Chapter IV: Multi-Port Driven Antennas
5.4 Polarization Modulation
In the previous sections, we showed that a multi-port driven integrated antenna and its driving circuitry can be designed in such a way to allow transmitting any desired arbitrary polarization (linear at any polarization angle, circular, and elliptical with any axial ratio) to enables dynamic polarization control. Now, we can use the same
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Figure 5.52: (a) HFSS simulation setup for the 2×2 integrated slot-based radiator and the similated gain patterns for (b) linear polarization mode and (c) circular polarization mode.
DPC antenna to propose a new communication scheme called Polarization Mod- ulation (Pol-M) that augments existing phase and amplitude modulation schemes in wireless communications by an additional degree of freedom. Pol-M substan- tially enhances the effective channel capacity. This additional feature opens up a new door to a variety of research problems in multiple areas of electromagnetics, communication circuits and systems as well as information theory, where innovative schemes for modulation and channel coding can be investigated. This approach can also be combined with MIMO systems to increase the number of transmit/receive paths without increasing the number of antennas [102].
The DPC feature of multi-port driven integrated antennas can be utilized to encode data in the polarization of EM waves and dynamically switch the transmitted polar- ization at very high rates; i.e., to “modulate” polarization and transmit polarization
“symbols” as data. Figure 5.53 shows an example of such a modulation scheme where the data is encoded in the polarization angle of the linearly polarized transmit-
Figure 5.53: DPC radiator can switch between different polarizations at symbol rate,Tsym, to transfer data through Polarization Modulation.
ted EM waves and is being transmitted at the rate of 1/Tsym. Here, the polarization symbols are the different polarization angles of the transmitted EM waves over each symbol period,Tsym.
When the data is solely transferred in the polarization itself, the security of the data stream would significantly improve too, because in this case the phase and amplitude of the signal can be intentionally adjusted to mask the true data set from undesired receivers. This is due to the fact that only a polarization agile receiver (introduced in [103]) with multi-port antenna or antenna arrays that can detect all polarizations is able to capture the true polarization angle of the received signal and decode the data regardless of the possible misinformation encoded in the phase and amplitude of the received signal. Any single-port receiving antenna or any multi-port antenna whose polarization is fixed cannot distinguish between a signal that has zero amplitude and a signal with finite amplitude that is in the orthogonal polarization with respect to the antenna polarization and thus would not be able to decode the data.
Since Pol-M is a spatial concept, it is completely independent and orthogonal to existing phase and amplitude modulation of the signal’s time-domain waveform.
Therefore, we can implement transmitter architectures, which incorporate simulta- neous phase and amplitude modulations, combined with Pol-M in any two orthog- onal polarizations (horizontal and vertical, clock-wise and counter clock-wise, etc.) as independent dimensions for modulation to achieve very high data rate mm-wave communications. As an example consider two orthogonal polarizations,Px andPy. Each polarization on its own can accommodate two quadrature components,(Ix,Qx) for polarizationPx, and(Iy,Qy)for polarizationPy. The set(Ix,Qx,Iy,Qy)creates
168 an orthogonal basis for a 4-D constellation. Since it is difficult to visualize a 4-D constellation on a 2-D plane, Figure 5.54 shows the projections of such a constella- tion on the six orthogonal planes of such 4-D space whereIx andQx form 16-QAM modulation andIyandQyform a QPSK modulation. It should be noted that such sce- nario substantially subsumes the previously used scheme [15] where two orthogonal antennas form two channels to send independent data streams with two orthogonal polarizations to double the channel capacity for point-to-point polarization-matched applications by only using two 2-D constellations of the existing 4-D constellation.
Figure 5.54: Projections of a 4-D constellation on orthogonal planes of the 4-D space for an exemplary 4-D data constellation resulted by performing simultaneous quadrature modulation schemes on two orthogonal polarizations.
Figure 5.55 shows the conceptual block diagram of a transmitter architecture capable of performing simultaneous phase, amplitude, and polarization modulation. In this block diagram, “Signal Modulator” refers to the entire set of blocks, which only modulate amplitude and phase of the signal. However, the signal modulator and the integrated antenna with DPC, as a whole, act as “Signal and Polarization Modulator”.
