Simulating Multipath
Contents
1. Figure 5 Manual Multipath Settings Window OK Help Cancel SPIRENT Application Note Page 9 User Actions Set the State parameter to Multipath and click the Settings button to open the settings window Select the multipath type and add the required number of echoes setting the parameters for each as required Figure 6 illustrates the method To apply a multipath signal using the User Action File first edit the file from the scenario tree gt options branch then select the Switch Simulated Satellite from the Command Type drop down list Specify the time into the scenario when you want the multipath to begin and set the vehicle antenna and which SV PRN en you want the multipath to apply to eee B Data display and logging fie L E Data streaming defintion file _ amp NMEA input file NMEA output file nmea_loop_1 nmea __ E RTCM definition file Ge User Actions File eke OoOo E none Forced Figure 6 Setting Up Multipath Using the User Actions File Remote Commands Remote commands can be sent in real time SimGENT s SimREMOTE feature allows you over a Selection of interfaces or from a file to set up multipath using the SWITCH_SAT n a either wee a i up multipath via SimREMOTE SPIRENT Application Note Page 10 Fixed Offset Multipath The most basic multipath model used in SimGEN is the Fixed Offset For this type of multipath the si2. Page 4 RF Simulation An RF Constellation Simulator reproduces the environment of a GNSS receiver on a dynamic platform by modelling the vehicle and satellite motion signal characteristics atmospheric and other effects such that the receiver will actually navigate according to the parameters of the test scenario By its very nature simulation is a representation of the real world Simulation cannot reproduce the full richness of real world conditions A common misconception is the need to exactly replicate real world conditions for a GNSS test to be valid However application of representative effects via simulation is proven over some 25 years of testing to exercise receivers and adequately identify their limitations allowing for design centring and optimisation More importantly it gives complete repeatability control and exact knowledge down to bit level of the signal stimulating the receiver Typical GPS Simulators All the tests discussed in this Application Note can be performed using any of Spirent s multi channel simulators The GNSS Environment A GNSS receiver works well when it has a clear un interrupted view of the orbiting satellites transmitting the ranging and navigation signals In many situations this is not the case and ranging measurements to the satellites are affected SPIRENT Application Note This is not possible in the real world We should look upon simulator t
3. Successive periods are mirror images of the first period for example You define the first period to be To gt T1 Ta gt T2 is identical to T1 gt To T2 gt T3 is identical to To gt T1 T3 gt T4 is identical to T1 gt To e Los none Forcea 0 Se SS Se eee ea ee aa Sree as ae es es Figure 12 Polynomial Multipath Settings Window SPIRENT Application Note Sinusoidal Multipath This model allows you to apply a sinusoidal variation to the delay and amplitude of a multipath signal As with the Legendre and Polynomial models this method simulates a representative effect for the multipath signals rather than being based on the vehicle and satellite geometry This method has the advantage that it gives a time varying multipath signal which is well bounded and easily defined For each Sinusoidal multipath you must enter values for the following sinusoidal coefficients in the Manual multipath settings dialog Attenuation Peak dB The min and max peak levels of the attenuation of the sinusoid ignal level dBm Normal signal level Attenuation Attenuation peak dB bias offset Attenuation Attenuation peak dB start phase Time Manual multipath settings Attenuation Freq Hz The frequency of the sinusoidal variation of the multipath Attenuation Phase deg The start phase of the attenuation of the sinusoid Attenuation Bias offset The offset between the sinusoid and the LOS s
4. Sarr t T a mu ti p ath Visible satellites Simulated satellit SVID Forcedon State Settings Channel SVID 1 none Normal Bettinas 1 EN il none Normal E 2 Ea EB tore Nomar AE e fia none Normal aez J a far none Normal Beta 5 i fa none Normal eA 6 l 15 none Normal JEE 7 Ar none Normal aea 8 E fro none Normal J 9 ES e E foe Fi Setings 11 rates using specific commands m i EENE Figure 4 Channel Assignment Window 14 15 e Channel Assignment allows manual application during scenario run time e User Actions allows you to set up multipath via pre scripted commands which are executed in a time ordered manner e Remote Control Commands Vehicle 1 Antenna 1 rs ula AID w Channel Assignment To access the Channel Assignment window click on the Channel Assignment button on the toolbar This brings up the Channel Assignment window as shown in Figure 4 Manual multipath settings xi Click on the state drop down arrow to reveal the various state options Select the Multipath state to bring up the Manual Multipath Settings window as shown in Figure 5 GPS SVID 4 Number of echos 1 Add Echo Forced channel Select the multipath type and add the required number of echoes setting the parameters for each as required
