Heterogeneous Networks in 5G
Challenges in mobile network systems
Over the recent decades, there is a rapid expansion of wireless communication systems due to which the research community has to face many challenges. The 4G communication technology offers various advances in wireless system design and provides improved spectral and energy efficiencies and good quality of service (QoS) to the users. But, instead of these advantages, it is expected that there will be capacity crunch that the wireless industry will face in the future. Despite spectrum reuse and interference coordination techniques, there is an unsatisfactory cellular coverage in the indoor environment. 5G systems are expected to be deployed around the year 2020 that will be an integration of diverse techniques and solutions. The increasing disproportionality between the user QoS and the available wireless resources can be mitigated by deploying femto and pico cells in the current architecture in the higher density. Hence, due to increase in the frequency reuse per unit area and the average data rate per transmission, the network capacity can be improved. There is a need for additional small cells as there is an expectation of increase in growth by 13-fold over the next 5 years. So, direct device-to-device (D2D) radio links are used by the clients to offload their traffic.
Moreover, the licensed spectrum is becoming scarce and expensive which is another major challenge. There is an increase in overlapping of the footprints of cellular, local, and personal area networks as cell sizes shrink. Hence, an opportunity has been created for capacity and connectivity improvement by utilizing multiple radio access technologies (RATs).
There is a requirement of changes in both network infrastructure and device architecture for any transformation of mobile user experience. Along with the development in cellular and WLAN technologies, another integral part of cellular network is Wifi. Hence, different radio technologies are aggregated as a part of common converged radio network which is transparent to the end user and techniques for efficient utilization of radio resources can be developed.
Therefore, an advanced networking architecture, as shown in fig. 1, of heterogeneous networks (HetNets) has been emerged that enables improved capacity and coverage in 5G networks.
Fig. 1. 5G heterogeneous network
For basic connectivity, wide‐area macrocells are deployed in the architecture in which densely deployed small cells are added from different RATs so that capacity can be boosted. In order to unlock advanced levels of interworking between cellular and WLAN RATs, there is an increase in management of unlicensed‐band technologies as a part of an operator’s cellular network.
There is a development of new interworking solutions between 3GPP cellular technologies and Wifi bt the Third Generation Partnership Project (3GPP). The more focus of current standardization efforts is on user‐centric interworking architectures that will assume that degrees of cooperation/assistance across the HetNet should be limited. The interworking Solutions have been targeted by several new work items that involve cooperation within the radio access network along with the previous schemes that have loosely defined functions, such as security and inter‐RAT mobility, within the core network given by 3GPP.
When there is an allocation of additional spectrum, diverse transmission technologies are required by the new frequencies in order to remain fragmented. Hence, the advanced architectures and protocols, leveraging the unlicensed spectrum, will provide increased gains.
Challenges of HetNets in 4G networks
The quality of experience (QoE) within cells in LTE-A is variable. Faster connectivity has been provided by addition of spectrum and improvement in link adaptation. The new requirements of efficiency of area spectral and distribution of user rate are the challenges of 4G and forthcoming 5G technologies. By increasing the eNB deployment density, per-user performance along the cell may also be improved. A single-tier homogeneous network approach of mobile networks is shifted to a multi-tier heterogeneous network (HetNet). A HetNet consists of Macro cells, pico cells, femto cells or small cells. The Mcells are placed along the geographical area so that they can provide maximum coverage and reduced interference and eNBs are of high power. Scells offload the MCells at hot-spot areas by improving capacity and eliminating coverage holes. There are two solutions based on the deployment of the frequency:
- Co-channel deployment, the same frequency is shared by both Mcells and Scells
- Dedicated deployment, different frequencies are transmitted by each type of cells
Radio planning in HetNets
Different approaches are required in network planning and design for hyper densification of cells with multiple tiers so that fundamental objectives of increasing data rate and per-user spectral efficiency can be obtained. The deployment of frequency is the major challenge in the network planning of HetNets when limited spectrum is available and the other tiers reuse the frequencies of MCell. An important constraint in the deployment of co-channel is the inter-cell interference. As compared to classical deployment of Mcell, a higher number of interfering nodes are there in both UL and DL. High levels of interference are generated in the large number of Scells by reusing the same carriers.the interference can be minimized by using radio resource management procedures in such cases. Whereas, in case of large bandwidth availabilities, dedicated deployments are used which can control the interference in a better way.
When there is very little migration of users from the Mcell, DL transmit power is low that results in low coverage of Scell. The size of Scell may increase with higher transmit power that may also increase the load of Scell. If the received power from the best UL serving cell and the best DL serving cell are different, the UE is said to be im DL/UL imbalance problem. In this case the UE is connected to Scell in UL and to Mcell in DL.
Also, providing energy efficient and low cost backhaul in the deployment of Scell is quite tricky. The Scell availability may be limited due to lack of high capacity backhaul. As a result, the availability of such resources can be considered by the cell association procedures so that sufficient quality of experience is provided to the users.
Improvement strategies in HetNets
The imbalance problem is generated due to selection of cell on the basis of reference signal received power (RSRP) as the Mcell provides larger coverage of DL as compared to that of Scell. The cell range extension (RE) increases the coverage of Scell by allowing the UEs to connect to the cells whose DL RSRP is not the highest and this is one strategy to reduce this problem. With the addition of cell selection offset and RSRP measured from the Scell, the range of Scell can be expanded and the UEs in the expanded area range are connected, in both UL and DL, to the Scell. In this situation, there is an improvement in the UL of UEs, whereas the association of DL is in suboptimal way. But, by increasing the range of low power nodes, the chances of UL interference and DL interference are also increased. Hence, eICIC mechanisms are investigated which are variations of frame muting and coordinated scheduling. Actually, almost blank subframes (ABS) are introduces since LTE-A Release 10 by 3GPP.
The use of radio resources across Mcells and Scells is one of the major design goals for HetNets so that higher per-user throughput and system capacity can be achieved. The concept of Dual Connectivity has been introduced in release 12 by 3GPP in which radio resources are consumed by the user which is provided by at least two different network points. It is one of the potential solutions provided by 3GPP in which the benefits of the coverage of MCell and the capacity of Scell are combined to improve the performance of the user. Shared resources are used to improve the performance of the UE.
The decoupling of both UL and DL can be motivated by the power of UL/DL, Mcell/Scell load and power imbalance which is beneficial in deployment of the co-channel heterogeneous.
Decoupling as a solution
Radio planning and interference of management is the main challenge of HetNets that are addressed by the current technologies which are classified into three main groups:
- Dual connectivity
In dedicated deployments UL has been improved. UL/DL imbalance problem is not completely removed by using dual connectivity as the association rules are proposed on the basis of DL RSRP.
- Cell RE with eICIC
UL/DL imbalance problem can be reduced with this strategy by maximizing the UL improvement while the inter-cell interference in DL can also be increased. Hence, eICIC solutions are used along with RE. Due to increase in interference in DL, RE technique is limited to moderate offset values. RE offset is added to cell selection rules on the basis of DL RSRP.
- UL/DL decoupling
UL has been improved in co-channels deployment by using DUDe. the different requirements of UL and DL are fulfilled by the decoupled association policies that can solve the UL/DL imbalance problem successfully in terms of coverage, load and interference.