《MIMO多进多出多天线》课程教学资源（文献资料）Polarization Diversity in Mobile Communications

S ON VEHICULAR TECHNOLOGY.VOL.39.NO.3.AUGUST 1990 177 Polarization Diversity in Mobile Communications RODNEY G.VAUGHAN,MEMBER,IEE ls in the ve in many six b for P b and the of R of has sp e dive has not been ver d a base shor-te was the ImoeleuCtricland rst step i mpl d as a horizo ontal wire elements sit ich is d e.it tur ut that the an s alor are hat the at he signals by the elation for the signals in each po nted depe The refe gethe he m (cf of on al ep path of the blish nd Bach nd incl refra d]r als mea hand The a te of th signal will be of the of Le bly le prac a ation I polariz ven r ex nle ourees of 0. nt of in fact he b and well over 100 w n an on a e)A slo olteandrcarwouldt patch na [7]offer y of desi rthogonal polarizations. This may permit ony two diversity g for a is may be n There are s regarding the propa 018-9545/90/0800-017701.00©1990EEE

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 177 Polarization Diversity in Mobile Communications RODNEY G. VAUGHAN, MEMBER, IEEE Abstmct- Signals in the vertical and horizontal polarizations at the base station have been measured by transmitting from a principally ver￾tically polarized mobile. There was no direct line-of-sight path between the mobile and base. The envelopes were uncorrelated and the means differed by 7 dB and 12 dB when the mobile was in urban and subur￾ban areas, respectively. The discussion of the results includes theoretical curves showing the relationship between the envelope correlation coef￾ficient and the mean levels difference of Rayleigh distributed signals in orthogonal linear polarizations at the base station. The variable param￾eters are the rotation angle of the base station antenna and the cross polar discrimination of the incident fields. I. INTRODUCTION LARIZATION diversity seems to have received dispro- p" portionately little attention in the literature. Lee and Yeh's [4] polarization path diversity proposal was the first step in this direction, but the idea of using just one transmit antenna at the mobile and receiving orthogonal polarizations at the base station was not considered. Kozono et al. [3] measured linear polarizations at a f 45" orientation, received from a verti￾cal dipole antenna on a mobile in Tokyo. Their base station antenna was variable in the sense that the arms could rotate in opposite directions, from both arms lying in the horizon￾tal plane to being orthogonal to each other at f45"; they also accounted for the azimuthal dependence. Herein, base station elements that are always orthogonal and rotatable to￾gether are considered. The measurements described are con￾fined to a vertical-and-horizontal configuration. The azimuthal dependence is not considered: incident signals are assumed to be broadside to the plane of the antenna. Preliminary results were published by Vaughan and Bach Andersen [8]. Bergmann and Arnold [l] have also discussed measurements for (hand￾held) polarization diversity. The aforementioned publications seem to be the only ones that regard polarization diversity. This is surprising, considering the advantages and simplicity of the scheme. At the base station, space diversity is considerably less prac￾tical than at the mobile because the narrow angle of incident fields calls for large spacings of the antennas. For example, Lee [5, p. 2011 notes that for an azimuthal angle width of sources of 0.4" and an envelope correlation coefficient of 0.7, base station antennas must be spaced by 25 wavelengths for the broadside propagation case and well over 100 wavelengths for the in-line propagation case. The high cost of space di￾versity at the base station prompts the consideration of using orthogonal polarizations. This may permit only two diversity Manuscript received August 16, 1986; revised February 1990. The author is with the Department of Scientific and Industrial Research, Physics and Engineering Laboratory, Gracefield Road, Gracefield, Private Bag, Lower Hutt, New Zealand. IEEE Log Number 9035261. branches (without resorting to antenna spacing), but it does allow the antenna elements to be colocated. Currently, there is considerable interest in many-branched diversity at the base station motivated by the potential for reduced interference. Glance and Greenstein [2] discuss six branches, for example. If the polarization diversity scheme can be made to work, then the physical extent of base station diversity antennas is halved relative to conventional space diversity. At the mobile, use of orthogonal polarizations only to pro￾duce diversity branches has not been very successful. Mea￾sured horizontal and vertical polarization paths between a mo￾bile and a base station are reported to be short-term uncorre￾lated by Lee and Yeh [4]. Their mobile polarization diversity antenna consisted of colocated vertical electrical and magnetic (implemented as a horizontal wire loop) elements sited 1.5 h above the vehicle's conducting roof. The presence of the ve￾hicle roof gives rise to an array pattern, which is discussed in the appendix. For an infinite, perfectly conducting ground￾plane, it turns out that the array patterns alone are sufficient to decorrelate the signals received by the mobile antenna ele￾ments. The mechanism of decorrelation for the signals in each po￾larization is the multiple reflections undertaken between the mobile and base antennas. The reflection coefficient for each polarization is in general different (cf. Fresnel's formulas), which results in the phases of orthogonal polarizations un￾dergoing different changes for each (or at least some) of the reflections. The path of the signal occupies three dimensions and includes reflection and refraction, which causes coupling between orthogonal polarizations. After sufficient random re￾flections, the polarization state of the signal will be indepen￾dent of the transmitted polarization. This is ideally what hap￾pens to signals propagating through an urban environment. In practice, and as noted from the measurements of Lee and Yeh, there is apparently some dependence of the received po￾larization on the transmitted polarization, even in urban en￾vironments. The multiple scattering cannot be sufficient for a given polarization to decouple half its power into the orthogo￾nal polarization. Still, an antenna at the mobile need not be of pure polarization (in fact, achieving true polarization purity from an antenna mounted on an average car would be virtually impossible). A sloping monopole and the roof-mounted circu￾lar patch antenna [7] offer some possibility of designing for a given (approximate) ratio of radiated polarizations. However, this may not be necessary to get a considerable return from a polarization diversity system. There are too many unknowns regarding the propagation from the mobile to the base station to establish from theo￾retical considerations how well a polarization diversity sys- 0018-9545/90/08oO-0177$01 .oO 0 1990 IEEE

