lte air interfacce

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LTE - Physical Layer

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Contents

1 Physical Layer in Downlink 2

1.1

Orthogonal Frequency Division Multiplex (OFDM) in LTE 2

1.2 Time Domain in LTE 9

1.3 Orthogonal Frequency Division Multiple Access (OFDMA) 11

1.4 Resource Blocks and Resource Grid 12

1.5 Downlink Reference Signals 15

2 Downlink physical channels 16

2.1 Usage of the Resource Block Grid 16

2.2 Physical Broadcast Channel (PBCH) 17

2.3 Physical Control Format Indicator Channel (PCFICH) 17

2.4 Physical Hybrid-ARQ Indicator Channel (PHICH) 17

2.5 Physical Downlink Control Channel (PDCCH) 18

2.6 Physical Downlink Shared Channel (PDSCH) 18

3 Physical Layer in Uplink 20

3.1 The Use of SC-FDMA in Uplink 20

3.2 Reference Signals in Uplink 23

3.3 Uplink Physical Channels 26

3.4 Physical Uplink Control Channel (PUCCH) 27

3.5

Physical Uplink Shared Channel (PUSCH) 27

3.6 Physical Random Access Channel (PRACH) 27

4 Supported Modulation schemes in LTE 28

5 Spectrum Allocation 32

6 Duplex methods in LTE 33

7 The Use of MIMO in LTE 34

7.1 MIMO and Smart Antenna Concepts 34

7.2 Variants of multi-antenna usage 36

7.3 Single User and Multi-User MIMO 39



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1 Physical Layer in Downlink

1.1 Orthogonal Frequency Division Multiplex (OFDM)in LTE

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier technology.

The basic idea is to spread the information on a lot of sub-carriers in order to createvery narrow band channels, which are orthogonal to each other.

frequency

10011101110011101010001100010110

Serial-to-parallel Converter

1001 1101 1100 1110 1010 0011 0001 0110

Fig. 1 OFDM: Multicarrier Technology

One approach to explain OFDM is by starting with its technological predecessorFrequency Division Multiplexing (FDM). While a multitude of different systemsemploy FDM, the easiest example is arguably the ubiquitous FM radio. In order toreceive a specific station, the radio needs to be tuned to the correspondingfrequency. However, when scanning for radio stations one can observe that there is afrequency band between two adjacent stations that appears to be unused. In theBoston metropolitan area, the classic rock station WZLX can be received at 100.7MHz, the station on the next lower frequency is WBRS, the campus radio of BrandeisUniversity, on 100.1 MHz. Tuning ones radio to 100.4 MHz in the Boston area wouldyield nothing more than the reception of static. In fact, this unused frequency band is

a so-called "Guard Band", which is always employed in FDM systems. A guard bandensures that two neighboring signals do not interfere with each other, so that


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interference between different carriers, the so-called Adjacent Carrier Interference(ACI)is minimized.

Yet, the use of a guard bandsignificantly decreases the spectral efficiency of thesystem. In OFDM spectral efficiency is optimized by allowing signals to overlap. Dueto the orthogonal nature of the individual signals, interference between signals isnullified. OFDM segments a frequency band into several subcarriers or frequencieswhich are equally spaced. Each frequency represents a sub-carrier.

While development of OFDM began as early as the 1950's and the structure ofOFDM was patented in 1966 by Robert Chang, the first OFDM-based standard, theDigital Audio Broadcast standard (DAB), was not published until 1995. DAB is astandard for digital radio transmission which was developed as a replacement for FM

audio broadcasting. OFDM was chosen as the underlying technology due to its hightolerance of multipath propagation, spectral efficiency and flexibility in assigningresources to users.

FDM

OFDM

frequency

frequency

Fig. 1 FDM versus OFDM

Subcarriers in OFDM must be orthogonalto each other in order to allow overlappingof the spectrum without interference. At this point, it is necessary to define"Orthogonality".

In radio communications, multiple access schemes are orthogonal when a receivercan (theoretically) completely reject an arbitrarily strong unwanted signal. In otherwords, orthogonality allows the transmission of several signals over one channelwhile each signal can be received perfectly without degradation of signal quality.