5. angles at particular azimuth angles such as adjacent buildings or at high elevations to simulate the case for a user positioned within a tall building Use of these segments maximises use of the available channels for meaningful signal simulation Page 23 e Category B LOS only e Category D Echoes only Satellites arriving at these segments are Satellites arriving at these segments simulated with a line of sight LOS signal are simulated as echoes only the LOS only These signals represent signals signals are obstructed depending that are generally unobstructed and not upon the number of channels available subject to reflections Satellites within Satellites within category D have category B suffer Rician fading modified Rayleigh fading applied for the echoes Category C LOS Echoes Satellites arriving at these segments You can create representative environments are simulated as a LOS plus echoes Surrounding the receiver antenna Figure depending upon the number of 16 shows a simple urban environment channels available These signals where three buildings of different height represent unobstructed signals that are are Surrounding the receiver s antenna subject to reflections Satellites within Four masks are present by default Default category C suffer Rician fading on the Urban Canyon Trees and Highway Flyover LOS channel and modified Rayleigh You can make copies of these and edit and fading on the echoes sav
6. operation of the receivers Spread spectrum correlation process means that reflections with long delays are attenuated and often completely eliminated If the delay of the multipath signal is long compared to a chip width approx 300m or 1microsecond for C A code the auto correlation properties of the code suppress the effect Short delay reflections however are much more of a problem which is unfortunate as most real life multipath tends to be the close in short delay type There are several multipath mitigation techniques employed by receivers Narrow correlators first introduced in the 1990 s are probably one of the best known techniques Other more up to date techniques include Strobe amp Edge Correlators High Resolution Correlators HRC and Gated Correlators The latest developments include A Posteriori Multipath Estimation APME which relies on an a posteriori estimate of the multipath error through use of a fourth replica of the PRN code see Reference 1 Other mitigation techniques include Carrier Smoothing and Multipath Limiting Antennas such as choke rings If the relative phase between the LOS and reflected signals changes rapidly the receiver can average carrier smooth the pseudorange measurements attenuating the erroneous measurements SPIRENT Application Note A Multipath Limiting Antenna can reduce ground reflections from satellites that are very low on the horizon by attenuating or b
7. s y s s a 6 s on esfs at ie LJ m 4 1 pg a Ann E alll i sa 2 M A aS ee a Pagi oe a ee AT F Peat gato A A eg OS ORs Me Pe aper tee A nn a a as e eaa TTUTVUVUTAC ETC CUT UTTER TTT TT eee ea 8 Ba BD Sestas Mesa ee e Pees ee KTT E pa T Be es Thee st SURD EES SRS S T wh ee E Tess PE EES PEE TPRI n tna i nae BUTE E a PE ARAC I E E s fe ee e Pe eT le Figure 9 Settings Window for Doppler Offset Multipath SPIRENT Application Note Page 13 Reflection Pattern Multipath In order to explain the function of Reflection Pattern Multipath we must first look at the Antenna Pattern feature of SimGEN as both use the same editor and principles The Antenna Pattern function allows you to model the electrical properties of a simulated antenna in your test When you are connecting the RF output from the simulator to the input of the receiver with a suitable RF coaxial cable the receiver s antenna is omitted GNSS signals arrive at the receiver s antenna from different directions because the satellites are spatially separated in their constellation a Antenna attenuation pattern editor Untithed Any variation of performance over the antenna s aperture its field of view Should be accounted for as signals arriving at the antenna from some directions may be affected differently to those arriving from other directions The Antenna Pattern Editor in it
