19-07-2012, 01:07 PM
Development of Smart Antenna Technology
[attachment=28061]
Disclaimer
This report was commissioned by Ofcom to provide an independent view on issues relevant to its duties as regulator for the UK communication industry, for example issues of future technology or efficient use of the radio spectrum in the United Kingdom. The assumptions, conclusions and recommendations expressed in this report are entirely those of the contractors and should not be attributed to Ofcom.
Acknowledgements
The authors would like to thank those people and companies in the wireless industry who have contributed information and advice during the course of the work presented in this report.
The authors would also like to thank Ofcom for sponsoring the work and for their helpful discussions and suggestions.
4
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
EXECUTIVE SUMMARY
Introduction
Wireless systems are undoubtedly an essential part of modern society and are becoming more so as we move towards the “information society” and demand access to more information, more immediately and in more places. Concurrently, technological developments are making new applications possible, opening up new markets, and promising significant economic benefits. In all cases, spectrum is an essential basic resource which, although reusable, cannot be created to meet demand. It is, therefore, increasingly important to improve the efficiency with which use is made of the spectrum. As an example, the next generation mobile telephone system may require an order of magnitude increase in capacity. Since present day systems are close to the Shannon limit, there is relatively little to be gained by improving the modulation and coding schemes, and smart antennas have been identified as one technique that may close the predicted performance gap.
A smart antenna is one that responds in some way to its electromagnetic environment in order to improve a specified performance metric; in so-doing, they can provide, for example, increased immunity to interference, or reduced signal level towards a vulnerable receiver. In general, provision of these capabilities requires the use of multi-element array antennas, with control over the excitation of each element. Smart antennas are not new in concept, yet, with a number of notable exceptions, they have not seen widespread adoption. This project, which was awarded as part of the Ofcom Spectrum Efficiency Scheme, set out to investigate smart antennas and their potential application areas, in an attempt to understand the reasons why they have not been implemented widely. Possible reasons for this may be that their forecast benefits are unobtainable, that the technology for their implementation is not mature, or that they cannot currently be implemented economically. These lead to the overall objectives of the project, namely, to assess and demonstrate the potential of smart antennas for enhancing spectrum efficiency in wireless systems.
To meet these aims a work programme was developed, which drew on the skills o f a number of organisations, namely, Vector Fields Limited, Quotient Associates, Plextek Limited, Loughborough University, and European Antennas Limited.
5
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
Work programme
The activities undertaken during this project divide into four main areas:
Application review
The first element of the work, which is described in Section 2 of the main report, considers the types of smart antenna available and the applications that are most suitable for their adoption. In addition, it must be remembered that the use of the spectrum sits within a regulatory framework. How this may affect the use of smart antennas, and whether their amendment may assist more widespread adoption of smart antennas is preliminarily considered in advance of the other activities.
Technology review
Section 3 details a review of the technology available for the implementation of smart antennas, the main sub-systems of which are the radiating structure and the beam forming network. Both areas are considered to determine the options presently available and possible future developments.
Technology development
The technology review was expected to highlight areas where more novel, less mature techniques could benefit the performance of a smart antenna system. Provision was made within the programme to select and develop such a technique; the results of this are discussed in Section 4.
Performance benefits analysis
The assessment of the benefits brought by smart antennas is an essential part of the programme. From the outset, it was intended to perform a hardware demonstration of a smart antenna operating in a wireless system.
An analysis of a wider area wireless system has also been performed, in this case by simulation rather than by hardware demonstration. The selected system, namely broadband wireless access (BWA), was chosen during the programme following the consideration of the most likely smart antenna applications.
The details of the simulation and hardware demonstration are given in Section 5 of the main report.
The results of the above activities allow us to form a view on why smart antennas have not achieved widespread adoption. This is addressed in Section 6 of the main report.
The remainder of this Executive Summary describes the work performed and the findings from the above activities, and concludes with a discussion of adoption issues.
6
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
Summary of activities
The work performed under each of the activities is summarized briefly below, and the principal findings highlighted.
