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Spatial Multiplexing Techniques and Antenna Diversity for Multiuser MIMO Capacity Improvement

Updated on May 31, 2012
antenna separtion of spatial diversity
antenna separtion of spatial diversity

Spatial Diversity: It is a technique to fight against signal fading. If a radio signal is received through one channel in a deep fading environment, then there is a possibility of losing that signal. Once a signal is lost then there is nothing that can be done to recover it. That is why diversity technique is used to improve system performance in the presence of fading channels. In this technique, signals are transmitted and received through a number of channels instead of only one channel. The main idea behind diversity is that when several copies of the same signals are passed through different channels then they experience independent fading--since each signal pass through different physical channels or paths. There is high probability that some signals will undergo deep fading while the others may not. When these signals reach the receiver, there will be significant amount of energy to make a decision that what was actually sent. The major types of diversities are:

  • Frequency diversity
  • Temporal diversity
  • Multipath diversity
  • Spatial diversity

Why diversity technique? The main reason if to fight against deep fading channels.

What is spatial diversity? In spatial diversity a number of antennas are used to pick up the transmitted signals coming from different multipath fades. To achieve spatial diversity the receiving antenna must be spaced more the half the wavelength of the transmitted signals. The main constraints of this type of diversity are the number of antennas.

Application of spatial diversity- this technique is very effective in frequency selective and time selective fading channels.

Diversity signals combining methods

The main idea of diversity technique is to compare and combine different copies of the received signals-coming from independent fading channels- to increase the received power at the receiver so as signals can be interpreted correctly at the receiving ends. The most popular diversity combining methods are:

  • Maximum ratio combining- selects the best signal to noise ratio.
  • Pure selection diversity-select the best signal to noise ratio in the antenna branch.
  • Equal gain combining- almost the same as maximum ratio combining in performance.

Spatial diversity receiving antennas
Spatial diversity receiving antennas

Spatial Multiplexing

What is spatial multiplexing? It is a special type of multiplexing where different signals or data bits are transmitted through several independent (spatial) communication channels by multiple antennas and at the same time the receiving side also use multiple antennas for receiving signals-this way the communication systems increase the date transmission rate which is in direct proportion to the number of antennas used for both transmission and receiving purpose. The higher the number of antennas, the higher the number of data transmission rate. It is a proprietary multiplexing techniques developed by Stanford university.

  • Benefits of spatial multiplexing
  • It does not require any additional power
  • No additional bandwidth requirement

How does spatial multiplexing work?

Say. You have M= 3 number of antennas in the transmitting side and have K (a1, a2, a3, a4, a5, a6) = 6 bits for sending. At first divide the bits into M=3 sub streams of data {(a1, a3), (a2, a4), (a3, a6)} and then multiply each sub stream of data with three carrier frequency in order to transmit them via three separate antennas. If all the sub-streams had to be transmitted by one carrier then the bandwidth consumptions would be three time greater-this is one of the great advantage of spatial multiplexing.

Now at the receiving end each sub-stream will have three spatial signatures-that means total 9 spatial signature will be at the receiving antenna-due to the multipath environment each sub stream will have its own spatial signature. Based on this spatial signature sub-streams of data will be demultiplexed and decoded in order to get back the original data stream-this is how spatial multiplexing works.

Spatial Multiplexing
Spatial Multiplexing

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