8. signals for each single simulator hardware channel SPIRENT Application Note Digital replica sub channels are created in the signal generator FPGAs and the level delay and phase of each can be independently defined The Fader Multipath model is currently only available via a SimREMOTE command and is not available on all systems Reference 4 has more information on this model Page 25 Conclusions This Application Note describes the fundamental performance parameters that apply to all GNSS receivers These parameters must be optimised at an early stage in a receiver design Optimisation requires suitable testing This Application Note shows that a GNSS simulator allows you to develop tests that optimise receiver design SimGEN offers very high resolution control of signals and Referenced Documents 1 Mitigating Short Delay Multipath a Promising New Technique Sleewsegen Boon Septentrio Satellite Navigation 2 Understanding GPS Principles amp Applications E Kaplan C Hegarty 2nd Ed 2006 3 Global Positioning System Signals Measurements amp Performance P Misra P Eng 2004 SPIRENT Application Note bit level manipulation of data reproducing the most complex error effects while its easy to use interface allows straightforward tests to be carried out with the same powerful modelling taking place in the background It shows that there are no practical alternatives to simulato
9. the selected Active multipath environment and Active multipath category mask The current selections define the environment and mask that apply at the start of the scenario Select the Environment editor or Category mask editor to view or edit the settings Environment Editor The Environment editor is used to define the characteristics of the Rician and Rayleigh models as a function of satellite elevation The operation of the multipath model is controlled by the contents of a number of look up tables driven by satellite elevation angle and satellite selection interval Figure 15 shows the Environment editor window Page 20 Land mobile multipath RIES Active configuration Environment editor Category mask editor ol SPEDE Ex om EEE g 1 2 24 8 E 29 97865 5 E 29 E 2 2 Q Oo N TELE N n so in foo fia 100000 Esa TTT Other parameters For reflected signals echoes Maximum near echo delay 400s Power level update interval fico ms Undo ox He cme Figure 15 LMM Environment Editor Window The Rician Rayleigh and Deep Fade model where parameters are manually entered in the is the received signal amplitude relative corresponding boxes These models are to the direct path described as follows K is the ratio of direct to multipath power received and is a constant Rician Fading Model I is the oth order modified Bessel The Rician model is used to
10. to the receiver For GNSS the problem of replica signals is significant as measurement of transit time for the signal in order to determine pseudorange is fundamental for calculating the navigation solution PER Several reflections may take place and their relative phase delays to each other and the LOS combine in either a constructively enhancing the multipath or a destructively cancelling out the multipath The perceived coming and going of the composite multipath is called fading According to Reference 5 a multipath completely in phase with the LOS signal generates the largest error and one which is 180 degrees out of phase generates the smallest error A simple representation of a single ray multipath reflection is shown in Figure 2 In this example a reflection of the wanted signal takes place on the surface of the building The receiver sees this delayed and attenuated signal as well as the direct wanted signal It also shows the two opposing states that create full constructive interference and full destructive interference Reflected ray arrives in phase and t interferes constructivety Direct ray Reflected ray arrives out of phase t And interferes destructively Figure 2 Example of a simple multipath SPIRENT Application Note Page 6 You may also logically assume that the longer the delay regardless of phase addition or subtraction the greater the pseudorange error but the
11. 5 Po t 1 Pi t t P2 t 1 5072 0 5 P3 t 2 513 1 5 Pa t 4 375t 4 3 7512 0 375 Ps t 7 87505 8 75t 3 1 8750 The Ai and Di terms are the multipath coefficients These are in the Pth coefficients of the expansion of the relative amplitude and delay functions respectively in terms of Legendre polynomials determined using the following equations for known R t and T t profiles p ROT de Se de Page 16 For each Legendre multipath you must Successive periods are images of the first enter values for the following polynomial period for example coefficients in the Manual multipath You define the first period to be To T1 Series Cine T1 T2 is identical to To T1 Ao to A5 coefficients Ao to A5 in the T2 13 is identical to To T1 Legendre polynomial for R t _ l T3 T4 is identical to To T1 Do to D5 coefficients Do to D5 in the Legendre polynomial for D t Figure 11 Shows the Legendre Multipath Duration s the period of the Legendre j 2 settings window cycle It is the period between which the echo is modelled Once a period has expired the echo pattern is continually repeated Manual multipath settings E3 Number of echos 1 Add Remove Remove LOS T Sa Geet at Gn Sn ei E re aT i foe loa o o f o f o f o fo f o f o fo f o fo f o f oOo o Hie ii it Figure 11 Legendre Multipath Settings Window SPIRENT Applicat