Applications
As a precursor to considering potential applications, the various types of smart antenna were reviewed. It was found to be useful to classify them broadly as operating in “beam space” or “signal space”. The former uses knowledge of the geometry of the antenna, combined with an ability to vary the excitation of each antenna element, to steer and shape the radiation pattern (ie the beam). The latter uses knowledge of the structure of parts of the received signals, and adjusts the antenna element weights to minimize the difference between the combined signal at the output and the known (or training) sequence. Basic implementations of these require relatively little integration with the receive chain or the link protocol, and are often suitable for overlaying on the system as an appliqué layer. More sophisticated implementations combine one or other of these with processing in the time domain (ie space-time processing and detection) in order to incorporate signal components that have been temporally dispersed. While these offer improved performance, they are correspondingly more demanding on the signal processing hardware. Figure E1 shows how these techniques may be viewed in relation to their complexity, level of integration and to the characteristics of the propagation environment.
“Beamspace”
Fixed beam switching
Narrowband adaptive
spatial beam-forming
Uplink only (HSR)
SFIR
SDMA
“Signalspace”
Diversity combining
Single channel
equalisation
“Space-time
Detection”
Space-time
equalisation
e.g. Advanced MIMO
Channel dispersion
Predictable antenna response
Integration with protocol / receive chain
“Space-time
Processing”
Wideband adaptive
beam-forming
Temporally spread
signal beam-forming
Simple MIMO
Complexity & performance
Figure E1: Main smart antenna methods and primary constraints for their application
A distinction is made between those “smart” systems that achieve improved performance using diversity methods, for example diversity combining and MIMO, and those that adapt weighted excitations to the elements of the antenna to optimise its pattern performance. Simple diversity systems are used widely, and the incorporation of MIMO techniques is rising rapidly, albeit in advance of a formal specification. This project hence concentrated on the weight adaptation techniques, which are further from widespread adoption, but which may offer additional or complementary benefits to the diversity methods.
7
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
A wide range of possible wireless applications that have high economic value to the UK have been considered as candidates for the early adoption of smart antennas. These include mobile and broadcast communications, wireless local area networks (WLAN) and broadband wireless access (BWA). In some cases, smart antennas based on various techniques are already available commercially, or have undergone trials. The application requirements, possible benefits and current implementations have been reviewed, and conclusions have been drawn on which systems show the most promise for the near and long term. These are summarized below:
Mobile communications:
Given the significant capital investment required to grow these mobile networks, any benefit that can result from smart antennas is likely to be applied in the future. However it should also be noted that the roll-out of 3G services in the UK has been relatively slow, and it is unlikely that the significant capacity increases that can be delivered using smart antennas will be needed until demand increases substantially. Development of beamformer-type smart antennas has been halted within 3GPP, and MIMO is seen as the primary enabler to increase the capability of 3G mobile systems (in high multipath environments) once demand has increased sufficiently. By this time the MIMO specification should have been finalised, with fully compliant hardware available for integration.
Retrospective application of smart antennas to existing 2G infrastructure is unlikely, given that the networks are at a mature level of deployment, and any expensive hardware upgrade would have a short period to recoup the costs before the expected re-farming to other technology (such as 3G).
Broadcast communications
Mobile receivers for DAB, DVB-T and DVB-H or other mobile broadcast variants could benefit from multiple antennas if they could be accommodated in mobile receiver systems. However the relatively low frequency at which these systems operate, results in a wavelength of the order of 1.5m for T-DAB or DVB-H at VHF, ~70cm for DVB-H at UHF band or a more manageable ~20cm for T_DAB at L-Band. With the possible exception of L-Band operation, none of these options are suited for the deployment of an array of sensors for use in a small portable device, and will not be considered further here.