178 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY.VOLNO.3.AUGUST 190 E 100 1Ea2+E,2 光时w2 larizaionisdominanmlyventicalforhelowerclc base station an te This shor gro which was e gnmalleveldi The base station an nna com sed two identical four Maximum ratio combning is assumed. on as a paramete 100m 160 II.MEASUREMENTS AND DISCUSSION A.Measurement Setup h was dri ven in urban terrain tal giving the layout around os. he antenna was a sloping monopoleof ement run n at Vestergade,in No

178 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 I E+I2 * Y J x Fig. 1. Pseudo-three-dimensional far-field linear power patterns for a sloping monopole of length 0.6 X and elevation angle 60'. Infinite ground plane and sinusoidal current distribution is assumed. Vertical polarization is clearly dominant at lower elevation angles. tem will work, so empirical techniques are called for. The measurement scheme is relatively simple to implement and is described in the following section. The ensuing discussion in￾cludes an analysis using Rayleigh distributed signals incident on the rotatable base station antenna. This shows the trade￾off between the envelope correlation coefficient and the mean signal level different between the signals of the orthogonal po￾larizations, with the cross-polar discrimination as a parameter. Maximum ratio combining is assumed. 11. MEASUREMENTS AND DISCUSSION A. Measurement Setup Any two orthogonal polarizations for receiving at the base station will suffice for the measurements. There seems little justification for not using vertical and horizontal components. The transmitting mobile antenna was a sloping monopole of length 0.6 X and elevation angle 60". The antenna pattern, as￾suming an infinite groundplane, is depicted in Fig. 1. The po￾larization is dominantly vertical for the lower elevation angles. The monopole was mounted on the center of an aluminium groundplane, which was larger than 2 x 1 wavelengths at the measurement frequency of 463 MHz. The base station antenna comprised two identical four￾element dipole arrays, one for vertical and the other for hori￾zontal polarization reception. These antennas were mounted adjacently, at a height of 100 m (-160 A), on a mast at Frejlev, Denmark. The land near the mast is basically rural. The mobile was driven in urban and suburban areas of the city of Aalborg, which is about 20 km from Frejlev. The terrain between Frejlev and Aalborg is that of slightly rolling plains with occasional foliage and dwellings. Fig. 2 is a map giving the layout around the measurement zone. The urban measurement run was taken at Vestergade, in Ngrresundby