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Many common multiplexing schemes are inherently orthogonal. Time DivisionMultiplexing (TDM) allows transmission of multiple information signals over a singlechannel by assigning unique time slots to each separate information signal. During

each time slot only the signal from a single source is transmitted preventing anyinterference between the multiple information sources. Hence, TDM is orthogonal innature.

In FDM, orthogonality can be achieved by the introduction of large guards bands,which obviates overlapping (and therefore interference) of signals.

Although orthogonality can be achieved in FDM, the term Orthogonal FrequencyDivision Multiplex is reserved for a specific access scheme. OFDM retainsorthogonality while minimizing carrier spacing. The frequency domain null of one

subcarrier corresponds to the maximum value of the adjacent subcarrier, whichallows the subcarriers to partially overlap without interference with each other.

In the frequency domain each OFDM subcarrier has a sinc, sin(x)/x,frequencyresponse, as shown in the figure below. The sinc-shaped frequency response is aresult of the carrier spacing being a multiple of the inverse of the symbol time andoriginates from the Inverse Fast Fourier Transformation (IFFT), which is used forpulse shaping and modulation.

The sinc shape has a narrow main lobe, with many side-lobes that decay slowly with

the magnitude of the frequency difference away from the centre.

Each carrier has a peak at the centre frequency and nulls evenly spaced with afrequency gap equal to the carrier spacingThe orthogonal nature of the transmissionis a result of the peak of each subcarrier corresponding to the nulls of all othersubcarriers.


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-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

-8 -6 -4 -2 0 2 4 6 8

Frequenz (in Sub-Carrier-Abstnden)

Leistung

Frequency (Carrier Spacings)

TX

Power

Fig. 2 OFDM signal consisting of some subcarriers

A complete OFDM signal consists of a sum of sinusoids, with each corresponding toa subcarrier.

The baseband frequency of each subcarrier is chosen to be an integer multiple of theinverse of the symbol time, resulting in all subcarriers having an integer number of

cycles per symbol.

The selection of subcarrier frequencies according to this relation to the symbolduration is a necessity for achieving orthogonality.

The figure below shows the time domain signals for four subcarriers as well as theresulting OFDM signal.


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-28

-24

-20

-16

-12

-8

-4

0

4

0 1 2 3 4 5 6 7 8

time

Am

plitude

Fig. 3 OFDM Signal in time domain

1.1.1 Guard period / Cyclic Prefix

The effect of Intersymbol Interference (ISI) on an OFDM signal can be improved bythe addition of a guard period to the start of each symbol. This guard period is acyclic copy that extends the length of the symbol waveform. Each subcarrier, in thedata section of the symbol, (i.e. the OFDM symbol with no guard period added, whichis equal to the length of the IFFT size used to generate the signal) has an integernumber of cycles. Hence, placing copies of the symbol end-to-end results in acontinuous signal, with no discontinuities at the joins. Thus by copying the end of asymbol and appending this to the start results in a longer symbol time. The figurebelow shows the insertion of a guard period.

In addition to protecting the OFDM from Intersymbol Interference, the guard periodalso provides protection against time-offset errors in the receiver.


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copy

Guard Period

Fig. 4 Continuous signal

The guard period is added to each symbol before transmission. The receiver simply

disregards the guard period.

Any unwanted effects, like Intersymbol Interference or phase jitter, that have affectedthe signal within the guard period are ignored and do therefore not influence thesignal part containing the information.


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1.1.2 Subcarrier Spacing

In order to obtain orthogonality between subcarriers, it is necessary to define the

subcarrier spacing as the inverse of the symbol time. Since the cyclic prefix isremoved at the receiver previous to performing the DFT, only the length of the dataportion of the symbol is of importance when calculating the carrier spacing

In LTE, the symbol time of 66.7s is defined - this refers to a subcarrier spacing of 15kHz.

Sub-carrier spacing

Bandwidth

Frequency

Sampling point for sub-carrier =Zero Value for all other sub-carriers

Fig. 5 Subcarrier Spacing

Note:

A reduced subcarrier spacing of 7.5 kHz has also been defined for MultimediaBroadcast Multicast Service (MBMS) specific parameterization.


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1.2 Time Domain in LTE

In LTE, two frame structures are defined:

Frame structure 1 for FDD mode

Frame structure 2 for TDD mode

In frame structure 1 for FDD mode the 10ms radio frame is divided into 20 equallysized slots of 0.5ms. Therefore a 1ms sub-frame consists of two consecutive slots.

