PCFICH spans four (4) locations across the bandwidth and are equidistant. The location of 4 REGs of PCFICH depends on PCI and system bandwidth But PCIs are 504, so cells with different PCI...
Case Study LTE Small Cell backhaul planning using CONNECT
AIRCOM International International Cassini Court, Randalls Way, Leatherhead, KT22 7TW United Kingdom www.aircominternational.com
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1 Executive Summary With wireless data already predicted to exceed wired data in the next few years and network capacity demands to increase 20-40 fold over the next 5 years, mobile operators are under pressure to dramatically increase their network capacity and maintain data throughput rates, in a cost effective manner. Embracing a Small Cell strategy seems to be the most common approach to achieve this across the World’s operators. Small Cell technology (which includes femto, pico and micro cells) is currently in use by 67% of operators according to the Small Cell Forum and its usage will increase from 4.3 million small cells to 36.8 million by 2016. Operators need a planning strategy to ensure capacity issues are addressed and throughput rates are maintained for continued positive customer experience. Small Cells can offer additional capacity and provide improved indoor and outdoor coverage but planning is required to ensure that backhaul transmission can be achieved with equipment that will typically be mounted on street furniture such as traffic lights, sign posts or lamp posts and that there is sufficient capacity to handle the increased data rates provided by LTE. A Case Study is presented here, on how LTE Small Cell backhaul transmission can be planned with AIRCOM’s Backhaul and Link Planning tool, CONNECT using Point-to-Point (PtP) Wireless Links, either micro- or millimetre-wave, Point-to-Multi-Point (PtMP) Microwave Links and Fibre Optic Links in a high-traffic, high-density area of central London, in the United Kingdom, which contains both Small Cells and Macro Cells. The RF planning of the Small Cell layer can be performed using AIRCOM’s ASSET Radio Planning tool or any other similar solution. AIRCOM’s I-VIEW DIMENSION End-to-End Capacity Planning tool can be used for Capacity Management and scenario planning, what-ifanalysis, future expansion planning and optimisation. These are not covered in this case study but were addressed in previous ones which can be obtained by contacting us if you missed them.
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2 Introduction This document demonstrates how an LTE Small Cells backhaul network can be planned with AIRCOM’s Backhaul and Link Planning tool, CONNECT. For this case study we are using a high-traffic, high-density area of central London, in the United Kingdom, which contains both Small Cells and Macro cells. The Macro Cell sites locations are real ones obtained from the UK’s regulator’s published data. We will show in CONNECT the topology of this network segment and we will discuss the various backhaul options listing their advantages and disadvantages. We will show that a number of different combinations can be used and there is not a single answer but a careful evaluation has to be performed taking into account a number of factors before the final approach is decided. We will then analyse the topography and the existing infrastructure of the study area and we will select the best backhaul option which can offer the required capacity with high availability and low cost. The required capacity on each cell is taken from an RF planning tool but it can also be taken from real network statistics or marketing data. For more details about the configuration and the simulations which were executed, please contact us.
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3 Link Planning Considerations 3.1 Case Study Area The Case Study Area is a dense urban area of central London that includes Covent Garden and Holborn and is bounded by Soho, Leicester Square and Tottenham Court Road.
Fig. 1 The area under study
The map above has a Bing Web Map backdrop, overlaid with a Building Vector layer. The buildings have been coloured by height (m) according to the ranges in the top left-hand corner. The buildings in the area are between 10-60m in height. The eight (8) large pink three-sector icons denote the existing LTE Macro Cell sites while the small pink vector icons represent the new Small Cells that were added manually in locations decided in the Radio Planning Tool to improve poor coverage and increase capacity. Twenty four (24) Small Cells were added in this manner, in order to complement the macrocell network and offer the required QoS to all the users. The Macro Cell sites are on building rooftops or façades between 22-53m whereas the Small Cells are at street level and all have a height o f 4m. The use of one or more transmission only sites located on high building (>50m) was considered during Line of Sight (LOS) analysis.
