UWB Spatial Multiplexing by Multiple Antennas andRAKE Decorrelation

Van Phuong TRAN and Alain SIBILLEUnite Electronique et Informatique (UEI)

Ecole Nationale Superieure de Techniques Avancees (ENSTA)Paris, France

van-phuong.tran(i)ensta.fr and alain.sibille(i)ensta.fr

Abstract-An Ultra Wide Band (UWB) spatial multiplexingtechnique by Multiple Elements Antennas (MEA) at both ends ofthe link or only at the emitter side using biconical antennas ispresented and analyzed in this paper from measured radiochannels. This work evaluates the performance of thedemultiplexing process at the receiver side, as ensured bymultiple parallel RAKE receivers in order to achieve channeldecorrelation efficiently. The quality of signals restitution, thesubchannels isolation caused by the selective RAKE combinersand the quality of the communication links are evaluated by anormalized similarity coefficient, an inter-channel interferenceslevel ratio and a Bit Error Rate (BER) curve respectively. Itappears that closely spaced antennas can give good performancesin complex multipath environments.

Biconical Antennas, MIMO Systems, Mutual Coupling, RAKEReceivers, Spatial Multiplexing, UWB Communications

I. INTRODUCTIONAccording to the Federal Communications Commission

(FCC), one speaks of Ultra Wide Band (UWB) technologywhen the transmitted signals occupy a frequency band greaterthan 20 % of the central frequency or more than 500 MIHz [1].This large spectral band can combat the multipath effects of theradio channel. In order to satisfy the increasing demand of highdata rate, UWB communication systems have to be able to holda big quantity of information as quickly as possible. Amultiplexing technique in spatial domain is one possiblesolution. Multi Element Antennas (MEA) have to be installedat both emission and reception side, leading to a Multiple InputMultiple Output (MIMO) system of several parallelsubchannels [2]. Indeed, the multipath characteristic of theradio channel is exploited, i.e. each path is assigned as asubchannel. MEA installation can be inconvenient for smallhandheld devices. So, a partial Multiple Input Single Output(MISO) solution is also investigated, where only one antenna isinstalled at the receiver side. From the works of Weisenhomand Hirt [3], it appears possible to successfully receivesequences of independent symbol transmitted simultaneouslyby each emission antenna. In this paper, we try to see if thediscrimination between the different transmitted signals isadequately ensured by several parallel RAKE receivers ofreasonable complexity [4], for measured channels employingbiconical antennas which are well known to radiate on asufficient bandwidth according to FCC requirements [5]. The

communication scheme is introduced in the next section. Then,some adequate quantities are defined to evaluate theperformances of the communication scheme. The results willbe analyzed from two measured channels corresponding to aLine Of Sight (LOS) and a Non Line Of Sight (NLOS)propagation environment. Finally, we will conclude about thepotentiality of this multiplexing/demultiplexing technique.

II. SYSTEM DESCRIPTIONThe multiplexing architectures principle involving RAKE

combiners are described by Figure 1 and Figure 2. In the firstscheme, the propagation environment is characterized by theChannel Impulse Responses (CIR) H,(t) relating the emissionantenna indexed by i (i = { 1, , NT}) to the reception antenna.NT is the number of emission antennas. The emission antennasspaced by a distance d are assumed to radiate independent dataflows in parallel using an identical FCC compliant transmittedreference pulse waveform Prad(t) (Figure 3 and Figure 4).

W- A< . Hi(t) KE I

HY2(0)2(t) ~ RAKE2

Radiated PulsePra(t)

Rac

DemultiplexedSignals

Figure 1. MISO 2x1 Configuration

z2(t)

4-> . [.I!(t). - . D3 -

.S '.

+< 2HI(t) -

diated PulsePrad(t) Z,(t) Demultiplexed

Signals

Figure 2. MIMO 2x2 Configuration

0-7803-9206-X/05/$20.00 ©2005 IEEE

272

Z(t)

The reception antenna captures a superposition Z(t) of thevarious transmitted signals affected by the radio channel.

NTZ(t)=Si (t) with Si(t)=Hi (t)*Pr,, (t) (1)

i=l

* designates the convolution product. Pre(t) is the receivedreference pulse waveform which is taken as the primitive ofP,.,d(t) in order to materialize the integrating behaviour ofantennas in recption mode [6].

