Heterogeneous network (HetNet) is considered as the main and eminent future communication technology, since it achieves high spectral efficiency per unit area and saves energy due to low transmission power. Mass deployment of small cells in cochannel mode increases overall system capacity, but it is also coupled with greater risk of cochannel interference. This paper overviews the interference model based on the Poisson point process (PPP) and analyzes the performance in terms of energy efficiency in multitier HetNet. As the promising solution for improving the performance of HetNet, both the cooperative communication and cognitive radio techniques to mitigate the interference in HetNet are surveyed. As one example of cooperative communication techniques, a hierarchical cooperation scheme on the spectrum allocation is presented and its energy efficiency performance is analyzed and evaluated. Meanwhile, the energy efficiency increases from the cognitive radio technique are demonstrated as well. The energy efficiency performance comparison between the presented cooperative communication and cognitive radio techniques is emphasized, which suggests that the cooperation communication technique is preferred to suppress the interference and increase the energy efficiency in HetNets.
Heterogeneous network (HetNet), which consists of a mix of generous macrocells and low-power nodes, for example, micro, pico, femto, relay node (RN), and remote radio head (RRH), has been seen as a promising approach for improving the spectrum and energy efficiency. The large macrocells provide mobility and basic coverage, while small cells and low-power nodes boost capacity and extend the range of cellular network [
However, the HetNet architecture also brings new issues and challenges compared with traditional homogeneous network, such as the impact of resource allocation and interference mitigation schemes on the performance [
In order to increase the resource utilization and overall system capacity, macro- and small cells are deployed in cochannel mode in HetNet usually, while it has to couple with greater risk of intertier and intratier interferences. A significant amount of research has been done to manage the cochannel interference (CCI) in a two-tier network, comprising a macrocell network overlaid with small cells [
However, to the best of our knowledge, the energy efficiency comparison between the cooperative and cognitive schemes in HetNet has not been studied much in related works. This motivates us to write this paper to investigate the energy efficiency comparison of cooperation and cognition in HetNet. In addition, the energy efficient transmission schemes are proposed with cooperative and cognitive techniques. Similarly, interference model in HetNet is presented to investigate the impact of interference on the performance of HetNet, and the energy efficiency improvements in HetNet achieved by interference cancelation and resource allocation schemes are analyzed. Lastly, the energy efficiency comparison of cooperation and cognition in HetNet is presented and some energy efficient transmission schemes are proposed, with the simulation results to evaluate the performance.
The remainder of this paper is organized as follows. The interference model of HetNet is investigated in Section
The performance of users is degraded due to the cochannel interference in the orthogonal frequency division multiplexing (OFDM) cellular network where the frequency resources are shared between neighboring eNBs. The interference problem is essentially challenging in HetNets with respect to the limited backhaul capability supporting interference coordination between macro-eMBs (MeNBs) and SeNBs, the restricted access control (e.g., closed access femtocells), and the demand of monitoring cochannel interference from different types of nodes [
Transmission in multiple access relay channels.
The major factors affecting the cochannel interference include the spatial distribution of interfering nodes and the propagation parameters. The spatial locations of homogeneous cellular nodes have been investigated in linear array models, square networks, triangle networks, and hexagon networks, where the topology is regular and the node locations are deterministic. In HetNet, however, the deployment of SeNBs is generally performed without careful network planning. Consequently, it is difficult to evaluate a tractable model for these randomly distributed nodes based on the aforementioned deterministic structures. To capture the random characteristic of spatial locations, a concise and tractable HetNet model has been recently discussed based on a point process, where the positions of eNBs are stochastically modeled and only the statistical description is available. The most widely used model is the Poisson point process (PPP), where the eNBs with intensity are uniformly distributed over the area. For the infinite planar network, the aggregate interference can be characterized with a shot-noise process, and the probability density function (PDF) is characterized by the intensity of eNBs instead of their specific locations. Extending this model to the HetNet, the eNBs in each tier can be modeled as independent PPP with different pathloss exponents and transmission power.
