ADAPTIVE MULTIBAND ARRAYFOR CANCELLING CO-CHANNEL INTERFERENCE AND IMAGE-BAND

INTERFERENCE

Masahiro Tamaoki(1), Satoshi Denno(1), Tatsuo Furuno(2) and Masahiro Morikura(1)

(1)Graduate School of Informatics, Kyoto-UniversityYoshida-Hommachi, Sakyo-ku, 606-8501 Kyoto, Japan

(2)Research Laboratories, NTT DOCOMO, INC.Hikarino-Oka 239-8536, Yokosuka-shi, Kanagawa, Japan

Abstract—We propose an architecture which reduce the circuitsize to concurrently cancel co-channel interference and image-band interference. We adopt analogue Hilbert transformers tocancel image-band interference and adaptive array antennas tocancel co-channel interference. Furthermore, we can miniaturizethe receiver by mixing all signals at the IF stage. We also showperformance of the proposed system by computer simulation. Asa result, image-band interference caused by some imperfectionof Hilbert transformer can be completely canceled. Moreover,co-channel interference can be simultaneously canceled.

I. INTRODUCTION

Various radio transmission systems have recently been com-mercialized. Basically, each of these systems uses differentfrequency band. Hence, frequency bands for new radio systemsbecome limited. Therefore, it is necessary to increase thefrequency utilization efficiency of radio systems as much aspossible. For this purpose, mobile transmission systems adoptcellular concepts. In cellular systems, a signal transmitted froma cell can be propagated to another cell. If the two cells usethe same frequency, the transmitted signal turn out to interferewith signals from the other cell and transmission performanceis impaired. Moreover, when several radio systems are allowedto share the same frequency band such as ISM (Industrial,Scientific and Medical) band, the interference becomes moreserious. Hence, various methods for suppressing co-channelinterference have been investigated. One of effective schemesis an adaptive array antenna that points null to undesiredsignals. Since different radio systems have different frequencybands, a radio frequency circuit dedicated to one radio systemusually differs from that dedicated to another system. Inparticular, because some RF (Radio Frequency) devices in theterminal are carefully selected to optimize the transmissionperformance in terms of quality of communication and powerconsumption, the RF devices for one system are definitelydifferent from those for another system. As is well known,various mobile systems are currently in use, hence, usersrequire multiband/mode transmitters to gain access to thesesystems. However, a multiband/mode transmitter has as manyRF chains as radio systems which multiband/mode transmit-ter has to communicate for the reason described above. Toovercome this problem, multimode/band receiver that enableone RF chain to receive some frequency band is important inthe field of mobile communication[1][2][3]. However, when

heterodyne reception is applied to multimode reception with-out RF-BPF (RF-Band Pass Filter), transmission performanceis impaired owing to image-band interference. However, inprinciple this interference can be cancelled by introducing aHilbert transformer at the RF stage[4]. Though this methodhas been experimentally confirmed to suppress interference byapproximately 40 dB[5] or 62 dB[6], it is not sufficiently ef-ficient to replace conventional single-mode receivers. Further,because the performance of RF devices depends on the RFfrequency, heterodyne receivers implemented with RF devicescannot always guarantee the desired level of performance.Hence, some researchers have proposed an adaptive image-band canceller for multiband/mode receivers[7][8][9].

This paper proposes a new heterodyne receiver with anarray antenna for enabling multimode communication. Inthis paper, we propose a receiver for canceling co-channelinterference and image-band interference simultaneously witha linear adaptive filter based on MMSE (Minimum MeanSquare Error)[10]. Moreover, we attempt to miniaturize thecircuit scale of the array antenna by using a receiver for mixingof signals at the IF (Intermediate Frequency) stage.

II. SYSTEM MODEL

Our proposed receiver is shown in Fig 1.

