LTE-A MIMO Concepts May 2015 Ashwani

8/9/2019 LTE-A MIMO Concepts May 2015 Ashwani

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Space-division multiple access (SDMA) uses directional or smart antennas to

communicate on the same frequency with users in different locations within range of the

same base station. An SDMA system was proposed by ichard oy and !"#rn $ttersten%

researchers at Array&omm% in ''.

unctions of M*M$

M*M$ can be sub-divided into three main categories% precoding% spatial multiple+ing or

SM% and diversity coding.

Spatial multiplexing requires M*M$ antenna configuration. *n spatial multiple+ing% a

high-rate signal is split into multiple lower-rate streams and each stream is transmitted

from a different transmit antenna in the same frequency channel. *f these signals arrive

at the receiver antenna array with sufficiently different spatial signatures and the receiver

has accurate &S*% it can separate these streams into (almost) parallel channels. Spatial

multiple+ing is a very powerful technique for increasing channel capacity at higher signal-

to-noise ratios (S,). he ma+imum number of spatial streams is limited by the lesser of

the number of antennas at the transmitter or receiver. Spatial multiple+ing can be used

without &S* at the transmitter% but can be combined with precoding if &S* is available.

Spatial multiple+ing can also be used for simultaneous transmission to multiplereceivers% nown as space-division multiple access or multi-user M*M$% in which case

&S* is required at the transmitter. /012 he scheduling of receivers with different spatial

signatures allows good separability.

Diversity coding techniques are used when there is no channel nowledge at the

transmitter. *n diversity methods% a single stream (unlie multiple streams in spatial

multiple+ing) is transmitted% but the signal is coded using techniques called space-time

coding. he signal is emitted from each of the transmit antennas with full or near

orthogonal coding. Diversity coding e+ploits the independent fading in the multiple

antenna lins to enhance signal diversity. !ecause there is no channel nowledge% there

is no beamforming or array gain from diversity coding. Diversity coding can be combined

with spatial multiple+ing when some channel nowledge is available at the transmitter.

http://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/ArrayCommhttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Diversity_Codinghttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/Multi-user_MIMOhttp://en.wikipedia.org/wiki/MIMO#cite_note-32http://en.wikipedia.org/wiki/Diversity_codinghttp://en.wikipedia.org/wiki/Channel_state_informationhttp://en.wikipedia.org/wiki/Space-time_codinghttp://en.wikipedia.org/wiki/Space-time_codinghttp://en.wikipedia.org/wiki/Array_gainhttp://en.wikipedia.org/wiki/ArrayCommhttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Diversity_Codinghttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/Multi-user_MIMOhttp://en.wikipedia.org/wiki/MIMO#cite_note-32http://en.wikipedia.org/wiki/Diversity_codinghttp://en.wikipedia.org/wiki/Channel_state_informationhttp://en.wikipedia.org/wiki/Space-time_codinghttp://en.wikipedia.org/wiki/Space-time_codinghttp://en.wikipedia.org/wiki/Array_gainhttp://en.wikipedia.org/wiki/Space-division_multiple_access


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MU-MIMO

*n radio% multi-user MIMO (MU-MIMO) is a set of

advanced MIMO (pronounced mee-moh or my-moh)% multiple-input and multiple-

output% technologies where the available antennas are spread over a multitude of

independent access points and independent radio terminals - each having one or

multiple antennas. *n contrast% single-user M*M$ considers a single multi-antenna

transmitter communicating with a single multi-antenna receiver. o enhance the

communication capabilities of all terminals% M3-M*M$ applies an e+tended

version of space-division multiple access (SDMA) to allow multiple transmitters

to send separate signals and multiple receivers to receive separate signals

simultaneously in the same band. 431& is a fundamental and practical M3-

M*M$ technology for broadcast and multiple access wireless communications.