#0#0 #1#1 #2#2 #3#3 #19#19

One slot, Tslot = 15360Ts = 0.5 ms

One radio frame, Tf= 307200Ts=10 ms

#18#18

One subframe

Fig. 6 Frame structure

Each slot then consists of a number of OFDM symbols including cyclic prefix.

In LTE, two cyclic-prefix lengths are defined

the normal cyclic prefix

the extended cyclic prefixfor cell scenarios with high delay spread andMultimedia Broadcast Multicast Service (MBMS) transmission


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1 subframe = 2 slots

Symbol 0CP Symbol 1CP Symbol 2CP Symbol 3CP Symbol 4CP Symbol 5CP Symbol 6CP

Normal CP:Symbol time: 66.7sCP for first symbol: 5.1sCP for other symbols: 4.7s

Extended CP:Symbol time: 66.7sCP: 16.7s

Symbol 0CP Symbol 1CP Symbol 2CP Symbol 3CP Symbol 4CP Symbol 5CP

time

Fig. 7 Timing structure in LTE


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1.3 Orthogonal Frequency Division Multiple Access(OFDMA)

OFDM in its primary form is considered as a digital modulation technique, and not amulti-user channel access technique, since it is utilized for transferring one bit streamover one communication channel using one sequence of OFDM symbols. However,OFDM can be combined with multiple access, using time, frequency or coding forseparation of the users.

Multiple access is achieved in OFDMA by assigning subsets of sub-carriers toindividual users as shown in the figure below. This allows simultaneous transmissionfrom several users.

Based on feedback information about the channel conditions, adaptive user-to-sub-carrier assignment can be achieved. If the assignment is done sufficiently fast, thisfurther improves the OFDM robustness to fast fading and narrow-band co-channelinterference, and makes it possible to achieve even better system spectral efficiency.

Different number of sub-carriers can be assigned to different users, in view to supportdifferentiated Quality of Service (QoS), i.e. to control the data rate and errorprobability individually for each.

OFDM allocates users in timedomain only

OFDMA allocates users in time andfrequency domain

Time domain Time domain

Frequen

cydomain

Frequen

cydomain

User3

User3

User2

User2

User1

User1

Fig. 8 Differenence OFDM - OFDMA

OFDMA provides more flexibility with regard to the data rate of simultaneous datastreams than pure OFDM.


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1.4 Resource Blocks and Resource Grid

The spectral resources are allocated as a combination of both time and frequencyunits - that means a combination of slots and subcarriers.

Resource elementis the smallest identifiable unit of transmission and consists ofone subcarrier for one symbol period.

Resource blockscomprise 12 adjacent subcarriers for one slot period. Therefore aresource block consists of 84 resource elements in case of normal cyclic prefix.

The minimum scheduling unitconsists of two resource blocks within one subframe.

This is also sometimes referred to as resource-block pair.


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One downlink slot

Resource block

Resource element

Subcarriers

symbols

Fig. 9 Resource Grid

Currently, 6 Resource Blocks are the minimum supported bandwidth and 100 is themaximum supported bandwidth:


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6 Resource Blocks* 12 (12 subcarriers per resource Block)* 15 kHz (15 kHz Subcarrier spacing in LTE)

1.08 MHz

100 Resource Blocks

* 12 (12 subcarriers per resource Block)* 15 kHz (15 kHz Subcarrier spacing in LTE)

18 MHz

Fig. 10 Relation bandwidth - subcarrier

In Release 8 3GPP the following Channel Configurations are defined:

Fig. 11 Transmission bandwidth configuration in LTE (3GPP, TS36.101)

Network operators can use scalable bandwidth between 1.4 and 20 MHz. Largebandwidths obviously provide higher data rates whereas small bandwidths can be

useful to reuse small available spectrum.


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1.5 Downlink Reference Signals

OFDM is very sensitive to frequency errors, which might result in Inter-Carrier-Interference. To be able to perform coherent demodulation of different downlinkphysical channels, a mobile terminal needs to estimate the downlink channel. LTEapplies therefore a downlink reference signal structure. Reference Signals are a setof resource elements used by the physical layer without carrying informationoriginating from higher layers.

time

frequen

cy

Fig. 12 Cell Specific Reference Signals - one antenna port (3GPP, TS36.211)


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2 Downlink physical channels

2.1 Usage of the Resource Block Grid

Now let's talk about how these physical resources are used.

