6260 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013

Fig. 15. Theoretical Rayleigh + Clarke Doppler spectrum lcr, and lcrs of ex-perimental single branch and 4-branch incremental MRC.

Also, in Fig. 15 the level crossing rate, lcr, in crossings per wave-length is presented for a single branch and various incremental levelsof MRC diversity. The lcr is much smaller than that of Clarke’s theo-retical model (also in the figure). The introduction of incremental di-versity gradually shifts the maximum toward higher levels, indicatingthe reduction in the fade depths brought about by diversity.

V. SUMMARY

A quantification of the space diversity gain for low elevation links inurban areas was presented. The analysis is based on a unique measure-ment campaign using a remotely controlled aerial platform for simu-lating the wanted path geometry.The analysis results show that significant gains are possible, espe-

cially for the low outage probability levels, slightly smaller than thoseobtained for uncorrelated Rayleigh fading channels. The fades are alsoquite long in duration due to the very low Doppler spread present; thisis also partially overcome by the use of diversity as shown in the cal-culated reductions in the average fade durations.With respect to other combining methods, MRC showed marginal

performance improvements at the expense of the required additionalcomplexity.As expected, the largest increment is achieved when the first diver-

sity branch is introduced. Further branches bring about smaller im-provements but, still, substantial onesThe achieved space diversity plus combining gain was found to be

practically independent of the elevation angle (within the narrow rangeconsidered), the street orientation or the street width.

REFERENCES[1] J. F. V. Valdes, M. A. G. Fernandez, A. M. M. Gonzalez, and D. S.

Hernandez, “The role of polarization diversity for MIMO systemsunder Rayleigh-fading environments,” IEEE Antennas WirelessPropag. Lett., vol. 5, no. 1, pp. 534–536, Dec. 2006.

[2] P. L. Perini and C. L. Holloway, “Angle and space diversity compar-isons in different mobile radio environments,” IEEE Trans. AntennasPropag., vol. 46, no. 6, pp. 764–775, Jun. 1998.

[3] S. R. Todd, M. S. El-Tanany, and S. A. Mahmoud, “Space and fre-quency diversity measurements of the 1.7 GHz indoor radio channelusing a four-branch receiver,” IEEE Trans. on Veh. Technol., vol. 41,no. 3, pp. 312–320, Aug. 1992.

[4] L. Lingfeng, C. Oestges, J. Poutanen, K. Haneda, P. Vainikainen, F.Quitin, F. Tufvesson, and P. D. Doncker, “The COST 2100 MIMOchannel model,” IEEE Wireless Commun., vol. 19, no. 6, pp. 92–99,Dec. 2012.

[5] P. R. King and S. Stavrou, “Low elevation wideband land mobile satel-lite MIMO channel characteristics,” IEEE Trans. Wireless Commun.,vol. 6, no. 7, pp. 2712–2720, Jul. 2007.

[6] P. D. Arapoglou, P. Burzigotti, A. B. Alamanac, and R. De Gaudenzi,“Capacity potential of mobile satellite broadcasting systems em-ploying dual polarization per beam,” in Proc. 5th Advanced SatelliteMultimedia Syst. Conf. and 11th Signal Processing for Space Commun.Workshop, 2010, pp. 213–220.

[7] E. Eberlein, F. Burkhardt, C. Wagner, A. Heuberger, D. Arndt, and R.Prieto-Cerdeira, “Statistical evaluation of the MIMO gain for LMSchannels,” in Proc. 5th Eur. Conf. on Antennas and Propagation,Rome, 2011, pp. 2848–2852.

[8] M. Simunek, P. Pechac, and F. P. Fontan, “Excess loss model for lowelevation links in urban area for UAVs,” Radioengineering, vol. 20, no.3, pp. 561–568, Sep. 2011.

[9] M. Simunek, F. P. Fontan, and P. Pechac, “The UAV low elevationpropagation channel in urban areas: Statistical analysis and time-se-ries generator,” IEEE Trans. Antennas Propag., vol. 61, no. 7, pp.3850–3858, Jul. 2013.

[10] J. Parsons, Mobile Radio Propagation Channel, 2nd ed. New York,NY, USA: Wiley, 2000.

[11] M. Simunek, P. Pechac, and F. P. Fontan, “Space diversity analysis forlow elevation links in urban areas,” in Proc. 6th Eur. Conf. on Antennasand Propagation, 2012, pp. 1165–1168.