12. APPLICATION NOTE M Uima gt ae Wr 7 177117 a Simulating Multipath a SPIRENT Spirent Communications PLC Paignton Devon TQ4 7QR England Web http www spirent com positioning Tel 44 1803 546325 Fax 44 1803 546301 Copyright 2011 Spirent All Rights Reserved All of the company names and or brand names and or product names referred to in this document in particular the name Spirent and its logo device are either registered trademarks or trademarks of Spirent plc and its subsidiaries pending registration in accordance with relevant national laws All other registered trademarks or trademarks are the property of their respective owners The information contained in this document is subject to change without notice and does not represent a commitment on the part of Spirent The information in this document is believed to be accurate and reliable however Spirent assumes no responsibility or liability for any errors or inaccuracies that may appear in the document SPIRENT Application Note Page 2 Contents Audience 3 Introduction 3 RF Simulation 3 Typical GPS Simulators 4 The GNSS environment 4 What is Multipath 5 Simulating Multipath 7 Multipath implementation in the simulator 7 Methods of applying a multipath 7 Fixed Offset Multipath 10 Ground Reflection Multipath 10 Doppler Offset Multipath 11 Reflection Pattern Multipath 12 Legendre Multipath 13 Po
13. Multipath type Fixed offset Number of echos 1 Add Remove Remove LOS ros none Forcea P n 3 bo Echo Forced channel Attenuation dB Range offset m ae Sr ay Figure 7 Fixed Offset Multipath Page 11 Ground Reflection Multipath Ground Reflection Multipath simulates the echo signal that may be caused by the LOS signal reflecting from the ground or sea surface in terms of the relative geometry of the transmitting satellite and the receiver The signal generated is based on the arrival angle at the WGS 84 ellipsoid height Ground WGS84 Ellipsoid a A flat plane surface is assumed for the reflection The receiver antenna position must have some height relative to the ellipsoid height associated with for the ground reflected signal to exist The amount of delay is automatically modelled as a function of the receiver antenna height and arrival angle of the satellite signal Figure 8 shows this concept Direct LOS signal Antenna Height Ground reflection signal Figure 8 Concept of a Ground Reflection Multipath As with Fixed Offset Multipath you can adjust the power level attenuation in dB of the Ground Reflection Multipath signal relative to the LOS This allows the reflection loss of the ground sea to be accounted for The relative delay in the reflected signal will vary with satellite position and as a result the interference with the direct signal will
14. also vary SPIRENT Application Note You can remove the LOS signal from the simulation by ticking the Remove LOS box It is not sensible to define more than One multipath signal for a given satellite using this method Page 12 Doppler Offset You can set the difference in initial carrier phase between the LOS and the multipath Multipath echo to a fixed value or randomised This multipath type was originally If the box is ticked the initial carrier phase developed to support a specific test in difference will vary run to run the 3GPP Mobile Phone test standard If un ticked it will remain fixed You can 3GPP 1525 171 However it is also useful set up Doppler Offset Multipath in the same for other testing It is an enhancement way as the previous examples of the Fixed Offset Multipath type Figure 9 shows the settings window Note In addition to the level and initial delay you can set the Initial Delay in either C A you can set a Doppler frequency offset chips code transitions are 1millisecond between the multipath and the LOS or in metres one chip is approx 293m which causes the delay between the If you specify several multipaths for the multipath and LOS to change dependent same satellite with different Doppler on the amount of Doppler applied values a greater disturbance of the LOS signal will result Manual multipath settings teen wee fe i J r a oe ae Ly See ee wee se ee eRe segatu r ie RET e
15. ath This model enables you to specify multipath signals using a fifth order Legendre Polynomial for the relative amplitude and delay of the multipath signal This is typically used to model multipath signals in a fairly static environment with gradually changing multipath characteristics The advantage of this model is that the same polynomial coefficients can be used over any period of time so the characteristics of the multipath signal are always kept within the same bounds In this model the reflected signals are not particularly representative of the relative geometry of the satellites and the receiver but give a typical effect The more multipaths defined the more complex the interference The polynomials used are R t AoPo t A1P1 t A2P2 t A3P3 t A4P4 t A5P5 t DoPo t D1P1 t D2P2 t D3P3 t D4Pa t D5P5 t Where R t Relative amplitude of the reflected signal with respect to the direct signal expressed as a ratio Limited to the range o to 1 4 of FORO de ee hrd SPIRENT Application Note t Delay of the multipath signal relative to the direct signal Seconds t Normalised time 2 t to T 1 t Time into simulation Seconds to Time into scenario at which to start generation of the multipath signal seconds T Duration of the multipath signal seconds Pit Legendre polynomial of ith order i o to