VAUGHAN:POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 179 Frejlev Base Station Fig.2. The st lin There co th for the ve polar on to deco an h ation the mobile environ plus the heigh of th a row of large trees in front of suburban houses. o th This suggests that future m not be in The measured envelopes,their ying pare thes rements with those taken with the ures is defined by adB- where mean an al has dB-477 n which is referred toasa verical-verticalystmcan mber (4 rem dB larized signal is about 31 dB,and departs from the Rayleigh and i ses to about 3 dB at the or the distribu ere is ove Rayleigh distrib ionatabrcie is0.03.The results are summarized in Tb

VAUGHAN: POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 179 Fig. 2. Map showing layout. (a) Between base station measurement site at Frejlev and urban transmitting route. (b) Between base station measurement site at Frejlev and suburban transmitting route. (Open terrain is gently rolling plains.) which is parallel to the water edge on the other side of a wide strip of water, the Limfjord, to Frejlev. The street is lined on both sides with 4-5 story buildings and runs at right angles to the direction of the base station. There is no direct line-of￾sight path to the base station. The suburban measurement was taken in Hobrovej, beside the Southern Hospital. Hobrovej lies in a shadow area, caused by a hill, and this street runs at near right angles to the direction of the base station. Looking toward the base station, there is suburban housing and some large buildings on the hill. In the opposite direction, there is a row of large trees in front of suburban houses. B. Measurement Results The measured envelopes, their lossless maximum ratio com￾bination, and the Rayleigh diagrams are given in Figs. 3 and 4 for the suburban and urban measurement runs. The noise level was chosen to be the lowest value of signal power that could be measured. The standard deviation noted in the fig￾ures is defined by U dB = 10 log (: ~ : $9, where p and U are the conventional mean and standard eviations of the envelope power. A Rayleigh signal has U dB = 4.77, and an uncorrelated two-branch maximum ratio has U dB = 3.2 1. For the suburban measurement, the horizontal polarization is limited at the noise level, restricting its dynamic range to about only 25 dB. The dynamic range of the vertically po￾larized signal is about 31 dB, and departs from the Rayleigh distribution at about 15 dB below its mean level. The envelope correlation coefficient is 0.02. For the urban measurements, the horizontal polarization curve follows a Rayleigh distribu￾tion over its full dynamic range of about 36 dB. The vertical polarization departs from the Rayleigh distribution at about 18 dB below its mean value. The envelope correlation coefficient is - 0.003. The results are summarized in Table I. The exact mechanism causing domination by the vertical po￾larization is not clear. There are several possible contributing factors: the urban and suburban environments are both rather transparent at 450 MHz so that an effective line-of-sight exists between mobile and base station; there is insufficient polar￾ization coupling in the multipath reflections for the vertically polarized transmission to decouple into equal levels of vertical and horizontal polarizations; and the open terrain between the base station and the mobile environment plus the height of the antenna act to favor propagation of the vertical polarization to the base station. This suggests that future measurements could involve repeating the experiment with mobile antennas of varying polarization. The mobile antenna need not be in a vehicle- a simple handheld apparatus could be sufficient to find some useful information. It would also be useful to compare these measurements with those taken with the base station sited in the same urban environment as the transmitting mobile, rather than being well separated from the urban area. In the Rayleigh diagrams of Figs. 3 and 4, the reference (SNR) is that of the vertically polarized signal. The diversity gain, which is referred to as a “vertical-vertical’’ system, can be read directly off the diagrams. The curves have not been smoothed; they represent a direct mapping from the finite number (4600) measurement points. For the suburban mea￾surements, the diversity gain is less than l dB at the 80% level (i.e., for 80% of the time), and increases to about 3 dB at the 95% level, and nearly 5 dB at the 95% level (a smoothed curve for the distribution is imagined). In the urban measurements, there is over 3 dB diversity gain at the 80% level, increasing to nearly 7 dB at the 99.5% level. In practice, there will be combination losses of perhaps 1 dB, so that the return from polarization diversity in suburban environments is negligible for this definition of diversity gain

