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Recently, several approaches were followed for the enhancement and better resource utilization in mobile networks; this is to achieve energy efficient consumption for production and delivery of an information bit. Using Cognitive Femto cells (as a member of the small base stations’ family) proves that, it is an efficient solution for achieving this goal[1]. The use of Energy Efficiency term
*η* has become one of the major indices for measuring the performance of these systems.
*η* is the measure of the overall system Capacity (
*C*) in bps/Hz versus the Consumed Energy (
*E*) in Joules [2]. In consistence with many researches, analytic models and empirical measurements,
*η *will be investigated throughout the course of this work. Cognitive Base Stations (CBS) (as an element of the system model) which performs the traffic offloading operations is proved to enhance
*η* performance. In this work, a combination of both analytic and simulation models are used to construct a practical system model. The obtained model is then used to illustrate the effect of different operational parameters that are involved in the
*η* problem. On the other hand, the current paper tries to focus on the selection criteria that may be used to design the cooperative cognitive networks in order to achieve the best
*η* indices. Both of CBSs radii as well as the inter-separation distances (between CBSs and MBS location) are examined to obtain best
*η* index for different operation scenarios; in addition, both of capacity and energy consumption are taken into consideration based on practical operating measures. This work proposed several nonlinear equations with fixed parameters to be used by field engineers to achieve the results with minimum reduced computation complexity. So, the current work may be of importance for the regulator bodies as well as the cognitive mobile operators.

Recently, huge interest has evolved for finding means to conserve the natural resources especially in communications. Two facts are realized by communication systems’ engineers and researchers: the rapid consumption of natural resources and the increase in global warming with the dependency growth of civilizations on energy for using wireless communication systems, together, the scarcity of spectrum resources of these wireless communication systems. “Green Communication” is currently one of the hottest topics in the field of wireless communication researches due to its contributions in saving the environment [^{3}F) [

The paper is organized as follows: Section 2 will introduce the CR based HetNet and its relation to the

In CR based HetNets, Green communications has become a hot topic in the wireless telecommunication era. EARTH is one of the major European research projects that is concerned about finding energy efficient solutions by the reduction of the overall energy consumption [^{3}F) has been established to find out means to increase the radio network’s energy efficiency. In 1^{st} ETSI TC EE workshop [^{3}F builds on the 3GPP evaluation framework for LTE [

The system model elements are shown in

where, _{,} is the received power at the receiver side, _{,} is the antenna output radiated power at the transmitter side,

User Equipments (UE) joining this system model are either Macro Users (MU), where they are served by the MBS, or, Cognitive Users (CU) where they are served by the nearest CBS. They are all deployed uniformly in the MBS coverage area (locations are shown as “+” sign).

usage by UEs, those UEs go through two states of operation, active and idle with active probability

where,

Theoretically, capacity

In this work, theoretical capacity is used to obtain the maximum capacity available by the system, further modifications may be applied to enforce conditions of the actual bit rate.

In order to calculate the system capacity

where,

where, src stands for “source” and dis stands for “destination”, and

where,

where, _{,}

Which are in consistence with [

where,

To evaluate the overall consumed energy in the proposed model, all the major system components are considered. In [^{3}F builds on the 3GPP evaluation framework for LTE based on a sophisticated power model. This framework considers two main measures, the base station (MBS, CBS) power consumption and user equipment (MU, CU) power consumption.

(mains supply) for connection to the electrical power grid.

For BS Variable Load, (i.e. the power consumption of PA depends on the traffic load), it was found that BS power consumption model is shown in [

where,

Assuming

multiple normalized time slots

Let

The total energy consumption by the system model is:

where,

Using Equations (13) and (18), energy efficiency

Numerical results are obtained using an algorithm which contains the previously mentioned analytical-based functions. Those functions calculate the overall system

The assumed parameters are initialized with values shown in

Model element | Parameter | Explanation | Value |
---|---|---|---|

MBS | R | Radius of cell | 500 m |

Max transmission power | 46 dBm | ||

FM | Number of frequencies | 10 | |

UE | Idle state consumption power | 200 mW | |

Receiving state consumption power | 600 mW | ||

Probability of active traffic state for MU | 0.6 | ||

Probability of active traffic state for CU | 0.6 | ||

CBS | Max transmission power | 30 dBm | |

Consumed power in idle state | 500 mW | ||

Consumed power for backhauling | 100 mW | ||

Consumed power in sensing state | 600 mW | ||

Average number of channel switching | 5 | ||

FCR | Number of CBS frequencies | 5 |

6 | 40.0 | 118.7 | 2.66 |

figure is useful for converting CBS number _{ }

due to increasing the available area for deploying this large number of CBSs (due to reduction of each cell’s coverage area). This will lead to more interferers to cause this capacity decay. The “turning point” that happens for different values of

mum obtained values of

To reduce the complexity of calculations to find the asymptotic behavior of the previous results, deeper analysis is conducted. A mathematical reverse interpolation of the results is obtained. A unified formula is suggested to approximate these results. This is performed in order to provide a short but effective tool to help engineers working in the field of CR field planning. The suggested formula is applied with several parameters relative to the distance used (

where

It is found that the best parameters to fit the results’ interpolation are:

・ maximum values of

・ and, maximum values of

Using a combination of an analytical model and practical simulation, several outcomes have been achieved. Using variable parameters of ^{s} which will in turn increase the interference). Increasing

Amr A.Fahmy,Asmaa M.Saafan,Hesham M.El-Badawy,SalwaEl-Ramly, (2015) Energy Efficiency Behavior in Heterogeneous Networks under Various Operating Situations of Cognitive Small Cells. Wireless Engineering and Technology,06,9-23. doi: 10.4236/wet.2015.61002