16. ating multipath signals in a variety of different ways but unlike the real world these are fully quantified and controlled by the simulator user Multipath implementation in the simulator With the exception of fader multipath all multipath signals that are simulated in addition to a given LOS are considered as discrete signals This means that the simulator uses a separate hardware channel to generate each signal For example a LOS with three multipath echoes will require four simulator channels It is important to remember this as there is a limit to the number of channels in the simulator 12 for the Spirent GSS6700 for example SPIRENT Application Note This section provides a set of test methodologies giving step by step instructions on how you can create simulator scenarios that include multipath effects Spirent s SimGEN software contains several multipath features and a demonstration of each one is given In general this is not a problem as there are usually fewer visible satellites in environments where there are more multipaths urban canyons for example and less multipaths where there are more visible satellites so the overall number of required hardware channels balances out Fader multipath techniques available on some Spirent systems uses digital replica signals giving up to four multipaths per LOS while using just one simulator hardware channel Page 8 Methods of applying
17. describe the function of the first kind fading on line of sight signals Snin 2Kvexpl K v7 1 2Kv v 0 i lt 0 SPIRENT Application Note Page 21 Rayleigh Fading Model A modified Rayleigh model is used to describe the fading on echo channels There is a deterministic mean power function an amplitude noise function Rayleigh and a delay function The deterministic mean power reduction in addition to Rayleigh noise is given by F t F 0 d t The user can define the iteration period minimum 1oms with a resolution of 10ms The delay on the echo channel is calculated at random with an exponential distribution where b is taken from a look up table An upper limit is imposed as determined by the Maximum Near Echo Delay parameter in the land mobile multipath environment editor window SPIRENT Application Note where Ph o and d are provided by a look up table and the delay of the echo signal The amplitude noise on the echo channel determined on every user defined iteration period is randomly calculated from a Rayleigh distribution given by Jaia V 2KV expl Ky It is listed as Power level update interval in the land mobile multipath environment editor l T foal o When the satellite is unchanged and satellite position modelling not enabled the delay remains fixed Page 22 Deep Fade Model Where On all echoes that are not the primary echo a carrier Doppler offset is appl
18. e them as user defined environments Land mobile multipath default_v1 0 lmm Active configuration Environment editor Category mask Eyer Canyon _ Copy Remove JR Category 4 Obscuration Category mask editor C E Category B LOS only C J Category c LOS Echoes ao Mew f o Curent category Em C __ Category D Echoes only Elevation degrees 50 ofo 20000 gg ofo ofo J a oo OG ofo 399 J g J off Boo Bo Ro Ro Ro Ro g 9g oS ggg em oOo oo 0 fo Mo Mo Mo fo fo fo Bo oo Mo Ro Rego of i iii 3999999 9 o Bo Boffo fo Bo e E fo Boffo fo Boffo o o 3090999 Off fo Boffo fo fo Mo Ro Ho RRR Ro Mofo Rofo GoGo Ro Rofo Rofo fo Rofo ofc Bo Rofo hoe 0 fo fo fo fo fo fo fo fo fo fo ho Bo fo fo Bo Mo fo ho Bo fo fo Bo fo ho ho fo Mo ho ho Mo oo fo Rofo SUDO OTS O COCO OOOO OOOO CSOD OOS SO OOS SEO D0DODODDDDDDDDDDDDODODODDDDD DDD DDD ODD DDDDDDDD DDD ODO DODODDDDDDDDDDDODDD00 60 20 30 60 30 0 30 60 30 T20 150 10 Azimuth deqrees yl ok Help undo Cancel Figure 16 Land Mobile Multipath Category Mask Editor SPIRENT Application Note Page 24 Fader Multipath The implementation of the multipath models so far described is accomplished by assigning an independent simulator hardware channel to each multipath echo signal as highlighted on page 9 The compromises made are also discussed Spirent s Fader Multipath implementations use a different approach where you can simulate up to four multipath
19. ebook Linkedin Twitter Technorati Google Buzz Digg Delicious Reddit Stumbleupon Spirent Communications Spirent Federal Systems 44 1803 546325 1 714 692 6565 globalsales spirent com info spirentfederal com www spirent com positioning www spirentfederal com SPI RE NT Rev 1 0 Jul 2011