5ie the relationship between $DM and $DMA% M3-M*M$ (and% similarly%

SDMA) can be thought of as an e+tension of M*M$ applied in various ways as a

multiple access strategy. A significant difference is that the performance of M3-

M*M$ relies on precoding capability than $DMA so that if the transmitter does

not use precoding% the performance advantage of M3-M*M$ is not achievable./'2

Multiple access M*M$% M*M$-SDMA%/12/'2 massive M*M$%/02 cooperative M*M$%

coordinated multipoint (&oM4) or in other wordsmacrodiversity%/62 and ad hoc

M*M$ are all family terminologies within M3-M*M$% as each of those

technologies leverages multiple users as a degree of freedom in achieving

successful radio transmission.

Technology /edit2

MIMO/edit2

o achieve M*M$ from a conventional S*S$ system% several technologies have

been proposed.

http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/MIMOhttp://en.wikipedia.org/wiki/Wireless_communicationshttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/OFDMAhttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-gesbert-1http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-jindal-2http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-gesbert-1http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-Hoydis-3http://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-fnt2013-4http://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=2http://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/MIMOhttp://en.wikipedia.org/wiki/Wireless_communicationshttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/OFDMAhttp://en.wikipedia.org/wiki/Precodinghttp://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-gesbert-1http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-jindal-2http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-gesbert-1http://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-Hoydis-3http://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Multi-user_MIMO#cite_note-fnt2013-4http://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=2


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• !eamforming alters the phase of each element in an antenna array to

create spatial beam patterns through constructive and destructive interference.

• Space-time coding7processing performs antenna diversity with multiple

antennas at either transmitter or receiver side or both sides% where every

antenna element is separated from its nearest element by around four to ten

times the wavelength to eep the signal through each multi-path independent.

he distance between two ad"acent antenna elements is relying on the angular

spread of the beam signal.

• Space-division multiple access (SDMA) is a common and typical multiple

input multiple output scheme in cellular wireless systems. SDMA is often

referred to as simply a M*M$ system since the half port of a SDMA system

also consists of multiple users. Although SDMA is indeed a M*M$ technique%

M*M$ is not necessarily SDMA.

• Spatial multiple+ing is performed by multiple antennas equipped at both a

transmitter and a receiver front end.

• &ooperation are nown as networ M*M$ systems% distributed M*M$

systems% macrodiversity M*M$% or virtual antenna array systems. Mobile

devices use the partnered mobile devices8 antennas% antenna arrays% or

antenna elements as virtual antennas.

• &ombinations of the above listed techniques

MU-MIMO/edit2

Multi-user M*M$ can leverage multiple users as spatially distributed transmission

resources% at the cost of somewhat more e+pensive signal processing. *n

comparison% conventional% or single-user M*M$ considers only local device

multiple antenna dimensions. Multi-user M*M$ algorithms are developed to

enhance M*M$ systems when the number of users% or connections% numbers

greater than one (admittedly% a useful concept). Multi-user M*M$ can be

generali9ed into two categories: M*M$ broadcast channels (M*M$ !&) and M*M$multiple access channels (M*M$ MA&) for downlin and uplin situations%

http://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Space%E2%80%93time_codehttp://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Cooperative_wireless_communicationshttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=4http://en.wikipedia.org/wiki/Multiple-input_multiple-output_communicationshttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Space%E2%80%93time_codehttp://en.wikipedia.org/wiki/Space-division_multiple_accesshttp://en.wikipedia.org/wiki/Spatial_multiplexinghttp://en.wikipedia.org/wiki/Cooperative_wireless_communicationshttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/w/index.php?title=Multi-user_MIMO&action=edit&section=4http://en.wikipedia.org/wiki/Multiple-input_multiple-output_communications


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respectively. Single-user M*M$ can be represented as point-to-point% pairwise

M*M$.

o remove ambiguity of the words receiver and transmitter % we can adopt theterms access point (A4; or% base station)% and user . An A4 is the transmitter and a

user is the receiver for downlin environments% whereas an A4 is the receiver and

a user is the transmitter for uplin environments.


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networ. he power control in either !S or MS is implemented in most modern

networs% especially on the MS% as this ensures a better battery life for the MS

and thus a better user e+perience (in that the need to charge the battery becomes

less frequent). his is why it may actually be safer to have a !S close to you as

your MS will be powered down as much as possible. or e+ample% there is more

power being transmitted from the MS than what you would receive from the !S

even if you are > m away from a mast.


