Uncertainties in TRP Measurements Due to Finite Range Lengths

Transcription

1 Uncertainties in TRP Measurements Due t Finite Range Lengths James D Huff Carl W Sirles The Hwland Cmpany, Inc 4540 Atwater Curt, Suite 107 Bufrd, Gergia Abstract Ttal Radiated Pwer (TRP) and Ttal Istrpic Sensitivity (TIS) are the tw metrics mst mnly used t characterize the ver the air (OTA) perfrmance f a wireless device The minimum range length fr these measurements has usually been determined using the arbitrary far-field criteria f R>2D 2 /λ This paper quantifies the changes in measured TRP as the range length is increased frm D 2 /λ t infinity (r thereabuts) TRP measurements n a UMTS diple bined with a phantm head have been made at different range lengths Additinally, numerical simulatins f TRP measurements n an array f pint surces have been made at different range lengths The result is a theretical determinatin f TRP measurement errrs versus range length supprted by actual measurement results Keywrds: far-field, OTA, TIS, TRP, wireless 1 Intrductin Ttal Radiated Pwer (TRP) is calculated frm measurements f EiRP taken ver the entire sphere surrunding the device under test (DUT) Histrically it has been assumed that the diameter f the measurement sphere, r range length, needs t be greater than 2D 2 /λ, where D is the diameter f the DUT and λ is the wavelength at the measurement frequency This paper investigates the validity f this assumptin by paring TRP measurements made at different range lengths Measurement simulatins have als been made using an array f pint surces and the change in TRP calculated fr different range lengths frm D 2 /λ t 1 millin kilmeters (DUT) in an anechic chamber with a tw axis psitining system and the DUT psitined t different crdinates in a spherical crdinate system The transmitted pwer is then sampled at equally spaced increments in phi and theta ver the entire sphere surrunding the DUT These relative pwer measurements are then cnverted t Effective Istrpic Radiated Pwer (EiRP) using a range reference measurement The EiRP values are then weighted by the sine f theta and summed t calculate the ttal radiated pwer 3 The Basic Cncept TRP is measured by sampling the radiated pwer ver a spherical surface surrunding the DUT In Figure 1 we have tw spheres surrunding a device transmitting pwer P 0 It is bvius that the same ttal pwer that radiates utward frm the DUT passes thrugh bth the large sphere and the small sphere n its way t infinity Therefre in cncept it shuld be pssible t make accurate TRP measurements regardless f the diameter f the sphere used t sample the transmitted pwer Large Sphere Small Sphere P 2 The TRP Measurement One f the mst mn parameters measured n a wireless device is the ttal radiated pwer (TRP) The measurement is made by placing the device under test Figure 1 The Basic Cncept

2 4 Experimental Measurements In rder t verify this cncept, measurements were made at three different range lengths The device under test (DUT) was a SAM phantm head with a UMTS diple The DUT is shwn in figure 2 This DUT was chsen because it fills up a 30 quiet zne and prvides a realistic test scenari parable t testing a cellular phne The UMTS frequency band (2110 t 2170MHz) was used because effects due t range length shuld be mre prnunced at higher frequencies, and the UMTS band is the highest frequency cellular phne band currently being utilized Measurements were made at five frequencies in the UMTS band at five degree steps f phi and theta The nly setup change between measurements at different range lengths was the spacer itself The RF instruments and cables were kept unchanged This meant that it was nly necessary t calibrate the range at ne range length The cal factrs fr the ther range lengths culd be calculated frm the change in range length using the fllwing frmula Change in Cal Factr = 20LOG(R 1 /R 2 ) where R 1 and R 2 are the tw range lengths The reasn fr using this prcedure was t remve as many uncertainties frm the data as pssible 5 Measurement Results Figure 2 The Device Under Test In rder t change the range length spacers were used n the bm f a distributed axis psitining system Three different spacers were used prviding range lengths f 65, 119 and This setup is shwn in Figure 3 Shrtened 65 Range Length 91 spacer The measurement results are summarized in Table 1 belw Fr a pint f reference, is apprximately equal t 2D 2 /λ while 65 is apprximately equal t D 2 /λ Referring t the table belw, ne will see that the largest change in TRP is apprximately 01 db and that there is n bvius trend as the range length is increased The effect f the change in range length is prbably being masked by the effect f extraneus signals in the range As the range length is changed the relative phase f the direct signal t the extraneus signal is changing causing the received signal level t change Freq MHz Table 1 Measurement Results TRP Delta t 119 t 65 NHRP ±45 t 119 t 65 NHRP±30 t 119 t Patch Antenna Nrmal 156 Range Length Values fr the near hrizn radiated pwer (NHRP) at ±45 degrees and ±30 degrees were als calculated These results are similar t the results fr TRP, with maximum changes n the rder f 01 db Again there is n clear trend in NHRP versus range length Figure 3 Measurement Setup