3.2 Link Types The available options while designing the Small Cell backhaul networks include a number of different link types, each of them with its strengths and weaknesses. Point-to-point microwave links take advantage of the numerous frequency bands that are available between 10-60 GHz and is a widely used and tested technology. They are
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suitable connections between high (macrocell) sites, offering medium-to-high capacity capabilities. However, a line-of-sight (LOS) between these sites is required, as well as a recurring OPEX, due to the licensing of these links. Point-to-point millimetre wave connections have a relaxed licensing procedure that reduces the spectrum OPEX in many markets. Additionally, due to the high-frequencies, 7080 GHz, the antennas have a small and discreet size that makes them perfect for their street level deployment. They can achieve very high capacities, but for short line-of sight links, and thus they are suitable for the connections between the small cells. Point-to-multipoint line-of-sight microwave links use the ETSI PtMP 10.5, 26 and 28 GHz frequency bands (other bands are in consideration). Up to 8 remote terminals can be connected per access point with up to 300 Mbps backhaul capacity. Therefore they can only be used as a backhaul option very close to the edge of the network, where the capacity needs are not very demanding, with an increase though on the OPEX costs. Non-line-of-sight (NLOS) PtMP links take advantage of the unlicensed 3.5 and 5 GHz bands and their excellent low frequency propagation interaction with the environment to connect multiple Small Cells. However, it is expected that these bands will be congested, with relatively moderate capacity capabilities, and thus they will be used as last-mile connections for low demanding Small Cells. Other non-wireless connections are possible, like ADSL lines, coaxial cables or optical fibres. Despite their high capacity and reliability (as in optical fibres), they add a significant cost especially if they are not readily available in the planning region. They are mostly suitable for connecting transmission hubs or Small Cells which cannot be accessed with a wireless link.
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4 Planning Methodology The backhaul connections of the Small Cells depend on a number of different factors and the network itself will include many different link types of those described above; a single size doesn’t fit all. The main drivers in the Small Cell planning are the traffic requirements and the existence of a line-of-sight path to a macrocell or a transmission hub. It is possible though, if the macrocell/transmission hub cannot be reached, to interconnect the small cells in a tree or daisy chain or even for lower requirements with NLOS PtMP connections. For those small cells that an adequate wireless path cannot be found, a wired one should be used, or an alternative location to be found. The interconnections between macrocells are planned with PtP microwave links, which are typically available in most real deployments.
4.1 Line-of-Sight Analysis As a first step, it is necessary to establish whether and where line-of-sight paths exist between the newly added small cells and the macrocells/transmission hubs locations. A valid LOS path should fulfil the Fresnel clearance criteria and consider all the available information about the area under study. Such information is the terrain heights (DTM), clutter heights, and building vectors and/or rasters. CONNECT offers three ways of performing such an analysis; the LOS Wizard, which provides a complete list of the possible LOS paths, the ability to save this information and create the links automatically; the Height Profile between two locations (Fig. 2), with a visual display of the actual path, user-obstruction editor and link budget/reporting functionalities; and the Site Visibility in the Map 2D View window, for a quick LOS assessment.
Fig. 2 The height profile between two properties
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4.2 Traffic Requirements The traffic needs for each small cell were calculated using AIRCOM’s RF planning tool, ASSET, which is tightly integrated with CONNECT, and thus seamlessly share this information. This information is also an important parameter on deciding where to place the small cell itself.
Parent Base Station
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5 Link Planning Solution The above requirements dictate the way the network, which now consists of eight LTE macrocell sites and twenty-four LTE small cells, should be interconnected. It is acknowledged that are needed:
1 x Transmission Only ‘High’ Site
8 x PtP (LOS) 38GHz Hybrid Microwave Links
9 x PtP (LOS) E-Band Microwave Links
1 x PtMP (LOS) 26GHz Hub with 4 x PtMP (LOS) Links
1 x PtMP (NLOS) 3.5GHz Hub with 8 x PtMP (NLOS) Links
3 x Fibre Optic Links
Fig. 3 Small Cell backhaul network
The requirement for one transmission only ‘high’ site was identified in the eastern half of the Case Study area, with a height of almost 60m it was one of the highest buildings in the area and provided a fibre optic Point-of-Presence (PoP) and good opportunities for both LOS and NLOS links.
5.1 PtP (LOS) 38GHz Hybrid Microwave Links PtP (LOS) 38GHz Microwave Links (in red) were created from two hubs, one transmission only and on existing Macrocell Site in order to provide inter-connectivity between the eight Macrocell sites and the fibre optic PoP. Hybrid Radios have been used as it is likely that the LTE Macrocells will be co-located with legacy technology sites for UMTS/HSPA and GSM/EDGE/GPRS.
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Fig. 4 The LOS links shown in Google Earth as exported by Connect
5.2 PtP (LOS) E-Band Microwave Links PtP (LOS) E-Band Microwave Links (in blue) were mainly used between Smalls Cells with lineof-sight along streets. In fact it was possible to link seven Small Cells in a tree topology with central location having access to optical fibre. In addition E-Band Microwave Links have been used for connection of a Macrocell Site to a Small Cell and from the transmission only hub to a another Small Cell.
