SC Superposition coding

Superposition coding (SC) is a technique used in wireless communication systems to improve spectral efficiency and increase data rates. It is commonly employed in multiple-input multiple-output (MIMO) systems where multiple antennas are used at both the transmitter and receiver.

In SC, the transmitter sends multiple data streams simultaneously by superimposing them on the same frequency band. Each data stream is encoded and transmitted using a different antenna at the transmitter. This superposition of multiple signals is made possible by taking advantage of the unique channel characteristics in MIMO systems.

The key idea behind SC is that the receiver can separate the different data streams by exploiting the spatial diversity provided by multiple antennas. Each antenna at the receiver picks up a linear combination of the transmitted signals, which includes contributions from all the antennas at the transmitter. By employing advanced signal processing techniques, the receiver can separate these mixed signals and recover the individual data streams.

Let's go through the steps involved in SC:

1. Encoding: The transmitter first encodes each data stream independently. This can be done using various coding schemes such as convolutional coding, turbo coding, or low-density parity-check (LDPC) coding. The encoding process adds redundancy to the data to enable error detection and correction at the receiver.
2. Mapping: After encoding, the data symbols are mapped onto complex modulation symbols. Common modulation schemes used in SC include quadrature amplitude modulation (QAM) and phase shift keying (PSK). Each data stream is associated with a specific constellation diagram.
3. Precoding: In MIMO systems, the transmitter applies precoding to the data symbols before transmission. Precoding involves multiplying each symbol by a complex weight or precoding matrix. The purpose of precoding is to exploit the channel state information (CSI) and maximize the received signal quality at the receiver.
4. Superposition: The precoded symbols from each data stream are then linearly combined and superimposed on the same frequency band. Each symbol is transmitted using a different antenna. The signals transmitted from different antennas are subject to different channel conditions due to spatial variations in the wireless channel.
5. Reception and Decoding: At the receiver, the signals received by each antenna are processed separately. The receiver uses advanced signal processing algorithms, such as maximum likelihood (ML) or minimum mean square error (MMSE) detection, to separate the mixed signals and recover the individual data streams.
6. Decoding and Demapping: Once the receiver separates the mixed signals, it performs decoding to detect and correct any errors introduced during transmission. The received symbols are demapped to obtain the decoded data symbols.
7. Demodulation and Decoding: The demapped symbols are demodulated to obtain the original data bits. The decoding process reverses the encoding performed at the transmitter, recovering the original data streams.

By superimposing multiple signals on the same frequency band, SC enables the simultaneous transmission of multiple data streams, thereby increasing the spectral efficiency of the system. The spatial diversity provided by MIMO techniques allows for improved performance in terms of data rates, link reliability, and resistance to fading and interference. However, SC requires sophisticated signal processing algorithms at the receiver to separate the mixed signals accurately.

Overall, SC is a powerful technique used in MIMO systems to enhance the capacity and efficiency of wireless communication systems by exploiting spatial diversity and leveraging multiple antennas at both ends of the link.