20. esting as representing the real world rather than replicating it Continued successful deployment of receiver designs in many applications prove that the simulators being used for their development and verification are accurate in their implementation of the GNSS environment Figure 1 shows the concept of simulation using a GSS6700 simulator Figure 1 RF Simulation Flow For further information on Spirent s range of Simulators please contact your local Spirent representative or visit www spirent com positioning The degree that performance is affected depends on the application and the environment of the receiver A variety of factors can affect a receiver s performance some Specific to or emphasised by certain applications Multipath is a potential problem which is worse in some application environments than others The discussions in this document are with regard to GPS unless otherwise stated Page 5 What is Multipath Multipath is described to some extent by its name A radio signal conveying information or on which radio ranging measurements are performed should travel in a direct single and un altered path from the transmitter to the receiver Often this is not the case Radio signals can be diffracted and reflected by physical Structures in the vicinity of the receiver creating unwanted replicas of the original desired LOS signal The composite signal is said to take multiple paths
21. ied W is the conversion factor from m s to offset rate v is the velocity being emulated listed So vy l as User velocity value in the land la AVI B Pv cosa ae mobile environment editor ae is the elevation angle of the satellite B is the bias value for zero peed listed as Zero speed offset in the land mobile ronment editor Category Mask Editor e Category A Complete obscuration You can use the Category Mask Editor to define a mask which is applied over the simulated antenna It is similar to the Antenna Pattern editor described in section 7 6 in that it represents the receiver s view of the sky as an array of azimuth and elevation It allows you to define the signal affecting properties of each portion of the sky view as one of the following four categories You can apply each of these categories independently for different test cases The signal arrival angle is resolved into satellite elevation and azimuth in 5 degree increments for positive elevations only The reference frame is local geographic and the orientation of the hemisphere created may be rotated in the azimuth plane SPIRENT Application Note Satellites arriving at these segments are not simulated at all and hence this category represents a visibility mask All satellites with elevation angles less than 5 degrees are automatically excluded Segments would be allocated with this category to simulate obstructions at low elevation
22. ignal level Delay Peak ns The peak offset delay to the sinusoid Delay Freq ns The frequency of the offset delay to the sinusoid Delay Phase The phase of the offset delay to the sinusoid Delay Bias offset The offset delay to the sinusoid The representation of the attenuation parameters is shown in Figure 13 Figure 13 Sinusoidal Multipath Attenuation Parameters GPS SVID 4 Multipath type Sinusoidal Number of echos 0 d Remove Remove LOS eS el Att peak dB Att frea Hz Att phase dea Att bias dB Delay peak ns Delay frea Hz Delay phase dea Delay bias ns a as a ea ie roe eee ae a a o ae _coea_ Figure 14 Sinusoidal Multipath Settings Window SPIRENT Application Note Page 19 Land Mobile Multipath LMM The Land Mobile Multipath model was developed to fulfil the need to simulate the signal environment effects experienced by a portable device such as a mobile phone The multipath models so far discussed are analytical in their approach which could present a significant challenge to the mobile phone tester as such tests per handset can number many hundreds The LMM model allows you to automatically define the signal conditions through selection from a database of pre defined environments for each test The analytical data relating to the delay and amplitude variation associated with the LOS signals and the multipath reflection
23. ill fall upon different parts of the antenna and will be adjusted accordingly Similarly in the context of Reflection Pattern Multipath the settings are attenuation and delay If you specify a Reflection Pattern Multipath on SV12 for example then an echo signal is generated according to the arrival vector of the signal for this satellite relative to the vehicle s reflection pattern and the appropriate delay and attenuation values are applied This does not imply that the multipath signal comes from the same direction as the LOS but that this LOS gives a reflection with these characteristics The LOS will be unaffected SPIRENT Application Note As the antenna and satellites move multipath signals will be varied as the LOS moves with respect to the reflection pattern This method can be useful where the source of the multipath signals is predominantly from the host vehicle The receiver antenna pattern can be used to attenuate signals which are blocked by parts of the vehicle structure and the reflection pattern used to give consistent changes in multipath signals as the vehicle changes its orientation Another benefit is that exactly the same multipath signals will be generated at exactly the same times when the scenario is rewound and re run This repeatability is not possible with live sky testing It is sensible to define only one multipath signal for each LOS using this model Page 15 Legendre Multip