WLAN:
WLAN offers higher speed access to nomadic users than 2 or 3G mobile systems can deliver. In addition WLAN offers benefits to enterprise networks in terms of reduced infrastructure (wiring) costs, and improved working flexibility. However the high penetration loss at 5GHz and the congestion experienced at 2.4GHz, particularly in busy public spaces (such as Heathrow) threaten to reduce the utility of WLAN. Significant effort is being invested in the development of MIMO capability for WLAN systems (in high multipath environments). The unit cost of including MIMO functionality with integrated single chip solutions is small (i.e. several dollars). However, simpler beam and signal space approaches are expected to have application as a complement to MIMO in the low and moderate multipath environments, provided that a similarly low cost impact can be achieved.
BWA:
The lack of enthusiasm demonstrated in the UK 28GHz broadband (fixed) wireless access spectrum auctions highlighted the potential difficulties in using conventional high frequency line-of-sight connections using proprietary systems. However equipment standardisation, such as ETSI’s HiperMAN II and the IEEE 802.16x and 802.20 standards, promise lower cost equipment and facilitate non-line of sight operation at lower frequencies, allowing additional savings by removing the requirement to 8
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
accurately align a high gain antenna. Together with spectrum liberalisation measures, BWA can offer the possibility of new, nomadic or even mobile, applications in the future. While the benefits of using smart antennas may not have been sufficient to kick-start the market in the past, there are signs that the market is now becoming more buoyant, and such antennas are expected to be helpful in lowering costs and improving link performance in these applications.
From the above, broadband wireless access and wireless LAN were selected as the most likely applications to benefit from the use of smart antennas in the near term. These applications were adopted as the basis for the benefits analysis activity of this project.
Regulatory issues
In advance of the work performed on the other areas of this project, the possible impact of the regulatory environment on the adoption of smart antennas was considered.
Since adaptive beam antennas modify the shape, and hence gain of the radiation pattern, the EIRP will vary on transmit unless the power is adjusted. However, to gain full advantage of the adaptive antenna, the maximum permitted power would normally be used. This may violate existing licence conditions that may specify maximum EIRP or maximum signal levels outside an agreed region. It is therefore considered that flexibility or redefinition of the licence conditions would be required as an enabler to smart antenna adoption.
[attachment=28061]
Disclaimer
This report was commissioned by Ofcom to provide an independent view on issues relevant to its duties as regulator for the UK communication industry, for example issues of future technology or efficient use of the radio spectrum in the United Kingdom. The assumptions, conclusions and recommendations expressed in this report are entirely those of the contractors and should not be attributed to Ofcom.
Acknowledgements
The authors would like to thank those people and companies in the wireless industry who have contributed information and advice during the course of the work presented in this report.
The authors would also like to thank Ofcom for sponsoring the work and for their helpful discussions and suggestions.
4
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
EXECUTIVE SUMMARY
Introduction
Wireless systems are undoubtedly an essential part of modern society and are becoming more so as we move towards the “information society” and demand access to more information, more immediately and in more places. Concurrently, technological developments are making new applications possible, opening up new markets, and promising significant economic benefits. In all cases, spectrum is an essential basic resource which, although reusable, cannot be created to meet demand. It is, therefore, increasingly important to improve the efficiency with which use is made of the spectrum. As an example, the next generation mobile telephone system may require an order of magnitude increase in capacity. Since present day systems are close to the Shannon limit, there is relatively little to be gained by improving the modulation and coding schemes, and smart antennas have been identified as one technique that may close the predicted performance gap.
A smart antenna is one that responds in some way to its electromagnetic environment in order to improve a specified performance metric; in so-doing, they can provide, for example, increased immunity to interference, or reduced signal level towards a vulnerable receiver. In general, provision of these capabilities requires the use of multi-element array antennas, with control over the excitation of each element. Smart antennas are not new in concept, yet, with a number of notable exceptions, they have not seen widespread adoption. This project, which was awarded as part of the Ofcom Spectrum Efficiency Scheme, set out to investigate smart antennas and their potential application areas, in an attempt to understand the reasons why they have not been implemented widely. Possible reasons for this may be that their forecast benefits are unobtainable, that the technology for their implementation is not mature, or that they cannot currently be implemented economically. These lead to the overall objectives of the project, namely, to assess and demonstrate the potential of smart antennas for enhancing spectrum efficiency in wireless systems.
