a macrocell means an easier way to roll out new software releases as the software release is targeted to only the macrocell. One can debate if a 5G end‐user device should or shouldn't be software defined but all 5G infrastructure from the femtocell to the largest tower are software‐defined entities where new software releases can upgrade the infrastructure and enhance its capabilities without the need to deploy new hardware.

19 19 If SAs, femtocells, and picocells are static and their geolocation information is known publically, there is no need to amend spectrum sensing information messages with geolocation information.

20 20 A block is represented by a frequency band and signal orthogonality at a time slot that is part of a repeated epoch and can be utilized and released dynamically.

21 21 In practical MIMO antennas, orthogonal signals energy can be detected across different dimensions. This is different from intersymbol interference or co‐site interference.

22 22 The development of fractional programming and heuristic technique is beyond the scope of this book.

23 23 Here, the term “optimum” is avoided and the term “feasible” is used instead because the algorithms that can be used may have to be heuristic and have to consider the speed of making decisions and the computational power constraints making them reach a “feasible” decision.