Revised version 04-09-2001 1

Experimental Investigation of the Joint Spatial and Polarisation Diversity forMIMO Radio Channel

Jean Philippe Kermoal1, Laurent Schumacher1, Frank Frederiksen2

Preben E. Mogensen1;2

Center for PersonKommunikation1

Niels Jernesvej 12, DK-9220 Aalborg, Denmarkfjpk,schum,pmg@cpk.auc.dk

Nokia Networks2

Niels Jernesvej 10, DK-9220 Aalborg, Denmarkfrank.frederiksen@nokia.com

AbstractThis paper presents analysis of MIMO radio channel mea-

surements. A description of both the measurement set-up andthe picocell environment is given. The correlation propertiesof the MIMO radio channel are investigated considering differ-ent diversity techniques such as spatial, polarisation and jointspatial and polarisation diversity techniques. It is shown that acombination of space and polarisation diversity is an attractivesolution for achieving compact MIMO implementation espe-cially when a large number of antenna ports is considered. Theissue of unbalanced Branch Power Ratio (BPR) is addressedand its influence on the power gain of the potential subchannelgenerated by the MIMO concept is analysed.

KeywordsMIMO radio channel measurement, joint spatial and polarisa-tion diversity, eigenanalysis.

1. IntroductionThe inherent demand for higher data rates is a topic of highattention in the research community. For some time now, theconcept of Multi-Input-Multi-Output (MIMO) has been underinvestigation as reported in the literature by [1],[2] among oth-ers and modelled by [3], [4] among others. Briefly, the MIMOstructure of the radio channel shows that a large capacity en-hancement can be achieved via parallel channelling. Only re-cently, experimental results have been presented in [5] [6] [7][8] [9].

This paper presents empirical results derived from MIMOradio channel measurements performed as part of the Euro-pean IST research project METRA (Multi Element Transmitand Receive antennas) [10]. The correlation properties of theMIMO radio channel is investigated considering different di-versity techniques such as spatial, polarisation and joint spatialand polarisation. The issue of unbalanced Branch Power Ratio(BPR) is addressed and its influence on the power gain of thepotential subchannel generated by the MIMO concept is anal-ysed. These results are derived from the measured complex nar-rowband information using the eigenvalue decomposition. Theuse of collocated polarised antenna is presented as a solutionto avoid the use of a large cumbersome antenna array for pic-ocell environment. Two case studies where a 4�4 MIMO set-up using single polarised antennas and a 4�4 set-up using a

combination of single polarised and dual polarised antennas arecompared.

The rest of the paper is organised as follows. The measure-ment set-up is described in Section 2. The environments wherethe measurement campaign was conducted are outlined in Sec-tion 3. Empirical results will be presented in Section 4 alongwith concluding remarks in Section 5.

2. The measurement set-upIn general, MIMO is understood as aM�N concept, whereMand N are the number of antenna ports at the base station (BS)and the mobile station (MS) respectively. A simplified sketchof a MIMO set-up is presented in Fig. 1, where the transmitter(Tx) at the MS is on the left side and the stationary receiver (Rx)located at the BS is on the right side. In this study, the measure-ment set-up provides experimental results whereM;N 2 [1; 4].

Figure 1: Illustration of the MIMO measurement configuration.

At the MS, an interleaved antenna array with 4 verticallypolarised sleeve dipoles, as one can see in Fig. 2, was movedalong a linear slide covered with a microwave absorber shieldover a distance of 11.8� over a duration of 5 s. Such antennaarray arrangement was used to reduce the mutual coupling ef-fect between the antenna ports. After post-processing, a lineararray1 is derived from it with a spacing of 0.4�. The Tx uses a1-to-4 switch with a switch interval of 50 �s between each an-tenna port of the antenna array, implementing a pseudo parallel

1Note that a mechanical artifact enabled the implementation of twoarrays at the MS. This eventually increases the statistical figure of thepropagation channel measurement campaign. Consequently the MSconsists of (4�2) vertical sleeve dipoles

Revised version 04-09-2001 2

transmission2 within 200 �s. Channel sounding measurementswere performed every 20 ms at a carrier frequency of 2.05 GHz(UMTS band) and a chip rate of 4.096 Mcps. The complex nar-rowband information was extracted from the wideband channeldata. More detailed descriptions of the MS and the testbed (i.e.Tx and Rx) are documented in [6] and [11] respectively.

