VectaStar 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT Cambridge Broadband Limited D000114 Issue A01 Mark Jackson
1 INTRODUCTION 3 1.1 The purpose of antennas 3 2 ANTENNA CHARACTERISTICS 4 2.1 Antenna radiation patterns 4 2.2 Reciprocation 5 2.3 Antenna front-to-back ratio 5 2.4 VectaStar antenna front-to-back ratios 6 2.5 The affects of the front-to-back ratio 6 2.6 Frequency re-use and the front-to-back ratio 7 3 IMPROVING ANTENNA ISOLATION 8 3.1 Worked example 8 3.2 Improving antenna isolation by physical obstruction 8 3.3 Improving antenna isolation by additional shielding 9 4 REFLECTIONS 10 4.1 Example of a reflection condition 10 4.2 Rejection of reflected signals using polarisation 11 5 RADIATION PATTERNS AND RADIO COVERAGE 12 5.1 Radiation patterns 12 5.2 VectaStar beamwidths 13 5.3 VectaStar beam-tilt 13 6 IMPROVING RADIO COVERAGE 14 6.1 Reducing fading through beam-tilt 14 6.2 Improving isolation through beam-tilt 15 7 DOCUMENT PROPERTIES 16 D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 2 OF 16
1 INTRODUCTION 1.1 THE PURPOSE OF ANTENNAS The purpose of an antenna is to radiate electromagnetic waves into free space. Antennas are also used to receive radiation from free space and deliver the energy in a propagating wave to a piece of electronic equipment. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 3 OF 16
2 ANTENNA CHARACTERISTICS An ideal antenna would exhibit perfect isolation characteristics and receive only the signals for which it is intended. In the real world, antennas receive both wanted and unwanted (interference) signals. The overall quality and performance of a radio system is determined by the ability of the antenna to accept wanted signals, and reject unwanted signals. An antenna receives interference by receiving signals not within the intended beamwidth of the radiation pattern reflection of unwanted signals into beamwidth of the antenna 2.1 ANTENNA RADIATION PATTERNS 2.1.1 ANTENNA RADIATION PATTERNS AND BEAMWIDTH Each antenna has a characteristic radiation pattern. The radiation pattern indicates the directional performance of the antenna. A directional antenna has a radiation pattern with bias in one direction called the antenna beamwidth measured between the 3dB points. For a radio system to provide radio coverage through 360 degrees using four antennas, each antenna requires a 90 degree beamwidth. In practice, it is difficult to design an antenna that maintains a 90 degree beamwidth at all power levels, and the resultant antenna will radiate in all directions including to the side and out of the back. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 4 OF 16
2.2 RECIPROCATION Most antenna designs exhibit a reciprocal characteristic. The reciprocal characteristic of an antenna means that it will transmit and receive signals of the same kind. An understanding of the reciprocal characteristic when deploying antennas helps to maximise the efficiency of the antenna and the reliable operation of the communication system. 2.2.1 RADIATION PATTERNS AND THE RECIPROCAL CHARACTERISTIC The radiation pattern and beamwidth of an antenna are independent of whether the antenna is used to transmit or receive signals. For a given antenna design, with a defined radiation pattern, the antenna will transmit and receive signals strongly in the beamwidth, but will also transmit and receive signals from the side and from the back. 2.3 ANTENNA FRONT-TO-BACK RATIO Antennas are imperfect devices and for a given antenna design and beamwidth, the front-toback ratio is a measure of the isolation performance when compared to an ideal antenna. It refers to the radiated signal level behind the antenna compared to the signal level directly in the centre of the main beamwidth 1. As the antenna exhibits reciprocity, it will both transmit and receive signals in all directions as defined by the radiation pattern. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 5 OF 16
2.4 VECTASTAR ANTENNA FRONT-TO-BACK RATIOS VectaStar dual linear polarised antennas have a front-to-back ratio of 30dB. VectaStar circular polarised antennas have a front-to-back ratio of 35dB. 2.5 THE AFFECTS OF THE FRONT-TO-BACK RATIO The characteristic of an antenna affects the installation and deployment of a fixed wireless access network such as VectaStar. In the radio system illustrated above, the Subscriber Unit SU1 is transmitting on frequency f2. The wanted signals from SU1 are strongly received by AP1. The signals from SU1 are also received as interference by AP2 at a reduced level owing to the front-to-back ratio of AP2. In order for AP2 to reject interference from SU1, the signals from SU1 must be below the noise floor of AP2. Increasing the distance (d) between the two Access Points will only slightly improve the isolation owing to the slight increase in free space loss for the extra distance from SU1 to AP2. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 6 OF 16
2.6 FREQUENCY RE-USE AND THE FRONT-TO-BACK RATIO In a radio system such as VectaStar, that employs frequency re-use for efficient use of the available frequency spectrum, signals from one sector must not interfere with signals of the same frequency in the diametrically opposed sector. It is vital that the deployment of the antennas is performed to maximise the isolation characteristics of the antenna, pushing their performance towards that of the ideal antenna. In the radio system illustrated above the Access Points AP1 and AP2 are diametrically opposed and are transmitting on the same frequency f1. The Subscriber Units SU1 and SU2 are transmitting on the same frequency f2. The wanted signals from SU1 (a) are strongly received by AP1 and also received as interference (b) by AP2 at a reduced level owing to the front-to-back ratio of AP2. The wanted signals from SU2 are strongly received by AP2 and also received as interference by AP1 at a reduced level owing to the front-to-back ratio of AP1. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 7 OF 16