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180 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 i! g -ao t Y H -110 LLLL 11 111 1 UL U 1111 LU LL . Horizontal CHANNEL 1 SIMAL POWER REAN SNR I081 I 15.07 RINIHUH VRLUE SNR 108) = 0.00 STANORRO DEVIATION I08 I = 5.48 HERN SIONRL LEVEL IOBHl = -104.93 1 -l¶O Vertical CHANNEL 2 SIMAL POYER RERN SNR 1081 = 27.32 HEAN SIGNAL LEVEL IOBNI = -92.68 RINIMUR VRLUE SNR IO81 = 4.53 I HRXU(IH(IIL RATIO 6IowRL POYER HEAN SNR IO81 = 27.57 HININUH VRLUE SNR IO81 = 5.84 STRNORRO OEVIRTION IOB 1 = 1.07 RERN SIONRL LEVEL IOBMI = -89.42 (a) Fig. 3. Measurements from mobile transmitting from suburban environment, Transmitted polarization was principally vertical. (a) Fading envelopes of each polarization and maximum ratio combination. (b) Rayleigh diagram

181 of horizonta E。=r1cos(at+中，l (1) and vertical polarization Ey =r2 cos(wt+) Vi=Ey cos a +E sin a =(r2 cos c cos 2 +ri sin a cos cos cof -(r2 cos a sin 2+r sin a sinsin wf (4) and V2=Ey sin a-Ex cos a =(r2 sin a cos 2 +r sin a cos cos of -(r2 cos a sin -r sin a sinsin cot (6) whose respective envelopes are 【SNR/SNR>EWDB. R1=[r cos a+ri sin a +2rn2c0 sa sin acos(1-月（⑦ and istributed.The ertical pola R2=[ri cos a+r sin'a +cos a sin a cos((8) nt component in the ho roblem of the square root can be circumvented by con eived po R-(R》R号-(R》 *TR-RR-(R》严 (9) particularly relevant. figure me (10 The required moments and moment products are In Ko re the in (R)=cos2a+rsin2a e rec +2r1r2 cos a sin a cos(12)) ourse t the seof raising the corre =》cos2a+sin2a ollected (R》=》cos2a+(sin2a: (12) (Ri)(R)=(r)(r)(cost a+sin'a) An in +cos a sin a((); (13)

VAUGHAN: POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 181 ISNRI/ IN OB. (b) Fig. 3. (Continued) In both the urban and suburban measurements, the re￾ceived horizontal polarization appears genuinely Rayleigh￾distributed. The vertical polarization looks more Rician, caused by a dominant component, as discussed in the pre￾ceding. The lack of a dominant component in the horizontal polarization is quite reasonable, since essentially all the re￾ceived power results from polarization coupling by reflection and refraction, of which no individual contribution continually dominates. It is noteworthy that the figure found here for the polarization cross-coupling agrees with that of Lee and Yeh [4]. This indicates that the effects of the open terrain may not be particularly relevant. The figure measured here also agrees with the measurements of Kozono et al. [3], albeit in an in￾direct sense (see Table I), further supporting this possibility. C. Discussion In Kozono et al.'s [3] results, the base station was in ur￾ban Tokyo. Their base station antenna was arranged so that the received polarizations were f 45", in order to equalize the received mean signal levels. This equalization works, of course, but only at the expense of raising the correlation coef￾ficient. Note that the total energy in the incident signal is being collected, assuming maximum ratio combining, irrespective of the angles (or senses) of the orthogonal polarizations. A trade-off between the mean level difference and the branch signal correlations is evident. An investigation simi￾lar to that of Kozono et al. [3] can be arranged by assuming an incident signal of horizontal polarization and vertical polarization in which rl and r2 are Rayleigh-distributed and uncorrelated. 41 and 42 are assumed to be random, uniformly distributed, and uncorrelated. An antenna receiving orthogonal linear po￾larizations, such as crossed dipoles broadside to the incident propagation vector (the situation is depicted in Fig. 5(a), re￾ceives voltages proportional to VI =Ey cos a +Ex sin a (3) = (r2 cos a cos 42 + rl sin a cos 41) cos ut - (r2 cos a sin 42 + rl sin a sin 41) sin ut (4) and V2 =Ey sin a -E, cos a (5) = (r2 sin a cos 42 + rl sin a cos 41) cos at - (r2 cos a sin 42 - rl sin a sin 41) sin ut (6) whose respective envelopes are RI = [r,' cos2 a + r: sin2 a +2r1r2 cos a sin a cos(41 - 42)1'/~ (7) and R2 = [r: cos2 a +ri sin2 a +2rlr2 cos a sin a cos(41 - 42)]1/2. (8) The problem of the square root can be circumvented by con￾sidering the power correlation coefficient (known to be similar to the envelope correlation coefficient [6]) The required moments and moment products are (R:) = (r,' cos2 a + r: sin2 a + 2r1r2 cos a sin a cos (41 - 42)) (R:)(R,~) = (r:)(t,2)(cos4 a + sin4 a) + cos2 a sin2 a( (r:)2 + (r:)2); (13)

