Antenna

Antenna

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Introduction An antenna is an electrical conductor or system of conductors  Transmission - radiates electromagnetic energy into space  Reception - collects electromagnetic energy from space  In two-way communication, the same antenna can be used for transmission and reception.  Antennas convert “Wire Line” signals to “Wire Less” • The physical size of the radiating element is proportional to the wavelength. The higher the frequency, the smaller the antenna size. 

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Types of Antennas –

Generally speaking, there are two ‘types’ of antennae:

1. Directional  this type of antenna has a narrow beamwidth; with the power being more directional, greater distances are usually achieved but area coverage is sacrificed  Yagi, Panel, Sector and Parabolic antennae

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2. Omni-Directional 

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this type of antenna has a wide beamwidth and radiates 360;  Radiates power equally in all directions (A=B) with the power being more spread out, shorter distances are achieved but greater coverage attained Also called as Isotropic antenna

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Dipole antennas 

Half-wave dipole antenna (or Hertz antenna)

Quarter-wave vertical antenna (or Marconi antenna) 

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Antenna Gain 

Antenna gain  The amount of energy the antenna can ‘boost’ the sent and received signal by is referred to as the antennas Gain.  Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

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Relationship between antenna gain and effective area

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4π Ae 4π f 2 Ae G= 2 = 2 λ c G = antenna gain Ae = effective area f = carrier frequency c = speed of light (≈ 3 x 108 m/s) λ = carrier wavelength

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Gain may be expressed as dBi or dBd When talking about gain it is always the main lobe that is discussed.

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dBi versus dBd 

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dBi indicates gain vs. isotropic antenna  Isotropic antenna radiates equally well in all directions,  spherical pattern dBd indicates gain vs. reference halfwavelength dipole  Dipole has a doughnut shaped pattern with a gain of 2.15 dBi

dBi = dBd + 2.15 dB 8

There are certain guidelines set by the FCC that must be met in terms of the amount of energy radiated out of an antenna. This ‘energy’ is measured in one of two ways: 1. Effective Isotropic Radiated Power (EIRP) measured in dBm = power at antenna input [dBm] + relative antenna gain [dBi] 2. Effective Radiated Power (ERP) measured in dBm = power at antenna input [dBm] + relative antenna gain [dBd] 9

Antenna System Elements Antennas Earthing kit

Mounting clamp

Wall gland

Cable trace

Feeder cable

Jumper cable

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Jumper Jumpercable cable

Connector Connector

Feeder Feederclamps clamps Grounding Grounding

Feeder FeederClamps Clamps 11

Feeder 

Technical summary: Inner conductor: Copper wire Dielectric: Low density foam PE Outer conductor: Corrugated copper tube Jacket: Polyethylene (PE) black

Outer Outerconductor conductor Inner Innerconductor conductor

Dielectric Dielectric

Jacket Jacket

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Trisectorial Site Antennas

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Polarization 

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Radio waves are built by two fields, one electric and one magnetic. These two field are perpendicular to each other. The sum of the fields is the electromagnetic field. The position and direction of the electric field with reference to the earth’s surface (the ground) determines wave polarization. 14

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Horizontal polarization - the electric field is parallel to the ground. Vertical polarization - the electric field is perpendicular to the ground.

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Polarisation of EM wave circular

vertical

Electrical field, E horizontal 16

Tx Power Tx is short for “Transmit” All radios have a certain level or Tx power that the radio generates at the RF interface. This power is calculated as the amount of energy given across a defined bandwidth and is usually measured in one of two units: 1. dBm – a relative power level referencing 1 milliwatt 2. W – a linear power level referencing Watts

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Rx Sensitivity  

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Rx is short for “Receive” All radios also have a certain ‘point of no return’, where if they receive a signal less than the stated Rx Sensitivity, the radio will not be able to ‘see’ the data. This is also stated in dBm or W. The actual level received at the radio will vary depending on many factors. 18

dB Units 

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Decibel (dB) is a mathematical expression showing the relationship between two values. Relative Measurement dB=10 log(Po/Pi) The RF power level at either transmitter output or receiver input is expressed in Watts, but it can also be expressed in dBm. The relation between dBm and Watts can be expressed as follows: P dBm = 10 x Log P mW

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For example: 1 Watt = 1000 mW; P dBm = 10 x Log 1000 = 30 dBm 100 mW; P dBm = 10 x Log 100 = 20 dBm dBW is wrt to Watt dBm is wrt to mill Watt dBi is wrt to Isotropic value  An Increase of 3dB = Increase by 2 Times  An Increase of 7dB = Increase by 5Times  An Increase of 10dB = Increase by 10 Times

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W-dBm -30 dBm

1μW

-20 dBm

10μW

-10 dBm

100μW

-7 dBm

200μW

-3 dBm

500μW

0 dBm

1 mW

3 dBm

2mW

7 dBm

5mW

10 dBm

10mW

13 dBm

20mW

20 dBm

100mW

30 dBm

1W

40 dBm

10W

50 dBm

100W

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Signal Propagation 

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As the signal leaves the antenna it propagates, or disperses, into space. The antenna selection will determine how much propagation will occur. At 2.4 GHz it is extremely important to ensure a that a path (or tunnel) between the two antennas is clear of any obstructions. Should the propagating signal encounter any obstructions in the path, signal degradation will occur.