3 IMPROVING ANTENNA ISOLATION The isolation of an antenna can usually be improved by obstructing unwanted signals from the Access Point. This can be achieved by physical obstructions additional shielding 3.1 WORKED EXAMPLE For Subscriber Unit SU2 to have no noticeable affect on the noise floor of Access Point AP2, the unwanted signal must be received at least 10dB below the noise floor ( 109dBm or less). The required isolation is 65dBm ( 109dBm) = 44dB The dual-linear polarised antenna has a front-to-back ratio of 30dB and the circular polarised antenna has a front-to-back ratio of 35dB. Additional isolation is required to achieve a 44dB front-to-back ratio. 3.2 IMPROVING ANTENNA ISOLATION BY PHYSICAL OBSTRUCTION Where possible, improved antenna isolation can be achieved by careful selection of the installation site. A large brick building such as a chimney can provide increased isolation when the Access Point antennas are installed orthogonally around the structure. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 8 OF 16
3.3 IMPROVING ANTENNA ISOLATION BY ADDITIONAL SHIELDING Cambridge Broadband can supply additional shielding for Access Points to increase the isolation characteristics of the antenna. The shields are lightweight and manufactured in two parts to allow installation around the antenna without the need to remove the antenna from the installation site. The specially designed antenna shields improve the front-to-back ratio of the Access Point. The shields have no adverse affects on the beamwidth. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 9 OF 16
4 REFLECTIONS Interference often occurs in dense urban environments where there are many RF reflective surfaces. Reflected signals are generally not affected by the isolation characteristics as reflected unwanted signals can arrive within the beamwidth of the antenna. Unwanted reflected signals can be avoided by preventing reflections through careful alignment and positioning of antennas reducing the effects of reflected signals through polarisation techniques 4.1 EXAMPLE OF A REFLECTION CONDITION In the radio system illustrated above, Access Points AP1 and AP2 are transmitting on frequency f1. The wanted signals (a) from AP1 are strongly received by Subscriber Unit SU1. The signals from AP2 are reflected from an RF-reflective surface (metalised-glass) and are received as interference (b) by Subscriber Unit SU1. Reflections can be difficult to avoid particularly in very dense urban deployments. VectaStar employs circular polarised antennas to help reject unwanted reflected signals. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 10 OF 16
4.2 REJECTION OF REFLECTED SIGNALS USING POLARISATION A circular polarised antenna uses a combination of vertical and horizontal polarisation to create a resultant circular polarised signal. 4.2.1 BENEFITS OF CIRCULAR POLARISATION In dense urban deployments where there are a lot of hard glass-sided and steel-clad buildings, powerful reflections can swamp the system. Using circular polarisation, strong primary reflections have completely opposite polarisation (right-hand becomes left-hand) to the incident signal enabling the system to reject the reflected signal. In the radio system illustrated above Access Points AP1 and AP2 are transmitting on frequency f1 using right-hand circular polarisation The wanted signals (a) from AP1 are strongly received by Subscriber Unit SU1.The signals from AP2 are reflected from an RF-reflective surface and exhibit a polarisation change to left-hand circular polarisation. The unwanted signals (b) rejected by the Subscriber Unit SU1. 4.2.2 CONSIDERATIONS FOR CIRCULAR POLARISED ANTENNAS A complete polarisation change will only occur when the primary incident wave strikes a flat highly RF-reflective surface. The reflected wave can strike a second RF-reflective surface that will change its polarisation to the original polarisation direction although its power will be significantly reduced. Imperfect reflective surfaces will result in scatter and incomplete polarisation change. This may mean that there are some resultant reflected waves that have the same polarisation as the primary wave, but their power will be significantly reduced. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 11 OF 16
5 RADIATION PATTERNS AND RADIO COVERAGE The purpose of an antenna in a communication system is to transmit and receive signals in a geographical area. The type of antenna used to provide radio coverage depends upon the geographical area of the network deployment. An understanding of the radiation pattern of an antenna can help in the selection of a suitable location, and ensure the reliable operation of the communication network. 5.1 RADIATION PATTERNS The complete radiation pattern of an antenna defines its performance in every possible direction. Directional antennas have specific sections of their radiation pattern called beamwidth. An antenna will transmit and receive signals strongly within the beamwidth. Radiation patterns extend in all directions. The beamwidth of a directional antenna also extends in three-dimensions outwards from the antenna. It is useful to examine both the vertical and horizontal radiation patterns of an antenna to give an indication of the radio coverage offered. Directional antennas are useful in communication systems to focus signals in a particular direction to cover geographical areas and enhance the performance of the system. Typically, an antenna is designed with a beamwidth that enables it to function at its best within the communication system. Base station antennas are used to define radio coverage for a geographical area called a cell. Typically base station antenna will have a radiation pattern that provides a wide horizontal beamwidth, and a narrow vertical beamwidth. An arrangement of base station antennas will provide radio coverage for subscribers in the geographical area. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 12 OF 16
