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CHAPTER 1. INTRODUCTION
Bottleneck
Nodes
(a) A network bottleneck.
Bottleneck
Node
(b) Network bottleneck in
a relay scenario.
Figure 1.3: Bottlenecks in a network.
Figure 1.4b, Alice first transmits a packet to the relay node, which relay the packet to
Bob. Next Bob transmits a packet to the relay node, which again relay the packet, this
time to Alice. This is a total of 4 transmissions in order for Alice and Bob to exchange
packets. Using network coding, Figure 1.4c, this is reduced to three transmissions,
since the relay node waits until it has received a packet from both Alice and Bob, then
it codes the two packets to one and broadcast it, so both Alice and Bob receives it.
Alice and Bob are both able to decode the packet using their own original packet. In
this example there is a coding gain of 43 , because 4 transmissions are reduced to 3 using
network coding.
A challenge with network coding in the relay scenario is; if Alice wish to transmit a
packet to Bob, and Bob does not desire to transmit anything to Alice. How long shall
the relay node wait for a packet from Bob to code the packet from Alice with, before
it relays Alice’s packet with no network coding. If it delays transmission for too long
it introduces a delay in the system, possibly lowering the overall throughput through
the relay.
There are multiple types of digital network coding, XOR, Linear Coding, Random
Network Coding, etc. [29]. This project mainly concerns binary XOR network coding,
which is XOR’ing of the binary data. In the example in Figure 1.4c, this would mean,
that the relay node makes a binary XOR of the packets from Alice and Bob, which
is then broadcasted. Upon reception Alice and Bob performs an XOR of the received
coded packet with their original transmitted packets to extract the packet from the
other party.
Physical Layer Network Coding
In conventional network routing it is desired, that each transmission does not interfere
with another transmission, since this results in a collision. However in physical layer
network coding this interference is embraced, and it is desired, that two given nodes
transmit at the exact same time [9]. When two nodes transmit at the same time, the
relay node receives an interfered signal. If this interfered signal is just forwarded to the
recipients, using a single broadcast, the two nodes may in principle be able to obtain
the other parties packet by subtracting their original transmission from the received
broadcast. This is called amplify-and-forward [11], and results in a gain of 24 .
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