Basically, there are three different types of usage:

Reference Signals to help demodulate and decode the signal

Control Channels to convey Control Information, for example scheduling

assignments User Data Channels for User Data

Each subframe in downlink is divided into a control region, which is occupied byControl Channels, followed by a data region for user data. The control regionoccupies 1,2 or 3 symbols (in small bandwidths 2,3 or 4 symbols).

Reference

Signal

1 subframe (1ms) = Scheduling Period

1 slot (0.5 ms)

Control Region(1 up to 3

OFDM Symbols)

DataRegion

Fig. 13 Control and Data region

The following Downlink Physical Channels exist in LTE:


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PHICH

:ACK

/NACK

PDCC

H:As

signment

sinDL

,Sche

duling

Grants

inUL

PDSCH:

UserData

PBCH: Broadcast

PCFIC

H:Siz

eofC

ontrolRe

gion

Fig. 14 Physical Channels in LTE - DL

2.2 Physical Broadcast Channel (PBCH)

The Broadcast Channel transmits system control information. It carries the mostimportant parameters to access the cell and the references where the other SystemInformation can be found. The PBCH is located in the data region.

2.3 Physical Control Format Indicator Channel

(PCFICH)The PCFICH provides the UE with the information about the size of the control regionwithin a subframe and is located in the control region. It carries the number of OFDMsymbols being used for the control region within a subframe (2 bits).

2.4 Physical Hybrid-ARQ Indicator Channel (PHICH)

PHICH is used to transfer acknowledgements in case of correct reception of atransmission and to request retransmissions in case of failed reception in Uplink


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(ACK/NACK messages). The HARQ indicator consists of 1 bit and is set to 0 for apositive Acknowledgement (ACK) and 1 for a Negative Acknowledgement (1).

The PHICH is also located in the control region.

2.5 Physical Downlink Control Channel (PDCCH)

The Physical Downlink Control Channel carries the so-called DCI (Downlink ControlInformation). DCI can include:

Downlink Scheduling Information

Uplink Scheduling Information

Uplink Power Control Commands

Each Scheduling message is transmitted on a separate PDCCH. The size dependson the information which has to be transmitted.

The PDCCHs are located in the control region.

2.6 Physical Downlink Shared Channel (PDSCH)

The PDSCH is used to transmit user data traffic. The PDSCH is located in the dataregion.


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3.eNBt

ransfe

rsassi

gnment

sviaP

DCCH

.UEs

earche

sPDC

CH

initsS

earch

Space

sfora

ssignm

ents

4.eNB

transf

ersUs

erDa

taviaP

DSCH

onthe

assig

nedresou

rces

(freque

ncy/tim

e).UE

decodes

data.

2.UEu

sesPC

FICHtog

etthe

sizeofC

ontrolRe

gion

1. eNB decides to schedule UE

Fig. 15 Physical Channels in DL


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3 Physical Layer in Uplink

3.1 The Use of SC-FDMA in Uplink

In the Uplink, SC-FDMA (Single Carrier Frequency Division Multiple Access) waschosen as transmission scheme in Uplink.

The use of OFDMA in Uplink was also investigated, but proofed to have two maindisadvantages:

High Peak-to-Average Power Ratio

With increasing number of subcarriers, the composite time-domain signal has a highpeak-to-average ratio (PAR) that can cause problems for amplifiers.

This is basically caused by the constructive addition of subcarriers.

The following figure shows the generated OFDM signal. Note how much it variescompared to the underlying constant amplitude sub-carriers.

OFDM signal

Underlying OFDM subcarrier

Fig. 16 Peak-Average-Ratio in an OFDM signal

On the downlink, this problem can be overcome by using high compression pointPower Amplifier and sophisticated peak-to-average ratio reduction mechanisms. Theuse of these methods in Uplink would lead to an increase in cost, size and powerconsumption for the user devices.

Another problem is the sensitivity of OFDM to frequency errors.