[12] [Online]. Available: www.airshipclub.com

Effect of Channel Correlation and Path Loss on AverageChannel Capacity of Body-to-Body Systems

Khalida Ghanem

Abstract—The channel capacity in body channels is of great importancesince it determines the amount and type of data that can be transmittedusing the body as a media for communication. In this communication, theaverage capacity of body-to-body channels, where the transmitter and thereceiver are on different human bodies, is investigated in an indoor wirelessenvironment when using multiple antennas at the transmitter and the re-ceiver, the so-called MIMO system. Different types of antennas are studied,namely PIFA, inverted IFA and monopole antennas, and the capacity inthese channels is compared to on-body case. The impingement of the spa-tial correlation due to the presence of a high line of sight (LOS) component,on the average capacity is also studied. The effect of path loss on averagecapacity is asserted as well.

Index Terms—Body-to-body channels, channel capacity, multiple-inputmultiple-output (MIMO), on-body channels.

I. INTRODUCTION

Body-centric wireless communications have emerged as a newparadigm and have developed since then even if it is still at the earlystage of development knowing their great potential [1]–[9]. Even if theused techniques are currently spanning different applications such asmilitary, entertainment, or at home, they have particularly targeted theinfrastructure of patient-centered medical applications. It has become

Manuscript received March 19, 2012; revised January 31, 2013; acceptedApril 16, 2013. Date of publication September 20, 2013; date of current ver-sion November 25, 2013.The author is with Centre De Développement Des Technologies Avancées,

Algeria (e-mail: kghanem@cdta.dz).Color versions of one or more of the figures in this communication are avail-

able online at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2013.2283035

0018-926X © 2013 IEEE

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013 6261

obvious that the healthcare quality of service needs serious improve-ments. Indeed, the hospital is no longer able to fulfill the patientsdemand and face the rapid pace the chronic diseases are spreading atand affecting the population. There are also other factors which areimpeding the flexibility of healthcare system among which the tooselective skillfulness and workload imposed on the medical staff, thetightness of the budgets allocated to healthcare services, and the agingof population. The emerging of new wireless technologies and theadvances of miniaturization techniques have opened new opportunitiesfor the implementation of very low-power wireless sensors. Theselatter can be placed on a human-body to ensure the smart monitoringof the vital signs anywhere and anytime, without disrupting the dailyactivities of the wearer. Obviously, the body channel capacity is thedirect factor impeding the usage of wireless techniques at their fullpotential and should be studied and carefully analyzed.The body devices at the transmitter and the receiver blocks can

be placed at the same body, on different bodies or in the body, thusgiving rise to the on-body, body-to-body and in-body channels, re-spectively. In the on-body channel, the processing of the transmitteddata is performed at the patient body and a local medical interventionis ensured via a close-loop bio-feedback system. In the body-to-bodychannel case, the processing is performed at a terminal placed on thecaregiver’s body. It is now known that the body channel characteris-tics are different from the conventional wireless channel in that theydepend not only on the antenna type and the local propagation envi-ronment, but also on the body size, shape and posture, as well as onthe antenna orientation and distance from the body [4]–[6], [8], [9].Naturalistic movements of the user induce random variations in theon-body antenna orientation and position, resulting in a polarizationmismatch between the antennas at the transmitter and the receiver.Spanning the literature on body-area networks (BANs), it is clear thatultra-wideband (UWB) communications on the human body has at-tracted a great deal of research interest [2], unlike the 2.45 GHz fre-quency band, which has not witnessed the same eagerness particularlyin channel modeling. For this band however, several measurementcampaigns were undertaken in anechoic environment [3]–[5], [7]–[9].In this work, we aim at reducing this gap, by investigating at the 2.45GHz frequency band, the effect of the antenna choice on the capacityof body-to-body channels.One efficient mean to significantly increase channel capacity is the

use of multiple transmit and receive antenna systems (MIMO). Earlyresearches carried out by Foschini [10] and Telatar [11], have predictedimportant spectral efficiencies for multiple antennas wireless channelsin the presence of a rich scattering environment. The effects of usingMIMO systems in BANs have been investigated in few studies such asin [12], [13]. It appears that the body channels are experiencing more orless a high correlation among the spatial subchannels, a power imbal-ance at the receive antenna elements, and a mutual coupling betweenthe spatially separated antennas. All these factors may have a drastic ef-fect the capacity of MIMO systems [12]–[14]. The other motivation ofthis work is that, according to the author’s knowledge, the capacity ofbody-to-body channels with MIMO body-worn antennas has not beenaddressed yet. This may be due to the aforementioned capacity lim-iting factors present in body channels. Furthermore, when the channelcapacity has been evaluated in on-body case such as in [12], [13], equalpower distribution was solely investigated. In this communication, wewill evaluate the achievable average capacity when adopting both uni-form power and waterfilling schemes in body-to-body channels usingdifferent types of antennas and will compare it to the on-body chan-nels. The communication is organized as follows. Section II describesthe measurement setup and channel characterization. Section III dis-cusses the simulation results. Finally, concluding remarks are offeredin Section IV.