Trees, buildings, poles, and towers are common examples of 22 path obstructions.

RF Planning 

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The radio access part of the wireless network is considered of essential importance as it is the direct physical radio connection between the mobile equipment and the core part of the network. In order to meet the requirements of the mobile services, the radio network must offer sufficient coverage and capacity while maintaining the lowest possible deployment costs.

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RF Planning 

Aims To Be Achieved(For Operator) 

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Operators Strategic Intentions should reflect in Planning First : Coverage Second : Tariffs ! Third , Fourth …. : Quality Of Network & Service, Value Added services, Content Provisions !!

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Inputs to be given to the OMC-R team by the RF team to design the RF Database Frequencies (BCCH,HSN), Base Station Colour Codes (BCC) Location Area Codes Neighbour Lists for each cell Transmitter Power for each cell etc..,

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General Approach for Radio Network Planning 

The radio network planning process can be divided into different phases as 

Preplanning phase  

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Main planning phase  

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Traffic & Coverage Analysis Nominal Cell Plan Surveys System Design

Adjustment phase  

Implementation & System Tuning System Growth

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Radio Cell Site planning process

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STEP 1: TRAFFIC AND COVERAGE ANALYSIS 

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Cell planning begins with traffic and coverage analysis. The analysis should produce information about the geographical area and the expected capacity (traffic load). The types of data collected are: Cost Capacity Coverage

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Grade Of Service (GOS) Available frequencies Speech quality System growth capability The basis for all cell planning is the traffic demand, i.e. how many subscribers use the network and how much traffic they generate.

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Erlang – a unit of traffic 

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An Erlang is a unit of telecommunications traffic measurement.  It Erlang represents the continuous use of one voice path.  In practice, it is used to describe the total traffic volume of one hour. Erlang traffic measurements are made in order to help telecommunications network designers understand traffic patterns within their voice networks.  30

Erlang contd.., 

It can be calculated with the following formula: A = n x T / 3600 Erlang Where, A = offered traffic from one or more users in the system n = number of calls per hour T = average call time in seconds

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Erlang contd.., 

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For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows: Minutes of traffic in the hour=number of calls x duration Minutes of traffic in the hour=30 x 5=150 Seconds of traffic in the hour=150 x 60 = 9000 Hours of traffic in the hour=9000 / 3600 Hours of traffic in the hour=2.5 Traffic figure=2.5 Erlangs

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Basic Data for Planning 

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The geographical distribution of traffic demand can be calculated by the use of demographic data such as: Population distribution Telephone usage statistics Topography Details of Roads & Towns from Maps Type of Buildings, People’s Profile.. Presence of Competitor, their Customer base

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Calculation of required no of BTS’s 

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To determine the number and layout of BTS’s the number of subscribers and the Grade Of Service (GOS) have to be known. The GOS is the percentage of allowed congested calls and defines the quality of the service. If n=1 and T=90 seconds then the traffic per subscriber is: A = 1 x 90 / 3600 = 25mE

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If the following data exists for a network: Number of subscribers: 10,000 Available frequencies: 24 Cell pattern: 4/12 GOS: 2% Traffic per subscriber: 25mE 35

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4/12 means that there are four three-sector sites supporting twelve cells.

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this leads to the following calculations: Frequencies per cell = 24 / 12 = 2 Traffic channels per cell = 2 x 8 - 2 (control channels) = 14 TCH Traffic per cell = 14 TCH with a 2% GOS implies 8.2 Erlangs per cell (see Table 10-1) The number of subscribers per cell = 8.2E / 25mE = 328 subscribers per cell

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If there are 10,000 subscribers then the number of cells needed is 10,000 / 328 = 30 cells. Therefore, the number of three sector sites needed is 30 /3 = 10

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Erlang Table

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Erlang Table contd..,

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Step 2: Nominal Cell Plan 

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A nominal cell plan can be produced from the data compiled from traffic and coverage analysis. The nominal cell plan is a graphical representation of the network and looks like a cell pattern on a map. Nominal cell plans are the first cell plans and form the basis for further planning.

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Nominal Cell Plan

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Nominal Cell Plan contd.., 

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Successive planning must take into account the radio propagation properties of the actual environment. Such planning needs measurement techniques and computer-aided analysis tools for radio propagation studies. Ericsson’s planning tool, TEst Mobile System (TEMS) CellPlanner, includes a prediction package which provides:

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Nominal Cell Plan contd..,    

Coverage predictions Co-channel interference predictions Adjacent channel interference predictions TEMS cell planner is a software package designed to simplify the process of planning and optimizing a cellular network.

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Frequency Planning 

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Frequency re-use means that two radio channels within the same network can use exactly the same pair of frequencies, provided that there is a sufficient geographical distance (the frequency reuse distance) between them so they will not interfere with each other. Tighter Re-use = More Capacity. But also results in increased interference issues. Trade Off.. Plan to achieve the capacity within tolerable interference levels

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Frequency Planning contd.., 

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The GSM specification recommends that the carrier-to interference (C/I) ratio (co channel interference) is greater than 9 decibels (dB). The GSM specification states that the carrier-to-adjacent ratio (C/A) (adjacent channel interference) must be larger than -9dB. By planning frequency re-use in accordance with well established cell patterns, neither co-channel interference nor adjacent channel interference will cause problems.