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182 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 -I -190 Horizontal CHANNEL 1 SIONAL POHER NEAN SNR (081 27.25 NEAN SINAL LEVEL (DBNI = -92.76 STANDARD DEVIATION (OB I = 5-31 NININUN VALUE SNR (081 = 0.00 I- - J -130 Vertical CHANNEL 2 8IONAL POYER NEAN SNR (081 = 34-08 MININUN VALUE SNR (081 = 9.24 NEAN SINAL LEVEL (OBNI = -85.92 STANDARD DEVIATION (OB 1 = 4-49 f -110 =i -IS0 MAXIHAL RATIO BIONAL POYER NEAN SNR (08) = 34.90 HININUN VALUE SNR (081 = 15.83 STANDARD DEVIATION ID8 I I 3-73 HEAN SIONAL LEVEL (DBHI = -82.09 (a) Fig. 4. Measurements from mobile transmitting from urban environment, Transmitted polarization was principally vertical. (a) Fading envelopes of each polarization and maximum ratio combination. (b) Rayleigh diagram

RIZATION DIVERSITY IN MOBILE COMMUNICATIONS 183 the result is (a.(uan an) (2 The limiting cases serve as checks: 1)x=0.=1.independent of o, (22) 2)x==0,independent of a, (23) 3)a=0.independent of x. (24 (25 tiger0mcwhhteelofKoaooetaLB.q 。 月牛 (26) (5MR1/45NR>IN DB. that is e.Cmd L(o.0 (2) (R1)=(r)cos'a+(r)sin'a The limiting case checks are as follows: +4(rr)cos'a sin2 a 1)a=0一L=x (28 =2(r)cos a+(r)sin'a). (14) 2)a=45°→L=l,independent of x. (29 because 3)X=0-L=am2a:g=0-L=0 (30 r)=2r2 =2 (15) a=45°-L=1. (31) (R)=2(()cos a+)sin'a): (16 4)x=1L=1,independent of a. (32 and finally, (RR)=()(r)cost a+()(r)sin'a vario -2()cos"a sin?a+2)cos?a sin?a +2()cos?a sin'a =2 cos'a sin?a((r+ +(r)(r2)(cos a-sin a (18 The envelope correlation coefficient becomes abou antenna to±45° (19) and using the cross-polar discrimination L0°，x=p(45°，X (33) (20)

