Physical-Layer Network Coding
Physical-layer network coding (PNC) leverages the network coding operation performed by nature when electromagnetic waves mix and superimpose with each other. With PNC, the network throughput can be enhanced by 100% and communication latency can be reduced by 50% in special cases.
The concept of physical-layer network coding (PNC) was proposed in 2006 by Prof. Soung Liew and his students at The Chinese University of Hong Kong. Since then, it has developed into a subfield of network coding with wide implications.
The basic idea of PNC is to exploit the mixing of signals that occurs naturally when electromagnetic (EM) waves are superimposed on one another. It is a simple fact in physics that when multiple EM waves come together within the same physical space, they add. This additive mixing of EM waves is a form of network coding, performed by nature. In particular, at a receiver, the simultaneous transmissions by several transmitters result in the reception of a weighted sum of the signals. This weighted sum is a form of network coding operation by itself. Moreover, the additive network coding operation performed by nature can be transformed and mapped to other forms of network coding after the reception. Exploiting this phenomenon turns out to have profound and fundamental ramifications.
Why use it?
PNC brings significant benefits in terms of throughput enhancement and latency reduction. Some of the scenarios are considered below.
- Range extension: A relay can be used to extend the range of the network. By using PNC, throughput can be enhanced by 100% concerning conventional routing.
- Point-to-point networks: A neighboring node can also be used to relay the transmission from the source to the destination, even when the source and destination are within the communication range of each other. The relay in this case helps in improving transmission reliability and may enable ultra-reliable communication in point-to-point networks.
- Multihop/Line networks: In multihop networks, the traffic flow is in general bidirectional. PNC can help in such networks to reduce the latency and improve bandwidth.
Given the above scenarios, PNC can play a significant role in a smart city, internet of things (IoT), and industrial internet of things (IIoT) applications.
Further Read on Network Coding
In many wireless communication networks today, interference is treated as a destructive phenomenon. When multiple transmitters transmit radio waves to their respective receivers, a receiver receives signals from its transmitter as well as from other transmitters. The radio waves from the other transmitters are often treated as interference that corrupts the intended signal. In Wi-Fi networks, for example, when multiple nodes transmit together, packet collisions occur and none of the packets can be received correctly. PNC was an attempt to turn the situation around. By exploiting the network coding operation performed by nature, the ‘‘interference’’ could be put to good use.
The easiest way to illustrate the concept of PNC is through a two-way relay channel (TWRC). TWRC is a three-node linear network in which two end nodes, nodes 1 and 2, want to communicate via a relay node R. There is no direct signal path between nodes 1 and 2. An example is a satellite network in which nodes 1 and 2 are the ground stations, and the relay R is the satellite. Another example can be of a transmitter and receiver that are out of range of each other. An additional node can assist to relay the signal of both transmitter and receiver to enable communication and to extend the range of the network. The half-duplex constraint is often imposed on wireless communication systems to ease engineering design. With the half-duplex constraint, a node cannot transmit and receive at the same time. This means that the relay in TWRC cannot receive from node 1 or node 2 and transmit to them at the same time. A corollary is that each packet transmitted from node 1 to node 2 via relay R (and similarly, each packet transmitted from node 2 to node 1 via relay R) must then use up at least two time-slots to reach its destination.
Without the use of network coding, and with a design principle that tries to avoid interference, a total of four time-slots are needed to exchange two packets, one in each direction. In time slot 1, node 1 transmits a packet A to relay R; in time slot 2, relay R forwards A to node 2; in time slot 3, node 2 transmits a packet B to relay R; and in time slot 4, relay R forwards B to node 1.
PNC reduces the number of time slots to two. It allows nodes 1 and 2 to transmit together and exploits the network coding operation performed by nature in the superimposed EM waves. The figure illustrates the idea. In the first time slot, nodes 1 and 2 transmit A and B simultaneously to relay R. Based on the superimposed EM waves that carry A and B, relay R deduces the network-coded packet C = A ⊕ B. Then, in the second time slot, relay R broadcasts the network-coded packet C = A ⊕ B to nodes 1 and 2.
By allowing the two end nodes to transmit simultaneously to the relay and not treating this as collision, PNC can boost the system throughput by 100%.