5.2 VECTASTAR BEAMWIDTHS 5.2.1 BEAMWIDTH OF VECTASTAR ACCESS POINT ANTENNAS The VectaStar Access Point antenna has a horizontal beamwidth of 90 degrees measured at the 3dB points. The vertical beamwidth is 13 degrees measured at the 3dB points. 5.2.2 BEAMWIDTH OF VECTASTAR OF SUBSCRIBER UNIT ANTENNAS The VectaStar Subscriber Unit antenna has a horizontal and vertical beamwidth of 23 degrees measured at the 3dB points. 5.3 VECTASTAR BEAM-TILT It is possible to tilt the beam of an antenna by either electrical or mechanical methods. VectaStar provides no electrical beam-tilt methods. The beam may be tilted by physically tilting the VectaStar unit. The installation bracket on each unit provides ±12 degrees of mechanical tilt. Additional tilt can be achieved in the installation. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 13 OF 16
6 IMPROVING RADIO COVERAGE An understanding of the beamwidth of an antenna can identify the need to mechanically tilt an antenna to increase radio coverage and improve system performance in a radio deployment. 6.1 REDUCING FADING THROUGH BEAM-TILT In the idealised 2 radio deployment above, Subscriber Unit SU2 strongly receives signals from Access Point AP2 as the beamwidths coincide. Subscriber Unit SU1 may experience fade as the signals from Access Point AP2 miss the beamwidth of SU1. Ten degrees of downward tilt at Access Point AP2 makes the beamwidth of Subscriber Unit SU1 coincide and reduces the chances of fade. To further improve performance Subscriber Unit SU1 could be tilted upward. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 14 OF 16
6.2 IMPROVING ISOLATION THROUGH BEAM-TILT In the idealised radio deployment illustrated, Access Point AP2 is transmitting on frequency f1. Access Point AP4 is re-using the frequency and also transmitting on frequency f1. Subscriber Unit SU2 receives strongly wanted signals from AP4 and interference from Access Point AP2. Twelve degrees of downward tilt of Access Point AP2 and 10 degrees of upward tilt of Subscriber Unit SU2 reduce the interference from Access Point AP2. Increased isolation could be achieved by further downward tilting of Access Point AP2. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 15 OF 16
7 DOCUMENT PROPERTIES Revision Action Reference A00 Initial release A01 Corrections following release DOCUMENT REVISION HISTORY The information contained within this document is subject to change without notice. Cambridge Broadband Limited makes no warranty of any kind with regard to this material, including, but not limited to, the implied warranties of merchantability and fitness for purpose. Cambridge Broadband Limited shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance or use of this material. Cambridge Broadband Limited assumes no responsibility for the use or reliability of interconnected equipment that is not supplied by Cambridge Broadband Limited. This document contains proprietary information, which is protected by copyright. All rights reserved. No part of this document may be photocopied, reproduced, or translated to another program or language without prior written consent of Cambridge Broadband Limited. Information contained herein has been prepared by Cambridge Broadband solely for the use of Cambridge Broadband employees, agents and customers. Any dissemination of the information and concepts contained herein to other parties is prohibited without prior written consent of Cambridge Broadband Limited. 2003 Cambridge Broadband Limited. Cambridge Broadband reserves the right to make changes to the specifications of the products detailed in this document at any time without notice and obligation to notify any person of such changes. VectaStar, Cambridge Broadband, and the Cambridge Broadband logo are trademarks of Cambridge Broadband limited. All other trademarks are acknowledged and observed. Mention of third-party products does not constitute an endorsement or recommendation. This document is based on template DOC-TMPL-0005-B01 1 Most antenna radiation patterns show the horizontal (azimuth) and vertical (elevation) and use the signal level at the point exactly behind the antenna to calculate the front-to-back ratio. An antenna will radiate energy at all angles in a three dimensional sphere surrounding the antenna. Adjacent antennas in a radio deployment are rarely exactly behind the antenna in question, and may well be at different heights above the ground. Radiation from the antenna to an adjacent antenna will almost certainly be at some angle relative to the point directly behind the antenna, and will frequently be at some elevation angle out of the plane in which the horizontal (azimuth) pattern is measured. The back lobe radiation characteristics of an antenna will change slightly from one frequency to another throughout the design bandwidth of the antenna. 2 The illustration shows an idealised beamwidth. In reality, the beam pattern will contain side lobes and radiation at all angles in a three dimensional sphere surrounding the antenna. D000114 A01 VECTASTAR 3500 METHODS FOR SUCCESSFUL ANTENNA DEPLOYMENT PAGE 16 OF 16