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where denotes


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Formal definition/edit2

5et and be the random variables representing the input and output of the

channel% respectively. 5et be the conditional distribution function of

given % which is an inherent fi+ed property of the communications channel. hen

the choice of the marginal distribution completely determines the "oint

distribution due to the identity

which% in turn% induces a mutual information . he channel

capacity is defined as

where the supremum is taen over all possible choices of .

Shannon capacity of a graph/edit2

Main article: Shannon capacity of a graph

*f G is an undirected graph% it can be used to define a communications

channel in which the symbols are the graph vertices% and two codewords

may be confused with each other if their symbols in each position are equal

or ad"acent. he computational comple+ity of finding the Shannon capacity

of such a channel remains open% but it can be upper bounded by another

important graph invariant% the5ovs9 number ./62

Noisy-channel coding theorem/edit2

he noisy-channel coding theorem states that for any B C @ and for any

transmission rate R less than the channel capacity C % there is an encoding

and decoding scheme transmitting data at rate R whose error probability is

http://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=1http://en.wikipedia.org/wiki/Conditional_distributionhttp://en.wikipedia.org/wiki/Marginal_distributionhttp://en.wikipedia.org/wiki/Joint_probability_distributionhttp://en.wikipedia.org/wiki/Joint_probability_distributionhttp://en.wikipedia.org/wiki/Mutual_informationhttp://en.wikipedia.org/wiki/Infimum_and_supremumhttp://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=2http://en.wikipedia.org/wiki/Shannon_capacity_of_a_graphhttp://en.wikipedia.org/wiki/Undirected_graphhttp://en.wikipedia.org/wiki/Lov%C3%A1sz_numberhttp://en.wikipedia.org/wiki/Channel_capacity#cite_note-4http://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=3http://en.wikipedia.org/wiki/Noisy-channel_coding_theoremhttp://en.wikipedia.org/wiki/Information_theory#Ratehttp://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=1http://en.wikipedia.org/wiki/Conditional_distributionhttp://en.wikipedia.org/wiki/Marginal_distributionhttp://en.wikipedia.org/wiki/Joint_probability_distributionhttp://en.wikipedia.org/wiki/Joint_probability_distributionhttp://en.wikipedia.org/wiki/Mutual_informationhttp://en.wikipedia.org/wiki/Infimum_and_supremumhttp://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=2http://en.wikipedia.org/wiki/Shannon_capacity_of_a_graphhttp://en.wikipedia.org/wiki/Undirected_graphhttp://en.wikipedia.org/wiki/Lov%C3%A1sz_numberhttp://en.wikipedia.org/wiki/Channel_capacity#cite_note-4http://en.wikipedia.org/w/index.php?title=Channel_capacity&action=edit&section=3http://en.wikipedia.org/wiki/Noisy-channel_coding_theoremhttp://en.wikipedia.org/wiki/Information_theory#Rate


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less than B% for a sufficiently large bloc length. Also% for any rate greater

than the channel capacity% the probability of error at the receiver goes to

one as the bloc length goes to infinity.

Example application/edit2

An application of the channel capacity concept to an additive white

=aussian noise (A=,) channel with


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where is the received signal-to-noise ratio (S,). his result

is nown as the ShannonE2

hen the S, is large (S, CC @ d!)% the

capacity is logarithmic in power and

appro+imately linear in bandwidth. his is called the band"idth-

limited regime.

hen the S, is small (S, GG @ d!)% the capacityis linear in power but insensitive to bandwidth. his is called

the po"er-limited regime.

he bandwidth-limited regime and power-limited regime are

illustrated in the figure.

http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theoremhttp://en.wikipedia.org/wiki/Channel_capacity#cite_note-6http://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theoremhttp://en.wikipedia.org/wiki/Channel_capacity#cite_note-6


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A=, channel capacity with the power-limited regime and bandwidth-limited

regime indicated.