The BS consisted of 2 sets of four parallel Rx channels,each connected to one specific antenna configuration. The firstconfiguration consisted of a uniform linear array with four ver-tically polarised sleeve dipoles with a spacing of 1.5�. The sec-ond set-up was composed of two dual polarised �45Æ patchantennas. Depending on the environment investigated, the an-tenna patches were orientated in a�45Æ or a +90Æ=0Æ position.Fig. 3 summarises the antenna configuration used in this paperwith respect to each measured environment.

interleavedantenna

array

microwaveabsorbers

Base Station

Mobile Station

Figure 2: MS with a microwave absorber shield covering thelinear slide during a measurement. The two antenna arraysat the BS can be seen at the background. A zoom on the MSpresents the interleaved antenna array used for MIMO radiochannel measurements.

3�

-45

3�

-45

+45

3�

Novi2

Nokia

Airport

MS BS (dipole) BS (patch)

Measuredenvironment

����

Antenna set-up

-45

Figure 3: Summary of the different antenna configurations.

3. Description of the investigatedenvironments

The results presented in this paper are extracted from measure-ment campaigns undertaken in different picocell environments.

2In this way, only one transmit antenna is active at a time, thus pro-viding isolation between the transmit antennas. Since the switching isrelatively fast and the environment stationary, it approximates a paralleltransmission for low mobile speeds.

The two main locations were the campus of Aalborg University(AAU) and the Aalborg International Airport (Denmark). Foreach environment several MS locations were selected to pro-vide a set of measurements where both line-of-Sight (LOS) andNon-LOS (NLOS) were present. Moreover, several BS loca-tions were selected in addition to the MS locations in order toincrease the statistical information of the environment. This isshown in Fig. 4 and Fig. 5 with the arrows representing the po-sitioning of the MS and the shaded area indicating the differentlocations of the BS.

Three of the measured environments characterised as pico-cell are presented in this study. The first measured environmentreferred to as NOVI2, provides an example of a building withseveral small offices on the same floor. The second environmentwhich is of the same size as NOVI2, denoted NOKIA illustratesa typical modern open office environment. The last measuredenvironment was the Aalborg International Airport, denoted inthis paper Airport, which provides a very large indoor openarea. Environment NOKIA and Airport are illustrated in Fig. 4and Fig. 5 respectively.

Figure 4: Example of the different positions of the BS and theMS for an open picocell environment (i.e. NOKIA). The arrowsrepresent the displacement of the MS (1 to 6). The 3 shaded ar-eas represent the different positions of the BS. In total 18 pairs(MS,BS) of measurement positions were investigated for this en-vironment.

Table 1 summarised the number of measurement positionswhich have been made during the measurement campaigns aswell as the mean signal-to-noise ratio (SNR) related to each en-vironment. Here the mean SNR is computed over all the mea-sured positions for each environment.

Environ-ment

Number Number Total meanof BS of MS number of SNR

positions positions positions in [dB]

NOVI2 3 7 21 31NOKIA 3 6 18 31Aiport 2 8 16 30

Table 1: Summary of the number of investigated environmentsand the mean SNR associated to each environment.

Revised version 04-09-2001 3

Figure 5: Example of the different positions of the BS and theMS for a very large open picocell environment (i.e. Airport). Intotal 16 pairs (MS,BS) of measurement positions were investi-gated for this environment.

4. Experimental results

In this section, the correlation properties of the MIMO radiochannels will be investigated for different BS antenna configu-rations. Direct comparison between single polarised and dualpolarised antenna configurations will be presented. The powercorrelation coefficient between antenna ports using different di-versity techniques such as space, polarisation and joint spatialand polarisation diversity will be analysed. In addition, issuesrelated to the BPR when dual polarised antennas are used willbe addressed. This will be presented first in a Single-Input-Multi-Output (SIMO) approach and then extended to a MIMOconfiguration to present the impact of the BPR. Following this,eigenvalue results will be presented when a 4�4 antenna con-figuration is considered. Finally, the power level of the differentsubchannels obtained in a MIMO perspective will be presentedfor all the picocell environments.

4.1. Power correlation coefficient

The power correlation coefficient j�j has previously been de-fined in [6]. In the rest of this paper, to keep the discussion sim-ple, j�j will be mentioned as the correlation coefficient. In thatrespect, when considering different diversity techniques, thenj�spacej is the spatial correlation coefficient associated to thespace diversity, j�xpolj is related to the cross polarisation cor-relation coefficient of a dual polarised antenna. Then finally,j�space+xpolj refers to the correlation coefficient which exists

between two antennas of different polarisation and separated bya certain spatial distance.