To meet these aims a work programme was developed, which drew on the skills o f a number of organisations, namely, Vector Fields Limited, Quotient Associates, Plextek Limited, Loughborough University, and European Antennas Limited.
5
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
Work programme
The activities undertaken during this project divide into four main areas:
Application review
The first element of the work, which is described in Section 2 of the main report, considers the types of smart antenna available and the applications that are most suitable for their adoption. In addition, it must be remembered that the use of the spectrum sits within a regulatory framework. How this may affect the use of smart antennas, and whether their amendment may assist more widespread adoption of smart antennas is preliminarily considered in advance of the other activities.
Technology review
Section 3 details a review of the technology available for the implementation of smart antennas, the main sub-systems of which are the radiating structure and the beam forming network. Both areas are considered to determine the options presently available and possible future developments.
Technology development
The technology review was expected to highlight areas where more novel, less mature techniques could benefit the performance of a smart antenna system. Provision was made within the programme to select and develop such a technique; the results of this are discussed in Section 4.
Performance benefits analysis
The assessment of the benefits brought by smart antennas is an essential part of the programme. From the outset, it was intended to perform a hardware demonstration of a smart antenna operating in a wireless system.
An analysis of a wider area wireless system has also been performed, in this case by simulation rather than by hardware demonstration. The selected system, namely broadband wireless access (BWA), was chosen during the programme following the consideration of the most likely smart antenna applications.
The details of the simulation and hardware demonstration are given in Section 5 of the main report.
The results of the above activities allow us to form a view on why smart antennas have not achieved widespread adoption. This is addressed in Section 6 of the main report.
The remainder of this Executive Summary describes the work performed and the findings from the above activities, and concludes with a discussion of adoption issues.
6
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
Summary of activities
The work performed under each of the activities is summarized briefly below, and the principal findings highlighted.
Applications
As a precursor to considering potential applications, the various types of smart antenna were reviewed. It was found to be useful to classify them broadly as operating in “beam space” or “signal space”. The former uses knowledge of the geometry of the antenna, combined with an ability to vary the excitation of each antenna element, to steer and shape the radiation pattern (ie the beam). The latter uses knowledge of the structure of parts of the received signals, and adjusts the antenna element weights to minimize the difference between the combined signal at the output and the known (or training) sequence. Basic implementations of these require relatively little integration with the receive chain or the link protocol, and are often suitable for overlaying on the system as an appliqué layer. More sophisticated implementations combine one or other of these with processing in the time domain (ie space-time processing and detection) in order to incorporate signal components that have been temporally dispersed. While these offer improved performance, they are correspondingly more demanding on the signal processing hardware. Figure E1 shows how these techniques may be viewed in relation to their complexity, level of integration and to the characteristics of the propagation environment.
“Beamspace”
Fixed beam switching
Narrowband adaptive
spatial beam-forming
Uplink only (HSR)
SFIR
SDMA
“Signalspace”
Diversity combining
Single channel
equalisation
“Space-time
Detection”
Space-time
equalisation
e.g. Advanced MIMO
Channel dispersion
Predictable antenna response
Integration with protocol / receive chain
“Space-time
Processing”
Wideband adaptive
beam-forming
Temporally spread
signal beam-forming
Simple MIMO
Complexity & performance
Figure E1: Main smart antenna methods and primary constraints for their application
A distinction is made between those “smart” systems that achieve improved performance using diversity methods, for example diversity combining and MIMO, and those that adapt weighted excitations to the elements of the antenna to optimise its pattern performance. Simple diversity systems are used widely, and the incorporation of MIMO techniques is rising rapidly, albeit in advance of a formal specification. This project hence concentrated on the weight adaptation techniques, which are further from widespread adoption, but which may offer additional or complementary benefits to the diversity methods.