5.3 PtMP (LOS) 26GHz Hub/Links The possibility to make four PtP (LOS) Links to Small Cells from a Macrocell site in the southeast quadrant of the Case Study area was identified. This provided a good opportunity to locate a PtMP (LOS) 26GHz Microwave Hub and four PtMP (LOS) 26GHz Microwave Links (in green). The PtMP Hubs consist of six Sectors with one Carrier per Sector.
5.4 PtMP (NLOS) 3.5GHz Hub/Links A PtMP (NLOS) 3.5GHz Hub was located on the transmission only ‘high’ site, from where it was possible to connect eight Small Cells using PtMP (NLOS) 3.5GHz Microwave Links (in orange). The key difference between the PtMP (LOS) and PtMP (NLOS) links is that it is not necessary to have clear line of sight between the Hub Sector Antenna and the Small Cell PtMP Antenna.
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Fig. 5 No LOS path exists for this Small Cell; a NLOS option has to be followed
5.5 Fibre Optic Links Fibre Optic Links (in yellow) were used to connect two Smalls Cells with no radio path (LOS or NLOS) that meet capacity and link budget requirements to the transmission only hub. A Fibre Optic Link was also used to connect a Small Cell hub location that was in turn connected to a further Small Cells in a tree topology. In CONNECT, as well as planning a range of PtP and PtMP Microwave Links, it is also possible to plan non-microwave connections, such as optical fibres, coaxial cables, twisted pairs and leased lines.
5.6 Link Performance All planned wireless links were ensured not only to offer enough capacity to fulfil the traffic requirements but also to operate within the availability targets, reducing the outage periods in acceptable levels. Transmit powers, operating modulation types and antennas were fine tuned to achieve this goal. Interference levels were made certain not to degrade the network performance, by running the Interference Analysis wizard, and make amendments in the configuration, where necessary. CONNECT offers adequate planning capabilities according to the ITU specifications to allow for accurate planning of the links. Link Budget, Fade Margin, Outage, Reliability and Objectives are calculated to ensure high availability of the links.
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Fig. 6 Link planning
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6 Conclusions It is evident that planning the backhaul of a Small Cell network will require a variety of wireless and wired link types based on the generated traffic, the location of the small cells, and cost, among other factors. Traditional link planning methods will not be sufficient as the environment and the propagation phenomena are becoming equally crucial for the backhaul as they are for the RF. This Case Study has shown that backhaul transmission for LTE Small Cells can be effectively planned and analysed in CONNECT. As the majority of Small Cells will be deployed in dense urban and urban environments it is highly recommended that Building Vector data is available for LOS analysis and the calculation of obstruction losses for NLOS links; the more detailed the environment information the more accurate the planning results. Not all the small cells locations can be reached with line-of-sight connections. Therefore, it is necessary to locate the hub locations, from where NLOS PtMP microwave links could be delivered with adequate capacity and availability. Similarly, not all small cells can be connected with any wireless connection, LOS or NLOS. Therefore, it may be necessary to identify them and plan the wired connections (coaxial cables, optical fibres, leased lines). The macrocells will be interconnected with LOS point-to-point microwave links, but the increased capacity due to the small cells means that they will need to be upgraded. Such knowledge would help to plan and perform the necessary actions timely, before leading to a degraded performance. Since the traffic needs is an important factor, cooperation between the radio-frequency and transmission planners would be necessary to ensure that the designed backhaul network would fulfil the requirements for both teams. Therefore, the integration with an RF planning tool, like the one that CONNECT offers with ASSET, would make that an easy task.
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7 Glossary AMC – Adaptive Modulation and Coding ATPC – Automatic Transmit Power Control DTM – Digital Terrain Map E-Band Extremely High Frequency bands from 71-76GHz, 81-86GHz and 92-95GHz EDGE – Enhanced Data Rates for GSM Evolution ETSI – European Telecommunications Standards Institute FDD – Frequency Division Duplexing GPRS – General Packet Radio Service GSM – Global System for Mobile Communications HSPA – High Speed Packet Access IP – Internet Protocol ITU – International Telecommunications Union LOS – Line of Sight LTE - Long Term Evolution, a technology from the 3GPP industry group NLOS – Non Line of Sight OPEX – Operational Expenditure PoP – Point of Presence PtMP – Point to Multi Point PtP – Point to Point Rx – Receive TDD – Time Division Duplexing TDM – Time Division Multiplexing Tx – Transmit UMTS – Universal Mobile Telecommunications System WMS – Web Map Service –