24. ion Note Page 17 Polynomial Multipath The Polynomial Multipath model is similar in application to the Legendre method In this case the polynomial coefficients may be more intuitively defined but the corresponding disadvantage is that the multipath profile is no longer bounded so for different durations the coefficients may need to be revised to prevent un realistic multipath offsets being applied In the Polynomial multipath model the following functions represent the relative amplitude and delay of the multipath signal RelAmp ao ait a2t2 a3t3 a4t4 a5t5 Delay do dit d2t2 d3t3 d4t4 d5t5 Where RelAmp Relative amplitude of the reflected signal with respect to the direct signal expressed as a ratio limited to the range o to 1 anual multipath settings Alternatively the relative amplitude in dB dBrel is 20 x logio RelAmp Delay Delay of the multipath signal relative to the direct signal seconds t seconds For each Polynomial multipath you must enter values for the following polynomial coefficients in the Manual multipath settings dialog A1 to A5 coefficient ao to a5 in the polynomial for RelAmp D1 to D5 coefficient do to d5 in the polynomial for Delay Duration s the period of the polynomial cycle It is the period between which the echo is modelled and prevents the echo going to infinity Once a period has expired the echo pattern is continually repeated
25. locking signals below a certain elevation GNSS signals can also be reflected from below ground They travel through dry ground and then reflect off more moist layers further below and can be particularly troublesome at high quality DGPS reference stations where specialised antennas are essential See References 2 and 3 In marine environments strong multipath from low elevation satellites is created by the surface of the sea which is a very efficient surface for reflecting L band signals There are many techniques for mitigating multipath too numerous for discussion here However there are also many books on the subject References 2 and 5 provide good introductions to multipath in relation to GNSS Correlation peak no multipath me delay Correlation peak constructive in phase interference with reflected ray Correlation peak destructive anti phase interference with reflected ray o Ceiy m Pseudorange my too short Figure 3 Correlation Peak with In phase and Anti phase Interference Page Simulating Multipath Proper multipath mitigation in receiver designs for all applications is essential Complementary to this is the need for proper testing Real world testing presents very complex and un quantifiable multipath environments that are un repeatable and can be time consuming and costly to trial A GNSS simulator provides you with powerful methods for gener
26. lynomial Multipath 15 Sinusoidal Multipath 16 Land Mobile Multipath LMM 17 Fader Multipath 21 Conclusions 21 Referenced Documents 21 Definition of Terms 22 Further Information 22 SPIRENT Application Note Page 3 Audience This Application Note is for users of Spirent simulators who are designing developing integrating and testing GNSS receivers or systems and need to ensure their products will perform in the intended application Introduction There is a steady growth in the use of GNSS in new and existing markets Consequently there is an increasing reliance on GNSS technology With this in mind it is important for designers manufacturers and consumers of these products to understand what to expect from such systems This includes formulating an understanding of the limitations and problems of GNSS technologies SPIRENT Application Note Spirent recommends you have a basic understanding of satellite navigation principles and RF simulation as a test method This application note discusses the problem of Multipath which is a phenomenon that can cause serious reductions in a GNSS receiver s performance in a range of applications Complementary to this it demonstrates how you can use Spirent s range of GNSS Test Solutions to create and run controlled and repeatable simulations that include multipath modelling It also includes test methodology specifically for users of Spirent s simulation systems