Fig. 1. Configuration of the PIFA antenna used in on-body and body-to-bodymeasurements [19].

II. MEASUREMENT SETUP AND CHANNEL CHARACTERIZATION

Themeasurements were performed, at the Antennas&Applied Elec-tromagnetics Laboratory of the university of Birmingham which isa 7.5 m 9 m-sized indoor environment containing equipment, ta-bles, and computers thus providing a rich multipath propagation en-vironment. In the body-to-body measurements, an array of either mi-crostrip-fed planar inverted-F (PIFA), printed-IFA, or monopole an-tennas has been placed on one human body, while the same type of an-tennas have been placed on a different body. Since the on-body channelhas been already investigated in different environments and with dif-ferent type of antennas [7], [13]–[16], it will be used herein to compareits achievable capacity with the body-to-body channels using differenttypes of antennas. Our studies have shown that using the designed PIFAantennas for the on-body case allows to give the best performance interms of path gain, alleviate the inherent on-body mounting detuning,and diminish the return loss compared to IFA antennas. In additionthey have quite low profile compared to monopole antenna. Therefore,when performing the comparison with on-body channels, only PIFAantennas, which are similar to the ones used in body-to-body case,will be investigated. The PIFA antennas used in the array, as shownin Fig. 1, were 10 mm-spaced with a radiating plate of 1-mm thick-ness, 3-mm distance between the short-circuit pin and the feeding pinand 0.8-mm thickness for the FR4 substrate. The ground plane sizewas the same as the substrate size, namely 45 mm 40 mm, which isbelieved to be small enough to fit into comfortable body-worn sensordevices [13]–[16]. Once mounted on the body, these antennas exhibita return loss inferior to 10 dB, a mutual coupling of dB and alow detuning. The printed-inverted F antenna (IFA) diversity antennaarray shown in Fig. 2, was designed such that the substrate size was 40mm 40 mm and the ground plane size was 30 mm 40 mm. In eachcase, the length of the longer arm of the inverted-F was 22 mm whilethe one of the shorter arm was 3 mm. The mutual coupling betweenthe IFAs was dB which is also considered as a low value. Forthe monopole, two thin wire antennas were placed on the same groundplane as shown in Fig. 3. The height was 30.6 mm, which correspondsto , where is the wavelength in the free space. The wire diameterwas 0.4 mm and the antenna spacing was 30 mm. The ground planewidth was similar to the antenna spacing and its length was twice thespacing. The mutual coupling was dB.For the body-to-body channel, the transmitting array was placed at

the belt position at the left side of the first body, while the receivingarray was placed at the left side of the belt position and at the rightside of the head, respectively, thus forming the belt-belt and belt-headbody-to-body channels.The two transmitting antennas were connected to a signal generator

through an RF switch while the two receiving antennas on the otherbody were connected to the two ports of a HP8753ES vector networkanalyzer (VNA) calibrated in tuned receive mode. In the calibrationstage, the total power delivered to the transmitting antenna has beennormalized to 0 dBm after connecting the signal generator to each port

6262 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013

Fig. 2. Configuration of the IFA antenna used in body-to-body measurements.(a) On the receiver, (b) On the transmitter.

Fig. 3. Configuration of the monopole antenna used in body-to-bodymeasurements.

of the VNA through the same cables used afterwards in the measure-ments. This has been done to ensure that low power levels are solelyused in body channels. In order to achieve a good synchronization ofthe signal generator and the VNA, the 10 MHz reference output signalof the signal generator was fed to the VNA. In the channel measure-ments, the magnitude and the phase of the parameter were collectedbetween each pair of transmit-receive antenna. A total of ten sweepswere performed for ten different types of movements chosen as ran-domly as possible to reproduce naturalistic body movements. For fur-ther details on the measurement procedures, we refer the reader to [6].Using these measurements, the channel is then characterized by sep-arating the large and the small-scale fading components. Usually, thelong-term fading is removed from each spatial subchannel componentby demeaning the signal received at this subchannel [11]–[13]. Thelong-term fading envelope which results from the path loss and shad-owing effects represents the local average power of the received signal[14] and is superimposed on the short-term fading corresponding to thecontribution of the multipath components. It is usually assumed that thelong-term fading is a multiplicative factor to the received signal enve-lope [11]–[13], [19]