Although the interference is mainly determined by the positions of eNBs, the impact of propagation environment is nonnegligible. Rayleigh channel is the simplest fading model which results in an exponentially distributed receive power of a single link [
Using the stochastic interference model, it is feasible to evaluate the performances of transmission reliability and capacity in large cellular networks. The success probability, defined as the probability that the signal to interference plus noise ratio (SINR) exceeds the predefined QoS threshold, is employed to evaluate the transmission robustness under the specific modulation and coding scheme (MCS). The capacity can be evaluated in terms of spatial average rate which denotes the mean rate of the instantaneous Shannon bound achieved by applying the adaptive MCS and area spectral efficiency which represents the network throughput achieved by using the specific MCS. The SINR for a two-tier HetNet with open access mode is depicted in Figure
SINR for HetNets with open access mode.
Andrews proposed a tractable framework for cellular networks, where the UE is associated with the nearest MeNBs. The distance of a UE to its serving MeNB can be obtained as a random variable according to the null probability of the Poisson distribution. Under this framework, the success probability and spatial average rate are derived for the homogeneous cellular network, which are observed to be independent with the density of MeNBs in the interference-limited scenario. This framework can be easily extended to the HetNets with differing transmit powers, coverage areas, and access policies.
In [
The two-tier femtocell model was investigated in [
On the other hand, based on the definition of energy efficiency, we analyze the impacts of the average data rate, eNBs transmit power, and the circuit power on the energy efficiency of HetNet. The average rate increases with the eNBs transmit power and density. However, the increasing eNBs transmit power and density will also result in the increasing interference which will deteriorate the received signal SINR and the UE QoS requirement. The eNBs circuit power is the monotonically increasing function of the eNBs density which is the major part of the network power consumption. On the other hand, the circuit power largely depends on the bandwidth of eNBs transmit spectrum which will increase the operation power consumption of building baseband unit and remote radio unit. In wireless networks, the traffic volume from mobile users fluctuates in both temporal and spatial domains, which brings enormous challenges to energy efficient design of HetNet. Hence, the SeNBs sleep and zooming strategies are considered as the most promising method for reducing energy consumption while meeting UEs’ increasing traffic demands.
In HetNet, the conflict between the scarcity of spectrum resource and low resources utilization increases the energy consumption significantly and impairs the energy efficiency seriously. Because of the intertier and intratier interferences, the independent performance optimization in the homogeneous and isolated cell cannot converge to the global optimal solution of HetNet. Furthermore, the coordination schemes among the heterogeneous nodes have a strong impact on the complexity and performance of the network, which must be designed and carried out carefully.
In order to solve these challenges efficiently, the cooperation of the multidimensional radio resource and the cognition of the complicated scenario in HetNet should be investigated comprehensively. Cooperative and cognitive network are new paradigms in wireless communications to enhance the utilization of limited spectrum resources and improve the performance. Although the cooperative and cognitive radio networks can be integrated together and implemented jointly between different nodes in HetNet, a higher degree of cooperation and cognition brings a higher degree of complexity and cost in terms of network operation and energy consumption. In this section, the energy efficiency improvements of the proposed cooperative and cognitive HetNet are investigated and evaluated separately for low complexity. Then, the energy efficiency comparison of these two techniques is presented under different scenario and deployment of HetNet. Furthermore, the energy efficient networking schemes with different network parameters are proposed in HetNet.
In HetNet, there is two-tier cooperation employed to optimize the performance: intratier and intertier cooperation. These two cooperative structures can be deployed separately or jointly with the constraints on backhaul link and complexity. It is a highly desirable characteristic for interference-limited networks that full spectrum reuse is exploited to improve the data rate and increase the utilization of resource, especially in heterogeneous network where amounts of different nodes are deployed in an overlapping area. The energy efficiency versus the density of low-power nodes in HetNet is shown in Figure
Energy efficiency in full reuse HetNet with different MBS load.