27. mulator produces an echo signal with constant user defined range and power offsets from the normal LOS signal Once defined the settings remain fixed in relation to the LOS during the scenario run The variable fields are as follows The Attenuation field specifies the difference in level between the main signal and the reflected echo The Range Offset field specifies the delay in meters of the multipath signal compared to the LOS always a positive number the delayed multipath signal cannot arrive before the LOS The change in relative phase between the reflected and LOS signals due to the vehicle and satellite motion is not modelled in this simple case so the net interference between these signals remains fixed Additional echo signals with different attenuation and delay values can be added The limitation is the number of channels available in the simulator hardware the simulator allocates a single separate hardware channel for each multipath echo added SPIRENT Application Note It is possible to remove the LOS signal from the simulation by ticking the Remove LOS box This simulates the situation where the LOS is completely obscured Figure 7 shows a multipath signal defined for satellite PRN4 that is 3dB lower than the LOS and delayed by 30m It is not sensible to define more than one multipath signal for a given satellite using this method Manual multipath settings GPS SVID 4
28. r testing in situations where the receiver must be tested while undergoing high dynamic motion 4 DGP792AAA SimREMOTE User Manual and ICD Spirent 5 Proposed Models and Methodologies for Verification Testing of AGPS Equipped Cellular Mobile Phones in the Laboratory P Boulton A G Read Et Al Page 26 Glossary of Terms 3GPP o 3rd Generation Partnership Project Coarse Acquisition code used by Standard service GPS receivers The time between transitions in the C A code not referred to as a bit because the code does not carry information Pseudo Random Number Position Velocity Time Scenario In this context a GNSS simulation running on either SimGEN or SimPLEX simulator control software 3GPP Chip GNSS Line of Sight i SV SV GPS Satellite Vehicle WGS 84 World Geodetic Survey 1984 SPIRENT Application Note Page 27 CONTACT US DANo04 ISSUE 1 02 Got a smartphone If you have a smartphone download a QR Code reader and then point your phone camera at the QR Code to read the graphic We are adding new content to our website on a regular basis Bookmark this link www spirent com positioning Visit the Spirent GNSS blog there are currently over 90 posts with 2 to 3 new posts added each week Catch up on what s new www spirent com Blog Positioning aspx Need more information gnss solutions spirent com Why not share this document f amp do g Fac
29. s are replaced with statistical models commonly used in laboratory testing of wireless communication equipment together with a bespoke channel allocation algorithm for management of the simulator hardware This allows the following effects to be realised e Direct LOS signals with Rician fading e Reflections echoes with Rayleigh fading power decay and exponential delay e Deep fading of echoes giving a carrier Doppler offset The relative numbers of the direct and reflected signals are determined using a Satellite visibility category mask which uses the azimuth and elevation of the LOS signal Reference 5 gives an in depth description of the approach taken in developing the LMM model SPIRENT Application Note The LMM model is enabled in SimGEN in a different way to the other models described in this note It is only available for use with the Static Vehicle model and is not available via the Channel Assignment settings window as described in section 7 2 You can also use User Actions and there is a SimREMOTE command LMM _ SELECT that allows you to define the LMM environment category mask You can also use these methods to change the category mask settings while the scenario is running See Reference 4 for more details of the remote command To enable LMM select Options and tick Land mobile multipath file to include the Land Mobile Multipath model in the scenario The Active Configuration area displays
30. s default State defines an omni directional isotropic spherical antenna theoretical antenna with uniform gain in all directions It takes the Surface area of this sphere and divides it into equal sized portions minimum resolution 10 by 10 The Antenna Pattern Editor represents the spherical antenna as a 2 D array in the same way that a globe map of the world can be represented as a 2 D map Figure 10 show the Antenna Pattern Editor File Edt View Options Help oe a M azs E 20 Ar ar ee jer er er fer Er er Er ET ME M s MR PS 230 230 230 60 0 eo eo eo eo so eo so R a ea 290 230 230 0 eo 0 eo eo so aso 50 O SR Par Pea Ta ar Pa Pee PP ea PO Pa Pe Pe oP gt MR 20 230 230 230 230 230 230 230 230 230 230 A a a H Ea 0 80 50 50 50 50 60 50 0 a 2 0 0 0 80 so so so 0 i A L375 0 0 0 50 80 feo so eo 0 A A d e E RS SS A E S E A S SSs i Figure 10 Settings Window for Doppler Offset Multipath Download the Spirent Application Note Keeping your eye on the sky The importance of antenna modelling in GNSS testing SPIRENT Application Note Page 14 You can enter attenuation and phase values for each field so that if the satellite signal falls on that part of the antenna it is modified according to the settings for that field As the satellites and antenna move the signals w
Download Pdf Manuals
Related Search