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SC-FDMA signals have better PAPR properties compared to an OFDMA signal,which is important for cost-effective design of UE power amplifiers. SC-FDMAcombines the low PAR of single-carrier systems with the multipath resistance and

flexible subcarrier frequency allocation offered by OFDM.

In LTE, for generation of a SC-FDMA signal DFT-spread-OFDM has been selected.

DFT-spread-OFDM is often referred to as a normal OFDM with a DFT-basedprecoding.

DFT: Discrete Fast Fourier Transformation: time -> frequency Domain

IFFT: Inverse Fast Fourier Transformation: frequency -> time domain

DFTSub-carrier

mappingIFFT

Additional Processingstep for DFT-spread-

OFDM

Fig. 17 DDFT-spread-OFDM in LTE - principle

OFDMA transmits a data stream by using several narrow band sub-carrierssimultaneously. The input bits are first grouped and assigned for transmission overdifferent frequencies (sub-carriers). In theory, each sub-carrier signal could begenerated by a separate transmission chain hardware block. The output of theseblocks would then have to be summed up and the resulting signal could then be sent

over the air. Due to the high number of sub-carriers used this approach is notpracticable. Instead, a mathematical approach is taken: A mathematical functionalcalled Inverse Fast Fourier Transformation (IFFT) is applied. The Inverse FastFourier Transformation thus does exactly the same as the separate transmissionchains for each sub carrier would do including summing up the individual results.

On the receiver side the signal is first demodulated and amplified. The result is thentreated by a Fast Fourier Transformation function which converts the time signal backinto the frequency domain. This reconstructs the frequency/amplitude diagramcreated at the transmitter. At the center frequency of each sub-carrier a detectorfunction is then used to generate the bits which were originally used to create the

sub-carrier.


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Despite its name, Single Carrier Frequency Division Multiple Access (SC-FDMA) alsotransmits data over the air interface in many sub-carriers but adds an additionalprocessing step. Instead of assigning the bits to the subcarriers directly as in OFDM

to form the signal for one sub-carrier, the additional processing block in SC-FDMAspreads the information of each bit over all the sub-carriers.

One uplink slot

Resource block

Resource element

Subcarriers

symbols

Fig. 18 Resource Grid in Uplink


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3.2 Reference Signals in Uplink

Similar to the downlink, Uplink Reference Signals are used for channel estimationand data demodulation at the eNB and as scheduling input for the scheduler.

Other than in the Downlink, the Uplink Reference Signals are not frequencymultiplexed but time multiplexed with other uplink transmissions.

Two types of uplink reference signals are supported:

Uplink Demodulation Reference signal (DRS)- associated with transmission ofPhysical Uplink Channel

Sounding Reference Signal (SRS)- not associated with transmission of PhysicalUplink Channel

The Uplink Demodulation Reference Signalis used for Channel estimation forDemodulation and is also transmitted together and covering the same frequencyband as the corresponding physical channel. It occupies the same bandwidth as itsphysical channel data transmission.


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TransmissionUE2

TransmissionUE1

C

arrier

time

Fig. 19 Uplink Reference Signal

The Sounding Reference Signal is transmitted on the Uplink to allow for thenetwork to estimate the Uplink channel quality at different frequencies and thereforeenable frequency selective scheduling. The Sounding Reference Signals are alwaystransmitted in the last symbol in the configured subframe.


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1 Subframe

Fig. 20 Uplink Sounding Reference Signal - Frequency Hopping


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3.3 Uplink Physical Channels

The following uplink physical channels are defined:

PRACH:

Rando

mAcce

ssChann

elforInit

ialAcc

ess

PUCC

H:Ch

annelS

tatusR

eports,

ACK/NAC

K,

Sched

uling

PUSCH:

Chann

elStatu

sRepo

rts,ACK/N

ACK,

User

Data

Fig. 21 Physical Channels in UL

Note:

PUCCH and PUSCH are never transmitted at the same time from one UE. ThePUCCH is used in case the mobile does not have a valid scheduling grant for aPUSCH. Otherwise, the uplink L1/L2 control signaling is time multiplexed with the

user data on the PUSCH.


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3.4 Physical Uplink Control Channel (PUCCH)

The PUCCH carries uplink control information, the so-called UCI (Uplink ControlInformation).This contains:

Channel Quality Indicator (CQI): The CQI is an indicator sent from the mobile tothe eNB of the data rate which can be supported by the channel, including arecommended modulation scheme and coding rate for downlink transmission.

