Orbital Angular Momentum (OAM) is an intrinsic property of electromagnetic waves. Great research has been witnessed in the last decades aiming at exploiting the OAM wave property in different areas in radio and optics. One promising area of particular interest is to enhance the efficiency of the available communications spectrum. However, adopting OAM-based solutions is not priceless as these suffer from wave divergence especially when the OAM order is high. This shall limit the practical communications distance, especially in the radio regime. In this paper, we propose a cooperative OAM relaying system consisting of a source, relay, and destination. Relays help the source to transmit packets to the destination by providing an alternative connection between source and destination. This cooperative solution aims on the one hand, through best-path selection, on increasing the communications range. On the other hand, through the parallel transmission orders allowed by OAM carrying waves, the system could raise its total transmission throughput. Simulation results show that combining a cooperative relay with OAM improves the system throughput compared to using each element separately. In addition, the proposed cooperative relaying OAM outperforms the cooperative relaying non-orthogonal multiple access scheme, which is a key spectrally efficient technique used in 5G technology.
In recent years, wireless connection has become the dominant form of communication, but also remains one of the most challenging. For example, the internet of things requires massive wireless connectivity, such as Fifth Generation (5G) technology, that cannot be realized with current network infrastructure, which cannot achieve sufficient throughput, where throughput measures the rate of successful transmission [
Cooperative communication is considered a key technology that can improve wireless network performance to meet future requirements [
There are two relay modes, named according to the method the relay node uses to process the received signal: Amplify-and-Forward (AF) and Decode-and-Forward (DF). In AF mode, the relay node amplifies the received signal and forwards it to users, this makes AF simple to implement, however, it also has the disadvantage of noise amplification [
In DF mode, the relay node decodes the received signal, re-encodes it, and then forwards it to users. The DF mode has no noise amplification drawback due to decoding operations at the relay node; however, to enable accurate decoding, a better channel is required compared to the one needed for AF mode [
Due to the relays’ capability in fighting path losses, combining cooperative relaying with other 5G techniques has become a trend in the current literature. For example, the authors in [
In addition to mm-wave and NOMA performance, other advanced radio spectrum efficiency enhancement techniques are expected to play a role in the transition to 5G technology, which were investigated under cooperative relay channels. Among these are massive multiple-input multiple-output, co-frequency, co-time, and full-duplex techniques [
In addition, a new and promising resource for spectrum efficiency hinges on Allen et al.’s [
Waves carrying OAM exhibit helical phase-fronts owing to the azimuthal dependence of their spatial phase distribution
In this paper, we combine an OAM multi-access technique with a relay cooperative network. To the best of the authors’ knowledge, no previous studies have employed cooperative communication to enhance OAM performance. The combining of OAM and relay shows several mutual advantages to both of OAM and relay. The design overcomes many of the limitations usually encountered in the utilization of OAM-based solutions in radio, mainly the problem of wave divergence where the size of the null amplitude region at the beam axis expands during propagation.
Such beam divergence demands the use of large receivers and may severely reduce transmission distance, particularly for high-order OAM orders, where the null region is quite large. Raising the frequency of transmission helps to confine the wave over larger distances, making OAM-based solutions more practical for high-frequency mm-wave networks [
The relay-based solution adopted in this paper reduces the distance between the transmitter and receiver, as the best (i.e., shortest) path is inherently offered by the relay selection. The distance reduction via relays makes OAM viable for applications with lower frequencies than mm-wave networks. In addition, fading is a limiting factor of the OAM orders that can be used in transmission.
The use of relays adds a degree of freedom; rather than having only a direct source–destination link, there is another path through the relay. Hence, if one link has deep fading or low channel gain, the other link is available to avoid bad transmission. Furthermore, the OAM transmission approach accommodates the relay–receiver link with the orthogonal orders used to raise the throughput via parallel channel transmission. In summary, this paper suggests a novel system that employs OAM technique in a relay cooperative network to overcome the beam divergence limitation and deep fading. Hence, higher throughout is achieved.
This paper is organized as follows: Section 2 includes a general overview of the proposed cooperative relay network based on the OAM technique and describes the signal model. Numerical results and comparisons between derived formulas and those obtained via other studies are shown in Section 3. Finally, conclusions are drawn in Section 4.
The system model of relay cooperative networks is shown in
We assume that the source always has sufficient information (i.e., it is saturated) to send to relays in all time slots. Due to path loss and shadowing, we assume that the source and destination are not directly connected. Without losing generality, we assume the transmission power to be
The data rate is assumed to be fixed at a value of
The channel state information at the receivers is assumed to be available. At time slot
where
The channel gains
Since the system throughput is a function of channel outage, we first describe the outage analysis of the relay network. An outage occurs if the link capacity is less than the target data rate:
Assuming a wave is carrying an OAM order,
The exponential term is a Gaussian envelope, where
The wave propagation in space is governed by the paraxial Helmholtz equation:
where
where
In this section, we present simulation results to validate our analysis and design. We study the performance of our proposed relay cooperative OAM scheme. In the simulations, we assume that the noise variance (
In the first part of the simulation, we demonstrate the effect of receiving aperture size on received power. A receiver consisting of a sufficient number of sensors distributed around a circular circumference is considered. For the different cases demonstrated in
Recall from
As shown in
Different power-threshold values (as percentages of maximum power, see
It can be observed that lowering the threshold makes the reception less restrictive and allows higher OAM orders than raising the threshold; hence, higher throughput is achieved at low thresholds. For instance, from
As mentioned previously, OAM suffers from divergence that accompanies the wave propagation.
Finally, we present a comparison between our proposed system and other state-of-the-art research in
We compared the relay cooperative NOMA system to our proposed cooperative OAM system. In [
This paper presented a cooperative relaying system based on the OAM technique. The results showed that the receiver could detect more orders as its size increased, which in turn enhanced the total system throughput. We enhanced the overall received power throughput of all orders by increasing the SNR. While the OAM system suffered from divergence as the wave propagated, we were able to reduce the received power as the distance between the relay and the receiver increased, an effect that was more notable for higher orders. In addition, the proposed model outperformed the cooperative relaying NOMA scheme, which is a key spectrally efficient technique in 5G technology.