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TEMS Cell Planner 

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With TEMS CellPlanner, traffic can be spread around on a map to determine capacity planning. The traffic can be displayed using different colors for different amounts of Erlangs/km or the user can highlight the cells that do not meet the specified GOS. 47

TEMS Cell Planner contd.., 

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It is possible to import data from a test MS and display it on the map. TEMS CellPlanner can also import radio survey files, which can be used to tune the prediction model for the area where the network is to be planned. Data can also be imported from and exported to OSS.

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Step3 : Surveys 

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Once a nominal cell plan has been completed and basic coverage and interference predictions are available, site surveys and radio measurements can be performed. Site surveys are performed for all proposed site locations. Radio measurements are to be made at all the locations.

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Surveys contd.., 

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Radio measurements are performed to adjust the parameters used in the planning tool to reality. That is, adjustments are made to meet the specific site climate and terrain requirements. For example, parameters used in a cold climate will differ from those used in a tropical climate.

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Surveys contd.., 

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A test transmitter is mounted on a vehicle, and signal strength is measured while driving around the site area which is called as Drive Test. Afterwards, the results from these measurements can be compared to the test tool values. Through drive tests the simulated results will be examined and refined until the best compromise between all of the facts is achieved.

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Step 4: System Design 

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Once the planning parameters have been adjusted to match the actual measurements, dimensioning of the site can be adjusted and the final cell plan produced. As the name implies, this plan can then be used for system installation. New coverage and interference predictions are run at this stage, resulting in Cell Design Data (CDD) documents containing cell parameters for each cell. During system design link budget is also supposed to be calculated.

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Link Budget Calculations – To establish the viability of a link prior to installing any equipment, a Link Budget Calculation needs to be made. – Performing this calculation will give you an idea as to how much room for path loss you have, and give you an idea as to link quality.

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Link Budget 

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The link budget is the table recording the power loss in the uplink or downlink of the network Power Budget Calculations  

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What is the max Path Loss? What can be the BTS Tx Power? : To balance both links

Cell size Evaluation Main factors : BTS & MS Powers, Sensitivity Slow, Fast Fade Margins, Penetration Losses

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Fade Margin – Defined as the difference between the Receive Signal Level RSL, and the Rx Threshold or other chosen reference Level.

Ie. If you have an RSL of –60dB and a Rx Threshold of –72dB, than your fade Margin would be 12dB

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Link Budget contd.., 

The link budget results can be improved by adopting some techniques like frequency hopping, using receiver diversity, implementing tilt, DTX and by choosing proper site location.

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Step 5 & 6: System Implementation and Tuning 

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Once the system has been installed, it is continuously monitored to determine how well it meets demand. This is called system tuning. It involves: Checking that the final cell plan was implemented successfully Evaluating customer complaints Checking that the network performance is acceptable Changing parameters and taking other measurements, if necessary

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TEMS 

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TEst Mobile Systems (TEMS) is a testing tool used to read and control the information sent over the air interface between the BTS and the MS. It can be used for radio coverage measurements. In addition, TEMS can be used both for field measurements and post processing.

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TEMS

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TEMS contd.., 

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TEMS consists of an MS with special software, a portable Personal Computer (PC) and optionally a Global Positioning System (GPS) receiver. The MS can be used in active and idle mode. The PC is used for presentation, control and measurements storage.

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TEMS contd.., 

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The GPS receiver provides the exact position of the measurements by utilizing satellites. TEMS measurements can be imported to TEMS CellPlanner. This means that measurements can be displayed on a map.

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TEMS graphical user interface

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Step 7: System Growth  

Cell planning is an ongoing process. If the network needs to be expanded because of an increase in traffic or because of a change in the environment (e.g. a new building), then the operator must perform the cell planning process again, starting with a new traffic and coverage analysis. 63

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Antenna Impedance A proper Impedance Match is essential for maximum power transfer. The antenna must also function as a matching load for the Transmitter ( 50 ohms). Voltage Standing Wave Ratio (VSWR), is an indicator of how well an antenna matches the transmission line that feeds it. It is the ratio of the forward voltage to the reflected voltage. The better the match, the Lower the VSWR. A value of 1.5:1 over the frequency band of interest is a practical maximum limit.

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Return Loss is related to VSWR, and is a measure of the signal power reflected by the antenna relative to the forward power delivered to the antenna. The higher the value (usually expressed in dB), the better. A figure of 13.9dB is equivalent to a VSWR of 1.5:1. A Return Loss of 20dB is considered quite good, and is equivalent to a VSWR of 1.2:1. 66

VSWR 1.0:1

Return Loss ∞

Transmission Loss 0.0 dB

1.2:1

20.83 dB

0.036 dB

1.5:1

13.98 dB

0.177 dB

5.5:1

3.19 dB

2.834 dB

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