Fig. 1. Configuration of the proposed receiver

At the RF stage, each signal received from the arrayantenna is amplified by LNA(Low Noise Amplifier). Then,these signals are converted into IF band analytical signals (I/Q

978-1-4244-2517-4/09/$20.00 ©2009 IEEE 1

signals) by the Hilbert transformer. However, since the Hilberttransformer usually comprises analogue devices, it suffersfrom imperfections in amplitude, γi, and phase, δi between in-phase (I) and quadrature-phase (Q) at the i-th antenna. Then,at the IF stage, the weight of the i-th antenna, which controlsdirectivity of the array antenna and compensates for theimperfections of the Hilbert transformer, W (i) is multiplied.Then, all the signals are mixed and down-converted by a CDC(Complex Down Converter). Finally, at the baseband stage, thedesired signal is extracted by a LPF (Low Pass Filter). And,the adaptive weight controller requires baseband signals thatbypass the weight circuit from the reasons explaining at II-B.

A. Multiband Array

Now, let θ(n) denote the data where n represents time. Forexample, the PSK (Phase Shift Keying)-modulated transmis-sion signal d(t) is represented as,

d(t) =∞∑

n=−∞cos(ωRFt + θD(n)). (1)

The signal received by the i-th antenna, which is denotedby y(i)(t), is written as

y(i)(t) =M∑

m=1

τ−1∑l=0

hi,lRe[d(t − lτ)ejωmt

]+ n(t)

=M∑

m=1

τ−1∑l=0

hi,l cos(ωm(t − lτ) + θm(t − lτ)) + n(t),

(2)

where j, M , τ , n(t), ωm, θm, and hi,l represent the imag-inary unit, the number of signals received at the receiver,the number of paths in the radio channel, AWGN (AdditiveWhite Gaussian Noise), carrier frequency, phase conveyinginformation, and the channel impulse response between thetransmitter and the i-th antenna, respectively. In addition,Re[ ] is a function for extracting only the real part froma complex number. Although conventional receivers extractonly signal predetermined by RF-BPF, which has been con-sidered indispensable for signal-mode reception, the RF-BPFcannot be implemented in multiband receivers this is becausemultiband receivers can dynamically extract the desired signalfrom the received signals. When the received signal is down-converted to IF signals, as is described above, the signals invarious frequency bands are possibly mixed with the desiredsignal. At the IF stage, the IF-BPF cannot distinguish positivefrequency from negative frequency. The desired signal in theωRF + ωIF band is down-converted to ωIF, while the signalon the ωRF − ωIF band (called image-band in this paper) isdown-converted to −ωIF. As a result, the image-band signalcauses interference. Therefore, by transforming the receivedsignals into an analytical form, image-band interference canbe suppressed. In other words, the Hilbert transformer possiblyhelps to cancel the interference in the receiver. In principle,the Hilbert transformer has to be set at the RF stage of thereceiver. After the received signals defined in (2) are directly

fed to Hilbert transformer with down-conversion, analyticalsignals composed of real and imaginary signals are obtained.Subsequently, the real and imaginary signals are fed to an IF-BPF to remove undesired signals such as higher harmonicsand interference signals. The imaginary and real outputs ofthe Hilbert transformer at the i-th antenna, r

(i)I and r

(i)Q , are

written as follows:

R(t) =

(r(i)I (t)

r(i)Q (t)

)= BPF

[y(i)(t)γi cos((ωIF)t)

y(i)(t) sin((ωIF)t + δi)

].

(3)

BPF[ ] is a function for rejecting all signals except those inthe ωIF band. Then, the weight matrix W (i) is multiplied bythe output signals of the Hilbert transformer. At the end ofthe IF stage, the outputs of all the antenna are mixed andprovided to the CDC. If the CDC transforms IF signals inthe ωIF band to the baseband, IF signals in the −ωIF bandare up-converted to −2ωIF. Therefore, the LPF placed afterCDC can completely reject the image-band signals if theHilbert Transformer is ideal. However, since the performanceof the Hilbert transformer implemented with an RF devicedepends on the frequency of the signals, it is impossible tokeep the performance over any RF on the multimode receiver.This imperfection of the Hilbert transformer degrades thetransmission performance. Thus, the output of LPF, denotedby Z(t), is written as

