Why MIMO?
More antennas sending same data will mean better signal quality ,and more reliability
More antennas sending different data will mean better throughput or you can say improved data speed.
One of the biggest factors limiting data speeds is bandwidth.
More than one antenna can be used to send the signal on the same frequency thus solving the bandwidth limitation.
More than one antenna can be used to send the signal on the same frequency thus solving the bandwidth limitation.
Speed/Throughput is also limited by signal-to-noise ratio (SNR), to which we can increase the power (or loudness) of the transmission so the phone tower can 'hear' us better.
But this could result in interference. Once we've squeezed all the performance we can from antenna-to-antenna transmission, we have to approach the problem differently.
This is where MIMO comes in to play - if we're unable to improve air transmission, why not increase the number of antennas?
But this could result in interference. Once we've squeezed all the performance we can from antenna-to-antenna transmission, we have to approach the problem differently.
This is where MIMO comes in to play - if we're unable to improve air transmission, why not increase the number of antennas?
What is MIMO?
A device with multiple antenna's would be called a MIMO device because it has more than one antenna and can support more than one data stream at once.
Figure above shows 2X2 MIMO
A 2×2 MIMO device has two antennas for two simultaneous data streams and a 4×4 MIMO device has four antennas for four data streams.Where the numbers 2x2,4x2,4x4 indicates the number of Tx antenna and the number of Rx antenna.
A 2×2 MIMO device has two antennas for two simultaneous data streams and a 4×4 MIMO device has four antennas for four data streams.Where the numbers 2x2,4x2,4x4 indicates the number of Tx antenna and the number of Rx antenna.
The figure below shows the multiple antenna strips on your phone (4X4 MIMO)
The network provider is also using 4 transmit antennas thus helping your mobile to take advantage of the MIMO feature available on your individual devices
The number of antennas helps to increase capacity.
Theoretically the more antennas, the faster you can transmit/receive or the more users you can serve.The more antennas your device has, the more data it can transfer at once and that means faster wireless download and upload speeds.
The figure above shows 2 X 2 MIMO on a handset
2 antenna strips done
MIMO Technologies Introduced
- Spatial Multiplexing
- Transmit Diversity/Receiver Diversity
- Beamforming
What is Spatial Multiplexing
Spatial multiplexing allows to transmit different streams of data simultaneously
on the same downlink resource block(s).
These data streams can belong to one
single user (single user MIMO / SU-MIMO) or to different users (multi user
MIMO / MU-MIMO).
While SU-MIMO increases the data rate of one user, MUMIMO allows to increase the overall capacity
Figure below shows an example of a 2X2 MIMO in SU MIMO mode and MU-MIMO mode
Theoretically for 2x2 MIMO, for example, two independent streams can be generated , while for 4x4 (four transmit antennas and four receive antennas), it can generate four streams, potentially quadrupling the throughput.
However practically the actual throughput will depend on Signal to noise Ratio that can be achieved in each of the streams.
MIMO connections use advanced signal processing algorithms.
To take advantage of the additional throughput offered, MIMO wireless systems utilize a matrix mathematical approach.
Data streams A1, A2 A3 can be transmitted from antennas 1, 2 and 3.
Where B1 = signal received at antenna 1, B2 is the signal received at antenna 2 and so forth.Then there are a variety of paths that can be used with each path having different channel properties from each of these antennas.These can be represented by the properties h12 , travelling from transmit antenna 1 to receive antenna 2 and so forth.
In this way for a three transmit, three receive antenna system a matrix can be set up:
B1 = h11 A1 + h21 A2 + h31 A3
B2 = h12 A1 + h22 A2 + h32 A3
B3 = h13 A1 + h23 A2 + h33 A3
In matrix format this can be represented as:
To recover the transmitted data-stream at the receiver it is necessary to perform a considerable amount of signal processing.
First the MIMO system decoder must estimate the individual channel transfer characteristic hij to determine the channel transfer matrix.
Once all of this has been estimated, then the matrix [H] has been produced and the transmitted data streams can be reconstructed by multiplying the received vector with the inverse of the transfer matrix.
C1,C2 and C3 is the final reconstructed signal at the mobile receiver end.
In any case for MIMO spatial multiplexing the number of receive antennas must be equal to or greater than the number of transmit antennas.
What is Spatial Multiplexing
Spatial multiplexing allows to transmit different streams of data simultaneously
on the same downlink resource block(s).
These data streams can belong to one
single user (single user MIMO / SU-MIMO) or to different users (multi user
MIMO / MU-MIMO).
While SU-MIMO increases the data rate of one user, MUMIMO allows to increase the overall capacity
Figure below shows an example of a 2X2 MIMO in SU MIMO mode and MU-MIMO mode
Figure below shows an example of a 2X2 MIMO in SU MIMO mode and MU-MIMO mode
However practically the actual throughput will depend on Signal to noise Ratio that can be achieved in each of the streams.