Precoder Matrix Indication (PMI): Recommended Precoder Matrix for closed-looped MIMO in Downlink.

Rank Indication (RI): Recommendation about the number of layers to be used inDownlink if MIMO is used.

ACK/NACK:Feedback from the mobile to the eNB concerning the previoustransmission. If the decoding has failed, a retransmission is requested using aNACK, otherwise ACK signals to the eNB that it can delete the sent data packetsin its buffer.

Scheduling Request (SR): Using the Scheduling Request the UE requests UL-SCH resources for new Transmission in uplink.

3.5 Physical Uplink Shared Channel (PUSCH)

The PUSCH is used for transmission of User Data and L1/L2 control data, which ismultiplexed on the PUSCH with the User Data:

User Data

Channel Quality Indicator (CQI)

Precoding Matrix Information (PMI)

Rank Indicator (RI)

ACK/NACK

3.6 Physical Random Access Channel (PRACH)

The PRACH is used for Random Access.


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4 Supported Modulation schemes in LTEIn mobile networks, information is transmitted using an electromagnetic wave, whichhas the form of a sinusoid.

Fig. 22 sine wave

Now information has to be transmitted via the electro-magnetical wave. This is doneby changing the characteristic of the sine wave, for example frequency, amplitude orphase:

Fig. 23 Amplitude Modulation


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Fig. 24 Frequency Modulation

Fig. 25 Phase Modulation

The modulation varies not only in the characteristics, which is changed, but also inthe definition of the number of different states.

For example, using two different states, you can convey 1 bit of information in a

digital system: 0 and 1. If you use four different states, you can already submit 2 bitsof information, 00, 01, 10, 11, hereby doubling the data rate, and so on. Therefore,using so called higher order modulations are a popular means to enhance data ratesin a mobile system. However, the more different states you define, the more complexthe demodulation for the receiver will be and the signal will be more vulnerable to badradio conditions. Additionally, the requirements for the hardware increase.

Therefore, higher order modulations are typically used in good radio conditionswhereas in bad radio conditions more robust types of modulations are used. Theadaption of the modulation to the respective radio channel conditions over time wasfirst introduced with HSDPA:


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16

4

4

8

2

Number of differentstates

Phase / Amplitude416 Quadtrative

AmplitudeModulation16QAM

HSDPA

Phase2QPSKHSDPA

Phase2Quadrature Phase

Shift Keying(QPSK)

UMTS

Phase38 Phase Shift

Keying(8PSK)

EDGE

Frequency1Gaussian Minimum

Shift Keying(GMSK)

GSM

Modulation MethodBit per symbolModulation

Fig. 26 Modulation in Mobile Networks

64

16

4

Number of differentstates

Phase / Amplitude664 QuadratureAmplitudeModulation

(64 QAM)

Phase / Amplitude416 QuadtratureAmplitudeModulation

(16QAM)

Phase2Quaterney PhaseShift Keying(QPSK)

Modulation MethodBit per symbolModulation

Fig. 27 Modulation in LTE

The use of higher order modulationsalso enables higher data rates:


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86.457.650Peak Data rate

64QAM16QAMQPSKModulation

Fig. 28 UL Peak data rates

In downlink, all mobiles need to support QPSK, 16 QAM and 64 QAM. In uplink, onlyone type of mobiles supports 64 QAM.


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5 Spectrum AllocationE-UTRA is designed to operate in the frequency bands defined in the following table:

Fig. 29 Defined Frequency bands for LTE (3GPP, TS36.101)

The channel raster is 100 KHz (the center frequency must be a multiple of 100 KHz).


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6 Duplex methods in LTELTE supports both, Frequency division Duplex and Time Division Duplex:

Control and User Data

Guard Period to UL

UL DL

FDD TDD

Fig. 30 FDD and TDD in LTE

Vendors already demonstrated eNB operating in both, TDD and FDD mode. LTE isthe only technology which can use the same platform for both, paired and unpairedspectrum.


