Experimental Evaluation of High-capacity Wireless Transmission Using Orbital Angular Momentum Multiplexing Technology

2.1 Principle

OAM is a physical property of electro-magnetic waves characterized by a helical phase front in the propagation direction. Since these characteristics can be used to create multiple orthogonal channels, wireless communication using OAM can increase radio-spectrum efficiency and expand the capacity of the scarce radio spectrum. The number of phase rotations is called an OAM mode. Different data signals can be transmitted at the same time by transmitting different signals using radio waves having different OAM modes [8].

Given an electromagnetic wave having this OAM property, the trace of the same phase takes on a helical shape in the direction of propagation (Fig. 2). For example, Figure 2(a) illustrates the trance of the same phase in OAM modes 1, 2, and 3. Radio waves having this OAM property cannot be received without a receiver (Rx) having the same number of phase rotations at the time of transmission. For this reason, if multiple radio waves having different OAM modes are superposed, they can eventually be separated without mutual interference as long as an Rx is prepared that can receive radio beams with the same phase rotations corresponding to each OAM mode. Figure 2(b) shows an example in which different signals are put on OAM modes 1, 2, and 3 and transmitted simultaneously. Technology for transmitting multiple data signals using this feature is called OAM multiplexing transmission technology.

Fig. 2. Principle of OAM multiplexing transmission technology.

2.2 Generation and separation

To generate a beam carrying the OAM mode n (L = n), antenna elements are connected with phase shifters that make n × 360 degrees of rotations. Figure 3(a) shows an example of beam generations of OAM modes 0, 1, and 2 using a uniform circular array (UCA) consisting of 8 antenna elements. Note that concurrent transmission of multiple OAM modes can be achieved by superposing multiple OAM signals, as in Fig. 3(c).

The separation of beams carrying OAM modes can be done in a similar manner as generation using antenna elements connected with phase shifters that make opposite direction of rotations. As far as the number of antenna elements are larger than 2n, n × 360 degrees of rotations are orthogonal one another. Therefore, the separation of each OAM mode from a mixed OAM mode signal can be done without aliasing. Figure 3(b) shows an example of the phase of each antenna element corresponding to the example above.

Fig. 3. (a) Generation of OAM modes with a UCA, (b) separation of OAM modes with a UCA, (c) intensity distribution (above) and phase distribution (below) of combined OAM modes (L is the number of OAM modes).

2.3 Transmission and reception

The beams with different OAM modes can be approximately generated or separated simultaneously using a UCA with a discrete Fourier transform (DFT). They are orthogonal as long as they propagate coaxially. Mathematically, the channel between UCAs is a circulant matrix, so the channel can be diagonalized using a DFT matrix as the singular value decomposition (SVD) method, meaning oppositely arranged transmitting and receiving UCAs can create several isolated channels regardless of the propagation distance. Theoretically, the OAM has an infinite depth of field, but the discretization of space sampling limits the maximum number of available OAM modes when using a UCA and DFT processor. The maximum number is equal to the number of antenna elements in a UCA. When using UCAs for transmitting and receiving antennas, the beam propagation with OAM modes can be analyzed using the standard narrow band MIMO theory.

We now discuss the theoretical background of an OAM multiplexing system with UCAs. Figure 4 shows a schematic chart of OAM multiplexing transmission. When UCAs are placed opposite each other, the antenna-array configuration is axisymmetric so that the channel matrix becomes circulant. When the number of antenna elements in a UCA is defined as n, the circulant matrix is diagonalized using the n × n DFT matrix

where l denotes OAM mode corresponding to the yth eigenvector, H and Σ denote the channel matrix between the receiving and transmitting UCAs and diagonal matrix, respectively, and Λ denotes the equivalent channel matrix including DFT processing, as shown in Fig. 4.

Fig. 4. Schematic of OAM multiplexing system.

One of the strengths of OAM multiplexing is that it only requires static processing and DFT, so we can use analog devices for OAM-mode generation and separation without digital signal processing. A Butler matrix is an analog circuit that can process DFT calculations with hybrid circuits and phase shifters, so we used it for OAM-mode generation and separation in our system [11, 17].

4.1 Experimental setup

Table 2 lists the specific parameters of our experimental setup. We multiplexed 15 data streams using 7 OAM modes and 2 UCAs with different linear polarizations in both transmitting and receiving antennas. We then designed antenna elements in the UCAs, which were patch antenna arrays with an antenna gain of 25 dBi, as shown in Fig. 5. The isolation between different linear polarizations of the antenna elements was more than 30 dB on average across the whole bandwidth. In accordance with the relationship between effective bits in a symbol and SNR of our digital signal processing and the symbol rate of 1.5 GHz, roughly a 20-dB SNR on average of 15 data streams is enough to obtain more than 100 Gbit/s. The amount of transmitting power and antenna gains was roughly 53 dB. The channel gain was then roughly –90 dB, and the noise floor including the receiver noise figure was −67.7 dBm. We thus determined that our experimental system was able to obtain more than a 20-dB SNR on average across all of OAM modes.

4.2 Experimental results

We conducted outdoor wireless transmission experiments at distances of 100 and 200 m. Each distance was a line-of-sight environment, as shown in Figs. 7 and 8, respectively. The alignment between the Tx and Rx was conducted using a laser rangefinder placed at the corner of the antenna. Error-free transmission with channel coding was 137.3 Gbit/s with 15 streams at a distance of 100 m. The spectral efficiency was 91.5 bit/s/Hz. The 200-m error-free transmission with channel coding was 117.5 Gbit/s with 14 streams. Figure 9 shows the representative measured constellations of 64 quadrature amplitude modulation (QAM) and 256 QAM. The SNRs were 21.03 and 24.55 dB, and the bit error rates (BERs) without FEC were 0.428 and 1.622%. We transmitted a few data frames and confirmed reproducibility. The spectral efficiency was 78.3 bit/s/Hz. The multiplex number of 14 at 200 m was due to the modified water-filling-power allocation and providing a margin for determining modulation and coding rates. Table 3 shows the transmission capacity, modulation, and coding rate of each stream. They are listed in descending order of transmission capacity, that is, the order of the receiving SNR. The modulations used were from quadrature phase-shift keying (QPSK) to 1024 QAM for each stream, and transmission rates were up to 11.25 Gbit/s. We previously reported the achievement of 119.7 Gbit/s transmission at a distance of 100 m and showed that similar capacity can be achieved at a distance of 200 m.

Fig. 7. Experimental environment for wireless OAM multiplexing transmission at a distance of 100 m in the field.

Fig. 8. Experimental environment for wireless OAM multiplexing transmission at a distance of 200 m in the field.

Fig. 9. Representative constellation diagrams of 64 QAM and 256 QAM.

Table 3. The receiving SNRs, modulation, and coding rate during transmission experiment at distances of 100 and 200 m.