126 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 2, 2003

Analysis of RF Radiation Interference on WirelessCommunication Systems

H. Y. David Yang, Fellow, IEEE

Abstract—The effects of radiated RF interference on wirelesscommunication systems are investigated. Noise source at theantenna input port induced by in-band external electromagnetic(EM) fields is determined based on full EM simulations of thecommunication systems. Signal bit error rate for a line-of-sightadditive white Gaussian noise (AWGN) channel and for a non-line-of-sight Rayleigh channel as a function of the signal-to-noiseratio is used to determine the susceptibility of the systems subjectto external power sources. Two specific examples are given,including a dongle for a wireless mouth and a PCMCIA card for alaptop computer.

Index Terms—Bit error rate (BER), electromagnetic (EM) sim-ulation, RF interference, wireless communications.

I. INTRODUCTION

RECENTLY, there is significant interest in understandingand mitigating the effects of high-power electromagnetic

(EM) source on electrical systems and equipment [1]. Possiblesources include ultra wide-band (UWB) or narrow-band pulses,whose frequency spectra extend well beyond several gigahertz.Complex electronic targets and sophisticated integrated circuitsare becoming electrically comparable to a wavelength. Theirresonances and EM coupling alter significantly the physics ofelectronic components, circuits, and functional systems. Thereis a need to understand and model the effects to electronic cir-cuits and systems at all scales. In recent years, there have beenmany theoretical and experimental investigations of the effectsof RF interference on digital circuits [2], [3]. EM radiationthat alters the logic states of specific digital circuits inducesequivalent noise sources that are subsequently used in eitherlarge-signal or small-signal simulations. Most of the focusof EM interference (EMI) investigation has been for specificcircuit components with interference well below gigahertzrange [4]–[6]. bIn microwave regimes and above, conventionalRF electronic devices are comparable to a wavelength andbecome distributed circuits. Moreover, small apertures orcracks on the system shielding are no longer negligible andmay become the salient features of the whole structure [4].Strong EM coupling between various components as well ascavity shield resonances further complicate the physics of theradiation effects on electronics.

On the other hand, wireless communication technology hasmade significant progress in the past few years. The capabilitiesto process, deliver, and receive multichannel voice, video, com-puter, and imaging signals at the same time, with high speed,

Manuscript received May 9, 2003; revised June 2, 2003. This work wassupported in part by the U.S. Department of Defense under MURI GrantF49620-01-1-0436.

The author is with the Department of Electrical and Computer Engi-neering, University of Illinois at Chicago, Chicago, IL 60607 USA (e-mail:hyang@ece.uic.edu).

Digital Object Identifier 10.1109/LAWP.2003.816634

Fig. 1. A dongle for wireless mouth including a complete mix-signal wirelesscommunication system.

Fig. 2. External microwave source interaction with a dongle for a wirelessmouth. Incident field is polarized in the x � y plane (� = 90 ).

have become feasible. Microwave (gigahertz) RF integrated cir-cuits (RFIC) that are the backbone of the wireless communica-tion systems serve this purpose. In future communication sys-tems, all the electronic circuits are on a small chip of dime sizeor smaller together with front-end antennas and tuning passivespackaged on a printed circuit board (PCB). Complete commu-nication systems including analog RF and digital circuits areminiaturized significantly.

In this letter, we study the problem of an external microwavesource interference with a complete communication systemresided on a PCB. The electronics circuits are shielded insidea packaged chip and we assume that the coupling of externalmicrowave source into the system is through the front-endantenna. Full-wave EM analysis based on the HFSS software(finite-element method) is applied to determine the inducedvoltage source at the antenna port. The printed circuit antennais generally narrow band. This induced voltage source can betreated as in-band noise. There are two major effects of thisin-band noise. When the noise level is sufficiently high, thelow-noise amplifier (LNA) nonlinearity becomes noticeableand significant high-order harmonics (IP3) will be introduced[7]. Large-signal nonlinear circuit simulation could determinethis IP3 level. When the radiation noise level is sufficientlysmall and the front-end low-noise amplifier (LNA) remains in

U.S. Government work not protected by U.S. copyright.

YANG: RF RADIATION INTERFERENCE ON WIRELESS COMMUNICATION SYSTEMS 127

Fig. 3. Induced radiation noise at the input pin of a dongle obtained by HFSS simulation. 0-dB directivity microwave source transmits 12.2-dBm power at 1 maway at f = 2:44 GHz.

Fig. 4. BER of the dongle as a function of external microwave source for f = 2:44 GHz and signal power �50 dBm.

the linear region, the radiation noise is added on to the systemnoise and its effect is modeled in terms of the signal-to-noiseratio (SNR) of the overall communication systems. In thisletter, we investigate two examples of mixed-signal systemsunder the illumination of external microwave sources. Thefront-end LNA is assumed in the linear region and the radiationinterference effects are determined in terms of the bit error rate(BER) of the base-band signals.

II. ANALYSIS OF A BLUETOOTH WIRELESS MOUTH

An example of a complete communication system for a wire-less mouth is shown in Fig. 1. It was built on a 1 3 cm FR4PCB. A chip antenna is placed on the left front corner and a7 mm 6 mm chip, with both RF and digital systems, is pack-aged in the middle of the board. A printed inverted F antenna,which is more cost effective, can replace the chip antenna. The

Fig. 5. External microwave source interaction with a PCMCIA card insertedon a laptop PC. Incident field is polarized in the x � y plane (� = 90 ).

128 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 2, 2003

Fig. 6. Induced radiation noise at the input pin of a PCMCIA card. 0-dB directivity microwave source transmits 12.2-dBm power at 1 m away at f = 2:40, 2.44,and 2.48 GHz, respectively.