Fig. 6 introduces the scatter diagram of j�spacej versusj�xpolj for the environment NOVI2. These correlation coeffi-cient results are extracted from all the 21 measurement posi-tions performed in this environment. This correlation coeffi-cient is computed between two antenna ports at the BS and forall the 8 dipoles of the MS to increase the statistical amount ofdata.j�spacej is derived for two antenna ports separated with 1.5� and j�xpolj considers two collocated �45Æ dual polarised an-tenna ports. It can be seen that for the spatial domain, j�spacejexhibits very low values whereas for j�xpolj they are signifi-cantly higher.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

|ρspace

|

|ρxp

ol|

Figure 6: Scatter diagram of j�spacej versus j�xpolj for NOVI2.Decorrelation exists in the spatial domain. Higher values ofcorrelation are noticeable in the polarisation domain.

Fig. 7 represents the scatter diagram where j�space+xpolj isplotted from both a theoretical and empirical perspective. Theenvironment is the same as for Fig. 6. The theoretical values areobtained from the equation defined in [12] such that

j�space+xpolj = j�spacej � j�xpolj (1)

The diagonal line represents the case where the equation (1) isvalid. It can be seen that for very low correlation values this isvery sensitive and therefore values lower than 0.5, for instance,will be disregarded. It can be seen that for higher correlationvalues, the measurement data confirm the work of [12] for theinvestigated picocell environment.

4.2. Branch Power Ratio impact

It is well known that the correlation coefficient drastically im-pacts the performance of a MIMO scenario. It is known thatwhen the antenna branches are decorrelated, the MIMO config-uration provides significant power gain for each parallel sub-channel. However, it is only true for equal branch power. Thepresent study investigates the impact of the BPR on the eval-uation of the parallel subchannel of the MIMO radio channel.A SIMO configuration will be used as a first study case to un-derstand the propagation mechanism involved when the BPRvaries from a low value to a high value. Then a second studycase will be presented using a 2�2 MIMO configuration for thetwo BPRs.

Revised version 04-09-2001 4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Measured |ρspace+xpol

|

The

oret

ical

|ρsp

ace+

xpol

|

High sensitivity

Figure 7: Scatter diagram of the measured j�space+xpolj ver-sus the theoretical j�space+xpolj for NOVI2. Sensitiveness veryhigh for low correlation value. Good matching for higher cor-relation value.

Fig. 8 summarises the antenna configuration with their as-sociated BPR and correlation coefficient. The value of j�spacej,j�xpolj, j�space+xpolj and the BPR for the two examples illus-trated in Fig. 9 and Fig. 10 are averaged over the 8 antenna portsof the MS. It can be seen that the correlation values are very low.

BPR[dB]

Antenna set-upat the BS

�

-45

+450.9 0.22

+90

0

-8 0.16

-45

+451.4 0.23

+90

0

-8 0.12

+45+45

0.7 0.22

+90 +90

0.3 0.11

Environ-ment

NOKIA

Airport

Figure 8: Summary of the antenna configuration with their as-sociated BPR and correlation coefficient for a 1�2 scenariowhere the antenna port at the MS is vertically polarised.

Fig. 9 and Fig. 10 present the cumulative distribution (cdf)of the eigenvalue computed for a 1 � 2 antenna configuration.The analysis is performed using one vertically polarised dipoleat the MS and a dual polarised patch antenna at the BS. In thefirst case, the patch antenna is orientated such that it exhibits a�45Æ polarisation (Fig. 9). This is the situation for the mea-surement undertaken in NOKIA. In the second case, for theAirport environment, the patch is tilted of 45Æ in the elevationplane such that a +90Æ (vertical) and 0Æ (horizontal) polarisa-tion (Fig. 10) is achieved. The reason for selecting these twomeasurement positions is that they are both open picocells ex-

hibiting a similar 1� 1 channel cumulative distribution (almostRayleigh).

The cdfs are computed over a full slide run of the Tx forone measurement position of picocell NOKIA and Airport re-spectively. In order to increase the statistical figures, the cdfswere computed over all the 8 antenna ports of the MS. The cdfsare normalized to the strongest 1� 1 radio channel.

Fig. 9 illustrates clearly that the power gain from using twoantennas spatially separated but with a low BPR is the samewhether the +45Æ or the �45Æ antenna port is used. Obviously,there is no gain added to the space diversity technique by us-ing polarisation diversity since j�spacej is low. Fig. 10, on theother hand, demonstrates an imbalance in the power gain avail-able depending on which branch used. On the lowest branch(0Æ), a gain is noticeable due to the use of polarisation diversitybut remains negligible compared to the power gain of the 90Æ

branch.