VAUGHAN: POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 183 (b) Fig. 4. (Continued) the result is The limiting cases serve as checks: 1) x = 0 + pe = 1, (22) independent of a, 2) x = 1 -+ pe = 0, independent of a, (23) 3) a = 0 -+ pe = 0, independent of x, (24) which is in agreement with the result of Kozono et al. [3, eq. (13) with /3 = 01. (26) - (r:) cos2 (Y + (ri) sin2 a (r;) cos2 a + (r:) sin2 a - that is, x + tan2 a 1 + x tan2 a Uff, x) = The limiting case checks are as follows: (R;) = (r4) a + (r:) sin2 (Y (14) 2) a = 45" + L = 1, independent of x. (29) = 2((r;) cos2 (Y + (r:) sin2 a), (30) because 3) X=~+~=tan~a; ~=o+L=o (32) (~4,) = 2( (r:) cos2 (Y + (ri) sin2 a)2; (16) 4) x = 1 + L = 1, independent of a. and finally, The ratio of powers L(a, x) is plotted against the envelope correlation coefficient pe (a, x) for various parametric values of x and a in Fig. 5. The range of a is from 0" to 45". The curves of constant x intersect the ordinate for the constant value of x. The value of a at this point is 0" and increases to 45" where the abscissa is intersected. The curves of constant a emanate from the origin where x = 1, and migrate out to pe = 1 where x = 0 (-cc dB). For the urban measurements at Frejlev, for example, the mean level difference of 7 dB between branches could have been traded with an envelope correlation coefficient of about 0.44 (for similar branch levels) by rotating the base station antenna to & 45" polarizations. The locus of the mean levels and the equivalent correlation coefficient is given by the curve (R:R;) = (r?)(r;) cos4 a + (r:)(r;) sin4 a - z(~;),.;) c0s2 (Y sin2 + 2(4) cos2 a sin2 a + 2(,.:) c0s2 a sin2 a (17) = 2 cos2 a sin2 a((r:) + (ri))2 + (r,)(r2)(cos2 a - sin2 a)2. (18) The envelope correlation coefficient becomes Pe cos2 a sin2 a((ri) - (r;)12 ((,.;) c0s2 (Y + (r;) sin2 c0s2 a + (,.;) sin2 a) (19) and using the cross-polar discrimination L(O", X) = ~e(45", XI- (33) By rotating the antenna, the mean level difference is traded with the correlation coefficient for constant (maximum) energy reception. In comparing Kozono et al.'s [3] result for a = (20)

184 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY.VOL9.NO.3,AUGUST19 -18dB -18 dB leve 12 dB 7 dB 009 -0.00 a·20 1.0

184 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 TABLE I SUMMARY OF RESULTS FOR THE POLARIZATION DIVERSITY MEASUREMENTS~ Suburban Urban Vertical Horizontal Vertical Horizontal Envelope dynamic range 31 dB 25 dB 32 dB 36 dB (limited) Level (cf. mean) where distribution departs significantly Mean level difference between polarizations 12 dB 7 dB from Rayleigh - 18 dB - - 18 dB - Envelope correlation coefficient between vertical and hori￾zontal polarizations 0.019 - 0.003 Vertical and horizontal polarizations were received at the rural base station and the mobile-transmitted principally vertical polarization. Each measurement run was 45 m (-70 X at 463 MHz). There was no direct line-of-sight between mobile and base in either run. r2 2 L=7 -I4 - -12 m - 0 I & -10 a X e r -8 2 = -6 -4 -2 I I O?O' 0.2 0.4 0 -6 0.8 I .n . - ENVELOPE CORRELATION COEFFICIENT (3) Fig. 5. (a) Polarization diversity antenna and incident field schematic and relations. (b) Branch power ratio L and envelope correlation coefficient pe behavior. (Circumferential lines are of constant cross-polar discrimination ,y (value of line at ordinate), with a increasing from 0' (on ordinate) to 45' (on abscissa); radial lines are for constant a, with x decreasing from 0 dB (at origin) to -M dB; thick radial line is mean branch power ratio/envelope correlation trade-off.)

VAUGHAN:POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 185 107 le with t this autho Returning to the Rayleigh di ()and 4(b) system des cribed.In discussin ony the(SNR)ga 0.8 1以h cally ause of the Lari pattern to ha by using more horizon ircular micro dual polariza as n of open. gnal when (R)at a ds to ered as minimum scattering ant e p) as(they are good appr Th the diversity gain is nated as [10] minimum va ssible for the assu po≈Em(Ee()dn (34 s was already where E(m)and Et are the normalized far-field patterns of n urban and conwenicntsourcemodg rtical and the mobile The scalar s no gain rather the smal cnts 7 dB at A(0)=1+eos (35 n u Aim(0)=1-eika2h cos 3.C or rotatabl given. station have bee %≈方A间(sin9a (37 sin (ko2h cos sin d (38 Signal Deco relation by array Pat of Electric and Magnetic Elements over a Ground Plane Here D is the normalization factor me (39 that the array factor is important for the -且-(r)门 (40 If the polarization diversity antennaelemensn be conid-