PCB is placed inside a PC mouth for wireless communicationat the frequency band of 2.4–2.47 GHz. In order to investigatethe external microwave interference with the system, a simpli-fied structure for full-wave EM (EM) simulation is shown inFig. 2. The small board tuning passives are neglected in EManalysis. The grounded FR4 PCB board is 40 mil thick and hasa dielectric constant of 4.2. A bended inverted F antenna is con-nected to the communication chip through a 50 metal trace.Note that the metal ground is removed underneath the F antenna.The whole structure is simulated with Ansoft HFSS software.Metal trace to the chip input pin is assumed terminated witha 50 load. Full-wave simulation is to determine the inducedelectric field (or induced voltage) at the chip input pin, wherethe input impedance to the entire electronic system inside thechip is assumed tuned to a 50 load. The incident field is as-sumed polarized in the plane of the PCB board ( ) (ver-tical polarization to the plane has little effect to the PCB) withstrength 1 V/m and . This field corresponds toa microwave source 12.2-dBm transmitted power at 1 m awaywith 0-dB directivity. Induced power at the chip pin input versusthe angle from simulation is shown in Fig. 3. This input poweris considered the noise input to the system. This radiation noiseadds to the system noise reducing the overall system SNR andthe BER of the base-band digital signals.

For binary phase shift keying (BPSK) in the additive whiteGaussian noise (AWGN) channel, the BER is given as [8]

BER

(1)

AWGN channel is for the line-of-sight (LOS) signal path andis a reasonable assumption here for a wireless mouth dongle.When the received signal is composed of the LOS signal anda random multipath component, the BER would be based on aRice channel model.

For a good design, the system noise in a small commu-nication system is less and the sensitivity of the re-ceiving system is defined as the minimum received noiselesssignal energy such that BER is less than 0.1%. If we ignore the

system noise (several order of magnitude less than the radiationnoise), SNR in decibels can be written as

SNR (2)

where is the signal, is the radiation noise induced atthe input pin given in Fig. 3, and is the microwave sourcepower (dBm) 1 meter away from the system. Radiation interfer-ence of the communication system is characterized by the BERat base band versus the external noise source power for a givenreceived signal. As an example, assuming that the signal poweris , Fig. 4 shows the BER as a function of the ex-ternal microwave power. Note that if we fix , increasingthe distance by a factor of two corresponds to increasing thenoise power source by 6 dB. Fig. 4 also shows that the noisepower due to the incident field from either the left or right sideof the dongle is much stronger than from the front, a result of thefront-end linear antenna orientation. For a signal, a20-dBm external source 10 m away will result in the worst caseof about 15% BER, far beyond a system can tolerate.

III. ANALYSIS OF A PCMCIA CARD FOR A LAPTOP COMPUTER

The second example is for a PCMCIA card inserted into alaptop computer. The card is a PCB board with packaged chipsand board circuitry. Two printed inverted F antennas are placedon the left and right board corners, respectively, for diversitypurpose. The card is inserted into the right-side of a laptop PC.The simplified geometry for EM simulation is shown in Fig. 5,where 50 traces are terminated by 50 loads at the chip inputpins. The board that stands out is 4.6 cm wide and 2.3 cm long.The metal backing is 1 in in height and 5 in wide. Antennas aretuned in 2.4–2.5 GHz for less than . A 0-dB direc-tivity microwave source with in-plane (in the plane of the board)polarization is assumed. The result of the finite-element simu-lation by HFSS is shown in Fig. 6 for three in-band frequencies.Note that the direction in front of the board is . The noisepower induced at the pin input is sensitive primarily to theantenna orientation. The radiation arm is on the trace extensionto substrate ground, explaining the minimum at inFig. 6.

YANG: RF RADIATION INTERFERENCE ON WIRELESS COMMUNICATION SYSTEMS 129

Fig. 7. BER of a PCMCIA card as a function of external microwave source for f = 2:44 GHz and signal power �50 dBm.

For a PCMCIA card, the receiving signals are mostly undernon-LOS (NLOS) environments. In narrow-band fast fading, theprobability density of the receiving signals follows a Rayleighdistribution, rather than the AWGN for a LOS case. The BERfor a Rayleigh distribution is given as [9]

(3)

The SNR is related to the radiation noise and external mi-crowave power according to (2).

As in the case of a dongle, assuming that the signal power isdBm, Fig. 7 shows the BER as a function of the external

microwave power. It is seen that the external radiation sourcefrom the side has worse BER than that from other direction.It is a result of antenna orientation. In fact, more receiving ra-diation noise also corresponds to more receiving signal power.Comparing Figs. 3 and 6, it is observed that a PCMCIA cardhas more radiation noise received. Also comparing, (2) and (3),we observed that the BER for a PCMCIA card is worst than adongle for the same SNR due to the NLOS environment of aPCMCIA card.

IV. CONCLUSION

Advanced wireless communications including both RFanalog and digital circuits have the features of small size, lowpower, low cost, and transmitting high date-rate signals. Signalinterference with external power sources is mostly throughthe front-end antenna and the effect appears on the BER ofthe base-band digital signals. In this letter, we combined EMand communication theory to investigate the radiation noiseeffects. First, EM simulation is performed to determine the

radiation noise power at the chip input pin due to the externalpower source. Communication theory taking into account thefast fading due to random multipath signals is used to relatethe BER to SNR of the systems. The method discussed in thisletter is primarily for the low-power case, where the front-endLNA is still operated in the linear region with the presenceof radiation noise. Two examples were given, a dongle fora wireless mouth and a PCMCIA card for a laptop PC. Theproposed method provides a statistic and much simplifiedsystem approach to evaluate the effects of radiation interferenceto a communication system.

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