STBC vs SM: Comparing MIMO Techniques

This page compares STBC vs SM and mentions the difference between STBC and SM used as MIMO techniques.

STBC stands for Space Time Block Coding, and SM stands for Spatial Multiplexing or Spatial Modulation. Advantages of both are mentioned.

STBC (Space Time Block Coding)

STBC (Space Time Block Coding) is a MIMO technique that transmits multiple copies of the same data symbol across multiple antennas and exploits the various received versions to derive (or decode) the transmitted data efficiently without any errors. This technique improves the reliability of transmission in a fading environment.

To perform the decoding function, Space Time Coding algorithms at the receiver combine all the copies of the received signals to extract the transmitted information.

Figure 1: STBC Transmitter part

The figure above depicts STBC 2T2R (2 Transmit, 2 Receive) configuration. Here, at time instant $t = T_1$, data symbols ($X_1, X_2$) are transmitted from antenna-1 and antenna-2. After some duration at time instant $t = T_2$, modified copies of $(-X_2^*, X_1^*)$ are transmitted from Ant-1 and Ant-2, respectively. As shown, after the appropriate modulation (BPSK/QPSK/16QAM etc.), the data symbols are parallelized as per Alamouti codes and the number of transmit antennas. After appropriate modifications, the data symbols are transmitted using multiple antennas.

There are many other MIMO configurations as per the code defined by Alamouti for multiple transmit antennas (2, 3, or 4) and the number of receive antennas used.

This STBC increases the coverage of the system compared to the SM technique. Following are the useful characteristics of the STBC (Space Time Block Coding) based MIMO system.

• The input will be a single stream, and the output will be multiple streams. (Single is split into multiple)
• At the Receiver, the number of Antennas can be a minimum of 1 and a maximum of any number (no limit).
• For Channel estimation, the least square (division logic either using preamble or pilots) method is used.
• For Channel Equalization, ML decoding (Almoutti) or the MRC method is used.

Figure 2: STBC receiver part

SM (Spatial Multiplexing / Spatial Modulation)

SM (Spatial Multiplexing) is a MIMO technique to transmit independent and separate copies of data symbols. These encoded data symbols are called streams from each of the transmit antennas. As they are transmitted over the space and multiplexed in time, it is known as Spatial Multiplexing. It is also known as Spatial Modulation.

Figure 3: SM Transmitter part

Here, at time instant $t = T_1$, data symbols ($X_1, X_2$) are transmitted from antenna-1 and antenna-2, while after some duration at time instant $t = T_2$, different data symbols ($X_3, X_4$) are transmitted from Antenna-1 and Antenna-2, respectively. Hence, data from multiple information sources are separately encoded and modulated before transmission from multiple antennas simultaneously.

This increases the data rate of the system compared to the STBC technique.

$N_s = \text{Min}(N_t, N_r)$

Where:

• $N_s$ = Maximum spatial multiplexing order or number of streams
• $N_t$ = Number of Transmitter Antennas
• $N_r$ = Number of Receiver Antennas

Following are the useful characteristics of the SM (Spatial Multiplexing) based MIMO system.

• The input will be multiple streams, and symbols will be mapped to antennas.
• At the Receiver, the number of antennas should be the same as or more than the number of transmit antennas but should not be less.
• For Channel Estimation, the least square (division logic either using preamble or pilots) method is used.
• For Channel Equalization, ZF/MMSE/ZF-SIC/MMSE-SIC/Sphere decoder/ML decoding method is used.

Conclusion

Both STBC and SM MIMO techniques are used together in wireless systems to have benefits (i.e., advantages) of both in terms of coverage and data rate, respectively.