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ast-#ading channel/edit2

*n a fast-fading channel% where the latency requirement is

greater than the coherence time and the codeword length

spans many coherence periods% one can average over many

independent channel fades by coding over a large number of

coherence time intervals. hus% it is possible to achieve a

reliable rate of communication of

/bits7s7


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Applications that employ the SHD include computing the pseudoinverse% least

squares fitting of data% multivariable control% matri+ appro+imation% and

determining the ran% range and null spaceof a matri+.

Statement of the theorem/edit2

SupposeM is a m × n matri+ whose entries come from the field K % which is

either the field of real numbers or the field of comple+ numbers. hen there e+ists

a factori9ation of the form

where U is an m × m unitary matri+ over K (orthogonal matri+ if K = R )% Σ is

a m × n diagonal matri+ with non-negative real numbers on the diagonal% and

the n × n unitary matri+ V∗ denotes the con"ugate transpose of

the n × n unitary matri+ V. Such a factori9ation is called a singular value

decomposition ofM.

he diagonal entries σ i of Σ are nown as the singular values of M. A

common convention is to list the singular values in descending order. *n thiscase% the diagonal matri+ Σ is uniquely determined byM (though the

matrices U and V are not).

LTE MIMO Concepts

Multiple Input Multiple Output (MIMO) systems form an essential part of LTE in order

to achieve the ambitious requirements for throughput and spectral eciency! MIMO

refers to the use of multiple antennas at transmitter and receiver side!

Downlink MIMO

"or the LTE do#nlin$% a &'& conguration for MIMO is assumed as baseline

conguration% i!e! & transmit antennas at the base station and & receive antennas at

the terminal side! ongurations #ith * antennas are also being considered!

Downlink MIMO modes

+i,erent MIMO modes are envisaged! It has to be di,erentiated bet#een spatial

multiple-ing and transmit diversity% and it depends on the channel condition #hichscheme to select!

http://en.wikipedia.org/wiki/Moore%E2%80%93Penrose_pseudoinversehttp://en.wikipedia.org/wiki/Least_squareshttp://en.wikipedia.org/wiki/Least_squareshttp://en.wikipedia.org/wiki/Rank_of_a_matrixhttp://en.wikipedia.org/wiki/Range_of_a_matrixhttp://en.wikipedia.org/wiki/Kernel_(matrix)http://en.wikipedia.org/w/index.php?title=Singular_value_decomposition&action=edit&section=1http://en.wikipedia.org/wiki/Matrix_(mathematics)http://en.wikipedia.org/wiki/Field_(mathematics)http://en.wikipedia.org/wiki/Real_numberhttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Unitary_matrixhttp://en.wikipedia.org/wiki/Orthogonal_matrixhttp://en.wikipedia.org/wiki/Diagonal_matrixhttp://en.wikipedia.org/wiki/Conjugate_transposehttp://en.wikipedia.org/wiki/Singular_valuehttp://en.wikipedia.org/wiki/Moore%E2%80%93Penrose_pseudoinversehttp://en.wikipedia.org/wiki/Least_squareshttp://en.wikipedia.org/wiki/Least_squareshttp://en.wikipedia.org/wiki/Rank_of_a_matrixhttp://en.wikipedia.org/wiki/Range_of_a_matrixhttp://en.wikipedia.org/wiki/Kernel_(matrix)http://en.wikipedia.org/w/index.php?title=Singular_value_decomposition&action=edit&section=1http://en.wikipedia.org/wiki/Matrix_(mathematics)http://en.wikipedia.org/wiki/Field_(mathematics)http://en.wikipedia.org/wiki/Real_numberhttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Unitary_matrixhttp://en.wikipedia.org/wiki/Orthogonal_matrixhttp://en.wikipedia.org/wiki/Diagonal_matrixhttp://en.wikipedia.org/wiki/Conjugate_transposehttp://en.wikipedia.org/wiki/Singular_value


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Spatial Multiplexin

.patial multiple-ing allo#s to transmit di,erent streams of data simultaneously on

the same do#nlin$ resource bloc$(s)! These data streams can belong to one single

user (single user MIMO / .01MIMO) or to di,erent users (multi user MIMO / M01

MIMO)! 2hile .01MIMO increases the data rate of one user% M01MIMO allo#s to

increase the overall capacity! .patial multiple-ing is only possible if the mobile radio

channel allo#s it! "igure 3 sho#s the principle of spatial multiple-ing% e-ploiting the

spatial dimension of the radio channel #hich allo#s to transmit the di,erent data

streams simultaneously!

