IEEE COMMUNICATIONS LETTERS, VOL. 18, NO. 2, FEBRUARY 2014 317 Channel Multiplexing Technique Utilizing Electric and Magnetic Components of a Radio Wave M. A. Nikravan, Student Member, IEEE, Hans G. Schantz, Senior Member, IEEE, Alfred H. Unden, and Do-Hoon Kwon, Senior Member, IEEE Abstract—A wireless channel multiplexing method which makes use of the electric and magnetic field components of an electromagnetic wave as separate information carriers is presented. It is based on the observation that the rates of attenuation for electric and magnetic fields with respect to distance are different for electric and magnetic sources in the near-field region. Capacity enhancement is obtained in free space without any scattering. Measured indoor channels at 1 MHz confirm capacity enhancement of the proposed 2 × 2 MIMO link compared with the reference SISO link. Index Terms—Dipole antennas, diversity methods, electromag- netic coupling, information rates, multiplexing, MIMO systems. I. I NTRODUCTION M ULTIPLE-input multiple-output (MIMO) technologies take smart advantage of a scattering environment to enhance the channel capacity in wireless communications using multiple antennas on both the transmitter and receiver sides [1], [2]. In rich scattering environments, the capacity increases almost linearly with the number of antennas, signif- icantly increasing the bandwidth efficiency compared with the traditional single-input single-output (SISO) channels. In current MIMO technologies, a scattering environment is needed to reduce the correlation among received signals at the antenna terminals of a receiving array. In the trivial free-space setting, no practical multiplexing gain can be obtained. Even the existence of a strong line-of-sight component significantly reduces the MIMO channel capacity [3]. It has been found that a multiplexing gain can be achieved only in a restricted, controlled configuration in free space—a known distance in a short range, large inter-element spacings, and specific array orientations [3], [4]. It is noted that all current MIMO technologies make use of the radiation phenomena, where the transmitting and receiving arrays are in the far-field regions of each other. The electric and magnetic fields carry exactly the same information without exception. This letter presents the channel capacity enhancement of a 2 × 2 near-field MIMO technique utilizing electric and mag- netic fields as distinct information carriers based on measured channels at 1 MHz. It takes advantage of the fact that the Manuscript received November 21, 2013. The associate editor coordinating the review of this letter and approving it for publication was K. K. Wong. This work was supported by the National Science Foundation under Grant No. 1217524. M. A. Nikravan and D.-H. Kwon are with the Department of Electrical and Computer Engineering, University of Massachusetts Amherst, Amherst, MA 01003, USA (e-mail: {nikravan, dhkwon}@ecs.umass.edu). H. G. Schantz and A. H. Unden are with Q-Track Corporation, Huntsville, AL 35805, USA (e-mail: {h.schantz, j.unden}@q-track.com). Digital Object Identifier 10.1109/LCOMM.2013.122713.132588 electric and magnetic fields attenuate at different rates for electric and magnetic radiators as a function of distance in the reactive near-field region. This field multiplexing provides a gain in free space for the same polarization. In this respect, the multiplexing approach is different from [5], where a scattering environment is required and a far-zone communication is considered. When the proposed scheme is used as a diversity communication technique, the resulting system will have a low probability of link loss. This method was also investigated in [6] from a different perspective, in which the channel characteristics and consequently channel capacities of a 2 × 2 MIMO system are computed using mode-based analysis. II. SMALL ELECTRIC AND MAGNETIC DIPOLE FIELDS We begin with a brief overview of the fundamental free- space near-field multiplexing technique that was discussed in [7]. Consider a small electric dipole and a small magnetic dipole with moments p e and p m radiating in free space at a time-harmonic angular frequency ω. At an observation point r =ˆ rr in the spherical coordinate system, the electric (E) and magnetic (H) fields generated by p e are equal to [8] E e = jkZ 0 4π e -jkr r ˆ r × ˆ r × p e  1 - j kr - 1 (kr) 2  - jkZ 0 2π e -jkr r ˆ r(ˆ r · p e )  j kr + 1 (kr) 2  , (1) H e = - jk 4π e -jkr r ˆ r × p e  1 - j kr  , (2) where k =2π/λ and Z 0 ≈ 120π Ω. The fields E m , H m produced by the magnetic dipole moment are given in a similar form using the duality theorem [8]. Let the complex wave impedances Z e and Z m represent the ratio between the E and H fields that are transverse to ˆ r. In Fig. 1, they are plotted with respect to r/λ (λ = free-space wavelength). In the far-field region (r ≫ λ), the two complex impedances are the same, i.e. Z e = Z m = Z 0 , making the H field a scaled copy of the E field. For Z e and Z m to be distinguished in practice, the distance from the antennas should be less than one wavelength, beyond which they may be considered identical. This picture changes dramatically in the near zone of an antenna, where both quantities become functions of r. For example, Fig. 1(b) shows that  Z e changes continuously from -90° to 0 as r is increased. This characteristic has been exploited in a near- field ranging application [9]. In general, the two impedances satisfy Z e Z m = Z 2 0 . This implies that the total E and H fields radiated by a combination of p e and p m contain different amount of information transmitted by the two antennas. 1089-7798/14$31.00 © 2014 IEEE