3 6 Measurement Simulatins A numerical simulatin was perfrmed using seven pint surces One pint surce was placed at the center f the quiet zne and the ther 6 were placed at ± 15 alng the X, Y and Z axes This is shwn schematically in Figure 4 belw Range Length Table 2 Simulatin Results TRP NHRP±45 NHRP±30 D 2 /λ D 2 /λ Z P 3D 2 /λ D 2 /λ X -Y Z Pint Surce (7 places) Figure 4 Pint Surce Array The vectr sum f the cntributin f each surce was calculated at every 5 degrees in phi and theta using ray ptics The resulting pattern was weighted and summed and crrected fr free space dispersin Simulatins were made fr range lengths f D 2 /λ, 2D 2 /λ, 3D 2 /λ, 4D 2 /λ, 5D 2 /λ and 1 millin kilmeters Fr each range length simulatin values fr TRP, NHRP ±45 and NHRP ±30 were calculated These calculated values were then nrmalized t the values calculated fr a range length f 1 millin km The results are shwn in Table 2 Althugh the effect f range length can be seen in the results, the calculated changes in TRP, NHRP ±45 and NHRP ±30 versus range length are fairly small R +Y +X 5D 2 /λ M km Summary Bth the measured data and the measurement simulatins shw nly small changes in TRP as the range length is varied frm D 2 /λ t 2D 2 /λ and beynd High perfrmance wireless test labs such as The Hwland Cmpany Mdel 5100 typically have an expanded TRP measurement uncertainty f apprximately 1 db Including the additinal uncertainty cntributin fr measurements at a range length f D 2 /λ wuld have a very small impact n the verall measurement uncertainty The real imprtance f these results is that larger devices can be tested at higher frequencies n existing ranges Wireless devices are being marketed that perate as high as 58GHz, and these devices will be installed in everything frm autmbiles t appliances 8 References [1] Test Plan fr Mbile Statin Over The Air Perfrmance, Revisin 22, CTIA Certificatin Prgram, Nvember 2006 [2] Micrwave Antenna Measurements, Secnd Editin; Lyn, Hllis, Claytn; Scientific-Atlanta, Inc; 1970 [3] Huff, J D and Sirles, C W The Effect f Range Length n the Measurement f TRP, 29 th Prceedings f the Antenna Measurement Techniques Assciatin (AMTA-2007), St Luis, MO, pp

4 Figure 5 Measured Radiatin Patterns Hrizntal Plarizatin Theta= MHz Relative Pwer R=65 R=119 R= Phi Angle (deg) Vertical Plarizatin Theta= MHz Relative Pwer R=65 R=119 R= Phi Angle (deg)

5 Figure 6 - Calculated Patterns fr Pint Surce Array Phi=0 degrees Relative Pwer Theta Angle (degrees) Range=D**2/lambda Range=2D**2/lambda Range=3D**2/lambda Range=4D**2/lambda Range=1M km Theta=50 degrees Relative Pwer Range=D**2/lambda Range=2D**2/lambda Range=3D**2/lambda Range=4D**2/lambda Range=1M km Phi Angle (deg)

6 Figure 7 Change in TRP vs Range Length Change in TRP vs Range Length Delta TRP Range Length (D^^2/lambda) Delta TRP NHRP(30) NHRP(45)