Baseband inputs are fed to the system. Depending on the implementation, either all or different sets of baseband inputs modulate amplitude and phase of the signal and polarization of the transmitted EM waves, thus resulting in simultaneous phase, amplitude, and polarization modulation.
Figure 5.56 illustrates detailed block diagram of one possible implementation of such transmitter that uses our previously discussed DPC antenna. Quadrature LO signals are generated by a quadrature oscillator. The baseband inputs which control the two 360◦phase rotators perform two simultaneous tasks: 1) setting the relative phases of the driving ports of the antenna with respect to each other (modulating
Figure 5.55: Simplified block diagram of a transmitter capable of simultaneous phase, amplitude, and polarization modulation.
Figure 5.56: One possible implementation of a transmitter capable of performing Pol-M.
170 the polarization [103]) and 2) setting the relative phases of the driving ports of the antenna with respect to the QVCO phase (modulating signal’s phase). In addition, other baseband inputs, which control variable gain amplifiers, modulate the ampli- tude of the signal resulting in simultaneous modulation of phase, amplitude, and polarization of the transmitted EM waves.
In order to receive the 4-D constellation of such transmitter, similar to the stand- alone Pol-M modulation, a polarization agile receiver with multi-port antenna that can detect all polarizations can simultaneously demodulate phase, amplitude, and polarization information. In these receivers, phases and amplitudes of the received signals at all ports of the receive antenna are used together to extract phase, ampli- tude, and polarization information.
5.4.1 Prototype: Transmitter and Receiver Architectures for Pol-M
In this section, we present prototype transmitter and receiver architectures for Pol-M, implemented as proof-of-concept printed circuit boards (PCB) for a 2.4 GHz Pol-M link.1
Any linear polarization of an electric field E can be expressed as a vector sum of two perpendicular linear polarizations Ex and Ey. Our prototype transmitter and receiver architectures for Pol-M utilize this orthogonal decomposition. The transmitter spatially combines two perpendicular linear polarizations of electric field with different gains to generate the desired linear polarization. Figure 5.57(a) shows the conceptual diagram of the architecture for the transmitter. Depending on the desired polarization angle for each symbol, the baseband signal is weighted properly in two independent paths (A1and A2) and is then upconverted. These two paths are then feed to the two ports of a dual-port antenna which can simultaneously radiate orthogonal polarizationsExandEy. The far-field linearly polarized electric field can thus be expressed as a superposition of the two radiated polarizations:
E= k A1cos(ω0t)uˆx+k A2cos(ω0t)uˆy (5.8) whose polarization angle is:
angle(E)=tan−1(A2/A1) (5.9) which indicates that depending on the choice for A1 and A2 the polarization an- gle of the transmitted electric field can be controlled. In 5.8, the parameter k
1This was a joint project with my colleague Kaushik Dasgupta. I was responsible for the electromagnetic design of the transmitter and receiver antennas and Kaushik was in charge of the circuit implementation for both transmitter and receiver units.
Figure 5.57: System architecture for the prototype polarization modulation (a) transmitter and (b) receiver.
5.4.1.1 Antenna Design
A dual port antenna which is capable of transmitting/receiving two orthogonal polarizations (Ex andEy) corresponding to the two driving/receiving ports (Port- X and Port-Y) can be used for electromagnetic radiation and reception for both transmitter and receiver in a Pol-M link. Furthermore, it should provide sufficient isolation between the two ports to minimize cross-polarization in both transmitting and receiving modes. Such isolation is also necessary to avoid input impedance variation at Port-X based on how strong the signal Port-Y is and vice versa.
A dual port patch antenna is designed to achieve these goals, as shown in Figure 5.58(a) [104]. The patch is designed to be resonant at 2.4 GHz, and quarter-wave transmission lines are used to match the input impedance of each port to 50 Ω. Figure 5.58(b) shows the simulated radiation pattern and antenna gain while the patch is driven only at one port with the other port shorted to ground. Similar radiation pattern and antenna gain are achieved for both cases of Port-X and Port-Y individual drive. Figure 5.58(c) shows the isolation between the two ports versus frequency. The simulated isolation of 37 dB ensures that in the transmitter each port can be driven almost independently with arbitrary amplitude and phase to transmit