(1)

where is the received signal envelope, is the short-term fadingenvelope, and is the long-term fading component defined as fol-lows [11]–[13], [19]

(2)

In (2), is the size of the local averaging sliding window. The choiceof the window size is an issue but is beyond the scope of this commu-nication. Herein it was selected such that there were sufficient short-term fading oscillations inside the window and small enough comparedto the time scale of the long-term variation. The window is slidingsuch that at each instant, the received signal envelope is normalizedto its local average value, hence the long-term variation is removed toachieve the short-term fading envelope . Our previous works haveshown that the fast fading component follows a Rician distribution,while the slow fading is log-normal-distributed [6], [8]. Table I com-

pares the slow and fast fading statistical parameters for the belt-headand the belt-chest on-body channels with the belt-head and belt-beltbody-to-body channels when using the same PIFA antennas. Table IIshows the same parameters corresponding to the belt-head and belt-beltbody-to-body channels with different types of antennas. Denoting

the MIMO antenna system such that , where

is the spatial subchannel between the th receive antenna and theth transmit antenna, it is shown from Table I that the on-body sub-channels for both belt-chest and belt-head channels do not exhibit thesame propagation characteristics. For the belt-head on-body channel,

and exhibit lower mean path loss and higher K values than thesubchannels and , while less disparity is noted for the case ofbelt-chest channel. The belt-chest channel experiences low path lossand high Rician -factor, thus tending to a strong Rice case. The belt-head channel, on the other hand, exhibits higher path loss and lower Kvalues particularly with the subchannels and , which representsa Rice case tending to a Rayleigh channel. For the case of body-to-bodychannels using either PIFA or IFA antennas as shown in Table II, thedifferent spatial subchannels exhibit the same statistical parametersfor a given body channel. The belt-head and the belt-belt channelsusing PIFA antennas show statistical parameters that are comparableto the belt-head on-body channel. The -factor for all subchannels isless than 1 which shows that the direct path is weak compared to theon-body case. On the other hand, it is seen that the body-to-body chan-nels are suffering from an important shadowing effect. This comes fromthemutual effect of the presence of two bodieswhich parts contribute inblocking the signal. Comparing the body-to-body channels when usingeither printed-IFA or PIFA antennas, it is seen that with IFA antennas,the channels are experiencing higher path loss compared to PIFA an-tennas while the shadowing and the -factors remain comparable. Inaddition to the configuration shown in Fig. 2 for the printed IFA an-tenna, which has been shown to be viable in on-body channels, a studyof different configurations of printed IFA antennas should be under-taken to minimize the path loss in the case of body-to-body channels.The statistical parameters of the body-to-body channels are close whenusing the PIFA antennas except that the shadowing value is higher inthe belt-belt channel. For the printed-IFA antennas, the path loss is lessin the belt-belt channel than in the belt-head but the shadowing is stillhigher. The shadowing values are higher because of the more impor-tant effect of the floor at the receiver in the belt-belt channel and thehigher path loss may be due to the polarization loss resulting from thedifference in the direction of the antennas in the transmitter and the re-ceiver at the belt positions.Thus in the BANs channels, as described above, there exist a dis-

parity in the propagation experienced in each subchannel owing to thevariation in the antenna position, orientation and the amount of shad-owing. In [6], we have derived a model which has been shown to wellfit the measurements and takes into account the difference of the sta-tistical parameters perceived at each subchannel as follows

...

...(3)

where is the Ricean factor, is the received power which corre-sponds also to the path-gain since the transmit power has been normal-

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013 6263

TABLE IFADING PARAMETERS OF THE ON-BODY AND BODY-TO-BODY CHANNELS

USING PIFA ANTENNAS

TABLE IIAVERAGE PATH LOSS, SHADOWING AND K VALUES FOR

BODY-TO-BODY CHANNELS

ized. is the phase of the - th subchannel in the constant channelcomponent which has been chosen randomly distributed overbecause of the variation in the orientation of transmit and receive an-tennas and the polarization losses. The vector of the NLOS correlatedmatrix components , is calculated by multiplying the symmetricmatrix extracted from the standard Cholesky factorization of the corre-lation matrix complex envelope, and a vector of zero-mean complex

random variables [6], [17].In conventional wireless indoor MIMO systems, the correlation ma-

trix is obtained by calculating the Kronecker product of the spatialtransmit and receive correlation matrices [17]. The assumptions whichwere under this derivation may not be satisfied for on-body channelsbecause, even if the transmit and receive antennas are chosen such thatthey have the same polarization, a polarization mismatch is imposed bythe unpredictable movements of the body and by the associated shad-owing. As an alternative, the joint correlation matrix is extracted frommeasurements as follows [7], [13], [18]