VAUGHAN: POLARIZATION DIVERSITY IN MOBILE COMMUNICATIONS 185 45", of L M 2 dB and pe M 0.3, the graph suggests x E - 5 dB, comparable with the 6 dB measured by Lee and Yeh [4], and the 7 dB measured by this author. Returning to the Rayleigh diagrams of Figs. 3(b) and 4(b), there is some diversity gain available from the polarization di￾versity system described. In discussing only the (SNR) gain, not a great deal of information is revealed regarding the chan￾nel capacity of a digital link. Bit error rate (BER) measure￾ments may have revealed a considerable improvement when using the diversity system-relative to using just the verti￾cally polarized signal-because of the reduced random FM. Lee and Yeh's measurements indicate that the transmitted po￾larization has more effect than the environment on the relative levels of the received polarizations. It is felt that by arrang￾ing the mobile antenna pattern to have, e.g., equal amounts of each polarization (for example, by using a more horizon￾tally oriented sloping monopole or by varying the diameter of a circular microstrip patch antenna [lo]), then dual polariza￾tion reception at the base station would offer significant gains over the usual "vertical-vertical'' system, even in suburban environments. The "vertical-vertical'' system effectively corresponds to the case when (RI) at a = 0 is much larger than (R2). There is no diversity action because the power contribution from the R2 signal is negligible. Maximum diversity gain is achieved when the envelope correlation coefficient is zero (this is the minimum value possible for the assumed Rayleigh distributed envelopes) and the branch power ratio is 0 dB. The antenna system must be arranged for this condition, as was already mentioned. 111. CONCLUSION Measurements of uncorrelated Rayleigh-like envelopes in the vertical and horizontal polarizations at the base station have been presented. There was no direct line-of-sight to the mobile. For suburban base stations, the dominance of the ver￾tical polarization makes the diversity gain rather small- only a couple of dB at the 99.5% probability level. In urban en￾vironments, however, the diversity gain is nearly 7 dB at the 99.5% level, offering much promise for system design us￾ing polarization diversity. The mean levels of the orthogo￾nally polarized signals agree well with existing measurements from different cities by other workers (viz. [4], [3]). General expressions and their graphical interpretation for a rotatable polarization diversity antenna at the base station have been given. APPENDIX Signal Decorrelation by Array Patterns of Electric and Magnetic Elements over a Ground Plane Lee and Yeh's 141 classic polarization path diversity mea￾surements automatically include signal decorrelation by the differing array factor of the electric and magnetic elements above the ground plane (roof) of the mobile. For an infinite, perfectly conducting ground plane, the following analysis in￾dicates that the array factor is important for the decorrelation of the signals. If the polarization diversity antenna elements can be consid- 1.01 ii 0.2 0% 0.26 tlCIOnT 0.60 RDOVE 0.76 WWMO 1.00 ?I" 1-26 IN WIVELCNOnT8 1.60 1.76 2.00 Fig. 6. Correlation coefficient expected as result of array patterns of elec￾tric and magnetic elements above infinite ground plane. Curve can be in￾terpreted as magnitude of open-circuit signal correlation coefficient (i.e., square root of envelope correlation coefficient) for two colocated antennas of same polarization with far-field patterns given by Ace)(@) and A("')(B). Inclusion of scalar element factor does not alter curve significantly. ered as minimum scattering antennas (they are good approx￾imations since they are operated in a single mode), then the open-circuit correlation coefficient at the mobile is approxi￾mated as [lo] (34) where E(m) and E@) are the normalized far-field patterns of the elements in the presence of the groundplane, and the in￾tegration is over the sources generating the incident fields. In an urban situation, a reasonable and convenient source model is an even-distribution model, surrounding the mobile, and from zero to 30" in elevation from the mobile [9]. The scalar element patterns are similar, and the correlation coefficient is unity. The presence of an infinite ground plane a distance h below the elements gives rise to array patterns for the electric and magnetic elements (35) A(e)(q = 1 +e~ko2hcose (36) A(m)(e) ~ 1 - eJk~2h cos0 which alone produce a correlation coefficient 1 *I2 pa El/3 ~(m)(e)~(e)(e) sin me (37) sin(ko2h cos e) sin 8de. (38) Here D is the normalization factor pa can be interpreted as the magnitude of the open-circuit correlation coefficient for two colocated antennas of the same