Pr (t)= frad (t)dt (2)

The discrimination between the different signals is ensuredby NT parallel RAKE receivers. The parameters of each RAKEstage i are adjusted in order to combine received signalsconstructively according to the corresponding CIR Hi(t). Theemitted signals are expected to be retrieved on each RAKEoutput Yi(t).

decorrelation thanks to path diversity. In the second scheme(Figure 2), NT = NR antennas are employed at both sides of thelink. The CIR Hi(t) relate the emission antenna i to thereception antenna i. Each RAKE stage i is also configuredaccording to each CIR Hi(t). Introducing MEA at the receptioncan enhance the robustness ofthe link.

III. MEASUREMENT PROCEDUREA LOS and a NLOS scenarios of propagation are

investigated to evaluate the performance of the communicationscheme (Figure 5 and Figure 6). The LOS scenarios arecommonly not favorable for MIMO communications but wecan expect to fully exploit the reflected path although the LOSpath is dominant. In the NLOS scenario, the two entities arehidden by a common wall. Very complex CIR have to beexpected resulting in many paths. The latter feature is favorablefor spatial multiplexing.

Figure 5. LOS Scenario

Tine (OF)

Figure 3. Reference Pulse Waveforms (Time Domain)

-1

Figure 4. Reference Pulse Waveforms (Frequency Domain)

It is clear that this multiplexing technique is efficient only ifthe diversity at the emitter side is sufficient, the elementantennas have to be sufficiently spaced i.e. the CIR have to beas different as possible. Indeed, all RAKE combiners must nothave the same parameters, so that the differing data flows couldbe extracted. The propagation environment also has to be"rich" (many paths), in order to guarantee a strong channel

Figure 6. NLOS Scenario

The measurements took place at night, in order to avoid anyhuman influence. They were realized in the frequency domain,with a frequency band from 2 GHz to 10 GHz. In a system ofNT antennas at the transmitter side and NR antennas at thereceiver side, NTXNR frequency responses were thus acquiredby a Vector Network Analyzer (VNA) (Figure 7). Theswitching between the different antennas was implemented bytwo identical RF switches. At the transmitter side, a poweramplifier with a 40 dB gain is used. At the receiver side, thereis a Low Noise Amplifier (LNA) with a 10 dB gain. The CIRwere calculated by Invert Fast Fourier Transform (IFFT) with asuitable windowing. The receiving array was fixed on a 3D

273

computer-controlled positioning device with less than 100 ,umresolution. In the present experiments, the array was movedover a horizontal X/Y grid, in order to obtain a full space-variant characterization of the channel matrix. The spatialsampling was chosen to be every 40 mm, for a total of 11positions per axis (121 spatial locations). This corresponds to asquare grid of dimension equal to 400 mm.

PowerAmplifier

Transmitter

*B2

Receiver

Propagation Environment

C,- -C2% I I% I RF Switch%I,

Low NoiseAmplifier

D __

Figure 7. Measurement Set-up

The VNA was calibrated in the planes A and D. Thecalibration type being "RESPONSE & ISOLATION", only thecomplex wave transmission coefficient S2, was measured inthese planes. The transmission coefficients from plane A toeach plane Bi and from each plane Ci to plane D were alsomeasured. Thus, the coefficient corresponding to the radiochannel including the antennas effects could be extracted.

function A(t) and B(t). Ideally of course Fi should be equal to1. Another issue is to ensure no crosstalk between the parallelmultiplexed data flows. For this purpose, another quantity isnow introduced in order to measure the isolation ratio (i.e.inter-channel interference ratio) between the varioussubchannels:

2

I7PrXik (Tl,i )

YPrOCxLi (Tl,i )2(6)

Ii,k (i # k) is the ratio between the power induced on theneighbouring subchannel k intended to the subchannel i and thepower delivered by its nominal RAKE receiver. X1,k(t) denotesthe received signal Si(t) from the antenna i in the case of asingle emission antenna affected by the RAKE processingappropriate for channel k:

n

Xi,k ( t)= aj,kSi (t-TI ,k +Tj,k )j=l

(7)

