Where is spatial multiplexing used

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Spatial multiplexing is a MIMO wireless protocol that sends separate data signals or streams between antennae to enhance wireless signal performance or functionality. A wireless access point uses multiple radios to enable more than one unique data stream to go between the transmitter and receiver.

In diversity techniques, same information is sent across independent fading channels to combat fading. When multiple copies of the same data are sent across independently fading channels, the amount of fade suffered by each copy of the data will be different. This guarantees that at-least one of the copy will suffer less fading compared to rest of the copies. Thus, the chance of properly receiving the transmitted data increases.

In effect, this improves the reliability of the entire system. This also reduces the co-channel interference significantly. Consider a SISO system where a data stream [1, 0, 1, 1, 1] is transmitted through a channel with deep fades. Due to the variations in the channel quality, the data stream may get lost or severely corrupted that the receiver cannot recover. The solution to combat the rapid channel variations is to add independent fading channel by increasing the number of transmitter antennas or receiver antennas or the both.

The SISO antenna configuration will not provide any diversity as there is no parallel link. Thus the diversity is indicated as 0. In this Single Input Multiple Output SIMO antenna system, two copies of the same data are put on two different channels having independent fading characteristics. Even if one of the link fails to deliver the data, the chances of proper delivery of the data across the other link is very high. Thus, additional fading channels increase the reliability of the overall transmission — this improvement in reliability translates into performance improvement — measured as diversity gain.

For a system with transmitter antennas and receiver antennas, the maximum number of diversity paths is. In the following configuration, the total number of diversity path created is. In this way, more diversity paths can be created by adding multiple antennas at transmitter or receiver or both.

The following figure illustrates a MIMO system with number of diversity paths equal to. In spatial multiplexing, each spatial channel carries independent information , there by increasing the data rate of the system. This can be compared to Orthogonal Frequency Division Multiplexing OFDM technique, where, different frequency subchannels carry different parts of the modulated data.

But in spatial multiplexing, if the scattering by the environment is rich enough, several independent subchannels are created in the same allocated bandwidth. Thus the multiplexing gain comes at no additional cost on bandwidth or power. The multiplexing gain is also referred as degrees of freedom with reference to signal space constellation [2]. The number of degrees of freedom in a multiple antenna configuration is equal to , where is the number of transmit antennas and is the number of receive antennas.

The degrees of freedom in a MIMO configuration governs the overall capacity of the system. Following figure illustrates the difference between diversity and spatial multiplexing. In the transmit diversity technique shown below, same information is sent across different independent spatial channels by placing them on three different transmit antennas. Here, the diversity gain is 3 assuming MISO configuration and multiplexing gain is 0. In the spatial multiplexing technique, each bit of the data stream independent information is multiplexed on three different spatial channels thereby increasing the data rate.

Here, the diversity gain is 0 and the multiplexing gain is 3 assuming MIMO configuration. While some improvements can be made in terms of optimising the modulation scheme and improving the signal to noise ratio, these improvements are not always easy or cheap and they are invariably a compromise, balancing the various factors involved. It is therefore necessary to look at other ways of improving the data throughput for individual channels.

MIMO is one way in which wireless communications can be improved and as a result it is receiving a considerable degree of interest. To take advantage of the additional throughput capability, MIMO utilises several sets of antennas. In many MIMO systems, just two are used, but there is no reason why further antennas cannot be employed and this increases the throughput.

In any case for MIMO spatial multiplexing the number of receive antennas must be equal to or greater than the number of transmit antennas. To take advantage of the additional throughput offered, MIMO wireless systems utilise a matrix mathematical approach.

Data streams t1, t2,. Then there are a variety of paths that can be used with each path having different channel properties. Capano November 26, Factory Technologies. Gray Solutions - Stone Technologies, Inc. By using this website, you agree to our use of cookies. This may include personalization of content and ads, and traffic analytics.

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