Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 101

Spatial modeling in 5G can use a set of metrics that can affect SIR. SIR becomes an instantaneous ratio of desired energy to all the additives of undesired interferences and noise. Thus SIR can be considered a random variable that depends on a set of factors that include the following:

1 The distance between the transmitting node and the receiving node. Much like traditional signals, this factor can be modeled by a path loss model. All path loss models follow an inverse‐power law with an exponent trend. For example, in a free space model, signal power decrease with distance in a quadratic trend. Other path loss models can use different exponent values to model signal scattering and signal absorption.

2 The number of active transmitters in a given proximity. For a given receiver, and during the time of reception, there are different potential combinations of active transmitters where their signal will appear as interference. The sum of interference power from all the transmitting nodes, taking into consideration their distances from the given receiver, has to be modeled. In a dense deployment of 5G cells, this sum of interference power from other transmitters can exceed the noise power threshold that is selected to ensure reliable connectivity.

3 The ambient noise. SIR will be affected by noise power and this noise will depend on the received signal and the interference power. Notably, if a 5G deployment resorts to lower transmission power, the ambient noise impact on SIR can be the larger factor. On the other hand, if a 5G deployment resorts to a high transmission power, the sum of interference power in step 2 above becomes the larger factor.

4 Other factors. There are many other factors that can affect the SIR calculation such as fading and shadowing, transceiver design (e.g., the use of multiple antenna and interference cancelation techniques), and adaptive power control.

A simple representation of SIR at a typical receiving node can be expressed as:

(6.1)

where o represents the origin in the spatial model assuming the receiving node is at the origin of the spatial plan, h_io is the fading coefficient of the channel at the receiving node for the signal transmitted from node i, ρ_i is the transmit power of transmitter i, N_o is the noise power, Φ is the set of all interfering nodes,⁷ and X_i expresses the distance between the ith interfering node and o.

Notice that Equation (6.1) performs the following:

1 It puts the receiving node at the origin of the spatial model o making the calculation with respect to o.

2 It creates a probabilistic spatial model where transmitting nodes can be randomly positioned with respect to the origin.

3 It consolidates path loss calculation of the subset of transmitting nodes presumed to cause interference with the receiving node at the origin.

Equation (6.1) can be used in a spatial model to calculate SIR, which can be used in a 5G dense deployment to calculate connectivity and coverage when assigning spectrum resources. In addition, SIR calculation can be used to estimate the capacity and throughput of a given deployment area. This calculation can further lead to collecting metrics that can measure the reliability of the 5G networks.⁸