Heterogeneous Cellular Networks, HetNet

- essential basics of heterogeneous cellular networks, HetNets detailing what they are and how they are used to enable better performance and reduced costs.

Heterogeneous cellular networks for a very compelling approach for cellular networks to provide the coverage and capacity needed to move forwards.

These heterogeneous networks, often labelled HetNets are typically composed of a variety of formats of base station, radio access networks, transmission solutions and power levels.

Combining such a variety of technologies together enables the best option to be chosen for a given area, but it also presents problems in terms of ubiquity and operation with such a variation of technologies and approaches.

Nevertheless mobile operators are looking to adopt the heterogeneous cellular network, HetNet approach to enable them to meet their goals for coverage and capacity as much greater demands are placed on mobile networks.

HetNet basics

The growth of data over the cellular networks is increasing at an exponential rate as users download more video, transfer more data and use smartphones and tablets as their main access point for mobile communications.

Operators need to meet the need while still retaining profitability as cost per bit levels are falling as data levels increase.

There are a number of factors that have forced operators to look for new ways of carrying data.

One driver is cell size. The bigger the cell site, the less capacity there is per person as more users need to share the same capacity. If the cell coverage is reduced then the cell capacity is shared between fewer users. This results in higher capacity levels and faster data speeds.

In addition to this, many operators have seen significant opportunities for offloading data to local Wi-Fi networks as well as using home or office based femtocells.

The natural progression of the reduction of cell sizes, offload of data from the central backhaul network and other technologies including the better use of the radio spectrum has developed into what is termed heterogeneous cellular networks, or Het-Net.

In this way HetNets have several aspects:

  • Use of multiple radio access technologies:   Although, HetNet technology is being talked about in conjunction with LTE, one of the key concepts of heterogeneous networks, HetNets is that they will be able to use a variety of radio access networks including LTE, HSPA and also Wi-Fi and CDMA2000.
  • Operation of different cell sizes and approaches:   In order to maintain the flexible operation of the network different cell sizes and approaches are being used. The different cell sizes and types are able to fulfil different applications and provide a different type of serve better.
  • Backhaul:   With operators needing to carry an ever increasing amount of data, they will need to be creative about ways of transferring this back to the network. With everything from the ADSL links typically used by femtocells to the more traditional methods used by macro-cells, the whole system must operate in a seamless heterogeneous manner.

It can be seen from this that the basic concept of the heterogeneous network, HetNet is to have a core macro wireless network which seamlessly cooperates with intelligent small format cells that improve coverage and augment overall network capacity. These small cells, typically in the format of femtocells or Wi-Fi hotspots can be deployed within users buildings - houses, offices, etc, and used to improve coverage and deliver capacity inside buildings as well as offloading backhaul from the main network.

Radio access technologies

Although LTE is now a major focus for the cellular operators, there are still very many other radio access technologies that are available. There are the 3G services including UMTS, HSPA and CDMA2000 all of which can provide significant data capacity, and are applicable for many services.

In addition to this there will be times when the data can be carried via a Wi-Fi access point.

In terms of the heterogeneous cellular network, this creates a few additional problems in terms of creating a seamless experience for the user as the backhaul methodology is very different as the data and calls migrate from the cellular network to a more public network where the data transmission environment is very different.

Multiple cell sizes

The different types of cell are used for different applications. The traditional macro cells are still needed to provide general coverage, and connectivity particularly for those in outlying districts or in moving fast in vehicles where multiple handovers are not wanted. However where high data rates are needed in buildings or urban areas, a variety of small cell technologies can be used. These may use one of a variety of backhaul technologies. All these need to appear as a single network to the user, proving the same high performance level.

Typically the cell may fall into the very broad categories outlined in the table below.

Comparison of Macro and Small (Metro) Cells
  Macro cells Small / Metro Cells
Max users per cell
~ 2 000
~ 200
Max range
~25km
~200 m
Max transmit power
50W
5W
Typical usage Fast moving and rural coverage Stationary / slow moving and urban / in-building coverage

HetNet backhaul

With a variety of forms of base station, one of the major considerations for heterogeneous cellular networks is the backhaul technology.

Introducing new forms of base station in the form of picocell or femtocell will help alleviate radio access network congestion, but then the backhaul network can become congested. Also congestion can start to move further into the core network. In addition to this, there is some communication required to organise and configure the network and as a result there needs to be communication between the core and the periphery for some aspects of operation.

HetNets and HetNet structure, therefore is considerably more than adding femtocells or standard pico cells to the network. Thus the Heterogeneous network, HetNet concept requires the whole network to operate in a more efficient and seamless manner.

One of the key elements of a heterogeneous network is the integration of different elements into the network. For LTE, the architecture of the macro-cells is very flat.

Also the macro and metro cells can connect through the S1 interface into Mobility Management Entities or S1 gateways. As the interface is standardised, these entities can be from any manufacturer.

It is also possible to perform S1 based handovers and eliminate the need for the X2 interface between base stations. This makes the X2 interface and communications optional, thereby simplifying the interface links.

The standardisation of these interfaces brings many benefits is a key element of the heterogeneous nature of networks. It is possible to utilise equipment from many manufacturers, knowing that the operation of the interfaces is the same whatever the source of the equipment. This brings benefits in terms of both operational costs and capital costs. Performance is also guaranteed and in addition to this, innovation is stimulated.

Requirements for heterogeneous networks

Heterogeneous networks are being used in a variety of areas. Cellular telecommunications is one of the most high profile, but they are also being used in many other areas, especially where ad-hoc networks are being deployed.

The network must be structured in such a way that the performance seen in all areas of the network meet the requirements. Aspects such as latency, performance and speed must meet the requirements across the network.

HetNet development

With the challenges for operators increasing with users demanding improved performance and lower costs, this will require operators to use and ever wider number of technologies to ensure satisfactory operation of their networks in a variety of scenarios.

To provide not only coverage, but the right form of coverage in terms of small and macro cells, operators need to deploy a variety of formats of base station, and also implement a heterogeneous format for the backhaul.

With over 60% of traffic occurring within urban areas it is necessary to have deployed a blend of macro and metro or small cells into the environment. With city planners wanting to keep the visual impact to a minimum, the days of the towers and mast for all base stations are over. Unobtrusive small cells will be deployed alongside Wi-Fi to provide the required coverage. All this needs to operate seamlessly to the user without knowing how the signal is being carried.

By Ian Poole

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