−20 −15 −10 −5 0 5 10 15−2

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

1x1

space: +45o space: −45o xpol: +/−45o

space+xpol

Figure 9: Illustration of the influence of the BPR on the powergain for a 1�2 SIMO configuration: �45Æ.

−20 −15 −10 −5 0 5 10 15−2

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

1x1

space: +90o space: 0o xpol: +/−90o

space+xpol

Figure 10: Illustration of the influence of the BPR on the powergain for a 1�2 SIMO configuration: +90Æ and 0Æ. Note theimbalance in the power gain compared to Fig. 9

Fig. 11 and Fig. 12 present a MIMO approach to the pre-

Revised version 04-09-2001 5

vious SIMO analysis. A 2�2 antenna set-up is considered.Here, two vertically polarised dipoles are considered at the MS.The same antenna topology is used at the BS as for the SIMOanalysis. The eigenvalues are normalized to the strongest 1�1channel. One can see in Fig. 11 that the two eigenvalues areoptimized since the BPR is low while when the BPR is muchlarger (-8 dB) the imbalance exhibited in the SIMO case is stillpresent as shown in Fig. 12. This clearly highlights the factthat although the correlation value exhibits strong decorrela-tion no optimum MIMO configuration can be achieved whenthe branch power is unbalanced. Therefore, it is recommendedto use an antenna arrangement such that both the BPR and thecorrelation coefficient are as low as possible when consideringimplementing a MIMO configuration.

−20 −15 −10 −5 0 5 10 15−2

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

λ1

λ2

1x1

space: +45o space: −45o xpol: +/−45o

space+xpol

Figure 11: Illustration of the influence of the BPR on the powergain for a 2�2 MIMO configuration: �45Æ.

−20 −15 −10 −5 0 5 10 15−2

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

λ1

λ2

1x1

space: +90o space: 0o xpol: +/−90o

space+xpol

Figure 12: Illustration of the influence of the BPR on the eigen-value for a 2�2 MIMO configuration: +90Æ and 0Æ.

4.3. Impact of the joint spatial and polarisation diversity

A major drawback in employing space diversity is usually theactual physical size of the antenna array implemented. Conse-quently, one may see the use of space diversity as a cumbersome

tool when implementing MIMO set-up in picocell environment.It will be shown that the use of collocated dual polarised antennacould solve this issue. Two 4�4 MIMO scenarios are presentedhere. These two set-ups differ in the type of antenna configura-tion used at the BS. The first scenario considers 2 dual polarised�45Æ patch antennas (with a low BPR) and the second scenariois implemented using 4 vertically polarised sleeve dipoles. Atthe MS, 4 vertically polarised dipoles are considered. The envi-ronment is the same as the one used in Fig. 11 (i.e. NOKIA).

−20 −15 −10 −5 0 5 10 15−2

−1.8

−1.6

−1.4

−1.2

−1

−0.8

−0.6

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−0.2

0

λ1

λ2λ

3

λ4

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

1x1

Figure 13: Eigenvalue MIMO result for a 4�4 antenna config-uration using 4 dipoles at the MS and 2 dual polarised �45Æ

patches. 4 eigenvalues can be identified, 2 of them being signif-icant (P>0 dB)

−20 −15 −10 −5 0 5 10 15−2

−1.8

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−1

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0

λ1

λ2

λ3

λ4

Eigenvalue [dB]

log

P(e

igen

valu

e<ab

scis

sa)

1x1

Figure 14: Eigenvalue MIMO result for a 4�4 antenna configu-ration using 4 dipoles at the MS and 4 dipoles at the BS. 4 eigen-values can be identified, 2 of them being significant (P>0 dB).Very similar behaviour to Fig. 13.

The first and the second MIMO scenario performances areillustrated in terms of the cdf of their eigenvalues in Fig. 13and Fig. 14 respectively. It can be seen when comparing bothfigures, that they exhibit very similar behaviour. Four eigenval-ues can be identified, two of them being significant (P>0 dB).This enables to conclude that using four dipoles or two dual

Revised version 04-09-2001 6

polarised patch antennas will provide the equivalent results interms of subchannel gain in the MIMO radio channel. The ad-vantage of using the joint spatial and polarisation solution is thepossibility of using a more compact antenna set-up when largea number of antenna ports is considered.