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7 The Use of MIMO in LTE

7.1 MIMO and Smart Antenna Concepts

In radio systems MIMO (multiple-input and multiple-output)is the use of multipleantennas at both the transmitter and receiver to improve communicationperformance. MIMO is effectively a radio antenna technology as it uses multipleantennas at the transmitter and receiver to enable a variety of signal paths to carrythe data, choosing separate paths for each antenna to enable multiple signal paths tobe used.

Tx Rx

HChannel Matrix

N input antennas M output antennas

Coding

ModulationWeighting/Mapping

Weighting/Demapping

DemodulationDecoding

Fig. 31 MIMO

A MIMO system consists of several antenna elements plus adaptive signalprocessing, at both transmitter and receiver.

MIMO technology has attracted attention in wireless communications, since it offerssignificant increases in data throughput and link range without additional bandwidthor transmit power. It achieves this by higher spectral efficiency (more bits per secondper hertz of bandwidth) and link reliability or diversity (reduced fading).

The terminologyfor these concepts refers to the communication channel. If there isone transmit antenna and two or more receive antennas, there is one input for thechannel and multiple outputs (SIMO), if there is only one receive antenna, it is calledSISO. If, however, multiple transmit antenna are used, the technique is called MISOor MIMO, depending on the number of receive antennas.


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SISO

SIMO

MISO

MIMO

Tx

Tx

Tx

Tx

Rx

Rx

Rx

Rx

Fig. 32: Antenna Systems


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7.2 Variants of multi-antenna usage

The benefits of multi-antenna systems can be sub-divided into the following maincategories:

Receive Diversity (SIMO)

Transmit Diversity (MISO)

Spatial Multiplexing (MIMO)

Beamforming (MISO, MIMO)

DiversityDiversity gain is the gain which will be gained by transmitting the same data onindependently fading branches. Each pair of transmit-receive antennas provides asignal path from transmitter to receiver. By sending the same information throughdifferent paths, multiple independently-faded replicas of the data symbol can beobtained at the receiver end. Therefore, more reliable reception is achieved. Theprinciple is to provide the receiver with multiple identical copies of a given signal tocombat fading.

Diversity

Data Stream 1

Data Stream 1

Data Stream 1

Fig. 33 Diversity Gain

Spatial MultiplexingSpatial Multiplexing Gain is achieved by transmitting streams of independent dataover different antennas, thus maximizing the average data rate over the MIMOsystem. Spatial Multiplexing increases spectral efficiency.


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Tx Rx

Data Stream 1

Data Stream 2

Data Stream nSpatial

Multiplexing (SM)

Fig. 34 Spatial Multiplexing

Beamforming

Beamforming is a general signal processing technique used to control thedirectionality of the reception or transmission of a signal on an antenna array.

Tx

RxBeamforming

Fig. 35 Beamforming

Using multiple antennas on either side of the communication channel allowenhancements (one or multiple, but not all at the same time) of the radio path

to enlarge the network coverageby obtaining a higher signal strength at thereceiver's position

to increase the reliabilityof the transmission by combining several decorrelatedsignals (diversity)

to enhance the Signal to Interference-plus-Noise Ratio(SINR) by spatialfocusing of several correlated signals (beamforming)

to increase the data rateand the spectral efficiencyby sending several datastreams in parallel (spatial multiplexing)

LTE differentiates between two transmission modes which may support beamformingfor the PDSCH:


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Close-loop rank 1 precoding: The UE feeds channel information back to the eNBto indicate suitable precoding to apply for the beamforming operation

UE specific Reference Signals: The UE does not feedback any precoding relatedinformation. UE specific Reference Signals are used.


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7.3 Single User and Multi-User MIMO

MIMO can be further be distinguished into

Single-User MIMO (SU-MIMO)

Multi-User MIMO (MU-MIMO).

S1S2

S3

SU-MIMO MU-MIMO

S1

S2

S3

Fig. 36 Single-User and Multi-User MIMO

Single User MIMO (SU-MIMO)is where two or more data streams are allocated toone user with the intent of increasing peak data rates. Throughput improves when theradio channel exhibits uncorrelated transmission paths. Single User MIMO is used indownlink in LTE.

Multi User MIMO (MU-MIMO)relies on the same principle of uncorrelatedtransmission paths, but in this case the paths belong to different users with the intentbeing to increase the capacity of the cell rather than increase peak data rates. Multi-User MIMO is used in Uplink in LTE.

lte air interfacce

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