(4)

Where the parameters are the correlation coefficients between themeasured subchannels and defined as

(5)

In (5) is the total number of samples and stands for the conju-gate of the operand. Using (4) and (5), the spatial complex correlationmatrix has been calculated for each body channel.The channel capacity normalized respective to the bandwidth, under

a uniform power allocation scheme, can be expressed in terms of theSNR as

(6)

where is the identity matrix which size depends on whether thenumber of receive antennas or the transmit antennas is higherand ’ is the normalized channel matrix. In addition to equalpower scenario, we also have considered waterfilling with which thecorresponding capacity is

(7)

where K is the number of independent eigenchannels, is the powerallocated to the th MIMO eigenchannel and is the correspondingeigenvalue.In the two power allocation schemes, two normalization approaches

are adopted to generate the normalized channel matrix from themeasured matrix . In the first one referred to as normalization 1,the Frobenius norm of each realization is maintained equal tothus overlooking the path loss variation with time and emulating a tightpower control procedure. The second approach, referred to as normal-ization 2, maintains the Frobenius norm of the channel matrix averagedover all its realizations at . This corresponds to a fixed transmitpower and a receive SNR varying with path loss.

III. SIMULATION RESULTS

The achievable equal power and waterfilling capacities using theaforementioned 2 2 on-body and body-to-body channels are investi-gated in the same indoor environment using three types of MIMO an-tennas, namely PIFA, IFA and monopole. Unless mentioned otherwise,the equivalent 2 2 Rayleigh channel is taken as a reference. In thefirst set of simulations, normalization 1 is adopted, thus the Frobeniusnorms of all subchannels realizations have been maintained at. Fig. 4 depicts the achieved capacity of the belt-belt body-to-body

channel along with the Rayleigh channel with different types of an-tennas when varying the SNR level. From this figure, it can be seenthat, at low SNR range, the belt-belt channel and the Rayleigh channelyield close achievable capacity. At this SNR range, waterfilling yieldsbetter capacity values than equal power scheme because it fights thebad channel conditions in a better way, by allocating the power, amongthe two eigenchannels, to the one that is in the best condition. It isseen that, when adopting normalization 1 such that the path loss ef-fect is removed, at low SNR, Rayleigh channel performs similarly tothe body-to-body channel. When the SNR increases, the good state ofthe channel results in the same capacity values for the waterfilling andequal power for either Rayleigh or belt-belt body-to-body channels.

6264 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013

Fig. 4. Waterfilling and uniform power average capacity of the belt-beltbody-to-body channel when using different antennas.

Fig. 5. Uniform power average capacity of the belt-belt body-to-body channel,when using different antennas and belt-head and belt-chest on-body channels.

However, the capacity gap between these two channels increases asSNR increases, because of the presence of a LOS component in thebelt-belt channel which reduces the order of its MIMO matrix and thusits capacity, compared to the Rayleigh full rankmatrix case. It is worthyto notice that when adopting channel matrix normalization 1, the an-tenna type has no effect on the capacity because the main differencebetween these antennas comes from the measured path loss and not thestructure of the matrix. This is consolidated by the results in Table IIwhere the subchannels are shown to experience the same fast fadingparameters.Fig. 5 compares the uniform power average capacity of the belt-belt

body-to-body channel along with the belt-head and belt-chest on-bodychannels when normalization 1 is adopted. For figure clarity, the resultson belt-head body-to-body channel has not been reported because theyare quite similar to the belt-belt channel. The same remark holds forwaterfilling which has been shown in Fig. 4 to behave similarly to equalpower allocation scheme.From Fig. 5 it can be noted that, as previously mentioned, the