An outage level of 10% is typically a good measure for sys-tem level performance. The power gain of each subchannel isextracted from this threshold and cumulated over all the mea-surement positions of the two picocell environments NOVI2and NOKIA. The cdf of the power gain of each subchannel ispresented in Fig. 15. It can be seen that the first two subchan-nels offer little variance in the power gain for all the variousmeasurement positions for the two investigated picocells. Forthe weaker eigenvalues, the slope is less steep, however theyhave practically no significant impact on the general power gain.This indicates that wherever a BS is considered to be placed,one can expect to have almost identical power gain.

−20 −15 −10 −5 0 5 10 15−1

−0.9

−0.8

−0.7

−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0

λ1

λ2

λ3

λ4

Power gain of the subchannel [dB]

log

P(p

ower

gai

n<ab

scis

sa)

Figure 15: Power gain of the parallel subchannels for4(dipoles)�2(�45Æ patch) MIMO configuration. The powergain is extracted from the 10% outage level of the eigenvalue(Fig. 13) for each measurement position for the two picocellenvironments using the �45Æ patch orientation. Note the littlevariance of the 2 significant parallel subchannels (P>0 dB)

5. Conclusions

This paper presented MIMO radio channel measurements aspart of the European IST project METRA. The measurementset-up and the investigated picocell environments were de-scribed. The investigation of space, polarisation and joint polar-isation diversity is presented. It was shown that a combinationof space and polarisation diversity is an attractive solution forcompact MIMO implementation especially when a large num-ber of antenna ports is considered. This was shown to be a veryrobust solution too, since the power gain of the significant par-allel subchannels exhibited a little variance. This indicates thatfor the investigated picocell environments wherever the BS isconsidered to be placed the power gains remain almost identi-cal. However, it was outlined that it is necessary to have equalBPR and decorrelated antenna branches to achieve an optimumimplementation of the MIMO concept.

6. References[1] G.J. Foschini, “Layered Space-Time Architecture for

Wireless Communication in Fading Environment WhenUsing Multi-Element Antennas”, Bell Labs TechnicalJournal, pp. 41-59, Autumn 1996.

[2] J. Bach Andersen, “Array Gain and Capacity forKnown Random Channels with Multiple Element Ar-rays at Both Ends”, IEEE Journal on Selected Areasin Communications-Wireless Communication Series, vol.18, no. 11, pp. 2172-2178, November 2000.

[3] K.I. Pedersen, J.B. Andersen, J.P. Kermoal and P.E. Mo-gensen, ”A Stochastic Multiple-Input Multiple-OutputRadio Channel Model for Evaluation of Space-Time Cod-ing Algorithms”, Proceedings of VTC 2000 Fall, pp. 893-897, Boston, United States, September 2000.

[4] Lucent Technologies,“Further link level results for HS-DPA using multiple antennas”, 3GPP TSG RAN WG1,TSGR1#17(00)1386, Stockholm, Sweden, November2000.

[5] J.P. Kermoal, P.E. Mogensen, S.H. Jensen, J.B. Ander-sen, F. Frederiksen, T. B. Sørensen, K.I. Pedersen, “Ex-perimental Investigation of Multipath Richness for Multi-Element Transmit and Receive Antenna Arrays”, IEEEProc. Vehicular Technology Conference, pp. 2004-2008,Tokyo, Japan, May 2000.

[6] J.P. Kermoal, L.Schumacher, P.E. Mogensen, K.I. Peder-sen, “Experimental Investigation of Correlation Propertiesof MIMO Radio Channels for Indoor Picocell Scenarios”,IEEE Proc. Vehicular Technology Conference, pp. 14-21,Boston, Massachusetts, September 2000.

[7] L. Schumacher, J.P. Kermoal, F. Frederiksen, K.I. Peder-sen, A. Algans, P.E. Mogensen,“MIMO Channel Charac-terisation”,IST Project IST-1999-11729 METRA Deliver-able 2, February 2001.

[8] M. Steinbauer, A. F. Molisch, A. Burr, R. Thomae,“MIMO channel capacity based on measurement results”,Proc. ECWT 2000, pp. 52-55, Paris, France, 5-6 October2000.

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[10] http://www.ist-metra.org

[11] F. Frederiksen, P. Mogensen, K.I. Pedersen, P. Leth-Espensen, “A “Software” Testbed for Performance Evalu-ation of Adaptive Antennas in FH GSM and Wideband-CDMA”, Conference Proceeding of the 3rd ACTS Mo-bile Communication Summit, Vol.2, pp. 430-435, Rhodes,Greece, June 8-11 1998.

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