In the first step, the intratier cooperation scheme is implemented among the MeNBs to reduce the intratier interference according to the fluctuation of MeNBs load. When there is some idle resource, the spectrum is allocated orthogonally among the MeNBs, which cause the most serious interference to others, by the cooperation schemes. In the second step, the intertier cooperation is explored between the MeNB and SeNB in the same macrocell to increase the utilization of spectrum while maintaining the QoS of macrocell UEs (MUEs). When the density of SeNBs is large, the reused frequency resource should be decreased for reducing the interference to MUEs. When there is small number of SeNBs located in the macrocell, more frequency resources can be reused for increasing the resource utilization efficiency. The reused spectrum resource should be optimized in a cooperated manner to decide the tradeoff between the impact of resource and interference on the energy efficiency.
Figure
Energy efficiency in cooperative HetNet with different MBS load.
Cognitive radio (CR) networks are designed to utilize the licensed spectrum when it is not used by the primary (licensed) users. The key enabling technologies of CR networks are the cognitive radio techniques that provide the capability to share the spectrum in an opportunistic manner. In traditional homogeneous network, the cognitive abilities have a wide range of potentials for improving the tradeoff among resources utilization, limited bandwidth, spectrum efficiency, and energy efficiency in future wireless communication network. The hierarchy in energy games for cognitive radio networks is investigated in [
With the development of heterogeneous nodes, MeNBs which are located for basic coverage have priority in utilizing the spectrum to satisfy the requirement of user’s QoS (as the primary users in common CR networks). If the SeNB can automatically sense the surrounding, the received signaling from the MeNB and other SeNBs (in the close vicinity), and intelligently allocate resources without any excessive intertier and intratier interferences, the network performance in terms of spectrum and energy efficiency will be enhanced significantly.
In cognitive HetNet, MeNBs are the licensed owner of the channel and SeNBs are allowed to seek transmission opportunities on the licensed spectrum of MeNB only when the macro-UEs are absent on these spectrum resources. It is assumed that these spectrum resources can be reused among the SeNBs. With this constraint, the intertier interference between MeNBs and SeNBs does not exist but the intratier interference among MeNBs or SeNBs cannot be neglected. Although more shared spectrum resources can be utilized with more cognitive SeNBs, the increasing intratier interference will impair the network performance seriously. The density of cognitive nodes is believed to be a crucial factor that affects the spectrum and energy efficiency of the HetNet, which cannot be neglected in the design of practical network with the consideration for the circuit power consumption and deployment cost.
As the simulation results illustrated in Figure
Energy efficiency in cognitive HetNet with different MBS load.
With the previous analysis, we can compare the energy efficiency of the proposed cooperative and cognitive schemes in HetNet. For the direct and clear description of the comparison, Figures
Energy efficiency comparison between cooperative and cognitive HetNet with different MBS load.
On the other hand, we cannot easily make conclusion that the cooperative schemes are preferred than cognitive schemes for improving the energy efficiency in HetNet. In the proposed schemes, the power consumptions of information exchange in cooperation and spectrum sensing in cognition are not considered. Meanwhile, the complexity of the centralized process in cooperation cannot be neglected. Therefore, the more practical power consumption model and cost estimation method in cooperative and cognitive HetNet should be investigated, in order to give some useful suggestions in the design of energy efficient communication networks.
In this paper, a theoretical framework based on the random spatial models is utilized to analyze the interference characteristic and energy efficiency performance in multitier HetNet. Meanwhile, the performance improvements and interference mitigation with cooperative and cognitive schemes in HetNet are surveyed. In particular, the energy efficiency of the proposed cooperative and cognitive schemes in HetNet is evaluated by the simulation. Furthermore, the comparison between them is presented with different traffic load and network deployment. In future work, more energy efficient cooperative or cognitive schemes will be proposed in HetNet and the practical power and cost model of cooperation and cognition should be investigated to give more reasonable advice in the design of energy efficient wireless communication systems.
This work was supported partially by the State Major Science and Technology Special Projects of China (no. 2013ZX03001001-003), the National Natural Science Foundation of China (no. 61222103), Beijing Natural Science Foundation (no. 4131003), and the New-Star of Science and Technology supported by Beijing Metropolis (xxhz201201).