Z(t) = LPF

[T C ·

∑i

W (i) ·(

r(i)I (t)

r(i)Q (t)

)](4)

T C =(

cos ωIFt sin ωIFt− sin ωIFt cos ωIFt

)(5)

W (i) =

(w

(i)11 w

(i)12

w(i)21 w

(i)22

), (6)

where T C represents the matrix of complex frequency con-version. In addition, LPF[ ] is a function for extracting onlythe baseband signal. Calculating (4), (5), and (6), they can berewritten as

Z =∑

i

{S(i) · W (i)

V

}(7)

W(i)V =

(w

(i)00 w

(i)01 w

(i)10 w

(i)11

)T

(8)

S(i) =

(s(i)1 s

(i)2 s

(i)3 s

(i)4

−s(i)3 −s

(i)4 s

(i)1 s

(i)2

), (9)

where the superscript T indicates transpose of the matrix.Hence, the received signal is represented as a linear vector.

B. Adaptive Algorithm for Weight W (i)

In proposed system, the adaptive weight W (i) is es-timated based on MMSE criterion. If a signal vec-tor is defined as D(n) = (Re[d(n)] Im[d(n)])T,S(n) = (S(1)(n) . . . S(M)(n)), and W V(n) =

2

(W (1)V (n) . . . W

(M)V (n))T, the cost function based on

MMSE is expressed by the following:

J = E[|D(n) − S(n) · W V(n − 1)|2] −→ minimize.

(10)

Im[ ] is a function for extracting only the imaginary part of acomplex number. Based on this criterion, W (i) is generatedby RLS algorithm.

C. Down Conversion of Received Signals Used for the Adap-tive Algorithm

In this section, we explain the method of generating controlsignals for adaptive control and miniaturizing the circuit onthe control side. We propose a receiver structure that mixesall the signal at the IF stage. However, in order to generatecontrol signals, the receiver needs signals which bypass theweight circuit, matrix S. It needs receiver chains as many asantenna and so circuit becomes huge. To avoid this, in ourproposed system, mixing of the control signal is performedafter spreading at the IF stage, and the mixed signal isdown-converted. By despreading the mixed signal the desiredsignals, matrix S(i) is separated at the baseband.

In our proposed system, when the outputs of M antennas atthe IF stage are spread and mixed, the output of the spreaderdenoted as X(t) is written as

X(t) =M∑i=1

∞∑n=0

(c(i)11 (t − nT ) c

(i)12 (t − nT )

c(i)21 (t − nT ) c

(i)22 (t − nT )

)· R(i)(t)

(11)

c(t) =

{ck (k − 1)Tc ≤ t ≤ kTc

0 t ≥ T, t < 0, (12)

where c(i)k (t), T , and L represent the spreading codes, symbol

duration of the spreading code, and length of the spreadingcode, respectively. Then, signals denoted by ZC(t), which aredown-converted and rejected higher harmonics are expressedas:

ZC(t) =(

zI,C(t)zQ,C(t)

)= LPF [T C · X(t)]

=M∑i=0

∞∑n=0

⎛⎜⎜⎜⎝

c(i)11 (t − nT )s(i)

00 (t) + c(i)12 (t − nT )s(i)

01 (t)−c

(i)21 (t − nT )s(i)

02 (t) − c(i)22 (t − nT )s(i)

03 (t)c(i)21 (t − nT )s(i)

00 (t) + c(i)22 (t − nT )s(i)

01 (t)+c

(i)11 (t − nT )s(i)

02 (t) + c(i)12 (t − nT )s(i)

03 (t)

⎞⎟⎟⎟⎠ .

(13)

At the baseband stage in the adaptive controller, these signalsare dispread, and these are expressed as follows:(

s(i)1 (t) s

(i)2 (t) −s

(i)3 (t) −s

(i)4 (t)

s(i)3 (t) s

(i)4 (t) s

(i)1 (t) s

(i)2 (t)

)

=∞∑

n=−∞ZC(t) · ( c

(i)11 (t − nT ) c

(i)12 (t − nT )

c(i)21 (t − nT ) c

(i)22 (t − nT ) ).