7
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
A wide range of possible wireless applications that have high economic value to the UK have been considered as candidates for the early adoption of smart antennas. These include mobile and broadcast communications, wireless local area networks (WLAN) and broadband wireless access (BWA). In some cases, smart antennas based on various techniques are already available commercially, or have undergone trials. The application requirements, possible benefits and current implementations have been reviewed, and conclusions have been drawn on which systems show the most promise for the near and long term. These are summarized below:
Mobile communications:
Given the significant capital investment required to grow these mobile networks, any benefit that can result from smart antennas is likely to be applied in the future. However it should also be noted that the roll-out of 3G services in the UK has been relatively slow, and it is unlikely that the significant capacity increases that can be delivered using smart antennas will be needed until demand increases substantially. Development of beamformer-type smart antennas has been halted within 3GPP, and MIMO is seen as the primary enabler to increase the capability of 3G mobile systems (in high multipath environments) once demand has increased sufficiently. By this time the MIMO specification should have been finalised, with fully compliant hardware available for integration.
Retrospective application of smart antennas to existing 2G infrastructure is unlikely, given that the networks are at a mature level of deployment, and any expensive hardware upgrade would have a short period to recoup the costs before the expected re-farming to other technology (such as 3G).
Broadcast communications
Mobile receivers for DAB, DVB-T and DVB-H or other mobile broadcast variants could benefit from multiple antennas if they could be accommodated in mobile receiver systems. However the relatively low frequency at which these systems operate, results in a wavelength of the order of 1.5m for T-DAB or DVB-H at VHF, ~70cm for DVB-H at UHF band or a more manageable ~20cm for T_DAB at L-Band. With the possible exception of L-Band operation, none of these options are suited for the deployment of an array of sensors for use in a small portable device, and will not be considered further here.
WLAN:
WLAN offers higher speed access to nomadic users than 2 or 3G mobile systems can deliver. In addition WLAN offers benefits to enterprise networks in terms of reduced infrastructure (wiring) costs, and improved working flexibility. However the high penetration loss at 5GHz and the congestion experienced at 2.4GHz, particularly in busy public spaces (such as Heathrow) threaten to reduce the utility of WLAN. Significant effort is being invested in the development of MIMO capability for WLAN systems (in high multipath environments). The unit cost of including MIMO functionality with integrated single chip solutions is small (i.e. several dollars). However, simpler beam and signal space approaches are expected to have application as a complement to MIMO in the low and moderate multipath environments, provided that a similarly low cost impact can be achieved.
BWA:
The lack of enthusiasm demonstrated in the UK 28GHz broadband (fixed) wireless access spectrum auctions highlighted the potential difficulties in using conventional high frequency line-of-sight connections using proprietary systems. However equipment standardisation, such as ETSI’s HiperMAN II and the IEEE 802.16x and 802.20 standards, promise lower cost equipment and facilitate non-line of sight operation at lower frequencies, allowing additional savings by removing the requirement to 8
Smart Antenna Technology CUL/EM/030854/RP/06
Final Report (Issue 2)
accurately align a high gain antenna. Together with spectrum liberalisation measures, BWA can offer the possibility of new, nomadic or even mobile, applications in the future. While the benefits of using smart antennas may not have been sufficient to kick-start the market in the past, there are signs that the market is now becoming more buoyant, and such antennas are expected to be helpful in lowering costs and improving link performance in these applications.
From the above, broadband wireless access and wireless LAN were selected as the most likely applications to benefit from the use of smart antennas in the near term. These applications were adopted as the basis for the benefits analysis activity of this project.
Regulatory issues
In advance of the work performed on the other areas of this project, the possible impact of the regulatory environment on the adoption of smart antennas was considered.
Since adaptive beam antennas modify the shape, and hence gain of the radiation pattern, the EIRP will vary on transmit unless the power is adjusted. However, to gain full advantage of the adaptive antenna, the maximum permitted power would normally be used. This may violate existing licence conditions that may specify maximum EIRP or maximum signal levels outside an agreed region. It is therefore considered that flexibility or redefinition of the licence conditions would be required as an enabler to smart antenna adoption.