Tl, is the delay introduced in the first branch of the nominalRAKE receiver and is also the instant where -yP x (T)| is

maximal. We indeed consider that each receiver reconstitutesits nominal digital information sequence by a specificallysynchronized detection. The quality of the communications isevaluated through Bit Error Rate (BER) curves as functions ofthe well known Signal to Noise Ratio (SNR) and takingaccount the parasitic collected power from the neighbouringsubchannels. A Binary Phase Shift Keying (BPSK) modulationwill be employed. It means a radiated signal +Prm(t) by theemission antenna i is associated to a digit B = I and a radiatedsignal -Prad(t) to a digit Bi = 0. The received digits B1 arecomputed for several SNR values by:

BU[ Uif x0 a,Y ( T[,j ) +N]U[x]-l if x>0 and U[x]=0 if x<0

IV. PERFORMANCES EVALUATIONInsight is provided by the evaluation of the quality of the

demultiplexing process through a normalized similaritycoefficient Fi between a really demultiplexed signal Yi(t) andthat which would be received for single isolated emissionantennas Xjj(t).

= max[yxi.y (T)] (4)jmax[ Tx)()lmaxB[yjy (T)]

n

Xi j (t)=aj,iSi (t-Tl,ij ej,i ) (S)j=1

n is the number of RAKE fingers. a, and j, are theintroduced weight and delay of the finger j of the RAKEmodule i. YAB(T) designates the cross-correlation between the

(8)

(9)

N being a noise realization. The statistic properties of Nis a centered Gaussian with a standard deviation equal to:

aN = 02

(10)

where No is the noise power which is obtained by the ratioof the mean received power after RAKE combining on theSNR. The channel will be normalized to keep the same totalemitted power whatever the number of emission antennas NT.Only the basic case of 2 elements MEA is discussed for thesimplicity of analysis. The number of RAKE fingers will befixed to n= 10. The Equal Gain Combining (EGC) algorithm isemployed meaning all weights have the same amplitude but thesigns are adjusted to combine the received signalsconstructively after a temporal synchronization. The elementantennas are spaced by d = 4 or 20 cm corresponding to astrong correlation/coupling and a low correlation coupling. TheCumulative Density Function (CDF) of the similarity

274

coefficient and the isolation ratio are plotted in Figure 8 andFigure 9 for the MISO and MIMO architectures in the LOSpropagation environment. We can state that the similarity isbetter if the antennas are closely spaced but the isolation isworse. This can be explained by a great similarity of the CIR.At the reception, the different signals are superposed withoutdistortion. The worse isolation is due to strong electromagneticinteractions between the element antennas as well known asMutual Coupling (MC) in the antenna community.

-------I ---------------- ------- .....

M-ISO 2xl d- '0 cm----- AISO 2xl - d 4 cnf

0. 8 -....----. SmI-O---2 --- --t - ---

b - -. --- ThIO' x2 - d 24 cmiT

ML02.x-d in

Smlrit Coefiiet

0A ---------------------------------------------------

1~~~~~~~~~~~~~~~~0 Sl L.- t40 !V 60 , so 90 100

Correlation Coefficeialt (0,)

Figure 8. CDF of Similarity Coefficient (LOS Scenario)

ISO 2xl - (1I= 20 cmi----- MISO 2x1 - d = 4 cm

0.8 . fmKIO '2 d= ?Occ----AMIM02x-2 -d 4cenif

2 06 -- -- --04' F ;C ' {

v A -- --

Isolatioi ( B)

Figure 9. CDF of Isolation Ratio (LOS Scenario)

100

-10o

10p:;1

X -3. 10

lo-4

-10 -s 0 5 10

Sipial to Noise Ratio (cB)15 20

Figure 10. BER Curves (LOS Scenario)

The MIMO configuration gives better performances in termof isolation. MC at the receiver side can also create morechannel diversity resulting in pattem diversity. It gives worseperformance in term of similarity except if the antennas arelargely spaced. The pattem diversity is ensured at the receiverside but there is more signal distortion induced by the antennasbehavior. Looking at the BER curves (Figure 10), we canobserve a right shift compared to the basic reference SingleInput Single Output (SISO) case which is obtained by:

BER=2erfc( SNR (11)

Where erfc( is the complementary error function. The BERwith the MISO and MIMO systems are approximately thesame. Large antenna spacing without MC at reception ispreferable. This configuration can be viewed as two parallelindependent SISO systems. The co-channel interferences haveno detrimental effects because of the array orientation.

