STBC vs SM | Difference between STBC and SM, MIMO techniques
This page compares STBC vs SM and mentions 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 Transmitter part | Space Time Block Coding
STBC (Space Time Block Coding) is a MIMO technique which transmit multiple copies of the same data symbol across multiple antennas and exploit the various received versions to derive (or decode) the transmitted data efficiently without any errors. This technique improves reliability of transmission under fading environment. To perform the decoding function, Space Time Coding algorithms at the receiver combines all the copies of the received signals to extract the information transmitted.
The figure-1 depicts STBC 2T2R (2 Transmit, 2 Receive) configuration. Here at time instant (t = T1) data symbols (X1, X2) are transmitted from antenna-1 and antenna-2 while after some duration at time instant (t = T2) modified copies of (-X2*,X1*) 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 number of transmit antennas. After appropriate modifications, the data symbols are transmitted using multiple antennas. There are many other MIMO configurations as per code defined by Alamouti for multiple transmit antennas (2, 3 or 4) and number of receive antennas used. This STBC increases the coverage of the system compare to SM technique.
Following are the useful characteristics of STBC (Space Time Block Coding) based MIMO system.
• The input will be single stream and output will be multiple Streams. (Single is split into multiple)
• At Receiver No. of Antennas can be min. 1 and maximum any (no limit).
• For Channel estimation, least square (division logic either using preamble or pilots) method is used.
• For Channel Equalization, ML decoding (Almoutti) or MRC method is used.
STBC receiver part
Refer MIMO MATLAB Code>> which mentions MATLAB code for 2T1R and 2T2R configurations of STBC MIMO with 2 Antennas at transmit side.
SM Transmitter part | 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. Here at time instant (t = T1) data symbols (X1, X2) are transmitted from antenna-1 and antenna-2 while after some duration at time instant (t = T2) different data symbols (X3, X4) 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 compare to STBC technique.
Ns = Min (Nt, Nr)
Ns = Maximum spatial multiplexing order or number of streams
Nt = Number of Transmitter Antennas
Nr = Number of Receiver Antennas
Following are the useful characteristics of SM (Spatial Multiplexing) based MIMO system.
• The input will be multiple streams and symbols will be mapped to antennas.
• At Receiver, number of antennas should be same or more than number of transmit antennas, but should not be less.
• For Channel Estimation, 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.