(14)

From the above mentioned observation, we conclude thateach signals required for adaptive control is separated at thebaseband stage.

III. COMPUTER SIMULATION

In this section, we show the performance of our proposedreceiver structure by computer simulation. Data is modulatedby BPSK and transmitted on a 1-path Rayleigh fading channelsimulated using a Jakes model[11]. At the receiver, it has 4antennas, and the Hilbert transformer is assumed to have animperfection of gI/gQ = 0.9 with δ = 10◦ at each array.

A. BER performance vs. Hilbert transformer error

First, we show the BER vs. I/Q imbalance performance inFig. 2 and Fig. 3, with Eb/N0 = 8.0 dB, CIRimg = −40.0 dBand CIRcci = −20.0 dB. In addition, the performance of thearray having 3 antennas and a perfect Hilbert transformer isshown as a reference. As is shown the figure, the gain imbal-ance is compensated for gI/gQ > 10−1, and phase imbalanceis compensated for |δ| < 80◦. Since the imperfection of theHilbert transformer depends on frequency of signal at the RFstage, this receiver can deal with the frequency bands whichgenerate imbalance for these limits.

0.01 0.1 1 10 100

BE

R

g /g

imperfect HTPerfect HT wiht no image interference

I Q

10-2

10-3

10-4

Fig. 2. BER performance vs. gain imbalance in Hilbert transformer, gI/gQ

B. BER performance vs. interference signal power

Next, we show the BER vs. CIR (Carrier to InterferenceRatio) performance in Fig. 4, with Eb/N0 = 8.0 dB. In thispaper, because of the two types of interferences, image-bandinterference and co-channel interference, we represent the CIRof image-band interference as CIRimg and that of co-channelinterference as CIRcci. In addition, the performance of aperfect receiver is shown as a reference. As is shown in thefigure, both the above mentioned interferences are suppressedwhen CIR > −80.0 dB.

3

-100 -80 -60 -40 -20 0 20 40 60 80 100

BE

R

[degree]

Imperfect HTPerfect HT wiht no image interference

10-1

10-2

10-3

10-4

Fig. 3. BER performance vs. phase imbalance in Hilbert transformer, δ

-100 -80 -60 -40 -20 0 20 40 60

BE

R

CIR [dB]

CIR vs BER / CIR = - 60.0 dBCIR vs BER / CIR = - 60.0 dB

Ideal performance with 1 Co-Channel interferenceIdeal performance with no interfernce

10-2

10-3

10-4

10-5

imgcci

cciimg

Fig. 4. BER performance vs. interference signal power

C. BER performance vs. Eb/N0

The BER performance of the proposed receiver is shownin Fig. 5. The performance is simulated with CIRimg = −80.0dB and CIRcci = − 60.0dB. In addition, the performanceof an ideal array antenna without interference is shown asa reference. As shown in the figure, the proposed receivermaintains the same performance of an ideal array antenna.

IV. CONCLUSION

We have proposed a novel heterodyne receiver structure formultiband/mode communications. In the receiver, co-channelinterference is suppressed by array antenna and image-bandinterference is suppressed by Hilbert transformer. This isaccomplished using only one adaptive linear filter for eachantenna based on the MMSE criterion. Moreover, becausethese filters are at the IF stage, all the signals can be mixed,and thus, circuit can be miniaturized. Computer simulation

10

0 2 4 6 8 10

BE

R

Eb/N0 [dB]

No interference at proposalIdeal performance using 4 antennas

1 Interference at proposalIdeal performance using 3 antennas

2 Interference at proposal

-1

10-2

10-3

10-4

10-5

Fig. 5. BER performance vs. Eb/N0

confirms the performance of our proposed receiver. Actually,the proposed receiver structure can extract desired signals fromsignals having high interference, CIR > −80.0 dB. Conse-quently, it is shown that using our receiver, multiband/modereception can be carried out using only one RF chain, althoughthe receiver can demodulate signals with any carrier frequency.

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