COrTelation Coefficient (0O)

Figure 11. CDF of Similarity Coefficient (NLOS Scenario)

Isolation (dB)

Figure 12. CDF of Isolation Ratio (NLOS Scenario)

The CDF of the similarity coefficient and the isolation ratiofor the NLOS environment are plotted in Figure and Figure12. The effects of the inter-antennas distance d are less visiblethan for the LOS scenario. A large spacing clearly gives thebest performance both in similarity and isolation. The CIR are

fully decorrelated (complex environment) and the MC is weak.With closely spaced antennas, the MISO configuration is

275

-; zz : zz :;: z :: ; : :: : : : : ; : : :: ::: :: ;: : : "-.- ---- ---- ----

-- - -------r-----------r--~------'T- ---------T ------------------------_-__----_-__-_--_-__-_

--------------------

:::::r.::::: :::x:::::----l~~~~~~~~~l~~----------

--*--~~ ~~ ~~

----'------------------- ---4----T----'----4----- ---------------------''------' ---'-------, ------\------ - - T---------T------------

:::::::::t:::::::: :::::: : S:::: --- - ---- --- --*--- -- -- - ----

.. ......____________L_...__2- _d __.___=20______cm_'--'---.------- ---------- -----------------T-----

preferred. MC at the receiver side introduced more signaldistortion and more induced power. It was not the case for theLOS scenario because the number of paths is very greater here.Concerning the BER, it appear the best results with MIMOconfiguration and small antennas spacing. MC seems to easilydecorrelate the CIR.

10°

o-110

o2

1010 -IPdSO2xl - d = 20 cm - --*---

- ISO2xl-d=4cm - ------------

NIMIO 2x2 -d20cmn~-*----M----XIMO2x2- d=24 Clllij --_-10 0 x2-0 d 4 c 1

- SISO ---------------------

-10 - F 0 10 15 2?0SiPeal to Noise Rafio (dB)

Figure 13. BER Curves (NLOS Scenario)

The MIMO configuration with d = 20 cm is worse than theMISO configuration with the same inter-antenna spacing. Thisis probably due to unequal received power between the twosubchannels characterized by H,(t) and H2(t). Indeed, thedistance between the emission antenna 2 to the receive antenna2 is clearly greater than the distance between each antenna 1 inthe measurement set-up.

V. CONCLUSIONA multiplexing/demultiplexing technique in the spatial

domain employing MEA and RAKE receivers was presented,and its performance in tenns of demultiplexing quality,subchannels isolation and communication quality was analyzedfrom realistic measured radio channels. In a LOS environment,small antenna spacings lead to the best signal restitution butmultiplexing is not possible due to bad subchannels isolation.In "rich" multipath environment, largely spaced antennas givegood performances in terms of similarity and isolation.

REFERENCES

[1] Federal Communications Commission (FCC), "Revision of Part 15 ofthe Commission's Rules Regarding Ultra-Wideband TransmissionSystems", Notice of Proposed Rules Making (NPRM), ET-Docket 98-153

[2] J.B. Andersen, "Antenna Arrays in Mobile Communications: Gain,Diversity, and Channel Capacity", IEEE Antennas and PropagationMagazine, Vol. 42, No. 2, pp. 12-16, April 2000

[3] M. Weisenhom and W. Hirt, "Performance of Binary AntipodalSignaling over the Indoor UWB MIMO Channel", IEEE IntemationalConference on Communications (ICC), Vol. 4, pp. 2872-2878, 11-15May 2003, Anchorage (USA)

[4] R. Price and P.E. Green, "A Communication Technique for MultipathChannels", Proceedings ofthe IRE, Vol. 46, pp. 555-570, March 1958

[5] H. Ghannoum, S. Bories, C. Roblin and A. Sibille, "Biconical Antennasfor Intrinsic Characterization of the UWB Channel", IntemationalWorkshop on Antenna Technology (IWAT), pp. 101-104, 7-9 March2005, Singapore

[6] C. Roblin, S. Bories and A. Sibille, "Characterization Tools of Antennasin the Time Domain", International Workshop on Ultra Wide BandSystem (IWUWBS), 2-5 June 2003, Oulu (Finland)

276