186 S ON VEHICULAR TECHNOLOGY.VOL,NO.3,AUGUST9 .New York 间1.RF of a sin 8 scalar ctor i ty the .Under the assumpio ns of Sep.1984.pp. any heightg than about0.5λproduce s decorrelation by aoooep6 pp. obse A”EE 49-2,Nw.19%7 The uch to Professo for suggesting the polarizatio epAdnatm seenwmg cering degree froe REFERENCES 020 has b He studied in aalb edp时

186 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 39, NO. 3, AUGUST 1990 polarization, with far fields given by A@)(8) and A(m)(8). It is plotted in Fig. 6 against the height of the elements above the groundplane. Inclusion of a sin 8 scalar element factor in (36)-( 38) does not alter the curve significantly, the main effect being a slight raising of the minima. Under the assumptions of the analysis (especially (34) and the scenario characteristics), any height greater than about 0.5 X produces decorrelation by elevation angle diversity. Any decorrelation by polarization alone must be observed in the absence of a ground plane for this type of polarization diversity antenna. ACKNOWLEDGMENT The author owes much to Professor J. Bach Andersen of Aalborg University, Denmark, for suggesting the polarization diversity measurements and for his assistance and encourage￾ment. The Danish Post and Telegraph Administrations and Storno AIS are gratefully acknowledged for providing base station facilities and a mobile radio unit for the measurements. REFERENCES S. A. Bergman and H. W. Arnold, “Polarization diversity in the portable communications environment,” in Proc. Nut. Radio Sci. Meet., Univ. Colorado, Boulder, 1986, p. 193. B. Glance and L. J. Greenstein, “Frequency selective fading effects in digital mobile radio with diversity combining,” IEEE Trans. Com￾mun., vol. COM-31, no. 9, pp. 1085-1094, 1983. S. Kozono, H. Tsumhara, and M. Sakamoto, “Base station polariza￾tion diversity reception for mobile radio,” IEEE Tmns. Veh. Tech￾nol., vol. VT-33, no. 4, pp. 301-306, 1984. W. C. Y. Lee and Y. S. Yeh, “Polarization diversity system for mobile radio,” IEEE Trans. Commun., vol. COM-20, no. 5, 1972. W. C. Y. Lee, Mobile Communications Engineering. New York: Wiley, 1981. J. R. Pierce and S. Stein, “Multiple diversity with nonindependent fading,” Proc. IRE, vol. 48, pp. 89-104, 1960. R. G. Vaughan and J. Bach Andersen, “A multiport patch antenna for mobile communications,” in Proc. 14th European Microwave Conj., Liege, Sept. 1984, pp. 607-612. - , “Antenna diversity in mobile communications,” in Pm. Nordic Seminar on Digital Land Mobile Radio Communications, Espoo, R. G. Vaughan, “Signals in mobile communications: A review,” IEEE Trans. Veh. Technol., vol. VT-35, no. 4, 1986. R. G. Vaughan and J. Bach Andersen, “Antenna diversity in mo￾bile communications,” IEEE Trans. Veh. Technol., vol. VT-36, pp. 149-172, Nov. 1987. Finland, kb. 1985, pp. 87-96. . Rodney G. Vaughan (M’82) received the B.E. and M.E. degrees in electrical engineering from the University of Canterbury, New Zealand, in 1976 and 1977, respectively, and the Ph.D. degree from Aalborg University, Denmark, in 1985. From 1977 to 1978 he was with the New Zealand Post Office working with toll traffic analysis and forecasting. Since 1979, he has been with the Physics and Engineering Laboratory, Department of Industrial and Scientific Research, New Zealand, developing a variety of computer-based industrial and scientific equipment. He studied in Aalborg from 1982 to 1985, as a recipient of a New Zealand government study award. His current interests include mobile and satellite communications, antenna arrays, and signal pro￾cessing. Dr. Vaughan is a member of the IEEE Antennas and Propagation and Vehicular Technology Societies