Small Cell Deployment Considerations

Published: 26 February 2014

  1. Introduction

    The ever increasing demand for mobile broadband services is driving network operators to provide more and more capacity from radio access networks (RAN). This can be achieved by:

    1. increasing spectrum,
    2. employing more spectral efficient technologies such as 4G/LTE,
    3. and/or increasing the density of the cell sites.

    Spectrum is an expensive resource and is limited by license anyway. Capacity gain from the macrocell network by optimisation, improving spectral efficiency and upgrading RAN architecture such as deploying MIMO may not be able to keep pace with demand. For reasons of environment and radio engineering, it may not be possible to increase network density by deploying more roof top sites which are running out anyway despite co-operation between operators on site share. It seems that the only option available for operators on cell densification is to deploy smaller cells with smaller form factor which should facilitate the deployment in a wide range of locations.

  2. Small Cell Technology

    Small cell encompasses many low power technologies which may be employed in licensed and unlicensed bands. These include:

    1. Femtocell – This is predominantly low power devices deployed mostly in a residential environment. Most products are used to provide indoor 3G services operating in an operator’s licensed band. It is designed for easy deployment and self optimisation. It uses IP backhaul for which domestic broadband is quite adequate. The capacity of femtocell is usually small, supporting 4 to 5 simultaneous calls. It can operate in either closed, open or both closed and open access mode permitting private or public use. Enterprise femtocell such as SpiderCloud and ip.access offer high capacity and more functionality. Its operation principle is similar to that of residential femtocell.
    2. Picocell – This is a term mostly associated with small conventional base stations deployed in an indoor environment. A distributed antenna system may be used to extend the coverage footprint and to overcome losses due to building complexities.
    3. Microcell – Small conventional base stations mostly deployed in an outdoor environment providing short range coverage to both outdoor and indoor. This is also used for providing additional capacity to ease traffic congestion.
    4. Metrocell – There is no clear definition for a metrocell, which is basically a large quantity of microcells deployed on street furniture in a metropolitan area, providing both coverage and capacity, in particular in areas where large bandwidth is required.
    5. Carrier WiFi and WiMax – These are broadband devices conforming to the IEEE standard. They are deployed by telecoms operators in public areas providing broadband services to their customers.

    Femtocells normally have their own controller and interface with the operator’s core network using a gateway or service integrator. This aggregates the traffic from multiple femtocells. The radio part of the femtocell must conform to 3GPP standards so that a standard mobile handset can be used. Depending on the design, the controller may offer direct access to the Internet.

    Picocell, microcell and metrocell are conventional base stations conforming to 3GPP standards. Currently cell configuration and optimisation are required for these cell types, though there is a strong demand for self configuration and optimisation.

    Equipment vendors and operators are talking about heterogeneous networks (HetNet), a mixture of macrocells, small cells and WiFi providing contiguous coverage with handoff capability between them.

    Bringing-the-signal-closer-to-the-user

  3. Advantages and Disadvantages

    Small physical size base stations mean smaller capacity and lower transmit power which is therefore only suitable for short range coverage. However, it offers the possibility of deployment in a wide range of locations making it more environmental and planning friendly.

    Typically 90% of the network data traffic is carried on 10% of the network cells. The capacity of these heavily loaded cells can be increased by deploying more spectrums (this does not come cheap). Alternatively, it is possible to upgrade to a technology such as LTE-Advanced in MIMO operation and sectorisation. Physical restrictions on sites, such as space and mechanical loading may limit the options. Small cells may offer a better option as they can deploy in a more targeted manner to relief traffic. Proximity to the user and shorter coverage range means better coverage and quality, thus higher throughput.

    In general, large macrocells deliver a more cost effective solution for mobile services per unit area. It is nevertheless a large investment with limited capability for handling traffic dynamics. For example large traffic volume may shift from urban centre to sub-urban areas after work. Therefore a large investment in macrocells may not be generating revenue for half the time. Smaller cells to supplement macrocell traffic capacity may offer better network economy as the investment is relatively smaller.