MIMO connections use advanced signal processing algorithms.
To take advantage of the additional throughput offered, MIMO wireless systems utilize a matrix mathematical approach.
Data streams A1, A2 A3 can be transmitted from antennas 1, 2 and 3.
Where B1 = signal received at antenna 1, B2 is the signal received at antenna 2 and so forth.Then there are a variety of paths that can be used with each path having different channel properties from each of these antennas.These can be represented by the properties h12 , travelling from transmit antenna 1 to receive antenna 2 and so forth.
In this way for a three transmit, three receive antenna system a matrix can be set up:
Data streams A1, A2 A3 can be transmitted from antennas 1, 2 and 3.
Where B1 = signal received at antenna 1, B2 is the signal received at antenna 2 and so forth.Then there are a variety of paths that can be used with each path having different channel properties from each of these antennas.These can be represented by the properties h12 , travelling from transmit antenna 1 to receive antenna 2 and so forth.
In this way for a three transmit, three receive antenna system a matrix can be set up:
In matrix format this can be represented as:
To recover the transmitted data-stream at the receiver it is necessary to perform a considerable amount of signal processing.
First the MIMO system decoder must estimate the individual channel transfer characteristic hij to determine the channel transfer matrix.
Once all of this has been estimated, then the matrix [H] has been produced and the transmitted data streams can be reconstructed by multiplying the received vector with the inverse of the transfer matrix.
C1,C2 and C3 is the final reconstructed signal at the mobile receiver end.
What is Receive/Transmit Diversity
Transmit diversity is the default MIMO mode.
When multiple copies of the same data are transmitted by the Enode B the handset receives those multiple copies.
The receiver can select the best one or combine them all together in such a way to improve data quality.
So we get less errors while receiving data and helps improving reliability.
This is called Receive /transmit diversity.
Transmit diversity scheme is applicable to all the physical channels such as PDSCH ,Physical Broadcast channel (PBCH),Physical control Format Indicator channel(PCFICH),Physical down link control channel (PDCCH)and Physical Hybrid ARQ indicator channel (PHICH).
Transmit diversity scheme is applicable to all the physical channels such as PDSCH ,Physical Broadcast channel (PBCH),Physical control Format Indicator channel(PCFICH),Physical down link control channel (PDCCH)and Physical Hybrid ARQ indicator channel (PHICH).
The other MIMO schemes are applicable to PDSCH (Physical downlink shared channel)
Note:No transmit diversity scheme is applied to the primary and secondary synchronization signals
UE can recognize the number of transmit antennas at Enode B among {1,2,4}by decoding PBCH
Once the number of transmit antennas at eNodeB is detected by the UE it
gives feedback to the EnodeB using the Rank Indicator parameter
RI value is very closely related to the number of Antenna.It determines the number of data streams detected .For example in 2x 2 MIMO
When the value is 2 it means that there is no interference between the antennas
If the value is 1, it implies that the signal from the two Transmission antenna is perceived by UE (Mobile) to be like single signal from single Antenna.
If transmit diversity scheme will be applicable to the other physical downlink channels will thus be determined using the Rank Indicator.
The eNodeB may decide the transmission mode , taking into account the RI reported by the UE.
Transmission mode is a combination of the MIMO schemes used by the eNode B depending on the channel state condition experience by the user equipment's.
Some telecom operators use only 2 modes in case of release 8 one is TM1(Transmission Mode 1) that uses transmit diversity in cases where interference is more
TM4 is used when the signal conditions are good (uses spatial multiplexing Technnology).
Appendix
How is the MIMO Transmission mode selected for a particular user equipment?
Even though the system is configured in transmission mode 4 for a particular UE and if the same UE reports the Rank Indication value 1 to eNB, eNB will start sending the data in Transmit diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in spatial diversity MIMO mode .
How does Category of Handset determine the throughput and MIMO functionalities?
How is the MIMO Transmission mode selected for a particular user equipment?
Even though the system is configured in transmission mode 4 for a particular UE and if the same UE reports the Rank Indication value 1 to eNB, eNB will start sending the data in Transmit diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in spatial diversity MIMO mode .
How does Category of Handset determine the throughput and MIMO functionalities?
A UE signals its category (or categories) within UE Capability Information message. So depending upon the handsets category eNode B will support only those features that can be handled by the UE.User Equipment/Handset Category determines if MIMO mode can be used
What is the use of CQI[Channel Quality Indicator]
The CQI( Channel Quality Indicator) feedback indicates a combination of modulation scheme (ex.QPSK,QAM)and channel coding rate (the redundancy bits/duplication of data/same copies of the data to be sent)that the eNodeB should use to ensure that the block error probability experienced at the UE will not exceed 10%.
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