"igure 3 .patial multiple-ing

In "igure 3% each transmit antenna transmits a di,erent data stream! Each receiveantenna may receive the data streams from all transmit antennas! The channel (for a

specic delay) can thus be described by the follo#ing channel matri- 45 6s above


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gure!

LTE!E-UT"#

In this general description% 7t is the number of transmit antennas% 7r is the number

of receive antennas% resulting in a &'& matri- for the baseline LTE scenario! The

coecients hi8 of this matri- are called channel coecients from transmit antenna 8

to receive antenna i% thus describing all possible paths bet#een transmitter and

receiver side! The number of data streams that can be transmitted in parallel over

the MIMO channel is given by min 97t% 7r: and is limited by the ran$ of the matri- 4!

The transmission quality degrades signicantly in case the singular values of matri-

4 are not suciently strong! This can happen in case the & antennas are not

suciently de1correlated% for e-ample in an environment #ith little scattering or

#hen antennas are too closely spaced! In LTE% up to & code #ords can be mapped

onto di,erent so1called layers! The number of layers for transmission is equal to the

ran$ of the matri- 4! There is a -ed mapping bet#een code #ords to layers!

;recoding on transmitter side is used to support spatial multiple-ing% see "igure &!

This is achieved by applying a precoding matri- 2 to the signal before transmission!

"igure & ;recoding principle

The optimum precoding matri- 2 is selected from a predened and 0E side! 0nitary precoding is used% i!e! the precoding matricesare unitary5 242? I! The 0E estimates the radio channel and selects the optimum


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precoding matri-! The optimum precoding matri- is the one #hich o,ers ma-imum

capacity! The 0E provides feedbac$ on the uplin$ control channel regarding the

preferred precoding matri- (precoding vector as a special case)! Ideally% this

information is made available per resource bloc$ or at least group of resource bloc$s%

since the optimum precoding matri- varies bet#een resource bloc$s "igure @ gives

an overvie# of E0TA6 do#nlin$ baseband signal generation including the above1

mentioned steps relevant for MIMO transmission!

"igure @ Overvie# of do#nlin$ baseband signal generation

LTE!E-UT"#

T$ansmit Di%e$sity

Instead of increasing data rate or capacity% MIMO can be used to e-ploit diversity!

Transmit diversity schemes are already $no#n from 2+M6 release BB and #ill also

form part of LTE as one MIMO mode! In case the channel conditions do not allo#spatial multiple-ing% a transmit diversity scheme #ill be used instead% so s#itching

bet#een these t#o MIMO modes is possible depending on channel conditions!

Transmit diversity is used #hen the selected number of streams (ran$) is one!

Uplink MIMO

0plin$ MIMO schemes for LTE #ill di,er from do#nlin$ MIMO schemes to ta$e into

account terminal comple-ity issues! "or the uplin$% M01MIMO can be used! Multiple

user terminals may transmit simultaneously on the same resource bloc$! This is also

referred to as spatial domain multiple access (.+M6)! The scheme requires only one

transmit antenna at 0E side #hich is a big advantage! The 0Es sharing the sameresource bloc$ have to apply mutually orthogonal pilot patterns! To e-ploit the

benet of t#o or more transmit antennas but still $eep the 0E cost lo#% antenna

subset selection can be used! In the beginning% this technique #ill be used% e!g! a 0E

#ill have t#o transmit antennas but only one transmit chain and amplier! 6 s#itch

#ill then choose the antenna that rovides the best channel to the e7ode>!

Documents

LTE-A MIMO Concepts May 2015 Ashwani