belt-belt-channel exhibits the same behavior regardless of the antenna

used in the measurements. Despite the difference in spatial subchannelcorrelation among the body channels, as studied in Section II, at lowSNR level the direct ray is not that strong to have a significant impacton the correlation whilst the multipath signals are the predominantcomponents. It follows that, at this range, the belt-head and belt-cheston-body channels have quite similar capacity values which are inferiorto body-to-body channel. The capacity gain in this latter may comefrom the fact that the Rician K-factor, as shown in Tables I and II, isless as compared to on-body channels, which reflects a richer scatteredpropagation. One would expect that the body-to-body channels exhibita higher degree of scattering compared to on-body channels becauseof the higher distance between the transmitter and the receiver, theadditional body-shadowing resulting from the presence of anotherbody and the nature of the movements performed in the sweeps whichblock the direct path. However, and as seen from the statistical pa-rameters in Tables I and II, the difference between the body channels,except for the belt-chest case, is not that significant, particularly forthe K factor. As reported in [1], [13] this is because the direct ray inon-body channels propagates on the surface of the body in the form ofa creeping wave and is thus attenuated.At high SNR range, the direct ray is emphasized and the belt-chest

channel which shows the highest K value, as shown in Tables I andII, sees its achievable capacity degrades compared to the belt-headchannel which tends to reach the same capacity as belt-belt channeldue to their high scattering and low LOS component.Fig. 6 compares the waterfilling and equal power average capac-

ities for on-body and belt-head body-to-body channels when eithernormalization 1 or 2 are adopted. This figure shows that, again, atlow SNR range waterfilling allows to offer a better throughput com-pared to uniform power whatever is the body channel, the antennatype or the normalization method. At high SNR, waterfilling and equalpower schemes bring no difference in capacity in the case of the belt-head on-body channel and the belt-head body-to-body channel whennormalization 1 is used. In other cases, because of the presence of ahigh LOS component and thus a high spatial correlation such as in thebelt-chest on-body channel, or the presence of an important path losssuch as in belt-head body-to-body channel with normalization 2, op-timizing the distribution of the power among antennas does not allowto overcome the correlation or the path loss effect. When normalizingthe channel using the first method, at low SNR range, the belt-headbody-to-body channel yields slightly better capacity values than theon-body belt-head channel. Even though it is not represented, the av-erage capacity curve of the belt-belt channel is quite similar. Further-more, the belt-chest channel shows comparable capacity values be-cause the effect of LOS has not yet a great impact on capacity whenSNR is low. At high SNR, the belt-chest sees a degradation in its ca-pacity compared to belt-head body channels because the effect of spa-tial correlation overcomes the effect of multipath propagation. How-ever, the capacity loss remains reasonable and within a range of 1.5 to2 bps. When the path loss is taken into consideration and normaliza-tion method 2 is retained, the effect of path loss and shadowing doesnot allow the uniform power scheme to exploit the high SNR values inachieving the same capacity gain as waterfilling except for the case ofthe belt-head on-body channel.The achievable capacity with all body channels is degraded com-

pared to their corresponding counterparts using normalization method1.In spite of its high subchannel correlation, the belt-chest channel

reaches the same average capacity as the belt-head body-to-bodychannel because its path loss is significantly less than this latter.Furthermore, the average capacity is not sensitive to the used antennaas it can be noticed from Fig. 6 where replacing the PIFA array by IFAarray brings no average capacity gain.

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 12, DECEMBER 2013 6265

Fig. 6. Waterfilling and uniform power average capacity of the body-to-bodyand on-body channels with normalization 1 and 2.

IV. CONCLUSION

In this communication, the average achievable capacity of on-bodyand body-to-body channels using MIMO antenna systems with dif-ferent antenna types has been investigated. Two power allocationschemes are studied, namely the waterfilling and the equal powerdistribution schemes; and two normalization methods are adopted forthe channel matrix. At low SNR range, channels in which waterfillinghas been incorporated were shown to reach better capacity valuesthan the ones using uniform power, regardless of the body channel,the antenna type or the normalization method. At this SNR range,the belt-belt body-to-body channel exhibits the same capacity as theequivalent Rayleigh channel, but as SNR increases the presence ofa LOS component makes the channel matrix order less than 2 in thebody channel. When using normalization 1, waterfilling was shown toyield the same channel capacity as the uniform power scheme at highSNR because of the already good state of the channel whatever is theused antenna and the channel. However, when adopting normalization2, due to the effect of path loss, the corresponding channels experienceaverage capacity loss compared to the same channels using normal-ization 1 and the average power scheme is not able to exploit the goodSNR value to perform similarly as the waterfilling. At high SNR level,the belt-head on-body channel exhibits the best capacity values.

ACKNOWLEDGMENT

The author would like to thank Prof. P. S. Hall and Dr. I. Khan fortheir valuable assistance, and the two institutions CDTA in Algeria andFQRNT in Canada for their funding.

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