    However, it is not all ‘rosy’ for small cells. A large quantity of small cells increases the need for careful network management. This is achieved both in terms of configuration, such as the creation of a neighbour list and also in terms of optimisation. Smaller cells also create more cell borders, increasing cell handovers as the mobile moves. This puts a demand on signalling capacity. A more intelligent network management capability is required before small cells can be deployed as a BAU process. Currently network intelligence is very much core network centric. An increase in network management requirements may create the requirement for a localised controller or shifting some the intelligence from core network to the radio front end.

    NEC_SmallCellHetNet

  4. Deployment Considerations

    The small form factor of the base stations may simplify the deployment as it can be installed in a wide range of locations and is much easier to mount.

    Individual small cell deployment is no different to that of traditional microcells. This paper focuses on the deployment of small cells in a cluster or a metrocell providing reasonably contiguous coverage.

    1. Sites – given the number of cells involved in a cluster, it would be very time consuming and expensive to negotiate with individual site owners to mount equipment on a wall. It is better to reach an agreement with the local authority to deploy the operator’s own street furniture equipment or even better to be able to mount the equipment on the authority’s own street furniture like lamp posts etc. It would be ideal if the small cells can use the electricity supply to the lamp posts.

      Buildings in city centres or town centres are usually higher than a suburban house. The street furniture approach may mean the coverage is confined within a street cannon. The antenna design may have to take into account coverage above the horizon for mobiles at higher floors.

      Of course antenna design and base station housing should be aesthetic, not to become an eye sore to the locals.

    2. Backhaul – This is probably the main barrier to the deployment of small cells. The experience of macrocell deployment tells us that transmission is very expensive, and the delivery by telecom operators is poor. There are many backhaul options ranging from fibre, satellite, bonded copper, WiFi/WiMax and microwave.

      Fibres, satellite and bonded copper may need to be ruled out due to cost and control of delivery. WiFi/WiMax uses an unlicensed spectrum and it is thus difficult to control interference which would affect the backhaul quality. This leaves microwave to be the preferred option where to some degree deployment is under the control of the mobile operators.

      There is plenty of microwave spectrum when the frequency moves into millemetric band but hardware becomes relatively expensive. Currently there are many products available in sub-40GHz band, in particularly in the 38GHz band which is already used by mobile operators for the backhaul.

      Microwave backhauls can operate in point-to-point (P2P) and point-to-multipoint (P2M) mode. P2P can be in the form of star, ring or tree with each link requiring two radio units. P2M is akin to mobile network with a sector antenna at the central station and directive antenna at the small cell end.

      Backhaul traffic of mainly data is quite bursty, rendering P2P microwave links inefficient and thus not cost effective. On the other hand P2M offers better efficiency and is thus more cost effective. The macrocell site can be used as the node for the P2M backhaul as it is normally high above ground for easy availability of line of sight (LOS) with the small cell sites. P2P may still have to be deployed as a ‘daisy chain’ in order to achieve LOS with the central station.

    3. Network performance – Given the number of cells in a cluster, and the potentially large number of small cells in a network, it would be a vast task to keep track of the continuous deployment and the corresponding cell configuration. This is particularly true of the dynamic elements controlling mobility, not forgetting the need for cell optimisation. The Self Organising Network (SON) feature becomes essential for rolling out small cell network.

      Another important feature is ICIC (Inter Cell Interference Cancellation) which is already part of the Release 8 specification for LTE. This feature is essential to improving network quality which is critical for maximising throughput.

    4. Deployment discipline – There may be a need to adjust current deployment processes for macrocells. To be cost effective, the process should be tailored for a one stop shop deployment, providing an end to end service from installation to cell deployment. This should take place within a short period, around two to three days. Installation engineers become proficient in multi-disciplinary tasks which may require further training and development.
  5. Conclusion

    This note provides a short introduction to small cell technology and the issues concerning deployment. Nearly all mobile operators state that small cell is their future in order to meet customers’ demands for capacity. Apart from developing more intelligence for small cell products, installation staff should have an open mind, preparing themselves to take on multi-disciplinary tasks which will not be restricted to mechanical installation. They will need to familiarise themselves and become proficient in dealing with software applications that support full turn-key services that will bring a cell into service.

References:

  1. NGMN White Paper, Small Cell Backhaul Requirements
  2. Small Cell Forum, Small Cell – What is the Big Idea?
  3. CBNL, Small Cell Backhaul – The Big Picture