Successive Interference Cancellation (SIC)

Ah, I see your point now more clearly. You're asking a crucial question: How can a device subtract the strong signal from the received signal if it doesn't know what the strong signal is?

This is the essence of Successive Interference Cancellation (SIC), and I’ll explain in detail how this actually works in practice:

The Key Idea Behind SIC

In SIC, the process of subtracting the strong signal to improve the weak signal relies on the fact that the receiver already knows the strong signal once it has decoded it. The confusion usually comes from the fact that it sounds like the receiver is magically subtracting something it doesn't know. Here's how it works:

Step-by-Step Breakdown of SIC

1. Superposed Transmission

Both users transmit their signals simultaneously, and the received signal at the receiver is the sum of both signals, as well as noise.

Received Signal = h1 * Signal1 + h2 * Signal2 + Noise

Here, h₁ and h₂ are the channel coefficients for each user, and the signals Signal₁ and Signal₂ are the transmitted signals from Users 1 and 2, respectively.

2. Decode the Strongest Signal First

The receiver first decodes the strongest signal (typically the user with the higher signal power or better channel conditions).
The receiver does this by applying an appropriate demodulation technique (like QPSK) to extract the data from the strongest signal. It knows the modulation scheme and can identify the transmitted symbols.
After decoding the strongest signal, the receiver now knows what that signal is. Importantly, this is where the receiver gains knowledge of the strong signal.

3. Subtract the Strong Signal

Once the strong signal has been decoded, it is subtracted from the received signal to eliminate its contribution as interference. This process effectively removes the strong signal from the received mixture.

Received Signal for User 2 (after SIC) = Received Signal - h1 * Decoded Signal1

The strong signal (Decoded Signal1) is multiplied by the channel coefficient h₁ and then subtracted.
This leaves only the signal of the weak user, along with noise, for further decoding.

4. Decode the Weak Signal

With the interference from the strong user subtracted, the receiver now attempts to decode the weak user's signal.
Since the strong signal is no longer present, the weak signal can be decoded with much less interference.

5. Iterative Process (More Than Two Users)

If there are more than two users, the same process is repeated: decode the strongest signal, subtract it, and then decode the next strongest signal. This is why it is called Successive Interference Cancellation.

How the Receiver "Knows" the Strong Signal

The receiver does not need to know the strong signal in advance. It only needs to know it after it decodes it.
Using the modulation scheme, the receiver extracts symbols and reconstructs the transmitted signal.
The process of demodulation and decoding provides all the information needed for subtraction.

Key Assumptions for SIC to Work

Channel Knowledge: The receiver must know the channel coefficients via pilot symbols or feedback.
Perfect Decoding: Errors in decoding the strong signal lead to imperfect cancellation.
Known Modulation Scheme: Required to correctly demodulate and reconstruct the signal.
Sufficient SNR: The strongest signal must have high enough SNR for reliable decoding.

Example Scenario

User 1 has a much stronger signal than User 2.
The receiver decodes User 1’s signal first.
The decoded signal is subtracted from the received mixture.
User 2’s signal is then decoded with reduced interference.

Without SIC

Without SIC, the strong signal is treated as noise, leading to a higher symbol error rate (SER) for the weak user.

Summary

The receiver does not know the strong signal beforehand.
It decodes the strongest signal first and then subtracts it.
This enables clean decoding of weaker signals.

This is the core of how SIC works. Accurate decoding followed by subtraction allows weak signals to be recovered with much less interference.

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